2009 Aleksandar Mišković

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Geological Society of America Bulletin

doi: 10.1130/B26488.12009;121;1298-1324Geological Society of America Bulletin

SchalteggerAleksandar Miskovic, Richard A. Spikings, David M. Chew, Jan Kosler, Alexey Ulianov and UrsPeruvian Eastern Cordilleran granitoidscharacterization and zircon U-Pb geochronologic constraints from theTectonomagmatic evolution of Western Amazonia: Geochemical

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1298

ABSTRACT

The results of a coupled, in situ laser

ablationinductively coupled plasmamassspectrometry (LA-ICP-MS) U-Pb study on

zircon and geochemical characterization of

the Eastern Cordilleran intrusives of Peru

reveal 1.15 Ga of intermittent magmatism

along central Western Amazonia, the Earths

oldest active open continental margin. The

eastern Peruvian batholiths are volumetri-

cally dominated by plutonism related to

the assembly and breakup of Pangea dur-

ing the Paleozoic-Mesozoic transition. A

Carboniferous-Permian (340285 Ma) conti-

nental arc is identified along the regional

orogenic strike from the Ecuadorian border(6S) to the inferred inboard extension of

the Arequipa-Antofalla terrane in southern

Peru (14S). Widespread crustal extension

and thinning, which affected western Gond-

wana throughout the Permian and Triassic

resulted in the intrusion of the late- to post-

tectonic La MercedSan Ramn-type ana-

tectites dated between 275 and 220 Ma, while

the emplacement of the southern Cordillera

de Carabaya peraluminous granitoids in the

Late Triassic to Early Jurassic (220190 Ma)

represents, temporally and regionally, a sepa-

rate tectonomagmatic event likely related to

resuturing of the Arequipa-Antofalla block.Volcano-plutonic complexes and stocks asso-

ciated with the onset of the present Andean

cycle define a compositionally bimodal

alkaline suite and cluster between 180 and

170 Ma. A volumetrically minor intrusive

pulse of Oligocene age (ca. 30 Ma) is detected

near the southwestern Cordilleran borderwith the Altiplano. Both post-Gondwanide

(30170 Ma), and Precambrian plutonism

(6911123 Ma) are restricted to isolated

occurrences spatially comprising less than

15% of the Eastern Cordillera intrusives.

Only one remnant of a Late Ordovician intru-

sive belt is recognized in the Cuzco batholith

(446.5 9.7 Ma) indicating that the Fama-

tinian arc system previously identified in Peru

along the north-central Eastern Cordillera

and the coastal Arequipa-Antofalla terrane

also existed inboard of this parautochtho-

nous crustal fragment. Hitherto unknownoccurrences of late Mesoproterozoic and

middle Neoproterozoic granitoids from the

south-central cordilleran segment define

magmatic events at 691 13 Ma, 751 8 Ma,

985 14 Ma, and 10711123 23 Ma that

are broadly coeval with the Braziliano and

Grenville-Sunss orogenies, respectively.

Our data suggest the existence of a continu-

ous orogenic belt in excess of 3500 km along

Western Amazonia during the formation of

Rodinia, its early fragmentation prior to

690 Ma, and support a model of reaccretion

of the Paracas-Arequipa-Antofalla terrane to

western Gondwana in the Early Ordovicianwith subsequent detachment of the Paracas

segment in form of the Mexican Oaxaquia

microcontinent in Middle Ordovician. A tec-

tonomagmatic model involving slab detach-

ment, followed by underplating of cratonic

margin by asthenospheric mantle is proposed

for the genesis of the volumetrically dominant

Late Paleozoic to early Mesozoic Peruvian

Cordilleran batholiths.

INTRODUCTION

Whereas the Cretaceous to recent Andean

orogenic cycle is well characterized (e.g.,Ramos and Aleman, 2000), our knowledge ofthe early Phanerozoic and Proterozoic evolu-tion of the Andes becomes increasingly frag-mentary with age due to paucity of exposedlithologies. The problem is less pronouncedalong the Peruvian segment of the orogenwhere a lacuna in the ubiquitous Cenozoic vol-canic cover is interpreted to have resulted fromthe flat-slab subduction of the Nazca ridge dur-ing the Neogene (Jaillard et al., 2000). Batho-liths of the Eastern Cordillera of Peru straddlethe tectonic boundary between the Western

Amazonian tectonic provinces of San Ignacio(1.571.24 Ga) and Sunss (1.190.92 Ga;Cordani and Sato, 2000) and parautochthonousto allochthonous crustal domains (1.91.8 GaArequipa-Antofalla; 150 Ma Olmos-Amotapeterrane), thus providing a continuous record ofthe nature and rate of crustal growth at a long-lived cratonic margin. Despite its fortuitoussetting, however, the timing of magmatism inthe central Andes is relatively poorly under-stood with most of the geochronological workto date relying heavily upon whole-rock Rb-Srand K-Ar techniques, both of which are knownto yield ambiguous dates due to low retention

temperatures and the possibility of isotopicdisturbance by subsequent thermal episodes(Dodson, 1973). This is a particularly acuteproblem in Peru when we consider ca. 150 Maof uninterrupted subduction during the lastAndean orogenic cycle (Benavides, 1999).

We use a combination of in situ U-Pb geo-chronology, major- and trace-element geochem-ical characterization of plutonic rocks along the1400 km of the orogenic strike of the Eastern

For permission to copy, contact [email protected] 2009 Geological Society of America

GSA Bulletin; September/October 2009; v. 121; no. 9/10; p. 12981324; doi: 10.1130/B26488.1; 12 figures; 1 table; Data Repository item 2009052.

*Current address: Earth, Atmospheric, and Plan-etary Sciences, Massachusetts Institute of Technol-ogy, 77 Massachusetts Avenue (54-1224), Cambridge,Massachusetts 02139, USA.

E-mail: [email protected]

Tectonomagmatic evolution of Western Amazonia:Geochemical characterization and zircon U-Pb geochronologic

constraints from the Peruvian Eastern Cordilleran granitoids

Aleksandar Mikovi1,*,, Richard A. Spikings1, David M. Chew2, Jan Koler3, Alexey Ulianov4, and Urs Schaltegger11Department of Mineralogy, Earth Sciences Section, University of Geneva, 13 rue des Marachers, CH-1205 Geneva, Switzerland2Department of Geology, Trinity College Dublin, College Green, Dublin 2, Ireland3Department of Earth Sciences, University of Bergen, Allegaten 41, N 5007 Bergen, Norway4Institute of Mineralogy and Petrography, University of Lausanne, BFSH 2, CH-1015 Lausanne, Switzerland

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U-Pb geochronology and geochemistry of the proto-Andean granitoids of Peru

Geological Society of America Bulletin, September/October 2009 1299

Cordillera of Peru to construct a detailed chrono-logic framework, and identify tectonic regimesthat shaped the central proto-Andean margin ofAmazonia. By relating the secular changes inmagma composition to the tectonomagmaticcycles of continental assembly and breakupover the past 1.1 Ga along the western Rodinia

and Gondwanaland, we can holistically test thecurrent geodynamic scenarios for the evolutionof the Amazonian shield, with particular focuson the poorly understood breakup of Rodinia(Cordani et al., 2003; Loewy et al., 2003; Meertand Torsvik, 2003; Fuck et al., 2008; Li et al.,2008; Ramos, 2008).

Our data demonstrate the existence of a com-posite continental margin heavily dominatedby three distinct intrusive pulses related to theassembly and breakup of Gondwana (middleCarboniferous to Late Triassic), together withvolumetrically subordinate plutons emplacedduring the initiation of the modern Andean

cycle of subduction in the Early Jurassic. Plu-tonic remnants belonging to the early PaleozoicFamatinian, middle Neoproterozoic Braziliano,and the late Mesoproterozoic Sunss orogensare located in south-central Peru and indicatethe presence of a periodically reworked latestMesoproterozoic crust, 200 km east of the con-firmed limit of the Arequipa terrane and 225 kmfrom the present-day coast, making them thewesternmost exposures of the Amazonia cratonin South America.

GEOLOGICAL SETTING

Morphogeological Units

The Peruvian Andes have been historicallydivided into six linear geological provincesstriking parallel to the Pacific coast. From westto east, these are (1) Coastal forearc, (2) West-ern Cordillera, (3) Puna-Altiplano, (4) EasternCordillera, (5) Sub-Andean fold and thrust belt,and (6) Foreland basin (Fig. 1; Jaillard et al.,2000). The Peruvian Eastern Cordillera is a1400-km-long belt straddling the east-vergingthrust and fold belts of the Maran, Ticlio, andMaazo complexes to the west and a 120- to

250-km-wide zone of deformed Mesozoic andTertiary foreland sedimentary rocks to the east(Fig. 2). Its basement lithologies are predomi-nantly comprised of the Paleozoic Marangreenschist facies metasediments, which under-went four successive stages of deformation(Bard et al., 1974; Mgard, 1978; Zeil, 1983).There is a general trend of increasing metamor-phic grade to the south resulting in a transitionfrom gray phyllites with subordinate graphite-mica schist intercalations in the north to poly-deformed paraschists and granulitic gneisses

located 90 km east of Hunuco in the centralsegment (Cardona et al., 2006). Whereas U-Pbgeochronology on detrital zircons from theMaran transect between 6S and 10S effec-

tively constrains the depositional age of thesedimentary protolith between the Early Ordo-vician and Carboniferous (Chew et al. 2007),the timing of emplacement of the numerousbatholiths that intrude the metasedimentarysubstrate and form a 1400-km-long intrusivechain between the two Andean oroclines, theHuancabamba deflection and Arica bend, ispoorly understood.

