thematic article age and petrogenesis of plagiogranite ... · metamorphic mélange unit is composed...

Island Arc

(2006)

15,

44–57

© 2006 The AuthorsJournal compilation © 2006 Blackwell Publishing Asia Pty Ltd

doi:10.1111/j.1440-1738.2006.00522.x

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1038-48712006 Blackwell Publishing Asia Pty LtdMarch 20061514457Thematic Article

Plagiogranites of the Ankara mélangeY. Dilek and P. Thy

*Correspondence.

Received 18 November 2005; accepted for publication 13 December 2005.

Thematic Article

Age and petrogenesis of plagiogranite intrusions in the Ankara mélange, central Turkey

Y

ILDIRIM

D

ILEK

1,

*

AND

P

ETER

T

HY

2

1

Department of Geology, 116 Shideler Hall, Miami University, Oxford, OH 45056, USA (email: [email protected])

and

2

Department of Geology, One Shields Avenue, University of California, Davis, CA 95616, USA

Abstract

The Ankara mélange within the Izmir–Ankara–Erzincan suture zone in north-central Turkey includes ophiolitic fragments that represent the remnants of an oceanicbasement evolved between the Sakarya and Kirsehir continental blocks in the early Meso-zoic. The serpentinized upper mantle peridotites and lower crustal rocks in these ophiolitesare cross-cut by dolerite and plagiogranite dykes, which show mutually intrusive relationsindicating their synchronous emplacement into the pre-existing oceanic lithosphere. Zircondating (U–Pb) of a plagiogranite dyke has revealed a concordia age of

∼

179

±

15 Ma thatis interpreted here as the crystallization age of this differentiated rock. A fourth fractionof the zircon separates from this rock has also shown an inherited component greater than1.7 Ga, possibly derived from the Precambrian core of the Rhodope–Strandja Metamor-phic Massif in the Balkan Peninsula. Models for plagiogranite formation were tested andit is concluded that a high extent (

<

70%) of anhydrous or water-undersaturated, earlyamphibole-free fractionation of a basaltic melt source may have readily produced theobserved REE concentrations for the Ankara mélange plagiogranites. The trace elementabundances and other geochemical features of the coeval dolerite dykes are similar to thoseof the plagiogranites, suggesting a common melt source. The Ta–Nb patterns shown byboth the plagiogranite and dolerite dykes are typical of arc-related petrogenesis and canbe explained by the addition of slab-derived components to a depleted mantle wedge. TheEarly Jurassic ophiolitic basement and the dyke intrusions were formed in a back-arcsetting between the Paleo- and Neo-Tethyan domains in the eastern Mediterranean region.The Izmir–Ankara–Erzincan Sea developed in this back-arc environment and the relatedsuture zone had a diachronous evolutionary history.

Key words:

Ankara mélange, Early Jurassic ophiolites, eastern Mediterranean, fractionalcrystallization, Izmir–Ankara–Erzincan suture zone, plagiogranite intrusions, Tethys,Turkey.

INTRODUCTION

The Ankara mélange in northern Turkey is part ofa Mesozoic oceanic tract separating a series ofGondwana-derived continental fragments to thesouth (i.e. Anatolide–Tauride Belt, Kirsehir block)

from the ancient continental margins of Laurasia(Sakarya continent, Pontide Belt) to the north(Fig. 1). It is situated within the Izmir–Ankara–Erzincan suture zone (IAESZ), which is in generalconsidered to be a major tectonic boundarybetween the Pontide and Anatolide–Tauride Beltsof Turkey (Dilek & Moores 1990; Okay & Tüysüz1999). The Ankara mélange has been interpretedas a typical convergent margin mélange thatevolved at a south-facing active margin of Laur-asia; it has played a major role in the development

Plagiogranites of the Ankara mélange

45


of ideas and models about mélange formation(Bailey & McCallien 1950, 1953; Sengör 2003; andreferences therein).

In its type locality west of the Kirsehir block,the Ankara mélange consists of three distinct map-pable mélange units, including, from northwest tosoutheast, a metamorphic mélange, a limestone-block mélange and an ophiolitic mélange (Fig. 2).These different mélange units are imbricatedalong ESE-vergent thrust sheets, and the ophi-olitic mélange constitutes the structurally lowesttectonic unit within the Ankara mélange. Themetamorphic mélange unit is composed of a mix-ture of variably metamorphosed sedimentary andmafic–ultramafic rocks in a phyllitic-graywackematrix that corresponds to the Upper Karakayaunit of the Sakarya continent (Koçyigit 1991;Floyd 1993; Tüysüz

et al

. 1995). This upper green-schist facies metamorphic mélange unit may be thetectonic equivalent of the Permian–Triassic(?)Nilüfer unit of Okay

et al

. (1996) located fartherwest near the Marmara Sea; the Nilüfer unit hasbeen interpreted as an arc–forearc assemblagethat was incorporated into the Sakarya continent

during the closure of the Paleo-Tethys in the LateTriassic (Okay

et al

. 1996). The limestone-blockmélange that rests tectonically on the ophioliticmélange unit is composed of Permo-Triassic neriticlimestone blocks together with blocks and clasts ofconglomerate, agglomerate, dolerite and flysch ina matrix composed of shale and volcaniclasticrocks (Norman 1984; Koçyigit 1991; Tüysüz

