u-pb geocronologic evidence for the evolution of the gondwanan margin of the north-central andes

8/13/2019 U-Pb Geocronologic Evidence for the Evolution of the Gondwanan Margin of the North-central Andes

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697

ABSTRACT

We investigated the Neoproterozoicearly

Paleozoic evolution of the Gondwanan mar-

gin of the north-central Andes by employing

U-Pb zircon geochronology in the Eastern

Cordilleras of Peru and Ecuador using a

combination of laser-ablationinductively

coupled plasmamass spectrometry detritalzircon analysis and dating of syn- and post-

tectonic intrusive rocks by thermal ionization

mass spectrometry and ion microprobe. The

majority of detrital zircon samples exhibits

prominent peaks in the ranges 0.450.65 Ga

and 0.91.3 Ga, with minimal older detri-

tus from the Amazonian craton. These data

imply that the Famatinian-Pampean and

Grenville (= Sunsas) orogenies were available

to supply detritus to the Paleozoic sequences

of the north-central Andes, and these oro-

genic belts are interpreted to be either buried

underneath the present-day Andean chain

or adjacent foreland sediments. There is evi-

dence of a subduction-related magmatic belt

(474442 Ma) in the Eastern Cordillera of

Peru and regional orogenic events that pre-

and postdate this phase of magmatism. These

are confirmed by ion-microprobe dating of

zircon overgrowths from amphibolite-facies

schists, which reveals metamorphic events

at ca. 478 and ca. 312 Ma and refutes the

previously assumed Neoproterozoic age for

orogeny in the Peruvian Eastern Cordillera.

The presence of an Ordovician magmatic

and metamorphic belt in the north-central

Andes demonstrates that Famatinian meta-

morphism and subduction-related mag-matism were continuous from Patagonia

through northern Argentina to Venezuela.

The evolution of this extremely long Ordo-

vician active margin on western Gondwana

is very similar to the Taconic orogenic cycle

of the eastern margin of Laurentia, and our

findings support models that show these two

active margins facing each other during the

Ordovician.

Keywords:Gondwana, Andes, Peru, geochro-nology, zircon, Paleozoic.

INTRODUCTION

The Andes represent the locus of continuedplate convergence through much of the Pha-nerozoic. Whereas Andean deformation andmagmatism have been extensively studied, theearly evolution of much of the proto-Andeanmargin remains poorly understood. The mainreason is because exposures of pre-Andeanbasement rocks are in many places extremely

limited because they are either obscured by latetectonic events along the convergent margin oburied by the ubiquitous volcanic cover.

This problem is particularly acute in thenorth-central Andes, where Precambrian basement is not exposed for over 2000 km alongstrike, from 15S in Peru to 2S in ColombiaThis corresponds to the distance between the

northern extent of the Arequipa-Antofalla basement (Fig. 1), a Proterozoic crustal block thaexperienced 0.91.2 Ga Grenville metamorphism (Loewy et al., 2004; Wasteneys et al.1995), and the southernmost basement exposures in Colombia, the Proterozoic Garzninlier (Restrepo-Pace et al., 1997; Cordani et al.2005). This zone is also characterized by substantial development of Andean foreland sediments to the east, so that the basement geologyperipheral to the orogen is not known with anydegree of certainty.

However, in the Eastern Cordilleras of Peruand Ecuador, Paleozoic metasedimentary

sequences are well exposed. Most of thesesequences, including the Maraon Complex inPeru (Fig. 2) and the Isimanchi and ChiguindaUnits of the Cordillera Real in Ecuador(Fig. 2), are considered to be autochthonouswith respect to the Gondwanan margin (Haeberlin, 2002; Pratt et al., 2005). Hence, theiheavy mineral assemblages, and, in particulartheir detrital zircon populations, should reveainformation regarding their source areasin

U-Pb geochronologic evidence for the evolution of the Gondwanan marginof the north-central Andes

David M. ChewUrs SchalteggerDepartment of Mineralogy, University of Geneva, Rue des Marachers 13, 1205 Geneva, Switzerland

Jan KolerDepartment of Earth Science, University of Bergen, Allegaten 41, N-5007 Bergen, Norway

Martin J. WhitehouseLaboratory for Isotope Geology, Swedish Museum of Natural History, S-104 05 Stockholm, Sweden

Marcus GutjahrInstitute for Isotope Geology and Mineral Resources, ETH-Zentrum, Clausiusstrasse 25, CH-8092 Zrich, Switzerland

Richard A. Spikings

Aleksandar MikovicDepartment of Mineralogy, University of Geneva, Rue des Marachers 13, 1205 Geneva, Switzerland

Present address: Department of Geology, TrinityCollege Dublin, Dublin 2, Ireland; e-mail: [email protected].

GSA Bulletin; May/June 2007; v. 119; no. 5/6; p. 697711; doi: 10.1130/B26080.1; 8 figures; Data Repository item 2007110.


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Chew et al.

698 Geological Society of America Bulletin, May/June 2007

this case the Gondwanan margin of the north-ern Andes. Furthermore, granitic magmas thatintrude these sequences can carry inheritedzircon, which will also provide source infor-mation, particularly with respect to deepercrustal levels that would otherwise be impos-sible to sample.

The age and number of orogenic episodesin these Paleozoic sequences are poorly con-strained, especially in the high-grade meta-morphic sequences of the Eastern Cordillera ofPeru. It is thus entirely unknown how this regionrelates to other zones along the proto-Andeanmargin of Gondwana, where the Paleozoicgeological history is significantly better under-stood (e.g., northern Argentina and Venezuela-Colombia; see Ramos and Aleman, 2000, for areview). However, the metasedimentary rocksof the Peruvian Eastern Cordillera are intrudedby several phases of Paleozoic magmatism thatcan be temporally related to the various regional

orogenic episodes. The combination of preciseU-Pb zircon geochronology from these intrusiverocks with the provenance information describedalready thus provides new constraints on boththe timing of orogeny and the paleogeographyof the Gondwanan margin of the north-centralAndes during the Paleozoic.

REGIONAL GEOLOGY

The Andes of Ecuador can be subdividedinto a Western Cordillera, consisting of accretedLate Cretaceous oceanic rocks, and an Eastern

Cordillera (the Cordillera Real), which consistsof Mesozoic plutons emplaced into probablePaleozoic metamorphic pelites and volcanics(Fig. 2). The southern boundary of the accretedoceanic rocks is marked by the Gulf of Guaya-quil (Fig. 2). The metamorphic belts of the Cor-dillera Real continue into northern Peru (Fig. 2),where there is a marked change in the orienta-tion of the Andean chain at ~6S, termed theHuancabamba deflection.

South of the Huancabamba deflection, thePeruvian Andes are composed of a WesternCordillera, consisting of Mesozoic sedimentsintruded by a Mesozoic arc (the Coastal Batho-

lith) with younger Tertiary volcanics and local-ized plutons (the Cordillera Blanca), and anEastern Cordillera, consisting of a sequenceof metasedimentary schists and gneisses (theMaraon Complex) intruded chiefly by Carbon-iferous and Permian-Triassic plutons (Fig. 2).The basement to the Western Cordillera northof 14S is unexposed but is believed to repre-sent accreted oceanic material (Polliand et al.,2005, and references therein); south of 14S, theCoastal Batholith is intruded into Proterozoic

gneisses of the Arequipa-Antofalla basement(Figs. 1 and 2).

Cordillera Real of Ecuador

There is very little age control on the Paleo-zoic metamorphic pelites and volcanics of theCordillera Real of Ecuador, and opinion dif-fers as to whether the majority of the units areallochthonous (e.g., Litherland et al., 1994)or autochthonous terranes (e.g., Pratt et al.,2005). The uncertainty relates to the tectonicsignificance attached to north-south faults thatrun along the spine of the Cordillera Real.The model of Litherland et al. (1994) invokesthese faults as major Mesozoic terrane-bound-

ing sutures that separate a series of suspect ter-ranes. These are illustrated on Figure 2 and are,from west to east: Guamote (continental), Alao(island arc), Loja (continental), Salado (islandarc), and Amazonic (continental craton). Themodel of Pratt et al. (2005) considers these unitsautochthonous as they share a similar structuralhistory, while the majority of the major terrane-bounding sutures are reinterpreted as intrusivecontacts between major plutons and pelites thatwere reactivated during Andean tectonics. Themajor phase of fault movement in the region

(which has apparent vertical displacements ofmany kilometers) is believed to be Miocene

Pliocene in age (Pratt et al., 2005).

Eastern Cordillera of Peru

Existing age constraints on the timing ofdeposition and metamorphism of metasedimen-tary schists and gneisses of the Eastern Cordi-llera (the Maraon Complex) are very poor. Ndmodel ages cluster between 1.5 and 2 Ga (Mac-farlane, 1999), while a poorly defined Neopro-terozoic age has been assigned to the metamor-phism based on bulk, unabraded U-Pb zircondata (ca. 630610 Ma lower intercepts) fromgranulitic gneisses in central Peru (Dalmayrac et

al., 1980). Recent research has demonstrated thepresence of an Early Ordovician (484 12 Ma)metamorphic event in the Maraon Complex(Cardona et al., 2006).

