3. Chapter 2_Metamorphic and Plutonic Basement Complexes

7/27/2019 3. Chapter 2_Metamorphic and Plutonic Basement Complexes

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2 Metamorphic and plutonicbasement complexesFRANCISCO HERV (coordinator), VICTOR FAUNDEZ,MAURICIO CALDERN, HANS-JOACHIM MASSONNE &ARNE P. WILLNER

The present-day Andes have formed in response to subduction-related processes operating continuously along the westernmargin of South America since the Jurassic period. When theseprocesses started, the continental margin was mainly formed ofmetamorphic complexes and associated magmatic rocks whichevolved during Proterozoic (?), Palaeozoic and Triassic times,and which now constitute the basement to the Mesozoicand Cenozoic Andean sequences. These older units are com-monly referred to in the Chilean geological literature as thebasement or the crystalline basement.

The basement rocks crop out discontinuously (Fig. 2.1) innorthern Chile, both in the coastal areas and in the main cordil-lera. In contrast, from latitude 34S southwards, they form analmost continuous belt within the Coastal Cordillera extendingto the Strait of Magellan. In addition, sparse outcrops occurboth in the main Andean cordillera as well as further east in theAysen and Magallanes regions. In the first maps and synthesesof the geology of Chile (e.g. Ruiz 1965) these rocks were gener-ally considered to be of Precambrian age, forming a westerncontinuation of the Brasilian craton. Later work has demon-strated that rocks first described as metamorphic basement

units show a wide range of metamorphic grades and agesextending from possible Late Proterozoic through Palaeozoicand even, in some cases, to JurassicCretaceous.

With regard to previous works that have attempted to syn-thesize data on Chilean basement geology, the reader is referredto those by Gonzlez-Bonorino (1970, 1971), Gonzlez-Bonorino & Aguirre (1970), Aguirre et al. (1972), Muoz Cristi(1973), Herv et al. (1981a), Herv (1988), Breitkreuz et al.(1988), Damm et al. (1990), Bahlburg & Herv (1997), Hervet al. (2000, 2003a) and Willner et al.(2005). These accountsreflect increasing progress in our understanding of the base-ment based on recent field studies, the application of radiomet-ric dating techniques, and new ideas concerning the evolutionof accretionary prisms and terrane geology.

A description of the different units that constitute the

Chilean basement geology is presented below, includinginformation about their age, metamorphic characteristics andgeological settings, largely based on the above-mentionedpublications as well as on more specific studies that wil l be citedappropriately. Particular emphasis will be on recent studies thathave attempted to determine PTt paths of metamorphismthrough mineralogical observations and thermodynamic calcu-lations which were developed in the last two decades, althoughstudies of some of the units are still in an immature state.

As a general framework, the description of the units will bebased on the terrane model as established by Bahlburg &Herv (1997) for northern Chile and northwestern Argentina,depicted in Figure 2.1. The older metamorphic complexes ofnorthern Chile will be treated first, the Late Palaeozoic accre-tionary complexes of the coastal areas of central Chile next,and finally the Late Palaeozoic to Mesozoic complexes of thePatagonian Andes.

North Chile (Norte Grande): ArequipaAntofalla and

Mejillones terrane areas

In the main Andes and in the Coastal Cordillera of northernChile, scattered outcrops of basement rocks occur sporadically

under the Meso-Cenozoic cover (Fig. 2.1). The isolation ofthese units has hindered detailed interpretation of their geologi-cal significance and correlations between them. Interpretationscan be assigned to two main types: (a) that they reflect terranetectonics, related to the collision of Laurentia and Gondwanain Early Palaeozoic times; and (b) that these rocks representin situ evolution of old cratonic units of the western margin ofGondwana.

