j. petrology 1996 petford 1491 521

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JOURNA L OF PETROLOGY VOLUME37 NUMBER 6 BVGES 1491-1521 1996

NICK PETFORD1 and MICHAEL ATHERTON**'SCHOOL OF GEOLOGICAL SCIENCES, KINGSTON UNIVERSITY, KINGSTON UPON THAMES KT1 JEE, UK"DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF LIVERPOOL, JANE HERDMAN LABORATORIES, BROWNLOW STREET,LIVERPOOL L69 SBX, UK

Na-rich Partial Melts from NewlyUnderplated Basaltic Crust: the CordilleraBlanca Batholith, PeruThe late Miocene Cordillera Blanca Batholith liesdirectly overthick (50km) crust, inboardof the older Cretaceous CoastalBatholith. Its peraluminous S*ypmineralogyand its positionsuggest recyclingof continental c rust, which is commonlythought to be an increasingly important componentinmagmasinboardof continental margins. However, the peraluminous,apparent S type characterof the batholithis an artefact ofdeformation and uplift along a major crustal lineament.Thebatholith is a metaluminous I typeandthe dominant high-silica rocks (>70 ) areNa rich with many of the char-acteristicso fsubducted oceanicslab melts. However, the positionof the batholith andageof theoceaniccrust at thetrench duringthe Miocene preclude slab melting.Instead, partial melting ofnewly underplated Miocene crust is proposed Inthis dynamicmodel newly underplated basaltic materialismeltedto producehigh-Na, low HREE , high-Al Hrondhjemitic'type melts withresidues of garnet, clinopyroxene an d amphibole.Such Na-richmagmas arecharacteristicofthick Andean crust; they are sig-nificantly different from typical calc-alka line, tonalite-grano-diorite magmas, and theirpresencealong the spine of the Andesprovokes questions about models of trondhjemite genesisbymelting of subductedoceanic crust,aswellasany generalized,circum-Pacific model involving consistent isotopic or chemicalchanges inboard rom the trench.KEY WORDS batholith; modifiedT type granite; Na-rich magma;thick crust

INTRODUCTIONMcsozoicTertiary calc-alkaline batholiths con-stitutea major componentof the magmatism of theAndean margin. Although the parent magmas havea substantial mantle component (Miller &Harris,

1989), recycling of crust is to be expected by analogywith theHimalayas, theother active continen talmargin characterized by very thick crust.Inactivecontinental margins thereiscommonlyaswitchto'S ' type or crustally derived granites inboardofgranites with a mainly mantle signature (Pitcher,1983). Here we describe the Cordillera BlancaBatholithofPeru . Th is lies inboard of the CoastalBatholith (Fig. 1), directly over the massivelythickened keelofthe A ndes. Given the un ique tec-tonic positionofthe Cordillera B lancaas the finalcomponent in an easterly younging plutonicsequence intruded into thickened continental crust,recycling of old continental crust might be expected.Reconnaissance studies (Cobbing et al., 1981;Pitcher, 1983)concluded that the peraluminousrocks of thebatho lith were indeed 'S' type,i.e.derived from a sedimentary source (Chappell&White, 1974). This thinking was partly based on thebelief that the Precambrian crystalline Arequipabasement 'is an essential element in Andeanstructure' (Cobbing, 1985,p. 5) which floorsthewholeofthe Andean margin and was probablythesource of the batholith magmas. However,sub-sequent studies demonstrated that the peraluminousor 'S' type character is confined to rocksof thewestern deformed margin of the batholith anditwasconsideredbyAtherton &Sanderson (1987) to berelatedtohigh-level late-stage fluid interaction withthe aluminous country rocks into which the batholithwas intruded. New major and trace element[including rare earth element (REE)] and Sr, Ndand O isotope data areused to define the geo-chemical variation within the batholith, to assess the

C orraponding author. Oxford Univenity Preu 1996


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JOURNA L OF PETROLOGY V O L U M E37 N U M B E R 6 DECEMBER1996

78W8S-

10S-

Trujilld

N

LeucogranodioriteTonalite -Qu artz dionteYungay ignimbrite 30 km

Fig . 1. Simplified mapofmidnorthern Peru showing the Cordillera Blanca Batholith, boundedto thewestbythe C ordillera BlancaFault System an d lying within the Jurassic bu in fillofthe Chicama Form ation. The eastern boundaryof this basinismarkedby theMaranon Thruit and Fold Belt (MTFB), whichisalso the eastern limit of the west Peruvian Trough of Wilsonft Garayar (1967). Insetshows the distribution of leucogranodiorite and tonalite-quartz diorite, and recent U/Pb zircon (Mukasa, 1984) and "Ar/ 5Ar(Petford& Atherton, 1992) ages. The518and6-3 Ml ages are from leucogranodioritic rocks from the Llanganuco traverse, whereas the olderages, including the U/Pb zircon ageof 13'7 Ma, are from the early basic fades rocksinthe southern Carhuish area.

relative role of crust and mantle components,torelate fluid infiltration todeformation, todeterminethe relationship between crustal thickeningand theorigin of the batholith andtodetermine and explainthe compositional changes inboard fromtheCoastalBatholith.

GEOLOGICAL FRAMEWORKThe Cordillera Blanca Batholith issituated in thehigh Andes of northwestern Peru between 8 and10S. It is >200 km long but mayextend evenfurther as it plunges beneath the covertothe south

(Fig.l). Radiometric ages indicate a Miocene-Pliocene age of intrusion (Cobbingetai, 1981).The batholith was intruded into the ChicamaFormation, a 2000 m thick basinal sequenceofUpper Jurassic shales intercalated with siltstones,quartzite and volcanic rocks (Fig. 1). The rocks weredeformed anduplifted in the Eocene, mostof thedeformation being concentrated to theeast in theMaranon Fold andThrust Belt (MFTB, Megard,1989). During theMiocene there were three shortperiodsofcompressional activity [Quechua 1-3 ofMegard (1989) at ~20 Ma, 9 Ma and 6-3 Ma]separated by neutral or extensional periods.The

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PETFORDANDATHERTON CORDILLERA BLANCA BATHOLITH, PERU

Cordillera Blanca Batholith andcoeval ignimbrites(Yungay Formation) intruded andejected overtheperiod 143Marepresentthefinal magmatic eventsin central Peru associated with the extensionalperiods between the Quechua events (Petford &Atherton, 1992).

The western margin of thebatholith isboundedand strongly deformed by the Cordillera BlancaFault System (Fig. 1), aNNW-SSE-striking normalfault complex extending >200 kminlength whichisstill active (Megard &Philip, 1976; Sebrier et al.,1988).This fault systemis thedominant structureinthis part ofwestern Peru (Cobbing et al.,1981),representing a long-lived crustal lineament whichmarksthewestern marginofthe Jurassic basin,andlater, controlled magma ascent, andpost-emplace-ment uplift of the batholith owing to buoyancyforces at high crustal levels (Petford &Atherton,1992).

The geology of thearea wasfirst described bySteinmann (1910) but it was notuntil 1956thatEgeler&De Booy publishedthefirst geologicalmapof an area between 920'and940'S (aboutasixthofthe batholith) invery high difficult country. Morerecent reconnaissance studies have provided limitedfield, chemical and agedata (Wilson & Garayer,1967; Cobbing etal., 1981; Atherton &Sanderson,1987),and theMiocene andRecent tectonics havebeen describedbyFrench workers (e.g. Sebrieret al.,1988;Megard, 1989).

THE STRUCTURE OF THECRUST OF NORTH-CENTRALPERURecently, Fukaoetal.(1989) publishedtheresultsofa4year gravity survey acrossthePeruvian Andes,supporting theearlier seismological work ofJames(1971) which showed that thecrust thickens south-wards from 45-50km inNorthern Peruto50-60kmin Central Peru (Fig.2),reachingamaximumof70km beneath theAltiplano.Themajor thickeningisassociated with material ofdensity 3-0 g/cm (P-wave velocity of6-57-0km/s). Adetailed gravitysurvey at latitude 9S (Couch et al., 1981), alsoemphasizes thedeep crustal keel beneath the Cor-dillera and alowerandmiddle crust dominatedby3-0 g/cm3material (Fig.2).These densitiesarecon-sistent withthepresenceof amafic underplateat thebase of the crust. Detailed modelling includingseismic refraction studiesandcrustal shortening esti-mates further south (21~24S) also indicates thatthethickened crust beneath the Western Cordillera

Cordillera Blanca Batholith

10 0 20 0 -300Distancekm 40 0

70 50 0

F ig .2. West-cast crois-icction throughthe batholith showingthecrustal itructureat9S,with deniitiej after Couch ital. (1981).Corresponding velocity data from Wiljon (1985).Thelow-velocityzones (LVZ) with velocities arefrom Ocola & Meyer (1972).Lined partofthelower cruitistheunderplated keel.

probably results from magma addition from theasthenospheric wedge (Schmitz, 1994).

At present,theNazca plateissubducting beneaththe Peruvian Andesat avelocityof 10mm/yi,andthedip ofthe slabis30 just off thecoast, thenitfollowsaquasi-horizontal trajectory inland (Suarezet al.,1983). Subduction angles during theMioceneare thoughttohave been similartothose today(seeKonoetal., 1989). Fukaoetal.(1989), usingathree-layer gravity crustal model and earthquake foci,showed that thereis aclear mantle wedge undertheCordillera Blanca which is absent further southowingtosubductionofthe buoyant Nazca ridge.