et al

.1995; Tankut

et al

. 1998). The matrix material inthis limestone-block mélange may have deposi-tional ages of Lias through to Cenomanian (Nor-man 1984), and the unit itself may be the tectonicequivalent of the ‘Jurassic and younger coverrocks’ (Okay

et al

. 1996) that were deposited on therecently accreted arc–forearc assemblage (Nilüferunit) of the Sakarya continent. The ophioliticmélange contains kilometer-size coherent blocksof ophiolitic material composed of serpentinizedupper mantle peridotites, gabbros, doleritic dykes,pillowed to massive lava flows, radiolarian chert,and blocks and clasts of volcanic rocks, sandstoneand limestone in a mainly serpentinite-madematrix (Akyürek

et al

. 1979; Norman 1984; Tankut

et al

. 1998; Ogawa 2002).

Fig. 1

Simplified tectonic map of the eastern Mediterranean region showing the location of the Ankara mélange, suture zones, Tethyan ophiolites andplate boundaries. AC, Antalya complex; AO, Aladag ophiolite; BHN, Beysehir–Hoyran nappes; IAESZ, Izmir–Ankara–Erzincan suture zone; IPSZ, Intra-Pontide suture zone; MO, Mersin ophiolite; PO, Pindos ophiolite; SC, Sakarya continent; VO, Vourinos ophiolite; VZO, Vardar zone ophiolites (modifiedfrom Dilek & Flower 2003; additional data from Okay & Tüysüz 1999).

0 100

km

v

v

vv

v

vv

vv

vv

v

v

v

vv

vv

v

vv

vv v v v v v v

v

v

vv v v v

v

vv

v

vv

v

vv v v

v

v v

v

vv

v

v

Balkanides

BLACK SEA

ARABIA

MEDITERRANEAN SEA

AdriaticSea

DiraridesMoesianPlatform

RhodopeMassif

AegeanSea

Hellenides

MirditaOph.

Rhodes

Troodos oph.

Gulemanoph.

Baskil arcIspendere-

Kömürhan oph.

Kizildagoph.

De

ad

Se

a F

au

lt

East Anatolian Fault

North Anatolian Fault

Central

Pontides

Hellenic Trench

Tauride Platform

Çangaldagarc

Küre oph.

Lycian Nappes

•

•

• •

Istanbul

Izmir

Crete

Ankara

Mélange

Munzur

POVO

IAESZ

BHN

AC MO

AO

IPSZ

20ºE 24ºE 28ºE 32ºE 36ºE 40º E44ºN

42ºN

40ºN

36ºN

34ºN

EURASIA

Cyprus Trench

EasternPontides

IAESZ

Tauride PlatformKIRSEHIR

BLOCKInner Tauride

Anatolides

AEGEAN

SEA

PELAGO

NIA

Istanbul Zone

StrandjaMassif

SAKARYA CONTINENT

Ankara

Suture Zone

VZO

PannonianBasin

Erzincan

SC

46

Y. Dilek and P. Thy


Paleontological ages of limestone blocks in theophiolitic mélange range from Permian to Cenom-anian (Gökçen 1977). Recent studies of the pelagiclimestone intercalations within the pillow lavas,and of radiolaria-bearing limestone blocks withinthe mélange have revealed biostratigraphic agesranging from late Norian to Late Albian–Turonian(Rojay

et al

. 2001; Bragin & Tekin 1996). Blocks ofalkaline basalts in the ophiolitic mélange representfragments of seamounts developed on an olderoceanic basement (Çapan & Floyd 1985; Floyd1993; Rojay

et al

. 2001; Tankut

et al

. 1998). Theophiolitic material within this mélange unit has

been interpreted to represent the remnants of anEarly Cretaceous oceanic lithosphere developed inthe northern branch of the Neo-Tethys (Çapan

et al

. 1983; Tankut & Gorton 1990; Koçyigit 1991;Tankut

et al

. 1998).Although the available biostratigraphic ages

from different units within the Ankara mélangerange from Permo-Triassic to the early Late Cre-taceous, the juxtaposition of the three mélangeunits and their tectonic imbrication is younger (lat-est Cretaceous), as indicated by the presence of aMaastrichtian flysch that unconformably overliesthe ophiolitic mélange (Norman 1984). This imbri-

Fig. 2

(A) Simplified geological mapof Turkey showing different tectonic beltsand continental blocks. ATB, Anatolide–Tauride Belt; EF, Ecemis Fault; IAESZ,Izmir–Ankara–Erzincan suture zone;ITSZ, Inner-Tauride suture zone; KB,Kirsehir continental block; RPCF,Rhodope–Pontide continental fragment;SB, Sivas Basin; SC, Sakarya continent.(B) Imbricate structure of the Ankaramélange with three different compo-nents, metamorphic block mélange,limestone block mélange and the ophi-olitic mélange (modified from Tankut

et al

. 1998). (C) Simplified geologicalmap of the Ankara mélange in the vicinityof Elmadag and Eldivan towns, east ofAnkara (modified from Senel 2002).Polygons 1 and 2 mark the study areawhere the samples came from.