Regional Basement

Despite lacking direct geochronologicalevidence, the Central Andes of Peru have

been inferred to rest upon the westernmostAmazonian craton, which formed parts of thelong-lived Mesoproterozoic Sunss orogen(1.20.95 Ga) that resulted from the collisionof Laurentia with Amazonia (Fig. 1; Ramoset al., 1986; Sadowski and Bettencourt, 1996;Tosdal, 1996; Jaillard et al., 2000). The parau-tochthonous granulitic basement of the south-western Arequipa-Antofalla terrane (Shackletonet al., 1979) was initially thought to representthe apex of a Laurentian promontory (togetherwith Labrador, Greenland, and Scotland) that

was subsequently detached from Laurentia andincorporated into the Grenville-Sunss orogen(Dalziel et al., 1994). Its overall allochthonoucharacter to cratonic South America was recon

firmed based on U-Pb zircon geochronologyand Pb isotope systematics (Loewy et al., 2004)although the exact relationship to Laurentiaremains unclear. The tip of the Huancabambadeflection along the northwestern coast of Peruis dominated by the Amotape continental parautochthonous block and floored by oceaniccrust where the intervening Lancones synclinorium separates the Amotape complex from theLoja-Olmos massif (Fig. 1; Mourier et al., 1988Spikings et al., 2005). A notable exception to thedominantly Proterozoic basement architectureof Peru is the isotopically juvenile root of theLima segment of the Peruvian Coastal batho

lith, which does not yield Amazonian basemensignatures (Mukasa and Tilton, 1985; Macfarlane et al., 1990; Petford et al., 1996). Togethewith the absence of inherited zircon ages inthe Western Cordillera between Chimbote andPisco (Mukasa, 1986), the juvenile isotopicratios have been explained as manifestation of afundamental change in the age and nature ofbasement rock, and the absence of cratonic crushas been proposed west of the Eastern AndeanCordillera between 7S and 14S (Beckinsaleet al., 1985; Polliand et al., 2005).

10o o

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Rio

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illera

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Phanerozoic platform

sediments

Sunssu

s

s

O Almos- motape 0.16-0.15 Gamos motape 0 16 0 15 Ga

F M Bamatinia obile elt 0.48-0.43 GaM Bmatin ia obile el t0 48 0 43Ga

S Pierras ampeanas 0.53-0.48 GaPerras ampeanas 0 53 0 48 Ga

A Arequipa- ntofalla 1.8-1.9 Gaequipa ntofalla 1 8 1 9 Ga

C Thilenia errane 0.46-0.36 GaTi lenia errane 0 46 0 36Ga

Juruenauruena

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amba

deflection

Figure 1. Map of the central South American continent with the generalized major tectonic

provinces and ages of their most recent tectonothermal episodes. Black triangles represent-

ing volcanic centers of the Northern and Central Volcanic Zone are superimposed on the six

morphogeological belts of Peru. Modified after Tassinari and Macambira (1999), Cordan

and Sato (2000), and Loewy et al. (2004).



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Mikoviet al.

1300 Geological Society of America Bulletin, September/October 2009

Nazca

Ridge

BRAZIL

BOLIVIA

12 So

6o

8o

10 So

Peru

-Chile

T

rench

AM

OT

APE

SANNICOLAS

BATH

OLITH

BALSAS

A-A TERRANE

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Cuzcouzco

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Titicaca

i

t

i

c

c

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Huallaga

Ro

Maran

WESTERN A MAZON IA

TARMA

CORD.DECARABAYA

PAUCARTAMBO

TICLIO

SIRA

SANTAROSA

ALLINCAPACLIMBANI

ABANCAYDEFLECTION

QUEROBAMBA(SUCRE)

HUANCAVELICA

HUANCAYO

CARRIZALES

SANRAMON

OXAPAMPACR.PASCO

HUANUCO

SITABAMBA

PATAZ

ALTIPLANO

WESTERNCORDILLERA

FORELAND

COASA

PTP

Transform

PARACAS

0 100 200 300 400 km

Paleozoic basement(Maraon phyllites, paraschists)

Ordovician- Early Silurian granitoids

Late Triassic Carabaya granitoids

Early Jurassic fold / quartz syenites

Neo/Mesoproterozoic granites

Carbo-Permian rhyodacites/I-type diorites - granitoids

Permo-Triassic granodiorites/S-type granites-monzogranitoids

76o 70 Wo72o74o78o80o

Triassic Mitu Gr volcano-sediments

Lima

Tarma

SanRamon

Sira

AnticlineUcayali Basin

EASTERNCORDILLERAALTIPLANOWESTERNCORDILLERACOASTALACCR.WEDGE THRUST-FOLDBELT FORELAND

10km

0

N

Figure 2. Geologic map of the Eastern Cordilleran plutonic belt of Peru illustrating the presently known extent of tectonic domains and

locations of the sampled intrusives. The upper Rio Maran of the northern Eastern Cordillera marks the inferred boundary between

Western Amazonia and craton-free zone underlying much of the coastal Western Cordillera (Haeberlin, 2002). The cratonic edge east

of the thickened Altiplano crust is suggested to lie along the Mitu Group (Gr.) basin (Sempere et al., 2002), while the eastern limit of the

Arequipa-Antofalla terrane is currently defined by 206Pb/204Pb isotopic contrast from the Neogene volcanic centers sampling the southern

Peruvian crust (Mamani et al., 2005). The E-W fault system subparallel to the Abancay deflection at the latitude of the Paracas Peninsula

(14S) bounds the AA terrane from the north. The NNE-striking tectonic lineament east of the Lancones basin in the NW coastal Peru

demarcates the extent of the Loja-Olmos terranes (Litherland et al., 1994). Geological cross section modified after Mgard (1967).



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Neoproterozoic Evolution

Following a poorly defined, ca. 300 Ma,post-Sunss tectonic lull, the breakup of Rodiniain the middle Neoproterozoic led to the open-ing of the Iapetus Ocean by westward drift ofLaurentia leaving the Arequipa craton attached

to Amazonia (Bond et al., 1984; Cawood et al.,2001). Subsequent westerly shift of the spread-ing center is interpreted to have resulted in thegeneration of intervening oceanic crust betweenboth the Arequipa-Antofalla and Pampean ter-ranes and mainland Amazonia that was con-sumed by eastward subduction of proto-Pacificlithosphere during the Pampean orogeny in theMiddle Cambrian (Rapela et al., 1998). Daciticdikes that crosscut layered amphibolites ofthe north Chilean segment of the Arequipa-Antofalla block were dated at 635 Ma, and arecurrently the only local temporal constraintson the timing of Arequipa detachment (Loewy

et al., 2004). During the extensional phase, apassive margin developed along the westernproto-Gondwanan margin while rifting propa-gated northward into present Bolivia in thelate Neoproterozoic times, eventually formingthe epeiric Puncoviscana depocenter for thickmarine sediments behind the parautochthonousArequipa-Antofalla craton (Keppie and Bahl-burg, 1999). Together with the Maran Riverlineament of north-central Peru, the paleo-suturebetween the Arequipa block and ancestral Ama-zonia constitutes a zone of crustal weakness thatwas inherited from the Sunss-aged orogens and

was periodically reactivated during the Phanero-zoic (e.g., Permo-Triassic extension and depo-sition of the volcano-sedimentary Mitu Group;Kontak et al., 1985, 1990; Forsythe et al., 1993;Sempere et al., 2002).

Phanerozoic Evolution

Beginning in the Early Cambrian, the west-ern margin of Amazonia was again character-ized by compressional tectonics due to globalplate reorganization and the final amalgamationof Gondwana (Cawood and Buchan, 2007). Thecollision of the Pampean terrane and reaccretion

of the Arequipa-Antofalla crustal block againstthe Ro de la Plata and Amazonian cratons,respectively, in the Early Cambrian closed thePuna aulacogen (Ramos, 2008). The resultantSierras Pampeanas orogeny of NW Argentinawas accompanied by low-grade metamorphismand deformation of shallow marine sediments,followed by emplacement of the post-orogenic,calc-alkaline Santa Rosa granitoids, andcoeval rhyodacitic pyroclastics (534523 Ma;Adams and Miller, 2007) as incipient mag-matic responses to a collapsing orogen (Rapela

et al., 1998). Although no clear chronometricevidence of the Pampean orogen presentlyexists in Peru, the diachronous rift-drift transi-tion recorded in clastic platform sedimentaryrocks at the latitude of the northern Arequipa-Antofalla terrane suggests the end of an exten-sional regime by the Late Cambrian (Sempere,

1995). Magmatism soon resumed in the Ordo-vician along the frontal Arequipa terrane givingrise to the coastal San Nicols batholith (Fig. 2),which is interpreted to represent the root of acontinental arc (i.e., Faja Eruptiva Occiden-tal) resulting from eastward subduction of theproto-Pacific oceanic crust beneath westernGondwana (Ramos, 1988; Mukasa and Henry,1990). A recent, single grain, U-Pb zircon geo-chronological survey of the San Nicols intru-sives (Loewy et al., 2004) yielded Ordovicianintrusive ages between 468 and 440 Ma that aretypical of the Famatinian orogenic cycle, whichdeveloped along the eastern Sierras Pampeanas

(Pankhurst and Rapela, 1998). The apparenteastward transposition of the Ordovician igne-ous belt north of the Arequipa-Antofalla terranein Peru is suggested by the presence of LatestOrdovician granodioritic gneisses within theRo Maran valley east of Cajamarca (Fig. 2).These are interpreted to mark the presence ofeither an original embayment along the westernGondwanan margin (Chew et al., 2007), or agap left by removal of Oaxaquia microcontinentleaving behind a speculative Paracas terrane,which collided with western Gondwana duringthe Famatinian cycle (Ramos, 2008).

A conspicuous lack of magmatic activityduring much of Late Silurian and Devonianin the central Andes, as well as the absence ofGrenvillian terranes along the margin from theArequipa massif to Chibcha terrane in Colom-bia has been interpreted as evidence for thedevelopment of a passive margin west of theFamatinian arc resulting from strike-slip detach-ment of the northern segment of the Arequipa-Antofalla (i.e., Oaxaquia) block during eitherthe Devonian (Mgard, 1973; Bahlburg andHerv, 1997) or the Permo-Triassic extension(Haeberlin et al., 2004). Resumption of arcactivity in the Early Mississippian was first

recorded by Mgard et al. (1971) and Dalmayracet al. (1980), who described arc magmatism inthe context of an Eohercynian orogeny, but thearc activity was only recently documented withcertainty in the Balsas-Callangate-Pataz aurifer-ous province of the northern Eastern Cordillera,based on 40Ar/39Ar geochronology of plutonichost rocks and ore deposits (Schreiber, 1989;Snchez, 1995; Haeberlin et al., 2004; Mikoviet al., 2005). These intrusives were emplacedsubparallel to the Rio Maran crustal linea-ment (Fig. 2), and appear to have undergone a

phase of regional uplift in the Early Pennsylvanian, associated with the regional orogenictype Au-Ag mineralization (Haeberlin, 2002).

