Reconstructing the Late Paleozoic - Peru.pdf

8/10/2019 Reconstructing the Late Paleozoic - Peru.pdf

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Abstract
http://archive-ouverte.unige.ch/unige:23095


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Universite de Geneve

Departement de MineralogieFaculte des sciences

Professeur Urs SchalteggerDr. Richard Spikings

Reconstructing the Late Paleozoic - Early

Mesozoic plutonic and sedimentary record

of south-east Peru: Orphaned back-arcs

along the western margin of Gondwana

THESE

presentee a la Faculte des sciences de lUniversite de Genevepour obtenir le grade de Docteur es sciences, mention Sciences de la Terre

par

Martje Jel Reitsma

de Boxtel (Pays-Bas)

These No 4459


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UNIVERSITE

DEGENVE

F A C U L T D E S S C I E N C E S

octorat

es

s ciences

Mention sciences de la Terre

T h s e de

Madame

Martje Je REITSM

intitule

:

Reconstructing th

LatePaleozoic

Early

Mesozoic

Plutonic and SedimentaryRecordof

South East

Peru:

Orphaned

Back arcs

alongth

Western

AAargino f

Gondwana

La

Facult des sciences, sur le pravis de

Messieurs

U. S C H A L T E G G E R ,professeur ordinaire

et directeur de thse (Dpartement deminralogie),R.SPIKINGS ,docteuretcodirecteur

de thse (Dpartement de minralogie), W.W I N K LE R , professeur (Geologisches

Institut,

Eidgenssische

Technische Hochschule Zurich, Schweiz),

D.C H EW , docteur (Trinity

Collge

Dublin, Ireland) et O. MUNTENER, professeur

(Institut

de

minralogie

et de

gochimie,

Universit de Lausanne), autorise l'impression de la prsente thse, sans exprimer

d'opinionsur es propositions qui y sont nonces.

Genve,

le

3

aot2012

Thse

4459

N.B.-

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Doyen/ean AAarc

T R I S C O N E

La

thse doit porter la

dclaration

prcdente et remplir les conditions numres

dans

les Informations relatives aux thses de doctorat l Universit de Genve .


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i

Reconstructing the Late Paleozoic - Early Mesozoic plutonic and sedimentary

record of south-east Peru:

Orphaned back-arcs along the western margin of Gondwana

ABSTRACT

This thesis investigates the sedimentary, plutonic and tectonic evolution of the Eastern

Cordillera region of south-east Peru during assembly of the Pangea supercontinent and

subsequent early pulses of break-up. We present a chronostratigraphic framework for the

Carboniferous to Triassic sedimentary and plutonic record using geochronological, geochemical

and isotopic methods, integrated with field observations and data published in the literature. The

work of this dissertation is presented over three chapters which each have a distinct focus.

The first chapter investigates the plutonic record and its relation to growth of the continental

crust. With U-Pb zircon dating we demonstrate that magmatism was intermittently active over a

period of nearly half a billion years and can be separated into six magmatic pulses with a

duration of 20 myr or less ranging from the Ordovician to the Miocene (figure I). The similar

mineralogy and whole rock geochemistry of the Ordovician, Carboniferous, Permian and

Triassic granitoids point to a common source and arguably a comparable geodynamic setting

during melt generation. Plutonic remnants of the Ordovician and Jurassic arcs are preserved on

the coastline of south Peru and hence lead to interpretation of the contemporaneous plutons in

the Eastern Cordillera as a back-arc. This study argues that also the plutons emplaced in the

intervening period were intruded in a back-arc setting based on the following arguments: 1) The


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Abstract

ii

minimum inferred distance to the paleo-trench, 2) the extensional tectonic setting in the

Carboniferous, Permian and notably the Triassic which cannot be resolved with granitoid

emplacement under flat slab conditions, 3) the peraluminous nature of the granitoids combined

with only minor quantities of hydrous minerals. The lack of Carboniferous to Triassic arc plutons

in coastal Peru is assigned to obliteration by vigorous subduction erosion that has been reported

for the Cenozoic.

Hf, Nd and Sr isotopic compositions demonstrate that magmatism generated in the back-arc

region mainly formed by remelting of the crust and thus did not contribute significantly to crustal

growth. The Ordovician to Triassic granitoids plot on the same crustal evolution path as melts

that separated from the depleted mantle during the Grenville/Sunsas Orogeny which thus makes

them the most probable source. Only the radiogenic Hf-isotopic signature of the volumetrically

minor Jurassic plutonic pulse cannot be accounted for by a dominantly Sunsas-aged source and is

interpreted to have formed by adiabatic decompression melting of an enriched mantle reservoir.

We conclude that far-field back-arc regions are inefficient in generating large amounts of new

continental crust because mantle melting in the absence of a slab-derived fluid can only be

achieved when extreme lithospheric extension occurs.

The second chapter presents sedimentological, geochronological and geochemical data

acquired from the sedimentary and volcanic rocks of the extension related Mitu Group. Those

data are used to develop a tectonic model for the rift sequence, and to propose driving forces for

its formation and termination. Sections through the Mitu Group were studied at four different

locations spread 670 km along orogenic strike from central to south-east Peru. U-Pb zircon


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Abstract

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dating of volcanic or sedimentary rocks at the base of each section demonstrates that deposition

initiated in the Middle Triassic, contrary to the previously assumed Permian start (figure I). The

Norian (Upper Triassic) termination of the Mitu Group makes deposition entirely coeval with the

voluminous Triassic plutonic pulse.

Alkaline volcanism and large thickness variations within the Mitu Group point to deposition

under an extensional regime, most likely in a back-arc setting. During the initial, amagmatic

stage of the Mitu Group, extension was spread over a large area. Subsequently deformation was

localized and volcanic activity commenced. Lithospheric thinning resulted in thermal doming of

the crust and accounts for subaerial deposition of the Mitu Group. Based on the lateral offset

between the syn- and post-rift basin axes, extension is proposed to occur with a large simple

shear component. Asymmetric extension of the lithosphere gave rise to uplift of a rift shoulder to

the east of the Mitu Group hosting grabens. U-Pb detrital zircon ages demonstrate that the rift

shoulder efficiently blocked craton derived sediments and instead zircons in sandstones of the

Mitu Group were derived from syn-depositional volcanism or from the rocks exposed on the rift

shoulder. Termination of the Mitu Group and associated plutonism is interpreted as closure of

the back-arc basin due to landward migration of the Chocolate arc of southern Peru in the Upper

Triassic.

In the third chapter we introduce the first chronostratigraphic framework for the

Carboniferous Early Permian period in Peru based on radio-isotopic dates on volcanic and

detrital samples (figure I). The model serves to reconstruct the paleogeographic and tectonic


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Abstract

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history of Peru as it formed part of the western Gondwana margin during assembly and suturing

of the Pangea supercontinent.

As Pangea amalgamated in the Mississippian, compressional stresses started to concentrate

on the fringes of the supercontinent resulting in a resumption of arc magmatism in north and

central Peru. Among the subaerial deposits of the Ambo Group of southern Peru no direct

evidence for contemporaneous volcanism was detected, likewise the detrital zircon record of

these fluvial sandstones attests that Mississippian volcanism was not pronounced in this region.

This lack of (back-arc) magmatism in comparison to north Peru could be ascribed to the absence

of an extensional back-arc, flat slab subduction or strike-slip convergence along the southern

Peruvian margin. Volcanism became more pronounced during deposition of the Pennsylvanian,

shallow marine Tarma Formation and peaked simultaneously with a deformational event that

disturbed sedimentation in central Peru. On the contrary, the basins in south-east Peru were not

affected and experienced continuous subsidence resulting in a build up of platform carbonates of

the Copacabana Formation. A regression in the Early Permian led to retreat of the epeiric sea to

the present-day subandean region and initiation of fluvial deposition in the Eastern Cordilleran

region. Regression coincided with a major pulse of back-arc plutonism and is ascribed to thermal

doming of the crust due to lithospheric thinning. U-Pb ages and Hf-isotopic ratios in detrital

zircons suggest that zircons were sourced from granitoids that are only slightly younger than the

stratigraphic age of the sandstone, corroborating an extensional setting that accounts for quick

exhumation of the plutonic rocks.


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Figure I: Generalized stratigraphy of the sedimentary and plutonic records for south-east Peru.

Now column based on data from this thesis and Miskovic et al. (2009).

Figure I: Stratigraphie gnrale de lenregistrement sdimentaire et plutonique pour le sud-est de

Prou. Colonne Now est bas sur des donnes de cette thse et Miskovic et al. (2009).


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Abstract

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Reconstruction de lenregistrement plutonique et sdimentaire du Palozoque

Suprieur au Msozoque Inferieur dans le sud-est pruvien:

Arrire-arcs orphelins le long de la marge ouest du Gondwana.

RSUM

Cette thse traite de lvolution sdimentaire, plutonique et tectonique dans la rgion de la

Cordillre est du sud-est pruvien lors de la formation du supercontinent de la Pange et lors

dpisodes ultrieurs de dislocation prcoce. Nous prsentons un cadre chronostratigraphique

pour les enregistrements sdimentaires et plutoniques allant du Carbonifre au Trias, au moyen

doutils gochronologiques, gologiques et isotopiques, combins aux observations de terrains

ainsi quaux travaux antrieurs publis dans la littrature scientifique. Ce travail est prsent en

trois chapitres qui prsentent chacun diffrents aspects de cette thse.

Le premier chapitre traite de lenregistrement plutonique en relation avec la croissance de la

crote continentale. Grce des datations U-Pb sur zircons nous dmontrons que le magmatisme

a t actif par intermittence sur une priode de prs dun demi milliard dannes qui peut se

subdiviser en six phases magmatiques, dune dure de 20 Ma ou moins, allant de lOrdovicien au

Miocne (figure I). Luniformit minralogique et gochimique des granitodes de lOrdovicien,

du Carbonifre, du Permien et du Trias semble indiquer une source commune et un contexte

godynamique comparable lors de la production magmatique.