Existing constraints on the timing of mag-matism and younger deformation events in thePeruvian Eastern Cordillera are also sparse.However, although based on a very limited geo-chronological data set (in particular U-Pb zir-con), two principal plutonic belts of Paleozoic toMesozoic age can be distinguished in the Peru-vian Eastern Cordillera (Mikovic et al., 2005):

SF

80 W

0

60 W 40 W

20S

40 S

N

SoFranciscoCraton(SF)

CentralAmazonianProvince(>2.3 Ga)

Maroni-Itacainas

Province (2.2 - 1.9 Ga)Ventuari-TapajsProvince (2 - 1. 8 Ga)

MI

VT

CA

Rio Negro-JurenaProvince (1.8 - 1.5 Ga)

Rondonia-San IgnacioProvince (1.5 - 1.3 Ga)

SunssProvince (1.3 - 1.0 Ga)

RNJ

RO

SS

Neoproterozoictectonic provincesin Amazonian craton

Riode la

PlataCraton

Fig.2

So LuisCraton

Garzn

ArequipaAntofallaBasement

SierraPampeanas

FamatinaArc

PrecordilleraTerrane

Precambrianbasement

AndeanBelt

Paleozoicsequencesreferred to in text

Santander

ROSS

VT

VT MI

MI

CA

CA

RNJ

RNJ

Brasliabelt

Figure 1. Map of South

America illustrating the

major tectonic provinces

and the ages of their most

recent metamorphic events

(adapted from Cordani et

al., 2000). Precambrian

and Paleozoic inliers in the

Andean belt are shown in

black and light gray, respec-

tively.


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Evolution of the Gondwanan margin of the north-central Andes

Geological Society of America Bulletin, May/June 2007 699

UP

UP

OS

OS

OS

OS

OS

C

PT

PT

PT

Amotape

Amazonic

craton

LateCretaceousaccretedoceanicterranes

Ala

o

Salado

LojaGolfodeGuayquil

OCEAN

PACIF

IC

Lima

Quito

12 S

8 S

4 S

076 W 72 W80 W

Huanc

abam

ba

Deflection

E C U A D O R

P E R U

Proterozoic gneisses of the Arequipa -

Antofalla Basement

EA

STERNCORDILLERA

Cordillera Real

Permo-Triassic

Carboniferous

Ordovician - Silurian

Early Palaeozoic

and Arequipa Terrane

Late Palaeozoic

Plutonic Belts

Metamorphic belts

0 100 200km

N

PT

UP

C

OS

SU 03-24 Carboniferous migmatiteyoungest inherited zircon

345 14 Ma (2), n = 75

Bin Width: 75 Ma

Efficiency: 82.7%

0

1

2

3

4

5

6

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9 Age (Ga)

Probability

x

10-3

0

2

4

6

8

10

12

14

Frequency

99RS28 Chiguinda Unit

youngest detrital zircon362 12 Ma (2), n = 66

Bin Width: 75 MaEfficiency: 90.6%

0

1

2

3

4

5

6

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

Age (Ga)

Probability

x

10-3

0

2468101214161820

Frequenc

y

DC 04/5-2 Sitabamba Orthogneissyoungest inherited zircon473 18 Ma (2), n = 54


0

1

2

3

4

0

.3

0

.5

0

.7

0

.9

1

.1

1

.3

1

.5

1

.7

1

.9

2

.1

2

.3

2

.5

2

.7

2

.9

Probability

x

10-3

2

4

6

8

10

12

Frequency

99RS65 Isimanchi Fm.youngest detrital zircon

368 14 Ma (2), n = 61


0

1

2

3

4

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

Probability

x

10-3

0

2

4

6

8

10

12

Frequency

AM076 Maraon Complexyoungest detrital zircon461 25 Ma (2), n = 75

Bin Width: 75 Ma

Efficiency: 84.7%

0

1

2

3

4

5

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

Age (Ga)Probability

x

10-3

02

4

6

8

10

12

14

1618

Frequen

cy

DC 05/5-4 Maraon Complexyoungest detrital zircon

749 52 Ma (2)n = 102


0

1

2

3

4

0.3

0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

2.7

2.9

Probabilit

y

x

10-3

Age (Ga)0

4

8

12

1620

24

28

Frequency

SU 03-25

Fig. 3A

Fig. 3B

A

B

C

D

E

F

Guamote

Maraon Complex

Figure 2. Geological map of Peru and Ecuador illustrating the major Paleozoic metamorphic and magmatic belts along with the Proterozoic

gneisses of the Arequipa-Antofalla block. Inset figures AF illustrate zircon probability density distribution diagrams for both metasedi-

mentary and magmatic (inherited cores) samples. Geology was adapted from Litherland et al. (1994) and Leon et al. (2000).


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Chew et al.


Mississippian I-type metaluminous to peralumi-nous granitoids, chiefly restricted to the segmentnorth of 11S, and Permian to Early TriassicS- to A-type granitoids in central and southernPeru (Fig. 2). Additionally, the significanceand extent of a Late DevonianPennsylvanianorogenic cycle (the Eohercynian orogeny ofMgard, 1978) remain uncertain. Samples fromthe Eastern Cordillera of Peru come from tworegions (Figs. 2, 3A, and 3B) in northern andcentral Peru, respectively.

SAMPLING

Ecuador

Sample 99RS28 (Fig. 2; grid coordinatesfor all samples are listed in the relevant datatables in the GSA Data Repository1) wasselected for detrital U-Pb age zircon analy-sis by laser-ablationinductively coupledplasmamass spectrometry (LA-ICP-MS); itis a quartzite from the Chiguinda Unit, part of

the presumed allochthonous Loja terrane ofLitherland et al. (1994). This unit consists ofquartzites and black phyllites of poorly con-strained age thought to be post-Silurian basedon undiagnostic miospore data (Litherland etal., 1994).

2km0 1

10km0 5

5000m

4000m

X X'

D

C

1000m

3000m

YY'

Pacococha AdamelliteBasalt dykes

Mississippiansediments

Maraon Complex

Maraon Complex

Ur. Permian- Lr.Triassic sediments

Hualluniyocc Adamellite Huacapistana Granite Utcuyacu Granite

La MercedGranite

UpperTriassic Upper Permian

- Lower Triassic

SU 03-19

474.2 3.4 Ma [S]

SU 03-24SU 03-25 SU 03-21SU 03-20 SU 03-22

D325.4 0.6 Ma [T]312.9 3.0 Ma [S], [D] 310.1 2.3 Ma [S] 307.1 0.7 Ma [T]

7534' W1120' S

7518' W1111' S

7552' W1110' S

7544' W1105' S

SU 03-19

SU 03-24

SU 03-25

SU 03-21

SU 03-20

SU 03-22

Fig. 3c

X

X'

Y

Y'

Fig. 3d

N

0 25kmB

D

325.4 0.6 Ma [T]

312.9 3.0 Ma [S], [D]

310.1 2.3 Ma [S]

307.1 0.7 Ma [T]

474.2 3.4 Ma [S]

Pacococha

(Andean?) Faults

Tarma

N

DC 05/5-10

DC 04/5-2

DC 05/5-4

DC 05/5-7

DC 05/6-5

AM076

100 km0 50A

Permo-Triassic intrusive

Carboniferous intrusive

Ordovician intrusive

Maraon Complex

Carboniferous andyounger sediments

T: TIMS

S: SIMS

L: LA-ICP-MS

D: detrital

sample

D

D

442.4 1.4 Ma [T],444.2 6.4 Ma [L], [D]

445.9 2.4 Ma [S]

343.6 2.6 Ma [S]

477.9 4.3 Ma [S]

Sitabamba

Pataz

Bolivar

Balsas

Figure 3. Geological maps, cross sections, and sample localities with ages from selected regions of the Eastern Cordillera of Peru (see Fig. 2).

(A) Geological map of the northern section of Eastern Cordillera with names of plutons discussed in the text in italics (adapted from Leon

et al., 2000). (B) Geological map of the northern section of Eastern Cordillera (after Mgard, 1978). Legend as in A. (C) Cross-section X-X

through the Pacococha adamellite (after Mgard, 1978). Line of section is indicated in B. (D) Cross-section Y-Y in the vicinity of Tarma

(after Mgard, 1978). Line of section is indicated in B.

1GSA Data Repository item 2007110, Tables DR1,DR2, DR3, and DR4, is available on the Web athttp://www.geosociety.org/pubs/ft2007.htm. Requestsmay also be sent to [email protected].


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Sample 99RS65 (Fig. 2) is a phyllite selectedfor detrital zircon LA-ICP-MS dating from the

Isimanchi Formation, which forms part of theAmazonian cratonic cover. It consists of verylow-grade phyllites and marbles and has a poorlyconstrained CarboniferousLate Triassic agebased on fish remains (Litherland et al., 1994).

