Reconstructing the Late Paleozoic : Early Mesozoic plutonic and sedimentary record of south-east Peru : Orphaned back-arcs along the western margin of Gondwana

Thesis

Reference

Reconstructing the Late Paleozoic : Early Mesozoic plutonic and sedimentary record of south-east Peru : Orphaned back-arcs along

the western margin of Gondwana

REITSMA, Martje Jel

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. 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. The plutons were mainly emplaced in a back-arc setting where magmatism was generated by remelting of pre-existing crust. The second chapter presents sedimentological, geochronological and geochemical data acquired from the sedimentary and volcanic rocks of the Mitu Group, demonstrating that the Mitu Group was deposited in the Middle to Upper Triassic in an extensional [...]

REITSMA, Martje Jel. Reconstructing the Late Paleozoic : Early Mesozoic plutonic and sedimentary record of south-east Peru : Orphaned back-arcs along the western margin of Gondwana. Thèse de doctorat : Univ. Genève, 2012, no. Sc. 4459

URN : urn:nbn:ch:unige-230955

DOI : 10.13097/archive-ouverte/unige:23095

Available at:

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

Disclaimer: layout of this document may differ from the published version.

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Dr. 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

TH ` ESE

pr´esent´ee `a la Facult´e des sciences de l’Universit´e de Gen`eve

pour obtenir le grade de Docteur `es sciences, mention Sciences de la Terre

par

Martje Jel Reitsma

de Boxtel (Pays-Bas)

Th`ese N^o 4459

Gen`eve

Atelier de reprographie ReproMail 2012

UNIVERSITE DE GENÈVE

FACULTÉ DES SCIENCES

Doctorat es sciences Mention sciences de la Terre

Thèse de Madame Martje Je/ REITSMA

intitulée :

" Reconstructing thé Late Paleozoic - Early Mesozoic Plutonic and Sedimentary Record of South-East Peru :

Orphaned Back-arcs along thé Western AAargin of Gondwana "

La Faculté des sciences, sur le préavis de Messieurs U. SCHALTEGGER, professeur ordinaire et directeur de thèse (Département de minéralogie), R. SPIKINGS, docteur et codirecteur de thèse (Département de minéralogie), W. WINKLER, professeur (Geologisches Institut, Eidgenôssische Technische Hochschule Zurich, Schweiz), D. CHEW, docteur (Trinity Collège Dublin, Ireland) et O. MUNTENER, professeur (Institut de minéralogie et de géochimie, Université de Lausanne), autorise l'impression de la présente thèse, sans exprimer d'opinion sur les propositions qui y sont énoncées.

Genève, le 13 août 2012

Thèse - 4459 -

N.B.-

Le Doyen/ïean-AAarc TRISCONE

La thèse doit porter la déclaration précédente et remplir les conditions énumérées dans les "Informations relatives aux thèses de doctorat à l'Université de Genève".

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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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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

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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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.

Abstract

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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 générale de l’enregistrement sédimentaire et plutonique pour le sud-est de

Pérou. Colonne ‘Now’ est basé sur des données de cette thèse et Miskovic et al. (2009).

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Reconstruction de l’enregistrement plutonique et sédimentaire du Paléozoïque Supérieur au Mésozoïque Inferieur dans le sud-est péruvien:

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

RÉSUMÉ

Cette thèse traite de l’évolution sédimentaire, plutonique et tectonique dans la région de la Cordillère est du sud-est péruvien lors de la formation du supercontinent de la Pangée et lors d’épisodes ultérieurs de dislocation précoce. Nous présentons un cadre chronostratigraphique pour les enregistrements sédimentaires et plutoniques allant du Carbonifère au Trias, au moyen d’outils géochronologiques, géologiques et isotopiques, combinés aux observations de terrains ainsi qu’aux travaux antérieurs publiés dans la littérature scientifique. Ce travail est présenté en trois chapitres qui présentent chacun différents aspects de cette thèse.