Les vestiges plutoniques des arcs ordoviciens et jurassiques sont prservs sur la cte sud du

Prou et ont ainsi conduit interprter le plutonisme contemporain de la Cordillre est comme


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Abstract

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tant darrire-arc. Cette tude soutient la thse selon laquelle lemplacement plutonique entre

ces deux priodes sest galement fait dans un contexte darrire-arc, ceci sur la base des

arguments suivant : 1) la distance minimum suppose jusqu la palo-fosse, 2) le contexte de

tectonique extensive au Carbonifre, au Permien et notamment au Trias qui ne peut pas tre

concili avec lemplacement des granitodes en contexte de subduction plane, 3) le caractre

pralumineux des granitodes, combin des quantits mineures de minraux hydrats.

Labsence des arcs carbonifres triasiques sur la cte pruvienne est attribue loblitration

induite par une vigoureuse subduction-rosion qui a t report pour le Cnozoque.

Les compositions isotopiques de lHf, du Nd et du Sr dmontrent que le magmatisme gnr

en zone darrire-arc sest majoritairement form par fusion crustale et na pas significativement

contribu une croissance crustale. Les granitodes de lOrdovicien au Trias prsentent le mme

chemin dvolution crustale que les magmas qui se sont diffrencis du manteau appauvri lors de

lorognse de Grenville/Sunsas, faisant de ces derniers la source la plus probable. Seule la

signature radiognique des isotopes de lHf de la phase plutonique mineure du Jurassique ne peut

pas sexpliquer par une source dont lge dominant rfre au Sunsas, et est plutt interprter

comme stant form par fusion par dcompression adiabatique dun rservoir mantellique

enrichi. Nous concluons que les rgions recules darrire-arc ne permettent pas de gnrer

dimportantes quantits de nouvelle crote continentale car la fusion mantellique en labsence de

fluides issus de la plaque plongeante peut seulement avoir lieu lors dune extension extrme de la

lithosphre.

Le second chapitre prsente les donnes sdimentaires, gochronologiques et gochimiques

des roches sdimentaires et volcaniques du Groupe Mitu dposs dans un contexte extensif. Ces

donnes sont utilises pour dvelopper un modle tectonique pour la squence de rift, et pour


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Abstract

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proposer les forces motrices pour sa formation et son interruption. Des sections travers le

Groupe Mitu ont t tudies sur quatre endroits rpartis sur 670 km le long de lorogne du

centre au sud-est du Prou. Les datations U-Pb sur zircons des roches volcaniques et dtritiques

la base de chaque section dmontrent que la dposition a commenc au Trias Moyen,

contrairement un dbut au Permien qui tait prcdemment suppos (figure I). La terminaison

du Groupe Mitu au Norien (Trias Suprieur) rend la dposition entirement contemporaine de la

volumineuse phase plutonique triasique.

Le volcanisme alcalin et les importantes variations dpaisseur du Groupe Mitu indiquent

que la dposition sest faite dans un rgime extensif, probablement dans un contexte darrire-

arc. Lors de la phase initiale sans activit magmatique du Groupe Mitu, lextension tait

remarquablement tendue, sensuivi de la dformation localise et le dbut de lactivit

volcanique. Lamincissement lithosphrique eu pour rsultat un soulvement thermique de la

crote et explique la dposition subarienne du Groupe Mitu. Sur la base du dcalage latral

entre les axes des bassins post-rift et syn-rift, il est propos que lextension prsente une

composante majeure de cisaillement simple. Lextension asymtrique de la lithosphre a donne

lieu au soulvement dun paulement de rift lest du graben li au Groupe Mitu. Les ges U-Pb

sur zircons dtritiques dmontrent que cet paulement a efficacement bloqu lafflux

sdimentaire provenant du craton et que les zircons dans les grs du Groupe Mitu drivent du

volcanisme contemporain de la sdimentation ou des roches exposes lpaulement. La

terminaison du Group Mitu et du plutonisme associ est interprte comme la clture du bassin

darrire-arc due la migration de larc Chocolate au sud du Prou vers lintrieur des terres au

Trias Suprieur.


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Dans le troisime chapitre nous introduisons le premier cadre chronostratigraphique pour la

priode du Carbonifre au Permien Infrieur au Prou, sur la base de datation radio-isotopiques

sur chantillons volcaniques et dtritiques (figure I). Le modle tend reconstruire lhistoire

palogographique et tectonique du Prou lorsquil se situait sur la marge ouest du Gondwana

lors de la formation et la suture du supercontinent Pange.

Durant lamalgamation de la Pange au Mississippien, les contraintes compressives ont

commenc se concentrer la priphrie du supercontinent ce qui induit un redmarrage du

magmatisme darc au nord et au centre du Prou. Parmi les dpts subariens du Groupe Ambo

du sud du Prou, aucune preuve de volcanisme contemporain na t dtecte de mme que

lenregistrement des zircons dtritiques dans les grs fluviatiles atteste que le volcanisme

mississippien na pas t trs marqu dans cette rgion. Cette absence de magmatisme (darrire-

arc) en comparaison au nord du Prou peut tre attribue labsence dextension darrire-arc,

une subduction plane ou une convergence dcrochante le long de la marge sud du Prou. Le

volcanisme commena tre plus prononc lors de la sdimentation au Pennsylvanien de

sdiments marins peu profonds de la Formation Tarma alors que la sdimentation au Prou

central tait perturbe par un vnement de dformation. Au contraire, les bassins du sud-est du

Prou nont pas t affects et ont connu une subsidence qui donna lieu la construction dune

plateforme carbonate correspondant la Formation Copacabana. Une rgression au Permien

Infrieur mena au retrait de la mer peirique jusqu la rgion subandine actuelle et linitiation

dune sdimentation fluviatile dans la rgion de la Cordillre Est. La rgression concide avec

une phase majeure de plutonisme darrire-arc et est attribue un soulvement thermique de la

crote en rponse un amincissement lithosphrique. Les ges U-Pb et les rapports isotopiques

de lHf sur les zircons dtritiques suggrent que les zircons proviennent de granitodes


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lgrement plus vieux que lge stratigraphique des grs, ce qui concorde avec un contexte

extensif capable dexhumer rapidement les roches plutoniques.


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Table of contents

Abstract i

Rsum en Francais vi

Introduction to the thesis 1

Chapter 1: Crustal reworking in Paleozoic Early Mesozoic orphaned back-arcs on the

western Gondwana margin, south-east Peru

1.

Introduction 9

2. Geology of the Eastern Cordillera of southern Peru 13

3. Analytical methods

3.1Whole rock geochemistry 16

3.2Zircon U-Pb geochronology 18

3.3Lu-Hf isotope analyses 21

4. Results

4.1Research area and sample material 22

4.2U-Pb geochronology and Hf isotopes 23


5. Discussion

5.1

Geodynamic setting: Arc, back-arc or rift? 40

5.2Crustal reworking along the western Paleozoic Gondwanan margin 53

5.3Relating geodynamic setting to pulses of crustal growth 57


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Table of contents

6. Conclusions 60

7. References 62

Chapter 2: Triassic simple shear, back-arc extension and deposition of the Mitu Group in

south-eastern Peru

1. Introduction 67

2. Geological framework and previous work 72

3.

Methods

3.1Sampling strategy 75



3.4Lu-Hf isotope analyses on volcanic zircon 78

4. Results

4.1Stratigraphy and U-Pb zircon dating 78


5. Discussion

5.1Age of the Mitu Group 95

5.2Geochemical characterization of Triassic-Jurassic volcanism 99

5.3

Geodynamic setting 102

6. Conclusions 110

7. References 112


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Table of contents

Chapter 3: A late Paleozoic back-arc basin along the western margin of Gondwana: Age

and paleogeography of Permo-Carboniferous sedimentary rocks of south-east Peru

1. Introduction 115

2.

Geological setting 119

3. Methods

3.1Sampling strategy 122

3.2Whole rock major, trace and rare earth element analysis 123


3.4Lu-Hf isotope analyses of zircon 125

4. Results

4.1Stratigraphy and geochronology 129

4.2Hf-isotopic ratios in detrital zircon 138


5. Discussion

5.1

Paleogeography 141

5.2Tectonic implications 148

6. Conclusions 155

7. References 157


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Table of contents

Conclusions and Outlook 159

Appendices chapter 1 165



Acknowledgement 212


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1

Reconstructing the Late Paleozoic - Early Mesozoic plutonic and sedimentary

record of south-east Peru:

Orphaned back-arcs along the western margin of Gondwana

INTRODUCTION TO THE THESIS

The Peruvian margin is characterized by a hyper arid coastal desert, conditions that have

prevailed since at least the Miocene (Dunai et al., 2005). The low erosion rates on the continent

lead to a reduced sediment flux into the trench and hence a high degree of coupling between the

subducting and overriding plates. The climatic conditions of coastal Peru are hence in a large

part responsible for the high elevation of the Peruvian Eastern Cordillera with its numerous

peaks over 6000 m. However, elevated shear stresses in the sediment starved subduction zone,

amplified by high plate velocities, have also resulted in vigorous subduction erosion (Clift and

Hartley, 2007; Clift et al., 2003; Stern, 2011). This accounts for exposure of Grenville aged

metamorphic basement and plutonic remnants of the Ordovician Famatinian arc (Loewy et al.,

2004) as well as the Middle Jurassic Ilo batholith (Boekhout et al., in press) on the coastline of

south Peru and is perhaps potentially responsible for complete obliteration of Carboniferous

through Triassic continental arcs.