STRUCTURE OF THEBATHOLITHDetails of the structure of the Cordillera BlancaBatholithand themodeofintrusion have been givenby Petford &Atherton (1992) and only a shortsynopsis isgiven here to facilitate relation of thedifferent granite types to deformation. Strongde-formation isconfined to thewestern margin and isclearly fault related, occurring during and afterbatholith emplacement. Pre-full crystallization, highmelt-fraction fabrics, including biotite schlieren,layering andaggregates ofmagmatic phenocrysts,are confined to theundeformed core of thebatho-lith. These pass into low-melt fraction-meta-morphic fabrics and ultimately late brittle andcataclastic structures at the intensely deformedwestern margin (see Petford & Atherton, 1992).Thewestward intensification of thedeformation fabric

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JOURNAL OF PETROLOGY V O L U M E37 N U M B E R 6 DECEMBER1996

culminatesinribbon quartzinthe plane of the micafoliation. Such textures only occur when thehostrock is in the solid state and reflect high shear strainsassociated with late uplift. Regional structuresindicatea dominantly tensional stress regime duringthe Oligo-Miocene in the Western Cord illera,which,on thebasisof the neotectonic structures,isstill undergoing extensional collapse (Sebrier et al.,1988).THE MAIN FACIES ANDMINERALOGY OF THEBATHOLITHThe major mineral phases are plagioclase, K-feldspar, quartz, amphibole and/or biotite (Egeler &De Booy, 1956; Athcrton &Sanderson, 1987) w ithindividual rocks ranging from quartz dioritc-mon-zonite to evolved granite (Fig.3). Therocksas awhole defineacrude calc-alkaline trend (Lameyrc &Bowden, 1982) on a QAP plot (Fig. 3).D etailedmapping inasmall area in the south of the batholithby Egeler & DeBooy (1956) defined three majorunits: (1) leucogranodiorite (>70 SiO 2); (2)granodiorite;(3) tonalite-quartz monzodiorite. Leu-cogranodiorite dominates (>85%of the batholith),an dtheother units form older subordinate-discreteoutcrops occurring mainlyin thesouthand at themargins of the batholith (Fig. 1). Although the field

Ksp PIFig .3. QAP plot for the rockiofthe Cordillera Blanca Bathloith.Open squares, granodiorite, tonalite andvarious d ior ita : rilleddiamonds, leucogranodioritej; open circles, pegmatiteaplite. Itihould be noted that the dior ita, tonalitcs and granodiorite]cannot bediitinguiihed completely on thij diagram. Becauseofthii and thesimilar age (Egeler 4 De Booy, 1956) theyaregrouped together.

definitions of Egeler & De Booy (1956) are notentirely consistent withtheQAP nomenclature,wehave retained the general three-fold division,as therock units they recognizedin thefield appearto berepresented throughout the batholith.Amphibole is present in all units, and is associatedwith clusters of euhedral zircon, apa tite, sphene,biotite and mag netite. Compositions vary frommagnesiohornblende to ferrohornblende (Petford,1990) with little changeinA11V,Si and Mg betweenthe different units.Biotite forms large primary euhedral platesorreplaces amphibole. Compositions straddle the phlo-gopite-annite divide (Petford, 1990) and evolvetowards siderophyllite in the acidic rocks. Plagioclaseis zoned (An58-i8) and is rather sodic, evenin thebasic rocks(at~ 55 % S iO2 An+g-jg; Petford, 1990).K-feldsparisphenocrysticinthe leucogranodiorites,whereitcan be up to 8-5 cm long and enclose all theother phases.Muscoviteisconfined to leucogranodiorite faciesrocks,and is of secondary origin. It occurs in thedeformed rocks of the western margin within strong,quartz ribbon fabrics (Atherton & Sanderson, 1987;Petford & Athe rton, 1992). M inera l compositionsare consistent with the textural evidenceinthattheTi contents arelow, as indicated by Miller et al.(1981) for secondary muscovite (Petford & Atherton,1992).Two aspects of the mineralogy are important.First, the primary assemblageismetaluminous, con-taining hornblende, sphene and magnetite.Sig-nificantly, muscovite is a late local overprint.Second, although the mineralogyinmost respectsissimilar to that of the Coastal Batholith both modallyand compositionally (see Mason, 1985), plagioclaseis clearly more sodic and there arc no An-rich cores(A n> 80 % ) so characteristic of infracrustally derivedgranites (Chappell & Stephens, 1988). These differ-ences are important and together with the chemistrydistinguish the Cordillera Blanca Batholith fromtheCoastal Batholith.Syn-plutonic basic dykes and associated maficenclaves have not been found in the CordilleraBlanca Batholith. Onelate dyke cutting therooffacies at Llanganuco is ~4 m wide and chilledagainsttheleucogranodiorite. The only other dykefound isa sheared microdiorite, again cutting leuco-granodioritein theCanyon del Pato (Fig.1,inset).The paucityofevidencefor contemporaneous basicmagma isa further significant featureinstrong con-trasttothe Coastal Batholith (and m ost CordilleranBatholiths), whereitis abu ndantinthe form of syn-plutonic dykes, enclaves and mixed rocks (seePitcheretal.,1985).

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AGE OF THE BATHOLITHK-Ar radiometric ages have been determinedby anumberofworkers (seeCobbing et al., 1981), withvalues showingaspread of apparent ages between16-5and 2-7 Ma. Thedata apparen tly definetwogroups, i.e. 16-59Maand62-7Ma. The older agesoccur in the southern an d eastern m arginintonalitesand diorites,andwere considered byStewart etal.(1974) to represent intrusion ages. The inter-pretation of theyounge r ages (6-2-7 Ma) in theleucogranodiorite is more problematical, not leastbecause the youngest occur along the westerndeformed margin. Recent ' 'Ar/^Ar dating (Petford& Atherton, 1992)has confirmedayoungage (518Ma)atLlanganucoandtheolderage(9-87Ma)atthe southern end of the batholith (Fig. 1). Uranium-lead zircon data (Mukasa, 1984)from a leuco-granodiorite from L langanuco formanisochron withthe relevant interceptat6-3 0-3M a,andtwo frac-tions froma tonalite fromtheCarhuish Stock givea13-7Maage (Fig. 1). These more recent data com-plement the K-Ar data and emphasize the agegrouping. *AT(39AT da ta also indicate that agesyoung from 5-18 0-5Maatthecentreofthe b ath-olithto2-7Matowardsthe western, fault-boundedmargin (Petford & Atherton, 1992). This 'younging'is believedto reflect argon loss from deformedandrecrystallized micas after granite emplacement.Thedeformation ages recorded by the muscovite foliationcoincide with Quechua3,thefinal stageofPliocenetectonic activityinnorthern Peru (Megard, 1989).No agesareavailablefor the granodiorite, whichisassigned to theolder units following Egeler & DeBooy (1956).

The tonalitequartz dioritc faciestothe southandon the margins are significantly older thantheleucogranodiorite (~8-5 Ma) and therefore theycannotbedirectly related.GEOCHEMISTRYSixty-six rocks were analysed for major andtraceelementsbyX-ray fluorescence(XRF)andinstru-mental neutron activation analysis (INAA) (U, Cs,T a,Hfand Th ). Sample collectionwasdeterminedby accessibilityandgeological interest, withacon-centration alongthe well-exposed glaciated valleys,perpendiculartothestrikeofthe batholithatLlan-ganuco, Olleros, Cojup, etc. (Fig. 1). Representativeanalyses and analytical procedures are given inTable1.Major elementsMajor element variation diagramsfor rocksofthebatholith showalarge compositional rang e (52-77%

SiO 2). Although thereissome scatter, A12O3, TiO 2,MgO, P2O5andCaOall show good negative corre-lations (Fig.4).OnlyK2O andNa2O increase withincreasing SiO 2)the latter showinga generally flattrend around a mean of ~4 with just a slightincrease in the leucogranodiorites (average4-7 ,n =32, range 319-5-32).The granodioritic rocks (67-70% SiO 2) showgood trendsfor MgO, CaO, TiO2,K2OandP2O 5,whereasthetonalitic-d ioritic facies w ith SiO2con-tents 6 %classify as granites (Wilson, 1989).The difference in total alkali chemistry betweenthe two batholiths is due almost entirely to the higherNa2O contentofthe Cordillera B lanca suite (see Fig.8 below),K2Ovalues being similartothoseoftheCoastal Batholith. These Na-rich plutonic rocks haveonly recently been recognized and distinguished fromrocks of the more voluminous Coastal Batholith(Atherton & Petford, 1993), althoughitis clear thereare similar rocks throughout the high Andes (e.g.seeParada, 1990). When plottedonanAb-An-Ornor-mative diagram, they trend from tonalite throughgranodioriteto a tight group just withinthetrond-hjemite field of O'Con nor (1965), although somestraddletheboundary and a few lie in thegranitefield (Fig. 5). The leucogranodioritesareclearlydif-ferent from granites witha similar SiO2 rangeof atypical Cordilleran superunit fromtheCoastal Bath-olith (Fig.5, which evolvedbyfractiona tion fromtonalite (Atherton, 1984). Theyare trondhjemitesortransitional trondhjemiteson thebasisofnormativeclassifications. These rocks are singularin that thereare also subtle differencesinmajor element chemistrycompared with granites (70-74% SiO2) from typicalCordilleran suites, with lower FeO (total), MgO,TiO 2andCaO,and significantly higher A12O3 thanequivalent rocks from the Coastal Batholith (Figs6and 8).