Le premier chapitre traite de l’enregistrement plutonique en relation avec la croissance de la croûte continentale. Grâce à des datations U-Pb sur zircons nous démontrons que le magmatisme a été actif par intermittence sur une période de près d’un demi milliard d’années qui peut se subdiviser en six phases magmatiques, d’une durée de 20 Ma ou moins, allant de l’Ordovicien au Miocène (figure I). L’uniformité minéralogique et géochimique des granitoïdes de l’Ordovicien, du Carbonifère, du Permien et du Trias semble indiquer une source commune et un contexte géodynamique comparable lors de la production magmatique.

Les vestiges plutoniques des arcs ordoviciens et jurassiques sont préservés sur la côte sud du

Pérou et ont ainsi conduit à interpréter le plutonisme contemporain de la Cordillère est comme

Abstract

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étant d’arrière-arc. Cette étude soutient la thèse selon laquelle l’emplacement plutonique entre ces deux périodes s’est également fait dans un contexte d’arrière-arc, ceci sur la base des arguments suivant : 1) la distance minimum supposée jusqu’à la paléo-fosse, 2) le contexte de tectonique extensive au Carbonifère, au Permien et notamment au Trias qui ne peut pas être concilié avec l’emplacement des granitoïdes en contexte de subduction plane, 3) le caractère péralumineux des granitoïdes, combiné à des quantités mineures de minéraux hydratés.

L’absence des arcs carbonifères à triasiques sur la côte péruvienne est attribuée à l’oblitération induite par une vigoureuse subduction-érosion qui a été reporté pour le Cénozoïque.

Les compositions isotopiques de l’Hf, du Nd et du Sr démontrent que le magmatisme généré en zone d’arrière-arc s’est majoritairement formé par fusion crustale et n’a pas significativement contribué à une croissance crustale. Les granitoïdes de l’Ordovicien au Trias présentent le même chemin d’évolution crustale que les magmas qui se sont différenciés du manteau appauvri lors de l’orogénèse de Grenville/Sunsas, faisant de ces derniers la source la plus probable. Seule la signature radiogénique des isotopes de l’Hf de la phase plutonique mineure du Jurassique ne peut pas s’expliquer par une source dont l’âge dominant réfère au Sunsas, et est plutôt interpréter comme s’étant formé par fusion par décompression adiabatique d’un réservoir mantellique enrichi. Nous concluons que les régions reculées d’arrière-arc ne permettent pas de générer d’importantes quantités de nouvelle croûte continentale car la fusion mantellique en l’absence de fluides issus de la plaque plongeante peut seulement avoir lieu lors d’une extension extrême de la lithosphère.

Le second chapitre présente les données sédimentaires, géochronologiques et géochimiques

des roches sédimentaires et volcaniques du Groupe Mitu déposés dans un contexte extensif. Ces

données sont utilisées pour développer un modèle tectonique pour la séquence de rift, et pour

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proposer les forces motrices pour sa formation et son interruption. Des sections à travers le Groupe Mitu ont été étudiées sur quatre endroits répartis sur 670 km le long de l’orogéne du centre au sud-est du Pérou. Les datations U-Pb sur zircons des roches volcaniques et détritiques à la base de chaque section démontrent que la déposition a commencé au Trias Moyen, contrairement à un début au Permien qui était précédemment supposé (figure I). La terminaison du Groupe Mitu au Norien (Trias Supérieur) rend la déposition entièrement contemporaine de la volumineuse phase plutonique triasique.

Le volcanisme alcalin et les importantes variations d’épaisseur du Groupe Mitu indiquent

que la déposition s’est faite dans un régime extensif, probablement dans un contexte d’arrière-

arc. Lors de la phase initiale sans activité magmatique du Groupe Mitu, l’extension était

remarquablement étendue, s’ensuivi de la déformation localisée et le début de l’activité

volcanique. L’amincissement lithosphérique eu pour résultat un soulèvement thermique de la

croûte et explique la déposition subaérienne du Groupe Mitu. Sur la base du décalage latéral

entre les axes des bassins post-rift et syn-rift, il est proposé que l’extension présente une

composante majeure de cisaillement simple. L’extension asymétrique de la lithosphère a donnée