33º 30’

40ºN

Kalecik

Elmadag

Ankara

Elmadag Edige

Klliclar

AE

GE

AN

SE

A

33ºE

OPHIOLITIC UNITS IN THE ANKARA MELANGE

Marine rocks (Upper Cretaceous)

Pelagic limestone

Extrusive rocks with sediment intercalations

Quartz diorite, trondhjemite

Gabbro, dolerite

Serpentinized peridotite

Marine rocks(Lower Cretaceous)

0 5 10 15 20

Km

0 10 20 30

Km

N

Ophiolitic mélange

Ultramafic massifs

Limestone block mélange

Metamorphic block mélange

OTHER MELANGE UNITS

Flysch (Upper Senonian)

Carbonate and clastic rocks(Permo-Triassic)

Metamorphic rocks(U. Paleozoic-Triassic)

•

•

•

•

•

•

•

• •

•

•

•

• •

•

Çankiri

Eldivan

Sabanözü

Eldivan

Haymana

BLACK SEAThrace

Izmir

Ankara Erzirncan

ATB

RPCF

IAESZ

KB

ITSZ

ARABIANPLATEMEDITERRANEAN SEA

ITSZ

Kalecik

2

1

A

BC

SC

SB

EF

IAESZ

NAFZ


47


cation likely took place during the closure of aTethyan ocean basin, although the age and thepaleogeography of this basin are not well estab-lished. The geological history of the individualmélange units and their tectonic affinity prior tothe formation of the Ankara mélange are poorlyknown, and therefore the geodynamic significanceof the Ankara mélange in the evolution of theIAESZ is not well understood. The igneous ageand the tectonic environment of formation of theancient oceanic crust preserved in the ophioliticmélange are of particular importance because thisinformation can help construct the geodynamicevolution of the ocean basin developed between theKirsehir and Sakarya continents.

In the present paper, the geochemistry and geo-chronology of plagiogranite dykes in the ophioliticmélange are reported. New data from these rocksprovide important new information on the igneousage and the tectonic affinity of the Tethyan oceaniccrust within the Ankara mélange and allowregional correlations with other Mesozoic paleo-oceanic domains in the eastern Mediterranean tobe made, in order to better constrain the geody-namic evolution of the IAESZ in space and time.

GEOLOGY OF THE ANKARA MÉLANGE AND PLAGIOGRANITE INTRUSIONS

The best exposures of the ophiolitic mélange cropout in a NNE-trending zone between the townsof Edige and Eldivan, east of Ankara (Fig. 2).Dismembered ophiolite complexes consisting ofserpentinized peridotites, cumulate to isotropicgabbros, quartz diorite and volcanic rocks occur inkilometer-size mega-blocks in a sheared andlocally brecciated serpentinite matrix or in a flyschunit (Fig. 3). In addition, large (locally kilometer-size) blocks and fragments of basaltic massive topillow lava flows, intercalated with turbiditic topelagic limestone, red argillite–mudstone, andradiolarian chert, occur in a tuffaceous to shalymatrix (Fig. 2). Large ophiolitic blocks locally dis-play the pseudostratigraphy of an incompletePenrose-type oceanic crust with missing sheeteddykes, extrusive rocks and sedimentary cover.Major coherent ophiolitic blocks are seen nearEdige, east of Kalecik, and between Eldivan andSabanözü towns (Fig. 2).

In these ophiolitic blocks, tectonized peridotitescomposed of serpentinized harzburgite and duniteare the most extensive rock unit. Dunite commonlyoccurs as irregular lenses and/or discontinuous

interlayers within the harzburgite and containsmeter-sized, pod-shaped chromitite lenses. Pyrox-enite dykes ranging in thickness from a fewcentimeters to a few meters are common in theharzburgite. The harzburgite–dunite peridotiteunit is overlain by ultramafic to mafic cumulaterocks composed of dunite, pyroxenite (clinopyrox-enite and websterite) and layered gabbro (Tankut& Sayin 1990; Tankut

et al

. 1998). Pyroxenite iscommonly found stratigraphically below the lay-ered gabbros. Layering in the cumulate gabbros isdefined by bands of olivine–pyroxene and plagio-clase. Stratigraphically upward in the gabbros,amphibole almost completely replaces the pyrox-ene. The mineral chemistry of these mafic–ultramafic rocks is discussed in Tankut

et al

. (1998).Doleritic and microgabbroic dykes, ranging in

thickness from 0.2 to 20.0 m, cross-cut the uppermantle peridotites and cumulates to isotropicgabbros at all structural levels and show well-developed, two-sided chilled margins. Some doler-ite dykes are locally brecciated along their chilledmargins and are hydrothermally altered. Thesedykes have northwest to northeast azimuths withsteep dips to the east. In places, they are accom-panied by plagiogranite dykes that are also intru-sive into the peridotites and gabbros and displaymutually intrusive relations with the doleriticdykes (Fig. 4). Pods and lenses of dolerite alsooccur within the plagiogranite dykes, and both thedolerite and plagiogranite dykes display locallywell-developed lineations of plagioclase and/oramphibole phenocrysts. Dolerite and plagiogran-ite dykes are transected by brittle faults and arelocally intensely broken and brecciated, formingsubelliptical bodies within the serpentinite. Thesespatial relations of the dolerite and plagiogranitedykes indicate that they were coeval in their intru-sive emplacement into the upper mantle and lowercrustal rocks in the ophiolitic assemblage, and thatthey were most likely derived from the samemagmas.