In contrast, the tonalites and granodioritesaresimilarinsome respectstothetonalitic suitesoftheCoastal Batholith. ThusonanAbAnOr classifica-

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JOURNAL OFPETROLOGY VOLUME 37 NUMBER 6 DECEMBER 1996

Table1:Major an dtrace element abundancesinthebatholith rocks

Sample:

Si O 2T I 0 2AI2O3Fe(T)MnOMgOCaON a 2 0K 20P 2O BLOI

Leucogranodlorites

88 0

71-450-28

14-641 0 90-040-181-924-743-610 0 80-52

88 4

72-250 2

14-690-840-O40-173-124-853-680-060-46

90 2

72-140-2

14-593-340 0 71-661-763-193-20 0 50-6

90 0

70-130-39

1 50 11-770-050 862-754-263-480O 80-8

916

72-480-2

14-540-760-050-111-76 0 23-660-050-61

88 8

72270-2

14-70-990-050-081-564-83-880-050-62

925

72-150-24

15-150-790 0 40-221-824 6 63-880-10-76

91 9

71-740-22

14-9510-O60-21-844-883-660-050-66

75

71-80-24

15-271 0 30-050-211-944-863-480 0 70-4

76

7 2 20-24

15-051 0 10 0 61-681-93-673-260 0 90-41

Total 9853 100-3 100-8 99-68 9906 99-1 99-81 99-26 99-35 99-57OrAbAn

BaCeCoC rCsEi TH fLaNdNlPbRbScSrrTSrTa*TbTh *UVYZnZr

21-340-1

8

6734775

95-80 8 43 6

16201819

14323-58

45 62-160 3 19-93 ^ 1

188

63131

21-741

7-4

81 6t

5 841

1-31t2-72tt

1818

13 32t

42 71-39t

10-91-76

118

2910 3

18-927

8-4

664335838

3-171-212-13

2021311781

43-36

44 00-790-18

20-46-37

757

6099

20-63611-6

619t

8198

629t3-14tt

1817

1221t

46 31-48t

169-87

308

5 9103

2142-3

6-6

592387497

2-10-72-8

12173022

11922-7

3801-70-29

102-7

7

82

22-94 0 ^

7-1

739466739

2-20-74-1

18191620

12223-1

35 51-40-35

121-4

1294

2 2 939-4

8-4

7 7 85 08434

513 2

18181523

14123-3

47 11 80-32

113-3

7

10 5

21-641-3

8-1

86 42668

1333-61

3-36

14175019

13324-8

4912-30-3374-27

12125 0

116

20-141-1

9-2

7 80416710

3-14-66

2626

721

10 046-2

5662-180-47

12-54-83

159

3414 2

19-331

8-8

7 283877

92 0 5

3-22

2224

820

13544-96

5 301-960-44

11-52-85

168

43112

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PETFORD AND ATHERTON CORDILLERA BLANCA BATHOLTTH, PERU

Table1:confirmed

Sample:

Si O 2Ti O 2AljO,Fe T)MnOMgOCaON820K2OP206LOI

Lsucogranodioritas

84

70-660-23

16-791-170060-242085-323-38007069

924

71-130-23

14-861-040-060-191-874-953-420-061 -91

927

71-720-28

14-681-10-040-2420 45023-180071-37

64

72-250-28

14-981-090-060 51 964-8532 90080-19

93

72-150-19

15 220-850-090-141-725-193-230-060-62

69

71-210-25

14-531-90-11-835-133-120-061-09

Pegmatites

885

74-960-1

13-910-240-13


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JOURNAL OF PETROLOGY V O L U M E 37 N UM B E R 6 DECEM BER 1996

Table 1:continued

Sample:

SiOjT1 02Fe(T)M nOM gOCaONazOK2OP jO BLO I

Quartz diorites

63

51-610-9918-12

7-350-124-727-393-941-680-223-46

90 1

55031-2716-94

70 14-026 9 83-612-240-382-12

90 5

58-540 9 217-87

5 0 40 0 72-75-764-242 0 20-32 0 4

90 6

68-471-2116-11

7-30-146-473-611-990-373-01

Tonalrtas

90 3

61-710-9815-83

5-350-083-465-243 821-980-232-04

68

63-370-7916 22

3 8 10-092-064-544-122-320-191-59

91 0

63-410-7116-85

3-560-081-824-274-12-440-191-99

91 4

68-380-4415-63

3-160-061-283-174-112-980-120-9

92 8

69-370-4616-4

2-230-060-83-23-823-590-151

Dyke

86

54-381-0817-16

8-440-13-55-89420-264 0 1

Total 99-6 99-69 99-5 100-6 100-7 99-1 99-42 100-2 1 0 0 0 100-8

OrA bA nBaCeCoCrCsEuH fLaNdN iPbRbScSm *SrTaTbThUVYZnZr

9- 933-326-8

41 2334 086

6-11-263-26

172619

9682 4

4-9374 3

0-750-6121 0 2

20 016

11 711 4

13-230-423-4

722t

4669

3-8t2-61tt

2 0117116

t89 5

0-83t8-73-8

18114

11 214 1

11-93 5 92 3 8

66 4t

5239

3-7t4- 4tt

201265

7t

96 91-3t

133-1

1211189

14 4

11-830-521-9

56 74461653 31-63-6

283521

46416

6-394 3

0-790-5532 6

17315

12 514 2

11-732-320-2

410t

5662

2-98t3-66tt

203

6212

t71 1

2-5t5-432-5

13 81296

11 8

13-734-918-9

47 4t

469

t

tt

191094

7t

62 7

t6

991093

11 5

14-434-719-9

60 2275163

2-121 0 54 0 6

221619

991

63-9

75 50-760-27

18-52-43

927

11 814 0

17-634-814-9

74 8385339

1-41-33-8

181618

711 9

54

57 31-70-47

113-3

471382

10 7

21-132-314-2

70 4205228

5-30-83-3

15172022

13 322-6

58 31-30-27

253

348

7612 3

11-83 3 823

47 7383788

21-234-35

223321

34618

4-7839 8

1-370-7541-75

15 326

11 816 1

Major and trace element analyses by XRF.*Data by IN AA tSee Table 2.The analysed rocks are fresh wit h little alteration apart from the cataciasites near the westem c onta ct Minor chloritization of h ornblendeand biotite is not uncom mon but in85%ofcasesit makes up


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20-18-16-14-12

_ D

Gtonal te-diorite

aaGO0 a

D

granodiorite

AI 2O 3

leuco-granodiorite

7*1oQ

6 -5 -4 '

32 -1 -50 60 70 80 50

5-4-3-2-1-50

MgOm DG

GG D G * .

O3

60 70 80

10

642 -50

G DP O

60 70 808-64 -2

50

CaO

60 70 80

6-5432 -1

Na 2O o - O

50

' . :

60 70 80

1.0-

0.5-

0.0

O3 GO n _ IT

T iO 2

50 60 70 80

0.4-0.30.2-0.1-0.0

GG

G DG G D

P2O5

GG G Gof

50 60 70SiC>2

80Fig.4. Htrker diagrarruformajor elemenUinrocki from the Cordillera Blanca Batholith; rymbolj asinFig.3.Guide lino are drawntojeparate granodioritic rocks showing coherent trendsin MgO, CaO, T1O2, P2O 5 (65-70% SiO2) fromthetonalitaanddiorita whichareofsimilar ag e.The K2O** Si O2plot indicatesthehigh-K characterofthe batholith.

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JOURNAL OF PETROLOGY V O L U M E 3 7 N U M B E R 6 DECEMBER 1996

An ^sou rceGranodiorite

Fig. 5. AnAbOr diagram showing the trondhjemitic character of the leucogranodiorites and the tonalitic, granodioritic character ofthe more basic rocks. Fields from O'Connor (1965). Fractionated granites (>70% SiO2) from the Santa Rosa Super Unit of the CoastalBatholith have compositions in the stippled field. Symbols as in Fig. 3. Inset shows Cordillera Blanca leucogranodiorites, Coastal Batho-lith granite field and the field (diagonal lines) for experimental melt compositions (Sen & Dunn, 1994) from amphiboUte dehydration at1 -5 and 2-0 GPa, with the arrow showing the change in composition of melts with increasing temperature. Filled triangle is the experi-mental starting composition ( s source). The amphiboUte assemblage was amphibole, plagiodas e, quartz, ilmenite and garnet.

tion diagram they also lie in the tonalite-grano-diorite field (Fig. 5). None the less, compared withthose of the Coastal Batholith they have slightlylower CaO and MgO, and higher Na 2O + K 2O (Fig.8) contents comparable with the leucogranodiorites,but the AI2O3 contents (see Fig. 8) are the same orlower and the TiO 2contents higher.Trace e lementsTrace elements show good correlations with SiO 2forsome elemen ts such as V bu t diffuse trends for Sr, Baand Rb (Fig. 7). Generally the trace elements showfar more scatter than the major elements, and eventhe granodioritic rocks show little coherent variation(Fig. 7). Rb/Sr increases very slightly (from 018 to0-59) over the range 52-73% SiO2, whereas K/Rbshows significant scatter about a mean of ~ 250overthe same SiO 2 range. Importantly, Sr is enrichedand Y is depleted in the Cordillera Blanca rocksrelative to similar rocks in the Cordillera Batholith[Fig. 8 and Atherton & Sanderson (1985)].

Aplites and pegmatites with SiO 2>74% [DI(norm ative qua rtz + orthoclase + albite) > 90 ], arecharacterized by marked depletions in Sr, Ba, La,Ce , Nd, Zr, Hf and Zn relative to the more basicfacies (Fig. 7, Table 1).

