paleozoic magmatism and crustal recycling along the ... · 281 paleozoic magmatism and crustal...

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Paleozoic magmatism and crustal recycling along the ancient Pacific margin of North America, northern Cordillera1, 2

Stephen�J.�PierceyMineral�Exploration�Research�Centre,�Department�of�Earth�Sciences,�Laurentian�University,��

933�Ramsey�Lake�Road,�Sudbury,�Ontario,�P3E�6B5,�Canada,�[email protected]

JoAnne�L.�NelsonB.C.�Geological�Survey,�P.O.�Box�9333,�Stn�Prov�Govt,�Victoria,�British�Columbia,�V8W�9N3,�Canada

Maurice�ColpronYukon�Geological�Survey,�P.O.�Box�2703�(K-10),�Whitehorse,�Yukon,�Y1A�2C6,�Canada

Cynthia�Dusel-BaconU.S.�Geological�Survey,�Mineral�Resources�Program,��

345 Middlefield Road, Menlo Park, California, 94025, USA

Renée-Luce�SimardDepartment�of�Geology,�Brandon�University,�Brandon,�Manitoba,�R7A�6A9,�Canada

Charlie�F.�RootsGeological�Survey�of�Canada,�P.O.�Box�2703�(K-10),�Whitehorse,�Yukon,�Y1A�2C6,�Canada

Piercey, S.J., Nelson, J.L., Colpron, M., Dusel-Bacon, C., Simard, R.-L. and Roots, C.F., 2006, Paleozoic magmatism and crustal recycling along the ancient Pacific margin of North America, northern Cordillera, in Colpron, M. and Nelson, J.L., eds., Paleozoic Evolution and Metallogeny of Pericratonic Terranes at the Ancient Pacific Margin of North America, Canadian and Alaskan Cordillera: Geological Association of Canada, Special Paper 45, p. 281-322.

AbstractDevonian to Permian igneous rocks in the Yukon-Tanana terrane (YTT) record six cycles of arc, arc-rift, continental

rift and back-arc basin magmatism, each set apart from the others by changes in the locus and/or character of igneous

activity, as well as deformational episodes and unconformities. The first four cycles, from mid-Devonian to Late

Mississippian, record largely bimodal arc magmatism above a west-facing (east-dipping) subduction zone, with or

without accompanying back-arc basin magmatism and continental margin rifting. The fifth, Pennsylvanian-Early

Permian cycle, involved more primitive, mafic to intermediate volcanism in a west-facing arc with a corresponding

marginal back-arc basin to the east. The sixth, Late Permian cycle reflects subduction reversal, and continental-arc

1Data�Repository�items�Piercey_DR1.xls (Table�DR1),�Piercey_DR2.xls�(Table�DR2)�are�available�on�the�CD-ROM�in�pocket.�

2GSC�Contribution�2004058

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magmatism associated with an east-facing (west-dipping) subduction zone. Mafic rocks in all cycles of both arc and non-

arc character were derived from variably enriched sources, with contributions from depleted mantle wedge or back-arc

asthenosphere, and enriched lithospheric mantle, with or without a subducted slab component. Felsic rocks in arc, arc-rift

and back-arc geodynamic settings were derived predominantly from melting and recycling of upper continental crustal

material (UCC; La/SmUCN

≈ 1). Arc felsic rocks have calc-alkalic and tholeiitic signatures, whereas non-arc rocks are

enriched in high field strength elements and rare earth elements (A-type or peralkaline signatures). Notably, throughout

the late Paleozoic magmatic history of the YTT that spanned over 150 m.y., there are no systematic temporal variations in

the composition of most mafic and felsic rocks. Igneous source regions and igneous processes were essentially unchanged

throughout the tectonic history of the YTT.

An important aspect of many mafic rocks from YTT intra-arc rifts and back-arc basins is their high Nb/Thmn

and Nb/Lamn

> 1 (mn = primitive mantle normalized), implying excess Nb relative to Th and La compared to primitive mantle ratios.

Excess Nb in these rocks implies a recycled oceanic crustal component in their genesis, and is a common feature of plume-

derived magmas and magmatic rocks from large igneous provinces. The recurrence of this recycled component over the

extended magmatic history in YTT, and the common occurrence as low volume eruptions, argues against a direct plume

or large-igneous province origin; however, it is possible that this signature reflects the reactivation of recycled plume or

large igneous province components in the YTT lithospheric mantle that were originally derived via lithospheric fertilization

by magmatism associated with Neoproterozoic breakup of Rodinia.

RésuméLes roches ignées dévoniennes à permiennes du terrane de Yukon-Tanana (YTT) témoignent de la présence de six cycles

de formation d’arc, d’ouverture d’un fossé tectonique d’arc, d’ouverture d’un fossé tectonique continental et d’un mag-

matisme de bassin d’arrière-arc, chacun se distinguant des autres par des changements dans l’emplacement ou le caractère

de l’activité ignée, ainsi que par des épisodes de déformation et des discordances. Les quatre premiers cycles, du Dévonien

moyen au Mississipien supérieur, témoignent en gros d’un magmatisme d’arc bimodal au-dessus d’une zone de subduction

orientée à pendage vers l’est, avec ou sans magmatisme d’arrière-arc et ouverture d’un fossé tectonique continental. Le

cinquième cycle, du Pennsylvanien au Permien inférieur, affiche un volcanisme de composition mafique à intermédiaire,

plus primitif, dans un arc orienté vers l’ouest avec un arrière-arc correspondant vers l’est. Le sixième cycle, au Permien

supérieur, reflète un renversement de la subduction, avec un magmatisme d’arc continental associé à une zone de subduc-

tion à pendage vers l’ouest. Les roches mafiques de tous les cycles, qu’elles soient de caractère d’arc ou non-arc provenai-

ent de sources diversement enrichies, avec des apports provenant d’un prisme de matériau mantélique appauvri ou de

l’asthénosphère d’arrière-arc, et d’un manteau lithosphérique enrichi, avec ou sans composante de plaque subductée. Les

roches felsiques de contexte géodynamique d’arc, d’ouverture de fossé tectonique d’arc et d’arrière-arc proviennent

principalement de la fusion et du recyclage de matériau de la partie supérieure de la croûte continentale (UCC; La/SmUCN

≈ 1). Les roches felsiques de contexte d’arc ont des signatures calco-alcalines et tholéiitiques, alors que les roches qui ne

sont pas de contexte d’arc sont enrichies en éléments à forts effets de champ et d’éléments de la famille des terres rares

(signatures de type A ou hyperalcalines). Il est remarquable qu’il n’y ait pas eu de variations systématiques temporelles

dans la composition de la plupart des roches mafiques et felsiques, durant l’ensemble de l’histoire magmatique du YTT au

Paléozoïque tardif, s’étendant sur plus de 150 millions d’années. Pour l’essentiel, les régions sources et les processus ignées

sont demeurés inchangés pendant toute la durée de l’histoire tectonique du YTT.

Une caractéristique importante de nombreuses roches mafiques des fossés intra-arc et des bassins d’arrière-arc du YTT,

est la valeur élevée des ratios Nb/Thmn

et Nb/Lamn

> 1 (mn = normalisées selon le manteau primitif), ce qui implique

l’existence d’un excès de Nb par rapport au Th et au La, comparé aux ratios des matériaux d’un manteau primitif. L’excès

de Nb dans ces roches implique l’existence d’une composante de croûte océanique recyclée dans leur genèse, et c’est une

caractéristique commune des magmas et des roches dérivées de panaches des grandes provinces ignées. La récurrence

d’une telle composante recyclée sur une longue période de l’histoire magmatique du YTT, et l’occurrence commune dans

des éruptions de petits volumes, plaident contre l’hypothèse d’une origine découlant directement d’un panache ou d’une

grande province ignée; cependant, il est possible que cette signature soit le reflet d’une réactivation d’un panache ou d’une

composante d’une grande province ignée recyclé du manteau lithosphérique du YTT, et dont l’origine serait une fertilisa-

tion lithosphérique par un magmatisme associé à la fragmentation de Rodinia au Néoprotérozoïque.

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Paleozoic magmatism and crustal recycling

INTRODUCTIONThe�Yukon-Tanana� terrane�(YTT)� is�a�vast�pericratonic�province�exposed� in� the� central� Cordillera� of� northern� British� Columbia,�Yukon�and�eastern�Alaska.�The�terrane�lies�between�rocks�of�the�cratonic�margin�of�North�America,�to�the�east,�and�oceanic�and�arc�terranes�that�were�accreted�to�the�YTT�and�cratonic�margin�in�the�Mesozoic,� to� the� west� (Fig.�1).� It� contains� metamorphosed� and�polydeformed�mid-�to�late�Paleozoic�volcanic,�plutonic�and�sedimen-tary rocks whose stratigraphic record differs significantly from co-eval�strata�of�the�North�American�miogeocline�(Colpron�et al.,�this�volume-a).�However,�the�rocks�of�the�YTT�have�some�geochemical,�isotopic,� metallogenic� and� geochronological� characteristics� that�suggest�that�its�older�parts�may�have�originated�as�part�of�the�North�American�craton�(Mortensen,�1992a;�Creaser�et al.,�1997;�Paradis�et al.,�1998;�Nelson�et al.,�2002;�Villeneuve�et al., 2003;�Mortensen�et al.,�this�volume).�Both�allochthonous�and�parautochthonous�ele-ments,�with�respect�to�the�North�American�craton,�make�up�the�YTT�(Fig.�1).�Yukon-Tanana�terrane�locally�hosts�important�volcanogenic�massive�sulphide�deposits.�

Regional�studies�undertaken�under�the�auspices�of�the�Ancient�Pacific Margin NATMAP (National Mapping Program) project have generated�a�wealth�of�new�stratigraphic,�geochronological�and�bio-chronological data from Yukon-Tanana and affiliated terranes in northern�British�Columbia,�Yukon�and�eastern�Alaska�(e.g.,�Colpron�et al.,�this�volume-a,�and�references�therein).�These�data�provide�a�stratigraphic�framework�that�is�critical�to�elucidating�the�tectonic�evolution�of�the�terrane�and�onto�which�detailed�metallogenic,�geo-chemical�and�isotopic�studies�can�be�based.�In�particular,�geochemi-cal�and�isotopic�data�allow�for�determining�the�relationship�between�the�timing�of�magmatism,�magma�composition,�and,�ultimately,�the�tectonic�setting�in�which�ancient�terranes�have�formed�(e.g.,�Stern�et al.,�1995;�Swinden�et al.,�1997).�These�data�give�insights�into�the�relative�roles�that�the�mantle,�continental�crust�and�subducted�slab�play�in�the�genesis�of�magmatic�rocks.�They�provide�important�in-formation�on�the�processes�governing�crustal�growth�and�assembly�of�continents�(e.g.,�Pearce,�1983;�Pearce�and�Peate,�1995;�Pearce�and�Parkinson,�1993),�and�therefore�on�the�Paleozoic�evolution�of�YTT�and the ancient Pacific margin of North America.

In�this�paper�we�present�an�overview�and�synthesis�of�the�pet-rologic,�geochemical�and�(in�some�cases)�isotopic�attributes�of�over�500� volcanic� and� intrusive� rocks� from� YTT� and� related� terranes�(Fig.�1).�Our�presentation�follows�a�series�of�time�slices�that�corre-spond to documented major tectonic and magmatic events within the terrane. In spite of significant dynamothermal metamorphism throughout�much�of�the�terrane,�primary�igneous�textures�are�pre-served�in�many�areas,�and�immobile�element�signatures�have�not�been�changed�appreciably�(e.g.,�Creaser�et al.,�1999;�Dusel-Bacon�and� Cooper,� 1999;� Piercey� et al.,� 2001a,� b,� 2002a,� b,� 2003).�Accordingly,� where� initial� compositions� can� be� determined,� the�protolith�names�are�used�to�emphasize�the�pre-metamorphic�igneous�evolution�of�these�rocks.�Many�of�the�data�sets�analyzed�in�this�paper�are� published� either� in� previous�papers� (e.g.,� Creaser� et al.,� 1997,�1999;�Piercey�et al.,�2001a,�b,�2002a,�b,�2003,�2004;�Colpron,�2001;�Simard�et al.,�2003;�Dusel-Bacon�et al.,�2004;�Nelson�and�Friedman,�

2004)� or� elsewhere� in� this� volume� (e.g.,� Dusel-Bacon� et al.,� this�volume);�the�reader�is�referred�to�these�original�studies�for�more�de-tailed�treatment�of�the�data.

Given�the�breadth�of�new�stratigraphic�information,�geochrono-logic�and�biostratigraphic�age�constraints,�and�associated�geochemi-cal�and�isotopic�data,�YTT�may�well�become�one�of�the�best�under-stood� ancient� continental� margin� geodynamic� systems� in� the�world.

METHODOLOGY AND APPROACHThis paper synthesizes over 500 high precision major, trace and rare earth�element�analyses,�and,�where�available,�Nd-isotopic�data,�from�various�locations�throughout�YTT;�where�applicable,�data�from�the�Stikine,�Quesnel�and�Slide�Mountain� terranes�are�also�discussed.�Representative�analyses�are�presented�in�Tables�DR1�and�DR2�(see�footnote� 1)� and� were� compiled� from� published� and� unpublished�sources.�Most� samples�were�analyzed�by�a�combination�of�X-ray�fluorescence (XRF), inductively-coupled plasma emission spectros-copy�(ICP-ES),�and�inductively-coupled�plasma�mass�spectroscopy�(ICP-MS;�see�Piercey�et al.,�2001b�for�an�example).�Descriptions�of�instrumentation,�analytical�techniques,�precision�and�accuracy�are�found�in�the�original�studies�cited�in�Tables�DR1�and�DR2.

The�following�sub-sections�explain�the�trace�element�diagrams�used�in�this�paper�and�the�rationale�behind�their�usage.�We�will�dis-cuss,�where�available,�Nd�isotopic�signatures,�because�they�highlight�the�relative�importance�of�continental�crust�and�mantle�sources�in�the�genesis�of�YTT�magmatic�rocks.

Petrology and GeochemistryTrace�element�and�radiogenic� isotope�geochemistry�are�useful� in�establishing�the�tectonic�setting�and�petrogenesis�of�rocks�in�both�modern� (e.g.,� Pearce,� 1983;� Pearce� and� Peate,� 1995;� Pearce� and�Parkinson,�1993)�and�ancient�(e.g.,�Stern�et al.,�1995;�Swinden�et al.,�1997)�geodynamic�environments.�Trace�elements�are�more�sensitive�to petrological processes than major elements, and provide a proc-ess-oriented fingerprint or signature suggestive of a given tectonic environment�(e.g.,�Pearce�and�Cann,�1973;�Pearce�and�Norry,�1979;�Shervais, 1982; Wood, 1980). Furthermore, most major elements (except�Al

2O

3,�TiO

2,�P

2O

5)�are�highly�mobile�during�hydrothermal�

alteration�and�metamorphism�(e.g.,�Gibson�et al.,�1983;�MacLean,�1990). In contrast, the high field strength elements (HFSE: Zr, Hf, Nb, Ta, Y), rare earth elements (REE: La-Lu, except Eu), transition elements (TE: Cr, Ni, Sc, V), and the low field strength element (LFSE)�Th�are�considered�immobile�during�metamorphism�(up�to�mid-amphibolite�facies)�and�hydrothermal�alteration�at�low�water-to-rock�ratios�(e.g.,�Campbell�et al.,�1984;�Whitford�et al.,�1988;�You�et al.,�1996;�Jenner,�1996;�Swinden�et al.,�1997;�Johnson�and�Plank,�1999). Previous geochemical studies of YTT also have confirmed that� these�elements�are� immobile�during�regional�metamorphism�and�alteration�(e.g.,�Creaser�et al.,�1997;�Dusel-Bacon�and�Cooper,�1999;�Piercey,�2001;�Piercey�et al.,�2001a,�b,�c,�2002a,�b,�2003,�2004;�Dusel-Bacon�et al.,�2004).

In addition to fingerprinting tectonic setting, trace element data can�provide�insights�into�the�nature�and�type�of�mantle�(e.g.,�enriched,�

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59°N

148°

W

68°N16

0°W

60°N

140°

W

56°N

67°N

124°W

128°W

Tintina

Kaltag

Denali

Wh

DD

EE

FbFb

TT

WL

SELWYN BASIN

YTYT

YTYT

YTYT

YTYT

YTYT

YTYT

CA

NA

CA

ST

WM

RBRBININ

TZTZ TZTZ

TZTZ

TZTZ

TZTZWS

NX

DLMY

MN

WSWS

NX

DLMY

MN

SM

AA

NA

NA

AA

AAAA

COCO

SDSD

RB

AGAG

PCPC

NA

NA

WS

NA

BSBS

BeaufortSea

KotzebueSound

P acificOcean

limit of

Cordilleran

eastern

deformation

Ak

Ak

YT

B.C.

B.C.

YT

NW

T

YT

FinlaysonLake

Delta

Bonnifield

Ambler

Gataga

MacmillanPass

Tulsequah

0 200

km

Scale

Arc assemblages

Oceanic assemblage

Continent margin assemblages

Devonian - Permian Stikine - Boswell assemblage

Devonian - Triassic Slide Mountain - Seventymile assemblages

Paleozoic - Jurassic Tozitna - Angayucham assemblages

PermianKlondike assemblage

Pennsylvanian - PermianKlinkit assemblage

Devonian - Mississippian Snowcap and Finlayson assemblages

Neoproterozoic - Devonian Selwyn basin

Neoproterozoic? - Miss. Alaska Range andY ukon-Tanana Upland

Proterozoic - Devonian North Americanplatformal facies Neoproterozoic - Paleozoic Other continent margin assemblages

SYMBOLSBlueschist / eclogiteoccurrence

Devonian magmaticrocks in North Americanmiogeocline

Devonian - Mississippianmineral districts

Yuko

n-Ta

nana

1

4

3

5

2

6

7

8

910

11

Area of map

AKYT

BC

Figure 1. Paleozoic lithotectonic terranes and assemblages of the northern Cordillera: AA – Arctic Alaska (includes Endicott Mountains, North Slope and Skajit allochthon); AG – Angayucham; CA – Cassiar; CO – Coldfoot (schist belt of southern Brooks Range); DL – Dillinger; IN – Innoko; MN – Minchumina; MY – Mystic; NA – North American miogeocline; NX – Nixon Fork; PC – Porcupine; RB – Ruby; SD – Seward; SM – Slide Mountain - Seventymile (includes Chatanika); ST – Stikine (Asitka); TZ – Tozitna; WM – Windy-McKinley; WS – Wickersham (includes Chena River, Fairbanks schist); YT – Yukon-Tanana. Areas discussed in this paper: (1) Alaska Range; (2) Yukon-Tanana upland; (3) Fortymile River; (4) Stewart River (including Dawson); (5) Finlayson Lake; (6) Glenlyon; (7) Teslin; (8) Wolf Lake – Jennings River (including Big Salmon Complex); (9) Sylvester allochthon; (10) Tulsequah; (11) Lay Range. Other abbreviations: Ak – Alaska; B.C. – British Columbia; D – Dawson; E – Eagle; Fb – Fairbanks; NWT – Northwest Territories; Wh – Whitehorse; WL – Watson Lake; T – Tok; YT – Yukon Territory. Blueschists and eclogite occurrences are from Dusel-Bacon (1994) and Erdmer et�al. (1998).

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depleted),�crust�(e.g.,�continental,�oceanic)�and�subducted�slab�com-ponents involved in mafic and felsic rock genesis. In this paper, trace element data have been plotted on diagrams that reflect the tectonic setting�of� these�rocks,�and�diagrams�that�provide�insight� into�the�nature�of�mantle,�crustal�and�subducted�slab�components�in�YTT�igneous�rocks.�

Rocks of mafic to intermediate and felsic to intermediate com-positions are treated separately. For mafic to intermediate rocks, data are�presented�on�primitive�mantle�(PM)-normalized�multi-element�diagrams�that�depict�elemental�abundances�for�a�suite�of�elements�from�different�chemical�groups,�including�REE,�HFSE�and�transition�elements (Fig. 2). In contrast with mafic rocks, the significance of geochemical�signatures�for�felsic�volcanic�and�intrusive�rocks�can�be�problematic� in�continental�margin�environments,�as� they�may�merely reflect the continental crust itself (e.g.,�Piercey�et al.,�2001b).�Nevertheless,�there�are�subtle�differences�between�granitoids�from�different�tectonic�environments�(e.g.,�Piercey�et al.,�2001b),�and�these�differences, used in conjunction with the compositional character-istics of coeval mafic magmatism, and the nature of volcanic and sedimentary�facies�(e.g.,�Piercey�and�Murphy,�2000),�can�provide�significant insight into the origin of the felsic rocks. Upper conti-nental�crust�(UCC)-normalized�trace�element�plots�(Fig.�3)�and�the�Nb-Y�diagram�of�Pearce�et al.�(1984;�Fig.�4)�are�used�to�portray�the�chemical�characteristics�of�felsic�rocks.�The�Nb-Y�diagram�is�par-ticularly�useful�in�deciphering�the�relative�HFSE�enrichment�(non-arc)�or�HFSE�depletion�(arc)�in�felsic�rocks,�which�can�be�used�to�differentiate�arc�from�non-arc�rocks�(e.g.,�Piercey�et al.,�2001b,�2003;�Fig.�4).�This�diagram,�however,�cannot�separate�rocks�that�have�a�true�arc�signature�from�those�that�have�inherited�an�arc�signature�from�interaction�or�melting�of�pre-existing�arc�crust;�thus,�when�we�describe�“arc”�signatures�in�felsic�rocks�we�have�relied�on�a�combina-tion of field relationships and the lithogeochemical signatures of both mafic and felsic magmatic rocks.