Metamorphic rocks cropping out in northern Chile aregrouped within the Beln, Sierra de Morena-Chojas, andLimon Verde complexes (all placed within the ArequipaAntofalla Terrane: Fig. 2.1), and the Mejillones MetamorphicComplex, which may be a separate terrane. In addition to thesemetamorphic complexes, a volcanic and sedimentary sequenceknown as the Cordn de Lila Complex (CLC) crops out on the

west side of the Andes in northern Chile.Bahlburg & Herv (1997) and Bahlburg et al. (2000) haveobserved that during the Palaeozoic era in northern Chile andnorthwestern Argentina, a magmatic and metamorphic lullofc. 100 million years, from the Lower Silurian to the earlyLate Carboniferous, allows separation of the rock units intotwo orogenic groups: (a) Cambrian to Early Silurian rocksdeformed during Lower Palaeozoic orogenic cycles in an activemargin setting; (b) rocks formed after Lower Palaeozoicorogenies but affected by the Late Carboniferous to PermianToco event, in which active margin conditions resumed aftera lull during which the area evolved as a passive margin(SilurianEarly Carboniferous).

The more detailed account that follows presents lithologicaland geochronological data from both published and unpub-

lished work, organizing the data within the framework of themodels provided by Bahlburg & Herv (1997) and Bahlburget al. (2000). The detailed data available from Argentina,Bolivia and Peru are beyond the scope of this chapter (seeBahlburg & Herv 1997; Loewy et al. 2004). The descriptionsgiven below are organized to deal initially with the generalgeology, then metamorphic grade, and finally geochronology.The geotectonic setting is discussed under a separate heading atthe end of this chapter.

Beln Metamorphic Complex

The Beln Metamorphic Complex (BMC) (Basei et al. 1996)forms a narrow outcrop of metamorphic and igneous rocksalong a high-angle west-vergent thrust system located on thewestern slope of the Chilean Altiplano plateau betweenChapiquia and Tignamar (Muoz & Charrier 1996). Along


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Fig. 2.1. Geological map showing the distribution of outcrops of the metamorphic complexes, the Palaeozoic and some Triassic sedimentary units,and associated plutonic belts in Chile.


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these faults the BMC is thrust westward over late Cenozoicdeposits. Uncomformably covering the BMC are Jurassicmarine deposits, and Cenozoic volcaniclastic and continentalsedimentary rocks (Pacci et al. 1980; Muoz et al. 1988a; Garca1996). The BMC mainly comprises foliated amphibolitesand subordinate quartz micaschists, orthogneisses andserpentinites. It is intruded by a small gabbro stock and bymafic, aplitic and felsic dykes.

Peak metamorphic conditions for the Beln MetamorphicComplex have been determined by Wrner et al. (2000a)at c. 700C and 7 kbar. The prograde metamorphic path isreflected in zoning patterns of garnets which indicate a simple,single-stage metamorphic event. Retrograde stages are marked

by lower grade overprinting in amphibolite and greenschistfacies. The resulting PTt curve is shown in Figure 2.2.

The first geochronological determination of the age ofthe BMC was by Pacci et al. (1980) who produced a RbSrreference isochron of 1000 Ma which has since been laterrecalculated to 500 Ma (Damm et al. 1990). These authorsdetermined a 1460P448 Ma NdSm whole-rock isochron formetabasites which was considered as the crystallization age ofthe protoliths. Basei et al. (1996) obtained UPb zircon upperintercept ages (conventional method) of 509P46 Ma for theQuebrada Achacagua orthogneiss and 486P32 Ma on graniticveins at Quebrada Saxamar which they interpreted as crystalli-zation ages of the igneous precursors. These authors alsoreported model SmNd model ages of 1746 Ma and 1543 Maon the Quebrada Saxamar schists testifying an ancient crustalresidency for the protoliths. Wrner et al. (2000a) considered

that higher intercept ages of 1877P139 Ma and 1745P27 Maobtained on zircons (UPb, conventional method) fromBeln reflect crystallization ages and that lower intercepts of366P3 Ma and 456P4 Ma suggest severe Palaeozoic lead loss.Loewy et al. (2004) presented a UPb zircon age of 473P2 Mafor the Saitoco granodiorite, and revealed the presence of 1.8 to1.9 Ga old zircons on a cross-cutting dyke in the micaschists.