Rare earth elementsEighteen rocks spanning the range of SiO 2 (5573-6%) of the batholith were analysed for REEs (byradiochemical neutron activation analysis, Table 2).Lajy/Ybjv ratios range from 3-3 to 77. The leuco-granodiorites are strongly depleted in heavy REE(HREE) compared with the more basic rocks(quartz diorite-tonalite), but there is overlap in thelight REE (LREE) from La to Pr although theleucogranite field lies at lower values (Fig. 9). Leu-cogranodiorite 884 from Paron (Fig. 1) has thelowest REE abundances and highest Lajy/Yb^ ratio(77) of all the rocks. The variation within the leuco-granodiorite facies is large (Fig. 9). Lajy/Ybjv variesfrom 19 to 77 over an SiO 2 range of 3%, in rockswhich crop out over a distance of 140 km. Only onesample (69) exhibits a negative Eu anomaly. A plotof Lajv/Ybjy vs SiO 2shows a steady increase in Lajy/Ybjy ratio in the basic-intermediate facies, i.e. withSiO2 contents up to ~ 6 5 % , then considerablescatter in the leucogranodiorites and granodiorites(Fig. 10). The consistent variation in rocks below65 % is due to a decrease in the HREE (e.g. Lujyvaries from 10 to near 2-2; Fig. 9) with increasingsilica content; a feature implying a consistent changein source mineralogy or amount of melt with time,

1500


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P E TF O RD A N D A T HE R TO N C O R DI LL E RA B LA NC A B AT HO LIT H, P ER U

0.50-1

0 .40 -

0 .30 -

0 . 2 0 -

0 . 1 0 -

0.00-

TiO 2 5.00 -i4 .00 -

3 .00 - &

2.00

1.00-

0.00

CaO

7 0 72 74S lO2 70 72 7 4Cordillera Blanca BatholithA Coas tal Batholith (Lima segm ent)

2.00-1

1.50-

1.00-

0.50-

o.oo-

MgO

2.00-i

1.50-

1.00-

0.50-

70 72 74S i O 20.00

FeOA A

A A

y70 72 74

Fig.6. Harker ploUofMgO, TiO2 , CaOandFeOoftheleuco-granodiorita of the Cordillera Blanca Batholithandgraniteiofthe Coastal Batholith [data from Atherton (1984, and unpub-lished)].

or fractionatdon of a phase such as garne t withahighKDfor theHREE.Radiogenic isotopes and the source of themagmasSr (whole rock) and Pb isotopic datafor separatedfeldspars from the Cordillera Blanca Batholith havebeen discussed by Atherton & Sanderson (1987)basedon theworkofMukasa & Tilton (1984)andBeckinsale etal. (1985),who thought the ultimatesource could wellbe enriched subcontinental litho-sphere. The Cordillera Blanca rocks (tonalite, grano-diorite, leucogranodiorite) define a compact groupwith very similar Pb isotope compositions to thegabbros, dioritesandtonalitesof the Lima segmentof the Coastal Batholith (Fig. 11), which liesinthincrustto thewest,and to the NVZ(NorthernVol-canic Zone) lavasofthe Andes. Theydo notappearto contain an old Precambrian crustal componentsuchas theArequipa massif (Fig. 11, see also oxygen

isotopes). Thus the trend towards low20 6Pb/204 Pbobserved in CVZ (Central V olcanic Zone) rocksbetween 16 and 18S, commonly interpreted as amixing between mantle-derived Pb and less radio-genic crustalPb(Davidsonetal.,1991)isnot seeninthe Cordillera Blanca rocks. This may not be sur-prising,as the Precambrian Arequipa massif isnownot thoughttomakeup lower crust northofLima(Atherton, 1990).Nor do they appear to lie on amixing line between Nazca plate mid-ocean ridgebasalt (MORB) and Pacific sediments. They areenrichedin20 7Pb relative to MORB and very similartothe homogeneous reservoirofTilton &Barreiro(1980) which was the source of plutonic and volcanicrocksin central Chile,and which they thoughtwasenriched subcontinental lithospheric m antle.

More recent Sr-Nd work on selected batholithrocks shows relatively small variations in87 Sr/ 86 Sr(0-7047-0-7057) andU 3N d/14 4Nd (0-5126-0-5125)ratios (Petford et al., 1996).The older (16-10Ma)quartz diorites and tonalites define a tight groupwith ejvjd varying from 0-2to amost enriched valueof-0-2 overasilica rangeof14 w t% (Fig.12).Theyounger (65 Ma) leucogranodioritesareless tightlygrouped and slightly more evolved (mean e a 1-38 0 -6, max ENJ -2-5) thanthequartz dioritestonalites (Fig. 12). However, they are more prim-itive thanthe baseline CVZ volcanic andesites(Fig.12), which Davidson et al. (1991) considered couldbe partial melts of subcontinental lithosphericmantle.

To sum up, the lead data are consistent withasource similartothatofthe NV Z,i.e. a subductionmodified mantle with very minor crustal con-tamination (Wilson, 1989), whereas the slightlyenriched Sr and Nd data compared withthe NVZlavas suggest involvement of a young crustalcom-ponentinthe petrogenesisof the magmas, especiallythe leucogranodiorites, which may have been locallyderived from the envelope rocks. However, themajor componentisconsideredto bemantle derived(see Mukasa & Tilton, 1984).

C A U S E S OF C H E M I C A LV A R I A T I O N IN THEB A T H O L I T HAlthoughit is clear that thedifference in age andinitial Sr and Nd isotope compositions (Fig. 12)preclude the direct derivation of the leuco-granodiorites from the tonalitesquartz dioritesthemajor element trends in Harker diagrams showaregularity similar to that seen in fractionatingsystems (particularlyforthe granodiorites, Fig. 4).It

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JOURNAL OF PETROLOGY V O L U M E 37 N UM B ER 6 DECEM BER 1996

4321-n-

D

aa D a o

Ta Rpm.o

i

o

50 60 701200-1000-800600400200

Ba ppm

50400-300200-100-

50

. a ao aao a

60

60

70

40 130-

20-10"

La Rp.m.

D ID

80 5050 -40 -30 -2010 -

60 70 8 0YRpm.

aa

aoD tP

80 50 60 70 80Rb Rpm.

o

1000-800-600400-200-

n -

o a aa D

o o

Sr Rpm

aa v

o

70 80 50 60 70 803001

200

100-o a o aD Qa

50 60

Zrppm. 2 0 0l

100-

Ooo

V ppm.

H

** o70 80 50 60 70

S i C80

Fig. 7. Harker diagrams for selected trace element contents of rocks of the Cordillera Blanca Batholith; symbols as in Fig. 3. [Note thelack of consistent variations (apart from V) in the granodionte and even more so in the leucogranodiorite.]

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JOURNAL OF PETROLOGY V O L U M E 37 N U M B E R 6 DECEM BER 1996

Table2:REE abundances p.p.m.) in selectedbatholith rocks

900 (L)884 (L)69 (L)901 (QD)906 (QD)88 (T)96 (T)903 (T)

Le

21-419 816-723-924-122-32620-9

Ce

4 3 742-23 7 861 160-464-657-445-5

Pr

4-14-44-35-97- 46- 45-55

Nd

15-715-314-32 6 626-325-221-219-7

Sm

2-32 72-95-94-74-53-35

Eu

0-610-550-471-631-291-20-761 0 5

Gd

1-561-391-73-723-132-12-54

Tb

0-190-170-270-490-380-400-220-34

Dy

1-20-81-532-31-91-21-9

Yb

0-30-170-620-730-690-60-430-6

Lu

0 0 60 0 30 0 80-130-120-080-070 0 8

L, leucogranodio rite; QD , quartz dicxrte; T, tonalite.REEdetermination involved irradiation, radiochemical grou p separation of REE andyspectrometry usingaGe (Li) detector, all at N orthern Universities R eactor Centre, Risley. Procedure and spectral analysis have been givenby Duffield & Gilmore (19 79). AGVI was used asaprimary standard and results of the analysis of this standard reference are (literaturevalues in parentheses): La 35 (3 5), Ce 62 (63 ),Pr8 (7), Nd 33 (3 9), Sm 5-2(5-9), Eu1-5(1-7), Gd 4 0 (5-5), Tb 0-6 (0 -7), Dy 3-5 (3-5),Ho 0-5 (0-6 ), Er1 -3(1-2),Yb1-5(1-7), Lu 0-2 (0 -3).

100

10

c-a

La Ce Pr N dP m Sm Eu G d Tb D y H o Er Tm YbT.uFig. 9. Chondrite-normalized REE ploti for roclu of the Cordil-lera Blanca Batholith. Envelop e] ihow n for quart diorites andtonalites (n = 8) and leucogranodioritcs n= 7).

ou

60 -

40 -

20

0 -

Granodioriteo

Diorite /20 inequivalent Coastal Batholithrocks which vary consistently with SiOj content(Atherton, Fig. 7, 1993), and the lack of an Euanomalyare notconsistent withanoriginbycrystalfractionation. The volume of leucogranodiorite( > 8 5 % byvolumeof the batholith: minimum 6000km 3) is in marked contrast to that of rocks withsimilar SiC>2 contents in the Coastal Batholith(~ 10% by volume, Atherton, 1984), and generationby fractionation from basalt would produce anamphibole-clinopyroxenegarnet residue (see Fig.19 below),forwhich thereis noevidence,of54000km 3 at the site of crystallization (Spulber &Rutherford, 1983). Thus there appears to be nosimple scheme involving fractionation of a dioriticormore mafic magma source to form the leuco-

granodioritesof the batholith. Even withinthemainleucogranodiorite facies there is little evidenceofcompositional continuity which would indicateanimportant rolefor crystal fractionation. Thus thereisa large spread and no consistent variation in rocktrace element contents, e.g. REE, Ba and Y, overtheSiO2 range 70-74% (Fig. 7). Furthermore, whole-rock Rb-Sr isotopic data fail to define isochrons,which indicated to McCourt (1978) and Mukasa(1984) that theleucogranodiorite wasmade up ofbatch melts fromaheterogeneous source.Oxygen isotopes, deformation andmagma sourcesOxygen isotopes have proved usefulindistinguishing