Arc and Non-Arc Geochemical Signatures

Mafic Geochemical SignaturesMafic rocks from non-arc settings (e.g.,�mid-ocean� ridges,�ocean�islands,�etc.)�are�characterized�by�very�smooth�PM-normalized�trace�element patterns and have flat to positive anomalies of Nb relative to Th and La (Fig. 2A). In the non-arc mafic group, there are three broad� subdivisions,� including� normal� mid-ocean� ridge� basalts�(N-MORB),�enriched�mid-ocean�ridge�basalts�(E-MORB)�and�ocean�island�(or�oceanic�intraplate)�basalts�(OIB).�

Normal-MORB (N-MORB) magmas are typified by flat PM-normalized� signatures� with� depleted� light� rare� earth� elements�(LREE),�Nb�and�Th�contents�relative�to�PM�(Fig.�2A).�They�are�de-rived� from� incompatible� element� depleted�mantle� source� regions�(McKenzie�and�Bickle,�1988;�McKenzie�and�O’Nions,�1991;�Sun�and�McDonough,�1989)�and�are�commonly�found�at�spreading�ridges,�but�can�also�be�found�in�back-arc�basin�settings�(e.g.,�Gribble�et al.,�1996;�Sun�and�McDonough,�1989).�

So-called�ocean�island�basalt�(OIB�or�within-plate)�signatures,�are� characterized� by� steep� PM-normalized� patterns� with� LREE-

enrichment,� high� HFSE� contents,� and,� in� particular,� positive� Nb�anomalies�relative�to�Th�and�La�(Fig.�2A).�Rocks�with�OIB�signatures�can�be�derived�from�a�variety�of�sources,�but�it�is�generally�assumed�that�they�represent�melts�from�incompatible�element-enriched�mantle�

1000/.1

1

10

100

1000

Roc

k/P

rimiti

veM

antle

N-MORB

E-MORB

OIB

1

10

100R

ock

/Prim

itive

Man

tle

CAB

IAT

L-IAT

BON

1000/0.1

Roc

k/P

rimiti

veM

antle

.1

1

10

100

T-NEBBABB

ThNb

LaCe

PrNd

SmZr

HfEu

TiGd

TbDy

YEr

YbLu

AlV

Sc

ThNb

LaCe

PrNd

SmZr

HfEu

TiGd

TbDy

YEr

YbLu

AlV

Sc

A)

B)

C)

Figure 2. Primitive mantle-normalized plots for representative ex-amples of arc and non-arc mafic rocks. (A) normal-mid-ocean ridge basalt (N-MORB), enriched-mid-ocean ridge basalt (E-MORB) and ocean island basalt (OIB; Sun and McDonough, 1989); (B) boninite (BON; Jenner, 1981), island arc tholeiite (IAT; Piercey, 2001; Piercey et�al., 2004), LREE-enriched IAT (L-IAT; Shinjo et�al., 2000) and calc-alkaline basalt (CAB; Stoltz et�al., 1990); (C) back-arc basin basalt (BABB; Ewart et�al., 1994) and Th-enriched OIB (T-NEB; Shinjo et�al., 2000). Primitive mantle values in this diagram, and all other primitive mantle normalized plots in this paper, are from Sun and McDonough (1989).

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sources,�such�as�mantle�plumes,�in�within-plate�environments�such�as�ocean�island�(e.g.,�Hawaii),�oceanic�plateau�(e.g.,�Ontong-Java)�and continental flood basalt environments (e.g.,�Lassiter�and�DePaolo,�1997;� Sun� and� McDonough,� 1989).� These� signatures� can� also� be�found�in�basalts�derived�from�enriched�lithospheric�mantle�source�melts� formed� in�plate-margin�environments�during� the� rifting�of�continental�arcs�or�continent�margins� (e.g.,�van�Staal�et al.,�1991;�Piercey�et al., 2002a, b; Dusel-Bacon and Cooper, 1999; Shinjo et al.,�1999; Shinjo and Kato 2000). These types of basalts also occur in modern�arcs�associated�with�slab�windows�(e.g.,�Kamchatka)�and�Archean�greenstone�belts,�and�have�been�termed�Nb-enriched�basalts�(Kepezhinskas�et al.,�1997;�Wyman�et al.,�2000).�In�this�paper,�rocks�with�OIB�signatures�may�be�described�as�“within-plate”�in�places.�This reflects their positions on discrimination diagrams and does

ThNb

LaCe

PrNd

SmZr

HfEu

TiGd

TbDy

YEr

YbLu

AlSc

V ThNb

LaCe

PrNd

SmZr

HfEu

TiGd

TbDy

YEr

YTT - A-type

Yellowstone, A-type

YTT - Peralkaline

Ethiopia, Peralkaline

.01

.1

1

10

100

Roc

k/U

pper

Con

tinen

talC

rust

YbLu

AlSc

V

A) B)

ThNb

LaCe

PrNd

SmZr

HfEu

TiGd

TbDy

YEr

YbLu

AlSc

V ThNb

LaCe

PrNd

SmZr

HfEu

TiGd

TbDy

YEr

YTT - ThR

YTT - CAR

Andes, CAR

Crater Lake

.1

1

10

Roc

k/U

pper

Con

tinen

talC

rust

YbLu

AlSc

V

C) D)

Figure 3. Upper continental crust (UCC)-normalized plots for felsic rocks from YTT and recent analogues (Cenozoic and younger). Typical UCC-normalized signatures for non-arc felsic rocks: (A) A-type and peralkaline rhyolites from YTT (Piercey et�al., 2001b; Dusel-Bacon et�al., 2004); (B) within-plate (A-type) felsic rocks from the Yellowstone Plateau (Hildreth et�al., 1991) and Quaternary peralkaline rhyolites from Ethiopia (Peccerillo et�al., 2003). Typical UCC-normalized signatures for arc felsic rocks: (C) tholeiitic rhyolite (ThR) and calc-alkaline rhyolite from YTT (CAR; Piercey et�al., 2001b); (D) calc-alkaline rhyolite built on mafic crust (similar to tholeiitic rhyolites in YTT?) from Crater Lake, Oregon (Bacon and Druitt, 1988) and calc-alkalic rhyolites from the central Andes, Chile (Lindsay et�al., 2001). Upper conti-nental crust values for this diagram, and all other continental crust normalized plots in this paper, are from McLennan (2001).

Y1 10 100 1000

1

10

100

1000

Nb

Y

Within-Plate(A-type)

Ocean Ridge(OR-type)

M-type

Volcanic Arc

Syncollisional

Within-Plate &

Anomalous Ocean Ridge

Figure 4. The Nb-Y discrimination diagram for felsic rocks from Pearce et�al. (1984). Symbols and data sources are as in Figure 3.

� 287


not�imply�that�they�formed�in�a�within-plate�setting.�Most,�if�not�all,�of�these�rocks�formed�along�plate�margins�due�to�continental�or�arc�rifting.�

Enriched-MORB�(E-MORB)�signatures�are�hybrid�signatures�between�N-MORB�and�OIB�(Fig.�2A)�due�to�mixtures�of�magmas�from�depleted�and�enriched�mantle�sources�(Sun�and�McDonough,�1989).�Enriched-MORB�magmas�can�be�found�at�so-called�plume-centered�spreading�ridges�(e.g.,�Iceland;�Sun�and�McDonough,�1989),�spreading�centres�with�heterogeneous�plum�pudding-type�mantle�(e.g., East Pacific Rise; Niu et al., 1999), where there is mixing of enriched�(OIB)�“plums”�within�a�depleted�(N-MORB-type)�mantle�matrix,�and�can�also�be�found�in�rocks�derived�from�the�mixing�of�enriched�lithospheric/asthenospheric�sources�during�the�evolution�of�continental�rifts�and�back-arc�basins�(e.g.,�Piercey,�2001;�Piercey�et al.,�2002a).�

Mafic to intermediate arc rocks, unlike non-arc rocks, do not have�smooth�PM-normalized�signatures.�They�generally�(although�not� universally;� e.g.,� Piercey� et al.,� 2002a,� b)� show� a� distinctive�negative�Nb�anomaly�relative�to�Th�and�La,�the�so-called�“arc�sig-nature”�(Fig.�2;�Swinden�et al.,�1997).�Arc�volcanic�rocks,�like�non-arc�rocks,�are�derived�from�similar�variably�enriched�mantle�sources;�however,�unlike�non-arc�rocks,�they�have�an�additional�component,�the�slab�component,�superimposed�upon�the�mantle�wedge�(Pearce�and�Peate,�1995,�and�references�therein).�During�the�subduction�of�oceanic�lithosphere,�dehydration�of�hydrous�silicate�minerals�within�the�slab�(and�from�sedimentary�rocks�atop�it)�results�in�the�transfer�of fluid-mobile elements to metasomatize the sub-arc mantle wedge (e.g.,�Pearce,�1983;�Pearce�and�Parkinson,�1993;�Pearce�and�Peate,�1995;�You�et al.,�1996;�Johnson�and�Plank,�1999).�Due�to�the�high�mobility of the LFSE in fluids, arc rocks are typically enriched in these�elements;�Th,�the�relatively�immobile�LFSE,�also�involved�in�slab�metasomatism,�is�used�here�as�the�indicator�of�the�slab-derived�fluid flux. Thorium, however, can also be enriched in mafic rocks contaminated�by�continental�crust.�In�these�instances,�Th�often�shows�systematic�relationships�with�other�indicators�of�crustal�contamina-tion�(i.e.,�εNd

t,�SiO

2, Zr), and these geochemical relationships were

used�to�screen�our�data�in�order�to�identify�samples�that�may�have�been influenced by crustal contamination versus samples that have a�subducted�slab�signature�(see�Piercey�et al.,�2002a,�2004,�for�ex-amples).�In�addition�to�Th-enrichment,�arc�rocks�are�typically�char-acterized�by�HFSE�depletions,�in�particular�Nb,�that�result�from�re-tention�of�HFSE�in�accessory�minerals�within�the�subducted�slab�(e.g.,�rutile;�Foley�et al., 2000), which can be intensified by HFSE-depletions� due� to� previous� melting� events� in� the� sub-arc� mantle�wedge�(e.g.,�Pearce�et al.,�1992;�Woodhead�et al.,�1992).�The�elevated�LFSE� and� depleted� HFSE� signatures� lead� to� high� LFSE/HFSE�(Th/Nb)�ratios�in�arc�rocks�(Fig.�2B).�Within�the�arc�group�of�YTT�mafic rocks, there are broadly four common signatures: island arc tholeiites�(IAT),�LREE-enriched�island�arc�tholeiites�(L-IAT),�calc-alkaline�basalts�(CAB)�and�rare�boninites�(BON;�Fig.�2B).�

Island�arc�tholeiitic�rocks�represent�melts�of�a�source�similar�to�N-MORB�but�include�a�subduction�zone�metasomatic�component.�The IAT suites have flat PM-normalized patterns and are character-ized�by�a�negative�Nb�anomaly�relative�to�Th�and�La,�HFSE�depletion�

(Fig.�2B),� and� follow� a� tholeiitic� (Fe-enrichment)� differentiation�trend�(Swinden�et al.,�1997);�in�some�cases�they�have�very�low�Ti�contents�(Brown�and�Jenner,�1989).�These�suites�of�rocks�are�com-monly�associated�with�the�early�construction�of�island�arcs,�usually�before the arc edifice accumulated up to a significant thickness.

Calc-alkaline basalts and andesites typically reflect the mature stages of arc development coincident with arc edifice growth to a significant thickness. They are also the most common mafic-inter-mediate�rocks�in�arcs�built�on�or�near�continental�crust�(Swinden�et al.,� 1997).� The� CAB� suite� is� characterized� by� PM-normalized�patterns�with�LREE-enrichment,�very�strongly�developed�negative�Nb�and�Ti�anomalies�(Fig.�2B),�and�calc-alkaline�fractionation�trends�(e.g.,�early�Fe-depletion).�Calc-alkaline�suites�can�also�be�formed�when�tholeiitic�magmas,�and�possibly�N-MORB�magmas,�become�contaminated�by�continental�crust�early�in�their�petrogenetic�history�(e.g.,�Swinden�et al.,�1997).�Calc-alkaline�magmatic�suites�can�also�be differentiated from tholeiitic suites by high Zr/Y, La/Yb and Th/Yb� values� (see� Barrett� and� MacLean,� 1999,� and� references�therein),�all�of�which�can�be�observed�on�a�standard�primitive�mantle�normalized�plot�(Fig.�2).

The�L-IAT�suite�of�magmas�is�believed�to�be�a�hybrid�and�part�of�a�continuum�between�the�IAT�and�CAB�suites,�and�likely�repre-sents�either�derivation�from�an�E-MORB�source�with�a�subduction�component (Shinjo and Kato, 2000; Shinjo et al.,�1999),�or�a�weakly�crustally�contaminated�IAT�(Piercey,�2001;�Piercey�et al.,�2004).�

Boninitic rocks are unique magmatic rocks that reflect deriva-tion�from�high�temperature�melting�of�ultra-depleted�mantle�sources�(i.e.,�more�depleted�than�N-MORB).�They�commonly�form�during�the�initiation�of�island�arcs�or�back-arc�basins�(Pearce�et al.,�1992;�Crawford�et al.,�1989;�and�references�therein),�where�they�are�com-monly�spatially�associated�with�IAT.�Although�generally�occurring�in�fore-arc�environments�within�intra-oceanic�arcs,�in�YTT�boninites�are�associated�with�the�initiation�of�intracontinental�(ensialic)�back-arc� basin� magmatism� (Piercey� et al.,� 2001a).� Boninitic� rocks� are�characterized�by�U-shaped�PM-normalized�trace�element�patterns�with�negative�Nb�anomalies,�middle-REE�depletions,�very�low�TiO

2,�

HFSE�and�REE�contents,�high�compatible�element�contents�(Cr,�Ni,�Co, Sc, V), and often, but not always, they have positive Zr and Hf anomalies�relative�to�Sm�(Fig.�2B).�

In addition, YTT also contains mafic to intermediate magmatic rocks�that�have�geochemical�signatures�transitional�between�arc�and�non-arc. Typically they have non-arc affinities but have a weak subduction signature, these include: back-arc basin basalts (BABB) and�Th-enriched,�Nb-enriched�basalts�(T-NEB).�The�BABB�suites�are�essentially�similar� to�N-MORB�but�have�a�weak�negative�Nb�anomaly�(Fig.�2C).�Similarly,�the�T-NEB�suite�has�a�signature�very�similar to OIB but with a flat to weakly negative Nb anomaly (Fig.�2C).�The�BABB�suite�commonly�form�during�back-arc�basin�development� and� represent� MORB-type� magmatism� with� minor�subduction zone fluid influence (Hawkins, 1995; Gribble et al.,�1996;�Piercey�et al.,�2004).�The�T-NEB�suite�is�interpreted�to�represent�arc�rift�rocks�with�a�minor�subduction�signature�(Kepezhinskas�et al.,�1997;�Piercey�et al.,�2004).�

288

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Felsic Geochemical Signatures Delineating arc and non-arc signatures in felsic rocks is very difficult. Commonly�felsic�rocks�have�non-distinctive�geochemical�signatures�that�may�be�derived�in�whole�or�in�part�from�melting�pre-existing�continental�crust�(Piercey�et al.,�2001b).�In�order�to�understand�the�origin�and�setting�of�felsic�rocks�in�YTT,�an�integrated�approach�is�required.�This�approach�includes�a�detailed�documentation�of�vol-canic�and�sedimentary�facies,�and�using�the�geochemical�signatures�of associated mafic rocks. This approach has led to establishing empirical�relationships�for�arc�and�non-arc�felsic�assemblages�within�YTT�(e.g.,�Piercey�et al.,�2001b).�In�general,�non-arc�felsic�rocks�from�YTT�have�PM-�and�UCC-normalized�REE�patterns�that�are�somewhat�different�than�that�of�arc�rocks�(Fig.�3). Non-arc�rocks�have�higher�total�REE�and�HFSE�contents�and�lower�compatible�element�contents�(Sc,�V,�Ti,�Ni,�Cr)�and�are�similar�to�crustally-derived�A-type�and�peralkaline�felsic�rocks�(Figs.�3,�4;�Piercey�et al.,�2001b,�2003;�Dusel-Bacon�et al.,�2004). Shown�for�comparison�on�Figure�3B�are�within-plate�(A-type)�felsic�rocks�from�the�Yellowstone�caldera�(Hildreth�et al.,�1991)�and�Quaternary�peralkaline�rhyolitic�rocks�from�Ethiopia�(Peccerillo�et al.,�2003).�Although�some�of�the�felsic�rocks�within�this�paper�are�described�as�having�“within-plate”�signatures,�this�reflects their position on a discrimination diagram, and does not imply�that�they�formed�in�a�within-plate�setting.�Most,�if�not�all,�of�these� rocks� occur� along� plate� margins� due� to� continental� or� arc�rifting.�

Arc�felsic�rocks�are�present�in�many�parts�of�the�terrane�through-out�its�evolution�(Mortensen,�1992a;�Colpron�et al.,�this�volume-a,�and�references�therein).�They�typically�are�of�crustally-derived,�calc-alkaline affinity (e.g.,�Mortensen,�1992a;�Piercey�et al.,�2003;�Grant,�1997). Their UCC-normalized patterns are relatively flat, often with depletions�in�Ti,�Sc,�V,�and�in�some�cases�Th�±�Nb�(Fig.�3),�features�consistent�with�derivation�from�upper�crustal�sources�accompanied�by�oxide�and/or�accessory�mineral�fractionation�(Piercey�et al.,�2001b,�2003).�Compared�to�the�non-arc�suites�in�YTT,�the�calc-alkalic�arc�suites�have�lower�HFSE�and�REE�contents�with�volcanic-arc�(I-type)�signatures�(Figs.�3,�4).�Tholeiitic�arc�felsic�rocks�are�rare�within�YTT�(Grant,�1997;�Piercey�et al.,�2001b,�2003).�They�have�LREE-�and�HFSE-depleted�UCC-normalized�patterns�when�compared�to�calc-alkalic�arc�and�A-type�felsic�rocks�(Figs.�3,�4;�Piercey�et al.,�2001b).�The�tholeiitic�arc�felsic�rocks�are�interpreted�to�have�been�derived�from melting of primarily mafic to andesitic crust more juvenile than the� source� for� the� A-type� and� calc-alkaline� suites� (Piercey� et al.,�2001b,�2003).�Illustrated�for�comparison�are�arc�felsic�rocks�built�upon�a�thick�crust�of�accreted�oceanic�crustal�material�from�Mount�Mazama,�Crater�Lake,�Oregon�(Bacon�and�Druitt,�1988),�and�arc�felsic� rocks� erupted� in� a� continental� arc� setting� with� thick� crust�(~calc-alkalic)�from�the�Central�Andes,�Chile�(Fig.�3;�Lindsay�et al.,�2001).�Notably,�there�are�no�transitional�felsic�suites�in�YTT.

Sm-Nd Isotopic Signatures: Crust vs. Mantle Components in MagmasSamarium-neodymium�isotope�geochemistry�is�commonly�used�in�ancient�belts�to�decipher�the�relative�contributions�of�crust�and�mantle�in�igneous�rocks,�because�the�Sm-Nd�system�is�very�resistant�to�al-

teration�and�metamorphism,�and,� in�most� cases,� is� insensitive� to�fractionation� during� magma� crystallization� or� partial� melting�(e.g.,�DePaolo,�1988).�This�technique�is�based�on�the�radiogenic�decay�of�147Sm�to�143Nd;�both�of�these�isotopes�are�commonly�presented�as�a�ratio�to�the�nonradiogenic�144Nd�(e.g.,�147Sm/144Nd,�143Nd/144Nd).�The�model�isotopic�evolution�of�the�Sm-Nd�system�involves�the�separa-tion�of�continental�crustal�(CC)�and�depleted�mantle�(DM)�reservoirs�from�a�chondritic�uniform�reservoir�(CHUR).�In�the�case�of�the�DM�reservoir,�the�separation�from�a�CHUR�reservoir�resulted�in�a�higher�Sm/Nd�ratio�in�DM�than�in�CHUR�reservoir,�which�leads�to�a�time-integrated� increase� in� 143Nd/144Nd� relative� to� CHUR� (Fig.�5).� In�contrast,�continental�crust�evolved�with�a�lower�Sm/Nd�ratio,�slower�increase�in�147Sm�and,�by�association,�resulted�in�a�time-integrated�143Nd/144Nd�ratio�lower�than�CHUR�(Fig.�5A).�Neodymium-isotopic�data�are�commonly�presented�in�the�shorthand�form,�epsilon�notation,�which�represents�the�147Sm/144Nd�variation�of�a�rock�relative�to�CHUR�at�a�given�point�in�the�past�(i.e.,�εNd

t;�Fig.�5B).�Rocks�derived�from�

crustal�reservoirs,�or�with�recycled�crustal�components,�have�εNdt�<0,�

whereas�rocks�derived�from�depleted�mantle-type�reservoirs�have�εNd

t�>0;�values�from�chondritic�reservoirs�have�εNd

t�~�0�(Fig.�5B).

Depleted Mantle

Continental Crust

1.0 2.0 3.0 4.0Time (b.y.)

143

144

Nd/

Nd

CHUR

La NdSm Lu

La Sm LuNd

La Nd Sm Lu

1.0 2.0 3.0 4.0Time (b.y.)

Nd CHUR-Sm/Nd=0.325

Depleted Mantle-Sm/Nd~0.36

Continental Crust-Sm/Nd~0.18

0

+

-

A)

B)

Figure 5. Isotope evolution diagrams for the Sm-Nd isotope system. Modified from Swinden et�al. (1997) based on concepts outlined in DePaolo (1988; and references therein). See text for discussion. CHUR = chondrite uniform reservoir.