Northern Peru

Sample AM076 (Figs. 2 and 3A) is a stronglyfoliated greenschist-facies Maraon Complexpsammite selected for detrital zircon LA-ICP-MS U-Pb dating from the type area of the Mara-on Complexthe Maraon valley north ofPataz in northern Peru. Here the Maraon Com-

plex is locally overlain by Middle Ordoviciangraptolite-bearing slates of the Contaya Forma-tion (Wilson and Reyes, 1964), but opinion isdivided on whether this contact is conformableor an orogenic unconformity (Haeberlin, 2002).

Samples DC 04/52 and DC 05/65 (Figs. 2and 3A) are strongly foliated granodiorites(defined here as the Sitabamba orthogneiss) withconspicuous augen of relic igneous plagioclaseand development of metamorphic biotite andgarnet (Fig. 4A). The Sitabamba orthogneissintrudes the Maraon Complex in the southwest

portion of the Maraon Complex outcrop(Fig. 3A; Wilson et al., 1995). Thermobaromet-

ric estimates for the metamorphic assemblagesare 700 C and 12 kbar (Chew et al., 2005).Sample DC 04/52 was selected for preciseU-Pb zircon dating using isotope-dilution ther-mal ionization mass spectrometry (TIMS) tech-niques, to constrain the timing of intrusion andLA-ICP-MS dating of inherited zircon cores.Sample DC 05/65 was selected for U-Pb ion-microprobe dating to constrain the timing ofintrusion further to the southeast (Fig. 3A).

Samples DC 05/54 and DC 05/57 (Figs. 2and 3A) were also taken from the MaraonComplex. The metamorphic grade of the Mara-on Complex in this region is substantially

higher than in that to the south (e.g., sampleAM076), and consists of biotite-garnet para-gneisses intruded by a series of leucosomesthat share the same foliation as the coun-try rock (Fig. 4B). DC 05/54 is a sample ofthe high-grade paragneiss host rock that wasselected for detrital zircon LA-ICP-MS U-Pbdating. Particular care was taken during sam-pling and subsequent sample preparation toensure that no visible leucosome vein mate-rial was present. DC 05/57 was sampled froma thick, foliated leucosome (5 m across) for

U-Pb ion-microprobe dating to constrain thetiming of leucosome development.

SampleDC 05/510 (Fig. 3A) is from a 10-cmthick fine-grained granodioritic dike that cuts theregional gneissic fabric in the Maraon Complexand is presumably related to the nearby Balsasgranodiorite pluton of presumed Mississippianage (Fig. 3A). The Balsas pluton has yielded aK-Ar biotite age of 347 7 Ma (Sanchez, 1983)This post-tectonic dike was selected for U-Pbion-microprobe dating to provide a minimum ageon the timing of deformation in the region.

Central Peru

Sample SU 0319 (Figs. 3B and 3C) was

taken from the Pacococha adamellite, whichcuts the Maraon Complex in central Peru. It ispost-tectonic with respect to the ductile deformation fabrics in the Maraon Complex andis overlain unconformably by Mississippiansediments (Fig. 3C; Mgard, 1978). It is undeformed on a hand specimen scale, but magmaticbiotite is pervasively chloritized, and the plutonis prominently jointed with a series of verticabasic dikes exploiting the fissures (Fig. 3CMgard, 1978). The sole existing age constrainis a K-Ar biotite age of 346 10 Ma (Mgard

C

Enclave

Foliation

Late fault

Figure 4. Field photographs from the northern part of the Eastern Cor-

dillera of Peru. (A) Sheared granodioritic gneisses of the Sitabamba ortho

gneiss. Sample DC 05/65, which yields a U-Pb secondary ion mass spec

trometry (SIMS) zircon intrusion age of 445.9 2.4 Ma, was collected at this

locality. Lens cap is 6 cm across. (B) Foliated leucosomes within high-grade

metasediments of the Maraon Complex. Sample DC 05/57, which yields

a U-Pb SIMS zircon age of 477.9 4.3 Ma for leucosome development, wa

collected at this locality. The Maraon Complex host rock was also sampled

for detrital zircon analyses (DC 05/54) in a zone that did not exhibit leuco

some development. Pencil is 15 cm long. (C) Hualluniyocc adamellite, dem

onstrating the foliated nature of the intrusion, basic enclaves, and late-stag

brittle deformation. Sample SU 0320, which yields a U-Pb thermal ioniza

tion mass spectrometry (TIMS) zircon intrusion age of 325.43 0.57 Ma

was collected at this locality. Lens cap is 6 cm across.

A

B C


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The crystallization of the granodioritic pro-tolith of the Sitabamba orthogneiss in northernPeru (DC 04/52; Figs. 2 and 3A) has beenconstrained by both U-Pb TIMS dating of a dis-tinct subpopulation of acicular magmatic zircon(two concordant single-grain analyses yieldinga concordia age of 442.4 1.4 Ma; Fig. 5A) and

LA-ICP-MS U-Pb dating of coarse (up to 50 mthick) rims of magmatic zircon (concordia age[Ludwig, 1998] of 444.2 6.4 Ma; Fig. 5B).Further TIMS data only yielded strongly discor-dant points and are not shown. There was only alimited amount of Hf isotopic data for this sam-ple (as there were only two TIMS data points),but the difference in

Hf(T) values of 8.8 and

+2.0 for the two dated zircon grains was sub-stantially larger than the range shown by othersamples (Table DR3 [see footnote 1]; Fig. 5A).

The Hualluniyocc adamellite (SU 0320;Figs. 3B and 3D) yielded a concordia age of325.4 0.6 Ma based on four concordant data

points (Figs. 5C and 5D). Dated zircons wereslightly pinkish, ~200 m long, and were pris-matic to long prismatic (length:width ratio [l:w]= 4:1); their morphology corresponded to typeS22 on a Pupin typogram (Pupin, 1980). Somecrystals contained elongate and rounded meltinclusions. One zircon (Fig. 5C) yielded a discor-dant analysis that pointed to an inherited compo-nent of a 481 230 Ma age. Although this upperintercept age was poorly constrained, it should benoted the ca. 480 Ma peak was very prominentin the LA-ICP-MS detrital zircon data. The Hf(T)

values of the four concordant data points ranged

from 2.3 to 0.1 (Table DR3 [see footnote 1];Fig. 5D). The two discordant analyses 4 and 5had lower

Hf(T)values of 3.4 and 4.3, consis-

tent with an older crustal source for the Hf.The Utcuyacu Granite (SU 0322; Figs. 3B

and 3D) yielded a concordia age of 307.1 0.7 Ma based on four concordant data points(analyses 2, 3, 6, 7; Fig. 5E). Dated zirconswere ~80 m long and were short prismatic.They also corresponded to a type S22 morphol-ogy on a Pupin typogram (Pupin, 1980). Mostcrystals contained abundant inclusions, someof which were found in the center of the crys-tal and which probably represented cores. Two

separate zircons yielded older concordia ages of316 Ma and 322 Ma (Fig. 5E). These zirconswere presumably xenocrysts derived locallyfrom the Carboniferous magmatic arc that hadbeen incorporated into the melt. The

Hf(T)values

of all the data points ranged from 3.2 to +0.5(Table DR3 [see footnote 1]; Fig. 5A).

U-Pb Ion-Microprobe Dating of Zircon

U-Pb isotopic data and calculated U-Pbages are listed in Table DR4 (see footnote 1).

Scanning-electron micrograph (SEM) CL imagesof representative dated zircons are illustrated inFigure 6, and U-Pb Tera-Wasserburg concordiadiagrams are illustrated in Figure 7. A commonPb correction was only applied to samples thatexhibited significant levels of 204Pb. Detailed ana-lytical techniques are described in the Appendix.

Central Peru

The Pacococha adamellite (SU 0319;Figs. 3B and 3C) yielded a single zircon popu-lation consisting of short prismatic, orange-brown grains (l:w of ~2:1). Grains were typi-cally ~150200 m long and were euhedralwith well-developed terminations (Fig. 6A).The grains were dark under CL and exhibitedoscillatory zoning. No inherited cores wereanalyzed, and 13 spots from the rims of nineseparate crystals yielded a concordia age of474.2 3.4 Ma. Th/U ratios of the magmaticrims typically clustered between 0.1 and 0.35,

and the spots were uncorrected for common Pb(Table DR4, see footnote 1).