In addition, Basei et al. (1996) have presented a 344P22 MaRbSr whole-rock isochron, with a 87Sr/86Sr initial ratio of 0.708for the quartz micaschists of Quebrada Saxamar. KAr datingon different minerals yielded ages of 536 Ma to 516 Ma (hb) inthe Quebrada Saxamar schists, of 417Ma (Ms) to 365 Ma (bt)on the Saitoco orthogneiss (Basei et al. 1996), and 358P10 Mato 457P7 Ma (hb) (Lucassen et al. 2000) at Beln.

All these data point towards the involvement of the BMCin Early Palaeozoic orogenies, time-equivalents to the EarlyCambrian Pampian and Ordovician Ocloyic/Famatinianorogenies, both of which are more extensively represented innorthwestern Argentina. The age of the protoliths seems to beEarly Palaeozoic, as no undisputed evidence of Proterozoicages has been produced (Damm et al. 1990; Wrner et al. 2000a;Loewy et al. 2004).

Sierra de Moreno Chojas metamorphic complexes

Basement exposures of quartz micaschists, greenschists,migmatites, granites and mylonites in the Sierra de Morenocrop out as a 70x20 km belt orientated NNESSW (Skarmeta1983). The western side of the outcrop comprises a 3-km-wide

Fig. 2.2. A compilation of PTt trajectories for the different units of the metamorphic basement complexes of northern Chile. Sources of data:Limn Verde Metamorphic Complex (CMLV), Lucassen et al. (1999); Beln Metamorphic Complex (BMC), Damm et al. 1990; MejillonesMetamorphic Complex (MMC1), Damm et al(1990); (MMC2), Baeza (1984); Lila Igneous and Sedimentary Complex (CISL), Damm et al. (1990);Chojas, Damm et al. (1990); Chaaral Mlange (CM), Marioth & Bahlburg (2003).


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mylonite zone produced during the thrusting of the metamor-phic complex over Jurassic rocks (Quinchamale Formation)after four superimposed deformation phases that affected thesebasement rocks. An additional smaller outcrop of basementmicaschists, gneisses, amphibolites, migmatites and alkalisyenogranites occurs in the higher reaches of Quebrada Chojas,north of Sierra de Moreno (Damm et al. 1990).

The high grade metamorphism indicated by Damm et al.(1990) at Quebrada Chojas, where schists have undergone

migmatization, has been deduced to have achieved a peakof 850C at a relatively low pressure (c. 4 kbar). Part of themetamorphic pathway recorded by these rocks is illustrated inFigure 2.2.

A Proterozoic age for the Chojas complex has been suggestedby Damm et al. (1990) on the basis of upper intercepts inUPb zircon diagrams, which indicate 1254+97/94 Ma and1213+28/25 Ma for a migmatite and an orthogneiss respec-tively, which have lower intercepts at 466+8/7 and 415+36/38 Ma, interpreted as reset ages due to the high grademetamorphism.

Loewy et al. (2004) have presented conventional UPb dataon zircons from migmatite and foliated megacrystic granitewhich have yielded upper intercept ages of 1067P4 Ma and1024P5 Ma, which they interpret as the age of crystallization of

the protoliths. The lower intercepts of the former, and the con-cordant ages of a granitic neosome in the migmatites, yieldedages in the range of 497P16 Ma and 470P2 Ma. In contrast,isotopic ages at Sierra de Moreno, obtained in a much earlierstudy by Skarmeta (1983), give only Palaeozoic ages:511P9 Ma and 485P12 Ma RbSr whole-rock isochrons inmigmatites and schists, respectively. These values are closer tothe KAr ages in the range 452P11 Ma (hb) to 372P3 Ma (ms)for minerals in the migmatites and schists. The granites yieldedKAr mineral ages in the range 295225 Ma. The age patternwas further corroborated by Lucassen et al. (2000) with aNdSm isochron of 505P6 Ma and KAr ages (hb) rangingfrom 382P11 to 478P15 Ma as well as KAr ages (bt) in therange of 284P6 to 311P7 Ma.