� 289


MAGMATIC AND PETROLOGIC EVOLUTIONThe�magmatic�and�tectonic�evolution�of�Yukon-Tanana�and�related�terranes� in� the� northern� Cordillera� is� recorded� in� six� Paleozoic�magmatic�cycles�(Tables�1�and�2;�see�also�Colpron�et al.,�this�vol-ume-a;�Nelson�et al.,�this�volume).�The�magmatic�cycles�include�two�pulses of felsic (plus mafic and intermediate) magmatism, separated by Pennsylvanian – Early Permian mainly intermediate to mafic activity (Fig. 6). The first pulse of felsic magmatism in YTT corre-sponds�primarily� to� voluminous�Late�Devonian� to� Mississippian�igneous� rocks� of� the� Finlayson� and� lower� Klinkit� assemblages�(Colpron�et al.,�this�volume-a).�It�contains�four�distinct�cycles�(I-IV;�Fig.�6)�that�are�punctuated�by�at�least�two�episodes�of�deformation�and�erosion.�The�Pennsylvanian�to�Early�Permian�lull�in�felsic�mag-matism corresponds to a cycle dominated by intermediate to mafic magmatism�and�basinal�sedimentation�in�the�Slide�Mountain�and�upper�Klinkit�assemblages�(Cycle�V;�Fig.�6).�The�second�pulse�of�felsic�magmatism,�and�the�last�Paleozoic�magmatic�cycle,�is�repre-sented�by�more�localized�Middle�to�Late�Permian�magmatism�of�the�Klondike�assemblage�(Cycle�VI;�Figs.�1,�6).�

Ecstall Cycle (Cycle I - Middle to Late Devonian - 390-365 Ma)The first magmatic cycle is most widespread in the Alaska Range and�Yukon-Tanana�Upland�of�eastern�Alaska�(Fig.�1).�Coeval�felsic�volcanism of probable within-plate affinity is locally present in mio-geoclinal�strata�of�Selwyn�basin�in�central�Yukon�(Hunt,�2002),�and�magmatism of probable arc affinity is documented in the Coast Mountains�of�British�Columbia�and�southeastern�Alaska,�including�parts� of� the�Tracy�Arm�and�Endicott�Arm�assemblages� (Gehrels�et al.,�1992)�and� the�Ecstall�belt� (Gareau�and�Woodsworth,�2000;�Alldrick�and�Gallagher,�2000;�Alldrick,�2001;�Figs.�1,�7).�Massive�sulphide�occurrences�and�deposits�of�the�Ecstall�belt,�Selwyn�basin�and the Bonnifield district and Delta mineral belt formed during this cycle.�

Limited�trace�element�data�presented�by�Dashevsky�et al.�(2003)�for�the�Delta�mineral�belt�of�the�Alaska�Range�suggest�that�these�rocks�formed�in�a�continental�arc�setting.�In�the�Coast�Mountains,�the�predominance�of�tonalitic�plutons,�and�limited�isotopic�data�from�the�Ecstall�belt,�also�suggest�a�continental�arc�setting�for�these�rocks�(Table�1;� Gareau� and� Woodsworth,� 2000).� However,� because� the�dataset� from� the�Delta�mineral�belt� is� limited,� and�because�high-precision�geochemical�data�are�not�available�for�magmatic�rocks�of�the�Ecstall�belt,�Tracy�Arm�and�Endicott�Arm�assemblages,�these�rocks�will�not�be�discussed�further�here.

Cycle�I�magmatism� in� the�Alaska�Range�and�Yukon-Tanana�Upland�likely�occurred�in�a�continental�rift�setting�(Dusel-Bacon�et al., this volume; Figs. 1, 7). In the Bonnifield mining district of the� Alaska� Range,� the� Late� Devonian� to� earliest� Mississippian�stratigraphic�sequence�consists�of�a�varying�succession�(Healy�schist,�Keevy�Peak�Formation,�Totatlanika�Schist�and�Wood�River�assem-blage) of felsic and mafic metavolcanic and shallow intrusive rocks associated� with� variably� carbonaceous� metasedimentary� rocks�(Wahrhaftig,�1968;�Dusel-Bacon�et al., 2004, this volume). Mafic

metavolcanic�rocks�within�the�Totatlanika�Schist�(Moose�Creek�and�Chute�Creek�members)�have�alkalic,�OIB-like�PM-normalized�sig-natures,�with�the�one�Moose�Creek�sample�exhibiting�a�minor�nega-tive�Nb�and�Ti�anomaly�(Fig.�8A).�These�features�are�consistent�with�derivation� from�a�moderately�enriched�mantle�source� region;� the�Moose�Creek�sample�exhibits�minor�crustal�contamination.�Felsic�rocks in the Bonnifield mining district occur throughout the succes-sion�from�the�Healy�Schist�through�the�Totatlanika�Schist�(Dusel-Bacon�et al.,�2004,�this�volume).�Most�felsic�rocks,�with�the�exception�of� the�Mystic�Creek�Member�of� the�Totatlanika�Schist� and�some�Wood� River� assemblage� samples,� have� very� similar� calc-alkalic�patterns�on�UCC-normalized�plots�but�with�differing�HFSE�and�REE�abundances�(Fig.�9A).�In�contrast,�the�Mystic�Creek�member�contains�a�very�distinctive�suite�of�peralkaline�rhyolites�with�very�high�HFSE�and�REE�contents�and�positive�Nb�anomalies�(Fig.�9A).�

In the Yukon-Tanana Upland, Cycle I comprises bimodal mafic (amphibolites)�and�felsic�(augen�gneiss,�felsic�schist)�meta-igneous�rocks�associated�with�variable�amounts�of�metasedimentary�rocks�(quartzite,�pelite,�marble,�phyllite�of�the�Lake�George�assemblage;�Weber� et al.,� 1978;� Smith� et al.,� 1994;� Dusel-Bacon� et al.,� 2004).�Mafic rocks in the Yukon-Tanana Upland show mostly OIB-like signatures�(Fig.�8B),�with�some�samples�(unit�Dag)�exhibiting�weak�negative�Nb�anomalies�that�are�likely�due�to�crustal�contamination�of�an�OIB-like�magma�(Dusel-Bacon�and�Cooper,�1999;�Dusel-Bacon�et al.,�2004,�this�volume).�Felsic�rocks�in�the�Yukon-Tanana�Upland�exhibit greater variability, but generally have flat UCC-normalized

Pro

bab

ility

den

sity

IVV IIIIIIVI

n = 129 U/Pb ages

n = 252 fossil ages

PERMIAN DEVONIANPENN.TRIASSIC MISSISSIPPIAN

240 260 280 300 320 340 360 380 400Age (Ma)

Figure 6. Probability density plot for 129 U-Pb zircon ages (solid curve) and 252 fossil age determinations (dotted curve) for the pericratonic terranes of the northern Cordillera. The peaks in prob-ability density of U-Pb ages outline the two main pulses of felsic magmatism in YTT. The probability density curve for fossil ages (mainly from conodonts) is shown to illustrate biostratigraphic control during the Pennsylvanian-Permian lull in felsic magmatism (Cycle V). Peaks in probability density of fossil ages in Mississippian and Pennsylvanian times correspond to main episodes of carbonate deposition in YTT. Fossil age determinations whose range exceeded ± 25 Ma were excluded from this computation. Magmatic cycles (I-VI) discussed in the text are labelled at top of the diagram.

290

Piercey et al.

multi-element patterns with depletions in Ti, Al, Sc, V (±Zr, Hf and Eu)�that�are�consistent�with�derivation�from�melting�of�continental�crust�accompanied�by�feldspar�and�oxide�fractionation�(Fig.�9).�Data�for�these�rocks�cluster�close�to�the�boundary�between�within-plate�and volcanic arc fields, with the exception of two felsic samples from the Nasina assemblage, which have peralkaline affinities and within-plate�signatures�(Fig.�9). On�UCC-normalized�plots,�some�felsic�rocks�from�the�Yukon-Tanana�Upland�(units�Pzsq,�MDmg�and�Butte�as-semblage�of�Dusel-Bacon�et al.,�2004,�this�volume)�all�have�similar�relatively flat calc-alkalic patterns, with depletions in Ti, Al, Sc, V (±Zr, Hf and Eu) consistent with derivation from melting of conti-nental�crust�accompanied�by�feldspar�and�oxide�fractionation�(Fig.�9).�In�contrast,�two�samples�of�felsic�rocks�from�the�Nasina�assemblage�are much more depleted in LREE, but are highly enriched in Nb, Zr, Hf�and�HREE�contents�(Figs.�9D,�E).�

The distinctive occurrence of peralkaline rocks in the Bonnifield mining district, the dominance of OIB-like mafic rocks, the lack of mafic rocks with arc signatures, and the dominance of metasedimen-

.C.B TY

kATY

631°W

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36 °N

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B.C.

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95 °NP

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ane

0 300

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Scale

?

?

SYMBOLSVolcanism

Massive sulphide deposit

Plutonism

Compressionaldeformation

367

374

375

370

382

pre-362

366

360 -360 -375375

360 -360 -375375

367

?

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357 -357 -375375

357-357-368368

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Delta

MargBonnifield

RIFT RIFT

RIFT

ARC

ARC

Cycle I - EcstallMiddle to Late Devonian

(Eifelian - Famennian)390-365 Ma

CACA

SBSB

Figure 7. Distribution of tectonic and magmatic events during the Middle to Late Devonian Ecstall cycle (Cycle I: 390-365 Ma). Events are located on a base map of Yukon-Tanana and related terranes prior to ~430 km of dextral displacement along Tintina fault. Dashed red line marks approximate limit between geodynamic environments. Dashed blue line represents relative position of subduction zone; teeth indicate dip of subducting slab. See Figure 1 for tectonic as-semblage legend. Relative position of Cassiar terrane (CA) and Selwyn basin (SB) are also shown.

Roc

k/P

rimiti

veM

antle

.1

100

1000

1

10

Alaska Range - Non-Arc Rocks(Totatlanika Schist)

Chute Creek Mbr

Moose Creek Mbr

ThNb

LaCe

PrNd

SmZr

HfEu

TiGd

TbDy

YEr

YbLu

AlV

Sc

A)

Roc

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rimiti

veM

antle

1000

Butte assembl.Nasina assembl.

Dag

Pzsq

MDgn

.1

1

10

100

Yukon-Tanana Upland - Non-Arc Rocks

ThNb

LaCe

PrNd

SmZr

HfEu

TiGd

TbDy

YEr

YbLu

AlV

Sc

B)

Figure 8. Primitive mantle normalized plots for Cycle I mafic rocks. (A) Non-arc mafic rocks from the Totatlanika Schist, Alaska Range (data from Dusel-Bacon et�al., 2004); (B) Non-arc mafic rocks from the Yukon-Tanana Upland (data from Dusel-Bacon et�al., 2004).

� 291


tary�rocks�has�led�Dusel-Bacon�et al.�(2004,�this�volume)�to�suggest�that�Cycle�I�magmatism�in�the�Alaska�Range�and�the�Yukon-Tanana�Upland�was�the�result�of�crustal�attenuation�and�rifting�of�a�conti-nental-margin�sequence�that�was�parautochthonous�with�respect�to�the�craton,�coeval�with�extension�in�Selwyn�basin�and�arc�magmatism�in�the�outboard�parts�of�YTT�(Fig.�7;�Dusel-Bacon�et al.,�2004,�this�volume;�Nelson�et al.,�this�volume).

Finlayson Cycle (Cycle II - Late Devonian to Early Mississippian - 365-357 Ma)The�second�magmatic�cycle�marks�the�onset�of�felsic�magmatism�in�many�parts�of�YTT�and�the�formation�of�massive�sulphide�deposits�in�the�Finlayson�Lake�district�(Figs.�6,�10).�Magmatic�activity,�which�began�during�Cycle�I�in�the�Alaska�Range�and�Yukon-Tanana�Upland�persisted�during�Cycle�II�(Fig.�10).�Localized�felsic�magmatism�of�probable within-plate affinity also occurs in miogeoclinal strata of the�Selwyn�basin�and�Cassiar�terrane�(Figs.�1,�10).�In�YTT,�this�cycle�

Cycle Age Range Arc Setting Sources of Data

(Figure) Locations* Geochemical Signatures Isotopic Signatures

I 390-365 Ma Ecstall�belt,�B.C. No geochemical data; mafic to intermediate (minor�felsic)�metavolcanic�rocks;�meta-tonalite�plutons.

Felsic: εNdt�=�-4.0.� Alldrick�and�Gallagher�(2000);�Alldrick�(2001);�Gareau�and�Woodsworth�(2000).

Coast�Mountains,�SE�Alaska N/A Uncertain.�Proterozoic-Archean�inheritance�in�some�zircon�populations.�

Gehrels�et al.�(1992).

Delta�mineral�belt,�Alaska�Range

Felsic: Limited trace element data suggest calc-alkalic�arc.

Uncertain.�Proterozoic-Archean�inheritance�in�zircon�populations.�

Dashevsky�et al.�(2003).

II 365-357 Ma (Fig.�10)

Finlayson Lake (5)�(Cleaver�Lake,�Waters�Creek�and�Fire�Lake�formations)

Felsic: calc-alkalic arc; �Mafic: IAT, L-IAT, MORB, NEB and BON.

Felsic: εNdt�=�+0.1�to�-4.8;��Mafic: εNdt�=�-5.1�to�+7.0.�

Piercey�et al.�(2001a,�b,�2002a,�2003,�2004);�Grant�(1997).

Stewart River (3) Mafic (amphibolite): IAT (MORB). Uncertain.�Proterozoic-Archean�inheritance�in�zircon�populations.�

S.�Piercey�and�J.�Ryan�(unpublished�data).�

III 357-342 Ma�(Fig.�13)

Finlayson Lake (5)�(Simpson�Range�suite)

Felsic: calc-alkalic metaluminous intrusions Felsic: εNdt�=�-7.4�to�-12.9. Piercey�et al.�(2001a,�b,�2002b,�2003);��Grant�(1997).

Stewart River (3) Felsic: Calc-alkalic intrusions. Uncertain.�Proterozoic-Archean�inheritance�in�zircon�populations.�

J.�Ryan�and�M.�Villeneuve�(personal�communication);��S.�Piercey�and�J.�Ryan�(unpublished�data).

Fortymile River (3) Felsic: calc-alkaline intrusions; �Mafic: MORB.

N/A Mortensen�(1990),�J.�Mortensen�(personal�communication).

Glenlyon (6)�(Little�Kalzas�suite)

Felsic: calc-alkaline andesites, rhyolites, and intrusive�rocks;��Mafic: OIB-like mafic volcanic rocks.

Uncertain.�Proterozoic�inheritance�in�zircon�populations.�

Colpron�(2001)

Teslin (7) Felsic: calc-alkalic metaluminous intrusions; �Mafic: rift-like mafic rocks?

Felsic: εNdt�=�-2.5�to�-6.2;��Mafic: εNdt�=�+4.1�to�+6.4.

Stevens�et al.�(1995);�Creaser�et al.�(1997).

Wolf-Jennings (8)�(upper�Dorsey�and�Ram�Creek�complexes;�Swift�River�Group)

Felsic: calc-alkaline intrusions; �Mafic: L-IAT to CAB; minor N-MORB to OIB.

Uncertain.�Proterozoic�inheritance�in�zircon�populations.�

Nelson�and�Friedman�(2004).

IV 342-314 Ma�(Fig.�17)

Glenlyon (6)�(Little�Salmon�fm.�and�Tatlmain�suite)

Felsic: calc-alkalic andesite-dacite-rhyolite; calc-alkaline�intrusions;�Mafic: OIB-like.


Colpron�(2001),�Colpron�et al.�(this�volume-b).

Wolf-Jennings (8)�(Ram�Creek�and�Big�Salmon�complexes)

Felsic: calc-alkaline schist;�Mafic: L-IAT, CAA, E-MORB/OIB.


Nelson�and�Friedman�(2004).�M.�Mihalynuk�(unpublished�data).

Tulsequah (10) Felsic: calc-alkalic rhyolites;�Mafic: N-MORB to E-MORB with subduction�signature.

Uncertain. Sebert�and�Barrett�(1996).�

V 314-269 Ma�(Fig.�21)

Wolf-Jennings (8)�(Klinkit�Group)

Mafic: calc-alkalic fragmental volcanic and volcaniclastic�rocks;�OIB.�

Mafic: εNdt�=�+6.7�to�+7.4. Simard�et al.�(2003).

Sylvester allochthon (9)�(Fourmile�succession)

L-IAT mafic and calc-alkalic felsic Nelson�(1993).

Lay Range, B.C. (11) Mafic: L-IAT to MORB. Ferri�(1997).

VI 269-253 Ma�(Fig.�24)

Stewart River (3)�(Klondike�schist;�Sulphur�Creek�orthogneiss)

Felsic: calc-alkalic volcanic and intrusive rocks.�

Uncertain.�Proterozoic-Archean�inheritance�in�zircon�populations.�Radiogenic�Pb�in�associated�syngenetic�occurrences.��εNdt�-�crustal.

S.�Piercey�and�J.�Mortensen�(unpublished�data);�Mortensen�(1990);�J.�Mortensen�(unpublished�data).�

Wolf-Jennings (8)�(Meek�pluton)

Felsic: calc-alkaline intrusion. N/A Nelson�and�Friedman�(2004).�

Fortymile River (3) Felsic: calc-alkalic schist. N/A J.�Mortensen�and�C.�Dusel-Bacon�(unpublished�data).

Note: * Bold characters indicate areas for which data are summarized in this paper; numbers refer to locations shown in Figure 1.

Table 1.�Summary�of�geochemical�attributes�of�arc�assemblages�for�Paleozoic�magmatic�cycles�in�YTT.

292

Piercey et al.

ended�with�a�deformational�event�recorded�in�rocks�of�the�Finlayson�Lake�district�(Murphy�et al.,�this�volume).�This�deformational�event�is�recorded�by�fabric�development,�uplift�and�erosion,�and�is�inter-preted�to�represent�contractional�deformation�in�an�intra-arc�setting�(Murphy�et al.,�this�volume).

The�Finlayson�cycle�is�characterized�by�arc,�arc-rift�and�back-arc�basin�magmatism� in�most�of�YTT,�with� the� exception�of� the�Bonnifield district and Yukon-Tanana Upland, where magmatic ac-tivity� is� interpreted� to� have�occurred� in� an� extended� continental�margin�setting�(Dusel-Bacon�et al.,�2004,�this�volume;�Fig.�10).�Arc�magmatism�predominated�in�the�western�parts�of�the�terrane�in�this�

cycle,� and� includes� rocks� in� the� western� Finlayson� Lake� district�(Murphy�et al.,�this�volume),�the�Stewart�River-Dawson�area�(J.�Ryan,�personal�communication),�and�possibly�also�the�Delta�mineral�belt�of�the�Alaska�Range�(Dashevsky�et al.,�2003;�Figs.�1,�10).�Coeval�back-arc�basin�magmatism�occurred�predominantly�in�the�eastern�Finlayson�Lake�district� and� in� the�Slide�Mountain� terrane� in� the�Sylvester�allochthon�(Fig.�10).

The� Finlayson� Lake� district� is� divided� into� two� contrasting�Devonian-Mississippian magmatic regions, which are juxtaposed along�the�Permian�Money�Creek�thrust�and�associated�faults�(see�Murphy�et al., this volume). Rocks of arc affinity occur primarily to

.1

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Yukon-Tanana Upland:Arc/Non-Arc Felsic Rocks

Roc

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pper

Con

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Healy SchistKeevy PeakWood River

California CreekMoose Creek

Mystic Creek

Alaska Range: Arc/Non-Arc Felsic Rocks

CePr

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Yukon-Tanana Upland:Non-Arc Felsic Rocks(Nasina assemblage)

Group 1

Group 2

D)

1 10 100 10001

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Y

1 10 100 10001

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within plate(A-type)

ocean ridge(OR-type)

M-type

volcanic arc(I-type)

syn-collisional(S-type) within plate &

anomalous ocean ridge

B)



M-type




E) Figure 9. Upper continental crust (UCC)-normalized and Nb-Y discrimination plots for Cycle I felsic rocks. (A, B) Alaska Range; (C, D, E) Yukon-Tanana Upland (data from Dusel-Bacon et�al., 2004).

� 293


the�southwest,�in�the�hanging�wall�of�the�thrusts;�coeval�back-arc�facies� occur� exclusively� to� the� east,� in� the� footwall.� Cycle�II� arc�magmatism�is�represented�by�the�~365-360�Ma�Waters�Creek�and�Cleaver Lake formations, which comprise mafic volcanic, volcani-clastic�and�intrusive�rocks,�with�lesser�felsic�volcanic�and�sedimen-tary�rocks�(Mortensen,�1992a,�b;�Murphy,�1998,�2001;�Murphy�and�Piercey,�1999;�Murphy�et al., 2002, this volume). Mafic arc rocks in the�Waters�Creek�and�Cleaver�Lake�formations�are�geochemically�diverse,�with�signatures�that� include�BON,�IAT,�L-IAT�and�CAB�(Fig.�11A).�Their�source�magmas�were�derived�from�a�variably�en-riched�mantle�wedge,�with�components�from�subducted�slab�and/or�crustal�contamination�(Fig.�11;�Piercey,�2001;�Piercey�et al.,�2001a,�2004).�They�have�εNd

t�values�that�range�from�–5.0�to�+7.1;�however,�

the�bulk�of�the�samples�(6�of�8)�have�εNdt >0, reflecting derivation

from chondritic to juvenile sources (Piercey, 2001; Piercey et al.,�2004).�Volumetrically�minor�felsic�rocks�of�the�Cleaver�Lake�forma-tion have tholeiitic and calc-alkalic affinities, consistent with an arc parentage�(Figs.�12A,�B;�Piercey�et al.,�2001b,�2003).�Neodymium�isotopic� data� for� the� felsic� rocks� is� limited.� A� tholeiitic� rhyolite�sample� has� εNd

t� =� +0.1,� and� a� calc-alkaline� rhyolite� sample� has�

εNdt�=�–4.8�(Piercey�et al.,�2003).�These�variations�in�Nd�isotopic�

attributes,�coupled�with�various�trace�element�ratios,�suggest�that�the�YTT�in�this�region�was�built�upon�a�heterogeneous�basement�of�both�oceanic�and�continental�basement�(Grant,�1997;�Piercey�et al.,�2001a,�2003;�see�also�Murphy�et al.,�this�volume).