Late Permian to Early Jurassic lithosphericthinning in Peru and Bolivia resulted in emplacement of the central San RamonLa Merced-typemonzogranitoids, associated with localized mig

matization (Soler, 1991). The plutonism predatedor was contemporaneous with synrift depositionof calc-alkaline, Mitu Group bimodal volcanicand continent-derived sediments in transcurrenhalf grabens and pull-apart basins that openedalong the western margin of the orogen (Figs. and 2; Sempere et al., 2002). The geneticallysimilar but younger pulse of Late Triassicsyntectonic, peraluminous granitic plutonismoccurred in a localized transpressional settingin the southern Cordillera de Carabaya, and wasbroadly coeval with mantle-derived Mitu Groupalkali basalts and shoshonites of the Cuzco basin(Kontak et al., 1985, 1990). With the onset of the

modern Andean tectonic regime, and renewedeasterly subduction of the protoPacific platebelow the western South American margin in theEarly Jurassic, the newly formed continental arcdeveloped along the present-day Coastal (Western) Cordillera, thus leaving the Eastern Cordillera in a backarc position, which consequentlyaffected both the volume and type of plutonismduring the past 150 Ma. Except for the easternmost porphyritic monzonite and granitoidstocks of the Miocene metallogenic belt alongthe central and northern Western Cordillera oPeru (Noble and McKee, 1999), the inferred

Early Jurassic age for the peralkaline Allincapacvolcano-plutonic complex in the Cordillera deCarabaya of southern Peru (Fig. 2; Kontak et al.1990) makes it the only volumetrically significant intrusive locality of the Andean cycle in thePeruvian Eastern Cordillera. In summary, alternating passive margin sedimentation, volcanicarcs and rift-related magmatism, including tectonothermal events associated with both strikeslip displacements and crustal shortening ovethe past 1.0 Ga, has led to a composite magmaticbelt displaying complex lateral and along-strikevariations throughout the Peruvian cordilleras.

Previous Geochronology

A compilation of radiometric ages fromthe Eastern Andes of Peru (Jacay et al., 1999reveals that the majority of the reported dateare based either on whole-rock Rb-Sr and K-Aisotopic analyses of biotite and K-feldspar, orU-Pb chronometry of bulk-zircon separates withrelatively few, high-precision, single-grain U-Pbzircon or 40Ar/39Ar dates. Moreover, previouwork was mainly of local focus, and no attemptswere made at a regionally integrated survey. A

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review of the available data indicates three mainpulses of intrusive activity, separated in time andspace; from north to south these are Carbonifer-ous, Permian to Triassic, and Late Triassic belts(Petersen, 1999). Intrusive activity in the northernsegment of the Eastern Cordillera was initiallydated by Snchez (1983, 1995), who reported

K-Ar dates of 346.7 7.3 Ma and 329 10 Mafor the Balsas and Callangate monzogranitic plu-tons. Recent 40Ar/39Ar dating of the auriferousPataz Batholith granodiorites by Haeberlin et al.(2004) confirmed the earlier 40Ar/39Ar geochro-nology of Schreiber et al. (1990), with plateauages ranging between 329.2 1.4 Ma (biotite)and 319 3.2 Ma (hornblende). Emplacement ofthe east-central leucogranitoids was crudely con-strained by bulk-zircon U-Pb dating of Lancelotet al. (1978) and Dalmayrac et al. (1980), andK-Ar work on biotites by Soler (1991) from theEquiscocha (253 11 Ma), San Ramon (246 10 Ma), La Merced (238 10 Ma), and Talhuis-

Carrizal plutons (245 11 Ma and 233 10 Ma).The youngest pre-Andean intrusives in the Peru-vian Eastern Cordillera were identified in thesouthern segment of the Cordillera de Carabaya,where S-type monzogranitoids yielded K-Ar(biotite and muscovite) and whole-rock Rb-Srages between 225 15 Ma in the Coasa Batho-lith at 14S, and the southernmost Limacpampapluton was dated at 199 10 Ma (15S; Clarket al., 1990; Kontak et al., 1990).

SAMPLING

Five orthogonal and three margin-parallelsampling transects were conducted through-out the Eastern Cordillera of Peru between thelatitudes of 6S and 15S. Traverses varied inlength between 70 and 250 km and spanned thetotal orogenic strike length of 1300 km (Fig. 2).

Northeastern Cordillera

The northern sector was surveyed along twotraverses covering: (1) the Balsas-Callangateplutons (6S to 7S), and (2) the intrusivebelt between the Pataz Batholith, boundedby the Maran River to the west and the Ro

Huallaga to the east, between 7S and 9S. TheBalsas, Callangate, and Pataz intrusive com-plexes are emplaced subparallel to a NNW-trending fault zone associated with the upperMaran River valley and locally intrude theOrdovician Maran biotite schists and phyllites,Contaya Formation meta-arenites and Ordovi-cian to Devonian Vijus metabasalts. The plutonicrocks define a compositional spectrum frommedium- to coarse-grained, hornblende-bearingdiorites, through amphibole-rich tonalites intovolumetrically dominant, medium-grained bio-

tite, hornblende granodiorites, and minor gran-ites. Fragmented diorite dikes and mafic magmapulses intrude most of the granodioritic rocksand occur together with abundant microgranularenclaves as well as partially fused xenoliths ofthe Maran Complex. There is widespread tex-tural evidence supporting the coexistence and

mingling of compositionally contrasting mag-mas (Fig. 3A). Similar intrusive facies prevailacross the belt at 8S with the upper HuallagaRiver granitoids exhibiting variable proportions

of amphibole and biotite. A feature unique to thenortheastern margin of the Eastern Cordilleranintrusive belt is the ubiquitous presence of NW-trending bimodal dikes of the Permo-TriassicMitu Group.

Central Eastern Cordillera

The central intrusive belt, including thenorthern Altiplano, was sampled along fourtransects between the latitudes of 10S and

A B

C D

FE

Figure 3. Field photographs illustrating the typical mineralogical and structural features of

the Eastern Peruvian intrusives; (A) mingling of microdioritic and granodioritic magmas

in the calc-alkaline Pataz Batholith (8S), (B) mineralogically monotonous monzogranite of

the San Ramon pluton (11S) cut by up to 1-m-thick aplite dikes, (C) a partially absorbed

metasedimentary xenolith (restite) in the area of high-grade migmatization along the Chan-

chumayo River segment of the San RamonLa Merced batholith, (D) medium-grained

Machu Picchu biotite granite (13S), (E) a close-up photograph of the Allincapac complex

nepheline syenite (13.5S) with arfvedsonite and biotite as the principal Fe-Mg phases,

(F) Coasa Batholith K-feldspar megacrystic leucogranites (14S) displaying the classic

horse tooth texture.



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14S. Progressively from north to south,these are (1) OxapampaCerro de Pasco,(2) TarmaLa Merced, (3) Satipo-Huancayo,and (4) Huancavelica-Sucre (Fig. 2). Despitethe textural heterogeneity that is locally mani-fested by widespread migmatization along theRio Chanchumayo, or the existence of a WNW-

oriented fabric found in the Talhuis pluton aswell as in the Querobamba intrusive suite ofthe eastern Sucre region, the intrusive faciesof the central Cordilleran segment exhibit over-all uniform silica enrichment compared to thenorthern Peruvian plutonic belt. The NNW-oriented, San RamonLa Merced batholith isthe type locality of Permo-Triassic plutonism ineast-central Peru (Mgard, 1978; Soler, 1991). Itcomprises an 80-km-long and in places 10-km-wide intrusive belt composed of characteristi-cally pink-colored, K-feldspar megacrystic,biotite, and subordinate amphibole-bearinggranodiorites to leucogranites (Fig. 3B) that are

emplaced into late Carboniferous metasedimentsof the Tarma-Copacabana Group, which arelocally strongly deformed. The interior granitoidfacies of each plutonic body include mica-richrestites, together with subordinate mafic micro-granular enclaves, whereas the volumetricallyminor biotite quartz monzonites occur along themargins of individual plutonic bodies and entrainubiquitous, partially fused, metasedimentaryxenoliths (Fig. 3C). The NNE-striking, subhori-zontal aplite and fine-grained monzonite dikesindiscriminately traverse the intrusive suites butare conspicuously absent from those that dis-

play strong structural fabrics. The mineralogi-cal trends across the orogenic strike, from eastto west, are characterized by an increase in themodal amount of amphibole accompanied bya decrease in K-feldspar and a larger propor-tion of microdioritic enclaves. In contrast to thenorthern segment, the central intrusives of theEastern Cordillera are bound inboard and out-board by contiguous, transtensional grabensthat accumulated interbedded Mitu Groupbimodal volcanics (rhyodacitic pyroclastics andminor pillow basalts), together with subarkosicarenites during inferred Triassic synorogenic,continental rifting (Sempere et al., 2002).