The Mejillones Metamorphic Complex: an independent terrane?

The Mejillones Metamorphic Complex (MMC) is an isolated60x10 km outcrop of basement rocks exposed along theMejillones Peninsula (Fig. 2.1). There is also a smaller outcropof similar lithologies at Caleta Coloso, a few kilometres tothe south. Baeza (1984) has described the metamorphic rocksas consisting of orthogneiss, paragneiss, amphibolites andmicaschists, intruded by mafic to silicic plutonic rocks anddykes. The MMC consists of two metamorphic areas, one witha regional metamorphic imprint and the other with a contactmetamorphic aureole developed in the lower-grade parts of theformer.

Baeza & Pichowiak (1988) divided the MMC into twoformations (although given the lack of lithostratigraphic

control, the term complex is more appropriate): the PuntaAngamos Formation (PAF) and the Jorgino Formation (JF).The latter exhibits a metamorphic zoning including biotite,garnet and kyanite zones, which they estimate to have reachedmetamorphic peak PT conditions of 46 kbar and 400600C.The PAF developed as a contact aureole with biotite,andalusite-sillimanite, and K-feldsparsillimanite zones, onpreviously metamorphosed chlorite zone rocks (Fig. 2.2)

The oldest radiometric ages obtained directly from the MMCare RbSr wr data ofc. 530 Ma (Diazet al. 1985), a 521P55 MaNdSm wr isochron (Damm et al. 1986) and a 525P10 MaNdSm mineral isochron (Lucassen et al. 2000). The igneouscomplex that generated the contact aureole has been dated byUPb zircon upper intercept as 561P12 Ma (Dammet al. 1986)which contrasts with mainly Mesozoic ages of 200 Ma (RbSrwr isochron age; Daz et al. 1985), 152 to 143 Ma (KAr ages in

ms and bt) and 144P1 Ma (UPb, zircon) of Baseiet al. (1996).The influence of this Jurassic intrusive event on the meta-morphic rocks is reflected by KAr ages of 147162 Ma (bt)and 159 Ma (hb) from Basei et al. (1996), and of 151P3 to189P4 Ma (bt) and 152P5 Ma (hb) from Lucassen et al.(2000).

Cordn de Lila Complex

In Ordovician times, the volcanic products of a magmatic arcwere deposited in a marine basin located above the ArequipaAntofalla terrane, in the present western flank of the Andes.The resulting sequences are basaltic, andesitic and daciticto rhyodacitic lavas interbedded with hemipelagic to shallowmarine sedimentary rocks, which together constitute theCordn de Lila Complex (CLC) (Niemeyer 1989; Damm et al.1990) and a thick silicic volcaniclastic apron, known asthe Aguada de la Perdiz Formation (Breitkreuz et al. 1988).Palaeozoic, subduction-related plutons intrude the CLC, andare at least partly coeval with the volcanism.

The rocks of this succession record only very low levels ofmetamorphism. Damm et al. (1990) described the presenceof mineral assemblages of the pumpellyiteactinolitechloritezone at the base of the succession, with the metamorphic gradedecreasing to higher stratigraphic levels. This burial-type meta-

morphism reached peak metamorphic conditions of 3 kbar and370C (Fig. 2.2).