In�the�Finlayson�Lake�district�(Figs.�1,�10),�bimodal�back-arc�magmatic�activity�is�represented�by�non-arc�and�transitional�volcanic�

and intrusive rocks of the Grass Lakes group (Fire Lake, Kudz Ze Kayah�and�Wind�Lake�formations;�Murphy�et al.,�this�volume).�The�stratigraphically lower Fire Lake formation is dominated by mafic volcanic�and�plutonic�rocks�with�non-arc�to�transitional�signatures�including�E-MORB,�back-arc�basin�basalts,�OIB�and�Th-rich�OIB�which are likely crustally contaminated or slab-influenced OIB (Fig.�11B;�Piercey,�2001;�Piercey�et al.,�2004).�Neodymium�isotopic�data�for� these�rocks�suggest�derivation�from�mantle�sources�with�near-chondritic to depleted affinities (εNd

t�=�-1.6�to�+8.5),�with�the�

lower�εNdt�values�associated�with�the�most�enriched�rocks,�suggest-

ing the influence of lithospheric mantle within these suites (Piercey, 2001;�Piercey�et al.,�2004).�

Back-arc magmatic rocks in the slightly younger Kudz Ze Kayah formation� include� felsic� volcanic,� volcaniclastic� and� volcanosedi-mentary� rocks� (Murphy� et al.,� 2002,� this� volume;� Piercey� et al.,�2001b, 2002a, 2003). Felsic rocks of the Kudz Ze Kayah formation and�coeval�intrusions�of�the�Grass�Lakes�plutonic�suite�are�character-ized� by� within-plate� (A-type)� geochemical� signatures,� with� very�high�HFSE�and�REE�contents�and�low�compatible�element�contents�(Figs.�12C,�D;�Piercey�et al., 2001b, 2003). The flat UCC-normalized patterns,� coupled� with� negative� Ti,� Eu,� Al,� Sc� and� V� anomalies�(Fig.�12C)�are�consistent�with�their�derivation�from�partial�melting�of� continental� crustal� sources� coupled� with� the� fractionation� of�feldspar�and�oxide�minerals.�An�origin�by�partial�melting�of�conti-nental�crust�is�also�supported�by�the�Nb/Ta,�Ti/Sc�and�La/Yb�ratios�in�these�rocks�(Piercey�et al.,�2001b,�2003),�neodymium�isotopic�data�(εNd

t� =� -7.8� to� -9.5;� Piercey� et al.,� 2003),� ubiquitous� Proterozoic-

Cycle Age Range Back-Arc or Non-Arc Setting (Rift?) Sources of Data

(Figure) Locations* Geochemical Signatures Isotopic Signatures

I 390-365 Ma��(Fig.�7)

Alaska Range (1) Felsic: within-plate and peralkaline; some low�HFSE-REE�crustal�melts;�Mafic: OIB (+ contaminated OIB).

Uncertain,�Proterozoic-Archean�inheritance�in�zircon�populations.��

Dusel-Bacon�et al.��(2004,�this�volume).

Yukon-Tanana Upland (2) Felsic: within-plate and peralkaline felsic; low�HFSE-REE�crustal�melts;��Mafic: OIB (+ contaminated OIB).

Uncertain,�Proterozoic-Archean�inheritance�in�zircon�populations.��

Dusel-Bacon�et al.��(2004,�this�volume).

II 365-357 Ma�(Fig.�10)

Finlayson Lake (5)�(Fire Lake, Kudz Ze Kayah and�Wind�Lake�formations)

Felsic: within-plate (A-type);�Mafic: OIB-like; NEB, T-NEB, BABB, �and�E-MORB�in�Fire�Lake�formation.��

Felsic: εNdt = -7.8 to -9.5 (Kudz Ze Kayah); �Mafic: εNdt�=�-2.8�to�+1.1�(Wind�Lake);��εNdt�=�-1.6�to�+8.5�(Fire�Lake).�

Piercey�et al.�(2001a,�b,�2002a,�2003,�2004);�Grant�(1997).

Sylvester allochthon (9) Mafic: MORB N/A Nelson�(1993);�Ferri�(1997)

III 357-342 Ma�(Fig.�13)

Finlayson Lake (5)�(Wolverine�Lake�group)

Felsic: within-plate (A-type) volcanic rocks; �Mafic: N-MORB to BABB volcanic rocks.

Felsic: εNdt�=�-7.1�to�-8.2;��Mafic: εNdt�=�+6.9.

Piercey�et al.��(2001a,�b,�2002b,�2003);��Grant�(1997).

Sylvester allochthon (9) Mafic: MORB Nelson�(1993).

IV 342-314 Ma�(Fig.�17)

No back-arc yet discovered. Paucity of basalts in Slide Mountain.

N/A

V 314-269 Ma�(Fig.�21)

Finlayson Lake (5);��(Campbell�Range�fm.)�Permian�eclogites

Mafic: N-MORB, E-MORB, OIB, and BABB; MORB-signatures�in�eclogites�derived�from�Campbell�Range.��

Campbell Range equivalent eclogites: �εNdt�=�+5.4�to�+9.3.��Juvenile�Pb-isotopes�in�VMS�associated�with�Campbell�Range.��

Creaser�et al.�(1999);��Mann�and�Mortensen�(2000);�S.�Piercey�and�D.�Murphy�(unpublished�data);��Mortensen�et al.�(this�volume).

Sylvester allochthon (9) Mafic: N-MORB Nelson�(1993).

Lay Range, B.C. (11)�(Nina�Creek�Group)

Mafic: N-MORB Ferri�(1997).

VI 269-253 Ma�(Fig.�24)

No back-arc yet identified. N/A

Note: *Bold characters indicate areas for which data are summarized in this paper; numbers refer to locations shown in Figure 1.

Table 2.�Summary�of�geochemical�attributes�of�non-arc�assemblages�for�Paleozoic�magmatic�cycles�in�YTT.

294

Piercey et al.

Archean�zircon�inheritance�in�geochronological�samples�(Mortensen,�1992a;�Piercey,�2001),�and�very�radiogenic�(high-µ�~12)�Pb�isotopic�systematics�in�sulphides�associated�with�felsic�volcanic-hosted�mas-sive�sulphide�deposits�(Mortensen�et al.,�this�volume).�

The� culmination� of� back-arc� magmatism� during� Cycle�II� is�represented by the metamorphosed mafic volcanic rocks of the stratigraphically�highest�Wind�Lake�formation�and�accompanying�high� level� intrusions� (Murphy� et al.,� this� volume;� Piercey� et al.,�2002a).�This�formation�is�characterized�by�OIB�signatures,�with�a�subsuite�of�rocks�that�have�been�contaminated�by�continental�crust�(CC-OIB;�Fig.�11C;�Piercey�et al.,�2002a).�Both�suites�have�high�TiO

2,�

P2O

5,�HFSE�and�REE�contents�and�LREE-enriched�signatures.�The�

uncontaminated�suite�shows�positive�Nb-anomalies�(and�εNdt�=�+1.1),�

and�the�contaminated�suite�has�slightly�negative�Nb�anomalies�and�higher�Th/Nb�ratios� (εNd

t�=� -2.8;�Fig.�11C;�Piercey�et al.,�2002a).�

The low volume of mafic magmatism in the upper parts of the Grass Lakes�group,�a�~357-360�Ma� intra-arc�deformation�event,�and�an�unconformity�overlying�the�succession�at�~357�Ma�(Murphy�et al.,�this� volume),� suggest� that� the� Grass� Lakes� back-arc� became� an�aborted rift and did not evolve to full seafloor spreading.

In the Stewart River area, Cycle II mafic magmatism is repre-sented� by� amphibolitic� rocks� that� are� interlayered� with� a� lower�metasedimentary�package�(Ryan�and�Gordey,�2001,�2002;�Ryan�et al.,�2003).� Preliminary� geochemical� data� (S.J.� Piercey� and� J.J.� Ryan,�unpublished�data)�for�amphibolitic�rocks�are�characterized�by�IAT�signatures,�with�one�sample�having�a�N-MORB�signature�(Fig.�11D),�suggesting�that�these�rocks�were�derived�from�depleted�mantle�source�

.C.B TY

kATY

631°W

75 °N

36°N

B.C.

YT

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65 °N

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Pci

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Scale

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SYMBOLSVolcanism

Fossil


Plutonism


F

Famennian - TournaisianF

360

360 -375360 -375

358 -361

357 -365

361 -365

363

358358

?

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357 -357 -375375

360 -360 -375375

357-368357-368

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358358

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357 -375

359

357

359359

360

362362

noisne

txe

Delta

Bonnifield

Kzk

FyreLk

CirqueSEDEXCirqueSEDEX

RIFT

RIFT

RIFT

MORB

ARC

ARC

BAB

Cycle II - FinlaysonLate Devonian - Early Mississippian

(Famennian - Early Tournaisian)365-357 Ma

CACA

SBSB

Figure 10. Distribution of tectonic and magmatic events during the Late Devonian – Early Mississippian Finlayson cycle (Cycle II: 365-357 Ma). Events are located on a base map of Yukon-Tanana and related terranes prior to ~430 km of dextral displacement along Tintina fault. Dashed red line marks approximate limit be-tween geodynamic environments. Dashed blue line represents relative position of subduction zone; teeth indicate dip of subduct-ing slab. See Figure 1 for tectonic assemblage legend. Relative position of Cassiar terrane (CA) and Selwyn basin (SB) are also shown.

Figure 11. (facing page, top) Primitive mantle normalized plots for Cycle II mafic rocks. (A) Average values for arc rocks of the Waters Creek and Cleaver Lake formations, Finlayson Lake district (BON = boninite; IAT= island arc tholeiite; L-IAT = LREE-enriched island arc tholeiite; CAB = calc-alkaline basalt; data from Piercey, 2001; Piercey et�al., 2004); (B) Average values for non-arc rocks of the Fire Lake and Cleaver Lake formations, Finlayson Lake district (OIB-1 = Nb-enriched basalt with low La/Yb; T-OIB = Th-rich, Nb-enriched basalt; BABB = back-arc basin basalt); (C) Average values for non-arc basaltic rocks from the Wind Lake formation, Finlayson Lake district (data from Piercey et�al., 2002a); (D) Arc and non-arc rocks from amphibolitic rocks from the Stewart River area (S. Piercey and J. Ryan, unpublished data).

Figure 12. (facing page, bottom) Upper continental crust (UCC) normalized and Nb-Y discrimination plots for Cycle II felsic rocks. (A, B) Cleaver Lake formation (CAR = calc-alkaline rhyolite, ThR = tholeiitic rhyolite; data from Piercey et�al., 2001b); (C, D) Grass Lakes group (KZK-Rhy = Kudz Ze Kayah formation rhyolite; KZK-FPI = Kudz Ze Kayah formation feldspar porphyritic intrusion; KZK-FT = Kudz Ze Kayah formation felsic tuff; GLS-Gr = Grass Lakes suite granitoid; data from Piercey et�al., 2001b).

� 295


LuAl

VScTh

NbLa

CePr

NdSm

ZrHf

EuTi

GdTb

DyY

ErYb

.1

CAB

L-IAT

BONIAT

1

10

100

1000Finlayson Lake - Arc Rocks

(Waters Creek and Cleaver Lake formations)

Roc

k/P

rimiti

veM

antle

A)

LuAl

VScTh

NbLa

CePr

NdSm

ZrHf

EuTi

GdTb

DyY

ErYb

Finlayson Lake - Non-Arc Rocks(Fire Lake and Cleaver Lake formations)

100

1

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OIB-1OIBE-MORB

BABBT-OIB

.1

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rimiti

veM

antle

B)

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LuAl

VScTh

NbLa

CePr

NdSm

ZrHf

EuTi

GdTb

DyY

ErYb

Finlayson Lake - Non-Arc Rocks(Wind Lake formation)

OIB

CC-OIB

1000

Roc

k/P

rimiti

veM

antle

C)

Roc

k/P

rimiti

veM

antle

1000

.1

1

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ThNb

LaCe

PrNd

SmZr

HfEu

TiGd

TbDy

YEr

YbLu

AlV

Sc

Stewart River - Arc and Non-Arc Rocks(lower amphibolites)

D)

.1

1

10

ThNb

LaCe

PrNd

SmZr

HfEu

TiGd

TbDy

YEr

YbLu

AlSc

V

Roc

k/U

pper

Con

tinen

talC

rust

CAR

ThR

Finlayson Lake - Arc Felsic Rocks(Cleaver Lake formation)

A)

.1

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LaCe

PrNd

SmZr

HfEu

TiGd

TbDy

YEr

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AlSc

V

Roc

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Con

tinen

talC

rust

KZK - RhyKZK - FPIKZK - FTGLS-Gr

Finlayson Lake - Non-Arc Felsic Rocks(Kudz Ze Kayah formation andGrass Lakes suite granitoids)

C)

1 10 100 10001

10

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Nb

Y



M-type




1 10 100 10001

10

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M-type


syncollsional(S-type) within plate &


B)

D)

Figure 11. Caption on facing page.

Figure 12. Caption on facing page.

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regions.�Furthermore,�the�spatial,�temporal�and�geochemical�simi-larities�to�rocks�of�the�Fire�Lake,�Cleaver�Lake�and�Waters�Creek�formations�in�the�Finlayson�Lake�district�suggest�that�the�Stewart�River�area�represents�a�continuation�of�the�arc�front�recorded�in�the�Cleaver�Lake�and�Waters�Creek�formations,�and�initial�back-arc�basin�development�recorded�in�the�Finlayson�Lake�district�during�Cycle�II�(Fig.�10).

Wolverine Cycle (Cycle III - Early Mississippian - 357-342 Ma)The�third�magmatic�cycle�corresponds�to�the�climax�of�felsic�mag-matism�in�YTT�(Fig.�6).�It�begins�at�a�marked�angular�unconformity�at�the�base�of�the�Wolverine�Lake�group�in�the�Finlayson�Lake�district�(Murphy�et al.,�this�volume;�Colpron�et al.,�this�volume-a),�and�is�therefore�named�after�this�succession.�The�base�of�the�Wolverine�Lake�group� comprises� quartz-� and� feldspar-pebble� conglomerate,�grit� and� sandstone� derived� from� the� underlying� Late� Devonian� –�Early�Mississippian�volcanic�and�plutonic�rocks�(Murphy�et al.,�this�volume).�This�unconformity�represents�a�hiatus�of�approximately�3-4�m.y.�The�Wolverine�massive�sulphide�deposit�formed�during�this�cycle. This cycle ends with another deformational event, best defined in�the�Glenlyon�area�of�central�Yukon�(Colpron�et al.,�this�volume-b),�but�also�inferred�in�many�parts�of�YTT�(Colpron�et al.,�this�volume-a).�This�deformational�event�is�recorded�by�fabric�development�in�

~347-343�Ma�volcanic�and�plutonic�rocks�that�are�intruded�by�an�un-deformed�ca. 340�Ma�pluton,�and�by�foliated�quartzite�clasts�in�basal�conglomerate�of�the�ca.�340�Ma�and�younger�Little�Salmon�formation�(Colpron�et al.,�this�volume-b).�

Cycle�III�magmatism�was�of�arc,�arc-rift�and�back-arc�types.�Arc� magmatism� occurred� in� the� western� Finlayson� Lake� district,�Stewart�River,�Fortymile�River,�Glenlyon,�Teslin�and�Wolf�Lake-Jennings River areas (Fig. 13). Magmatism of probable arc affinity is�also�documented�in�the�Stikine�assemblage�of�northwestern�British�Columbia�(Logan�et al.,�2000;�Gunning�et al.,�this�volume)�and�the�Endicott� Arm� assemblage� of� southeastern� Alaska� (Fig.�13;�McClelland�et al.,�1991;�Gehrels,�2001).�Back-arc�magmatic�activity�was�restricted�to�the�eastern�Finlayson�Lake�area,�in�the�footwall�of�the� Money� Creek� thrust� (Fig.�13;� Piercey� et al.,� 2001b,� 2002b,�2003).�

Arc�magmatism�in� the�Finlayson�Lake�district� is�manifested�primarily� by� hornblende-� and� biotite-bearing� granitoids� of� the�Simpson�Range�plutonic�suite.�They�range�in�age�from�~357-345�Ma,�but� are� mostly� in� the� range� of� ~350-345� Ma� (Mortensen,�1992a;� J.K.�Mortensen�and�D.C.�Murphy,�unpublished�data).�The�UCC-normalized�plots�for�Simpson�Range�granitoids�show�slight�LREE-enrichment,�weakly�negative�Nb�anomalies�(Fig.�14A)�and�low Y, consistent with an arc affinity (Fig. 15A; Piercey et al.,�2003;�Grant,�1997).�These�patterns�are�also�consistent�with�derivation�from�upper�crustal�sources.�This�is�further�supported�by�their�Nd�isotopic�signatures�(εNd

t�=�-7.4�to�-12.9;�Grant,�1997;�Piercey�et al.,�2003),�

upper�crust-like�Pb-isotope�systematics�(Grant,�1997)�and�inherited�Proterozoic�(±Archean)�zircon�(Mortensen,�1992a,�b;�Grant,�1997).�

Cycle�III�back-arc�magmatic�rocks�occur�in�the�Wolverine�Lake�group� in� the� eastern�Finlayson�Lake�district� (Fig.�13).�Wolverine�

.C.B TY

kATY

631°W

75 °N

36°N

B.C.

YT

B.C.

Ak

65 °N

841°W

821°W

041°W

46 °N

95 °N

Pci

fica

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O

421W°

Tni

anit

Dil

ane

0 300

km

Scale0 300

km

Scale

?

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346

353353

Tournaisian- Viséan

Famennian - Tournaisian

F

F

357

340 -350

348

344 -347

345 -350

350

355

350

346 - 349

354

345

354

355

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356

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356356

357357

351351

353 - 355353 - 355

347 - 356347 - 356

350 -357

350 -357

343343

347

Wolv

RIFT

MORB

MORB

ARC

ARC

ARC

ARC

BAB

cifi

caP

naec

O

Cycle III - WolverineEarly Mississippian

(Tournaisian)357-342 Ma

SYMBOLS

Blueschist / eclogite occurrence

Volcanism

Fossil


Plutonism

F

Figure 13. Distribution of tectonic and magmatic events during the Early Mississippian Wolverine cycle (Cycle III: 357-342 Ma). Events are located on a base map of Yukon-Tanana and related terranes prior to ~430 km of dextral displacement along Tintina fault. Dashed red line marks approximate limit between geodynamic environments. Dashed blue line represents relative position of subduction zone; teeth indicate dip of subducting slab. See Figure 1 for tectonic as-semblage legend.

� 297


.1

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tinen

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rust

SRPS-G

SRPS-P

Finlayson Lake - Arc Felsic Rocks(Simpson Range plutonic suite)

.1

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PrNd

SmZr

HfEu

TiGd

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Con

tinen

talC

rust

Stewart River - Arc Felsic Rocks(tonalitic-dioritic orthogneiss)

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SmZr

HfEu

TiGd

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pper

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tinen

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WV-5fWV-6f-fwWV-6f-hw

Finlayson Lake: Non-Arc Felsic Rocks(Wolverine Lake group)

A) B)

D)C)

.1

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LaCe

PrNd

SmZr

HfEu

TiGd

TbDy

YEr

YbLu

AlSc

V

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Con

tinen

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rust Glenlyon area - Arc Felsic/Intermediate Rocks

(Little Kalzas)

LK-And

LK-Rhy

LKS-Gr

E)

.1

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LaCe

PrNd

SmZr

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TiGd

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tinen

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Upper Dorsey - rhyolites

Ram Creek - tonalites

Wolf Lake / Jennings River: Arc Felsic Rocks(Ram Creek intrusions, upper Dorsey complex rhyolites)

F)

Roc

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tinen

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LaCe

PrNd

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AlSc

V

Teslin: Arc Felsic Rocks(Simpson Range plutonic suite)

G)

.1

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ThNb

La

Eastern Alaska: Arc Felsic Orthogneiss(Fortymile River assemblage)

CePr

NdSm

ZrHf

EuTi

GdTb

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ErYb

LuAl

ScV

Roc

k/U

pper

Con

tinen

talC

rust

Calc-Alkalic (>70% SiO )2

Transitional (~64% SiO )2

Figure 14. Upper continental crust (UCC) normalized plots for Cycle III felsic rocks. (A) Plutonic rocks from the Simpson Range plutonic suite (SRPS), Finlayson Lake district (SRPS-P from Piercey et�al., 2003; SRPS-G from Grant, 1997); (B) Non-arc rocks from the Finlayson Lake district (WV-5f - Wolverine Lake group felsic volcanic rocks; WV-6f-FW - Wolverine Lake group - felsic volcanic rocks in footwall of Wolverine deposit; WV-6f-HW - Wolverine Lake group - aphyric rhyolites in hanging wall of Wolverine deposit; data from Piercey et�al., 2001b); (C) Tonalitic-dioritic orthogneiss from Stewart River area (S. Piercey and J. Ryan, unpublished data); (D) Orthogneiss from the Fortymile River area (Dusel-Bacon et�al., this volume); (E) Felsic plutonic and volcanic rocks from the Glenlyon region (LK-And = Little Kalzas andesite; LK-Rhy - Little Kalzas rhyolite; LKS-Gr - Little Kalzas suite granite; data from Colpron, 2001); (F) Felsic plutonic and volcanic rocks from the Wolf Lake-Jennings River area (data from Nelson and Friedman, 2004); and (G) Simpson Range plutonic suite equivalents from the Teslin area (data from Stevens et�al., 1995).

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1 10 100 10001

10

100

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within-plate(A-type)

ocean-ridge(OR-type)

M-type


syncollisional(S-type) within-plate &


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syncollisional(S-type)

within plate &


E)

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within plate &


G)

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D)

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M-type



within plate &


F)

Y

Figure 15. Nb-Y discrimination plots for Cycle III felsic rocks. Data sources and symbology as in Figure 14.