Southeastern Cordillera

An overall sinistral offset along theENE-WSWtrending Patacancha-Tamburco-Puyentimari transform fault system duringPermo-Triassic rifting is considered respon-sible for a drastic change in strike of the Peru-vian Eastern Cordillera (Carlotto et al., 2006).The resultant displacement of the intrusive beltand the Mitu graben by 200 km eastward at thelatitude of 13S, known as the Abancay deflec-

tion (Fig. 2), corresponds with the appearanceof the thickened Altiplano crust to the west thatis interpreted as a collage of lithospheric struc-tural blocks that are stacked vertically alongE-Wdipping, doubly vergent, Miocene thrusts(Allmendinger et al., 1997). We have surveyedthe discontinuously outcropping intrusive facies

along a strike-parallel traverse over 8300 km2

starting from the northern Cuzco batholith, inbetween the two principal transform lineaments.The traverse passes through the western marginof a poorly delineated intrusive belt near Pau-cartambo, via the central San Gabn pluton, andinto the southernmost segment represented bythe Cordillera de Carabaya specifically, the west-ern Ayapata suite of the Coasa pluton (Fig. 2;Kontak et al., 1990). The Cusco and Paucar-tambo batholiths are composed of texturallyhomogeneous, medium-grained, biotite-bearing,alkali feldspar granites (Fig. 3D) intruded intothe Paleozoic basement phyllites that are equiva-

lent to the Maran basement in the northeasternPeruvian Cordillera. There is a gradual increasein the proportion of modal mica as well as thefirst appearance of muscovite in the granitoidassemblages southward of Paucartambo. Thecentrally located San Gabn biotite granite dis-plays numerous, 3- to 5-cm-long mafic enclavesand mica-rich restites. Of particular importanceis the occurrence of peralkaline intrusives suchas the Nevado de Allincapac volcano-plutoniccomplex covering 350 km2. It is composed ofporphyritic leucite, nepheline phonolites, andperalkaline volcanic breccias that are intruded

by the coarse-grained, nepheline-bearing, alkalifeldspar syenite core (Fig. 3E). The centraland southern Carabaya intrusive suite is pre-dominantly composed of medium-grained toK-feldspar megacrystic, two-mica monzogran-ites (Fig. 3F), granodiorites, and muscoviteleucogranites, occasionally containing cordieriteand locally displaying intermediate to maficfacies (diorite to quartz diorite). The Carabayagranitoids exhibit a sporadic magmatic fab-ric and are cut by sinuous and discontinuousbiotite quartz diorite dikes that are themselvestraversed by the host granitoids indicating syn-intrusive emplacement (Pitcher, 1997).

RESULTS

Zircon Characterization

A total of 738 grains extracted from 60intrusives were imaged and examined formorphology and internal textures. Representa-tive subsets were analyzed for trace-elementcontent and were dated by the U-Pb method.Ages are shown in Table 1. The results ofindividual U-Pb spot analyses are presented

in the GSA Data Repository (Table DR11)The analyzed crystals are colorless and transparent ranging in size from 50 to 250 m andwith ratios of length to width between 1:1 and3:1. In cathodoluminescence (CL) images95% of zircons exhibit well-developed, yefrequently blurred or convoluted oscillatory

growth zoning, characteristic of a magmaticorigin (photomicrograph insets in Figs. 6M6S, and 6X; Corfu et al., 2003). The zonedrims tend to discordantly overgrow CL-brightoscillatory-zoned cores. In 25% of the zirconpopulation, thin rims mantle patched or sectorzoned xenocrystic cores that are characterizedby moderate CL intensities. Recrystallizationand abundant mineral inclusions or melt trailare observed in less than 8% of the imagedgrains. All identified domains within texturallycomplex zircons such as cores, rims, or sectorswere analyzed by separate laser traverses.

U-Pb Geochronology

The U-Pb ages from the intrusive rocksof the Eastern Peruvian Cordillera fall intomore than six distinct groups, with the geographical and temporal distribution presentedin Figures 4 and 5. Plots based on 476 concordant analyses are presented in Figure 5The identified age populations are Oligocene(2834 Ma; n(grains)= 8), Early to Middle Jurassic (167174 Ma; n(grains)= 12), Late Triassicto Early Jurassic (178217; n(grains) = 48)Permian-Triassic (ca. 220270 Ma; n(grains) =

126), Carboniferous-Permian (ca. 275360Ma; n(grains) = 207); Ordovician (ca. 440530 Ma; n(grains)= 9); middle NeoproterozoicCryogenian (ca. 650770 Ma; n(grains) = 24)and a broadly defined Late Mesoproterozoicearly Neoproterozoic (Stenian-Tonian) agespan between 960 and 1200 Ma (n(grains)= 25)An additional 17 zircons, with ages clusteringaround 1300 Ma but also extending to 1700 Mado not correspond to pluton crystallizationages, but are instead interpreted as an inheritedMesoproterozoic component (Fig. 5A).

Middle JurassicOligocene

The middle Cenozoic (Andean) ages clustering near 30 Ma were obtained from twogranodiorites (SCAM-04 and 08) in the southcentral Huancavelica region of the EasternCordillera (Fig. 6A). The Oligocene zirconare long prisms displaying blurred primary

1GSA Data Repository item 2009052, In situzircon U-Pb isotope data and whole-rock geochemistry of the Peruvian Eastern Cordilleran intrusives, is available at http://www.geosociety.org/pubsft2008.htm or by request to [email protected]



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Mikoviet al.


TABLE 1. SUMMARY OF THE LA- ICP-MS IN SITU ZIRCON U-PB GEOCHRONOLOGYOF THE EASTERN PERUVIAN CORDILLERA PLUTONS (CUMULATIVE CONCORDIA AGES)

Sample Locality Latitude

S

Lithology n (analyses) Concordia age(Ma)

(2) Inheritance(Ma)

(2) Common Pb Pb loss

SCAM-04 Colcabamba-Huancavelica

12.5 Granodiorite 19 29.37 0.78 Moderate Minor

SCAM-08 Colcabamba-Huancavelica

12.5 Granodiorite 15 31.36 0.64 94 7 Minor

NAM-11a Huayillas-Pataz 8.2 Quartz syenite 19 172.9 2.0 Minor SAM-21 Chillacori-Puno 13.8 Nepheline syenite 11 184.1 3.7

SAM-20 Chillacori-Puno 13.8 Nephelinemonzosyenite

2 195 11

COCA302

Ayapata-Carabaya 13.8 Granite 11 190.3 2.5 Minor

SAM-22a San Gabn 13.7 Granite 12 191.2 3.5 MinorCOCA262

E. CoasaCarabaya 13.8 Monzodiorite 7 207.0 3.4 281 20

COCA269

E. CoasaCarabaya 14.0 Monzodiorite 13 208.4 4.9 294 25 Minor

COCA298

S. San Gabn 13.8 Alkali feldspargranite

12 216.1 3.1 274 13 Minor

CAM-11a San Ramn 11.3 Granite 11 223 12 279 21 Moderate CAM-12 San Ramn 11.3 Granite 17 265.5 7.3 Strong CAM-15 La Merced 11.2 Alkali feldspar

granite8 250.0 5.9 268 12

CAM-16 Satipo 11.0 Granite 13 256.3 4.8 309 8 Strong CAM-24 Sacsacancha 11.5 Alkali feldspar

granite10 260 18 Minor

CAM-33 Oxapampa 10.7 Granite 13 238.0 3.6 Minor MinorCAM-35 Oxapampa 10.8 Granite 17 246.8 4.6 Minor CAM-39 Rio Huachn 10.7 Alkali feldspar

granite9 227.4 3.8 351 25

CAM-45a Paucartambo-Pasco 10.7 Granite 13 258.4 4.4 288 9 NAM-05 SE Pataz 8.1 Granite 7 244.5 3.3 MinorSCAM-01 Ayacucho 13.3 Quartz monzonite 6 243.4 6.7 Strong SCAM-02 Ayacucho 13.0 Granite 21 217.8 3.5 Moderate SCAM-22 Ayacucho 13.4 Granite 7 238.4 4.9 Moderate CAM-19a Mariposa-Junin 11.4 Granodiorite 19 254.5 4.2 Minor MinorCAM-40 Rio Huachn 10.7 Granodiorite 18 260.9 4.2 289 7 COCA268

C. CoasaCarabaya 14.0 Granodiorite 5 227.7 5.6 291 35

CAM-45c Paucartambo-Pasco 10.7 Tonalite 14 253.6 6.3 296 14 SAM-08 Urubamba 13.3 Quartz syenite 8 284.8 4.6 Minor CAM-10 San Ramn 11.3 Alkali feldspar

granite2 259.7 8.1 Strong

CAM-41 Rio Huachn 10.7 Granite 18 255.6 7.4 402 11 Minor COCA

362

Limbani-Carabaya 14.2 Granite 10 227.4 5.4 278 28

COCA362

Limbani-Carabaya 14.2 Granite 468 23

COCA358

Aricoma-Carabaya 14.1 Granite 7 227.4 4.2 245 12 Minor

SAM-17 Santa RosaPuno 13.3 Rhyolitic tuff 8 226 10 SCAM-06 Colcabamba-

Huancavelica12.4 Granite 8 257.6 8.6 Strong Minor

CAM-03 Parcamayo 11.3 Alkali feldspargranite

18 315.2 4.3 Moderate Minor

CAM-26 Huancayo 11.8 Granite 5 292 20 Strong CAM-30 Parihuanca 12.0 Granite 15 284 15 343 24 Moderate CAM-49a Huachn 10.7 Granite 15 309.4 4.0 349 7 Moderate CAM-52 Tingo Maria 9.2 Granite 17 313.4 5.2 363 24 Moderate MinorCAM-54 Upper Rio Huallaga

valley8.7 Granite 17 293.3 5.0 513 17

CAM-54 Upper Rio Huallagavalley

8.7 Granite 934 26

CAM-55b Nuevo Progresso 8.6 Granite 16 316.7 5.9 Strong CAM-57 Nuevo Progresso 8.5 Granite 10 304.5 7.2 Moderate NAM-02a SE Pataz 8.1 Quartz monzonite 12 301 5.2 497 23 SAM-09 Urubamba 13.3 Alkali feldspar

granite5 291.5 5.8 351 8

SAM-12a Machu Picchu 13.2 Granite 14 324.1 5.3 MinorCAM-04 Parcamayo 11.3 Granite 19 317.4 4.4 376 7 CAM-04 Parcamayo 11.3 Granite 687 27 ModerateAM-80 Central Pataz 7.8 Monzogabbro 5 333.2 7.7 NAM-28a San Vincente

Amazonas7.0 Tonalite 16 313.5 4.5

CAM-44a Junin 10.8 Granodiorite 9 303.8 5.3 NAM-27a Balsas 7.0 Granodiorite 17 313.9 4.3 MinorNAM-30 Golln-Callangate 7.2 Granodiorite 15 313 4.3 354 8 NAM-18 West Balsas 6.9 Granodiorite 17 320.0 3.8 NAM-22 East Balsas 6.8 Granodiorite 17 309.0 4.0 1303 35 Minor

(Continued)



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zoning developed around rare planar cores.Only one inherited zircon was identified with

a U-Pb concordia age of 94 Ma. The MiddleJurassic grains were extracted from two geo-graphically dispersed localities at the southernand northern extremes of the study area (Figs.6B and 6C). The concordia age recorded by aquartz syenite sample collected at the marginof the northern Cordilleran Pataz batholith(Huayillas; NAM-11a), and two SiO

2 under-

saturated nepheline (monzo)syenitic intrusivesof the southern Allincapac volcano-plutoniccomplex in the southernmost Cordillera deCarabaya (SAM-20, 21) yielded Middle toEarly Jurassic ages between 172.9 2.0 and

184195 Ma, respectively. Whereas the zir-cons from the north exhibit well-preservedoscillatory growth zones within large andeuhedral crystals, their southern equivalentsfrom the peralkaline Allincapac suite take theform of irregularly truncated crystal fragmentsof larger zircon grains. Not surprisingly, bothof the two zircon populations are devoid ofxenocrystic cores, which tend to be dissolvedin Zr undersaturated magmas such as the hostsyenites (Watson, 1996; Baker et al., 2002).

