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Abstract
http://archive-ouverte.unige.ch/unige:230958/10/2019 Reconstructing the Late Paleozoic - Peru.pdf
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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.
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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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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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iv
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
vi
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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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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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
4.3Whole rock geochemistry 34
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
3.2Whole rock geochemistry 76
3.3Zircon U-Pb geochronology 77
3.4Lu-Hf isotope analyses on volcanic zircon 78
4. Results
4.1Stratigraphy and U-Pb zircon dating 78
4.2Whole rock geochemistry 89
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
3.3Zircon U-Pb geochronology 123
3.4Lu-Hf isotope analyses of zircon 125
4. Results
4.1Stratigraphy and geochronology 129
4.2Hf-isotopic ratios in detrital zircon 138
4.3Whole rock geochemistry 139
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
Appendices chapter 2 195
Appendices chapter 3 211
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
3
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
4
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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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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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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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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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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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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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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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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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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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
xenocrystic zircon are plotted. n: number of analyses used in weighted mean calculation,
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
xenocrystic zircon are plotted. n: number of analyses used in weighted mean calculation,
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