� 299


Lake�group�strata�below�the�Wolverine�VHMS�deposit�contain�felsic�tuffs and flows. Above the deposit, silicified, aphyric rhyolites are overlain by massive mafic lava flows (Murphy and Piercey, 1999; Piercey�et al.,�2001b,�c,�2002b).�The�felsic�rocks�in�the�footwall�of�the�Wolverine�deposit�are�characterized�by�HFSE-�and�REE-enriched�signatures, which are virtually identical to the rocks of the Kudz Ze Kayah�formation,�and�are�consistent�with�formation�within�an�en-sialic� back-arc� basin� environment� (Figs.�14B,� 15B;� Piercey� et al.,�2001b,�2002b).�Above�the�Wolverine�deposit,�the�rhyolites�become�less�HFSE-�and�REE-enriched�exhibiting�more�arc-like�patterns�and�Nb-Y�contents�(Figs.�14B,�15B;�Piercey�et al.,�2001b,�2002b).�The�position of these thin, silicified, aphyric rhyolite flows, above non-arc felsic�rocks�and�below�MORB-type�basalts�(see�below),�would�require�an�unacceptably�transient�arc�location.�The�relatively�depleted,�arc-like signatures of the aphyric rhyolites could result from silicification, which�could�have�diluted�their�trace�element�abundances�(Fig.�15);�however, this would not appreciably change the trace element profiles of�these�rocks.�Alternatively,�Piercey�et al.�(2001b)�suggested�that�trace�element�depletions�in�the�aphyric�rhyolites�may�be�due�either�to mixing of HFSE- and REE-depleted mafic magmas with conti-nental�crust,�or�to�lower�temperature�melting�of�continental�crust.�In either case, a continental crustal influence is evident in both the hanging�wall�and�footwall�felsic�rocks�of�the�Wolverine�deposit,�as�both�have�distinctly�negative�εNd

t�values�(εNd

t�=�-7.1�to�-8.2;�Piercey�

et al.,�2003).�Capping�the�entire�felsic�back-arc�sequence�in�the�Wolverine�

Lake�group�are�massive�basalts�(Murphy�et al.,�this�volume).�They�have�MORB-like�signatures�that�have�been�derived�from�a�depleted�to�weakly�enriched�mantle�source�with�a�weak�subduction�signature�(Fig.�16A;�Piercey�et al.,�2002b);�a�single�Wolverine�basalt�sample�yielded�εNd

t�=�+6.9,�consistent�with�derivation�from�depleted�mantle�

sources�(Piercey,�2001).�The�presence�of�N-MORB�to�E-MORB�like�signatures�in�the�Wolverine�basalts�suggests�that�they�were�derived�by�melting� in� response� to�crustal� thinning�during�back-arc�basin�formation and seafloor spreading (Piercey et al.,�2002b).�

In�the�Stewart�River�area,�Cycle�III�magmatism�is�dominated�by�an�arc-like�assemblage�of�tonalitic-dioritic-granodioritic�orthog-neiss,�which�intrudes�a�lowermost�pre-Late�Devonian�metasedimen-tary�package�and�Late�Devonian�amphibolites�(Ryan�and�Gordey,�2001,�2002;�Ryan�et al.,�2003).�The�orthogneissic� rocks�have�pre-liminary� U-Pb� ages� (Villeneuve� et al.,� 2003)� and� petrological� at-tributes�that�are�similar�to�the�Simpson�Range�plutonic�suite�in�the�Finlayson�Lake�district.�Preliminary�geochemical�data�for�the�tona-litic orthogneiss show relatively flat UCC-normalized patterns with flat REE but weak positive anomalies in Nb, Eu, Ti and Al, and arc-like�Nb-Y�systematics�(Figs.�14C,�15C;�S.�Piercey�and�J.�Ryan,�un-published�data).�There�are�no�Sm-Nd�data� for� felsic� rocks�of� the�Stewart�River�area,�but�their�higher�Ti,�Nb�and�Al�relative�to�UCC�suggest�they�may�have�been�derived�from�a�source�more�depleted�than� UCC,� or� as� a� result� of� mantle-UCC� mixing� (Fig.�14C).�Nevertheless,�samples�of�the�orthogneiss�in�the�Stewart�River�area�contain� zircons� with� Proterozoic� inheritance� (Mortensen,� 1990;�Villeneuve�et al., 2003), and have flat REE relative to UCC (Fig. 14C), suggesting�that�a�component�of�continental�material�was�involved�in�their�genesis.�

In the eastern Yukon-Tanana Upland, just northwest of the Stewart� River� area,� Cycle�III� magmatism� is� represented� by�

~357-341�Ma�(J.�Mortensen�and�C.�Dusel-Bacon,�unpublished�data;�Day�et al., 2002) mafic amphibolites (gabbro/diorite?), intermediate-composition� quartz� diorite/tonalite� orthogneiss,� and� subordinate�coeval�granodiorite�and�felsic�metatuff�of�the�Fortymile�River�as-semblage.�These�igneous�rocks�are�interlayered�with�metasedimen-tary�rocks�(Dusel-Bacon�et al.,�this�volume,�and�references�therein).�Amphibolitic�rocks�have�trace�element�signatures�that�have�LREE-enriched island arc and, to a lesser extent, MORB affinities (Fig. 16B; Dusel-Bacon�and�Cooper,�1999;�Day�et al.,�2002).�Plutonic�orthog-neiss�samples�are�subdivided�into�those�with�intermediate�composi-tions�(~64%�SiO

2) and more felsic affinities (>70% SiO

2).�On�UCC-

normalized�diagrams,�the�lower�SiO2�group�are�depleted�in�LREE�

and�Th�relative� to�UCC�and�enriched� in�Al,�Sc�and�V�(Fig.�14D)�suggesting�derivation�from�sources�more�primitive� than�UCC.�In�addition, these samples likely have an affinity transitional between tholeiitic� and�calc-alkaline� (see�Barrett� and�MacLean,�1999)�and�were�most�likely�intruded�within�an�arc�setting�(Fig.�15D).�The�higher�SiO

2 group have calc-alkalic patterns with flat UCC normalized

patterns�and�minor�depletions�in�Nb,�Sc�and�V,�consistent�with�deri-vation�from�UCC-like�sources�within�an�arc�setting�(Figs.�14D,�15D).�Derivation� from� UCC-like� sources� is� supported� by� Precambrian�inheritance� in� some� zircons� from� this� region� (J.� Mortensen� and�C.�Dusel-Bacon,�unpublished�data),�as�is�the�case�with�nearby,�and�likely�related,�magmatic�rocks�of�the�Stewart�River�area.�The�occur-rence�of�several�amphibolites�with�MORB�characteristics�may�be�evidence�for�local�intra-arc�or�back-arc�magmatism.

Cycle�III�magmatism� in� the�Glenlyon�area� is� represented�by�volcanic� and� sedimentary� rocks�of� the�347-344�Ma�Little�Kalzas�formation�and�coeval�granitoids�of�the�Little�Kalzas�plutonic�suite�(Colpron�et al.,�this�volume-b).�Volcanic�rocks�in�the�Little�Kalzas�formation� consist� of� andesites� with� lesser� rhyolites� and� basalts,�interbedded�with�epiclastic�and�basinal�sedimentary�strata�(Colpron,�2001;�Colpron�et al.,�this�volume-b).�Granitoids�of�the�Little�Kalzas�suite�are�metaluminous,�in�part�K-feldspar�megacrystic,�diorite�and�tonalite,�and�resemble�the�Simpson�Range�plutonic�suite�(Colpron,�2001).�

Little Kalzas formation andesites have relatively flat REE rela-tive�to�the�UCC�but�have�lower�Th,�and�higher�Eu,�Ti,�Al�and�V�rela-tive to UCC (Fig. 14E), yet have Zr/Y ratios (~4-9) that are transi-tional to calc-alkalic in affinity (Barrett and MacLean, 1999). These features suggest possible derivation via mixing between mafic (i.e.,�Eu,�Ti,�V-enriched)�material�and�continental�crust,�within�a�continental arc environment. Little Kalzas rhyolites have flat UCC-normalized� patterns� consistent� with� derivation� from� a� UCC-like�source� (Fig.�14E).� Depletions� in� Ti,� V� and� to� a� lesser� extent� Nb�(Figs.�14E,�15E)�are�consistent�with�oxide�fractionation�at�high�levels�in�the�crust�(cf.�Lentz,�1998;�Piercey�et al.,�2001b).�Plutonic�rocks�of�the�Little�Kalzas�suite�have�a�similar,�but�less�erratic�UCC-normal-ized pattern, likely reflecting derivation from continental crust-man-tle�mixing�within�a�continental�arc�setting�(Fig.�14E).�Both�intrusive�and�extrusive�rocks�have�relative�low�Nb�and�Y,�and�plot�within�the�volcanic arc field in Nb-Y space (Fig. 15E). Tracer isotopic data are not�available�for�the�Little�Kalzas�formation�and�Little�Kalzas�suite�

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rocks;�however,�most�U-Pb�geochronological�samples�from�this�re-gion�have�inherited�Proterozoic-Archean�zircon�(Colpron�et al.,�this�volume-b), suggesting that there is a significant older crustal com-ponent in these rocks. Mafic rocks associated with the Little Kalzas formation,�and�the�underlying�Pelmac�formation,�have�smooth�PM-normalized�patterns�with�OIB-like�signatures�consistent�with�deriva-tion�from�enriched�mantle�sources�(Fig.�16C);�no�isotopic�data�are�

presently available for these rocks. These mafic rocks likely reflect rifting�of�the�Little�Kalzas�arc�(Colpron,�2001),�which�may�have�been�related�to�coeval�arc-rifting�and�ensialic�back-arc�magmatic�activity�in�the�Finlayson�Lake�district�(Piercey�et al.,�2001b,�2002b).

In�the�Wolf�Lake�–�Jennings�River�area,�Cycle�III�is�represented�by�intrusive�rocks�structurally�associated�with,�and�possibly�base-ment�to�the�Ram�Creek�Complex,�metavolcanic�rocks�in�the�upper�

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Figure 16. Primitive mantle normalized plots for Cycle III mafic rocks. (A) Mafic rocks from the uppermost part of the Wolverine Lake group (data from Piercey et�al., 2002b); (B) Amphibolites from the Fortymile River assemblage (Dusel-Bacon and Cooper, 1999); (C) Mafic rocks from the Glenlyon region (data from Colpron, 2001); (D) Arc and (E) non-arc mafic rocks from Wolf Lake – Jennings river area (data from Nelson and Friedman, 2004); (F) mafic rocks from the Teslin region (data from Creaser et�al., 1997).

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Dorsey Complex, and minor mafic-intermediate rocks in the Swift River�Group�(Roots�et al.,�this�volume;�Nelson,�1999,�2000;�Roots�and�Heaman,�2001).�The�Dorsey�Complex,�which�structurally�over-lies�the�Ram�Creek�complex,�consists�of�a�lower�unit�of�quartzofeld-spathic� metasedimentary� rocks� and� metabasites� (basalt/gabbro�flows/sills), and an upper unit of siliciclastic rocks, marble and minor felsic�tuffs�(Roots�et al.,�this�volume;�Nelson�and�Friedman,�2004).�Early�Mississippian�ages�have�been�obtained�for�felsic�metavolcanic�rocks�from�the�upper�Dorsey�Complex�(Roots�and�Heaman,�2001).�The Swift River Group lies in structurally modified depositional contact�with�Early�Mississippian�rocks�of�the�Dorsey�Complex,�and�is�overlain�by�the�mid-Mississippian�(Viséan)�Screw�Creek�Limestone�(Nelson,�2000;�Roots�et al.,�2000).�The�Swift�River�Group�consists�predominantly�of�basinal� sedimentary� rocks,�with�minor�volcani-clastic� debris� and� tuffaceous� rocks� (Nelson,� 2000;� Roots� et al.,�2000).�

Early� Mississippian� tonalitic� intrusions� associated� with� the�Ram Creek Complex have relatively flat UCC-normalized patterns but�lower�Th,�and�higher�Ti,�Al�and�V�relative�to�UCC�(Fig.�14F).�These�features�are�consistent�with�potential�derivation�from�a�more�mafic source, or from crust-mantle mixing within an arc environment (Figs.�14F,�15F;�Nelson�and�Friedman,�2004).�Early�Mississippian�UCC-normalized�patterns�for�coeval�rhyolitic�metavolcanic�rocks�from� the�upper� Dorsey Complex� have�UCC-normalized� patterns,�with relatively flat REE but with elevated Th, Zr and Hf relative to UCC,�and�Nb-Y�systematics�that�are�transitional�between�arc�and�non-arc (Figs. 14F, 15F). One sample of mafic rock from the Upper Dorsey complex has an L-IAT to CAB affinity (Fig. 16D). Mafic tuffs�from�the�Swift�River�Group�are�dominated�by�L-IAT�to�CAB�signatures�consistent�with�derivation�from�variably�enriched�mantle�sources�coupled�with�a�subduction�zone�component�with�or�without�minor�crustal�contamination.�In�addition,�two�samples�have�non-arc�affinities with N-MORB and OIB signatures (Fig. 16E; Nelson and Friedman,�2004).�Collectively,�these�geochemical�data�are�consistent�with�arc�magmatism�that�was�interrupted�by�intra-arc�rifting.�

In� the� Teslin� area,� Cycle�III� magmatism� is� represented� by�Simpson�Range�plutonic�suite�equivalents�(Stevens�et al.,�1995)�and�possibly by mafic metavolcanic rocks (unit PMgr of Stevens, 1994;

“Anvil�assemblage”�of�Creaser�et al., 1997). The mafic rocks have uncertain�age�and�stratigraphic�relationships�and�are�considered�to�be�part�of�Cycle�III�due�to�their�spatial�association�with�intrusions�of�that�age;�however,�they�could�be�part�of�any�cycle�in�YTT.�The�Simpson�Range�plutonic�suite�equivalents�in�the�Teslin�region�consist�of�variably�deformed�hornblende-bearing�tonalite�to�quartz-diorite�(Figs.�14G,�15G;�Stevens�et al.,�1995).�These�granitoids�have�very�erratic signatures that are broadly flat with positive Eu, Ti and Al anomalies,�low�La�and�Th�relative�to�Nb�(Fig.�14G)�and�low�Nb-Y�(Fig.�15G)�that�are�consistent�with�formation�within�an�arc�environ-ment.�Their�relative�LREE-depletions�and�high�Nb,�Eu,�Ti�and�Al�(Fig.�14G)�suggest�derivation�from�a�more�primitive�source�than�the�UCC;�however,�εNd

t�values�for�these�Simpson�Range�plutonic�suite�

equivalents�(εNdt�=�-2.5�to�-6.2;�Stevens�et al.,�1995)�clearly�point�

to an ancient crustal component. Mafic rocks from the Teslin region have been interpreted to reflect calc-alkaline basaltic protoliths

(Creaser�et al.,�1997);�however,�a�re-evaluation�of�the�geochemical�data�suggest�that�these�rocks�more�closely�resemble�E-MORB�with�a�weak�subducted�slab�component�(Fig.�16F).�Their�signatures�are�consistent�with�derivation�from�moderately�enriched�sources�during�arc�development�or�arc-rifting,�as�they�are�transitional�between�arc�and�back-arc�magmatism.�Furthermore,�the�Nd-isotopic�data�for�the�greenstones�of�the�Teslin�area�are�consistent�with�derivation�from�a�moderately�enriched�mantle�source,�as�they�exhibit�εNd

t�=�+4.1�to�

+6.4�(Creaser�et al.,�1997),�which�is�less�than�the�depleted�mantle�at�350�Ma�(εNd

t�=�+9.5;�Goldstein�et al.,�1984).�

Little Salmon Cycle (Cycle IV - Late Mississippian - 342-314 Ma)The�fourth�magmatic�cycle� is�mostly� represented� in� the�southern�part�of�YTT�(Fig.�17).�It�begins�at�an�unconformity�beneath�the�Little�Salmon� formation�of� central�Yukon�and� is� thus�named�after� this�succession.�The�sub-Little�Salmon�unconformity�is�locally�marked�by� a� basal� conglomerate� containing� foliated� quartzite� clasts� and�Early�Mississippian�detrital�zircons�(Colpron�et al.,�this�volume-b).�Cycle�IV�magmatism�was�dominated�by�arc,�and�lesser�intra-arc�rift�magmatism;�no�corresponding�back-arc�magmatism�has�yet�been�identified for this cycle. It is represented by the Little Salmon forma-tion�and�associated�plutons�in�the�Glenlyon�area�(Colpron�et al.,�this�volume-b),�the�Ram�Creek�and�Big�Salmon�complexes�in�the�Wolf�Lake-Jennings� River� area� (Roots� et al.,� this� volume;� Nelson� and�Friedman,�2004),�and�bimodal�magmatism�in�Stikine�terrane�(Sebert�and�Barrett,�1996;�Logan�et al.,�2000;�Gunning�et al.,�this�volume;�Fig.�17).�The�Tulsequah�Chief�VMS�deposit�in�northwestern�British�Columbia�formed�during�this�cycle�(Fig.�17).�

Coeval�successions�of�YTT�in�the�Finlayson�Lake�district�and�parts�of�Teslin�and�Wolf�Lake�areas�are�characterized�by�deposition�of�carbonates�in�the�arc�marginal�regions.�Basinal�sedimentary�rocks�are�predominant�in�back-arc�areas�rather�than�igneous�rocks�(Fig.�17;�Slide�Mountain/Seventymile�assemblages;�Nelson,�1993;�Murphy�et al.,�this�volume;�Dusel-Bacon�et al.,�this�volume).

In�the�Glenlyon�area,�volcanic�rocks�of�the�Little�Salmon�forma-tion�consist�of�basalt,�andesite,�and�lesser�dacite,� tuff�and�quartz-feldspar� porphyry,� interbedded� with� volcano-sedimentary� rocks�(Colpron,�2001).�Plutons�of�the�Tatlmain�suite�are�coeval�and�coge-netic�with�them.�Little�Salmon�andesites�and�volcaniclastic�rocks�have�LREE-depleted�UCC-normalized�patterns�with�higher�Eu,�Ti,�V�and�HREE�relative�to�UCC�(Fig.�18A).�These�features�are�consist-ent�with�derivation�from�a�source�more�depleted�than�UCC,�or�with�mixing� between� mantle-derived� magma� and� crustal� material.� In�contrast,�the�dacites�and�quartz-feldspar�porphyries�from�the�Little�Salmon formation have broadly flat UCC-normalized patterns with relatively�weak�depletions�in�Nb�and�V,�and�higher�LREE�and�total�REE�(Fig.�18A),�all�of�which�are�consistent�with�derivation� from�melting�of�continental�crust�accompanied�by�high-level�oxide�frac-tionation. The Tatlmain suite plutons have fairly flat, slightly LREE-depleted signatures (Fig. 18A), reflecting derivation from mixed mantle-crust�sources.�All�of�the�felsic�to�intermediate�rocks�of�the�Little�Salmon�formation�and�the�Tatlmain�suite�intrusions�have�low�Nb-Y, typical of arc settings (Fig. 19A). Mafic rocks from the Little

302

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Salmon� formation� are� dominated� by� OIB-like� alkalic� signatures,�which� indicate� derivation� from� enriched� mantle� source� regions�(Fig. 20A; Colpron, 2001). This geochemical signature likely reflects intra-arc�rifting�episodes,�which�are�also�indicated�by�the�presence�of�spatially�associated�Mn-rich�exhalite�horizons�(Colpron,�2001).�

Only limited geochemical data are available for mafic volcanic rocks�of�the�Big�Salmon�Complex�(M.�Mihalynuk,�unpublished�data;�Nelson�and�Friedman,�2004).�In�the�Wolf�Lake-Jennings�River�area,�the� Big� Salmon� Complex� contains� a� stratigraphic� succession� of�greenstone�overlain�by�marble,�Mn-rich�siliceous�exhalite�and�sil-iciclastic�sedimentary�rocks�with�minor�felsic�tuff�(Mihalynuk�et al.,�1998, 2000, this volume). Mafic volcanic rocks from the Big Salmon Complex�are�dominated�by�LREE-enriched�signatures�with�negative�Nb�anomalies,�typical�of�L-IAT�suite�magmas�(Fig.�20B);�andesite�with�a�calc-alkaline�signature�is�locally�present�(Fig.�20B).�The�Big�Salmon�Complex�also�contains�a�few�samples�that�exhibit�E-MORB�to OIB affinities with positive Nb anomalies (Fig. 20B). These non-arc magmas probably reflect intra-arc rift events. As in the Little Salmon� formation,� the� presence� of� Mn-rich� exhalative� horizons,�interpreted�to�have�formed�during�volcanic�hiatuses,�within�the�Big�Salmon�Complex�also�supports�this�interpretation�(Mihalynuk�and�Peter,�2001).

The�Late�Mississippian�Ram�Creek�Complex�in�the�Wolf�Lake-Jennings�River�area�comprises�intermediate�to�felsic�tuff,�interbed-ded�with�basinal�and�siliciclastic�sedimentary�rocks�and�rare�marble�(Roots� et al.,� this� volume;� Nelson� and� Friedman,� 2004).� Quartz-sericite�schist�from�the�Ram�Creek�Complex�exhibits�a�calc-alkalic�arc affinity (Figs. 18B, 19B), consistent with formation within an arc�environment.

The� Tulsequah� Chief� VMS� deposit� in� northwestern� British�Columbia�provides�the�most�comprehensive�geochemical�dataset�for�the�Stikine�portion�of�Cycle�IV�magmatism.�The�Tulsequah�Chief�deposit�is�hosted�by�a�Mississippian�(327�±�1�Ma�and�330�+10/-1�Ma,�U-Pb�zircon�on�rhyolites;�Childe,�1997)�bimodal�assemblage�of�ba-saltic� and� rhyolitic� volcanic� and�volcaniclastic� rocks� (Sebert� and�Barrett,�1996).�The�footwall�succession�to�the�Tulsequah�Chief�de-posit is bimodal: calc-alkalic arc rhyolitic rocks are interbedded with basalts�that�display�E-MORB�signatures,�and�in�some�cases�include�a�subordinate�subduction�zone�component�(Figs.�18C,�19C);�hanging�wall�basaltic�rocks�exhibit�similar�signatures�(Fig.�20C).�Collectively,�these�rocks�probably�represent�derivation�from�a�weakly�enriched�mantle�source,�coupled�with�crustal�melting�to�form�a�bimodal�vol-canic�assemblage�in�response�to�arc�rifting�(e.g.,�Sebert�and�Barrett,�1996).�

Klinkit Cycle (Cycle V - Pennsylvanian to Early Permian - 314-269 Ma)The fifth cycle is primarily represented by arc magmatism of the Klinkit� Group� and� Fourmile� succession� in� southern� Yukon� and�northern�British�Columbia�(Roots�et al.,�2002,�this�volume;�Simard�et al.,�2003;�Nelson�and�Friedman,�2004;�Fig.�21)�and�the�Lay�Range�assemblage�of�central�British�Columbia� (Fig.�1;�Ferri,�1997).�Arc�magmatism�is�also�inferred�in�the�Semenof�Hills�of�south-central�Yukon�(Fig.�1;�Tempelman-Kluit,�1984;�Simard�and�Devine,�2003;�

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Figure 17. Distribution of tectonic and magmatic events during the Late Mississippian Little Salmon cycle (Cycle IV: 342-314 Ma). Events are located on a base map of Yukon-Tanana and related terranes prior to ~430 km of dextral displacement along Tintina fault. Dashed red line marks approximate limit between geodynamic environments. Dashed blue line represents relative position of subduction zone; teeth indicate dip of subducting slab. See Figure 1 for tectonic as-semblage legend.