Late Triassic

The Late Triassic ages were obtained from

peraluminous, highSiO2 (monzo)granites ofthe central and northern Cordillera de Carabaya,namely the Coasa, Ayapata, and San Gabnplutons (Fig. 4). Concordia ages from samplesCOCA 302, 262, 269, 298, and SAM-22a showboth minor Pb loss and the presence of elevatedlevels of common Pb. The ages obtained straddlethe Triassic-Jurassic boundary from 190.3 2.5to 216.1 3.1 Ma (Figs. 6D6F). A unique fea-ture of this zircon group is widespread evidencefor post-magmatic, solid-state recrystallizationrepresented by ubiquitous blurring and occa-

sionally convoluted zoning within otherwiseeuhedral prismatic crystals. The CL-dark rims

occasionally have sporadic bright domains thatare subparallel to the growth zoning and arelikely due to a combination of metamictizationby U-induced radiation and localized recrys-tallization (Kempe et al., 2000; Nasdala et al.,2002). The inherited component from mod-erately to strongly recrystallized, CL-bright,xenocrystic cores defines an early Permian ageinterval between 274 and 281 Ma.

Permo-Triassic

The emplacement of voluminous, partiallymigmatized granitoids within the south-central

segment of the Eastern Cordillera of Peru is con-strained to the Permian and Triassic (223 12 Mato 284.8 4.6 Ma). Although encompassingmore than 65 Ma and stretching over 800 km oforogenic strike, from the central Peruvian dis-trict of Hunuco to the southernmost Cordillerade Carabaya (Aricoma pluton) (Fig. 2), 80%of the Permo-Triassic plutonism by area wasemplaced within a geographically restricted beltbetween 10S and 12S with a peak in magmaticactivity occurring from 240 to 260 Ma (Figs.6G6L). Zircon grain morphology is dominatedby euhedral and fragmented prisms that havepreserved primary, oscillatory growth zones.

Exceptions to the dominant magmatic growthpattern are zircons from three granitoids of thesouthern Cordillera de Carabaya (COCA 268,358, and 362). Here, as in the Late Triassic mag-matic episode, uniformly low CL rims surroundpartially recrystallized cores and show wide-spread convoluted zoning. Overall, in situ U-Pbisotopic analyses reveal the minor to moderatepresence of common Pb. Half of the analyzedgrains display CL-bright xenocrystic cores, withthe dominant peak in the inherited age spectrumclustering between 288 and 296 Ma (Table 1).

Late OrdovicianEarly Permian

The Carboniferous to early Permian ages are

recorded by the compositionally heterogeneouand regionally most extensive intrusive belof the Eastern Cordillera of Peru. This arrayof plutons dominates the orogenic marginin its northern segment between the cities ofBolvar and Hunuco (6S to 10S) but alsoextends farther southward, west of the PermoTriassic domain, as a narrow suite of plutonand stocks that ultimately terminates withthe outer Cuzco batholith at 13.5S (Figs. 2and 3). A 5 Ma lacuna at the MississippianPennsylvanian boundary separates two dominant magmatic pulses. The shorter, early Car

boniferous intrusive episode progressivelyincreases in number of concordant analysesfrom 350 to 325 Ma before a sharp decreasereemergence at 315 Ma, and final terminationat 285 Ma (Figs. 6M6R). Furthermore, thediachronous nature of Carboniferous-Permianplutonism is confirmed by the progressivelysouthward-younging trend from an averageintrusive age of 314.5 Ma in the Pataz regionto 299.9 Ma near Cuzco (Fig. 4). The patternof decreasing crystallization ages from northto south roughly corresponds to a similar trendin the ages of inherited zircons. Namely, theBalsas granodiorite (NAM-22) at the latitude

of 7S retains a Mesoproterozoic age as old as1303 35 Ma, while CAM-54 and CAM-04granites of the central Ro Huallaga valley(9S) and Parcamayo district (11S), respectively, record Neoproterozoic dates of 934 26and 687 27 Ma. Subhedral to euhedral, occasionally fragmented zircon prisms show mostlypristine to faintly blurred primary zoning aroundCL-bright cores. An age of 293.3 5.0 Mafrom the Huallaga granite (CAM-54) wasobtained on zircons that have clearly undergone a phase of metamorphic overprint, a

TABLE 1. SUMMARY OF THE LA- ICP-MS IN SITU ZIRCON U-PB GEOCHRONOLOGYOF THE EASTERN PERUVIAN CORDILLERA PLUTONS (CUMULATIVE CONCORDIA AGES) (CONT.)

Sample Locality Latitude

S

Lithology n (analyses) Concordia age(Ma)

(2) Inheritance(Ma)

(2) Common Pb Pb loss

SAM-04a East Cusco batholith 13.2 Alkali feldspargranite

1110 26 Minor

CAM-22 Carrizales 11.5 Quartz monzonite 16 752 21 1011 34 Moderate MinorCAM-23 Carrizales 11.5 Alkali feldspar

granite

7 691 13 Minor

SCAM-18 Querobamba-Sucre 13.9 Alkali feldspargranite

13 751.7 8.1 Minor Minor

CAM-17 Satipo 11.3 Tonalite 18 985 14 1149 17 CAM-17 Satipo 11.3 Tonalite 1226 18 CAM-18 Mariposa-Junin 11.4 Alkali feldspar

granite17 1071 23 1305 33 Minor

CAM-18 Mariposa-Junin 11.4 Alkali feldspargranite

1668 28 Moderate

SCAM-17 Querobamba-Sucre 14.0 Granite 8 1123 13 1200 38

Note:The concordia ages are calculated using the routines of Ludwig (2003) and following Th and U decay constants of Steiger and Jger (1977). See Figure 2 forsample locations. LA-ICP-MSLaser ablationinductively coupled plasmamass spectrometry.

SAM-04a East Cusco batholith 13.2 Alkali feldspargranite

10 446.5 9.7 508 12



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http://gsabulletin.gsapubs.org/


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Late Mesoproterozoic

Closely associated with the Neoproterozoicgranitoids are partially foliated and compo-sitionally more diverse late Mesoproterozoicto earliest Neoproterozoic intrusives. Theseinclude: (1) the Satipo tonalite (CAM-17),which yields an age of 985 14 Ma, with astrong inherited component between 1150 and1250 Ma, (2) the Mariposa alkali feldspar gran-ite (CAM-18) with a Mesoproterozoic age of1071 23 Ma and exhibiting bimodal inheri-tance recorded by xenocrystic cores at 1305 Maand 1668 Ma, and (3) the Querobamba gran-

ite (SCAM-17), emplaced at 1123 13 Ma,

with loosely defined inheritance clustering at1200 Ma (Figs. 6V6X). In cathodolumines-cence images, the Mesoproterozoic grains dis-play ubiquitous dark rims and moderately tostrongly recrystallized, CL-bright cores.

Whole-rock Geochemistry and

Tectonic Affinity

Whole-rock, major-, minor-, and trace-element data are presented in Table DR2 (seefootnote 1). The Early Jurassic Allincapac

nepheline monzosyenites are the only sampledintrusives from the Eastern Peruvian Andethat are silica undersaturated and lack normative quartz. In the standard International Unionof Geological Sciences (IUGS) classification oStreckeisen (1974), the majority of the Carboniferous-Permian and half of the Late Triassic Cor

dillera de Carabaya plutonic rocks have modamineralogies corresponding to granites, whilethe Permo-Triassic plutons together with threeNeoproterozoic samples straddle the boundarybetween the granite and alkali-feldspar granitefields (Fig. 7). Both of the Oligocene (Andeansamples are quartz syenites, while the Neoproterozoic intrusives plot within the alkali feldspar granite field. The most diversified intrusivesuite is comprised of the Ordovician sampleswhich span the modal spectrum from quartzmonzonites through granodiorites to alkali feldspar granites. According to a recently proposedgeochemical classification scheme for granitic

rocks based on major-element chemistry (Froset al., 2001), the Carboniferous-Permian, Ordovician, and the Late Triassic granitoids are almagnesian, but transgress different boundariebetween the calcic, calc-alkaline, and alkalicalcic differentiation trends, respectively. Theyspan a wider compositional spectrum (6077 wt% SiO

2) than the predominantly alkali

calcic, magnesian to ferroan Neoproterozoicand Permo-Triassic plutons (7078 wt% SiO

2

Fig. 8A). The calcic Oligocene quartz syenitestogether with the strongly alkalic and ferroanEarly Jurassic nepheline syenites, define end

member compositions on this modified alkalilime index of Peacock (1931; Fig. 8B). In terms oaluminum saturation (Maniar and Piccoli, 1989)the Eastern Peruvian granitoids are peraluminouto mildly metaluminous (A/CNK = 0.951.13)The exceptions are the strongly peraluminouLate Triassic Carabaya suite (A/CNK = 0.981.42), volumetrically minor Oligocene stockthat show a metaluminous character (A/CNK =0.910.97), and the metaluminous to peralkaline Early Jurassic Allincapac Group nephelinebearing (alkali feldspar) syenites (Fig. 9). Peculiar features of both the Early Ordovician andLate Triassic granites are their anomalously high