Another characteristic of the south Peruvian margin is the lack of terrane accretion

throughout the Phanerozoic. In contrast to the segments to the north and the south, the North

Peruvian margin experienced only accretion of the Paracas Terrane in the Ordovician (Ramos,

2010) while the Chilean and Ecuadorian-Colombian margins both have complicated geological


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Thesis introduction

2

histories including accretion of multiple terranes (Ramos, 2010; Rapalini, 2005; Spikings et al.,

2001, 2005, 2010). This leaves the geology of southern Peru comparatively simple although the

rocks exposed in the Eastern Cordillera have still been disturbed by two major orogenic events:

the Eohercynian Orogeny that affected the Devonian and older rock units and the Cenozoic

Andean Orogeny that deformed the entire Phanerozoic sequence (figure I).

In this thesis we investigate plutonic rocks and volcano-sedimentary sequences in the area

of the Eastern Cordillera of southern Peru between the city of Cerro de Pasco (10.8S) and Lake

Titicaca (16.1S). A detailed investigation of the Ambo, Tarma-Copacabana and Mitu groups,

which overlie the metamorphic basement generated during the Late Devonian Early

Carboniferous Eohercynian Orogeny (figure I), will lead to the reconstruction of the

Carboniferous to Triassic evolution of the Western Gondwana margin during assembly and early

break-up of Pangea (figure II). However, reconstruction of the sedimentary record and its

relation to the plutonic record is hampered by scarce age constraints and limited geochemical

data. The aim of this thesis is to i) provide more accurate and precise age constraints for the

sedimentary, plutonic and tectonic record by U-Pb zircon dating and ii) decipher the magmatic

record using whole rock and isotope geochemistry. These new data are integrated with published

information from the literature and field observations to reconstruct the geodynamic evolution of

the western Gondwana margin during the Carboniferous to Triassic.

The new sedimentary, magmatic and tectonic temporal framework (figure I) will be used to

answer the following general questions:

Under which geodynamic conditions were the plutons and sedimentary rocks

emplaced?


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Thesis introduction

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Can changes in the sedimentary, plutonic and tectonic record be related to the

supercontinent cycle?

Are we able to distinguish between main arc and back-arc plutonism?

Did plutonism in the Eastern Cordillera of Peru contribute to crustal growth?

Which clues can we deduce from the stratigraphic record to reconstruct the mode of

lithospheric stretching?

Analytical Methods

U-Pb dating of zircon is generally considered the most accurate method to determine the

emplacement age of a granitoid, because zircon has extremely low initial Pb/U ratios and a high

closure temperature for lead diffusion. Zircon has the additional advantage of being a common

accessory mineral in granitoids that cover a wide range of compositions. Consequently we

employed U-Pb zircon dating to constrain the plutonic pulses in the research area.

On the contrary, constraining the age and duration of the Late Paleozoic Early Mesozoic

sedimentary units of central and south-east Peru proved to be more challenging. Radio-isotopic

dating of volcanic deposits intercalated between sedimentary rocks is the most reliable method to

obtain direct age constraints for a sedimentary sequence. The abundance of volcanic material in

the Carboniferous to Early Jurassic units was not the limiting factor in this study, though its

intense alteration due to weathering made40

Ar/39

Ar dating of lavas using either their groundmass

or phenocrysts impossible. Conversely, zircon is highly resistant against weathering but only

encountered in intermediate to felsic volcanic rocks. Over the course of three field campaigns

only five zircon-bearing volcanic intervals were discovered.


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Thesis introduction

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Although less accurate, dating of detrital zircons obtained from sandstones was used to

obtain more age constraints for the sedimentary record. As mentioned above, zircon is extremely

resistant against both chemical and mechanical weathering and is therefore highly concentrated

in many clastic sedimentary rocks. The disadvantage of detrital zircon dating is that only a

maximum age is obtained for the sandstone, and it remains uncertain how closely the age of the

youngest detrital zircon approximates the actual stratigraphic age of the rock. However, the

benefit of detrital zircon dating is that it not only provides an age estimate for the sandstone but

also unravels its provenance history which can provide valuable information on the source

region.

All zircons in this study were dated by Laser Ablation Inductively Coupled Plasma Mass

Spectrometry (LA-ICP-MS) at the University of Lausanne, except for four granitic and two

volcanic samples that were selected for high precision dating by Isotope Dilution Thermal

Ionisation Mass Spectrometry (ID-TIMS) at the University of Geneva.

To reconstruct the melt sources of magmas and the geodynamic setting under which they

formed we use whole rock geochemical analyses of major oxides, trace and rare earth elements.

As discussed above, the volcanic rocks are highly affected by weathering and therefore for these

samples only immobile trace and rare-earth-elements could be used for interpretation.

The initial Hf isotope composition of magmatic zircon was used to distinguish between

magmas that formed by melting of the mantle, crust or a combination of both. Lu-Hf isotopic

analyses on dated zircons were performed by Multi-Collector ICP-MS at the Johann Wolfgang

Goethe (JWG) University in Frankfurt. For several plutonic samples the Hf-isotope analyses


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Thesis introduction

5

were complemented by determination of whole rock Sr and Nd isotopic ratios. Samples of

zircon-free Triassic lavas were analyzed for whole-rock Nd isotopes alone due to the mobile

character of Sr. Sr and Nd isotope analyses were undertaken by TIMS at the University of

Geneva.

Outline of the thesis

The geochemically very similar granitoids of the Peruvian Eastern Cordillera have been

assigned to a variety of geodynamic settings. A back-arc position is assumed for the Ordovician

(Bahlburg et al., 2011) and Jurassic (Miskovic et al., 2009) plutons in south-east Peru based on

the preservation of remnants of the main continental arc in the coastal area. The Permo-

Carboniferous plutons are regarded as representing the main arc axis at the time (Chew et al.,

2007; Miskovic et al., 2009) and the Permo-Triassic plutons were supposedly emplaced in a

continental rift setting based on apparently coeval graben formation and alkaline volcanism

(Dalmayrac et al., 1980; Kontak et al., 1990; Sempere et al., 2002; Vivier et al., 1976).

In chapter 1 Crustal reworking in Paleozoic Early Mesozoic orphaned back-arcs on

the western Gondwana margin, south-east Peru the timing and duration of the plutonic

pulses in the Eastern Cordillera of south-east Peru is further constrained. In addition, the

geodynamic setting of each time period is re-assessed based on geochronological, geochemical,

geometric and tectonic arguments. The second aim of this chapter is to investigate the

contribution of Paleozoic and Mesozoic plutonism to growth of the continental crust. We use Hf-

isotopes in zircon and whole rock Sr and Nd isotopic ratios to determine the involvement of

juvenile mantle melts in the final magmatic product. We further compare the different


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Thesis introduction

6

geodynamic settings and the associated melt forming processes and judge their efficiency to

contribute to crustal growth.

Disassembly of western Gondwana initiated in the Triassic. Extension along the northern

margin of western South America resulted in the formation of oceanic lithosphere of the western

Tethys Ocean, whereas extension along the Peruvian margin terminated before oceanic

lithosphere could form. The Mitu Rift of southern Peru experienced the highest amount of

extension along the Peruvian margin and remnants of syn-rift sedimentary and plutonic rocks are

still partly preserved in the Eastern Cordillera of Peru.

The Mitu Group consists of red sandstones, conglomerates and interbedded alkaline lavas.

Due to its dominantly coarse-grained clastic nature, the Mitu Group is nearly devoid of fossils

and therefore the onset and duration of deposition are largely based on the ages of the bracketing

formations. However, an angular unconformity related to the Jurua (Late Hercynian) Orogeny

(Audebaud and Laubacher, 1969; Laubacher, 1978; Rosas et al., 2007) separates the Upper

Carboniferous - Lower Permian Copacabana Group from the overlaying Mitu Group (figure I)

and renders the age estimate for the basal Mitu Group imprecise.

The aim of chapter 2 Triassic simple shear, back-arc extension and deposition of the

Mitu Group in south-eastern Peru is to generate more accurate and precise age constraints for

the onset and duration of deposition of the Mitu Group using U-Pb zircon geochronology of

rhyolitic lavas and sedimentary rocks. In addition, we develop a tectonic model for the rift

sequence using the geochronological data in combination with sedimentological observations


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Thesis introduction

7

and Nd-isotope and whole rock geochemistry acquired from the volcanic rocks, to propose a

geodynamic setting and driving forces for the initiation and termination of extension.

The changing sedimentary environment from fluvial sandstones of the Ambo Group to

marine deposits with increasingly abundant intercalated volcanic material of the Tarma-

Copacabana Group (figure I) has never been put into a regional perspective incorporating the

tectonic evolution of the western Gondwana margin. Although previous work has constrained

deposition to occur between the Mississippian and the Early Permina based on plant and marine

fossils (Azcuy and Di Pasquo, 2005; Azcuy et al., 2002; Crdenas et al., 1997; Doubinger and

Marocco, 1981; Iannuzzi et al., 1998), a connection with the simultaneous assembly and suturing

of the Pangea supercontinent has never been made (figure II).

In chapter 3 A late Paleozoic back-arc basin along the western margin of Gondwana:

Age and paleogeography of Permo-Carboniferous sedimentary rocks of south-east Peru

we present the first radiometric age data for the sedimentary and volcanic rocks of the Ambo and

Tarma-Copacabana Groups in order to establish an improved chronostratigraphic model for the

Late Paleozoic period. With the use of radiometric age data, provenance information and field

observations we aim to reconstruct the paleogeographic and tectonic evolution of southern Peru

and make a link with the supercontinent cycle.

Figure II: Plate tectonic reconstruction from Scotese for the A. Late Carboniferous; B. Late

Permian and C. Early Triassic. Red arrow indicates paleo-location of the present-day south

Peruvian margin.