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Figure 18. Upper continental crust (UCC) normalized plots for Cycle IV felsic rocks. (A) Tatlmain suite arc felsic rocks from the Glenlyon region (data from Colpron, 2001); (B) Arc felsic rocks from the Wolf Lake – Jennings River area (data from Nelson and Friedman, 2004); and (C) Arc felsic rocks from the footwall of the Tulsequah Chief VMS deposit, Stikine terrane (data from Sebert and Barrett, 1996).

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M.�Colpron�and�J.K.�Mortensen,�unpublished�data;�Figs.�1,�21).�This�cycle�corresponds�to�a�lull�in�felsic�magmatism�in�YTT�(Fig.�6)�and�important�deposition�of�carbonate�and�basinal�sedimentary�rocks�in�the�Finlayson�Lake�district�and�Stikine�terrane�(Colpron�et al.,�this�volume-a).�

Coeval,�back-arc�magmatism�is�present�in�the�Campbell�Range�formation�of�the�Finlayson�Lake�district�(Murphy�et al.,�this�volume;�Plint� and�Gordon�1997;�Piercey�et al.,� 1999)� and�Slide�Mountain�terrane�in�the�Sylvester�allochthon�and�Nina�Creek�Group�(Nelson,�1993;�Ferri,�1997;�Figs.�1,�21).�The�beginning�of�this�cycle�is�arbitrar-ily�set�at�the�Mississippian-Pennsylvanian�boundary,�a�time�marking�the�end�of�felsic�magmatism,�and�the�maximum�preservation�of�fossil�faunas� in� sedimentary� strata� of� YTT� (Fig.�6;� Colpron� et al.,� this�volume-a).�

The� Klinkit� Group� outcrops� discontinuously� from� northern�British�Columbia�into�southern�Yukon,�a�strike�length�of�over�250�km�(Figs.�1,�21).�It�is�characterized�by�the�predominance�of�Pennsylvanian�to�Permian�volcaniclastic�rocks�overlying�carbonates�of�Viséan�to�Bashkirian�age�(Roots�et al.,�this�volume).�The�Klinkit�Group�un-conformably�overlies�the�rocks�of�the�Swift�River�Group�(Nelson,�2002;�Roots�et al.,�this�volume).�Within�the�Klinkit�Group,�the�vol-caniclastic�members�of�the�Butsih�and�Mount�McCleary�formations�are�composed�of�crystal�and�lithic�tuff.�They�are�mainly�basaltic�in�composition,� with� calc-alkaline� arc� PM-normalized� signatures�

(Fig.�22A),�but�TiO2�and�Ti/V�ratios�akin�to�modern�island-arc�tho-

leiitic�basalts�(Gamble�et al.,�1995),�suggesting�that�the�rocks�have�transitional�signatures�between�tholeiitic�and�calc-alkaline�(Simard�et al.,�2003).�Their�εNd

t�values�(+6.7�to�+7.4)�and�Th/La�ratios�(0.13�

to�0.17)�indicate�that�these�rocks�were�derived�from�depleted�mantle�sources�with�minimal�crustal�contamination�(Simard�et al.,�2003).�Based�on�geochemical�and�geological�evidence,�the�volcaniclastic�members�of�the�Klinkit�Group�are�interpreted�as�primitive�arc�lavas�erupted�either�through�relatively�young�crust�that�consists�of�slightly�older�arc�basement,�or�rapidly�emplaced�through�coated�conduits�and/or relatively thin continental crust without significant crustal contamination�(Simard�et al.,�2003).�The�alkali-basalt�member�of�the� Mount� McCleary� Formation� comprises� scarce� discontinuous�lenses�within�the�volcaniclastic�member�(Roots�et al.,�this�volume).�These mafic rocks have OIB-like alkalic signatures with high TiO

2,�

HFSE�and�LREE�(Fig.�22A).�They�are�interpreted�to�have�been�de-rived�from�an�enriched�mantle�source�associated�with�episodic�intra-arc�rifting�of�the�Klinkit�arc�(Simard�et al.,�2003).

In� the� Sylvester� allochthon,� Cycle�V� magmatic� rocks� occur�within�the�Fourmile�succession.�It�occurs�as�thrust�panels,�in�which�undeformed�volcanic�and�epiclastic�units,�minor�limestone,�overlie�polydeformed�black�phyllite,�siltstone�and�argillite�(Nelson,�2002;�Nelson�and�Friedman,�2004).�It�forms�part�of�Division�III,�the�struc-tural�division�of�the�allochthon�assigned�to�the�Harper�Ranch�(late�

Figure 20. Primitive mantle normalized plots for Cycle IV mafic rocks. (A) Non-arc basaltic rocks from the Glenlyon region (data from Colpron, 2001); (B) Arc and non-arc rocks from the Wolf Lake – Jennings River area (data from Nelson and Friedman, 2004; and M. Mihalynuk, unpublished data); (C) Non-arc basaltic rocks from the Tulsequah Chief VMS deposit, Stikine terrane (data from Sebert and Barrett, 1996).

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Paleozoic�arc)�subterrane�of�Quesnellia�(Nelson,�1993).�Basaltic�to�andesitic�rocks�of�the�Fourmile�succession�have�predominantly�L-IAT affinities, consistent with derivation from weakly-enriched mantle�sources�within�a�subduction�zone�environment� (Fig.�22B).�Andesitic�to�dacitic�volcanic�rocks�of�the�Fourmile�succession�have�calc-alkalic� arc� signatures,� slightly� depleted� relative� to� the� UCC�(Fig.�23).�

Pennsylvanian-Early�Permian�arc�magmatism�within�the�Lay�Range�assemblage�of� the�Quesnel� terrane� (Ferri,� 1997)� is� coeval�with,�and�inferred�to�be�correlative�to,�the�Klinkit�Group�(Simard�et al.,�2003).�In�the�Lay�Range,�Ferri�(1997)�has�described�both�L-IAT�(Fig.�22C)�and�MORB�signatures�in�basaltic�rocks�from�the�Upper�Mafic Tuff division (Fig. 22D). These signatures are very similar to magmatism�in�the�Klinkit�Group�(Simard�et al.,�2003)�and�Fourmile�succession�(Nelson,�2002;�Fig.�20).�Simard�et al.�(2003)�suggested�that�by�the�late�Paleozoic,�YTT�was�the�basement�to�the�Quesnel�arc.�Furthermore,�they�argued�that�the�Klinkit�Group�represented�distal�turbiditic�sedimentation�in�response�to�arc�magmatic�activity�within�a�large�YTT�arc�system.�The�presence�of�MORB-type�magmatism,�although�minor,�in�the�Lay�Range�assemblage�suggests�that�the�Lay�Range, Klinkit Group and Fourmile succession reflect an arc system that�was�subsequently�rifted�during�renewed�back-arc�extension,�as�recorded�in�the�Campbell�Range�formation�of�the�Finlayson�Lake�district�(Fig.�21).

In�the�Campbell�Range,�Pennsylvanian-Permian�massive�and�pillowed lava flows, chert and carbonates unconformably overlie older�rocks�of�the�Money�Creek�formation,�the�Wolverine�Lake�and�Grass�Lakes�groups,�and�the�Fortin�Creek�group�east�of�the�Jules�Creek�fault�(Murphy�et al.,�2002,�this�volume).�Diabase,�gabbro�and�ultramafic rocks intrude both the basalt succession and its basement. Basalts�of�the�Campbell�Range�formation�have�an�array�of�non-arc�signatures,� including�N-MORB,�E-MORB�and�OIB,�representing�derivation� from� a� variably� enriched� mantle� wedge� (Fig.�22E;�S.�Piercey�and�D.�Murphy,�unpublished�data).�Additional�data�from�basalts�of�the�Campbell�Range�formation�(Plint�and�Gordon,�1997;�Piercey�et al.,�1999),�and�from�eclogitic�rocks�which�are�interpreted�to�be�recycled�Campbell�Range�basalts�(Creaser�et al.,�1999),�provide�additional�evidence�in�support�of�derivation�from�variably�enriched�mantle�in�a�back-arc�basin�environment.�At�present,�limited�isotopic�data�exist�to�elucidate�the�nature�of�the�crust-mantle�interaction�in�the�Campbell�Range�formation.�Nd�isotopic�data�for�the�MORB-like�Permian�eclogites�show�εNd

t�=�+5.4�to�+9.3,�suggesting�derivation�

from� depleted� mantle� to� weakly� enriched� sources� (Creaser� et al.,�1999).�Furthermore,�Pb-isotopic�signatures�on�sulphides�from�VMS�occurrences�hosted�by�the�Campbell�Range�basalts�in�the�Finlayson�Lake�district�are�non-radiogenic�and�suggestive�of�mantle�sources�(Mann�and�Mortensen,�2000).�These�primitive�isotopic�signatures,�coupled�with�the�prevalence�of�N-MORB�and�E-MORB�lavas,�sug-gest that full seafloor spreading occurred in the Campbell Range back-arc�basin.�

In�north-central�British�Columbia,�magmatism�coeval�with�the�Campbell�Range�formation�is�recorded�within�the�Slide�Mountain�terrane�(Nina�Creek�Group�and�Sylvester�allochthon;�Ferri,�1997;�Nelson,�1993,�2002).�Overall,�Slide�Mountain�basalts� range�from�

.C.B TY

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Plutonism


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Viséan - Moscovian

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ARC

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Cycle V - KlinkitPennsylvanian - Early Permian

(Bashkirian - Kungurian)314-269 Ma

Figure 21. Distribution of tectonic and magmatic events during the Pennsylvanian – Early Permian Klinkit cycle (Cycle V: 314-269 Ma). Events are located on a base map of Yukon-Tanana and related ter-ranes prior to ~430 km of dextral displacement along Tintina fault. Dashed red line marks approximate limit between geodynamic en-vironments. Dashed blue line represents relative position of subduc-tion zone; teeth indicate dip of subducting slab. See Figure 1 for tectonic assemblage legend.

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Lay Range Assemblage: Arc Rocks(upper mafic tuff)

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Finlayson Lake: Non-Arc Rocks(Campbell Range formation)

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antle Sylvester Allocthon: Non-Arc Mafic Rocks

(Slide Mountain terrane)

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A) B)

C) D)

E) F)

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Glenlyon area: Mafic Rocks(Slide Mountain terrane)

H)

Figure 22. Caption on facing page,

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latest� Devonian� through� mid-Permian� in� age,� corresponding� to�Cycles�II-V�in�YTT.�Pennsylvanian-Permian�units�include�the�Pillow�Ridge� succession� in� the� Nina� Creek� Group,� and� Pennsylvanian-Permian�units�in�Division�II�of�the�Sylvester�allochthon.�Basalts�from�the�Pillow�Ridge�succession�all�show�N-MORB�signatures�(Fig.�22F),�as do rocks of the older Mount Howell and Mafic-Ultramafic units. N-MORB�signatures�are�also�ubiquitous�in�the�Slide�Mountain�ter-rane�in�the�Sylvester�allochthon�(Fig.�22G;�Nelson�1993).�These�data�are�similar�to�the�Campbell�Range�N-MORB;�all�are�consistent�with�deriviation�from�depleted�mantle�sources�within�an�oceanic�or�back-arc�basin�environment�(Nelson,�1993,�2002;�Ferri,�1997).�Back-arc�magmatism�in�the�Campbell�Range,�however,�also�included�more�enriched-mantle�products�(OIB�and�E-MORB).�

The predominant N-MORB signatures and the juvenile nature of the isotopic systematics of Pennsylvanian – Early Permian mafic rocks�in�the�Slide�Mountain�terrane�suggest�that�magmatism�occurred�in�a�back-arc�basin�setting�with�no�interaction�with�continental�crust.�However,�local�interbedding�of�terrigenous�clastic�rocks�(e.g.,�Nelson,�1993),�and�evolved�isotopic�systematics�from�sedimentary�rocks�of�Slide� Mountain� terrane� (Patchett� and� Gehrels,� 1998),� indicate� a�continental� source� for� this� detritus.� This� implies� that� the� Slide�

Mountain�ocean�was�a�marginal�sea�that�developed�in�proximity�to�a�cratonic�region;�this�situation�is�similar�to�the�Japan�Sea�relative�to� the� island�of� Japan�and� the�Sino-Korean�craton� (Pouclet�et al.,�1995;�Creaser�et al.,�1999).�

Klondike Cycle (Cycle VI - Middle to Late Permian - 269-253 Ma)The�last�Paleozoic�magmatic�cycle�is�primarily�represented�in�the�Klondike�region�of�western�Yukon,�and�is�therefore�named�after�this�area�(Fig.�22).�It�corresponds�to�the�second�pulse�of�felsic�magmatism�in�the�pericratonic�terranes�(Fig.�6),�and�its�beginning�is�marked�by�the� formation� of� eclogites� in� Yukon� (Erdmer� et al.,� 1998).� The�Klondike�cycle�ends�with�the�cessation�of�magmatism�in�YTT,�and�a�period�of�depositional� and� igneous�hiatus� in� the�Early�Triassic�(Fig.�6).�Subsequent�deposition�of�Middle�to�Upper�Triassic�clastic�sedimentary�rocks�overlap�Yukon-Tanana�and�Slide�Mountain�ter-ranes,�and�also�the�North�American�miogeocline�(Colpron�et al., this�volume-a;�Nelson�et al., this�volume).�

Cycle�VI�magmatism�is�represented�by�the�mid-�to�Late�Permian�Klondike�Schist�(~263-253�Ma),�a�sequence�of�felsic�volcanic�and�volcaniclastic rocks with lesser interlayered mafic rocks, and coeval and�probably�cogenetic�monzonitic�to�quartz-monzonitic�granitoids�of�the�Sulphur�Creek�orthogneiss�(Mortensen,�1990).�Late�Permian�felsic�schist�layers�occur�also�within�carbonaceous�rocks�assigned�to� the�Nasina� assemblage� in� the�Fortymile�River� area�of� eastern�Alaska�(Figs.�1,�24;�J.�Mortensen�and�C.�Dusel-Bacon,�unpublished�data;�Dusel-Bacon�et al.,�this�volume).�Biotite-bearing�granitic�or-thogneiss�of�Late�Permian�age�also�intrudes�Devono-Mississippian�metasedimentary�rocks�of�the�Nasina�assemblage�(Ryan�et al., 2003;�J.� Mortensen,� unpublished� data).� Although� limited,� geochemical�data for felsic rocks of the Klondike Schist consistently exhibit flat calc-alkalic�signatures,�with�low�Nb,�Eu,�Ti,�Sc�and�V�relative�to�UCC�(Fig.�25),�indicative�of�an�arc�setting�(e.g.,�Mortensen,�1990;�S. Piercey and J. Mortensen, unpublished data). The UCC-profiles for�the�Sulphur�Creek�orthogneiss�exhibit�less�systematic�behaviour�than�the�Klondike�Schist�(Fig.�25A),�perhaps�due�to�mobility�of�some�

Figure 22. (facing page) Primitive mantle normalized plots for Cycle V mafic rocks. (A) Arc and non-arc mafic lavas from the Klinkit Group (data from Simard et�al., 2003); (B) Arc rocks from the Sylvester allochthon (data from Nelson and Friedman, 2004); (C) Arc and (D) non-arc rocks from the Lay Range area (data from Ferri, 1997); (E) Non-arc rocks from the Campbell Range formation (S. Piercey and D. Murphy, unpublished data); (F) Non-arc rocks from the Nina Creek Group (data from Ferri, 1997); and (G) Non-arc rocks from the Sylvester allochthon (data from Nelson, 1993; Nelson and Friedman, 2004); (H) Cycle VI greenstones from the Slide Mountain terrane in Glenlyon area (data from Colpron et�al., 2005).

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Sylvester Allochthon: Arc Felsic Rocks(Fourmile succession)

A) B)

Figure 23. (A) Upper continental crust normalized and (B) Nb-Y discrimination plots for felsic rocks in the Sylvester allochthon (data from Nelson and Friedman, 2004).

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elements� due� to� alteration� or� metamorphism,� or� cumulate�processes.

In�the�Fortymile�River�area,�felsic�schist�layers�within�carbona-ceous� rocks� of� the� “Nasina� assemblage”� have� UCC-normalized�profiles that can be broken into two groups: a group with relatively flat patterns with depletions in Eu, Zr, Hf and compatible elements, and�a�second�group�with�similar�characteristics�but�with�additional�LREE-depletion (Fig. 25B). The first group is consistent with an arc environment;� however� metarhyolite� with� LREE-depletion� and�slightly�higher�Nb�(and�Ta)�contents�suggest�a�possible�within-plate�origin�for�those�samples�(Fig.�25D).�Alternatively,�these�may�repre-sent�residual�enrichment�of�Nb�and�Ta�at�the�expense�of�LREE,�due�to�loss�of�LREE�during�hydrothermal�alteration�leading�to�closure-related�gains�in�Nb�and�Ta�(see�Stanley�and�Madeisky,�1994).

Permian�magmatic�activity� is� recorded�by�a�suite�of�plutons�that�intrude�YTT�and�related�allochthons�in�the�Wolf�Lake-Jennings�River�area�and� the�Sylvester�allochthon� (Fig.�24).�A�suite�of�mid-Permian�(270-262�Ma)�intrusions,�including�the�Ram�stock,�and�Nizi�and� Meek� plutons,� intrudes� the� Dorsey� Complex� and� panels� of�Cycle�V� arc� rocks� in� far� northern� British� Columbia� (Nelson� and�Friedman,�2004).�One�body�in�this�suite�stiches�a�post-Early�Permian�thrust�fault�in�the�Sylvester�allochthon.�Compositionally,�the�suite�ranges� from�gabbro� to�granite;� tonalite� is� the�dominant�phase.�A�sample�of�granite�from�the�Meek�pluton�exhibits�a�calc-alkalic�arc�signature,� like� the� rocks� of� the� Klondike� region� (Fig.�25C);� this�scenario�is�consistent�with�a�southerly�continuation�of�arc�magmatism�during� the� Klondike� cycle�throughout� eastern� YTT.� Carbonate�deposition�dominates�the�mid-�to�late�Permian�record�of�the�Stikine�terrane�(Gunning�et al.,�this�volume).

Mafic volcanism of back-arc basin affinity (N-MORB) persisted into�mid-Permian�time�in�Slide�Mountain�terrane�in�the�Sylvester�allochthon�and�Seventymile�terrane�(Figs.�1,�24;�Dusel-Bacon�et al.,�this�volume;�Nelson,�1993).�In�the�Glenlyon�area,�the�Slide�Mountain�terrane�is�represented�by�a�narrow�belt�of�chert,�argillite,�greenstone�and� serpentinite� of� Middle� Permian� age� (Colpron et al.,� 2005).�Greenstones� from�Slide�Mountain� terrane� in�Glenlyon�area�have�mixed�N-MORB�and�calc-alkaline�signatures�indicative�of�crustal�contamination�of�an�N-MORB�parent�magma�(Fig.�22H;�Colpron et al.,�2005).�

DISCUSSION

Petrochemical Constraints on the Regional Tectonic Evolution of Yukon-Tanana and Related TerranesModels� for� the� tectonic� evolution� of� Yukon-Tanana� and� Slide�Mountain terranes have three common themes: (1) rifting of part of YTT�from�the�western�margin�of�Laurentia�in�mid-Paleozoic�time�(~390-365�Ma);�(2)�mid-�to�late�Paleozoic�arc�activity,�back-arc�exten-sion�and�opening�of�the�Slide�Mountain�ocean�(~365-269�Ma);�and�(3)�subduction�of�Slide�Mountain�crust�and�lithosphere�(~269-253�Ma),�and�the�subsequent�accretion�of�YTT�back�onto�the�western�margin�of� North� America� (Tempelman-Kluit,� 1979;� Mortensen,� 1992a;�Hansen�and�Dusel-Bacon,�1998;�Nelson,�1993;�Nelson�et al.,� this�volume).�The�geochemical�and�isotopic�data�reviewed�in�this�paper�

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Artinskian -GuadalupianArtinskian -Guadalupian

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253

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Cycle VI - KlondikeMiddle Permian - Early Triassic

(Guadalupian - Olenekian)269-253 Ma

Figure 24. Distribution of tectonic and magmatic events during the Middle Permian – Early Triassic Klondike cycle (Cycle VI: 269-253 Ma). Events are located on a base map of Yukon-Tanana and related terranes prior to ~430 km of dextral displacement along Tintina fault. Dashed red line marks approximate limit between geodynamic environments. Dashed blue line represents relative position of subduction zone; teeth indicate dip of subducting slab. See Figure 1 for tectonic assemblage legend.

� 309


provide further constraints that help refine our understanding of the mid-� to� late� Paleozoic� evolution� of� Yukon-Tanana� and� related�terranes.

It is important to note that the present configuration of YTT (Fig.�1)�and�the�distribution�of�magmatic�belts�discussed�in�this�paper�are� the� result� of� the� protracted� deformational� history� of� Yukon-Tanana,�Slide�Mountain�and�the�North�American�miogeocline�be-tween�late�Paleozoic�and�early�Cenozoic�times�(e.g.,�Murphy�et al.,�2002;�Roddick,�1967).�On�Figures�7,�10,�13,�17,�21�and�24�we�show�the�distribution�of�the�main�Paleozoic�magmatic�and�tectonic�events�on�a�series�of�maps�of�Yukon-Tanana�and�related�terranes�prior�to�

~430�km�of�early�Cenozoic�dextral�displacement�along�Tintina�fault�(Murphy� and� Mortensen,� 2003;� Gabrielse� et al.,� in� press).� These�maps�essentially�show�the�Late�Cretaceous�paleogeography�of�Yukon-Tanana�and�related� terranes.�The�effects�and�magnitude�of�offset�associated�with�late�Paleozoic�to�Mesozoic�faulting�within�YTT�are�for�the�most�part�poorly�constrained,�and�have�not�been�removed�from�the�maps.�It�should�also�be�noted�that�displacement�along�the�Denali�and�Border�Ranges�faults�have�not�been�restored.�In�the�fol-lowing�discussion�we�assume�that�the�Late�Cretaceous�distribution�

of�magmatic�and�tectonic�events�in�YTT�approximates�their�relative�original�distribution�in�the�Paleozoic.�

Alaska� Range� cycle�(I� –� 390-365� Ma)� magmatic� activity� in-volved� crustal� extension� and� attenuation� of� the� North� American�continental�margin�in�the�Alaska�Range�and�Yukon-Tanana�Upland�(Dusel-Bacon�et al.,� this�volume,�2004),�and�continental�arc�mag-matism�in�the�Coast�Mountains�of�southeastern�Alaska�and�British�Columbia (Fig. 7). Cycle I magmatic rocks in the Bonnifield mineral belt�and�correlative�assemblages�(Lake�George�assemblage�and�sili-ceous,� carbonaceous� and� volcanic� assemblage)� have� diagnostic�crustally-derived peralkaline rhyolites and associated OIB-like mafic rocks�(Fig.�8).�These�geochemical�characteristics,�and�the�predomi-nantly�older�ages�of�these�Alaskan�rocks,�resemble�coeval�volcanic�rocks�of�the�North�American�miogeocline�(Selwyn�basin�and�Cassiar�terrane,� Fig.�1;� Mortensen,� 1982;� Mortensen� and� Godwin,� 1982;�Goodfellow�et al.,�1995).�Mid-Paleozoic�alkalic�volcanism�and�exten-sion�of�the�continental�margin�is�likely�associated�with�coeval�arc�magmatism�in�YTT�in�the�Coast�Mountains,�and�initiation�of�the�Slide�Mountain�rift�(Paradis�et al.,�1998;�Nelson�et al.,�2002).