K2O/Na2O ratios. Taken together with the cationic classification of de la Roche et al. (1980)which relates changes in silica saturationFe/(Fe + Mg) ratio and plagioclase composition during differentiation to tectonomagmaticsettings of emplacement (Fig. 10), the EasternCordilleran plutons can be grouped into: (1) Neoproterozoic anorogenic and transitional late- topost-orogenic granitoids of Permo-Triassic age(2) Cordilleran-type, continental-arc relatedintrusives from the Carboniferous to Permianand Late Ordovician and late Mesoproterozoic

0

5

10

15

20

25

30

35

40

45

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600206Pb/238U age (Ma)

Freque

ncy

Andeancycle

Gondwanides

Pampean-Famatinian

orogeny(0.44-0.53 Ga)

E. Braziliano /Pan-African

orogeny(0.63-0.75 Ga)

Nova Brasilndia /Sunss belts

(1.07 - 1.19 Ga)Aguape belt(0.96-92 Ga)

(Sunss orogeny)

San Igncioorogeny

(1.32 -1.34 Ga)

n= 278Bin width: 20 MaMean efficiency: 68.7 %

Serra daProvidncia /

LomasMeneches

suite(1.52-1.57 /

1.66-1.69 Ga)

0

2

4

6

8

10

12

14

16

150 175 200 225 250 275 300 325 350206Pb/238U age (Ma)

Frequency

Permo-Triassic Gondwanides(Pangea break-up)

Carbo-Permian Gondwanides(Pangea assembly)

Late Triassic(Cordillera de Carabaya)

Early Jurassic(Andean back arc)

n= 217Bin width: 5 MaMean efficiency: 33.5%

A

B

0.65 0.77

476

393

Figure 5. Zircon 206Pb/238U age histograms summarizing the known intrusive epi-

sodes represented by the granitoids of the Eastern Cordillera of Peru; (A) cumulative

Mesoproterozoic-Oligocene diagram, (B) blow-up of the volumetrically most dominant plu-

tonism of Gondwanide age (350160 Ma). Empty bins represent crystallization ages; black

boxes correspond to the ages of inherited zircons.



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Mikoviet al.


100 m

. D

100 m

E

100 m

100 m

.B

ierror ellipses are 2


.E

100 m

ierror ellipses are 2i ierror ellipses are 2

.C

100 m


.A

100 m


.F

SCAM-08 NAM-11a

SAM-21 COCA-302

COCA-262 COCA-298

Figure 6 (on this and following three pages). Chronologically arranged concordia diagrams for selected samples from the Figure 4. Dashed

lines correspond to the inherited zircon component. Cathodoluminescence (CL) images of zircon crystals illustrate the respective mineral

domains analyzed. MSWDMean square of weighted deviates.



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100 m

.

100 m

.I

50 m

50 m

.K

100 m

50 m

100 m

ierror ellipses are 2ierror ellipses are 2

ierror ellipses are 2 ierror ellipses are 2

ierror ellipses are 2ierror ellipses are 2

. H

.J

.

L

CAM-11a CAM-33

NAM-05 CAM-35

CAM-40CAM-45c

G

Figure 6 (continued).



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Mikoviet al.



100 m

. M

100 m

100 m100 m

.O

50 m

100 m

.

100 m





CAM-54

.NNAM-02a

CAM-03NAM-30

.P

NAM-18

.RSAM-12a

Ia

Q



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100 m

50 m 100 m

100 m

.

100 m

100 m 100 m

100 m

ierror ellipses are 2 error ellipses are 2




SAM-04a CAM-23

SCAM-18 CAM-17 . V

. W .X

CAM-18 SCAM-17

. S .T

U




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Mikoviet al.


periods, (3) Late Triassic syncollisional plu-tons and an Early Ordovician post-collisional

intrusive, and (4) Early Jurassic within-plate,peralkaline plutons forming the cores of alka-line volcanic complexes, and usually located inbackarc settings. The metaluminous granite por-phyries of the modern Andean orogenic cyclethat were emplaced in the Oligocene are mag-nesian and calcic, and exhibit cationic ratios thatoverlap those associated with continental arcsettings (Fig. 10).

Trace-element data from the Eastern Peru-vian intrusives broadly corroborate tectonicregimes inferred from the major-elementchemistry. Although primarily dependent onthe sources and crystallization history of the

melt (Frost et al., 2001), trace-element com-positions of granitoids have long been usedas first-order tectonic discriminators for gran-itoids (Harris et al., 1986; Barbarin, 1999).The oceanic plagiogranite normalized, multitrace-element plots (Pearce et al., 1984) for thePeruvian granitoids reveal a lack of heavy rare-earth element (HREE) fractionation character-istic of a garnet-dominated source, and displayan overall positive correlation between theextent of the large-ion lithophile to high field-strength element enrichment (LILE/HREE)

and known episodes of subduction-related mag-matism (Fig. 11). Negative Nb-Ta anomalies

are observed in both the late Mesoproterozoicplutons and within the Carboniferous-Permiansuite but are also mildly present in the LateOrdovician and Oligocene intrusive rocks(Figs. 11A, 11E, 11F, and 11H). In contrast,the Permo-Triassic, Early Ordovician quartzmonzonite, and especially Neoproterozoic(monzo)granitoids, exhibit strong Ba/Th andBa/Rb anomalies characteristic of anorogenicmagmatism, in addition to the lack of a typi-cal high LILE/HFSE trace-element patternassociated with subduction (Figs. 11D, 11F,and 11G). However, such time-dependenttectonomagmatic classification is somewhat

complicated by the regional variability withincoeval intrusive suites along the proto-Andeanmargin. For example, uniformly elevated HFSelements that are characteristic of rift- orbackarc-related plutonism displayed by thesouthern, Early Jurassic, peralkaline nephelinesyenites are markedly different from a mildsubduction-zone, trace-element signature ofthe contemporaneous quartz syenites from thenorthern Pataz region (Fig. 11B). On a morelocal scale, the early Mississippian, northernBalsas-Callangate pluton shows consider-

ably higher Rb/Ba values than the rest of theCarboniferous-Permian plutonic belt (Figs. 4and 11E). Given that the ratio of Rb/Badecreases with increasing whole-rock SiO

2con-

tent, this trend cannot be explained by simplefractional crystallization but likely reflects anincreased proportion of K-bearing phases in

the source. The most recent (Oligocene) intru-sive pulse is uniquely LILE enriched, withmildly depressed Nb-Ta concentrations indica-tive of a contaminated I-type magma that wasemplaced marginally inboard of the principalarc batholiths (Walawender et al., 1990).

DISCUSSION

Due to protracted and sometimes overlap-ping orogenic episodes that shaped the centralAndes, the interpretation of their Precambrianevolution is fragmentary and often speculative.Nevertheless, to shed additional light on the

evolution of the Peruvian segment of the west-ern Gondwanan margin, we place the newlyconstructed geochronological framework forthe Eastern Cordillera of Peru in a wider contextof magmatism along the proto-Andean marginof Amazonia, and ultimately link it to tectoniccycles affecting the western edge of the cratonsince the Mesoproterozoic.

Mesoproterozoic (Sunss-Grenvillian)

Two granitoids (CAM-17 and 18) from thecentral Satipo-Mariposa transect (Fig. 2) rep-

resent previously unrecognized, late Meso-proterozoic to early Neoproterozoic magmaticoccurrences in the Peruvian Andes outside of theknown extent of the Arequipa-Antofalla terrane.Although limited to a few samples, the peralu-minous, highSiO

2, calc-alkaline to alkali-calcic

granitoids exhibit uniform Nb-Ta anomalies,characteristic of subduction-related intrusivesuites. The 1071 23 Ma and 1123 13 Maages hence suggest an active segment ofnorth-central Amazonia during the peak of the1.30.9 Ga Rondonia-Sunss orogen, and pro-vide direct evidence for the existence of autoch-thonous South America within 150 km from

the eastern limit of the Arequipa-Antofallablock (Fig. 2). Blurred and partially convo-luted zircon rims from the late MesoproterozoicQuerobamba granite (SCAM-17; Fig. 6X) ofthe southwestern Peruvian Cordillera (Fig. 2),which yield a concordia age of 1123 12 Maand mantle extraction ages of 1200 Ma frominherited cores, also exhibit extreme U enrich-ments. They are seen as reflecting a period ofpost-emplacement metamorphism coeval withboth syntectonic orthogneisses from the easternBolivian Nova Brasilndia belt (U-Pb zircon

Modal A(vol.%)

Modal Q (vol.%)

Modal F (vol.%)

Modal P(vol.%)

Granite

Alk.-feldspar

Granite

Grano-diorite

Qtz. MonzoniteQtz. SyeniteQtz.Monzodiorite

Neph.Monzosyenite

Neph.-bearMonzonite

NephelineSyenite

Neph.-bearAlk.-feldsparSyenite

Neph.-bearSyenite

NeoproterozoicMesoproterozoic

Oligocene (Andean)E. Jurassic(N)

Early Ordovician

Carbo-Permian

Permo-Triassic

L. Triassic

E. Jurassic (S)

Late Ordovician

Figure 7. Modal composition of the Eastern Peruvian Cordilleran intrusives plotted on

the International Union of Geological Sciences (IUGS) classification diagram (Streckeisen,

1974). Modal proportions were determined by staining 10 10 cm polished rock slabs

for K-feldspar and plagioclase with Amaranth (C20

HllN

2Na

3O

lOS

3) and Na-cobalt nitrite

[Na3CO (NO

2)

6] respectively. Slabs were digitally scanned and an image processing software

JMicroVision created by Nicols Roduit was used for calculating the modal surface areas

that were projected into the ternary Qtz-Ab-Plag space.



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ages of 1113 56 Ma and 1110 8 Ma; Riz-zotto, 1999), and a Ro Pichari granulite-grade,garnet-bearing charnockite from southeasternPeru that yielded an upper intercept U-Pb zirconage of 1140 30 Ma (Dalmayrac et al., 1988).