A whole-rock NdSm isochron from 11 basalts and andesitesof the CLC yields an age of 448P145 Ma (Damm et al. 1986),which is consistent with the palaeontologically derivedLlanvirnian age for the CLC (Cecioni 1982). With regard tothe Palaeozoic intrusives, Mpodozis et al. (1983) reportedRbSr whole-rock isochrons of 441P8 Ma (Tuccaro pluton),452P4 Ma (Tilopozo pluton) and a 288P15 Ma errorchronfor the Pingo Pingo pluton, which, however, yielded KAr agesof 425P11 (bt) and 429P12 Ma (hb). Damm et al. (1990)presented UPb zircon ages of 502P7 Ma (conventional,discordant) for the Choschas pluton, and lower interceptages of 434P2 Ma for the Cerro Lila pluton, and of338+14/18 Ma for the Pingo Pingo pluton. All ages were

interpreted as crystallization ages.

Late Palaeozoic sedimentation and volcanism

DevonianCarboniferous passive margin sedimentationThick successions of shale, sandstone and rare conglomerateand limestone crop out in the Coastal Cordillera. Thesesequences have been given different lithostratigraphic namesin different areas: El Toco Fm (TF), Sierra del Tigre Fm, LasTortolas Fm (LTF), and the Chaaral Melange (CM: Fig. 2.1).Coeval siliciclastic shallower marine platform successions weredeposited further east, in the main Andean cordillera (ZorritasFm). The turbiditic rocks have abundantly preserved sedi-mentary features, such as cross- and graded bedding, obscuredlocally by deformation and low grade metamorphism. They

have been interpreted by Bahlburg & Breitkreuz (1991) as aturbidite system representing environments ranging from aproximal depositional lobe (TF) in the north, and a distaldepositional lobe to basin plain (TF and LTF) environments tothe south.

Progressive synsedimentary deformation and metamorphismof these marine units took place during the Carboniferousperiod (El Toco event of Bahlburg & Breitkreuz 1991). Theculmination of this process produced the Chaaral Melange(Bell 1982), a dismembered portion of the turbiditic complex.

The fossil record in the turbidite complex is poor but indi-cates a Devonian to Early Carboniferous depositional age. LateDevonian plant remains occur in the El Toco Fm (Bobenrieth1980; Boric 1980), Devonian brachiopods in the Sierra del TigreFm (Niemeyer et al. 1985, 1997b) and Lower Carboniferousconodonts (Bahlburg 1987) and brachiopods (Bell 1987b) in theLas Tortolas Fm have all contributed to the palaeontological


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database. The only radiometric age determination available isa 280 Ma RbSr whole-rock isochron (Brook et al. 1986) forthe Las Tortolas Fm and Chaaral Melange which is inconcordance with the radiometric ages of the plutonic rocksthat intruded the already folded turbidite succession. Finally,the metamorphic grade of the turbidite succession is invariablylow to very low grade, and little is known of the PT conditionsexcept in the Chaaral melange, with Marioth & Bahlburg(2003) establishing 2.22.8 kbar and 300350C for themetamorphic peak (Fig. 2.2).

Permian subaerial volcanismThick subaerial volcaniclastic rocks, known as the PeineGroup, crop out in the Precordillera and in the high Andes in anorthsouth elongated discontinuous belt (Breitkreuz et al.1989). The successions consist of felsic volcaniclastic rocks andandesiticbasaltic volcanic rocks, interbedded with limnic andshallow marine sediments. They lie with angular unconformityon the DevonianCarboniferous turbidites, and interfinger tothe east and west with the Early Permian marine carbonatesof the Cerros de Cuevitas (Breitkreuz 1986b) and Arizaroformations, in Chile and Argentina, respectively. These rocksare weakly folded and have been subjected to extensivesericitization and chloritization, particularly near plutonicintrusions.

Late Palaeozoic to Triassic batholithsThese intrusions occur in two belts, one in the high Andes andone in the Coastal Cordillera. Brown (1990) studied these plu-tonic belts at latitude 26S and concluded that they were partof the same subduction-related magmatic arc, later affected bycrustal extensional processes. The presence of subvolcanicdomes and high level intrusive bodies led Davidsonet al. (1985)to recognize the existence of former caldera systems in theImilac area dated at 290 to 217 Ma, some of which have coppermineralization. Camus (2003) has presented a synthesis of themineralized intrusive bodies north of 24S in the Precordilleraand High Andes, indicating that they range in age from 190 to307 Ma (K-Ar).