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Figure 25. Upper continental crust normalized plots for Cycle VI felsic rocks. (A) Felsic rocks from the Klondike region (S. Piercey and J. Mortensen, unpublished data); (B) Felsic schist in Nasina assemblage, Fortymile River area (Dusel-Bacon et�al., this volume); (C) Meek pluton (data from Nelson and Friedman, 2004); and (D) Nb-Y plot for Klondike and Meek pluton; SCO = Sulphur Creek orthogneiss.

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Finlayson�cycle�(II�–�365-357�Ma)�magmatism�was�character-ized�by�arc�and�back-arc�environments�in�the�Finlayson�Lake�district�and�Stewart�River�area.�Rift-related�magmatism�in�the�Alaska�Range�and�Yukon-Tanana�Upland�persisted�during�this�cycle.�Arc�magma-tism�occurred�predominantly�in�a�western�belt,�now�preserved�pri-marily�in�the�Stewart�River�region�and�in�the�southwestern�Finlayson�Lake� district� (Table�1;� Fig.�10).� Arc� sequences� are� dominated� by�felsic magmatic rocks with calc-alkalic signatures and mafic rocks with�arc�signatures�(Table�1;�Figs.�11-12).�This�magmatic�arc�belt�is�coeval� with� back-arc� magmatism� in� the� eastern� Finlayson� Lake�district�and�MORB-dominated�marginal�basin�facies�of�the�Slide�Mountain�assemblage�in�the�Sylvester�allochthon�(Fig.�10;�Nelson,�1993),� implying� a� west-facing,� east-dipping� subduction� zone�(Mortensen,�1992a;�Nelson,�1993;�Nelson�et al.,�this�volume).�

Wolverine�cycle�(III�–�357-342�Ma)�magmatism�is�most�wide-spread� in� the�central�and�southern�parts�of� the�YTT�(Fig.�13).� Its�configuration is similar to that of Cycle II, with arc sequences located in�the�western�portions�of�the�terrane,�in�front�of�the�more�easterly�back-arc� region� in� eastern�Finlayson�Lake,� and�even�more�distal�back-arc�of�the�Slide�Mountain�assemblage�in�the�Sylvester�alloch-thon�(Fig.�13).�There�is�little�Cycle�III�arc�magmatic�activity�recorded�in�the�Yukon-Tanana�Upland�of�eastern�Alaska.�The�intermediate�to mafic volcanic rocks of the Fortymile River assemblage span the Alaska-Yukon�border�and�have�arc�and�MORB�signatures�(Fig.�16B).�They� may� represent� an� arc� and� back-arc� sequence,� although� the�tectonic� implications�of� these�rocks�have�yet� to�be�resolved.� It� is�important�to�note�the�paucity�of�magmatism�in�the�Alaska�Range�from�Cycle�III�onward�(Fig.�13).�On�the�other�hand,�arc�magmatic�activity�was�voluminous�and�is�well�represented�in�the�Glenlyon�and�Teslin� regions� (Fig.�13;� Table�1).� These� latter� arc� sequences� also�contain alkalic mafic rocks (Fig. 16; Table 1), suggesting that the arc or�arcs�underwent�a�period�of�extension�during�Cycle�III.�Furthermore,�the�presence�of�arc-dominated�successions�in�the�central�and�western�portions of YTT, and the occurrence of seafloor spreading recorded by�MORB-type�and�BABB-type�basaltic�rocks�in�the�easternmost�part�of� the�terrane�in�the�Finlayson�Lake�district�(e.g.,�Wolverine�basalt;� Piercey� et al.,� 2002b)� and� in� the� Sylvester� allochthon�(e.g.,�Slide�Mountain�basalts;�Nelson,�1993),�imply�the�likelihood�of�westerly rollback of the slab and the induction of back-arc seafloor spreading.�

The�Little�Salmon�cycle�(IV�–�342-314�Ma)�marks�a�fundamental�shift in the configuration of the arc system. Both arc and back-arc magmatism�essentially�ceased�in�the�Finlayson�Lake�and�Stewart�River�areas,�and�the�locus�of�arc�activity�shifted�southwards�to�the�Glenlyon�and�Wolf�Lake-Jennings�River�areas�(Fig.�17).�Coeval�arc�magmatism�also�occurs�in�the�Stikine�assemblage�of�northwestern�British�Columbia�(Fig.�17).�Cycle�IV�sequences�are�dominated�by�crustally-derived,�calc-alkalic�magmatic�rocks�with�OIB-like�effu-sions of mafic material (Figs. 18-20), implying continental arc magmatism�with� intra-arc�rift�episodes.�The� lack�of�documented�Late�Mississippian�magmatic�activity�in�the�Slide�Mountain�assem-blage�of�the�Sylvester�allochthon�has�led�Nelson�and�Bradford�(1993)�to�suggest�that�there�was�a�lull�in�volcanic�activity�in�the�back-arc�basin.�Alternatively,�this�may�be�a�function�of�poor�preservation�of�

Late�Mississippian�Slide�Mountain� crust.�Nevertheless,� Cycle�III�magmatism�most�likely�occurred�above�an�east-dipping�subduction�zone,�as�with�previous�magmatic�cycles�(Fig.�17).

Klinkit�cycle�(V�–�314-269�Ma)�magmatism�represents�another�shift�in�the�style�and�nature�of�magmatism�in�YTT.�Arc�magmatism�was�widespread� in� the�Wolf�Lake-Jennings�River�areas,�as� repre-sented�by�the�Klinkit�Group�(Simard�et al.,�2003),�and�within�the�Fourmile� succession� in� the� Sylvester� allochthon� (Nelson� and�Friedman,�2004;�Fig.�21).�Arc�volcanic�rocks�within�the�Lay�Range�assemblage�of�Quesnellia�in�north-central�British�Columbia�are�also�correlated�with�this�magmatic�cycle�(Fig.�1;�Ferri,�1997;�Simard�et al.,�2003).�Coeval�back-arc�magmatism�is�present�in�the�Campbell�Range�formation�in�the�Finlayson�Lake�district�and�in�the�Slide�Mountain�assemblage�and�the�Nina�Creek�Group,�in�the�Sylvester�allochthon�and�the�Lay�Range�area�respectively�(Figs.�1,�21;�Nelson,�1993;�Ferri,�1997;�Plint� and� Gordon,� 1997;�Piercey� et al.,� 1999;� Creaser� et al.,�1999).�The�location�of�these�back-arc�rocks�to�the�northeast�and�north�of�the�dominantly�arc�assemblages�suggests�that�subduction�polarity�was�east-�to�northeast-dipping�(Fig.�21).�This�is�likely�a�continued�evolution�of�the�east-dipping�subduction�zone�that�characterized�all�previous�magmatic�cycles.�

An�interesting�feature�of�this�phase�of�magmatism�is�the�domi-nance of mafic to intermediate material with very little felsic mag-matism�compared�to�earlier�YTT�episodes�(e.g.,�Nelson,�1993;�Ferri,�1997;�Plint�and�Gordon,�1997;�Murphy�and�Piercey,�1999;�Piercey�et al.,�1999;�Simard�et al., 2003; Fig. 6). Furthermore, Cycle V mafic magmatism is characterized by very juvenile isotopic characteristics. In the Klinkit Group, arc-related samples have juvenile εNd

t�values�

of�+6.7�to�+7.4�(Simard�et al.,�2003).�Permian�eclogites�near�Faro�and�Ross�River,�north�of�the�Finlayson�Lake�district�(Figs.�1,�24),�which�have�back-arc-related�protoliths� and� are� inferred� to�be�Campbell�Range�equivalents,�have�εNd

t�=�+5.4�to�+9.3�(Creaser�et al.,�1999).�

Back-arc-related�VMS�occurrences�in�the�Campbell�Range�belt�have�juvenile Pb-isotopic systematics (Mann and Mortensen, 2000). Collectively,�these�features�suggest�that�both�arc�and�back-arc�related�rocks�had�minimal� interaction�with�continental�crustal�materials.�Two potential mechanisms can be invoked to explain this: (1) both arc and back-arc regions were built (at least in part) upon juvenile substrates;�and/or�(2)�that�the�rate�of�extension�was�rapid,�and�coupled�with�rapid�effusion�rates�and/or�conduit�armouring�that�prevented�any� substantial� interaction� with� continental� crustal� material.� Nd�isotopic�and�trace�element�geochemical�evidence�suggests�that�both�mechanisms�were�operative�in�YTT�(Piercey�et al.,�2001a;�Simard�et al.,�2003);�and�it�is�likely�that�both�were�important�in�explaining�the dominance of juvenile material during Cycle V magmatism.

Klondike�cycle�(VI�–�269-253�Ma)�tectonic/magmatic�patterns�mark a significant departure from that of all previous cycles in YTT. Cycle�VI�is�characterized�by�a�change�in�subduction�polarity�such�that�the�arc�now�faced�east�above�a�west-dipping�subduction�zone�(Fig.�24;�Mortensen,�1990).�This�east-facing�subduction�geometry�is�indicated�by�pairing�of�a�belt�of�mid-Permian�eclogite�and�blues-chists� along� the� eastern� edge�of� the� terrane� (Dusel-Bacon,� 1994;�Erdmer�et al.,�1998)�with�mid-�to�Late�Permian�arc�rocks�that�occur�primarily� in� the� Klondike� district� to� the� west� and� southwest�

� 311


(Mortensen,�1990,�1992a;�Fig.�24).�This�shift�from�east-�to�west-dip-ping�subduction�marks�the�onset�of�the�closure�of�the�Slide�Mountain�marginal�ocean�(Mortensen,�1992a;�Nelson,�1993).�The�occurrence�of� calc-alkalic� felsic� rocks� in� both� the� Klondike� district� and� in�northern� British� Columbia� suggest� that� this� magmatism� was� of�continental arc affinity (Fig. 25). However, geochemical data for these�rocks�are�sparse,�and�further�work�is�required�to�fully�docu-ment�their�character.�Cycle�VI�culminated�with�cessation�of�volcan-ism�in�YTT�and�the�deposition�of�synorogenic�conglomerate,�which�continued�into�Late�Triassic�time�and�represents�an�overlap�assem-blage� that� links� Yukon-Tanana� and� Slide� Mountain� terranes� to�western�North�America�(Fig.�24;�Nelson�et al.,�this�volume).�

The�combination�of�geological,�geochronological�and�geochemi-cal�data�reviewed�here�indicates�that�the�middle�to�late�Paleozoic�evolution�of�YTT�(~390-269�Ma)�is�characterized�by�extension�of�a�continental�margin�behind�a�west-facing�arc�system�(Figs.�7,�10,�13,�17�and�21).�Development�of�this�arc�system�was�punctuated�by�epi-sodic�arc�rifts�and�formation�of�a�back-arc�basin,�which�eventually�led to the opening of the Slide Mountain marginal ocean. The final stage�of�the�Paleozoic�evolution�of�the�terrane�is�marked�by�a�fun-damental� shift� in� geodynamic� setting� from� an� east-dipping� to� a�west-dipping�subduction�zone.�During�this�last�cycle�(269-253�Ma),�the�Slide�Mountain�ocean�was�ultimately�consumed�and�rocks�of�the�Klondike� arc� formed� above� the� west-dipping� subduction� zone�(Fig.�24).

Mantle Sources and Mixing During YTT EvolutionMafic magmatic rocks along convergent margins often have complex petrological�histories� involving� the� interaction�of� subducted� slab,�mantle�wedge,�lithospheric�mantle�and�continental�crust�(e.g.,�Gill,�1981;�Pearce,�1983;�Rogers�and�Hawkesworth,�1989;�Pearce�and�Peate,�1995; Shinjo and Kato, 2000; Shinjo et al.,�1999).�In�order�to�evaluate�the mantle sources of mafic rocks, discriminants that specifically characterize�mantle,� as� opposed� to� slab-metasomatic� and� crustal�components must be used. In Figure 26 the element ratio Zr/Yb is plotted against Nb/Yb for mafic rocks from each magmatic cycle in YTT.�This�diagram�is�particularly�useful�in�discriminating�the�rela-tive�incompatible�element�enrichment�of�the�mantle�source�region�for basaltic rocks, because Zr, Nb and Yb are all moderate to highly incompatible�elements�and�ratios�of�these�elements�to�one�another�are�largely�insensitive�to�slab�metasomatism,�crustal�contamination�and�fractionation.�

The broad range of Nb/Yb and Zr/Yb in Figure 26 shows that, regardless of age, mafic rocks in YTT were derived from heterogene-ous�mantle�sources�that�ranged�from�depleted�to�enriched.�In�YTT,�a�depleted�end-member�mantle�source�played�a�role�in�the�genesis�of�arc,�and�to�a�lesser�extent,�non-arc�rocks.�On�Figure�26,�most�arc�rocks�cluster�near�or�slightly�below�the�N-MORB�end-member�and�extend�toward�E-MORB�compositions,�whereas�non-arc�rocks�gener-ally�plot�between�E-MORB�and�OIB�end-members.�In�Cycles�III�and�V�(Figs.�26B,�D),�non-arc�rocks�show�more�variability�and�have�values�that�extend�toward�the�N-MORB�end-member,�implying�that�some� of� these� rocks� were� derived� from� depleted� mantle� sources�(e.g.,�BABB).�The�role�of�the�depleted�mantle�in�many�modern�arc�

and� back-arc� magmatic� environments� is� well� documented�(McCulloch�and�Gamble,�1991;�Woodhead�et al.,� 1992;�Hawkins,�1995;�Pearce�and�Peate,�1995;�Gribble�et al., 1996; Shinjo et al.,�1999).�The�presence�of�depleted�mantle�wedge�sources�for�YTT�arc�and�back-arc�magmatism�suggests� that� the�arc�mantle�wedge�and� the�back-arc�mantle�beneath�YTT�were�similar�to�modern�magmatic�arc�and�back-arc�environments.�

In�addition�to�depleted�mantle,�a�component�of�enriched�mantle�is�evident�in�most�YTT�arc�and�non-arc�rocks�(Fig.�26).�The�occur-rence of mafic rocks with more enriched compositions is a feature that� is�commonly�present� in�continental�arc�and�back-arc,�and� in�continental�rift�geodynamic�environments�(e.g.,�Pearce,�1983;�Pearce�and Peate, 1995; Shinjo et al.,�1999).�However,�the�nature�and�origin�of this enriched component is a matter of debate. Shinjo et al.�(1999)�proposed� an� asthenospheric� source� (i.e.,� new,� upwelling� astheno-spheric�mantle)�for�the�enriched�component,�whereas�Pearce�(1983),�Hawkesworth�et al.�(1990)�and�McDonough�(1990)�favour�a�lithos-pheric� origin� (i.e.,� ancient,� subcontinental�mantle).�Pearce� (1983)�further� argued� that� the� higher� HFSE� contents� of� continental� arc�magmas�relative�to�intra-oceanic�arc�magmas�are�a�result�of�a�sub-continental lithospheric contribution to continental arc mafic mag-mas,� in� addition� to� depleted� mantle�wedge� and� slab� components�(cf.�Pearce�and�Parkinson,�1993).�Rogers�and�Hawkesworth�(1989)�illustrated�a�correlation�between�increasing�incompatible�element�enrichment�(e.g.,�Nb/La

mn,�Nb/Th

mn�>1)�and�decreasing�εNd�and�in-

creasing�εSr in Andean basalts, indicating the influence of old sub-continental�lithosphere�in�their�genesis.�Data�for�enriched�rocks�in�YTT�favours�a�lithospheric�origin.�Most�Nb-enriched�rocks�in�YTT�(e.g.,�OIB-rift,�NEB-suite�rocks),�with�positive�Nb�anomalies�relative�to�Th�and�La�(Nb/Th

mn,�Nb/La

mn�>�1),�have�low�εNd

t�values�ranging�

from�+1.7�to�-1.6.�Furthermore,�a�broad�relationship�between�increas-ing�Nb/Th

mn�and�Nb/La

mn�and�decreasing�εNd

t in some YTT mafic

rocks�are�features�that�cannot�be�explained�by�crustal�contamination�and� thus� require� an� enriched,� older,� subcontinental� lithospheric�component�(Piercey,�2001;�Piercey�et al.,�2002a,�2004).�

Any�model�that�explains�the�enriched�component�in�the�YTT�subarc�mantle�wedge�requires�that�it�operated�for�most�of�the�mid-�to�late�Paleozoic�evolution�of�the�terrane�(Fig.�26).�We�favour�a�simple�two-component�mixing�model�between�magmas�derived�from�a�de-pleted�mantle�wedge,�or�depleted�back-arc�asthenosphere,�and�an�enriched� lithospheric� component,� to� explain� the�heterogeneity�of�YTT mafic rocks. The linear array between depleted and enriched components on the Zr/Yb-Nb/Yb diagram supports this hypothesis. Nevertheless,�there�are�clearly�end-member�magmas�that�do�not�re-quire� a�mixed� component.�For� example,� the�boninitic,� island� arc�tholeiitic and N-MORB-type magmas likely reflect derivation from solely� depleted� to� ultradepleted� mantle� sources� (Piercey,� 2001;�Piercey�et al.,�2001a,�2004),�which�would�probably�be�involved�in�normal�arc�and�back-arc�magmatic�activity.�In�contrast,�OIB-rift�type�rocks�(within-plate/extensional)�probably�represent�derivation�from�solely�enriched�lithospheric�sources�in�response�to�arc�rifting�and�the�initiation�of�back-arc�magmatic�activity�in�the�bulk�of�the�YTT,�as�well�as�continental�margin�extension�in�the�Yukon-Tanana�Upland�and�Alaska�Range�(Piercey�et al.,�2002b;�Colpron,�2001;�Simard�et al.,�

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Figure 26. Zr/Yb-Nb/Yb plots for mafic rocks from the various magmatic cycles in YTT evolution. Cycles I through VI represented in (A) through (F) respectively. Diagram modified after Pearce and Peate (1995). Average values of mantle reservoirs are from Sun and McDonough (1989): N-MORB = normal mid-ocean ridge basalt (depleted); E-MORB = enriched MORB (moderately enriched); and OIB = ocean-island basalt (enriched).

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2003).�Mixing�between�these�depleted�and�enriched�end-members�is�an�expected�response�to�concurrent�tectonic�activity.�The�presence�of an enriched component in arc rocks likely reflects mixing of mate-rial�from�a�depleted�mantle�wedge�with�subcontinental�lithospheric�mantle�during�migration�of�the�melts�to�their�ultimate�destination�within�and�upon�the�crust.�Similarly,�the�occurrence�of�E-MORB-type�signatures�within�YTT�back-arc�basins�would�signal�mixing�between�upwelling� depleted� N-MORB� type� asthenosphere� and� enriched�lithospheric�material,�en-route�to�emplacement�within�the�back-arc�regions�(e.g.,�Campbell�Range�basalts�and�Sylvester�allochthon).�

It�is�notable�that�there�are�no�temporal�variations�in�the�incom-patible element behaviour of YTT mafic rocks from the mid- to late Paleozoic. This suggests that mantle sources for mafic rocks of YTT did�not�change�appreciably�throughout�the�terrane’s�evolution,�and�that�the�mantle�heterogeneity�exhibited�by�YTT�is�a�fundamental�feature�of�the�terrane.�

Importance of Recycled Continental Crust During YTT EvolutionThe�importance�of�continental�crustal�recycling�in�modern�and�an-cient�continental�margin�arc�and�back-arc,�and�in�rifted�continental�margin�geodynamic�environments�is�well�established�(Rogers�and�Hawkesworth, 1989; Shinjo et al.,�1999;�Whalen�et al.,�1998;�Piercey�et al., 2003). In YTT, the influence of evolved continental crust is shown�by�the�Pb,�Nd�and�Sr�isotope�systematics�of�granitic�rocks�(Mortensen,�1992a).�Uranium-lead�zircon�data�from�geochronologi-cal�studies�of�granitoid�rocks�from�the�terrane�commonly�indicate�Proterozoic� and� Archean� inheritance� (Mortensen,� 1990;� 1992a;�Dusel-Bacon�et al.,� 2004).�Similarly,�Nd� isotopic� studies�of�YTT�sedimentary�and� felsic� igneous� rocks�exhibit� strong�evidence� for�recycled� ancient� continental� crust� in� their� genesis� (εNd

t<0� and�

Proterozoic�TDM

�ages;�e.g.,�Stevens�et al.,�1995;�Creaser�et al.,�1997;�Piercey�et al.,�2003).

Figure�27�shows�plots�of�La�and�Sm�(both�normalized�to�UCC�values)�for�felsic�rocks�from�all�magmatic�cycles�in�YTT.�Rocks�de-rived�from�crustal�sources�lie�on�or�near�the�line�with�a�slope�of�1,�equivalent� to� La/Sm

UCN�(UCN� =� Upper� Crust� Normalized)� =� 1�

(Fig.�27).�Samples�that�plot�below�this�line,�with�La/SmUCN

�<�1,�likely�come�from�sources�that�are�depleted�relative�to�UCC,�for�example,�mafic crust; or they may have fractionated LREE-bearing accessory phases.�The�case�of�La/Sm

UCN�>>1�is�very�rare,�as�it�would�require�

crustal� sources�with�much�higher�LREE� than� the�UCC;�however,�high�La�to�Sm�ratios�could�be�achieved�through�preferential�mobili-zation of LREE in hydrothermal fluids, or through kinetically con-trolled�melting�of�LREE-enriched�accessory�phases�at�high�tempera-tures�(e.g.,�Bea,�1996a,�b).