Taken together, the Peruvian Mesoprotero-zoic intrusives represent the northern extensionof the 1.100.92 Ga collisional Sunss province

in eastern Bolivia (Boger et al., 2005), and maytemporally link the 1098 9 Ma gneissic base-ment inliers of the eastern Colombian Andes(Garzn Massif; Restrepo-Pace et al., 1997) withthe 1.21.0 metamorphosed Arequipa-Antofallabasement (Martignole and Martelat, 2003) andultimately, the 1.2 Ga granulite-grade metamor-phic basement of the Argentinean western Sierras

Pampeanas (Casquet et al., 2006). Consequentlya contiguous orogenic belt must have existed foover 3500 km along the Amazonian margin during the assembly of Rodinia, involving a continental collision with the Laurentian Llano beltincluding Labrador, and Baltica between ca. 1080and 970 (Fig. 12A; Hoffman, 1991; Sadowskand Bettencourt, 1996; Sadowski, 2002).

S-typeLachlan FB

Global A-type granitoids

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Fe#(FeO/FeO+MgO)

A-typeLachlan FB

Peraluminoushigh-SiOgranites

2

Magnesian

Ferroan

Cordilleran I-typegranitoids

A

50 70 80

-8

-4

0

4

8

12

NaO+K

OCaO

2

2

SiO (wt%)2

Alkalic

Alkali-c

alcic

Peraluminoushigh-SiO granites2

Cordilleran I-typegranitoids

S-typeLachlan FB

Global A-type granitoids

60

Calcic

A-type Lachlan FB

Increasingf(H O)

in source2

B

Calc-

alkaline

Oligocene (Andean)

E.Jurassic(S)

E. Jurassic(N)

L. Triassic

Permo-Triassic

Carbo-Permian

Late Ordovician

Neoproterozoic

Mesoproterozoic

Early Ordovician

Figure 8. Major-element chem-

istry of the Eastern PeruvianCordillera intrusives plotted

on the geochemical classifica-

tion diagrams for granitoids

of Frost et al. (2001): (A) FeO

number vs. wt% SiO2 dia-

gram showing the boundary

between ferroan and magne-

sian plutons based on a global

compilation of 175 A-type, 344

Mesozoic Cordilleran I-type

granitoids, and 95 peralumi-

nous leucogranites, as well as

the Lachlan Fold Belt (LFB),

A (n = 67), I (n = 1155) and

S-type granitoids (n = 720;

Landenberg and Collins, 1996).

Shaded and stippled areas out-

line 95% of the analyses and the

bold dashed line is the bound-

ary between calc-alkaline and

tholeiitic magmas as defied by

Miyashiro et al. (1970). Whole-

rock ferrous iron determined

by 2,2 bipyridine [(C5H

4N)

2]

complex solution photometry.

(B) Modified alkali-lime index

of Peacock (1931) showingranges for the alkalic, alkali-

calcic, calc-alkaline, and calcic

rock series. For any given suite,

the SiO2value where the modi-

fied Na2O + K

2O CaO index

(MALI) equals 0 corresponds

to the original alkali-lime index

of Peacock.



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Mikoviet al.


Neoproterozoic (pre-Braziliano)

Three middle Neoproterozoic A-type quartzmonzonite and alkali feldspar granites fromthis study are the first identified outcrops ofNeoproterozoic granitoid plutonism along thewestern margin of ancestral Amazonia. The752691 Ma, mostly metaluminous, alkalic to

alkali-calcic, K-feldspar-bearing granitoids arefound in spatially distinct localities proximal tothe Sunss-age plutonic rocks (Fig. 4), and exhibithigh LILE and HFSE abundances characteristicof anorogenic, A-type plutonism (Bonin, 2007;Figs. 10 and 11). Other intrusives of similar ageshave been identified in the northern segmentof the parautochthonous Arequipa-Antofallaterrane of southern Peru, where zircons fromsyntectonic dacitic dikes that cut the gneissicbasement yield lower concordia intercepts at635 5 Ma (Loewy et al., 2004). Middle Neo-

proterozoic ages were also recorded by A-typeorthogneisses intruded into basement of theLaurentian Precordillera (Cuyania) Terrane innorthwestern Argentina (774 6 Ma; Baldoet al., 2006), and a nephelinite-carbonatitebody reported from a Grenville age terrane inthe Sierra de Maz of the western Sierras Pam-peanas (570 Ma; Casquet et al., 2008). Here,

middle Neoproterozoic magmatism was classi-fied as anorogenic and attributed to a phase ofextensional tectonics during the proto-Iapetusrifting of Rodinia. The Peruvian granitoids,however, mark the northernmost and one of theoldest Neoproterozoic occurrences of the pre-Brazilianoaged anorogenic magmatism alongthe western margin of ancestral South America.This activity might have commenced as early asca. 800 Ma in the form of synrift, ultrapotassicmafic dikes and HFSE-enriched alkaline lavaflows underlying the intracontinental Puncovis-

cana basin of northwestern Argentina (Omariniet al., 1999). It complements the 765680 Maages from the Appalachian Blue Ridge A-typegranites and bimodal volcanics of Tollo et al.(2004), predates the Laurentia-Gondwana sepa-ration at ca. 615570 Ma sensu stricto (Cawoodet al., 2001), and overlaps with the initiation of

Cawoods (2005) rift-to-drift related sedimen-tary sequences along the Pacific margin of proto-Gondwana, where the opening of the IapetusOcean is better constrained. Taken cumulatively,the timing of emplacement of anorogenic intru-sives in southwestern Amazonia suggests aprotracted period of discrete, north-migratingextensional events that might have heraldedthe incipient fragmentation of Rodinia and for-mation of the proto-Iapetus oceanic crust ear-lier in Neoproterozoic than the conventionallyaccepted 620550 Ma rifting of the Iapetan mar-gin of Laurentia (Hoffman, 1991; Cawood et al.,2001). This contention is further supported by

recent paleomagnetic models and tectonic syn-theses that invoke an early breakup of WesternRodinia by, or prior to 725 Ma, involving theseparation of Australia and Eastern Antarcticafrom Western Laurentia and consequent openingof the paleo-Pacific (Panthalassic) ocean (Powellet al., 1993; Li et al., 2008). It appears that asyn-chronous northward separation of present-dayWestern Amazonia similarly took place alongthe Appalachian margin of then southern, andpresently, Eastern Laurentia (Hartz and Torsvik,2002; Fig. 12B). Moreover, the distribution ofthe Peruvian intrusives places spatial constraints

on the Laurentiaproto-Gondwana conjugatemargin during the Neoproterozoic. In particu-lar, it gives credence to the recently proposedpaleogeographic reconstructions where WesternAmazonia, Arequipa-Antofalla, western SierrasPampeanas, and East Laurentia are juxtaposedwithin Rodinia (Loewy et al., 2003).

Early Paleozoic (Famatinian)

The extent of early Paleozoic plutonismwithin the Eastern Peruvian Andes in our studyis limited to a single alkali feldspar granite(SAM-04a) from the eastern margin of the

Cuzco batholith, which yields a Late Ordoviciancrystallization age of 446.5 Ma, with significantMid-Cambrian inheritance (508 Ma; Table 1).The central Peruvian Carboniferous plutons alsocontain abundant inherited Late Ordovician andLate Cambrian zircons (446 Ma; 490514 Ma).These ages are broadly coeval with those fromthe Sitabamba granodioritic orthogneisses iden-tified in the north-central segment of the EasternCordillera (445.9 2.4 Ma; 442.4 1.4 Ma),the nearby Balsas paragneiss (477.9 4.3 Ma),and the central Cordilleran Pacococha quartz

0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.50.5

1.0

1.5

2.0

2.5

3.0

Al/(Na+K)

Al/(Ca+Na+K)

Peralkaline

Metaluminous

Peraluminous

Excess H O melting of mafic source /

melting (semi)pelitic source2

Figure 9. Aluminum saturation index (ASI) plot for the Eastern Cordillera plutons of Peru

(Maniar and Piccoli, 1989). Al/Na + K and Al/Ca + Na + K are defined as molecular ratios

and take into account the presence of apatite so that rocks with ASI>1.0 are corundum nor-

mative and are termed peraluminous (Zen, 1988). For weakly peraluminous rock additional

Al-bearing in addition to feldspars may be Al-biotite, but more strongly peraluminous gran-

itoids are usually characterized by muscovite, magmatic cordierite, and garnet or Al2SiO

5

polymorph, depending on pressure of formation. Intrusives with ASI


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monzonite dated at 474.2 3.4 Ma (Chewet al., 2007). Whereas the orthogneisses and theCuzco alkali granite exhibit high LILE/HFSEratios coupled with profound Nb-Ta anomalies,the Pacococha quartz monzonite shows none

(Fig. 11). Its trace-element pattern is, however,characteristic of late- to post-collisional, calc-alkaline suites akin to the Cordillera de Carabayagranitoids. Combined with the refined ages forthe emplacement of the arc-derived San Nicolsbatholith between 473 3 and 464 4 Ma(Mukasa and Henry, 1990; Loewy et al.,2004), the early Paleozoic age data imply thatthe northern projection of the Famatinian arcsystem of NW Argentina extended along theArequipa-Antofalla terrane, whereas the LatestOrdovician arc granitoids lay inboard through-out the Eastern Cordilleran of Peru up to 6S.Although a detailed paleogeographic recon-

struction is outside the scope of our study,this poorly understood time period deservesa closer tectonodynamic examination. Here amodel is offered for the apparent early Paleo-zoic orogenic bifurcation, by relating thetiming and spatial distribution of granitoidemplacement to metamorphism recorded in thebasement lithologies.