The Huacapistana Granite (SU 0321Figs. 3B and 3D) and the Maraon Complexmigmatitic paragneiss (SU 0324; Figs. 23B, and 3D) closely resembled each other interms of their zircon populations. They will beconsidered together, and the CL images of SU0321 (Fig. 6B) can be considered represen

tative of SU 0324. The population consistedof medium-sized (between 100 and200 m indiameter), colorless, well-faceted grains. Thegrains were typically subspherical or stubbyprisms. Under CL, all grains contained inherited cores that formed the majority of the crystal. The inherited cores were nearly alwayrounded but exhibited a wide variety of internazoning and CL intensity patterns. The boundary between the inherited core and the magmatic rim was commonly characterized by athin (15 m) black zone under CL, which wa

Figure 6. Representative zircon petrography for secondary ion mass spectrometry (SIMS

analyses: scanning electron microscopecathodoluminescence images from (A) Pacococha

adamellite, (B) Huacapistana Granite, (C) leucosome within the Maraon Complex, (D)

Sitabamba orthogneiss, and (E) Carboniferous microgranite dike. Sample identification o

SIMS data points corresponds to Table DR4 (see text footnote 1). All quoted ages are concordia ages, and uncertainties are at the 2level. All images use the scale bar in E.


8/15

Chew et al.


560

520

480

440

400

360

320

0.051

0.053

0.055

0.057

0.059

11 13 15 17 19 21

238U/

206Pb

207Pb

206Pb

6

DC 05/5-10 SIMS

(all data)

500490

480

470

0.0545

0.0555

0.0565

0.0575

0.0585

12.2 12.4 12.6 12.8 13.0 13.2 13.4

238U/

206Pb

207Pb

206Pb

483.8 3.6 MaMSWD = 1.4

33

22

36

8

18

DC 05/5-10 SIMS

(oscillatory

zoned)

358 354 350346 342 338 334

0.0515

0.0525

0.0535

0.0545

0.0555

17.4 17.8 18.2 18.6 19.0

238U/

206Pb

207Pb

206Pb

343.6 2.6 MaMSWD = 0.16

27

16

24

1412

13

DC 05/5-10 SIMS

(sector

twinned)

1300

1100

900

700

500

0.05

0.06

0.07

0.08

3 5 7 9 11 13 15

238U/

206Pb

207

Pb206

Pb

27

7

DC 05/5-7 SIMS

(all data)

490480

470 460

0.054

0.055

0.056

0.057

0.058

0.059

12.4 12.6 12.8 13.0 13.2 13.4 13.6

238U/

206Pb

207Pb

206Pb

477.9 4.3 MaMSWD = 1.8

17

21

19

14

DC 05/5-7 SIMS

(magmatic rims)

470460

450

440 430

0.053

0.054

0.055

0.056

0.057

0.058

13.2 13.4 13.6 13.8 14.0 14.2 14.4 14.6

238U/

206Pb

207Pb

206Pb

445.9 2.4 MaMSWD = 0.42b36

43

11

14

40

632

b17

13

36

31

DC 05/6-5 SIMS

(magmatic

rims)

500490

480470

460450

0.054

0.055

0.056

0.057

0.058

0.059

12.3 12.5 12.7 12.9 13.1 13.3 13.5 13.7 13.9

238U/

206Pb

207Pb

206Pb

474.2 3.4 MaMSWD = 0.069

20

124

b36

b32

25

12

23

10

SU 03-19 SIMS

(magmatic rims) 500460

420

380340

300

0.047

0.049

0.051

0.053

0.055

0.057

0.059

11 13 15 17 19 21 23

238U/

206Pb

207Pb

206Pb

03

b51

66

70SU 03-21 SIMS (all data)

326 318 310 302 294

0.047

0.049

0.051

0.053

0.055

0.057

19.0 19.4 19.8 20.2 20.6 21.0 21.4

238U/

206Pb

207Pb

206Pb

310.1 2.3 MaMSWD = 0.75

134

153

109

183

02

73

196

106

129

SU 03-21 SIMS

(magmatic

rims)

540

500460

420

380 340

300

0.045

0.047

0.049

0.051

0.053

0.055

0.057

0.059

0.061

10 12 14 16 18 20 22

238U/

206Pb

207Pb

206Pb

b18 108

130

6

SU 03-24 SIMS

(all data)

330 322 314 306298

0.046

0.048

0.050

0.052

0.054

0.056

18.8 19.2 19.6 20.0 20.4 20.8 21.2

238U/

206Pb

207Pb

206Pb

312.9 3.0 MaMSWD = 7.567

b138

177

12

138

218

SU 03-24 SIMS

(magmatic

rims)

304312320328

0.050

0.054

0.058

0.062

0.066

0.070

18.8 19.2 19.6 20.0 20.4 20.8

238U/

206Pb

207

Pb206Pb

Intercept at 313.8 3.0 MaMSWD = 0.84

67

124

b138

138216

177

SU 03-24

(No Pb

corr)

A B C

D FE

J K L

G H I

Figure 7. Tera-Wasserburg concordia diagrams (AL) showing zircon ages for samples dated by secondary ion mass spectrometry (SIMS).

Spot numbers are those given in Table DR4 (see text footnote 1). MSWDmean square of weighted deviates.


9/15



sometimes overgrown by a bright white zoneof irregular thickness. The succeeding outerportions of the crystal typically displayed anirregular zoning pattern and were gray underCL (Fig. 6). The total thickness of the rim wasseldom greater than 40 m. Where possible,analyses were confined to the gray outer por-

tions of the rim. Inherited cores for both sam-ples yielded ages that ranged between 500 and340 Ma (Figs. 7B and 7D). Eight spots from therims on eight separate grains yielded a concor-dia age of 310.1 2.3 Ma (Fig. 7C) for sampleSU 0321. Six spots from the rims of six sepa-rate grains yielded a concordia age of 312.9 3 Ma (Fig. 7E) for sample SU 0324, albeitwith a relatively high mean square of weighteddeviates (MSWD) of 7.5. Th/U ratios from therims of both samples were typically extremelylow, clustering around 0.01 (Table DR4, seefootnote 1). Three out of eight rims in sampleSU 0321 were corrected for common Pb, while

all six rims in sample SU 0324 were correctedfor common Pb (Table DR4, see footnote 1).Five of these six analyses are shown uncor-rected for common Pb on an inverse concordiadiagram (Fig. 7F). They yield an intercept ageof 313.8 3.0 Ma (MSWD = 0.84), assumingthe anchor for the intercept has a present-dayaverage terrestrial common Pb composition of207Pb/206Pb = 0.83 (Stacey and Kramers, 1975),which corroborates the accuracy of the com-mon Pb correction.

Northern Peru

The Maraon Complex leucosome (DC05/57; Figs. 2 and 3A) yielded an extremelydiverse population of zircons. The majority ofthe population was composed of large (often upto 200 m in diameter) faceted grains (whichwere either long prismatic, short prismatic, orsubspherical in habit, with a discrete subpopu-lation of small prismatic, euhedral zircons thatrarely exceeded 100 m in length). Under CL,the distinction between the two subpopulationswas clear. The small euhedral prisms were CLdark and had faint idiomorphic zoning (Fig. 6C)with rare inherited cores. The coarser grainsshowed a thin (1030 m) CL dark rim with

faint idiomorphic zoning, which surrounded alarge inherited core that made up the majority ofthe zircon crystal (Fig. 6C). The inherited coresexhibited a wide variety of forms, zoning pat-terns, and CL intensities, which reflect the vari-able nature of the precursor detrital zircon popu-lation. Four spots from the CL dark regions onfour separate grains yielded a concordia age of477.9 4.3 Ma (Fig. 7H), which is interpretedas the crystallization age of the leucosome. Boththe large faceted grains and the small prismaticgrains were analyzed (Fig. 6C). One inherited

core (27) was analyzed and yielded a 206Pb/238Uage of 1093 16 Ma, while another analysis (7),which yielded an age marginally older than theconcordia age (206Pb/238U age of 494 10 Ma),is interpreted as a mixture between a magmaticrim and inherited core. Th/U ratios of the mag-matic rims were typically extremely low, aver-

aging around 0.05 (Table DR4, see footnote 1).All four magmatic spots were corrected forcommon Pb (Table DR4, see footnote 1).

The Sitabamba orthogneiss (DC 05/65;Figs. 2 and 3A) yielded one population of longprismatic (up to 300 m long) zircons that werepredominantly euhedral with sharp terminations(Fig. 6D). CL images exhibited bright oscilla-tory zoned rims that commonly surrounded aslightly darker oscillatory zoned core. The per-centage of the population that contained poten-tially inherited cores is estimated at less than 5%,although no such grains were analyzed. Elevenspots from the CL bright marginal regions on

nine separate grains yielded a concordia age of445.9 2.4 Ma (Fig. 7J), which is interpreted tobe the age of intrusion. Th/U ratios of the datedzircons clustered between 0.1 and 0.4, althoughone outlier (13) had a Th/U ratio >1 (Table DR4,see footnote 1). Analyses were uncorrected forcommon Pb (Table DR4, see footnote 1).