Most�felsic�rocks�from�YTT�cluster�close�to�the�La/SmUCN

≈1 control�line,�implying�that�recycling�of�continental�crust�has�been�an� important� process� throughout� the� history� of� the� terrane.�Furthermore,�although�there�are�variations�in�the�absolute�La�and�Sm�concentrations�between�arc�and�non-arc�rocks,�there�is�no�sig-nificant separation in terms of La/Sm

UCN� ratios� between� the� two�

groups,�suggesting�that�both�arc�and�non-arc�felsic�rocks�were�derived�from�predominantly�recycled�upper�continental�crustal�material.�

This�geochemical�data�for�felsic�rocks,�unfortunately,�can�only�point to an UCC-like source and cannot specifically delineate what the�source�of�that�crust�was.�In�particular,�it�cannot�delineate�whether�this�crust�was�once�part�of�the�North�American�craton,�or�an�exotic�crustal�fragment�with�UCC�geochemical�attributes.�The�similarity�in� stratigraphic� and�geochemical� attributes�between� rocks�of� the�Yukon-Tanana�Upland�and�parts�of�the�Alaska�Range�and�those�of�Selwyn�basin�in�Yukon�does�suggest�that�the�basement�to�YTT�may�have�once�been�part�of�the�North�American�miogeocline.�Other�as-pects�of�YTT�also�share�similarities�with�the�North�American�mio-geocline including: (1) Nd isotopic attributes for YTT felsic and sedimentary� rocks� (Mortensen,� 1992a;� Stevens� et al.,� 1995;�Boghossian�et al.,�1996;�Grant,�1997;�Creaser�et al.,�1997;�Garzione�et al.,�1997;�Patchett�et al.,�1999;�Piercey�et al.,�2003);�(2)�detrital�zircon�geochronology�(Gehrels�et al.,�1995);�(3)�Pb�isotopic�composi-tion�of�syngenetic�sulphide�deposits�(Nelson�et al.,�2002;�Mortensen�et al.,�this�volume);�and�(4)�broadly�similar�Devonian-Mississippian�geodynamic�history�(Paradis�et al.,�1998;�Nelson�et al.,�2002,�this�volume).�Collectively�these�features�point�to�a�likely�link�between�the�North�American�miogeocline�and�YTT�crust,�at�least�prior�to�Devonian-Mississippian back-arc initiation along the ancient Pacific continental�margin�(see�also�Nelson�et al.,�this�volume).

Importance of Recycled Oceanic CrustIt�is�well�established�that�basaltic�rocks�can�provide�probes�to�their�past�mantle�history,�their�mantle�sources,�and�the�relative�importance�of oceanic and continental crustal recycling in their genesis (Zindler and�Hart,�1986).�The�abundance�of�magmatic�rocks�with�incompat-ible�element-enriched�OIB�signatures�throughout�the�evolution�of�YTT�points�to�the�potential�for�a�recycled�oceanic�crustal�component�in�YTT�magmas.

It has been shown that mafic rocks erupted in ocean islands and other�large�igneous�province�(LIP)�environments�commonly�exhibit�geochemical�and�radiogenic�isotopic�evidence�for�past�oceanic�and�continental crustal recycling (White and Hofmann, 1982; Zindler and�Hart,�1986;�Hart�et al.,�1992;�Hofmann,�1997,�and�references�therein).�Of�particular�interest�to�this�paper�is�the�presence�of�excess�Nb�and�Ta�relative�to�Th,�La�and�other�LREE/LFSE�compared�to�primitive�mantle�values�(i.e.,�positive�Nb�and�Ta�anomalies),�features�interpreted�to�indicate�a�recycled�oceanic�crustal�component�in�these�magmas�(e.g.,�McDonough,�1991;�Niu�and�Batiza,�1997;�Niu�et al.,�1999).

The�origin�of�positive�Nb�and�Ta�enrichments�is�attributed�to�the�recycling�of�oceanic�crust�in�subduction�zones�and�its�re-incor-poration�into�the�mantle�wedge�via�convective�stirring�into�the�upper�mantle� (e.g.,� Allègre� and� Turcotte,� 1986;� Meibom� and� Anderson,�2003),� and/or� by� incorporation� into� mantle� plumes� (e.g.,� Weaver,�1991;�McDonough,�1991;�Hart�et al.,�1992;�Stein�and�Hofmann,�1994;�Hofmann,�1997;�Niu�and�Batiza,�1997;�Niu�et al.,�1999;�Condie,�1998,�2000).�As�the�subducted�slab�descends�into�the�subduction�zone,�it�undergoes�dehydration�and�transport�of�Th,�U,�LFSE�and�in�some�cases�LREE,�to�the�overlying�mantle�wedge,�leading�to�the�charac-teristic�“arc”�signature� in�subduction� related�basalts� (e.g.,�Pearce,�1983;�You�et al.,�1996;�Jenner,�1996;�Swinden�et al.,�1997;�Johnson�

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(e)

(b)(a)

(c) (d)

(f)

0 1 2 3 4 50

1

2

3

4

5

LaU

CN

SmUCN

Cycle I

La/SmUCN > 1Enriched

La/SmUCN < 1Depleted

Non-Arc

0 1 2 3 4 50

1

2

3

4

5

LaU

CN

SmUCN

Cycle II



Non-Arc

Arc

0 1 2 3 4 50

1

2

3

4

5

LaU

CN

SmUCN

Cycle III



Non-Arc

Arc

0 1 2 3 4 50

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2

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LaU

CN

SmUCN

Cycle IV



Arc

0 1 2 3 4 50

1

2

3

4

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LaU

CN

SmUCN

Cycle V



Arc

0 1 2 3 4 50

1

2

3

4

5

LaU

CN

SmUCN

Cycle VI



Arc

Figure 27. LaUCN

-SmUCN

plots for YTT felsic rocks. Cycles I through VI represented in (A) through (F). Details of the diagram provided in the text. Solid (filled) symbols in each plot represent non-arc rocks and open (unfilled) symbols represent arc rocks. Normalization values from McLennan (2001).

� 315


B)

C) D)

E) F)

A) Enriched Mantle(Recycled Oceanic Crust)

Slab Inputor

CrustalContamination

Cycle VIEnriched Mantle

(Recycled Oceanic Crust)

Slab Inputor


OIB

E-MORB

N-MORBNon-Arc

0 1 2 30

1

2

3

Nb/

Thm

n

Nb/Lamn

Cycle III

Enriched Mantle(Recycled Oceanic Crust)Slab Input

orCrustal

Contamination

OIB

E-MORB

N-MORBNon-Arc

Arc

0 1 2 30

1

2

3

Nb/

Thm

n

Nb/Lamn

Cycle IIEnriched Mantle


Slab Inputor


OIB

E-MORB

N-MORBNon-ArcArc

Cycle I

0 1 2 30

1

2

3

Nb/

Thm

n

Nb/Lamn

Cycle IVEnriched Mantle


Slab Inputor


OIB

E-MORB

N-MORBNon-Arc

Arc

0 1 2 30

1

2

3

Nb/

Thm

n

Nb/Lamn

Cycle VEnriched Mantle


Slab Inputor


OIB

E-MORB

N-MORBNon-Arc

Arc Arc

0 1 2 30

1

2

3

Nb/

Thm

n

Nb/Lamn

OIB

E-MORB

N-MORBNon-Arc

0 1 2 30

1

2

3

Nb/

Lam

n

Nb/Thmn

Figure 28. Nb/Thmn

-Nb/Lamn

diagram for YTT mafic rocks. Symbols as in Figure 22. Cycles I through VI represented in (A) through (F). Details of the diagram provided in the text. Diagram constructed from the concept of Niu et�al. (1999) (e.g., Ta/U

pm-Nb/Th

pm diagram).

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Piercey et al.

and�Plank,�1999).�In�contrast,�the�stability�of�phases�such�as�rutile�during�eclogitization� results� in� the� retention�of�HFSE� in� the� sub-ducted� slab� (in� particular� Nb,� Ta� and� Ti;� Saunders� et al.,� 1988;�McDonough,�1991;�Rudnick�et al.,�2000;�Foley�et al.,�2000).�The�preferential�retention�of�Nb,�Ti�and�Ta�in�the�slab�results�in�arc�basalts�with�characteristic�Nb�depletions�relative�to�Th�and�La�(and�other�LFSE� and� LREE),� and� primitive� mantle� normalized� Nb/La� and�Nb/Th�ratios�of�less�than�one�(Nb/Th

mn,�Nb/La

mn�<�1;�Pearce�and�

Peate,�1995).�The�subducted�slab,�in�contrast,�has�antithetical�Nb/La�and�Nb/Th�ratios�relative�to�arc�basalts,�with�primitive�mantle�nor-malized�values�greater�than�1�(Nb/Th

mn,�Nb/La

mn�>1).�It�has�been�

argued�that�the�Nb/Thmn

�and�Nb/Lamn

�>1�in�OIB�and�E-MORB�imply�a�recycled�oceanic�crustal�component�in�the�source�of�these�basaltic�rocks�(e.g.,�Saunders�et al.,�1988;�McDonough,�1991;�Niu�and�Batiza,�1997;�Niu�et al.,�1999;�Rudnick�et al.,�2000).�

On Figure 28, the data for YTT mafic rocks are shown on a Nb/Th

mn�vs.�Nb/La

mn�diagram. This diagram divides mafic rocks

derived�from�magmas�of�arc�parentage,�or�that�have�been�contami-nated�by�continental�crust�(Nb/Th

mn�and�Nb/La

mn�<1),�from�rocks�

that�come�from�enriched�sources�with�an�inherited�oceanic�crustal�component�(Nb/Th

mn�and�Nb/La

mn�>1;�Fig.�28).�Arc�and�crustally�

contaminated rocks invariably lie within the arc/contamination field (Nb/Th

mn�and�Nb/La

mn<1;�Fig.�28).�Non-arc�rocks�are�more�variable.�

Those�with�a�weak�subduction�signature�(e.g.,�BABB)�or� that�are�crustally contaminated typically plot in the arc/contamination field (Nb/Th

mn�and�Nb/La

mn<1; Fig. 28). Mafic rocks with N-MORB ±

E-MORB geochemical signatures lie near the junction between Nb/Th

mn�<�1�and�Nb/La

mn�>�1,�and�those�with�OIB�±�E-MORB�sig-

natures fall within the enriched field (Fig. 28). These�types�of�enriched�rocks�with�oceanic�crustal�components�

are�commonly�associated�with�large�igneous�provinces�(LIP).�The�occurrence�of�enriched�rocks�in�all�cycles�of�YTT�magmatism�sug-gests the influence of a recycled oceanic crustal component, but may also�highlight�the�possibility�that�these�rocks�represent�formation�within�a�LIP.�If�this�is�the�case,�then�this�LIP�must�have�existed�dur-ing� the�entire�mid-� to� late�Paleozoic�magmatic�evolution�of�YTT.�This�is�highly�unlikely,�as�it�would�require�that�the�LIP�persisted�for�more than 150 m.y. and influenced an area in excess of 250,000 km2�

in�the�northern�Cordillera.�Furthermore,�the�geological�events�that�dominated�YTT�history�such�as�arcs,�arc�rifts,� intra-arc�compres-sional�events,�and�extensional�back-arc�basin�activity�(e.g.,�Nelson�et al.,�this�volume),�coupled�with�the�lack�of�evidence�for�voluminous,�high-temperature�basaltic�magmatism,�are�inconsistent�with�a�LIP�geodynamic�environment�such�as�an�ocean�island�or�oceanic�plateau�(e.g.,�Hawaii,�Ontong-Java).

Although�formation�within�a�LIP�can�be�ruled�out�for�the�mid-�to�late Paleozoic evolution of YTT, the presence of mafic rocks with signatures�similar�to�rocks�from�such�environments�highlight�the�potential�role�that�a�LIP�may�have�had�in�its�ancestry.�In�particular,�the�recycled�oceanic�lithospheric�signatures�in�enriched�YTT�basalts�could�be�explained�by�inherited�LIP�melts�residing�as�veins�within�a�subcontinental�lithospheric�mantle�domain�in�the�terrane.�For�ex-ample,�Stein�and�Hofmann�(1992,�1994)�argued�that�in�the�Arabian-Nubian�shield,�past�LIP-related�material�(frozen�plume�heads)�froze�

against�and/or�was�incorporated�into�the�lithospheric�mantle.�With�subsequent�rifting�and�melting�of�the�lithospheric�mantle,�these�an-cient�LIP�components�were�recycled�into�younger�basalts�with�sig-natures�akin�to�the�preexisting�LIP�(Stein�and�Hofmann,�1992,�1994).�Pre-Paleozoic� fossilized�LIP�material� in� the�YTT� subcontinental�lithospheric�mantle�could�also�explain�the�recycled�oceanic�crustal�component in YTT enriched mafic magmas. An interesting conse-quence�of�this�model�is�the�question�of�the�provenance�of�the�LIP�magmatism (plume?) that fertilized the YTT lithospheric mantle.

Figure 29. The occurrence of OIB-like signatures in YTT rocks over 150 m.y. points to a potential plume component in their genesis. Enriched rocks associated with the Gunbarrel and Franklin large igneous provinces (LIP) resulted in widespread magmatism and igneous activity along the northwestern edge of Laurentia in Neoproterozoic time (~780-720 Ma; Heaman et�al., 1992; Ernst and Buchan, 2001; Harlan et�al., 2003). During this event, partial melts from the Franklin plume fertilized the subcontinental lithospheric mantle of northwestern Laurentia resulting in a veined lithospheric mantle, which retained the geochemical signature of plume-derived rocks (i.e., OIB to E-MORB). These veins were later liberated during partial melting related to YTT arc and back-arc magmatism, which developed on top of western Laurentian continental margin frag-ments during the middle to late Paleozoic. SCLM = Subcontinental lithospheric mantle.

Continental CrustContinental Crust

SCLM

Franklin-GunbarrelLIP Event

Low Degree Melts Fertilization of

Lithosphere

Neoproterozoic Breakup of Rodinia(~780-720 Ma)

CCCC

SCLM

Subduction and Arc-Rifting / Back-Arc Basin Magmatism

Mid- to Late-Paleozoic Convergent Margin Magmatism.

Recycling theAncient LIP Component

A)

B)

� 317


The ancient Pacific margin of North America was the locus of a� LIP� in� the� Late� Proterozoic.� In� particular,� two� Neoproterozoic�events,�the�Gunbarrel�(~780�Ma;�Harlan�et al.,�2003)�and�Franklin�igneous�events� (~720�Ma;�Heaman�et al.,� 1992;�Park�et al.,� 1995;�Dupuy�et al.,�1995;�Dudas�and�Lustwerk,�1997),�resulted�in�more�than�1,250,000�km2 of mafic magmatism extending from Wyoming, along�the�cratonic�margin�of�western�North�America,�through�Arctic�Canada�and�into�Greenland�(Ernst�and�Buchan,�2001).�These�events�resulted in widespread mafic dike swarms, mafic sills and flood basalt extrusions�over�this�extended�region�and�collectively�have�been�in-terpreted�to�be�a�product�of�the�Neoproterozoic�breakup�of�Rodinia�along�the�western�margin�of�Laurentia�(Fig.�29;�Heaman�et al.,�1992;�Park�et al.,�1995;�Dudas�and�Lustwerk,�1997;�Harlan�et al.,�2003). It�is�possible�that�the�widespread�magmatic�activity�associated�with�these�LIP�events�was�responsible�for�the�fertilization�of�the�subcon-tinental�mantle�of�the�northern�Cordillera,�resulting�in�a�lithospheric�mantle�with�veins�of�Nb-enriched�material�within�it,�which�upon�later�reactivation�during�continental�rifting�or�arc�rifting�resulted�in�the�OIB-like�signatures�observed�in�both�the�miogeocline�and�YTT�(e.g.,�Goodfellow�et al.,�1995;�Dusel-Bacon�and�Cooper,�1999;�Piercey�et al.,�2002a;�Nelson�and�Friedman,�2004).�Although�equivocal,�the�presence�of�Neoproterozoic�depleted�mantle�model�ages�in�uncon-taminated�alkalic�basalts�supports�the�hypothesis�that�these�rocks�had� their� initial� origins� as� part� of� the� LIP� associated� with� the�Neoproterozoic�breakup�of�Rodinia�(e.g.,�Piercey�et al.,�2002a,�2004;�S.J.�Piercey�and�R.A.�Creaser,�unpublished�data).

SUMMARY AND CONCLUSIONSYukon-Tanana� terrane�has�had� a�varied�petrological�history�with�complex�interactions�between�crust,�mantle�and�subducted�slab;�ul-timately�these�petrological�variations�can�be�tied�to�the�geodynamic�evolution�of�the�terrane.�The�main�conclusions�of�this�paper�can�be�outlined as follows:(1)� YTT� comprises� six� cycles� of� magmatic� activity� prior� to� its�

Mesozoic accretion to the North American craton. The first five cycles� occurred� above� an� east-dipping� subduction� zone� that�developed�along�the�distal�edge�of�the�North�American�craton,�whereas�the�sixth�(Klondike)�magmatic�cycle�took�place�above�a�west-dipping�subduction�zone.�

(2)� Felsic�magmatic�rocks�associated�with�the�extending�continental�margin�in�the�Alaska�Range�and�Yukon-Tanana�Upland�(Cycle�I)�are�dominantly�crustally-derived�peralkalic�rocks�with�lesser�crustally-derived�subalkalic� rocks;� they�are�accompanied�by�mafic rocks of intraplate affinity.

(3)� During�cycles�II�through�V,�magmatism�was�largely�bimodal�in nature, with mafic and felsic end-members. Mafic rocks with arc� signatures�are�predominantly�calc-alkalic�and� island-arc�tholeiitic,�with�lesser�LREE-enriched�island�arc�tholeiites�and�boninites. In corresponding back-arc environments, mafic rocks are�dominated�by�normal�mid-ocean� ridge�basalts,� enriched�mid-ocean� ridge� basalts� and� ocean� island� basalt� signatures.�Ocean� island�basalt-like� rocks� are� also� present� in� many� arc-dominated�successions,�and�are�interpreted�to�record�intra-arc�rift�events�within�YTT.�Felsic�rocks�in�arc�environments�are�

dominated by calc-alkalic affinities with lesser tholeiitic rocks; back-arc�rocks�are�characterized�by�HFSE-�and�REE-enriched�(A-type) affinities.

(4) Mafic rocks from both arc and non-arc environments in YTT originate�from�variably�enriched�mantle�domains,�ranging�from�ultra-depleted� (boninites)� to� enriched� (OIB).�There� is� a� con-tinuum�of�compositions�between�these�end-members,�but�with�arc� rocks� tending� toward� more� depleted� compositions,� and�non-arc� rocks� tending� toward� more� enriched� compositions.�Rocks�with�intermediate�signatures�can�be�explained�by�mixing�between�the�enriched�and�depleted�end-members�in�the�mantle�source�region.�There�are�no�inter-cycle�variations�in�the�com-position of the YTT mafic rocks, implying that both the underly-ing�mantle�and�the�processes�of�magma�generation�remained�similar�throughout�its�mid-�to�late�Paleozoic�evolution.

(5)� Felsic�rocks�from�the�YTT,�although�varying�in�absolute�HFSE�and�REE�contents,�are�predominantly�derived�from�recycled�upper�continental�crust�(UCC).�Most�felsic�rocks�have�LREE-enrichment and relatively flat REE patterns relative to UCC (La/Sm

UCN ≈1). This, coupled with published Nd, Sr and Pb

isotopic�data,�and�the�common�occurrence�of�inherited�zircon,�all�imply�derivation�from�crustal�protoliths.�This�UCC�protolith�was�similar�to�the�North�American�craton;�however,�more�data�is required to establish definitively whether the basement of YTT�was�the�North�American�craton.�

(6) Many mafic rocks from YTT intra-arc rifts and back-arc basins have�Nb/Th

mn�and�Nb/La

mn�>�1,�implying�excess�Nb�relative�to�

Th�and�La�relative�to�the�primitive�mantle.�Excess�Nb�relative�to�Th�and�La�in�the�primitive�mantle�in�these�rocks�implies�a�recycled�oceanic�crustal�component�in�their�genesis.�Recycled�oceanic� crust� is� a� feature� common� in� rocks� associated�with�many�large�igneous�provinces�(LIP);�however,�since�this�com-ponent�exists�in�all�cycles�of�YTT�magmatism,�it�is�highly�un-likely�that�these�rocks�represent�a�LIP,�because�YTT�magmatism�spans� more� than� 150� m.y.� and� affects� an� area� in� excess� of�250,000�km2�over�this�time�frame.�This�signature�in�YTT�alkalic�basalts likely reflects the reactivation of recycled LIP compo-nents�resident�in�the�lithospheric�mantle�that�were�originally�derived�via�lithospheric�fertilization�by�LIP�magmatism�during�the�Neoproterozoic�breakup�of�Rodinia.�

ACKNOWLEDGEMENTSSteve� Piercey� has� been� supported� by� Natural� Sciences� and�Engineering�Research�Council�(NSERC)�post-graduate�scholarships,�the� Yukon� Geological� Survey� (YGS),� the� Geological� Survey� of�Canada�(GSC),�a�Geological�Society�of�America�student�research�grant,� the�Society�of�Economic�Geologists�Hickok-Radford�Fund,�Atna,�Expatriate�Resources�and�an�NSERC�Discovery�Grant.�Mitch�Mihalynuk� is� thanked� for� providing� unpublished� data� from� the�Big�Salmon�Complex.�Funding�for� the�analytical�work�presented�herein�was�provided�by�the�GSC�(operating�funds�and�NATMAP�projects), U.S. Geological Survey, YGS and NSERC. We thank our colleagues� for� invigorating� and� stimulating� discussions� over� the�years and continued collaboration, in particular: Don Murphy,

318

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Jim�Mortensen,�Jim�Ryan,�Steve�Gordey�and�Rob�Creaser.�Reviews�by�Dwight�Bradley,�Edward�DuBray�and�Brendan�Murphy�are�grate-fully�acknowledged.�

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