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JOURNAL OF PETROLOGY V O L U M E 3 7 NUMBER 6 DECEMBER 1996

0.5135

0.5130

0.512530.5120

0.5115

Cordillera14-10 Ma O

OLeucogranod writesAOtz diorites-tonalrtes

Sr/^Sr D0.7045 0.7050 0.7055 0.7060

CVZ baselinecompositions

J i

Arequipa massif( 1 4 3 N d / 1 4 4 N d = 0.511

8 7 S r / 8 6 Sr = 0.7504)1 i

0.702 0.704 0.706 0.708 0.710 0.712

Fig. 12. Plot of H SN d / l 4 + Nd vs " S r / ^ r for Cordillera Blanca Batholith rocks. The field of Miocene to Recent lavas from the CentralVolcanic Zone (CVZ) ii also shown, as well as that for the Precambrian Arequipamusif. Data from Davidsonital. (1991) and Mukasa(1984), respectively. The CVZ baseline compositions, i.e. the last isotopically evolved basaltic andesites from this zone, are more evolvedthan all but one of the Cordillera Blanca rocki. There is none of the extreme enrichment in isotopic composition characteristic of theCVZ magmas. Inset shows the isotopic composition of the batholith rocks with respect to age. The older (1410 Ma) quartz diorites andtonalitcj have compositions which cluster around bulk Earth. The shift towards more negative CN dand higher Sr/^ Sr ratios in theyounger (65 Ma) leucogranodiorite* is consistent with derivation from an enriched source in subcontinental lithosphere with a minorupper-crustal component.

magma source regions and assessing the extent ofcrustal involvement and late-stage fluid-rock inter-action in granite petrogenesis (Taylor, 1986). Ingeneral, melts formed directly from the mantle, orindirectly by partial melting of unweadiered meta-basaltic material, will have + 10%o[Taylor (1986) and Fig. 14].The oxygen isotope compositions of 15 quartz andmica (biotite and muscovite) mineral separates andtheir host rocks from an ~ 5 km east-west traversealong the Llanganuco section of the batholith intothe Cordillera Blanca fault zone (Fig. 1), togetherwith a further 17 whole rocks (granitic rocks,Chicama sediments and metasedimentary xenoliths)from the Cordillera Blanca are given in Table 3.The leucogranodiorite cataclasite 921 is from thefault-bounded western margin of the batholith (Fig.14). Analyses were made both of qua rtz grainswithin the cataclasite matrix, and the deformed

matrix itself. X-ray diffraction (XR D) analysisshows that the matrix (mean grain size < 100 /zm) ismade up of quartz and albite, with minor chloriteand illite. The deformed leucogranodiorite (918)from inside the fault zone contains secondary mus-covite related to deformation and was partiallymylonitized during and after magma emplacement(Petford & Atherton, 1992). The undeformed leuco-granodiorite (879) is from the interior of the batho-lith. A sample of undeformed Yungay ignimbrite(889) and a garnet-bearing pegmatite (92) fromoutside the fault margin were analysed for com-parison with the batholith rocks.The range in isotopic composition of the batholithminerals is bracketed by biotite and quartz. Ingeneral, there is a decrease in


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P E T FO R D A N D A T HE R TO N C O R D IL L E RA BL AN CA B AT HO LT TH , P ER U

1000

10 0

(a)

U Th Enr ichmentin LILE onfractionation

Sr spike -no residualplagioclaseCordi l lera Blancaleucogranodiorite

Depletions owingto fractionatingphases

Ba

5.04.03.0-2. 0

1.0

>0.S-

0. 1

(b)I accessoryp / \ minerals,residual in CBB- ' I source or fractionating

Ba Rb U Th K Ta Nb L a C e Nd H f Z r Sm

Fig. 13. (a) Mantle-normalized trace element diagrams ofaleuco-granodiorite (~72% SiO 2) from the Cordillera Blanca Batholithcompared with a granitic rock (~72% SiO 2) from the CoastalBatholith, which was produced by crystal fractionation. The lackof LILE enrichment and major depletions in Sr, P and Ti in theCordillera Blanca rocks is in contrast to the granitic rock of theCoastal Batholith, which evolved by early precipitation (frac-tionation) of plagioclase, apatite and titaniferous magnetite, (b)Incompatible element enrichment (acton of a Cordillera BlancaBatholith leucogranodiorite compared' with a C oastal Batholithgranitic rock with similar SiO 2 contents (70% and 72%) andmineral assemblages.

coexisting quartz and its whole rock (Table 3). Thepegmatite (92) has elevated c$18O values by ~l%ocompared with the batholith rocks, consistent withthe permitted maximum isotopic shift associatedwith magmatic fraction in hydrous granitic melts(O'Neil, 1986).Although the data from the fault traverse arelimited, they differ from other studies where de-formation and subsequent resetting of oxygen isotoperatios led to higher8 8O values in the most deformedrocks (Cartwright et al., 1993). The generalreduction in 51O values in the b atholith rockstowards the faulted margin of the batholith mayreflect limited interaction between the leuco-granodiorites and low Sl8O meteoric fluids eitherduring or after emplacement (e.g. Taylor, 1986).This interpretation is supported by the higher quartzand biotite values in the pegmatite and Yungayignimbrite that lie to the west of the fault zone and

an identical whole-rock value of +9-2%o for theYungay ignimbrite and the least deformed leuco-granodiorite. Further evidence for interactionbetween the batholith rocks and low S1BOmeteorichydrothermal fluids are seen in the low


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JOURNAL OF PETROLOGY V O L U M E 37 NUMBER 6 DECEMBER1996

tompmturoftre)942qtz-bt -73 8 q tz-mu -

588qtz-bt -561 q tz- .593qtz-mu

A Wo

o 9g n f a n b r t t

CanBOenBttnc* FmH

D O I6

cto tormed mncovt tsleucogranodiortte

loutuyj'BJiodJorfta

10 116 O % .Fig . 14. 50 km)crust inboard of the Coastal Batholith emplacedwithin thin ( 1-0 and only sevenhaveavalue >1-10.'S' type granites have A/CNKvalues >1-1 (Chappell &Wh ite, 1974). ThustheCordillera Blanca Batholith is no more peraluminousthan 'I' type granites which lack peraluminousminerals,e.g.Coastal B atholith (Fig.8). The per-aluminous rocks (A/CN K > 1-0) are the deformedrocks of the western margin, most of which are mus-covite bearing (Petford & Atherton, 1992).Muscoviteissecondary, and the primary mineralassemblage, including hornblende, magnetiteandsphene, indicates that the deformed faciesis amod-ified T type (Chappell & Stephens, 1988). Leadisotope compositions for the undeformed rockssupport this; they are nearly identicaltothe relatedmetaluminous hornblende-bearing Surco Stocktothe south whichhas an A/CNK ratioof0-86, and

which couldnot have originated in oldcontinentalcrust. Accordingto Mukasa (1984), the source was'enriched' mantleorunderplated lower crust(Fig.11). Furthermore,


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PETFORD A NDATHERTON CORDILLERA BLANCA BATHOLTTH,PERU

Table3:Oxygen isotope data/or Cordillera Blanca rocksSample Mineral

87 9

91 8

921

88 9

92

Whole-rock dataBsthoBth rocks880 (L)884 (L)900 (L)921 (L)925 (L)915 (L)93 (L)901 (QD)920 (T)916 (P)Mean

Sadimentt89189 289389598Mean

XanoTnhs912913

QuartzBlotiteMuscoviteWhole-rockQuartzBio theWhole-rockQuartzmatrixQuartzBlottoWhole-rockQuartzMuscoviteWhole-rock

+ 10-8+ 6-1+ 8-4+ 9-2

+ 10-1+ 5-7+ 9-2+ 9-3+ 8-2+ 9-4+ 7-5+ 9-2

+ 10-1+ 8-7

+ 10-1

+ 8-8+ 4-1+ 8-1+ 8-7+ 9-2+ 9-2+ 8-5+ 8-2+ 9-2+ 9-5+ 8 -11 -7

+ 14-1+ 14-4+ 14-2+ 13-6+ 15-4+ 14 -30 -66

+ 2 3+ 13-3

Distance from Commentsfault (km)

Undeformed leucogranodiorfte

Partially deformed mmcovrte-bearing leucogranodlorite

Catadastic leucogranodlorite

Yungay Ignimbrite to west of fault

Pegmatite cutting leucogranodlorite

Mineral oxygen isotope rat ios were determined at the N ERC Isotope Geosciences Laboratory ( UK ) fol lowing the oxyg en l iberat iontechnique of Clayton & M ayeda ( 19 63 ) but ut i li zing a CIF 3reagent (BorthwicfcSi Harmon , 198 2) . Oxygen yields were converted to CO2by react ion wi th a platinized graphi te rod heated to 67 5C by induct ion furnace and analysed on a Ph oenix 39 0 mass spectrometer. Dataare given as per mi lle (%o) derivat ions relative to S M O W (Standard M ean Ocean W ater) . Standard sample N BS 28 gave an average 1 8 0value of + 9-63 0-14%o {n =3, analyst R. Greenwo od) during this study. Wh ole-rock samples determined at SU RR C (analyst A. B oyce) .L, leucogran odiorite; Q D , quartz diorite; T, tonalite; P, pegmatite.