The Balsas microgranite dike (DC 05/510;Fig. 3A) yielded two major populations of zir-cons. The first population consisted of elongateprismatic zircons (typically between 200 and250 m long) that were euhedral with sharpterminations (Fig. 5E). They were CL dark and

displayed a characteristic sector zoning. Thesecond population consisted of stubby prismaticzircons (l:w typically of ~2:1) that were alsoeuhedral with well-developed terminations. CLimages exhibited a prominent oscillatory zon-ing. Neither population exhibited clear inher-ited cores. These two morphologies comprised~95% of the total zircon population. Six spotsfrom six separate grains from the sector-zonedpopulation yielded a concordia age of 343.6 2.6 Ma (Fig. 7L), which is interpreted to bethe intrusion age of the dike. These analyseswere characterized by Th/U ratios between 0.35and 0.5 (Table DR4, see footnote 1). Five spots

from five separate grains from the oscillatoryzoned population yielded a concordia age of483.8 3.6 Ma (Fig. 7J). These analyses weretypically characterized by Th/U ratios less than0.1 (Table DR4, see footnote 1). One grain thatdid not conform to the two zircon morphologiesdescribed above was also analyzed. It yielded a206Pb/238U age of 453 10 Ma (Fig. 7K, spot 6)and was characterized by a high Th/U ratio of>0.9 (Table DR4, see footnote 1). The spotswere uncorrected for common Pb (Table DR4,see footnote 1).

TECTONIC EVOLUTION OF THE

EASTERN CORDILLERA OF PERU

Central Peru

U-Pb zircon data from the sampling transecin the Eastern Cordillera of Central Peru clearly

demonstrate that the Maraon Complex habeen affected by two major orogenic events. Thepost-tectonic Pacococha adamellite (SU 0319Figs. 3B and 3C) constrains all polyphasedeformation within the Maraon Complexprior to 474.2 3.4 Ma (Fig. 7A). However25 km to the southeast, the syn-D2 migmatiticHuacapistana granite (SU 0321; Figs. 3B and3D) and syn-D2 migmatic paragneisses in theMaraon Complex (SU 0324; Figs. 2, 3B, and3D) yielded ages of 310.1 2.3 Ma and 312.9 3.0 Ma, respectively (Figs. 7C and 7E). Thetiming of this Pennsylvanian (ca. 312 Ma) tectonothermal event is corroborated by the pres

ence of deformed plutons at 325.43 0.57 Ma(Hualluniyocc adamellite, SU 0320; Figs. 3B3D, and 5D) and entirely undeformed plutons a307.05 0.65 Ma (Utcuyacu granite, SU 0322Figs. 3B, 3D, and 5E).

Detrital zircon data have been obtained froma Maraon Complex meta-sandstone (SU 0325) from the same transect (Figs. 3B and 3Dthat had been affected by the late Carboniferou(ca. 312 Ma) tectonothermal event. Althoughthe age spectrum is not illustrated on Figure 2due to the small number of analyses (n = 15)the presence of young grains at ca. 470 Ma

(Table DR1, see footnote 1) implies that thissample has undergone post470 Ma metamorphism. The data from the Maraon Complexin central Peru suggest that it is composed otwo separate units: an older unit that underwent metamorphism prior to 474 Ma, and ayounger unit (which contains detritus presumably derived from the ca. 470 Ma magmaticbelt) that was deformed ca. 312 Ma. The contacbetween these two units remains uncertain, buit should be noted that the region was stronglydeformed during the main compressional phaseof the Andean orogeny (the late Eocene Incaicphase; Steinmann, 1929) and that the precise

locations of many of the Andean thrusts remainspeculative, particularly where they affect thelithologically monotonous metasediments of theMaraon Complex.

Northern Peru

U-Pb zircon data from the sampling transecin the Eastern Cordillera of northern Peru alsodemonstrate that the Maraon Complex has beenaffected by at least two major orogenic eventsA foliated leucosome from garnet-bearing


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Chew et al.


paragneisses in the Maraon Complex yieldeda concordia age of 477.9 4.3 Ma (DC 05/57;Figs. 2, 3A, and 7H), which is interpreted as dat-ing leucosome generation. It is suggested that thisevent correlates with the pre474 Ma deforma-tion in central Peru, i.e., there was a ca. 478 Maorogenic event that affected part of the Maraon

Complex for several hundred kilometers alongthe Eastern Cordillera. Importantly, the para-gneiss host rock (DC 55-4) to the leucosomescontained no zircons younger than ca. 750 Ma(Fig. 2C; Table DR1 [see footnote 1]).

The presence of a younger orogenic event innorthern Peru is indicated by two separate linesof evidence. First, the protolith to the Sitabambaorthogneiss (samples DC 04/52 and DC05/6/-5; Figs. 2 and 3A) crystallized between442 and 446 Ma (Figs. 5A, 5B, and 7J), and,therefore, the high-grade metamorphic assem-blages (700 C, 12 kbar; Chew et al., 2005) inthe orthogneiss must postdate this. Second, a

strongly deformed greenschist-facies MaraonComplex psammite (AM076; Figs. 2D and 3A)contains a prominent peak in the detrital zirconpopulation at ca. 470 Ma (Fig. 2D; Table DR1[see footnote 1]) not seen in the older MaraonComplex rocks. For the purposes of discussionhere, these two units are termed the Old andYoung Maraon Complexes, respectively(e.g., Fig. 8D). The young Maraon Complexrocks must have undergone post470 Ma oro-genesis, and we tentatively correlate this eventwith the post442 Ma event experienced by theSitabamba orthogneiss. However, this event is

older than the Pennsylvanian (ca. 312 Ma) eventseen in central Peru because the young MaraonComplex rocks in northern Peru (with the prom-inent ca. 470 Ma detrital zircon peak) are uncon-formably overlain by Mississippian (ca. 340 Ma)sediments (Wilson and Reyes, 1964).

Interpretation and Correlation with Events

in the Arequipa-Antofalla Basement

With the present data, we can reliably con-strain two orogenic events in the Eastern Cordi-llera of Peru at ca. 478 Ma and ca. 312 Ma, andloosely constrain a third to 440340 Ma. The

ca. 478 Ma event is interpreted to be of regionalextent, and the Neoproterozoic age previouslyassigned to this metamorphism (Dalmayrac etal., 1980) is interpreted to be a result of unre-solved multiple inheritance in bulk, unabradedzircon fractions.

These data also demonstrate the existenceof an Ordovician-Silurian magmatic belt(474442 Ma) in the Eastern Cordillera ofPeru. These plutons (the Sitabamba orthogneissand the Pacococha adamellite) have chemis-try compatible with formation at a continental

subduction zone (high large ion lithophileelement [LILE]/high field strength element[HFSE] ratios, negative Nb anomalies) simi-lar to the Ordovician-Silurian magmatic rocksthat intrude the Proterozoic gneisses of the Are-quipa-Antofalla basement (Loewy et al., 2004;Mukasa and Henry, 1990). Recent geochrono-

logical studies employing U-Pb TIMS singlezircon dating have demonstrated that the major-ity of early Paleozoic arc-related magmatism inthe Arequipa-Antofalla Block falls within the476440 Ma age range (Loewy et al., 2004),very similar to the age range for magmatism inthe Eastern Cordillera quoted here. Additionally,there is evidence for Ordovician metamorphismin the Arequipa-Antofalla basement (Loewy etal., 2004). Greenschist-facies metamorphism insouthern Peru occurred after the intrusion of a464 4 Ma megacrystic granite at Ocona andprior to the emplacement of undeformed gran-ites at Mollendo at 468 4 Ma (Loewy et al.,

2004), and amphibolite-facies metamorphismoccurred at Beln in northern Chile prior to theintrusion of massive granodiorites at 473 2 Maand 473 3 Ma (Loewy et al., 2004).

Combined, these data demonstrate the pres-ence of an Early Ordovician magmatic and met-amorphic belt that runs along the western mar-gin of the Arequipa-Antofalla basement and isoffset northeastward into the Eastern Cordillera(Fig. 8). We suggest that the change in strike ofthe belt results from the presence of an originalembayment on the western Gondwanan marginduring the early Paleozoic. This embayment

was then filled by subsequent accretion of oce-anic material (which represents the basement tothe Western Cordillera; Polliand et al., 2005),probably during the Carboniferous (Mikovic etal., 2005).