lieu au soulèvement d’un épaulement de rift à l’est du graben lié au Groupe Mitu. Les âges U-Pb

sur zircons détritiques démontrent que cet épaulement a efficacement bloqué l’afflux

sédimentaire provenant du craton et que les zircons dans les grès du Groupe Mitu dérivent du

volcanisme contemporain de la sédimentation ou des roches exposées à l’épaulement. La

terminaison du Group Mitu et du plutonisme associé est interprétée comme la clôture du bassin

d’arrière-arc due à la migration de l’arc Chocolate au sud du Pérou vers l’intérieur des terres au

Trias Supérieur.

Abstract

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Dans le troisième chapitre nous introduisons le premier cadre chronostratigraphique pour la période du Carbonifère au Permien Inférieur au Pérou, sur la base de datation radio-isotopiques sur échantillons volcaniques et détritiques (figure I). Le modèle tend à reconstruire l’histoire paléogéographique et tectonique du Pérou lorsqu’il se situait sur la marge ouest du Gondwana lors de la formation et la suture du supercontinent Pangée.

Durant l’amalgamation de la Pangée au Mississippien, les contraintes compressives ont

commencé à se concentrer à la périphérie du supercontinent ce qui induit un redémarrage du

magmatisme d’arc au nord et au centre du Pérou. Parmi les dépôts subaériens du Groupe Ambo

du sud du Pérou, aucune preuve de volcanisme contemporain n’a été détectée de même que

l’enregistrement des zircons détritiques dans les grés fluviatiles atteste que le volcanisme

mississippien n’a pas été très marqué dans cette région. Cette absence de magmatisme (d’arrière-

arc) en comparaison au nord du Pérou peut être attribuée à l’absence d’extension d’arrière-arc, à

une subduction plane ou à une convergence décrochante le long de la marge sud du Pérou. Le

volcanisme commença à être plus prononcé lors de la sédimentation au Pennsylvanien de

sédiments marins peu profonds de la Formation Tarma alors que la sédimentation au Pérou

central était perturbée par un événement de déformation. Au contraire, les bassins du sud-est du

Pérou n’ont pas été affectés et ont connu une subsidence qui donna lieu à la construction d’une

plateforme carbonatée correspondant à la Formation Copacabana. Une régression au Permien

Inférieur mena au retrait de la mer épeirique jusqu’à la région subandine actuelle et à l’initiation

d’une sédimentation fluviatile dans la région de la Cordillère Est. La régression coïncide avec

une phase majeure de plutonisme d’arrière-arc et est attribuée à un soulèvement thermique de la

croûte en réponse à un amincissement lithosphérique. Les âges U-Pb et les rapports isotopiques

de l’Hf sur les zircons détritiques suggèrent que les zircons proviennent de granitoïdes

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légèrement plus vieux que l’âge stratigraphique des grès, ce qui concorde avec un contexte

extensif capable d’exhumer rapidement les roches plutoniques.

Table of contents

Abstract

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Résumé 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