Essential mineral phases in doleritic dykesinclude clinopyroxene, plagioclase and aciculargreen amphibole (Tankut

et al

. 1998). Plagioclasecompositions range from bytownite An

90

−

80

toalbite An

10

−

20

. Extensive spilitization of the plagio-clase with An

<

20 is not uncommon in these rocks.The main alteration products are kaolinite fromthe feldspars, and chlorite, uralite, epidote andtremolite from the pyroxenes. The plagiogranitesare leucocratic rocks composed mainly of plagio-clase and quartz with minor amounts of amphi-bole and biotite. Hypidiomorphic granular to

48

Y. Dilek and P. Thy


Fig. 3

(A) Alkaline basalt lava blocks in a serpentinite matrix in the Ankara mélange, near Kiliçlar south of Kalecik; (B) dominantly serpentinizedharzburgite-derived ophiolitic fragment in the Ankara mélange east of Elmadag. Ophiolitic units are in places unconformably overlain by Miocene lacustrinedeposits (marl and evaporates).


49


granophyric textures are common, althoughamphibole locally shows a steeply plunging linea-tion giving the rock a mineral fabric. Plagioclase isgenerally subhedral and locally zoned, and quartzforms a granular mosaic texture enclosing feld-spar grains. Titanite and apatite are typical acces-sory phases, whereas chlorite and epidote-groupminerals occur as secondary phases replacingamphibole and biotite.

The minimum accretionary age of the ophioliticmélange has been established as the latest Creta-ceous because a Maastrichtian flysch unit uncon-formably rests on the ophiolitic rocks. The igneousage of the ophiolitic units is not known. However,radiolarian chert and biomicritic limestone spa-tially associated with the extrusive rocks withinthe ophiolitic mélange have yielded reliable bio-stratigraphic ages that can be used to date the lavaflows occurring as blocks in the tuffaceous–shalymatrix. Bragin and Tekin (1996) found upper

Norian, Lower Jurassic, Kimmeridgian–Titho-nian, Lower Cretaceous and Albian–Turonianradiolarian assemblages from the chert–mudstoneblocks in the mélange; they reported a lack of Mid-dle Jurassic radiolarians from the analyzed sam-ples. Rojay

et al

. (2001) documented the presenceof thin-shelled ‘

Protoglobigerina

’ and

Cadosina

associated with miliolids and epistominid foramin-ifers recovered from the biomicritic limestoneintercalations within the alkali basalt pillow lavas.These fossils collectively yield a time interval ofCallovian–Hauterivian for the age of the pelagiclimestone interlayers and the associated lavaflows. The geochemistry of the lavas suggests theirorigin as ocean-island alkali basalts of a seamount(Rojay

et al

. 2001). This inferred seamount wasclearly built on a pre-existing, older ocean floor.Thus, it appears that an igneous basement ofMiddle Jurassic and older age must have existedwithin the ocean basin in which the ophiolitic

Fig. 4

(A) Plagiogranite dyke intrusions within the doleritic dyke rocks; (B) mutual cross-cutting relations between plagiogranite and dolerite dykes.Notice the septas of dolerite dykes within the plagiogranite.

50

Y. Dilek and P. Thy


assemblages and their dyke intrusions in thepresent study had formed.

Zircons were separated from a plagiogranitedyke (97-AM-5) intrusion west of Eldivan for U–Pb isotopic analysis, conducted at the San DiegoState University. The isotopic data are given inTable 1. Three fractions of zircon separates give aconcordia age of approximately 179

±

15 Ma (lowerintercept at

∼

0), which is adopted here as the bestestimate of the crystallization age of the plagio-granite. A fourth fraction appears to have aninherited component possibly greater than 1.7 Ga.Thus, the ophiolitic basement into which the highlydifferentiated plagiogranite dykes were emplacedis likely to be Early Jurassic or older in age. Thisinterpretation is consistent with the older UpperTriassic–Lower Jurassic radiolarian ages availablefrom the ‘seamount’ rocks in the ophioliticmélange.

PETROGENESIS OF DYKE INTRUSIONS

The plagiogranite intrusions in the Ankaramélange (Table 2) are leucocratic tonalites charac-terized by low potassium (0.2–0.8 wt% K

2

O) andhigh sodium (4.0–5.6 wt% Na

2

O) contents. Theyare peraluminous and quartz normative and plotwithin the trondhjemite and albite granite field ofthe An–Ab–Or diagram (Barker 1979). The con-centrations of K, Ti and P, normalized to mid-oceanic ridge basalt (MORB), are stronglydepleted (Fig. 5A). The very low potassium con-tent has been suggested as a diagnostic feature ofoceanic and ophiolitic plagiogranites (Aumento1969; Coleman & Peterman 1975; Coleman &Donato 1979; Aldiss 1981; Pedersen & Malpas1984; Kanaris-Sotiriou & Gibb 1989; Jafri

et al

.1995). The rare earth elements (REE) show rela-tively low absolute concentrations and flat chon-drite normalized patterns (4–30

×

chondrite) withdistinct, but variable, Eu anomalies (Fig. 6A).