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9

Chapter 1

Crustal reworking in Paleozoic Early Mesozoic orphaned back-

arcs on the western Gondwana margin, south-east Peru.

Maril Reitsma, Richard Spikings, Alexey Ulianov, Cyril Chelle-Michou, Axel Gerdes,

Massimo Chiaradia, Urs Schaltegger

1. INTRODUCTION

The Peruvian segment of the western South American continental margin was facing the

Iapetus Ocean after break-up of Rodinia. The western Gondwana margin, as part of the larger

Terra Australis Orogen, became active with the inception of arc magmatism in the Early

Cambrian (~530 Ma, Cawood, 2005), although the detrital record suggests that subduction

might have initiated as early as the Neoproterozoic (~650 Ma, Chew et al., 2008). The South

Peruvian margin has not experienced terrane accretion throughout the Phanerozoic, in contrast

to the North Peruvian margin (Ramos, 2010), the Chilean margin to the south (e.g. Ramos,

2010; Rapalini, 2005) and the Ecuadorian-Colombian margin to the north (Spikings et al.,

2010; Spikings et al., 2005; Spikings et al., 2001), thus keeping its geological history

relatively simple. Consequently, southern Peru is an ideal region to study the evolution of a

long-lived, active continental margin, which opened within the Iapetus Wilson cycle and now

forms part of the Pacific cycle. We examine the relationship between geodynamics at an

active margin, melt forming processes and crustal growth or reworking by investigating the

intrusive history of the margin.

The plutons in the South Peruvian Eastern Cordillera (SPEC, 12 14S, figure 1 and 2)

were previously assigned to a single magmatic stage based on their mineralogical and textural


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Crustal reworking in back-arcs along the western Gondwana margin

10

Figure 1: A. Overview of western South American margin and plutonic remnants of

Ordovician, Carboniferous, Permian, Triassic and Jurassic. P Paracas terrane, Ar Arequipa

Block, An Antofalla Block. B. Overview of Jurassic and older plutonic rocks in Peru.

Triangles indicate presently active volcanoes.

Figure 2 (next page): Research area in south-east Peru. U-Pb zircons ages in Ma,Italic

obtained by LA-ICP-MS, regularby ID-TIMS. Ages in green - LA-ICP-MS zircon data from

Miskovic et al. (2009), age in blue - biotite40

Ar/39

Ar cooling age (Rodriguez et al., 2009),

yellow (Cenozoic) pluton - Andahuaylas-Yauri batholiths, PTFZ Patacancha-Tamburca

fault zone, ZSGZ Zongo-San Gaban fault zone.


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similarities (Dalmayrac et al., 1980; Laubacher, 1978; Marocco, 1978). Further, limited U-Pb

multi-fraction zircon and40

Ar/39

Ar biotite and K-feldspar dating of the granitoids yielded

Permo-Triassic ages (Kontak et al., 1990a; Lancelot et al., 1978). However, recent,

comprehensive U-Pb zircon dating has shown that the SPEC granitoids range in age from the

Ordovician to the Miocene, with the majority of plutons yielding ages within the

Pennsylvanian to Early Jurassic period (Miskovic et al., 2009).

The SPEC granitoids were classically assigned to a phase of Permo-Triassic rifting, based

on apparently coeval horst and graben formation and alkaline basaltic volcanism (Dalmayrac

et al., 1980; Kontak et al., 1990a; Sempere et al., 2002; Vivier et al., 1976). However,

previous authors have suggested that the proto-Eastern Cordillera region was in a back-arc

position in the Ordovician (Bahlburg et al., 2006) and Jurassic (Miskovic et al., 2009), with

the corresponding arcs preserved in coastal, southern Peru (figure 1). We propose, based on

geochemical and isotopic data that this back-arc setting also persisted in the intervening

period from the Carboniferous to the Triassic, even though magmatic arcs for these times have

not been identified in the rock record. The high rates of subduction erosion reported for the

Peruvian margin (Clift et al., 2003; Stern, 2011) probably obliterated the arcs, leaving behind

the Carboniferous to Triassic SPEC plutons as orphaned back-arc intrusive rocks.

It has been suggested that the processes of crustal growth by addition of mantle melts and

crustal recycling by subduction erosion are balanced in continental arcs (Hawkesworth et al.,

2010; Stern and Scholl, 2010). Therefore we investigate the contribution of back-arc

plutonism, which has a higher preservation potential, to the growth of the continental crust.

Given that the plutons in the research area span almost half a billion years, a variety of

geodynamic settings can be compared and their contribution to crustal growth tested. This

leads to a better comprehension of the efficiency of mantle melt forming mechanisms and

related gross crustal growth in the back-arc versus those processes operating in the main arc.


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2. Geology of the Eastern Cordillera of southern Peru

Gondwana had assembled by the beginning of the Phanerozoic, and the western margin

of South America formed part of the Terra Australis Orogen (Cawood, 2005). Plutonic

remnants and associated metamorphism of the Ordovician Famatinian arc are well preserved

in Peru (fig. 1b), e.g. on the Arequipa-Antofalla terrane of south Peru (Boekhout et al.,

submitted; Loewy et al., 2004) and in the Eastern Cordillera of north and central Peru

(Cardona et al., 2009; Chew et al., 2007). The Ordovician sedimentary rocks in the Eastern

Cordillera of south-east Peru have been interpreted as back-arc deposits (figure 2; Bahlburg et

al., 2006). The back-arc basin was part of an epeiric sea in which sandstone and shale

accumulated. Shallow marine fossils plus detrital zircon ages (max. 445 13 Ma) indicate a

Middle to Upper Ordovician age for these sedimentary rocks (Bahlburg et al., 2011;

Dalmayrac et al., 1980; Maletz et al., 2010). Intercalated lapilli tuffs and a 447 10 Ma age

for a alkali granite in the Machu Picchu Inlier (Miskovic et al., 2009) can hence be considered

as the extrusive and intrusive igneous components of the back-arc. However, Chew et al.

(2007) and Miskovic et al. (2009) regard the 474.2 3.4 Ma 442.4 1.4 Ma peraluminous

granitoids of central and north Peru as the main arc axis that stepped inland from the Arequipa

block due to an original embayment on the western Gondwanan margin (figure 1b).

After the Late Devonian Early Carboniferous Eohercynian Orogeny (Laubacher, 1978;

Marocco, 1978; Mgard, 1978), subduction-related plutonism in the Eastern Cordillera of

north Peru recommenced in the Mississippian (343.6 2.6 Ma, Chew et al., 2007; Miskovic et

al., 2009). Calc-alkaline granodiorites and monzogranites are hornblende- and biotite-bearing

and were emplaced in major fault zones (Haeberlin et al., 2004; Schreiber et al., 1990).

Contemporaneously, clastic sediments accumulated in dominantly subaerial basins throughout

Peru with plant remains indicating a late Visan earliest Serpukhovian age (Azcuy and Di

Pasquo, 2005; Iannuzzi and Pfefferkorn, 2002).


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A Pennsylvanian marine transgression covered a significant part of Peru and largely

overflowed the limits of the Mississippian basin (Laubacher, 1978). Shallow marine deposits

are mixed with reworked tuffs, tuffaceous sandstones and volcanoclastic deposits, which are

evidence of contemporaneous explosive volcanism (Laubacher, 1978; Dalmayrac et al.,

1980). Pennsylvanian peraluminous plutonism associated with a main arc axis located in the

Eastern Cordillera is dominantly granodioritic and granitic in respectively north and central

Peru (Miskovic et al., 2009). A short lived phase of high-grade metamorphism and crustal

anatexis is reported at ~312 Ma in Central Peru (Chew et al., 2007).

A phase of magmatic quiescence in the plutonic record of Peru (301 5 Ma 284 15

Ma, Miskovic et al., 2009) coincides with the maximum extent of the marine transgression

(Marocco, 1978). Platform carbonates of the Copacabana Group accumulated in the

epicontinental sea while deposits of volcanic origin are scarce (Marocco, 1978; Dalmayrac et

al., 1980). Resumption of alkali-calcic plutonism in the Machu Picchu Inlier in the Artinskian

(275.6 0.7 Ma 284.4 0.7 Ma; figure 2, this study) seems coeval with increased

siliciclastic input in the Copacabana Group of south-east Peru (chapter 3; Doubinger and

Marocco, 1981; Laubacher, 1978), interpreted as the result of thermal doming of the crust and

retreat of the epeiric sea to the subandean region (chapter 3).

The geodynamic setting of the Eastern Cordillera region changed in the Permo?-Triassic

to a continental rift (Vivier et al., 1976 and Dalmayrac et al., 1980). This hypothesis is based

on the presence of alkaline basalts and andesites with an intraplate signature that are

intercalated with red terrestrial, clastic rocks and conglomerates of the Mitu Group (Cenki et

al., 2000; Kontak et al., 1990a). The Mitu Group accumulated in an extensional setting,

resulting in half-graben formation and large thickness variations (Dalmayrac et al., 1980;

Mgard, 1978; Rosas et al., 2007).


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Magmatic intrusions continued more or less continuously from the Late Permian to the

Triassic (Miskovic et al., 2009) in the Eastern Cordillera of central Peru, although the

intrusive volume of Triassic granitoids is much lower. Some of the Late Permian plutons have

intraplate signatures while other granitoids display well pronounced negative Nb-Ta

anomalies usually interpreted as a subduction signature. The volume of Triassic plutons

increases in south-central Peru, while the Late Permian plutons are minor (Miskovic et al.,

2009; this study). The Triassic granitoids step abruptly 150 km inland from the Cordillera de

Andahuaylas to the Cordillera de Carabaya near the city of Abancay (figure 2). Except for a

few Early Jurassic plutons, the Carabaya plutons are Middle to Upper Triassic in age (235.6

4.2 Ma 207.0 3.4 Ma; Miskovic et al., 2009). The Triassic plutons are dominantly felsic (>

69 wt.% SiO2), peraluminous S-type granites (Miskovic et al., 2009; Kontak et al., 1990a).