In Peru, the Paracas fault boundary (Fig. 2)sharply truncates the Arequipa-Antofalla ter-rane at 14S (Dalmayrac et al., 1980) with alack of the Grenvillian basement ages to the

north, interpreted by Ramos (2008) to representa separate crustal block, i.e., the Paracas ter-rane. This terrane would account for the EarlyOrdovician compressional event in northeasternPeru, and possibly simplifies the outline of the

central Gondwanan paleoPacific margin thatdoes not require an embayment north of theParacas Peninsula, as suggested by Chew et al.(2007). The presence of any significant amountof sialic basement beneath the Peruvian WesternCordillera since at least the Middle Cretaceous,however, is refuted on the grounds of the iso-topically extremely primitive intrusives of theCoastal Batholith and the Cordillera Blanca ofPeru, a dominantly mafic underlying crust withaverage density of ca. 3.0 g/cm3and a completeabsence of zircon inheritance (Couch et al., 1981;Polliand et al., 2005; de Haller et al., 2006). Inour model, somewhat analogous to the Argen-

tinean Puna (Ramos, 1988), the Early Ordovi-cian Peruvian margin experienced re-dockingof the parautochthonous Paracas-Arequipa-Antofalla terrane against the Gondwananmargin dominated by the protoArica bend(Fig. 12C). We interpret the Vijus Group calc-alkaline andesites overlying the Maranon Com-plex metapelites in the northern Pataz region ofthe Eastern Cordillera (Haeberlin, 2002) as evi-dence for a Late Cambrian arc developing on athinned forearc crust similar to the Faja Eruptivade la Puna Oriental of NW Argentina (Ramos,

1988). The resuturing of Paracas-ArequipaAntofalla terrane closed the marginal basin thahosted the pre-Famatinian Old Maran sediments of Chew et al. (2007), and resulted in theemplacement of peridotite lenses as thrust sliceswithin thermally overprinted Maran metasediments (Haeberlin, 2002). To the south, the

original west Gondwana (passive) metasedimentary sequences inboard of the presenArequipa terrane are probably only preservedas thrust slices flanking the inverted Triassic riffrom Ayacucho to the northern Bolivian border

Following the collision, a new continental arcwould have been established upon the matureand thickened Paracas-Arequipa-Antofalla crusat ca. 473464 Ma, in response to westwardjumping of the subduction zone over the newlyaccreted block (Fig. 12D). Granitoids intrudedinto the western margin of Famatina, in responseto subduction of the intervening Iapetus oceaniclithosphere during the approach of the Cuyania

terrane farther south (Ramos, 2004), correspondto arc-derived granites of the San Nicols batholith emplaced along the coastal Arequipa blockand would have likely extended northwardalong the suspect Paracas terrane. We suggesthat a major shift in plate motions that produceda rapid northward drift and clockwise rotationof the Antofalla segment during the MiddleOrdovician (>12 cm/a; Forsythe et al., 1993)effectively ceased arc activity, and impingedthe Paracas-Arequipa-Antofalla block upon thesouth Eastern Cordillera due to the shape othe Arica orocline. The resultant detachmen

of the Oaxaquia terrane left behind the (Paracasmafic and isotopically primitive lower crustasubstrate that has been modeled by Couch et al(1981) and Polliand et al. (2005) as 3.0 g/cmdense basaltic underplate. The northeastwardtransport might have occurred along a strikeslip fault system that is represented by the600-km-long Ro Maran crustal lineamen(Fig. 12E). The present-day location of Paracaterrane as the Oaxaquia-Acatln microcontinent in Mexico is suggested on the basis of theearly Paleozoic faunal and paleomagnetic correlations with Gondwanan of NW Argentina(Snchez Zavala et al., 1999; Keppie et al., 2008)

The 20 Ma magmatic gap in the Late Ordovician was characterized by turbiditic sedimentation within newly formed depocenters that wereunderlain by isotopically juvenile lower crustboth north of the Arequipa (Contaya Formation, Dalmayrac et al., 1980; Young MaranChew et al., 2007), and behind the rotated Antofalla block (Puna turbidites; Bahlburg and Herv1997). In the terminal phase of the Maranorogeny, and following a reversal in subductionvectors, the trench stepped in to the modern-dayposition along the Peruvian coastline and the

R2=6Ca+2Mg+Al(millications)

R1 = 4Si 11[Na+K] 2[Fe+Ti] (millications)0 1000 2000 3000

0

500

1000

1500

Continental arc

Post-collisional

uplift

Late orogenic

Anorogenic

Post-orogenic

Peralkalineintrusives

Figure 10. Geotectonic discrimination plot for the Peruvian Eastern Cordilleran granitoid

rocks based on de la Roche et al. (1980) multicationic R1R2 diagram (after Batchelor and

Bowden, 1985). Symbols are the same as those in Figures 7, 8, and 9.



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Mikoviet al.


1

10

100

0.1

Rock/OceanRidgeGranite

1

10

100

0.1

Rock/OceanR

idgeGranite

K RbBa Th Ta NbCe P SmTi Y YbZrHf

Pataz (north)Cordill. Carabaya (south)

1

10

100

Rock/OceanR

idgeGranite

K RbBa Th Ta NbCe P SmTi Y YbZrHf

Oligocene(Andean)

Neoproterozoic

Permo-Triassic

L. Mesoproterozoic

E. Jurassic

Late Triassic

0.1

1

10

100

Rock/OceanR

idgeGranite

0.1

Carbo-PermianBalsas-Callangate (north)

Late OrdovicianEarly Ordovician

.A .B

.C .D

.E .F

.G .H

Figure 11. Ocean-ridge, granite normalized, selected trace-element patterns for the Eastern Peruvian

Cordillera intrusives through time. Normalizing values taken from Pearce et al. (1984).



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arc reestablished along the length of the East-ern Cordillera from 7S to 14S (Fig. 12F), asindicated by the Cuzco batholith sample andpossibly comagmatic, calc-alkaline Ollantay-tambo pyroclastic volcanics (Bahlburg et al.,2006). The arc-derived granitoids in the north-ern Eastern Cordillera subsequently recorded

a high-grade metamorphic event equivalent tomid-crustal conditions of 750 C and 12.8 kbar(Chew et al., 2005). According to 40Ar/39Ar agesfrom metamorphic micas (Cardona et al., 2006),this last orogenic episode of the Early Phanero-zoic along this segment of the proto-Andeanmargin may be only slightly younger than the442 Ma emplacement age, or alternatively itmay be related to the Devonian Chanic orogenythat accompanied docking of Chilenia againstCuyania (Keppie and Ramos, 1999).

Middle Paleozoic (Gondwanide)

Following the Devonian magmatic lull, acontinental arcderived intrusive belt formedalong the Peruvian margin of Gondwana inthe Early to Middle Mississippian (Fig. 12G).Locally, this marked the onset of the pan-Pacific Gondwanide Orogeny, a terminal phasein the Neoproterozoic Terra Australis Orogen(Cawood, 2005). Here, we address the entireGondwanide cycle (350190 Ma) as a geneti-cally linked succession of magmatic flare-upsthat can be related to discrete steps in the finalagglomeration and initial breakup of the Pangeasupercontinent. The beginning of the orogenesis

in the Carboniferous, known as the Eohercynianphase in Peru (Mgard et al., 1971), was char-acterized by the synchronous emplacement ofcomposite calc-alkaline hornblende, biotite-bearing granodioritic and granitic batholithsover 1200 km of strike of the Eastern Cordillera(320.0 3.8 Ma at 6.9S and 324.1 5.3 Maat 13.2S). The similarity between the width ofthe early Carboniferous belt (2030 km) andmodern arc plutonic tracts of central Chile andthe Aleutians, together with generally flat HREEprofiles, argue for intrusions within a relativelythin crust (


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http://gsabulletin.gsapubs.org/


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Gondwana between 200 and 300 Ma (Kay et al.,1989; Veevers et al., 1994).

An alternate hypothesis invoking slab shal-lowing has been proposed in several studies toexplain the diminishing volume of the magma-producing mantle wedge below the Chilean seg-ment of the (proto)Andean margin in the EarlyPermian (Pankhurst et al., 1988), and Neogene(Kay and Mpodozis, 2002). This model alsoconflicts with the apparent normal crustal thick-nesses, and fails to explain a number of other

magmatic and sedimentary patterns in Peruduring the Late CarboniferousEarly Permian:(1) a minimal inboard shift in the locus ofmagma production from the Carboniferousarc to the adjacent Permian-Triassic granitoidprovince, which is not expected if 60% of thecontinental mantle is displaced up to 600 kminland from the trench during a phase of shal-low subduction (Kay and Abbruzzi, 1996),(2) a lack of syndepositional deformation inthe early Permian shallow marine mudstones

of the Copacabana Group is incompatible withstrong plate coupling as the angle of subductionfalls below 10 (Dumitru et al., 1991; Gutscher2002), (3) the formation of post-tectonic granitoids preceded and overlapped with Triassicextension (Ramos and Kay, 1991). Instead of acollisional event or a scenario that appeals to tectonic underplating, we favor a model similar tothat proposed by Franzese and Spalletti (2001for the pre-rift evolution of the Neuqun Basin incentral Chile. We suggest that the transition from

.G .H

.I .J .K

N

AMRNSU

RPP

SFBR

SL

PC

A-A

C

PanthalassicOcean

Mississippian(320345 Ma)

PT PanthalassicOcean

Pennsylvanian(310315 Ma)

N

AMRN

SU

RPP

SFBR

SL

PC

A-A

C

PT

RheicOcean

N. America

Africa

Flat

subductio

n

PermianTriassic(225280 Ma)

Ouachita Alleghe

nian

L. TriassicE. Jurassic(190220 Ma)

N

PacificOcean

AMRN

SU

RPP

SFBR

SL

A-A

C

PT

EuramericaAfrica

PanthalassicOcean

SouthAfrica

N S slabroll-back &break off

Mitu Gr.

C

EarlyMid. Jurassic(160180 Ma)

N N

AMRN

RPP

SFBR

SL

PC

C

PT

Africa

SouthAfricaC

Paleo-PacificOcean

Euramerica

A-A

SU

PC

Carabayaanatexis

C.AtlanticOcean

AM

RPP

SFBR

SL

PC

C

PT

SouthAfricaC

SU

RN

Euramerica

A-A

Allincapac

Choco

lateFm

arc

Magmatism arc/rift-extension

Plate vector

Ophiolitic lenses

SL SO LUIS

SF SO FRANCISCO

PAA PARACAS-AREQUIPAP PAMPIA

RP RIO DE LA PLATARN RIO NEGRO PROV.

SU SUNSAS ARC / PROV.

BR BRASILLIANO PROV.

2009 Aleksandar Mišković

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