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on Sr/Yvs Y(Fig. 15b)diagrams have been mod-elled (see Jahn et al., 1984; Martin, 1986;Drummond &Defant, 1990). Source compositionsvary from eclogite (qz/cpx/ga: 15/55/30) toamphi-bolite (hble/plag/bio: 70/25/5) with sourceREEpatterns ranging from flattoslightly LREE enriched(Lajv/Ybjy= 4). TheLa^/Yb*-and Sr/Y plots(Fig.(a ) 150

100

50 H

L e u c o g ra n o d io r i t eD To n a l i t e

(b)

Sr /Y

(c)

Sr/Y

120

K H80

60

4020

0

120

100

80

60

40

20 -0-

1eclogiteArchaean trondhjemite-tonalite-granodiorite field

130% garnet amphibolite>7% garnet amphibol i te

^amphibolite J post-Archa -v f \ gr nitic fit

10Y b N 15 20* 30% garnet' amphibolite i

7%garnetamphibolite

Leucogranodioritea TonaliteArchaean high-AItrondhjemite-tonalite-dacite field

Andesite-daciterhyolite field

10 20Y 30 40

A Coastal Batholith

\ |post-Archaean andesite-dacite-rhyolite field

' .A10 20Yppm 30 40

15) suggest that leucogranodiorite melts could haveformed from garnet amphibolitetoeclogite sources,whereas mostofthe tonalites may have formed froma source with less garnet.Continental crust vs slab sourceThe critical requirementofthe sourcesofthe tonal-ites andleucogranodiorites isthat they containedgarnet and amphibole (with littleor noplagioclase)in the residue so that partial melts wouldbehigh-AItypes with high Lajy/Ybjv ratios and high Na2OandSr contents. There are twoenvironments inwhichthis might occur:(1)subducted oceanic lithosphere;(2) lower continental crust (thicker than 40 km).With regard to (1),Martin (1986) hasmaintainedthat Archaeanandpost-Archaean granitic rocks are.significantly different; theformer related topartialmelts of a garnet-hornblende source whereas moderngranitic rocks show no indication of a prominent rolefor either ofthese minerals. This heconsiders is adirect consequenceofthe progressive coolingoftheEarth. Because Archaean oceanic crust wasyoungand warm it melted before dehydration, leavingagarnet + hornblende residue. Modern oceanic crustof major ocean basins, beingold(average 60 Ma)and cold, dehydrates before reaching thesolidusofhydrated tholeiite, and so is subducted withoutmelting (Peacock et al., 1994). In this lattersituation, it is the 'fluid metasomatized mantlewedge' above theslab that melts to producethemagmas, and olivine and pyroxene are the dominantresidual phases.

Slab melting has also recently been put forwardtoexplainagroupofCenozoictoRecent high-AI vol-canicarcrocks with similar chemical characteristicsto the leucogranodiorites, which are apparentlyrestricted tosites similar tothosein theArchaeanwhere young (


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JOURNAL OF PETROLOGY V O L U M E 37 N U M B E R 6 DECEMBER 1996

(Defant & Drummond, 1990). The slab meltingmodelisattractive w ith respectto thegenesisoftheCordillera Blanca leucogranodiorites, assubductionhas beena major featureof the continental marginsinceat least the Jurassic (Fukaoet al., 1989),andthe rocks have characteristics apparently consistentwith slab melts. However, geophysical, isotopicandage dataand thermal modelling suggest that partialmeltingof a mafic protolithin thebaseofthe thickcrust(>40 km) beneaththebatholithismore likelyin this case [a brief discussion has been givenbyAtherton & Petford (1993)].Herewediscussthe two most compelling reasonswhy we consider tha t lower-crustal m elting rathe rthan slab melting producedthemagmasoftheCor-dillera Blanca Batholith:

(1) Num erical modelling of pressure-tem-peraturetime paths of subducting amphiboliticoceanic crust indicates that partial meltingcanonlyoccur on subduction of very young (5-10 Ma)oceanic lithosphere (Peacock etal., 1994). Altern-atively, with high rates of shear heating, shearstresses >100 MPa must be maintained by rockscloseto their melting temperature to cause partialmelting. Thisweconsiderto beimplausible,and sotheage of the subducting slab becomes critical.Asthe Nazca plateat 205Ma was not at theinitialstage ofsubduction and was 55 to 65 Ma at thetrench (Wortel, 1984),it wasclearly too old andcoldformeltingto occur before dehydration.(2) The thermal structureinsubduction zoneshasbeen much studied, with slab-induced convectionanimportant aspect of the models (Peacock,1991;Davies & Bickle, 1991). Local high temp eraturegradients are produced along the slabwedgeinterface. Various assumptions as to the slabsub-duction angle, etc. place slab melting,ifit occurs,inthe cornerofthe m antle wedgeat ~6070 km depth(Peacock et al.,1994) but importantly some80 kmfromthe trench, outboardofthe main plutonic-vol-canicarc, i.e. in the arctrenchgap (Fig. 16, andDavies& Stevenson, 1992).In Peru,thereverseisthe case,theCordillera Blanca Batholith lying~300km inboardofthe trenchand thevolcanic 'arc 'anddirectly abovethethickened keelofthe Andes. Sucha geometryis more consistent with derivation fromhot, newly thickened basaltic crust beneath thebatholith,butnot fromthesubducted slab.REE modelling of the batholithWe present herea two-stage model that relatesthegenerationof the Cordillera Blanca batholithto thedynamic underplating of the Andean crust duringMiocene times. Stage 1involvesthe creation of a

mafic underplate by partial melting of slightlyenriched upper mantle. In stage 2 this newlyaccreted m aterial is itself partia lly melted, giving risetotheolder tonalitesand, later after theunderplatehas thickened through the garnet-in transition,themore voluminous leucogranodiorites. Thusthe evo-lution of the batholith dynamically 'mirrors' thethickening, and formationofthe keelofthe Andesincentral Peru.Stage1:Generationofthe mafic crustal underplateInthemodelling we usea late high-Al, calc-alkalinebasaltic dyke rock (Table 1) from the CordilleraBlanca with an Lajy/Yb^- value of near four as aguide to the composition of the accreted materialthat might form the deep crustal keel. This rockissimilar to the EoceneMiocene (14-6 Ma) calc-alkaline basalts immediatelyto thewestoftheCor-dillera Blanca fault (Fig. 1), which have values nearfour (Athertonetal, 1985, table 25.2).Thereare two possible sourcesfor basalt magma,both of which may occur in the subcontinentallithosphere depending on depth: spinel or garnet( spinel) peridotite. Xeno liths of both typeshave REE patterns which are not very dissimilar(McDonough & Frey, 1989). Anhydrous fertileperidotites with high A12O3and CaOcontents tendto have La_y/Ybjy ratios near unityor less, whereasinfertile mantle types with low valuesofthese oxidesowing to basalt extraction often have enhancedLRE E, indicatingalater, separate enrichment event(McDonough & Frey, 1989).

Two end-member type mantle compositions fromMcDonough & Frey (1989) were usedin the mod-elling (Fig. 17): an infertile garnet peridotitexenolith, witha relatively steepREEpattern (La_y/Ybjv value of 40) and a fertile garnet peridotitexenolith withanLajy/Ybjy rat io of jus t over two.No partial melts (F values 0 -01-0-2) could beproduced from infertile mantle which approximatesthe REE patternofthe late basalt. H owever, pa rtialmelting (7-10%) of the fertile mantle source couldproducea melt similarto thebasalt (Fig. 17), andthiswastestedas a probable basalt sourcein mod-elling partial melting giving rise to the quartzdiorite-tonaliteand theleucogranodiorite.Stage2:GenerationofbatholithmagmasFrom previous discussions on fieldand age con-straints, andfrom the observed trace elementandREE profiles of the batholith rocks, any proposedmodel for their origin must account for the fol-lowing:

(1)The quartz diorites and tonalites are older

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PETFORD AND ATHERTON CORDILLERA BLANCA BATHOLITH, PERU

ANDESITICARCkmOr

- 5 0 k m - TRONDHJEMITIC ARC

50

10 0

150 -

OCEANICLITHOSPHERE

AMPHIBOLE CHLORITEDECOMPOSITION

Fig. 16. Diagram showing subduction zone geometryat acontinental margin, illustratingtherelative positionsofa trondhjemitcto atonalite (or andesitic) 'arc'. Fluid-absent meltingof oceanic lithospherecan only occur during subductionofvery young(


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J OUR NALOFPETROLOGY VOLUME37 NUMBER6 DECEMBER1996

1000

10 08No D

o 10-

basalt dykeunderrate?

Residual mineralogy10%amphlbote

2%cUnopyroxene23 %orthopyroxene64%oGvine

1%garnet

1000

LaCe Nd S m E u Q d T b D y Er Yb LaFig. 17. Mantle melting model for the generation of the under-plate involves 9 batch melting of an enriched (X 2'5) hydrouifertile mantle source similar to that proposed by Lopez-Eicobarit al.(1979) as a source for the central Chile granitic rocks. Thijproduces a melt similar in composition to the Cordillera Blancabasaltic andesite 86, and similar to late dykes in the Eocene-Miocene lavas to the west. Mantle Ko values are from Martin(1986), after Hanson (1980). Residual mineralogy is similar tothat seen in mantle xenoliths from the Sierra Nevada Batholith(Batcman, 1992), and expected after large-scale hydration of the

lithosphere above subducting slabs (Peacock, 1993).

100;113o Doo

Residual mineralogy40 %amphibole36 %dinopyroxeno

8% garnet8%K-feWspar6% quartz1%ilmenlte0.5%m agnetite

U C e Nd SmEu QdTbDy Er Yb LaFig. 18. Batch melting model of garnet amphibolite basalticunderplate to produce a typical tonalite. Melt proportions andresidual phases are comparable with the experimental partialmelts of amphibolite-edogite, giving tonalitic melts at 16 kbar(Rapp it al., 1991) although residual amphibole here is higherand garnet lower in amount. [See also the experimental data ofSen & Dunn (1994) at 15 GPa, where residual amphibole issimilar in amount.] Mineral-melt distribution coefficients forplagiodase, garnet, amphibole, clinopyroxcnc, magnetite andilmenite from the compilation of Martin (1987); quartz and

K-feldspar from Nash & Crecraft (1985).