The Cordillera de Carabaya continues into the Cordillera Real in Bolivia where Triassic

plutons are recognized as far south as 17S (Brad et al., 1974; Gillis et al., 2006).

After Triassic extension ceased, subsidence of the crust induced by thermal relaxation

resulted in a marine invasion of the grabens that host the Mitu Group in north and central

Peru, resulting in deposition of Norian Toarcian limestones and evaporites (Jaillard et al.,

1990; Rosas et al., 2007). However, in south-east Peru a depositional hiatus stretches from the

Late Triassic to the Jurassic / Lower Cretaceous. The plutonic record partly fills this gap with

the emplacement of the volumetrically minor, Lower Jurassic, peralkaline, SiO2 under-

saturated syenites of the Cordillera de Carabaya (figure 2; 195 11 Ma 184.1 3.7 Ma;

Miskovic et al., 2009). These plutons are interpreted to have been emplaced in an extensional

back-arc. The corresponding arc is preserved along the western margin of the Arequipa

Terrane where the metaluminous quartz-diorites of the Coastal Batholith of the Arequipa

region (200 1.1 Ma to 175.8 1.2 Ma; Demouy et al., submitted) and gabbro to


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granodiorites of the Ilo Batholith (173.3 1.3 Ma - 151.5 0.8 Ma, Boekhout et al., 2012)

indicate an active margin setting (figure 1b).

After the break-up of Gondwana in the latest Triassic and the opening of the South

Atlantic Ocean in the Early Cretaceous, the Peruvian margin remained active but changed to a

dominantly compressive regime, driving crustal shortening during the Andean Orogeny, along

with significant amounts of subduction erosion (Clift and Hartley, 2007).

The first major phase of the Central Andean Orogeny is traditionally referred to as the

Incaic stage, which is characterised by abrupt shortening, thrusting and rock uplift attributed

to a period of flat slab subduction (Sandeman et al., 1995). Slab flattening started in southern

Peru in the Early Eocene. The hornblende-rich, metaluminous monzodiorites and

granodiorites of the Andahuaylas-Yauri batholith were emplaced between ~48 and 32 Ma at

the inflection point between flat slab subduction to the south and normal subduction to the

north (Perell et al., 2003). Simultaneously, coupling of the overriding and down going plate

drove rock uplift and thrusting of the Cordillera de Carabaya over the foreland via the crustal

scale Zongo-San Gaban fault zone (figure 2; Farrar et al., 1988; Sandeman et al., 1995).

Between ~30 Ma and 24 Ma the subduction angle steepened and the arc migrated trench-

ward, stabilizing its position northeast of the Western Cordillera (Mamani et al., 2010).

3. ANALYTICAL METHODS

3.1 Whole rock geochemistry

A total of 36 samples were processed for geochemical analysis. Weathered zones were

removed prior to crushing and grinding. Samples were crushed using a jaw crusher, hydraulic

press and agate mill.


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3.1.1 Major, trace and rare earth element analysis

Whole rock powders were transformed into lithium tetraborate glass discs on which

major oxides and trace elements were determined using a x-ray fluorescence Philips PW 2400

spectrometer. Certified reference materials NIM-G (granite), NIM-N (norite) and SY-2

(syenite) were used for quality control.

Analyses of rare earth and additional trace elements (e.g. Th, U, Ta, Cs, Hf) were

performed by Laser Ablation ICP-MS with an Elan 6100 DRC quadrupole mass

spectrometer (Perkin Elmer) interfaced to a GeoLas 200M 193nm excimer ablation system

(Lambda Physik). Spot analyses were done on glass discs previously used for major oxide

determinations, using ablation parameters of 10 Hz, a 120 m pit size and ~10 J/cm2 on-

sample density and helium as a carrier gas. The acquisition times for the background and the

ablation interval were ~70 and 35 s, respectively. Dwell times per isotope ranged from 10 to

20 ms and peak-hopping mode was employed. The ThO+/Th

+ and Ba

2+/Ba

+ ratios were

optimized to 4.27*10-3

and 2.10*10-2

, respectively. The NIST SRM 610 synthetic glass

standard was analysed for external standardisation (Pearce et al., 1997).

Raw data were reduced offline using the LAMTRACE software (Jackson et al., 2004).

CaO or Sr concentrations measured by XRF were used for internal standardization. Three

analyses per sample were acquired and the results averaged to obtain the final concentrations

of trace and rare earth elements. All measurements were carried out at the Institute of

Mineralogy and Geochemistry, University of Lausanne, Switzerland.

3.1.2 Sr and Nd whole rock isotopic analyses

The twelve least altered of the dated SPEC granitoids were selected for whole rock Sm-

Nd and Rb-Sr isotope analyses.


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Approximately 150 mg of powdered rock (


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Chapter 1

19

magnetic separator with a side slope of 10 and heavy liquid (DIM, =3.32 g/cm3). Zircons

extracted from samples to be dated by ID-TIMS were separated with a Frantz horizontal

magnetic separator with the side slope reduced to 2 to obtain the least magnetic zircons.

3.2.1 Laser Ablation ICP-MS dating

Zircons were handpicked with a preference for large, euhedral grains, mounted in epoxy

and polished with diamond paste to expose the internal surface of the grains. A total of 465

zircons extracted from 19 granitoids were imaged by panchromatic cathodoluminescence

(CL) acquisition using the CamScan MV2300 and JEOL JSM7001F scanning electron

microscopes at the universities of Lausanne andGeneva, respectively. CL images were used

to characterize zircon grains in terms of growth zoning, xenocrystic cores, inclusions and

cracks.

Isotopic measurements were acquired with a Thermo Scientific Element XR sector field

single-collector ICP-MS interfaced to a NewWave UP-193nm excimer ablation system (ESI)

at the University of Lausanne. Operating conditions were similar to those described in

Ulianov et al. (2012) and included a 25-35 m spot size combined with a relatively low on-

sample energy density of 2.2-2.3 J/cm2 and a repetition rate of 5 Hz to minimize the

fractionation. Zircon standard GJ-1 (CA-ID-TIMS206

Pb/238

U age of 600.5 0.4 Ma;

Schaltegger et al., unpublished) was used for external standardization. Zircon standards 91500

(1065.4 0.3, Wiedenbeck et al., 1995) or Plesovice (Slmaet al., 2008) were measured along

with sample zircons on a routine basis to control the accuracy of results.

Ablation spots were located in parts of the zircon grains that exhibit oscillatory magmatic

zoning, avoiding inclusions and cracks where possible. Xenocrystic cores were analysed

where size allowed. Between 18 and 34 spots were analyzed per zircon population. An

analytical series consisted of 4 spot analyses on primary standard GJ-1 to establish laser-


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20

induced U-Pb fractionation and instrumental mass discrimination, 8 analyses on unknown

zircons and 3 spot analyses on secondary standard 91500 or Plesovice as an independent

control for the age calibration. LAMTRACE (Jackson et al., 2004) was used for offline data

reduction. The data treatment procedures are discussed in detail in Ulianov et al. (2012). No

quantitative common lead correction was applied, a qualitative control of the intensities for

masses 202 and 204 and a careful inspection of the cathodoluminescence images were used

instead, following the approach of Jackson et al. (2004).

A weighted mean age was calculated using the206

Pb/238

U ratio of analytically concordant

zircons. Discordant ages, inherited zircons and zircons showing signs of Pb-loss were

discarded from the age calculation. UPb data are plotted on the concordia diagram as 2

error ellipses.

In the majority of samples, the range in206

Pb/238

U dates observed is beyond pure

analytical scatter indicated by MSWD values in excess of the acceptable range of values for n

analyses (Wendt and Carl, 1991). The errors consists of analytical uncertainties only and do

not contain propagated errors on the GJ-1 CA-ID-TIMS age, nor on the reproducibility of

secondary standard measurements. However, the data scatter can also be produced or

enhanced by geological phenomena such as minor amounts of Pb-loss, inheritance or the

presence of antecrystic or xenocrystic material.

3.2.2 CA-ID-TIMS

Zircons from four samples (MR25, 71, 80 and 81) were dated by chemical abrasion-

isotope dilution-thermal ionisation mass spectrometry. The methodology described in

Schoene et al. (2010b) was followed for zircon annealing, leaching, dissolution and chemical

separation of U and Pb, isotope analysis and data treatment. Zircons were spiked with the

Earthtime 205Pb-233U-235U tracer solution. Isotopic analyses were performed at the University


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Chapter 1

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of Geneva on a TRITON mass spectrometer equipped with a MasCom electron multiplier in

ion counting mode.

Due to the high precision of chemical abrasion ID-TIMS analyses, only the youngest

206

Pb/238

U date or the weighted mean of the youngest data cluster was used (Schaltegger et al.,

2009; Schoene et al., 2010a).