INTERPRETATION OF THE DETRITAL

ZIRCON DATA

In interpreting these data, the relationship ofthe sedimentary sequences (or the crustal blocksfrom which the granitic melts inherited theirzircon) with respect to the Gondwanan marginis critical. If the units are autochthonous (or

parautochthonous), then the detrital zircon agesignatures obtained can be used to constrain thepaleogeography of the hinterlandthe Gond-wanan margin of the north-central Andes. Thedetrital zircon spectrum itself cannot be usedto assess the exotic status of the units in ques-tion. Currently, there is a very limited data setfor the early Paleozoic sequences within theAndean belt, so the characteristic detrital zirconsignature of this margin (if one indeed exists)is not known. Detrital zircon data do exist forEarly Cambrian sandstones from the Argentine

Precordillera (Thomas et al., 2004), but this ter-rane is exotic to the Gondwanan margin (DallaSalda et al., 1992). In considering the detrital zir-con data presented here, we consider the Isiman-chi Formation of the Cordillera Real of Ecuador(99RS65) to be autochthonous with respect to theGondwanan margin, since it is a cratonic cover

sequence (Litherland et al., 1994), and thus itsdetrital zircon spectrum can be used to recon-struct the evolution of this segment of the Gond-wanan margin. Additionally, the Maraon Com-plex is commonly accepted as an autochthonousGondwanan margin sequence (e.g., Haeberlin,2002). The status of the other units and crustalblocks is uncertain. The Chiguinda Unit of theCordillera Real was thought to be allochthonous(Litherland et al., 1994), but recent research sug-gests an autochthonous origin (Pratt et al., 2005).The similar detrital age distributions in the twosamples would be consistent with an autochtho-nous origin for the Chiguinda Unit. Clearly, it is

difficult to assess whether the crustal blocks thatwere the source of the sampled granitic melts areautochthonous or allochthonous, but the countryrock at the depth of emplacement is composedof presumed autochthonous Maraon Complexmetasediments.

The majority of samples exhibit a prominentpeak between 0.45 and 0.5 Ga, which tempo-rally overlaps with the onset of subduction-related magmatism in the Eastern Cordillera ofPeru (this study), the Famatinian arc (Fig. 1) ofnorthern Argentina (Lork and Bahlburg, 1993;Pankhurst et al., 1998), Patagonia (Pankhurst

et al., 2006), the Arequipa-Antofalla basementof southern Peru and northern Chile (Fig. 1;Loewy et al. 2004), Venezuela (the Caparo Arcof Bellizzia and Pimentel, 1994), and Colombia(Boinet et al., 1985). Further prominent peaksare found within the ca. 0.50.65 Ga age range,and these ages are typical of the BrasliaPanAfrican orogenic cycles. Significantly, lessdetritus lies within the ca. 0.70.9 Ga age range(this trend is particularly evident in samples DC04/52 and SU 0324). Between a third and ahalf of all grains in each sample lie within the0.91.3 Ga age range, and the largest peaks arecommonly encountered between 1 and 1.2 Ga.

These ages are comparable to the Elzevirianorogeny of the Grenville orogen, or the Sun-sas orogen of the southwest Amazonian craton(Fig. 1; Santos et al., 2002). With the exceptionof sample DC 04/52, there is a very limitedamount of detritus in the 1.31.7 Ga age range.Very minor peaks are encountered at 1.71.9 Gaand at ca. 2.6 Ga in some samples, but theamount of detritus older than ca. 2 Ga is mini-mal. The tectonic significance of the detrital zir-con data is discussed next and combined withthe U-Pb zircon magmatic and metamorphic


11/15



constraints described previously to produce amodel for the evolution of the proto-Andeanmargin of the north-central Andes.

EVOLUTION OF THE GONDWANAN

MARGIN OF THE NORTH-CENTRALANDES

Pre-Gondwanan history (1000600 Ma)

Figure 8 is a schematic reconstruction of theevolution of the Gondwanan margin of the north-central Andes. The Sunsas orogen of the south-west Amazonian craton is invoked as a potentialsource region for the abundant ca. 0.91.3 Gapeaks observed in the detrital zircon popula-tions. We infer that the Sunsas orogen continues

in a northwestern direction underneath thethick pile of foreland sediments to the east ofthe northern Andes (Fig. 1), where it is likely tobe contiguous with the ca. 1 Ga gneissic base-ment inliers in the Colombian Andes (Fig. 8A;

Restrepo-Pace et al., 1997; Cordani et al., 2005).The Arequipa-Antofalla basement is likely tohave accreted at this time (Loewy et al., 2004).Since the amount of older detritus is minimal,the Sunsas orogen might have served as a topo-graphic barrier to the transport of detritus fromthe PaleoproterozoicArchean core of the Ama-zonian craton (Fig. 8A).

The time period between 800 and 600 Ma(Fig. 8B) is represented by a very small amountof detritus in the detrital zircon spectra (Fig. 2),but the source is uncertain. During this time,

Laurentia detached from the Rodinian supercontinent, contemporaneous with the amalgamation of the East and West Gondwana craton(Hoffman, 1991; Dalziel, 1991; Figures 11 and12 in Dalziel 1997; Meert and Torsvik, 2003

Pisarevsky et al., 2003). There is little evidenceon the western Gondwanan margin for magmatism at this time, although this is at least partlydue to the restricted amount of Precambrianbasement exposed within the Andean belt. Juvenile extensional magmatism (dacite dikes) habeen dated at 635 4 Ma in the Arequipa-Antofalla basement of northern Chile (Loewy et al.2004). This rifting event probably involved thepartial detachment of the Arequipa-Antofallabasement, which was subsequently reaccretedduring the Pampean orogen (Loewy et al., 2004)

SRS

RNJ

ca. 1 Ga - Grenville - Sunsas orogeny

ca. 480 Ma - Famatinian orogeny

Metamorphism and subduction-

related magmatism.

ca. 340 - 310 Ma. Carboniferous arc

in Peru. "Gondwanide" orogeny.

ca. 800 - ?600 Ma - Rodinia breakup ca. 530 Ma. Final assembly of Gondwana,active margin, Pampean orogen in south

RP

"OLD"AMAZONIA

"OLD"AMAZONIA

"OLD"AMAZONIA

"OLD"

AMAZONIA

"OLD"

AMAZONIA

"OLD"

AMAZONIA

AAB

AAB

AABAAB

AAB

AAB

RNJ

SRS

RP

RNJ

SRS

RP

RNJ

SRS

RP

RNJ

SRS

RP

RNJ

SRS

RP

RP

RNJ

PC

"OLD AMAZONIA" > 2 Ga

Rio Negro - Jurena: 1.8 -1.5 Ga

Sunss - Rondonia -San Ignacio: 1.5 - 1.0 Ga

Arequipa - Antofalla Basement

So Francisco Craton

Rio de la Plata

Precordillera (Laurentian)

Pre-Famatinian (480 Ma) marginsequences, includes "Old" Maraon

Post-Famatinian (480 Ma) marginsequences, includes "Young" Maraon

Subduction-related magmatismPC

SRS

AAB

Brasiliano Belts

RNJ

PC

SF

SF SF SF

SFSF

Braslia

belt

Braslia

belt

Braslia

belt

Braslia

belt

ca. 470 - 440 Ma. Continued magmatism

in Peru. Deposition of Young Maraon.

Collision of Precordillera to south.

CBA

E FD

Figure 8. Schematic recon

struction of the evolution o

the Gondwanan margin of the

north-central Andes. Basemen

geology is from Cordani et al

(2000), sequence of tectonic

events on the south-centra

proto-Andean margin is fromthe summary of Ramos and

Aleman (2000), and sequence

of tectonic events on the north

central proto-Andean margin

is from this study.


12/15

Chew et al.


A-type orthogneisses of mid-Neoproterozoicage (774 6 Ma) have been documented fromthe Grenvillian basement of the Precordilleraterrane and are interpreted as representing thebreakup of Rodinia (Baldo et al., 2006). Exten-sion-related volcanism related to the breakup ofRodinia has been identified in the Puncoviscana

fold belt of northwestern Argentina (Omarini etal., 1999), but this volcanism is primarily basicin nature and is therefore unlikely to be a signifi-cant source of zircon.

Other potential sources include the Precam-brian (ca. 650580 Ma) basement of easternVenezuela (Marechal, 1983) or the north-southtrending Braslia belt, which developed inresponse to the convergence of the Amazonianand So Francisco cratons (Fig. 1). The Bras-lia belt contains both 0.80.7 Ga syncollisionalgranitoids and 0.90.63 Ga arc meta-tonalitesand meta-granodiorites (Pimentel et al., 1999).However, the southwestward extensions of the

Braslia belt toward the western Gondwananmargin (the Paraguay belt and the Tucavaca beltof Bolivia) are younger (0.60.5 Ga; Pimentelet al., 1999) and thus cannot have been a sourceof detritus for the time period in question. It isdifficult to envisage how the Braslia belt couldhave been a source for the early Paleozoic basinsof the western Gondwanan margin basins whenthere is minimal detritus from the interveningAmazonian craton.

The Active Gondwanan Margin

By the onset of the Phanerozoic (Fig. 8C),the entire western part of the Gondwana marginwas active. This was the inception of the TerraAustralis orogen (Cawood, 2005; see Fig. 3). Asynthesis of the Paleozoic accretionary historyof the proto-Andean margin of Gondwana isgiven by Ramos and Aleman (2000).