The spatially associated dolerite dykes are madeof evolved basalts with an MgO content between4.5 and 7.5 wt%. The variable silica content is likelyto be related to hydrothermal alteration and may

Table 1

U–Pb data for zircons from a plagiogranite dyke sample from the Ankara mélange

Sample

†

97-AM-5Concentration

‡

Isotopic compositions

§

Calculated Ages (Ma)

[U] [Pb] 208/206 207/206 204/206 206

¶

/238 207

¶

/235 207

¶

/206

¶

<

325 m 144.35 4.1262 0.18977 0.05093 0.00007 170.09 171.25 187.33200–325 m 114.86 3.5078 0.20915 0.06102 0.00077 175.06 175.67 183.89140–200 m 331.41 9.508 0.18366 0.05194 0.00015 171.16 172.08 184.61

>

140 m 32.253 1.0302 0.21158 0.05694 0.00037 184.59 190.54 264.83

†

Zircon analyses both by size separation and bulk fraction;

‡

total U and Pb concentrations in p.p.m.;

§

observed isotopic compositionscorrected for mass fractionation;

¶

ages calculated using the program by Ludwig (1993).

Table 2

Major (wt%) and trace element (p.p.m.) composi-tions of representative plagiogranite and associated doler-ite dykes

AM3 AM6 AM7 AME3

SiO

2

71.21 74.31 53.86 50.24TiO

2

0.20 0.31 0.66 1.71Al

2

O

3

16.29 12.44 16.03 15.95FeO 2.04 3.38 8.54 11.13MnO 0.02 0.03 0.16 0.19MgO 0.40 0.82 7.46 5.66CaO 2.74 2.58 7.47 9.70Na

2

O 5.56 5.50 3.92 4.00K

2

O 0.75 0.04 0.93 0.24P

2

O

5

0.03 0.06 0.06 0.18Total 99.24 99.47 99.09 99.00LOI 1.17 1.45 2.55 1.51Ni 7 7 37 56Cr 114 69V 13 19 270 365Sc 19 12 45 36Sr 228 30 201 159Rb 9 2 12 2Ba 89 2 71 104Zr 196 84 30 101La 5.41 4.60 1.41 8.14Ce 12.72 9.54 3.08 17.92Pr 1.90 1.19 0.50 2.47Nd 10.60 5.78 2.74 12.09Sm 4.36 2.14 1.21 4.08Eu 1.20 0.73 0.54 1.50Gd 6.27 2.95 1.92 5.34Tb 1.27 0.57 0.39 1.01Dy 8.72 3.83 2.69 6.62Ho 1.92 0.84 0.58 1.44Er 5.35 2.42 1.66 3.94Tm 0.80 0.35 0.24 0.58Yb 5.21 2.26 1.61 3.54Lu 0.86 0.35 0.26 0.55Th 1.30 1.28 0.21 0.89Nb 1.93 2.77 0.87 3.67Hf 6.30 2.73 0.88 2.77Ta 0.16 0.14 0.06 0.24

LOI, loss on ignition.


51


not be a primary characteristic feature. The traceelement abundance in the dolerite dykes showsmany features in common with the plagiogranites.The REE show the same relatively low concentra-tions (6–30

×

chondrite) and flat patterns (Fig. 6B)as the plagiogranites. In addition, distinct deple-tions in both Nb and Ta, typical for arc-relatedpetrogenesis, are observed for both the doleritesand the plagiogranites (cf. Tankut

et al

. 1998).The similarities of many of the trace element

features argue for genetic relationships betweenthese two groups of dyke rocks. It has been exper-imentally demonstrated that silicic melts can formfrom partial melting of metabasaltic and amphi-bolitic sources (Helz 1976; Sigurdsson 1977; Beard& Lofgren 1989; Thy

et al

. 1990; Rushmer 1991;Sen & Dunn 1994; Springer & Seck 1997; Koepke

et al

. 2004; Berndt

et al

. 2005). The actual compo-sition of these silicic melts is strongly dependenton the source and restite compositions as wellas on partial water pressures. Melts similar tooceanic plagiogranites have been produced at arange of water-saturated conditions, and partialmelting is often invoked to explain the formationof plagiogranites and tonalites (Arth 1979; Peder-sen & Malpas 1984; Flagler & Spray 1991; Peters