LeucogranodioritesAsthe leucogranodiorites have LREE similarto, orlower than,thetonalitesand diorites(Fig. 9),lowerdegrees of partial meltingof the same sourcepro-duced model liquid compositions with LREEenrichments far greater than those observed.Changesin melt fraction have little effect. Neitherdoesthe source mineralogy, as lower-crustal LREEK-Q valuesfor clinopyroxeneandgarnetare notverydifferent, so the change from a dominantly clino-pyroxene (+amphibole) to a dominantly garnet(+ amp hibole + clinopyroxene) residue as indicatedinthe Sr/Y vs Y modelling should not significantlyreduce the LREE enrichment for smaller partialmelts.The LREE abundancesin the source regionmust therefore be lower than those of the oldertonalites. Furthermore,the lower HREE abundances

in the leucogranodiorites indicate more residualgarnetin thesource. Thu s,theleucogranodioritecanbe modelledby 16-20% partial meltingof a maficsource slightly depleted in LREE, relative to thesourceof the older tonalites, leaving residuesof 35and 25% garnet, plus amphiboleandclinopyroxene.Modelled profiles are similar to those of the tworocks definingtheenvelopeof the leucogTanodiorites(see Fig. 9). For simplification, the summarydiagram (Fig. 19) refersto the most evolved leuco-granodiorite.The residual mineralogyiscom patiblewiththe Sr/Y modelling and is not very differentfrom the results of Martin (1987) for similarArchaean high-Na rocks from Finland.Onan Sr/Y vs Y (Fig. 15b) the tonalites-quartzdioriteslie to therightofthe leucogranodiorites witha very similar range in Sr/Y ratios but higherYvalues. Modelling of partial melting trends clearly

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P ETF ORD AND ATHERTON CORDILLERA BLANCA BATHOLTTH, PERU

1000

100-ioI

T3O6 10-

1modelled melt

Residual mineralogy3 1 % a m p h l b o t e17% cflnopyroxene4 8 % g a m e t

1% K-feldspar1% quartz1 % Dmenlte1 % magnetite

Yb LaL a C e Nd S m E u G d TbDy Er

Fig. 19. Batch melting model of garnet amphibolite underplatetoproduce the most HREE depleted leucogranodiorite. Melt propor-tions and residual phases are comparable with experimentalpartial meltsofamphibolitic edo gite which areNarichat >16kbar (Rappita l. 1991) although, again, the more hydrous natureof the residue means amphibole exceeds dinopyroxene. [See Sen &Dunn (1994) forexperimental dataonamphibole-rich residues.]Mineral-melt distribution coefficients as in Fig. 19.

shows this relatestosources with higherSrcontentsthan those oftheleucogranodiorite (Fig. 15b). Usingthe parameters determined from the REE modellingand theSrandY data, the sourceofthe tonalitesquartz diorites hadan Sr content of 351-364 p.p.m.andaY content of2531p.p.m., whereas the leuco-granodiorite source had an Sr content of 99-132p.p.m. andaY content of 26-30 p.p.m.EXPERIMENTAL STUDIESAs interstitial pore fluidisunlikelyto be presentinthe lower crust, partial melting is consideredtooccur in fluid-absent conditions. Consequently,experimental studies, post-1990, have concentratedon the melting behaviourofhydrous rocks acrossabroad rangeoftemperatureandfluid saturationatpressures equivalentto the lower crustordeeperinsubducted slab. Low melt fraction liquid composi-tions relevant to the Cordillera Blanca magmas have

been determined for amphibolite, garnet amphi-bolite and eclogite sources (Beard & Lofgren, 1991;Rapp et al., 1991; VanderLaan & Wyllie, 1992;Rushmer, 1993; Sen & Dunn, 1994;Rapp &Watson, 1995). Thus partial meltingofmafic crustwith the mineralogies given above has been proposedas a common mechanism for the large-scalepro-duction of tonalitic continental crust m agmas,aswell as the high-Na magmas (TTG) of theArchaean. These magmas can be distinguished frompost-Archaean magmas by their strongly fraction-ated REE patterns anddepleted HR EE (Ma rtin,1987), reflecting the presence of garnet and/oramphibole as important residual phases in the source(Arth etal., 1978). The liquids produced in theexperimentsarebroadly similartoArchaean T TGrocks and some more recent 'arc' magmas includingthose of the Cordillera Blanca (Fig.5,Sen & D unn,1994; Rapp & Watson, 1995). Residues formedduring the experimental studies for fluid-absentmelting andwater-undersaturated melting to 1-2GPa include amphibole, clinopyroxene, garnetandplagioclase. These are similarto assemblages foundin lower-crustal xenoliths froma variety of areas(Toft etal.,1989; Rushmer, 1993; Rapp & Watson,1995). Partial melting of such mafic rocks occursbya few impo rtant peritectic reactions ( Rap p &Waton, 1995). Garnet formsby thebreakdownofamphibole plus plagioclase under fluid absent condi-tionsatpressuresof12-18 kbar:

amph plag qtz=garnet + cpx + melt + newplagioclase (albitic).

At the bulk compositions of these experiments(Rushmer, 1993)amphibole remains stable withgarnet up toatleast 15 kbar. At higher pressure (upto ~22kbar) plagioclase andamphibole are de-stabilized as garnet and clinopyroxene are stabilizedto eclogitic assemblages. The presenceofgarnetinthe experimental studiesat pressures from12 to 18kbar has a major impact on melt composition,causing an increase ofSiand decrease in Fe, Mg,Tiand Ca, and the production of a high-Al type char-acter (Rushmer, 1993). These are preciselythecharacteristics notedinthe leucogranodiorites of theCordillera Blanca Batholith compared withequivalent rocksofthe Co astal Batholith (Fig.6),where the mafic parental magmas were generated bypartial melting ofspinel peridotite within the mantlewedge (Atherton & Sanderson, 1985). For less acidiccompositions (2) these rather subtle dis-tinctions areless clear, indicating that theleuco-granite magmas mark the incoming of significantresidual garnet in the source.

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JOURNAL OF PETROLOGY V O L U M E37 N U M B E R 6 DECEMBER1996

GEOPHYSICALCONSEQUENCES OF REEMODELLINGUsing the mineral proportions of the residuesobtained from the REE modelling,it ispossibletoestimate the physical propertiesofthe sourceoftheCordillera Blanca magmas, and to compare theseestimates with measured geophysical propertiesofthe lower crust. The residues for both quartzdioritestonalites and leucogranodiorites aremadeup mostlyofclinopyroxene, amphibole and g arnet,in varying proportions with minor quartz andfeldspar (Figs 18 and 19). Estimated densities [usingpressure corrected values from Christenscn (1982)]of the solid residues range from ~2-9 5 g/cm for thetonalite source to 3-01 g/cm3 for the leuco-granodiorite source, the latter (higher) valuereflecting the greater amount of garnet These valuesare closetothe measured value of 3-0 g/cm forthemidtolower crust (Jam es, 1971; Fukaoetal.,1989,and Fig. 2). The seismic velocityof the residue canbe estimated from measured P- and S-wave velocitiesfor individual minerals (Toft et al., 1989; Rushmer,1993). When expressedintermsof Poisson's ratio,our estimated valuesof0-26for the quartz diorite-tonalite residueand0-27for theleucogranodioriteresidue compare favourably with measured lower-crustal valuesof~O29 (James, 1971).

Seismic velocities {Vp)of mantle-derived maficunderplate have been estimatedatbetween 7-0 and8-1 km/s, depending upon chemical composition andthermal structure (Furlong & Fountain, 1986). Therelatively lowmeasured P-wave velocities beneathcentra l-north ern Peru of between ~6-5 and 7-0 km/s(James, 1971)support the earlier propositionofcrust that is mafic, relatively dense and very hot.

DISCUSSIONIn our modelforthe evolution of the magmas of theCordillera Blanca Batholith the source changed widitime from that producing theearly quartz dioritesand tonalitesof the westernandsouthern margins(1410 Ma)toone producing the leucogranodioritesat ~6-5 Ma. The later magmas left residues withmore garnet than theearlier magmas, indicatingthat thesource deepened over this period and thewell-documented rapid uplift overtheperiod 26-5M a (with very rapid uplift from 14 to 5 Ma ; Sebrieret al., 1988) wascaused by thecrustal thickeningover this period. Thus the intrusionofthe batholithclosely followed crustal thickening. Kono et al.(1989), from geophysical andgeological evidence,considered that the crustal thickening in the Western

Cordillera wasessentially by magmatic addition.Their model, which contains elements of earliermodels, e.g. those of Jam es (1971) an d Suarez etal.(1983), envisageda partitioningof the deformationso that tectonic thickening dominatedinthe EasternCordillera, whereas magma accretion dominatedinthe Western Cordillera. However, magmatismwasnot confinedtothe western Andes,asshownby theextensive volcanism across the Altiplano.Asimilarcontinuous, dual model for crustal thickening for theSouth Central Andes (~21S) has recently been putforward by Schmitz (1994).