3.3 Lu-Hf isotope analyses

3.3.1 LA-MC-ICP-MS

Lu-Hf isotopic analyses on ~6-7 concordant, dated zircons were performed with a

Thermo-Scientific Neptune multi-collector ICP-MS at JWG University, Frankfurt with a New

Wave Research UP-213 laser and teardrop-shaped, low-volume ablation cell (see Gerdes and

Zeh, 2006, 2009) with helium as a carrier gas. The Lu-Hf laser spot was drilled close to or

partially overlapping the U-Pb laser spot. The laser beam parameters were 40-50 m spot size,

5 Hz firing repetition rate, and xx J/cm2energy density. To correct for isobaric interferences

of Lu and Yb on mass 176 the isotopes172

Yb,173

Yb and175

Lu were simultaneously

monitored. The 176Yb and 176Lu signals were calculated using a 176Yb/173Yb of 0.796218 (Chu

et al., 2002) and176

Lu/175

Lu of 0.02658 (JWG in-house value). The instrumental mass bias for

Hf isotopes was corrected using an exponential law and a179

Hf/177

Hf value of 0.7325. In the

case of Yb isotopes, the mass bias was corrected using the Hf mass bias of the individual

integration step multiplied by a daily Hf/Yb offset factor (Slama et al., 2008; Gerdes and

Zeh, 2009). All zircon LA-MC-ICP-MS analyses were adjusted relative to the JMC 475

176Hf/

177Hf ratio of 0.282160 and the reported uncertainties (2) were propagated by quadratic

addition of the external reproducibility of GJ-1 (2, n = 18 / 30) and Temora (2, n=12) or

Plesovice (2, n=20) and the within-run precision of each analysis (2 SE). The external

reproducibility (2, n > 50) over more than 6 months of reference zircon 91500, GJ-1, and


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Pleovice (176

Hf/177

Hf = 0.282298 0.000026, 0.282003 0.000018, and 0.282482

0.000015, respectively) at JWG is about 0.005- 0.009% (


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Chapter 1

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hornblende over biotite, and its distinct magmatic contact with Cretaceous sedimentary rocks.

The SPEC granitoids are very poor in hornblende and mainly intrude pre-Carboniferous

metamorphic basement.

We present U-Pb ages and Hf-isotopic ratios on zircons from 16 SPEC granitoids and 3

samples from the Andahuaylas-Yauri Batholith (appendices 1.1; 1.2; 1.4; 1.5). Whole rock

geochemistry was performed on 32 SPEC granitoids and 4 granitoids from the Andahuaylas-

Yauri Batholith (appendix 1.3). Due to the similar mineralogy and geochemistry of the SPEC

granitoids, an age cannot be assigned to undated samples based on these two factors alone.

Therefore they are assigned to the same group as the nearest outcrop for which age data is

available. Twelve dated SPEC samples were selected for whole rock Nd and Sr isotopic

analyses.

4.2 U-Pb geochronology and Hf isotopes

4.2.1 Ordovician

Ordovician ages were obtained from 3 granitoids within the southern half (MR109, 176

and 215), and from a small outcrop along the eastern extremity (MR127) of the Machu Picchu

inlier (figure 2; table 1). The samples are granodioritic to tonalitic in composition, with traces

of muscovite, while only tonalite MR176 contains hornblende.

Approximately one quarter of the zircons that yielded Ordovician ages contain an

inherited core that was identifiable in the CL images. Seven concordant analyses of

xenocrystic cores and grains with virtually no overgrowths of Ordovician zircon yielded ages

between 512 Ma and 1485 Ma, while207

Pb/206

Pb ages of discordant zircon analyses span the

same range and are probably the result of mixed core-rim analyses.


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Table 1: Overview of weighted mean U-Pb zircon ages and Hf isotope ratios.

Lat. Long. Concordia

Sample Location (S) (W) Rock type Age (Ma) 2a MSWD n / Ntotb Hfi

MR127 Manto 13.0 72.1 granodiorite 479.9 2.3 4.0 18 / 22 6.04 0

MR215 SE. Vilcabamba 13.2 72.9 granodiorite 475.4 4.6 7.0 21 / 29 -5.09 0

MR176 SE. Machu Picchu 13.2 72.5 tonalite 473.5 3.2 6.5 15 / 20 -5.43 0

MR109 SE. Machu Picchu 13.2 72.4 granodiorite 472.2 4.8 2.9 11 / 18 -4.55 0

MR138 NW. Machu Picchu 13.2 72.6 monzogranite 320.1 1.8 3.0 13 / 27 0.79 0

MR130 NW. Machu Picchu 13.1 72.6 syenogranite 315.3 2.4 3.2 10 / 20 -0.19 0

MR81 Urubamba 13.2 72.1 alkali granite 304.25 0.11 0.63 3 / 7 2.78 0

MR194 N. Ayacucho 12.8 74.3 granodiorite 287.7 2.4 2.6 19 / 22 1.18 0

MR80 Quillabamba 12.8 72.6 monzogranite 282.21 0.13 0.29 2 / 6 1.86 0

MR210 Vilcabamba-Santa Maria 13.0 72.8 granodiorite 279.1 1.8 3.3 24 / 26 1.14 0

MR218G N. Ollantaytambo 13.1 72.3 monzogranite 276.9 1.6 4.9 21 / 23 1.35 0

MR25 Vilcabamba 13.1 72.9 syenogranite 271.29 0.28 - 1 / 61.87 0

3.43 0

MR198 Andahuaylas 13.6 74.0 alkali granite 260.7 2.8 1.8 18 / 22 1.45 0

MR213 S. Vilcabamba 13.2 73.0 monzogranite 253.6 1.3 3.0 18 / 20 1.33 0

MR120 Marcapata 13.6 70.9 gabbro 236.3 0.9 2.7 25 / 25 3.47 0

MR122 Ocongate 13.5 71.1 granodiorite 235.1 1.8 4.2 22 / 23 -1.06 0

MR91 Abancay 13.8 72.9 granodiorite 226.1 2.2 6.5 25 / 30 -0.63 0

MR71 W. Coasa 14.0 70.3 syenogranite 223.73 0.09 0.46 3 / 4 -0.5 1

MR163 N. Coasa 13.9 70.2 granodiorite 226.5 4.2 26 7 / 25 1.31 0

CC55 Yauri 15.0 71.3 gabbro 40.8 0.2 1.1 34 / 34 5.53 0

CC56 Yauri 15.0 71.3 rhyodacite 36.5 0.4 3.0 26 / 27 4.11 0

CC12 Yauri 15.0 71.3 hbl monzonite 35.5 0.3 2.5 31 / 33 3.15 0

MR78 S. Vilcabamba 13.2 73.0 syenogranite 16.1 0.01 0.24 3 / 18 -1.93 0

a 2 standard deviations, absolute value; b number of analyses used in weighted mean calculation over to

analyses, xenocrystic cores or analyses demonstrating slight inheritance or Pb-loss are not used in t


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Chapter 1

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Figure 3: LA-ICP-MS U-Pb zircon data on Ordovician granitoids. Ellipses are 2 sigma, black

ellipses are used in weighted mean calculation. Not all discordant data points and analyses on

xenocrystic zircon are plotted. n: number of analyses used in weighted mean calculation,

MSWD: mean square weighted deviation. A. Granodiorite MR127, B. Granodiorite MR215,C. Tonalite MR176, D. Granodiorite MR109.

The weighted mean U-Pb ages of all four samples are very similar and are barely

distinguishable at the 2level, ranging between 472.2 4.8 Ma and 479.9 2.3 Ma (figure

3). The Hf-isotope composition of the granodiorites and tonalite from the southern half of the

Machu Picchu Inlier span a narrow range between Hfi-5.43 0.29 and -4.55 0.50 (figure

4, table 1). Granodiorite MR127 from the eastern part of the inlier yields a positive Hfivalue

of 6.04 0.59 (figure 4, table 1).


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Figure 4: Age (Ma) versus Hfion zircons, error bars are 2 sigma. Data on Jurassic samples

from Miskovic and Schaltegger (2009) and Reitsma et al. (chapter 2) are added for

comparison.

4.2.2 Carboniferous

Syenogranite MR130 and monzogranite MR138 were sampled from the Carboniferous

Machu Pichu pluton and alkali granite sample MR81 was acquired from the Pennsylavanian

Nevado Chicon pluton located less than 50 km to the east, in the Sacred Valley (figure 2).

Approximately one third of the zircons from the Machu Picchu pluton contain inherited

cores (appendix 1.1), some of which constitute a majority of the volume of the grain and have

thin overgrowths of younger zircon. Xenocrystic grains lacking overgrowths have also been

identified. Concordant xenocrysts and inherited core ages range between 974 Ma and 1412

Ma (n = 6; appendix 1.1). Syenogranite MR130 and monzogranite MR138 yield weighted


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mean zircon U-Pb ages of 315.3 2.4 Ma and 320.1 1.8 Ma, respectively (figure 5a-b), and

Hfivalues of -0.19 0.46 and 0.79 0.74, respectively (figure 4).

Zircons extracted from granite alkali MR81 were selected after a second horizontal

Frantz magnetic separation at a low angle, which efficiently removed zircon with xenocrystic

cores. Seven whole grain ID-TIMS analyses on pristine zircons yield concordant U-Pb ages

that range between 304.2 0.3 Ma and 305.7 0.2 Ma (figure 5c), and a weighted average

age of the youngest three grains yields an age of 304.25 0.11 Ma. Initial Hf values vary

between 0.7 0.2 and 3.1 0.6 (n = 7; figure 4).

4.2.3 Permian

Permian plutons have been identified in the Machu Picchu inlier (MR25, 80, 210, 213

and 218) and the Ayacucho region (MR194 and 198) (figure 2), and span a range in

compositions from granodiorite to alkali granite.

Approximately 17% of the Permian grains were observed to have xenocrystic cores,

which is significantly lower than the proportion found in the Ordovician and Carboniferous

intrusive rocks. Furthermore, inherited cores in the Permian aged intrusions are also smaller

than those found in the older rocks. A single inherited core yielded a U-Pb age of 1041 11

Ma (granodiorite MR194), and one xenocrystic grain (332 6 Ma) was identified in the same

sample. Most samples yielded a maximum of three grains that gave discordant U-Pb ages,

with207

Pb/206

Pb ages ranging between 378 46 Ma and 2022 168 Ma, indicating mixing

with minor amounts or relict zircon and/or partial diffusive resetting of xenocrystic material.