& Kamber 1994; Barnes

et al

. 1996; Koepke

et al

.2004; Berndt

et al

. 2005).In contrast, it has been demonstrated that pla-

giogranite magmas can also form from a highextent of fractional crystallization from a basalticsource under dry or water-undersaturated condi-tions (Pedersen & Malpas 1984; Peters & Kamber1994; Amri

et al

. 1996; Borsi

et al

. 1996; Floyd

et al

. 1998; Berndt

et al

. 2005). Berndt

et al

. (2005)have pointed out that even though the partial melt-ing of a MORB-type basaltic source was involvedin producing plagiogranite magmas, a substantialamount of fractional crystallization would still berequired to obtain the low contents of many minorelements (Fe, K, Ti) displayed by plagiogranites(cf. Floyd

et al

. 1998).These two main explanations for the formation

of plagiogranites were tested in the presentstudy. The first model, shown in Figure 7, con-siders the partial melting model, assuming ametabasaltic source and a melting reaction in thegeneral form of quartz

+

plagioclase

+

amphibole

=

clinopyroxene

+

melt. The resultant melts havea relatively steep REE pattern for reasonableextents of partial melting (10–20%). The presenceof garnet in the source (eclogitic) would further

Fig. 5

Selected trace elements normalized to normal mid-oceanic ridgebasalt (N-MORB; Sun & McDonough 1989) and arranged in order ofdecreasing compatibility toward the liquid phase.

Fig. 6

Rare earth elements normalized to CI chondrite (Sun & McDon-ough 1989).

52

Y. Dilek and P. Thy


steepen the REE pattern as a result of theretainment of the heavy REE in garnet. Suchsteep patterns are inconsistent with the flat REEpatterns seen for all the analyzed plagiogranites.An alternative gabbro source, as suggested byKoepke

et al

. (2004) for the Mid-Atlantic Ridgeplagiogranites, also fails to reproduce the REEpatterns of the Ankara mélange plagiogranites,and is not supported by the close spatial associa-tion between the dolerite and plagiogranite dykerocks.

A simplified fractional crystallization modelappears to have far greater success in duplicatingthe observed REE patterns (Fig. 7). Assuminganhydrous or water-undersaturated, earlyamphibole-free fractionation, the observed REEconcentrations can be reproduced at a relativelyhigh extent of fractionation (

<

70%). Significantamounts of amphibole fractionation would havethe effect of steepening the REE trends. Morerealistic late-stage modeling would require albite,quartz, amphibole, Fe–Ti oxide minerals and apa-tite fractionation (Pedersen & Malpas 1984), andmay modify the end results (cf. Fig. 2A), but wouldnot greatly redistribute the REE. Note, however,that the parental magma of the plagiogranites waslikely more depleted in REE than that of thedolerites.

However, the relatively low potassium contentsof many oceanic plagiogranites are difficult toattribute to fractional crystallization alone, forwhich a K

2

O content of approximately 3 wt% atF

=

0.7 can be predicted. This is a common prob-lem for genetic models for plagiogranites and hasbeen interpreted to result from hydrothermalleaching of potassium among other elements (Arth1979; Coleman & Donato 1979; Pitcher 1997). Thenegative correlation between the

δ

O

18

and

87

Sr/

86

Srratios and K have been attributed to interactionsbetween seawater and crystallizing magma, and toleaching of K during the late stages of solidification(Spooner

et al

. 1974; Arth

et al

. 1978; Arth 1979;Coleman & Donato 1979). Apparently, the REEare unaffected by leaching, whereas Sr, Rb and Baare affected (Coleman & Donato 1979).

The Ta–Nb anomalies shown by both the pla-giogranite and dolerite dykes in the Ankaramélange are diagnostic for arc-related petrogene-sis and can be attributed to the addition of slab-derived components to depleted mantle wedges(Davidson 1996; Thirlwall et al. 1996). This inter-pretation is consistent with the earlier geochemi-cal and tectonic models and suggest that theophiolitic rocks in the Ankara mélange likelyevolved in a supra-subduction zone environment(Tankut et al. 1998). Tankut et al. (1998) haveshown the existence of two geochemically differentgroups of doleritic dyke rocks intruding the Edigeand Kalecik Ultramafic Massifs in the mélange.One group of dykes has subalkaline characteristicsand displays incompatible element abundances ofnormal (N)-MORB or possibly enriched-MORBtype asthenospheric mantle source. The secondgroup shows typical island arc tholeiite signatureswith large-ion lithophile element (LILE) enrich-ment and high field strength element depletioncompared to N-MORB. They inferred that themagmas of the second dyke group might have beenderived from a depleted asthenospheric mantlesource similar to that of the first group but modi-fied by a hydrous component associated with sub-duction. The dolerite and plagiogranite dykes inthe present study are cogenetic with this secondgroup of dykes of Tankut et al. (1998).

TECTONOMAGMATIC ORIGIN OF THE OPHIOLITIC MÉLANGE

The occurrence of coeval dolerite and plagiogran-ite dyke intrusions in the ophiolitic componentof the Ankara mélange indicates the presence of

Fig. 7 Rare earth element modeling of plagiogranite (AM3) using dol-erite AM7 as the source. Models are shown for 10% and 20% batchpartial melting assuming a metabasaltic source (35% plagioclase, 3%quartz, 48% amphibole, 14% augite) and the melting reactionquartz + plagioclase + amphibole = clinopyroxene + melt (0.85 amph,0.41 plag, 0.04 qz, −0.30 cpx) (Rushmer 1991; Sen & Dunn 1994). Therestite at 20% melting is 38% amphibole, 34% plagioclase, 3% quartzand 25% augite. Fractional crystallization model is shown for 10%, 30%,50% and 70% crystallization using AM7 as the parent composition. Thecrystallization assemblage is simplified to 60% plagioclase and 40%pyroxene. Trace element distribution coefficients are after Martin (1987)and normalizing chondrite after Sun and McDonough (1989).