3.1 Whole rock geochemistry 16

3.2 Zircon U-Pb geochronology 18

3.3 Lu-Hf isotope analyses 21

4. Results

4.1 Research area and sample material 22 4.2 U-Pb geochronology and Hf isotopes 23

4.3 Whole rock geochemistry 34

5. Discussion

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

5.2 Crustal reworking along the western Paleozoic Gondwanan margin 53

5.3 Relating geodynamic setting to pulses of crustal growth 57

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

3.1 Sampling strategy 75

3.2 Whole rock geochemistry 76

3.3 Zircon U-Pb geochronology 77

3.4 Lu-Hf isotope analyses on volcanic zircon 78 4. Results

4.1 Stratigraphy and U-Pb zircon dating 78

4.2 Whole rock geochemistry 89

5. Discussion

5.1 Age of the Mitu Group 95

5.2 Geochemical characterization of Triassic-Jurassic volcanism 99

5.3 Geodynamic setting 102

6. Conclusions 110

7. References 112

Table of contents

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

1. Introduction 115

2. Geological setting 119

3. Methods

3.1 Sampling strategy 122

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

3.3 Zircon U-Pb geochronology 123

3.4 Lu-Hf isotope analyses of zircon 125

4. Results

4.1 Stratigraphy and geochronology 129

4.2 Hf-isotopic ratios in detrital zircon 138

4.3 Whole rock geochemistry 139

5. Discussion

5.1 Paleogeography 141

5.2 Tectonic implications 148

6. Conclusions 155

7. References 157

Conclusions and Outlook 159

Appendices chapter 1 165

Appendices chapter 2 195

Appendices chapter 3 211

Acknowledgement

212

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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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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 (10.8°S) and Lake Titicaca (16.1°S). 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?

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 made

Ar/

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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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

Thesis introduction

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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

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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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

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

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; Cárdenas 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.

8   

9

Chapter 1

Crustal reworking in Paleozoic – Early Mesozoic orphaned back- arcs on the western Gondwana margin, south-east Peru.

Mariël 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 – 14°S, figure 1 and 2) were previously assigned to a single magmatic stage based on their mineralogical and textural

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, regular by ID-TIMS. Ages in green - LA-ICP-MS zircon data from Miskovic et al. (2009), age in blue - biotite ⁴⁰Ar/³⁹Ar cooling age (Rodriguez et al., 2009), yellow (Cenozoic) pluton - Andahuaylas-Yauri batholiths, PTFZ – Patacancha-Tamburca fault zone, ZSGZ – Zongo-San Gaban fault zone.

11

12

similarities (Dalmayrac et al., 1980; Laubacher, 1978; Marocco, 1978). Further, limited U-Pb multi-fraction zircon and ⁴⁰Ar/³⁹Ar 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.

Chapter 1

13

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; Mégard, 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 Viséan – earliest Serpukhovian age (Azcuy and Di Pasquo, 2005; Iannuzzi and Pfefferkorn, 2002).

14

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;

Mégard, 1978; Rosas et al., 2007).

Chapter 1

15

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 17°S (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

16

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 (Perelló 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.

Chapter 1

17 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 μm 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 Ba²⁺/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.

18

Approximately 150 mg of powdered rock (<70 µm) were dissolved in closed Teflon vials for 7 days on a hot plate at 140°C with a mixture of 4 ml concentrated HF. The solution was dried on a hot plate and re-dissolved in 3 ml of 15 M HNO3(aq) in closed Teflon vials at 140°C, and dried down again. Sr and Nd separation was carried out using cascade columns with Sr- spec, TRU-spec and Ln-spec resins following a modified method after Pin et al. (1994). Sr and Nd isotope ratios were measured on a Thermo Scientific TRITON mass spectrometer on Faraday cups in static mode. Sr was loaded on single Re filaments with a Ta oxide solution and measured at a pyrometer-controlled temperature of 1480°C in static mode using a ‘virtual amplifier’ to cancel out biases in gain calibration among amplifiers. ⁸⁷Sr/⁸⁶Sr values were internally corrected for fractionation using a ⁸⁸Sr/⁸⁶Sr value of 8.375209. Raw values were further corrected for external fractionation by a value of 0.03‰, determined by repeated measurements of the SRM987 standard (⁸⁷Sr/⁸⁶Sr = 0.710250). External reproducibility of the

87Sr/⁸⁶Sr ratio for the SRM987 standard is 7 ppm. Nd was loaded onto double Re filaments with 1M HNO3(aq) and measured in static mode using virtual amplifier switching. ¹⁴³Nd/¹⁴⁴Nd values were internally corrected for fractionation using a ¹⁴⁶Nd/¹⁴⁴Nd value of 0.7219 and the

144Sm interference on ¹⁴⁴Nd was monitored on the mass ¹⁴⁷Sm and corrected using a

144Sm/¹⁴⁷Sm value of 0.206700. External reproducibility of the JNdi-1 standard (Tanaka et al., 2000) was <5 ppm.

87Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd whole rock compositions were corrected for time-integrated decay of ⁸⁷Rb and ¹⁴⁷Sm using Rb, Sr, Sm and Nd concentrations determined by LA-ICP-MS on the glass whole rock discs and ages obtained in this study.

3.2 Zircon U-Pb geochronology

Approximately 3 kg of rock per sample was crushed to a size fraction < 300 μm using a jaw crusher and tungsten mill. Zircons were extracted using a Wilfley table, Frantz horizontal

Chapter 1

19

magnetic separator with a side slope of 10° and heavy liquid (DIM, ρ = 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 µm 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-TIMS ²⁰⁶Pb/²³⁸U 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 (Sláma et 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-

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 the ²⁰⁶Pb/²³⁸U ratio of analytically concordant zircons. Discordant ages, inherited zircons and zircons showing signs of Pb-loss were discarded from the age calculation. U–Pb data are plotted on the concordia diagram as 2σ error ellipses.

In the majority of samples, the range in ²⁰⁶Pb/²³⁸U 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

Chapter 1

21

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

206Pb/²³⁸U 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 μm spot size, 5 Hz firing repetition rate, and xx J/cm² energy density. To correct for isobaric interferences of Lu and Yb on mass 176 the isotopes ¹⁷²Yb, ¹⁷³Yb and ¹⁷⁵Lu were simultaneously monitored. The ¹⁷⁶Yb and ¹⁷⁶Lu signals were calculated using a ¹⁷⁶Yb/¹⁷³Yb of 0.796218 (Chu et al., 2002) and ¹⁷⁶Lu/¹⁷⁵Lu of 0.02658 (JWG in-house value). The instrumental mass bias for Hf isotopes was corrected using an exponential law and a ¹⁷⁹Hf/¹⁷⁷Hf 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 βHf/βYb 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 (2σ) were propagated by quadratic addition of the external reproducibility of GJ-1 (2σ, n = 18 / 30) and Temora (2σ, n=12) or Plesovice (2σ, n=20) and the within-run precision of each analysis (2 SE). The external reproducibility (2σ, n > 50) over more than 6 months of reference zircon 91500, GJ-1, and

22

Plešovice (¹⁷⁶Hf/¹⁷⁷Hf = 0.282298 ± 0.000026, 0.282003 ± 0.000018, and 0.282482 ± 0.000015, respectively) at JWG is about 0.005- 0.009% (<1σ).

3.3.2 Hf isotope analyses on solutions of ID-TIMS samples

Hf isotopic compositions were measured on the same volume of zircon dated by CA-ID- TIMS techniques. This involves retaining the waste eluted solution obtained from ion exchange chemistry. The dried residue was dissolved in 500 to 700µl 2% HNO3(aq) and introduced into the plasma with an Aridus II desolvating nebulizer, and analyzed for Hf isotope composition using the same multicollector ICP-MS techniques as described above for laser ablation analyses. Data acquisition of the solution analyses lasted for 5 minutes per sample (55 ratios) and was performed in automatic mode, bracketing groups of 5 unknowns by JMC-475 standard measurements. Six solution aliquots of standard zircon GJ-1 were analyzed as an external control (0.282016 ± 0.000002, 2).

4. RESULTS

4.1 Research area and sample material

Data is presented from granitoids exposed in the southern Peruvian Eastern Cordillera (SPEC), located between 12°S and 14°S, which can be divided into the northwest – southeast striking Cordillera de Carabaya in the southeast, the oval shaped Machu Picchu Inlier in the centre, and the northwest – southeast striking Cordillera de Andahuaylas between Abancay and Ayacucho in the northwest (figure 2).

We also present data from the Eocene Andahuaylas-Yauri Batholith, which is not exposed in the SPEC but forms part of the Altiplano (figure 2). Its distinct mineralogy and geochemistry are well understood and assigned to a period of flat slab subduction (Perelló et al., 2003). The Eocene batholith is easily identifiable in the field due to the dominance of

Chapter 1

23

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, while ²⁰⁷Pb/²⁰⁶Pb ages of discordant zircon analyses span the same range and are probably the result of mixed core-rim analyses.