Concordant zircon U-Pb ages range between 271.29 0.28 and 282.21 0.13 Ma

(MR25, 80, 210, 218, figure 6b-e, table 1) in the northern half of the Machu Picchu inlier, and

a Late Permian monzogranite was identified (253.6 1.3 Ma, MR213, figure 6g, table 1) in

the southern half of the inlier. A granodiorite and alkali granite from the Ayacucho area


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Figure 5: U-Pb zircon data on Carboniferous granitoids. Ellipses are 2 sigma, black ellipses

are used in weighted mean calculation. Not all discordant data points and analyses on


MSWD: mean square weighted deviation. A. LA-ICP-MS analyses on monzogranite MR138,B. LA-ICP-MS analyses on syenogranite MR130, C. ID-TIMS analyses on alkali granite

MR81.

Figure 6 (next page): U-Pb zircon data on Permian granitoids. Ellipses are 2 sigma, black

ellipses are used in weighted mean calculation. Not all discordant data points and analyses onxenocrystic zircon are plotted. n: number of analyses used in weighted mean calculation,

MSWD: mean square weighted deviation. A. LA-ICP-MS analyses on granodiorite MR194,

B. ID-TIMS analyses on monzogranite MR80, C. LA-ICP-MS analyses on granodiorite

MR210, D. LA-ICP-MS analyses on monzogranite MR218G, E. ID-TIMS analyses on

syenogranite MR25, F. LA-ICP-MS analyses on alkali granite MR198, G. LA-ICP-MS

analyses on monzogranite MR213.


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yielded concordant zircon U-Pb ages of 287.7 2.4 Ma (MR194, figure 6a, table 1) and

260.7 1.8 Ma (MR198, figure 6f, table 1), respectively.

Nearly all in-situ Hf data overlap within 2 error (figure 4, table 1) and yield a Hfi

weighted mean of 1.29 0.18 (MSWD = 1.8, n = 29; appendix 1.4). Only syenogranite MR25

and monzogranite MR80 yielded higher Hfivalues ranging between 0.9 and 3.5 (appendix

1.5).

4.2.4 Triassic

Intrusive rocks that yielded Triassic concordant zircon U-Pb ages are concentrated in the

Cordillera de Carabaya (MR120, 122, 163, 71) and the Abancay Ayacucho region (MR91)

(figure 2). All Triassic zircons exhibit oscillatory magmatic zoning under CL, except for

gabbro MR120, which has faint and broad zoned zircons that are typically found in mafic

rocks (Corfu et al., 2003). The Triassic zircons are poor in xenocrystic material, which only

occurs in significant amounts in granodiorite MR163 where numerous cores can be

recognised, although they were too small to be dated.

Gabbro MR120 and granodiorite MR122, located in the north-western part of Cordillera

de Carabaya, yield overlapping concordant zircon U-Pb ages of 236.3 0.9 Ma and 235.1

1.8 Ma, respectively (figure 7a-b; table 1), despite their contrasting whole rock geochemistry

and Hf-isotopic compositions with average Hfivalues of 3.47 0.56 (MR120) and -1.06

0.60 (MR122; figure 4, table 1).

Syenogranite MR71 forms part of the Coasa Batholith in the Cordillera de Carabaya

(figure 2) and yielded a concordant zircon U-Pb ID-TIMS age which averages 223.73 0.09

Ma for the youngest 3 grains (figure 7c, appendix 1.2).The U-Pb data obtained from

granodiorite MR163, which was also forms part of the Coasa Batholith, plots along an inverse

discordia, although seven (nearly) concordant zircons give a206

Pb/238

U age of 226.5 4.2 Ma


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(figure 7d). Granodiorite MR163 is located in a sheared zone which displays evidence of

Eocene K/Ar resetting (Zongo-San Gaban zone; Kontak et al., 1990b; figure 2), which may

have disturbed the U-Pb systematics. Initial Hf values are 1.31 0.27 and -0.5 1.1 for

MR163 and 71, respectively (figure 4).

Granodiorite MR91, located close to the city of Abancay, has an oriented magmatic

fabric which we interpret to have formed during intrusion into the Patacancha - Tamburco

fault zone (figure 2). It yields a concordant zircon U-Pb age of 226.1 2.2 Ma, with a Hfi of

-0.63 0.36 (figure 7e and 4; table 1).

4.2.5 Cenozoic

The Andahualyas-Yauri batholith located within the Altiplano consists of rocks that

range from gabbro to monzo- and granodiorite, which are easily distinguishable from the

SPEC plutons due to the dominance of hornblende over biotite. Transpressional vein patterns

suggest that the batholith was emplaced under compression.

A gabbro (CC55), subvolcanic rhyodacite (CC56) and hornblende monzonite (CC12)

from the district of Yauri (figure 2) all yielded Eocene concordant zircon U-Pb ages ranging

from 40.8 0.2 Ma to 35.5 0.3 Ma (figure 8a-c), with a positive correlation with Hfithat

ranges from 5.53 0.53 to 3.15 0.73 (figure 9). Cathodoluminesence images of zircons

from rhyodacite CC56 and monzonite CC12 show magmatic oscillatory zoning with frequent,

small xenocrystic cores while gabbro CC55 lacks inhereted cores and displays strong sector

zoning of unzoned to broadly zoned zircons.

Syenogranite MR78 with large K-feldspar phenocrysts from the Machu Picchu Inlier

yielded a Miocene U-Pb age, although the U-Pb data is complicated by minor amounts of

inheritance causing more than half of the zircons to be discordant. The youngest concordant

cluster of zircons yield a weigthed mean age of 16.12 0.01 Ma (n = 3; MSWD = 0.24; figure


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8d). Hf isotope data obtained from five grains define one statistical population with a Hfiof

1.93 0.28, and a MSWD of 0.96 (figure 9).

Figure 7: U-Pb zircon data on Triassic granitoids. Ellipses are 2 sigma, black ellipses are used

in weighted mean calculation. Not all discordant data points and analyses on xenocrystic

zircon are plotted. n: number of analyses used in weighted mean calculation, MSWD: mean

square weighted deviation. A. LA-ICP-MS analyses on gabbro MR120, B. LA-ICP-MS

analyses on granodiorite MR122, C. ID-TIMS analyses on syenofranite MR71, D. LA-ICP-

MS analyses on granodiorite MR163, E. LA-ICP-MS analyses on granodiorite MR91.


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Figure 8: LA-ICP-MS U-Pb zircon data on Eocene granitoids. Ellipses are 2 sigma, black

ellipses are used in weighted mean calculation. Not all discordant data points and analyses on


MSWD: mean square weighted deviation. A. Gabbro CC55, B. Rhyodacite CC56, C.

Hornblende-monzonite CC12, D. Syenogranite MR78.

Figure 9: Age (Myrs) versus Hfiobtained by LA-ICP-MS on Cenozoic samples. Error bars

are 2 sigma.


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4.3 Whole rock geochemistry

4.3.1 Major elements

Whole rock major element data show that samples range from gabbro to granite with the

majority of samples having >69 wt.% SiO2 (figure 10). Rocks are generally alkali-calcic to

calcic and peraluminous (figure 10 and 11).

Compared to the other granitoids, the Permian and Eocene granitoids have the broadest

range in SiO2, extending to mafic and metaluminous compositions. For the same SiO2content

the Permian and Eocene samples generally have a higher modified alkali-lime index (Frost et

al., 2001) even though compositions span from alkalic to calcic (figure 10).

The Ordovician granodiorites and tonalite are calc-alkalic to calcic in composition and

the Carboniferous alkali-, monzo-, syenogranite and granodiorite plutons are alkali-calcic to

calc-alkalic. Triassic granitoids extend to higher SiO2 and Na2O concentrations than the

Ordovician granitoids, but are also calc-alkalic to calcic. The two Miocene syenogranites are

alkali-calcic (figure 10).

4.3.2 Rare earth and trace elements

REE and multi-element plots for all age groups are rather similar, with LREE enriched

relative to HREE and moderate to high LILE/HFSE ratios (figure 12). Eocene granitoids are

among the least enriched in REE and yield the most pronounced negative Nb-Ta anomalies as

well as a slight negative Zr and positive Ba anomaly on a primitive mantle normalized multi-

element plot (figure 12c-d). Apart from a few exceptions, these latter two anomalies are not

recognised in the SPEC samples (figure 12). Ordovician and Permian samples are most

enriched in REE. On the contrary, Ordovician samples have well pronounced negative Eu

anomalies indicating plagioclase fractionation, while similar anomalies are recognised in less


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Figure 10: SiO2 versus Modified Alkali-Lime Index (Na2O+K2O-CaO) after Frost et al.

(2001) plotted per age group.

Figure 11: Aluminium saturation index (ASI) for SPEC plutons (Maniar and Piccoli, 1989).For comparison: grey field: Jurassic arc (Ilo batholith, Boekhout et al., in press), striped field:

Jurassic back-arc (Miskovic et al., 2009). See legend in figure 10.


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Figure 12: Whole rock REE normalized to chondritic (A, C) and trace element normalized to

primitive mantle values (B, D) from Sun and McDonough (1989) for A-B. Ordovician,

Carboniferous, Permian granitoids, C-D. Triassic, Eocene granitoids

Figure 13: Spider diagram normalised to primitive mantle (Sun and McDonough, 1989) ofgabbroic samples.


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Table 2: Whole rock Rb-Sr and Sm-Nd isotope data

Figure 14: Initial86

Sr/87

Sr versus A. Initial Nd and B. SiO2(wt.%). Permian samples G and E

are granite MR218 and its mafic enclave, resp. See legend in figure 10.