Figure 8C illustrates the margin sequences ofthe north-central Andes, which would includethe old Maraon Complex identified in this studyand the Chiquero and San Juan Formations,which rest unconformably on the Arequipa-Antofalla basement in southern Peru (Caldas,1978). No magmatism has been identified in the

north-central Andes at the onset of Phanerozoic(Fig. 8C) to account for the ca. 550 Ma peaksseen in some of the detrital zircon spectra (e.g.,samples 99RS28, AM076), but there is abun-dant subduction-related granitoid magmatismand high-grade metamorphism, which initiatedat ca. 530 Ma in the Sierra Pampeanas (Fig. 1)in northern Argentina (Rapela et al., 1998;Lucassen and Becchio, 2003). We tentativelysuggest that our detrital zircon data documentthe existence of an early (pre-orogenic) phaseof Pampean subduction-related magmatism in

the north-central Andes that is presently buriedunderneath the present-day Andean chain oradjacent foreland sediments.

The new ages for major phase of metamor-phism in the Eastern Cordillera of Peru, pre-sented here as Famatinian (ca. 480 Ma) andnot Neoproterozoic, as previously thought

(Dalmayrac et al., 1980), demonstrate that thisEarlyMiddle Ordovician orogenic cycle runsthe length of the western Gondwanan mar-gin (Fig. 8D) and is broadly contemporaneouswith the onset of Famatinian metamorphismto the south (1834S) (470440 Ma; Lucas-sen and Franz, 2005, and references therein)and in Colombia and Venezuela (Bellizzia andPimentel, 1994). Additionally, the existenceof a subduction-related magmatic belt in theEastern Cordillera of Peru is demonstrated, andU-Pb zircon dating of arc plutons constrains thisevent to 474442 Ma. It is also represented byan important Early Ordovician peak in the detri-

tal zircon record. This demonstrates that Fama-tinian-age magmatism spans the length of themargin (Figs. 8D and 8E) from Venezuela (theca. 495425 Ma Caparo Arc of Bellizzia andPimentel, 1994) and Colombia (Boinet et al.,1985) to northern Argentina (ca. 490470 Ma;Pankhurst et al., 2000).

The Argentine Precordillera, a terrane derivedfrom southern Laurentia (Dalla Salda et al.,1992) that collided with Gondwana in the Mid-dle Ordovician (Ramos et al., 1986), shows thatLaurentian crustal fragments were clearly inter-acting with Gondwana during the early Paleo-

zoic. The nature of this interaction has provedcontentious (see Thomas and Astini, 2003, fora review), and various authors favor either (1) a Laurentiawestern Gondwana collision, pro-ducing a continuous TaconicFamatinian oro-genic belt (e.g., Dalla Salda et al., 1992; Dalzielet al., 1994), (2) a connection between the Pre-cordillera and a distal promontory on Laurentia(the Ouachita embayment), which then collidedwith Gondwana (e.g., Dalziel, 1997), or (3) aPrecordillera microcontinent, which detachedfrom Laurentia during the Cambrian and driftedacross the Iapetus ocean basin to collide withthe proto-Andean margin of Gondwana during

the Ordovician (Astini et al., 1995). Althoughit is somewhat beyond the scope of this articleto present detailed arguments for the competinghypotheses, Figure 8E depicts the Precordilleraterrane as a detached Laurentian microcontinent,in keeping with recent research, which empha-sizes a wide separation between Laurentia andGondwana during the EarlyMiddle Ordovician(Thomas et al., 2002) and evidence that the Pre-cordillera acted as an indenter during the MiddleOrdovician collision episode (Astini and Dvila,2004). Evidence from this study that Famatinian

metamorphism and magmatism was continuousalong the western Gondwanan margin showsthat the early Paleozoic evolution of westernGondwana was similar to that of Laurentia (theTaconic orogenic cycle) and that they had com-parable tectonic histories over several thousandkilometers along strike.

The latest stages of the evolution of the north-central proto-Andean margin involved the depo-sition of post-Famatinian cover sequences (theyoung Maraon Complex of this study; Fig. 8E).These sequences contain abundant Famatinianzircon (Fig. 2). A prominent magmatic gap thenappears to follow along the margin for nearly100 m.y. The youngest phase of pre-Carbonif-erous magmatism in the Eastern Cordillera ofPeru is dated to 442 Ma (DC 04/52; Fig. 5A),while the oldest Carboniferous magmatism isthe 344 Ma microgranite dike (DC 05/510;Fig. 7L) associated with the Balsas pluton inthe northern Eastern Cordillera. An identical

magmatic gap has been reported in the proto-Andean margin further to the south in northernChile and northwestern Argentina accordingto Bahlburg and Herv (1997), who invoked apassive-margin setting during this period. Thisperiod of magmatic quiescence along the mar-gin from northern Peru (5S) to northern Chile(30S) is slightly complicated by the SilurianDevonian bulk fraction U-Pb zircon ages (lowerintercept ages) for arc-related magmatism inthe Arequipa-Antofalla basement of southernPeru (Mukasa and Henry, 1990). However,more recent single-crystal U-Pb zircon dating

from the same batholith did not yield any agesyounger than 440 Ma (Loewy et al., 2004), andit is very likely that the lower intercept ages ofMukasa and Henry (1990) were anomalouslyyoung due to the difficulty in deconvoluting thecombined effects of Pb loss and inheritance inbulk zircon U-Pb data.

The Mississippian (Fig. 8F) was marked bythe renewal of subduction-related magmatism.However, the tectonic setting of the north-cen-tral proto-Andean margin is complicated. Thenorthern parts of the margin (e.g., Venezuela)record continental collision (the Alleghenianorogeny) between Gondwana and Laurus-

sia (Ramos and Aleman, 2000). Farther to thesouth, in the Eastern Cordillera of Peru, there isa transition to an active-margin setting. A phaseof I-type subduction-related granitic magma-tism at ca. 342 325 Ma is followed by Penn-sylvanian (ca. 312 Ma) high-grade metamor-phism and crustal anatexis. This event has beenrelated to the accretion of an outboard oceanicterrane that is now buried beneath the WesternCordillera (Mikovc et al., 2005). This eventis broadly contemporaneous with the Toco tec-tonic event and the high-pressure metamorphism


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in the Sierra Limon Verde Complex of northernChile (Bahlburg and Herv, 1997), which isinterpreted as a deformed convergent-marginaccretionary complex. Additionally, a similarphase of Mississippian I-type granitic magma-tism followed by Pennsylvanian S-type granitesrepresenting crustal anatexis has been identified

in Patagonia (Pankhurst et al., 2006), where col-lisional orogeny is related to the accretion of theDeseado terrane. Combined, this phase of Penn-sylvanian tectonism can be considered to be partof the Gondwanide orogeny, a pan-Pacific oro-genic event that represents the terminal phase inthe Terra Australis orogen (Cawood, 2005) priorto the final assembly of Pangea.

CONCLUSIONS

The detrital zircon data from the north-cen-tral proto-Andean margin demonstrate that thebasement to the western Gondwanan margin

was likely composed of a metamorphic belt ofGrenvillian age, upon which an early Paleozoicmagmatic belt was situated in a similar way tothe Sierra Pampeanas and Famatina terranes ofnorthern Argentina. This is based on the pres-ence of prominent peaks between 0.450.65 Maand 0.91.3 Ga in the detrital zircon record ofPaleozoic sequences in the Eastern Cordillerasof Peru and Ecuador.

Plutons associated with this early Paleozoicsubduction-related magmatic belt have beenidentified in the Eastern Cordillera of Peru andhave been dated by U-Pb zircon TIMS and ion

microprobe to 474442 Ma. This is in closeagreement with the ages of subduction-relatedmagmatism in the Arequipa-Antofalla basement(Loewy et al., 2004). This early Paleozoic arc isclearly not linear since it jumps from a coastallocation in the Arequipa-Antofalla basement toseveral hundred kilometers inland in the East-ern Cordillera further to the north (Fig. 2). Thisis interpreted as an embayment on the proto-Andean margin at the time the arc was initiated;if this is the case, the northern termination of theArequipa-Antofalla basement in the vicinity ofLima (Fig. 2) is an Ordovician or older feature.

The arc magmatism pre- and postdates phases

of regional metamorphism in the Eastern Cordi-llera of Peru. U-Pb zircon ion-microprobe datingof zircon overgrowths in high-grade leucosomesdemonstrates the presence of a metamorphicevent at ca. 478 Ma and refutes the previouslyassumed Neoproterozoic age for orogeny in thePeruvian Eastern Cordillera. The presence of anEarly to Middle Ordovicianage magmatic andmetamorphic belt in the north-central Andesdemonstrates that Famatinian metamorphismand subduction-related magmatism were con-tinuous from Patagonia (Pankhurst et al., 2006)

through northern Argentina and Chile to as farnorth as Colombia and Venezuela, a distanceof nearly seven thousand kilometers. The pres-ence of an extremely long EarlyMiddle Ordo-vician active margin on western Gondwanainvites comparison with the TaconicGrampianorogenic cycle of the eastern Laurentia margin

(which is of similar age and strike length) andsupports models that have these two active mar-gins facing each other during the Ordovician.