Over the period 26-6Maplate convergence wasshallow (10-30) and fast, ~10cm/yr (Pilger, 1984).This would generate magma acrossawide zone froma mantle which was undergoing marked secondaryconvection and large-scale hyd ration producingamphibole, etc. (Tatsum i, 1989; Peacock, 1993).This is important in replenishment of themantlewedge (Thorpe et al., 1981) aswell as promotingpartial melting (Davies & Bickle, 1991). Suchaprocess could also be effectivein transporting litho-spheric material from the internal partsofthe con-tinent. Kono et al. (1989) considered that magmagenerated under these conditions was responsible forthe crustal thickening.The amount of material needed to producethethickening over the period 255 Ma by this methodaloneis large (see Konoet al., 1989; Petford et al.,1996). However, twoaspects of theproblemarerelevant. First, assessments of the magmatic additionto crust have commonly used volumesofvolcanicand plutonic rocks seen at the surface [see Kono etal.(1989)for areview]. Thisislikelyto be anunder-estimate, and could be grossly wrongifwe considerthat mande-derived magmas may well be arrestedatmajor density interfaces, especially at convergentmargins (Herzbergetal., 1983). Indeed, the notableabsenceof any basalticandesitic syn-or late-plu-tonic dykes (incontrast to the Coastal Batholith)and the large, essentially granitic batholith,as wellasthestrong Oligo-Miocene volcanism (Sebrier etal., 1988) associated with MASH (melting, assim-ilation, storage, homogenization; Hildreth &Moorbath, 1988) processesatdepth have been usedto infer, from thermal arguments, major basaltstalling in the deep crust.Second, there is strong evidence today for theanomalous thermal state of thecrust in Peru.Astriking feature of the James (1971) model is the lowshear velocity in the upper mantle beneaththeAndes (4-25-4-30 km/s).Hotupper mantlein thiszoneis in accordance withthehigh electrical con-ductivity zone (0-1 S/m)following the mountainrange just to the south of the Cordillera Blanca

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PETFORDANDATHERTON CORDILLERA BLANCA BATHOLITH, PERU

(Schmucker, 1969). It lies ~50 km below thepresent surface and may be correlated with a low-velocity zone (Ocola & Meyer, 1972) below 40 km.Conductivity values are similar to those in the Basinand Range province of North America, which Bott(1982) thought likely to be a melt fraction in highlyconducting rock. Aqueous fluids are ruled out atthese depths. A hot crust above the hot upper mantleis indicated by the high heat flow measurementsacross this zone and the Altiplano (40-60C/km,Uyeda & Watanabe, 1970), and active thermalsprings along the line of the Cordillera Blanca faultsystem.

The conjunction of rapid uplift, extension, rapidheating of the lower crust, increased magmatismover the period 26-5 Ma and the extensional col-lapse is consistent with some form of delamination or'catastrophic' mechanical thinning of the lithospherebeneath the Western Cordillera [see Dewey (1988),who considered only three places world wide, ofwhich the Andes is one, where delamination isoccurring]. It is also consistent with the undefinedMoho in the Western Cordillera further south at2124S (Schmitz, 1994). Isostatic uplift is generatedwhen hot, less dense asthenosphere replaces cold,dense lithosphere. This results in basaltic accretionin the lower crust generated by decompressionpartial melting of asthenosphere (Nelson, 1992), fol-lowed by considerable lower-crustal fluid-absentmelting. Under such lithospheric regimes up to 25km of basaltic material may be accreted, by under-plating, to the crust (Furlong & Fountain, 1986). Inthe region northeast of Lima, where the crust is ~55km thick, the magmatic addition would be ~20 km,assuming a pre-26 Ma crust of 35 km.

According to Pilger (1984) and Wortel (1984),slab dip in Northern Peru progressively shallowedfrom ~30 at ~25 Ma to 10 today. This wouldaccount for the lithospheric erosion indicated by theuplift, etc. Melts from a dynamically thickeningcrust should show a sequence indicating theincoming of garnet and the disappearance of plagio-clase as a residual phase. Diagnostic trace elementsignatures of delamination-related magmas in thickcontinental crust (Kay & Kay, 1991), high La /Ybjvand Sr/Y ratios, and the lack of a negative Euanomaly mirror this process and are characteristic ofthe rocks of the Cordillera Blanca.

Thus we envisage that the dynamic evolution ofmagma type relates to the building of the thickcrustal keel. Underplated mafic magmas from ~25Ma or earlier produced the proto keel of the WesternAndes. Continued accretion, possibly with a com-ponent of magmatic over-accretion, i.e. magmapiercing the Moho to form a layered lower crust,

would displace the new root down through garnet-into finally produce the garnetiferous rocks at pressuresat depths of >40 km, at the base of the crust. Overthe period 19-3 Ma major plate movement occurred,which produced strike slip, transtensional structuresof crustal dimensions, i.e. the Cordillera Blanca faultsystem (Petford & Atherton, 1992), which tappedbasic magmas at 14 Ma and ~ 10-9 Ma, and finallyNa-rich granitic magmas at ~65 Ma in thethickened, garnetiferous deep crust (>50 km) (Fig.20).In this model we envisage little residence time ofthe basaltic material in the crust before partialmelting. This is compatible with the isotopic data,which are indistinguishable from the mantlereservoir defined by Til ton & Barreiro (1980) incentral Chile.

The rocks of the Cordillera Blanca are in manyrespects similar to Archaean trondhjemites, but wemaintain here that the source is not subductedocean crust as suggested by Martin (1987). Magmaswith apparent 'Archaean' chemical signatures canbe produced in a post-Archaean setting in deepcrust (~50 km) where garnet and amphibole arestable.Thus in Peru Na-rich rocks of the CordilleraBlanca Batholith occur over thick crust and inboardof the more common tonalite-granodiorite-granitesuites of the Coastal Batholith lying over thin crust.The magmas formed from newly underplatedmaterial by a multistage process which involved theproduction of hydrous basalt from the hydratedmantle wedge (Peacock, 1993), which was quicklymelted after the formation of an underplate to formthe Cordillera Blanca Na-rich suite. The CordilleraBlanca Batholith is not 'S' type as previously sug-gested; peraluminosity is slight and related to high-level deformation. It is a modified T type anddiffers in chemistry, isotopic composition and pro-portion of acid (>70 SiC ) material from theCoastal Batholith, which lies nearer the continentalmargin. Simplified models of granitic belts arc notuseful in modelling circum-Pacific belt magmatism.Sources may be in 'thick' or 'thin' crust and vary intime as well as in space, laterally and longitudinallywithin the Andean belt. Continental margin bath-oliths are dynamically related to major crustalevents such as rifting in the case of the CoastalBatholith (Atherton, 1990) or magmatic accretionand uplift in the case of the Cordillera Blanca.There is no simple, common pattern at Pacific con-tinental margins.

ACKNOWLEDGEMENTSWe thank Marge Wilson and Andrew Saunders forthorough, thoughtful, incisive reviews. We are

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JOURNA L O F PETROLOGY V O L U M E 3 7 N UM BE R 6 DECEM BER 1996

Peru 9S W CoastWesternCordillera EasternCordillera

Cordillera Blanca

Nazca Plate> 30 km

| New, Miocene crustaiunderplate,source of tona litesource of granite

Fig. 20. Schematic representation of the crustai evolution in the region of the Cordillera Blanca Batholith near 9S during late Miocenetimes based on the geophysical data of Couch et al. (1981), who defined a three-layer crust. Crustai thickening and uplift was at amaximum during the period 15-3 Ma [see Kono it al. (1989) for a review], coinciding with the emplacement of the Cordillera Blancabatholith, whose source is this newly thickened crust made of hydrated basalt. The existence of a mafic underplate is consistent withgeophysical and geochemical data. Major tectonic thickening was confined to the eastern Andes (Kono it al., 1989). The scheme showsthat the crust thickens with time and thus that the source of the magmas becomes deeper, such that the tonalitic melts were generated atlower pressures than the later leucogranodioritic melts, which formed from a more garnet-rich source.

grateful to Alex Halliday, Barbara Barreiro andRichard Greenwood for providing support in radio-genic and stable isotopic analysis, to Helen Orme forthe modelling, and Kay Lancaster for most of thedraughting. Special thanks are due to Marge Wilsonfor patient and encouraging support. The errors stillremaining are, of course, ours.

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Atherton, M. P., 1984. The Coastal Batholith of Peru. In:Harmon, R. S. & Barreiro, B. A. (eds) Andean Magmatism:Chemical an dIsotopitConstraints. Nantwich, UK: Shiva, pp. 168-179.

Atherton, M. P., 1990. The Coastal Batholith of Peru: the productof rapid recycling of new crust formed within rifted continentalmargin.Geological Journal 25, 337-349.

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Borthwick, J. & Harmon, R. S., 1982. A note regarding C3Fj asan alternative to BrF 3 for oxygen isotope analysis. GcoMmiea ttCasvwchimicaActa46, 1665-1668.

Bott, M. P. H., 1982. Tht Interior of tiu Earth: its Structure,Constitution andEvolution. London: Arnold, 403 pp.

Brown, G. C , H ughes, D. J. & Esson, J., 1973. New X .R.F. dataretrieval techniques and their application to U.S.G.S. standardrocks.Chemical Geology11, 223-229.

Cartwright, I., Valley, J. W. & Hazelwood, A. M., 1993.Resetting of oxybarometers and oxygen isotope ratios in granu-lite fades orthognciiicj during cooling and shearing.Adirondack Mountains, New York. Contributions to Mintrologyand Petrology113, 208-225.

Chappell, B. W. & Stephens, W. E., 1988. Origin of infracrustal(I) type granite magmas. Transactions of tht Royal Sociity ofEdinburgh Earth Scitnci 79, 71-86.

Chappell, B. W. & White, A. J. R., 1974. Two contrasting gran-ite types.Pacific Geology 8, 173-174.

Christenjen, N. I., 1982. Seismic velocities. In: Carmichael, R. S.(ed.) Handbook ofPhysical Pnptrtits ofRocks 2. Boca Raton, FL:CRC Press, 228 pp.Clayton, R. N. & Mayeda, T. K., 1963. The use of brominepentafluoride in the extraction of oxygen from oxides and silic-ates for isotopic analysis. GeochimUa it Cosmochimica Ad a 27 , 4352 .

Cobbing, E . J., 1985. The tectonic setting of the Peruvian Andes.In: Pitcher, W. S., Atherton, M . P., Cobbing, E . J. &Bcckinsale, R. D. (eds) Magmatism at a Platt Edgt: tht PtrumanAndts. Glasgow: Blackie Halstead Press, pp. 167-176.

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