Sample Age Rb Sr Sm Nd

(Ma) (ppm) (ppm)87

Rb/86

Sr87

Sr/86

Sr 1SE87

Sr/86

Sri (ppm) (ppm)147

Sm/144

Nd143

N

MR127 480 73 144 1.477 0.713769 2.6*10-6

0.703666 4.08 21.49 0.11437 0.5

MR109 472 184 214 2.488 0.724702 2.0*10-5

0.707970 6.37 31.28 0.12250 0.5MR130 315 189 93 5.931 0.736264 1.6*10

-6 0.709677 4.70 22.23 0.12721 0.5

MR81 304 172 61 8.199 0.733140 3.7*10-6

0.697672 5.51 27.62 0.12019 0.5

MR194 288 78 409 0.552 0.707455 2.6*10-6

0.705202 4.73 21.45 0.13284

MR210 279 44 314 0.406 0.709914 2.3*10-6

0.708304 5.54 29.92 0.11146 0.5

MR218G 277 228 116 5.698 0.727830 2.6*10-6

0.705374 6.05 30.19 0.12065 0.5

MR218E 277 210 163 3.732 0.720124 1.7*10-6

0.705416 7.62 37.66 0.12175 0.5

MR120 236 16 375 0.120 0.704641 4.0*10-6

0.704239 3.11 13.73 0.13637 0.5

MR122 235 141 126 3.229 0.717083 1.8*10-6

0.706290 4.36 22.63 0.11605 0.5

MR91 226 103 454 0.656 0.707988 2.3*10-6

0.705888 3.35 22.20 0.09074 0.5

MR71 224 46 111 1.200 0.712313 1.4*10-6

0.708491 2.71 13.64 0.11962 0.5


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than half of the Permian samples. The Carboniferous samples have REE and trace elements

patterns that are very similar to those of the less enriched Permian and Ordovician granitoids.

The Triassic samples show a wide range in REE concentrations with alkali granite

MR186 being the most enriched sample and monzogranite MR192 the least. Gabbro MR120,

syenogranite MR71 and alkali granite MR186 have low LILE/HFSE ratios, moreover, the

latter two granitoids show strong negative Eu anomalies (figure 12c-d).

Comparing gabbros from different intrusive events, it is evident that Eocene gabbro

CC55 has a well pronounced negative Nb-Ta anomaly, as well as slight negative Th and Zr

anomalies that are observed to a lesser extent in Permian gabbro MR221 and hardly in

Triassic gabbro MR120 (figure 13). We interpreted these contrasting anomalies in the Eocene

versus SPEC gabbros as a fundamental difference in the parental melts, most likely as the

result of the presence of a slab derived fluid in the source of the Eocene granitoids.

4.3.3 Nd and Sr isotopes

Nd and Sr whole rock isotopic results show an inverse correlation (figure 14a), which is

expected when both the Sm-Nd and Rb-Sr systems behave as closed systems. The 87Sr/86Sri

ratio ranges from 0.709677 to 0.703666 and Ndi from -3.67 to 3.42 (table 2). Only alkali

granite MR81 gave an unrealistically low87

Sr/86

Srivalue (0.697672), which is probably due

to open system behaviour of mobile Sr.

The granodiorite enclave monzogranite host pair, MR218E and G, has very similar Sr

and comparable Nd isotopic ratios (figure 14a) indicating that they either formed from the

same source by varying degrees of partial melting and fractional crystallisation or that the Sr

and Nd isotopic ratios of granodioritic enclave MR218E re-equilibrated with those of its host

after mingling of the two magmas. The bimodality in zircon Hf isotopic ratios discovered in

the Ordovician samples (section 4.2.1) is also reflected by the whole rock Nd and Sr isotopes


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of granodiorites MR109 versus 127 (figure 14a), with the latter sample having the most

juvenile Hf, Nd and Sr compositions.87

Sr/86

Sriratios of Permian and Triassic granitoids show

a well developed positive correlation to more radiogenic values with increasing SiO2, which

suggests that significant crustal assimilation has occurred (figure 14b).

Figure 15: Tectonic discrimination diagram after Pearce et al. (1984). For comparison: grey

field - Jurassic arc (Ilo batholith, Boekhout et al., in prep.), striped field - Jurassic back-arc

(Miskovic et al., 2009). Syn-COLG syn-collisional granite, WPG within plate granite,

VAG volcanic arc granite, ORG oceanic ridge granite. See legend in figure 10.


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5. DISCUSSION

5.1 Geodynamic setting: Arc, back-arc or rift?

5.1.1 Ordovician

The Famatinian arc stretched from Patagonia to as far north as Colombia (Villagomez et

al., 2011) and Venezuela (Van der Lelij et al., in prep.), spanning the Iapetus margin of

Gondwana. Famatinian arc magmatism commenced at ~510 Ma and continued until the

earliest Silurian (Cawood, 2005; Chew et al., 2007), with a peak in magmatic activity between

c. 480 and 460 Ma (Bahlburg et al., 2009). The Ordovician granitoids from the Machu Picchu

Inlier coincide with peak activity (479.9 2.3 Ma - 472.2 4.8 Ma), and also record the final

phases of the Famatinian system (447 10 Ma; Miskovic et al., 2009).

A combination of geochronological data and their spatial distribution suggests the

Ordovician plutons that are exposed within the Machu Pichu Inlier formed part of a back-arc.

First, zircon U-Pb ages of Early Ordovician plutons within the Machu Pichu Inlier indicate

they were emplaced synchronously with arc intrusions that are currently exposed along

coastal Arequipa (473 3 Ma to 464 4 Ma, Loewy et al., 2004; 461 2 Ma, Boekhout et

al., submitted), 350 km to the west (figure 1b). We argue that the Ordovician plutons that are

exposed along the present-day coast of Arequipa and in the Machu Picchu Inlier are in the

same relative position as they were during emplacement because 1) The Arequipa block

sutured to the western Gondwana margin during the Grenville Orogeny (Loewy et al., 2004),

and 2) the Famatinian arc continues undisrupted in space and time from the region of

Arequipa onto the Antofalla Block to the south (Ramos, 2008; Zimmermann et al., 2010;

figure 1a), and the apparent polar wander path of the Antofalla Block has paralleled that

obtained from the Eastern Puna belt since the Early Ordovician (Rapalini, 2005). Therefore, it

is unlikely that there has been significant lateral displacement of the Arequipa-Antofalla

Terrane along the western Gondwana margin since the cessation of the Famatian arc. The


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Ordovician plutons exposed in southern Peru are thus preserved in their approximate arc -

back-arc geometry.

Bahlburg et al. (2006) suggested that the volcano-sedimentary Ollantaytambo and

Umachiri formations, which are exposed in the Machu Picchu Inlier and on the Altiplano,

respectively (figure 2), were deposited within back-arc basins because their stratigraphic ages

are synchronous with the emplacement ages of the Famatinian-aged plutons along coastal

Arequipa. Detrital zircon U-Pb ages and graptolite faunas indicate that the Ollantaytambo

Formation spans the Lower to Upper Ordovician (Bahlburg et al., 2011). Therefore, the

reworked lapilli tuffs that are concentrated in the lower part of the formation, and the

Ordovician plutons from the Machu Picchu Inlier probably represent extrusive and intrusive

components of the back-arc.

The geochemistry of Ordovician plutons within the SPEC does not provide unambiguous

evidence for their geodynamic setting, although their compositions do not exclude a back-arc

position. The Ordovician granitoids are calc-alkaline (figure 10) and peraluminous (figure

11), suggesting they comprise a substantial amount of reworked continental crust (see also

5.2), which can occur in a variety of geodynamic settings including incipient back-arcs (e.g.

(Collins and Richards, 2008; Kemp et al., 2009). Furthermore, trace element data indicate that

the plutons straddle the boundary between arc and within-plate granitoids (figure 15), as

defined by Pearce et al. (1984), which is an intermediate environment that could be expected

in a back-arc.

Bahlburg et al. (2006) show that lower Ordovician lapilli tuffs in the Machu Pichu Inlier

follow a calc-alkaline (Ollantaytambo Fm.) or transitional calc-alkaline - tholeiitic (Umachiri

Fm.) differentiation trend that originated in a volcanic arc setting. The reworked character of

the tuffs suggests that they may have erupted within the arc that erupted through the Arequipa

Block, and were subsequently transported into the back-arc basin (Bahlburg et al., 2006).


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Early to Middle Ordovician lavas in northeastern Argentina are more conclusive of a back-arc

setting. The subalkaline to alkaline basalts that are located ~85 km to the east of the

Famatinian arc have an intraplate signature (Coira et al., 2009a; Coira et al., 2009b), and their

trend to MORB-like compositions suggests that the back-arc basin behind the Antofolla block

was under greater extension than the basin in south-eastern Peru.

Ordovician plutons in the Eastern Cordillera of north and central Peru (Cardona et al.,

2009; Chew et al., 2007) are interpreted as the main Famatian arc that stepped inward from

the Arequipa block due to a contemporary embayment along the western Gondwanan margin

(Chew et al., 2007). Ramos (2010) proposed that this embayment was subsequently removed

due to the accretion of the Paracas Terrane in the late Early Ordovician, accounting for a

metamorphic event at ~478 Ma (Chew et al., 2007; Cardona et al., 2009, figure 1a). However,

the similar geochemical properties of Ordovician plutons throughout the Peruvian Eastern

Cordillera suggest that the whole Eastern Cordillera may have been in a back-arc position

during the Ordovician. The Famatinian arc of northern and central Peru may have been

removed by either terrane translation, e.g. detachment of the Oaxaquia Terrane in Late

Ordovician Silurian times (Ramos, 2010), or by extensive subduction erosion (further

discussed below).

5.1.2 Carboniferous

After a period of magmatic quiescence in the Siluro-Devonian (Bahlbu

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