U-Pb zircon ion-microprobe dating of zirconovergrowths in migmatites have been dated atca. 312 Ma, and they represent a previouslyunreported high-grade Gondwanide event thataffected the Peruvian segment of the proto-Andean margin. The original relationshipbetween the Carboniferous and Ordovician met-amorphic belts is uncertain because they werelater affected by Andean (EoceneOligocene)thrusting, but, overall, the pattern of crustalgrowth in the north-central Andes (Fig. 8F)

implies that this area was dominated by a seriesof progressive crustal accretion events, whichresulted in a series of age domains that becomeyounger away from an old Amazonian core.

APPENDIX

U-Pb Zircon Geochronology

Zircons were separated from several kilogramsof sample by conventional means. The sub300 mfraction was processed using a Wilfey table, and thenthe Wilfey heavies were passed through a Frantz mag-netic separator at 1 A. The nonparamagnetic portionwas then placed in a filter funnel with di-iodomethane.

The resulting heavy fraction was then passed againthrough the Frantz magnetic separator at full current.A side slope of 1 was used to separate nonparamag-netic zircons for TIMS and ion-microprobe analysisto maximize concordance, while a side slope of 10was used for LA-ICP-MS analyses to prevent poten-tial fractionation of the detrital population. All zirconswere handpicked in ethanol using a binocular micro-scope, including those for detrital zircon analysis. Zir-cons for TIMS analyses were air-abraded to removemarginal zones that are prone to Pb loss.

TIMS Analytical Technique

The zircon grains were washed in dilute HNO3

and rinsed several times in distilled water and acetone

in an ultrasonic bath. A mixed205

Pb-235

U spike wasadded prior to dissolution in a mixture of HF andHNO

3 using steel-jacketed Teflon bombs. Chemical

separation of Pb and U was done on anion exchangeresin using minimal amounts of ultrapure acids. Isoto-pic analysis was performed on a MAT262 mass spec-trometer equipped with an ion counting system. Thelatter was calibrated by repeated analysis of the NBS982 standard using the 208Pb/206Pb ratio of 1.00016 formass bias correction (Todt et al., 1996) and second-order nonlinearity calibration of the electron multiplier(Richter et al., 2001). Total procedural Pb blank wasestimated at 0.8 0.5 pg. Common Pb in excess ofthis amount was corrected with the model from Staceyand Kramers (1975) for the respective 206Pb/238U age

of the zircon. The uncertainties of the isotopic composition of the spike, blank, and common Pb were takeninto account and propagated to the final uncertaintieof isotopic ratios and ages. Ages were calculated usingISOPLOT (Ludwig, 2003). U-Pb data are plotted a2error ellipses (Fig. 5).

Determination of Hf Isotopic Composition

The Hf fraction was isolated from the Zr + Hf +rare earth element (REE) fraction of the Pb columnchemistry using Eichrom Ln-spec resin and measured in static mode on a NuPlasma multicollectoICP-MS using MCN-6000 and ARIDUS nebulizerfor sample introduction. The Hf isotopic values werecorrected for a 176Lu/177Hf value of 0.0005, typical ozircon. The Hf isotopic ratios were corrected for masfractionation using a 179Hf/177Hf value of 0.7325 andnormalized to 176Hf/177Hf of 0.28216 of the JMC-475standard. The isotopic ratios for the chrondritic uniform reservoir (CHUR) are those of Blichert-Toft andAlbarde (1997).

Ion-Microprobe Analytical Technique

Zircons were mounted in a resin disk along withthe zircon standard and polished to reveal the graininteriors. The mounts were gold-coated and imagedwith a Hitachi S-4300 scanning electron microscop(SEM), using a cathodoluminescence probe (CL) toimage internal structures, overgrowths, and zonationSecondary electron mode (SE) imaging was employedto detect fractures and inclusions within the grainsU-Th-Pb zircon analyses (Table DR4, see footnote 1were performed on a Cameca IMS 1270 ion microprobe following methods described by Whitehouse eal. (1999), which were modified from Whitehouse eal. (1997). U/Pb ratio calibration was based on analyses of the Geostandards zircon 91500, which has anage of 1065.4 0.3 Ma and U and Pb concentrations o80 and 15 ppm, respectively (Wiedenbeck et al., 1995)

Replicate analyses of the same domain within a singlzircon were used to independently assess the validity othe calibration. Data reduction employed Excel macrodeveloped by Whitehouse at the Swedish Natural History Museum, Stockholm. Age calculations were madusing Isoplot version 3.02 (Ludwig, 2003). U-Pb dataare plotted as 2error ellipses (Fig. 7). All age errorquoted in the text are 2unless specifically stated otherwise. Common Pb corrections were only applied tosamples that exhibited significant levels of 204Pb, andthey are indicated in Table DR4 where applied (seefootnote 1). Corrections assume a present-day averageterrestrial common Pb composition (Stacey and Kramers, 1975), i.e., 207Pb/206Pb = 0.83. A detailed rationalfor choosing present-day Pb as a contaminant is givenby Zeck and Whitehouse (1999).

LA-ICP-MS Analytical Technique

Zircons were mounted in 2.5-cm-diameter epoxyfilled blocks, polished, and cleaned in deionized wateand ethanol. Isotopic analysis of zircons by laser-ablation ICP-MS followed the technique described inKosler et al. (2002). A Thermo-Finnigan Element 2sector field ICP-MS coupled to a 213 NdYAG lase(New Wave UP-213) at Bergen University was usedto measure Pb/U and Pb isotopic ratios in zircons. Thsample introduction system was modified to enablesimultaneous nebulization of a tracer solution and laseablation of the solid sample (Horn et al., 2000). NaturaTl (205Tl/203Tl = 2.3871; Dunstan et al., 1980), 209Bi, and


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Chew et al.


enriched 233U and 237Np (>99%) were used in the tracersolution, which was aspirated to the plasma in an argon-helium carrier gas mixture through an Apex desolva-tion nebulizer (Elemental Scientific) and a T-piece tubeattached to the back end of the plasma torch. A heliumgas line carrying the sample from the laser cell to theplasma was also attached to the T-piece tube.

The laser was set up to produce energy density of~3 J/cm2at a repetition rate of 10 Hz. The laser beam

was imaged on the surface of the sample placed in theablation cell, which was mounted on a computer-drivenmotorized stage of a microscope. During ablation, thestage was moved beneath the stationary laser beam toproduce a linear raster (~15 50 m) in the sample.Typical acquisitions consisted of a 45 s measurementof analytes in the gas blank and aspirated solution,particularly 203Tl-205Tl-209Bi-233U-237Np, followed bymeasurement of U and Pb signals from zircon, alongwith the continuous signal from the aspirated solu-tion, for another 150 s. The data were acquired in timeresolvedpeak jumpingpulse counting mode with 1point measured per peak for masses 202 (flyback), 203and 205 (Tl), 206 and 207 (Pb), 209 (Bi), 233 (U), 237(Np), 238 (U), 249 (233U oxide), 253 (237Np oxide), and254 (238U oxide). Raw data were corrected for deadtime of the electron multiplier and processed offline in

a spreadsheet-based program (Lamdate; Kosler et al.,2002) and plotted on concordia diagrams using Iso-plot (Ludwig, 2003). Data reduction included correc-tion for gas blank, laser-induced elemental fraction-ation of Pb and U, and instrument mass bias. Minorformation of oxides of U and Np was corrected forby adding signal intensities at masses 249, 253, and254 to the intensities at masses 233, 237, and 238,respectively. No common Pb correction was appliedto the data. Details of data reduction and correctionsare described in Kosler et al. (2002) and Kosler andSylvester (2003). Zircon reference material 91500(1065 Ma; Wiedenbeck et al., 1995) and an in-housezircon standard (338 Ma; Aftalion et al., 1989) wereperiodically analyzed during this study.

ACKNOWLEDGMENTS

This study was funded by the Swiss National Sci-ence Foundation. Use of the NordSIMS was supportedby the European Communitys program Structuringthe European Research Area under SYNTHESYS atthe Swedish Museum of Natural History, project no.SE-TAF 663. The NordSIMS facility is financed andoperated under an agreement between the researchcouncils of Denmark, Norway, and Sweden, theGeological Survey of Finland, and the SwedishMuseum of Natural History. We are extremely grate-ful to J. Machar, R. Mucho, A. Sanchez, J. Galdos,A. Zapata, S. Carrasco, and R. Mamani of the Geo-logical Survey of Peru (INGEMMET), C. Moreno ofSan Marcos University in Lima, and Y. Haeberlin and

S. Beuchat of the University of Geneva for scientificand logistical support during the field seasons in Peru.The financial and logistical support of L. Seijas andCompania Minera Poderosa S.A. is also gratefullyacknowledged. We are very grateful to A. von Quadtfor technical assistance with the mass spectrometryat ETH Zrich. The careful and insightful reviews ofRicardo Astini, Robert J. Pankhurst, and AssociateEditor Peter Cawood are gratefully acknowledged.This is NordSIMS publication 173.

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