Plagiogranites of the Ankara mélange 53


a pre-existing oceanic basement between theSakarya and Kirsehir continental blocks by theEarly Jurassic. The geochemistry of the doleritedykes and the associated plagiogranite intrusionsin the serpentinized upper mantle peridotites andgabbros suggests a depleted asthenospheric man-tle wedge source modified by subduction-derivedhydrous components (cf. Tankut et al. 1998). Theoceanic lithosphere into which these dykes wereemplaced probably evolved in a supra-subductionzone setting between the Sakarya and Kirsehircontinental blocks.

Recent tectonic interpretations and reconstruc-tions of the Paleo- and Neo-Tethyan domains inthe eastern Mediterranean region suggest that byLate Triassic time, the Paleo-Tethyan ocean wasclosed as a result of a series of collisions along the

southern margin of Laurasia (Okay et al. 1996;Stampfli & Borel 2004). Several back-arc basins(i.e. Küre, Maliac) had evolved in the meantimeabove the retreating Paleo-Tethyan subductionzone as the Paleo-Tethys was consumed. A widerNeo-Tethyan ocean was evolving to the southbetween the Gondwana and Laurasia superconti-nents during this time. By 200 Ma, the Küre andMaliac back-arc basins were in the process of col-lapsing as their ocean floor started subductingsouthward, while the Neo-Tethyan ocean floor wasbeing consumed beneath Laurasia at a north-dipping subduction zone (Fig. 8). These subduc-tion events (and associated slab rollback pro-cesses) likely facilitated the opening of a back-arcbasin, the Izmir–Ankara Sea, that propagatedeastward (in the present coordinate system) sepa-

Fig. 8 Paleogeographic reconstruction of the Tethyan domains, Western Gondwana and Laurasia approximately 200 Ma (modified from Stampfli &Borel 2004). The green, stippled pattern defines incipient rift zones within Laurasia that eventually become embryonic seas and ocean basins with continuedrifting and sea floor spreading. The Meliata and Küre Basins represent the restricted back-arc basins of the diminished Paleo-Tethys. RSB defines theRhodope and Strandja Metamorphic Massifs mentioned in the paper. DO, Dobrogea Massif; EP, Eastern Pontides; GC, Greater Caucasus; IAO, Izmir–Ankara Sea; KB, Kirsehir continental block; SK, Sakarya continent; SL, Slavonia; TC, Trans-Caucasus; TZ, Tizia block.

DO

GC

TC

IAO

TZ

SLSK

EPRSB

L A U

R A

S I A

W. G

O N

D W

A N A

Africa

Arabia

IBERIA

Apulia

Tauride

PelagoniaPINDOS

MALIAC

MELIATA KURE¨

Sanandaj - Sirjan

NEO-TETHYS

Equator

N E

O - T E T H Y S

~ 200 Ma

60º

50º

40º

30º

20º

10º N

ER

KB

54 Y. Dilek and P. Thy


rating the Sakarya and Kirsehir continents(Fig. 8).

This Izmir–Ankara Sea can be envisaged as acontinental margin back-arc basin, analogous tothe evolution of the Late Jurassic–Early Creta-ceous Rocas Verdes Basin in southernmost SouthAmerica (Stern & De Wit 2003). The Rocas VerdesBasin opened by unzipping from the south to north,following the formation of a supra-subduction riftzone along the western edge of the South Americancontinent, and then evolved as an intracontinentalbasin. The upper crustal rocks of the Tortuga andSarmiento ophiolite complexes developed withinthis continental back-arc basin show tholeiitic dif-ferentiation trends similar to MORB, suggestingthat they may have formed from magmas derivedfrom LILE- and light (L)REE-depleted MORBmantle source. This observation is similar to thegeochemical evolution of the gabbros and the firstgroup of dyke rocks in the Edige and KalecikMassifs in the Ankara mélange, as shown byTankut et al. (1998). Hence, the initial evolution ofthe Izmir–Ankara Sea was similar to that ofthe Rocas Verdes Basin. The Permo-Triassic arc–forearc assemblages (Nilüfer unit) accreted to theSakarya continent in the Late Triassic may havebeen the volcanic arc of this Early Jurassic back-arc basin. The ophiolitic basement, now preservedas the lowest structural unit within the Ankaramélange, represents the remnant of the oceaniclithosphere developed in this Izmir–Ankara back-arc basin.

The westward extension of this Early Jurassicbasin formed the Vardar Sea east of Pelagonia,whereas the eastern segment extended into theLesser Caucasus developing the Erzincan zone ofthe larger Izmir–Ankara–Erzincan Seaway. Thiseast–west-trending ocean basin was terminallyclosed later in the Paleocene–Eocene during thediachronous collision(s) of the Sakarya–Pontide (tothe north) and the Anatolide–Tauride and Kirsehircontinental blocks (to the south) along the IAESZ(Okay & Tüysüz 1999; Dilek 2006). Thus, it can beinferred that the IAESZ is polygenetic in origin,and that the ophiolitic units along-strike withinthis suture zone may have different sources oforigin in different oceanic tracts, which appear tohave opened and closed diachronously between thebounding continental blocks.

The possible early stage evolution of the Izmir–Ankara Sea as a back-arc basin above a south-dipping subduction zone is supported by theinherited zircon age of ∼1.7 Ga from the plagiog-ranite dyke in the Ankara mélange that has been

dated in the present study. The only plausibleprovenance for the zircons of this Paleoproterozoicage is the Rhodope–Strandja Massif in northwestTurkey and southeast Bulgaria (Chen et al. 2002;Natal’in et al. 2005) that represents a polygeneticVariscan–Alpine orogenic belt built on a Precam-brian basement (Okay et al. 2001; Carrigan et al.2004; Gerdjikov 2005). Detrital material derivedfrom the exhumed Rhodope–Strandja Metamor-phic Massif was recycled into the lower mantle viaa south-dipping subduction zone within the Küreand Maliac Basins, adding to the heterogeneity ofthe depleted regular source for MORB beneaththe spreading center of the Izmir–Ankara back-arc basin.

Recycling of old zircon grains into the mantleand then back into the young oceanic crust form-ing at mid-ocean ridges has significant implica-tions for mantle contamination and heterogeneity.Pilot et al. (1998) have shown that the zircons theydiscovered in the gabbros recovered from the ODPLeg 153 cores drilled in the western wall of theMid-Atlantic Ridge near the Kane transform faultzone display ages of ∼330 my and 1600 my. Thisdiscovery demonstrates the significance of crustalcontamination in the evolution of melt beneathmid-ocean ridges and other oceanic spreading cen-ters. The presence of inherited Precambrian zir-cons (with ages of 1.70 Ga, 1.78 Ga and <2.10 Ga)in the plagiogranite dykes intruded into the lher-zolites and gabbros has also been shown from theJurassic Ligurian ophiolites in the northern Apen-nines (Borsi et al. 1996). These occurrences of Pre-cambrian zircons in much younger asthenosphericmelts of Phanerozic ages indicate that crustal con-tamination of the mantle via subduction recyclingis a widespread phenomenon and an importantmechanism for LREE enrichments of a MORB-type mantle source.

CONCLUSIONS

1. The ophiolitic component of the Ankaramélange within the IAESZ in north-centralTurkey represents the remnants of an oceanicbasement that evolved between the Sakaryaand Kirsehir continental blocks in the earlyMesozoic.

2. The serpentinized upper mantle peridotites andlower crustal rocks in these ophiolites are cross-cut by coeval dolerite and plagiogranite dykesthat display Ta–Nb patterns typical of arc-related petrogenetic evolution affected by the



addition of slab-derived components to adepleted mantle wedge.

3. Zircon dating (U–Pb) of a plagiogranite dykehas revealed a concordia age of ∼179 ± 15 Mafor the crystallization age of this differentiatedrock. A fourth fraction of the zircon separatesfrom this rock shows an inherited componentgreater than 1.7 Ga. This inherited componentlikely represents a detrital grain originatedfrom the Precambrian core of the Rhodope–Strandja Metamorphic Massif in the BalkanPeninsula. This discovery indicates the signifi-cance of the recycling of crustal material intothe mantle for the extent of crustal contamina-tion of the mantle via subduction processes dur-ing the evolution of Tethyan basins.

4. A high extent (<70%) of anhydrous or water-undersaturated, early amphibole-free fraction-ation of a basaltic melt source was most likelyresponsible for the formation of the magmasof the Ankara mélange plagiogranites and fortheir REE concentrations. The trace elementabundance and other geochemical features ofthe coeval dolerite dykes are similar to those ofthe plagiogranites and suggest a common meltsource.

5. The Early Jurassic ophiolitic basement and thedyke intrusions were formed in a back-arcsetting between the Paleo- and Neo-Tethyandomains in the eastern Mediterranean region.The Izmir–Ankara–Erzincan Seaway and therelated suture zone had a diachronous evolu-tionary history.

ACKNOWLEDGEMENTS

Our work in the Ankara mélange has been sup-ported by research grants from the NATO ScienceProgramme (CRG-970263) and The Scientific andTechnical Research Council of Turkey (TUBI-TAK), which we gratefully acknowledge. Thisstudy was part of a collaborative research projectinitiated by the late Ayla Tankut at the MiddleEast Technical University in Ankara who madesignificant contributions to our understanding ofthe geochemistry of ophiolitic rocks in the Ankaramélange; we have been saddened by her suddenpassing. We thank Barry Hanan at SDSU for zir-con dating of our plagiogranite sample. Construc-tive reviews by S. Altunkaynak, K. Hauer and Y.Ogawa have helped us improve the paper. Wethank Akira Ishiwatari for his editorial handling ofour paper.

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