The Coastal Cordillera plutons have given RbSr wr isoch-ron ages (Brook et al. 1986) in the range of 278P16 Ma and221P14 Ma. In addition, UPb determinations on zircon haveyielded 292P14 Ma, 230P6 Ma and 217P12 Ma (Damm &Pichowiak 1981). Thus, these rocks span Early Permian to LateTriassic times, similar to those of the High Andes belt whichranges in age from 270P10 Ma to 209P19 Ma (Brook et al.1986; wr RbSr isochrons). A 268+5/3 Ma age (Damm et al.1990; UPb zircon, lower intercept) was obtained for theCordn Chinquilchorro pluton, located further north, in theCordn de Lila area. In both belts, the age groupings suggestthere is a natural break in the ages during Early Triassic times.

Limn Verde Metamorphic Complex

A 12 km long and 2 km wide outcrop of metamorphic rocksis exposed on the western flank of the Sierra de Limn Verde(Baeza 1984), where it is intruded by a Late Palaeozoicbatholith and unconformably overlain by Triassic sedimentaryand volcanic rocks. The Limn Verde Metamorphic Complex(LVMC) comprises metabasites and metapelites with meta-morphic grade varying from greenschist to amphibolitefacies.

Lucassen et al. (1999), using conventional geothermobaro-metry and multi-equilibria calculations, established that thepeak conditions of metamorphism attained by the LVMC werec. 14 kbar and 660 to 720C, conditions unique in the CentralAndes for metamorphic complexes of this age (Fig. 2.2). Thesedata, combined with the age and stratigraphic data, pointtowards a very rapid exhumation of the LVMC during LatePermianEarly Triassic times. The tectonic environment for theattainment of these conditions is not well understood. Lucassen

et al. (1999) suggested a transpressional strike-slip environmentwhereas a collisional or subduction zone setting was previouslysuggested by Herv et al. (1985) and Bahlburg & Herv (1997).

Radiometric dating has not supported previous suggestionsthat the unit might be of Precambrian age (Baeza 1984; Rogers1985). Herv et al. (1985), Cordani et al. (1988) and Lucassenet al. (1999) established that the metamorphism took place inLate Palaeozoic times. Calculations of residence times usingRbSr isotopic data suggest that the protoliths could not have

been older than 405 Ma (Silurian). Three NdSm mineral iso-chrons indicate a range between 287 and 255 Ma (Lucassenet al. 1999). These ages are younger than the 309P11 Ma and300P20 Ma ages obtained using RbSr whole-rock isochrons(Herv et al. 1985; Cordani et al. 1988). All these authors inter-preted the ages as indicative of the metamorphism. DecreasingKAr mineral ages in hornblende, muscovite and biotite in therange 310229 Ma indicate cooling through Permian and EarlyTriassic times.

North-central Chile (Norte Chico)

The basement rocks of Norte Chico comprise the PampaGneisses (PG), the Trnsito Metamorphic Complex (TMC)and the coastal El Teniente (ETMC) and Choapa (ChMC)metamorphic complexes. The Pampa Gneisses have beenplaced within the Chilenia Terrane (Ramos et al. 1986), whichwould have docked to South America in the Devonian, and therest lie west of it. In addition, Mpodozis & Kay (1990, 1992)have suggested the existence of an Equis Terrane (not exposed),lying west of Chilenia, in order to explain a westerly jumpin subduction-related magmatic foci from Permo-Triassic toJurassic times (Fig. 2.1).

Pampa Gneiss

Ribba (1985) first described the presence of bandedorthogneisses in a small outcrop in the upper reaches of RioTrnsito (Fig. 2.1). They are intruded by Late Palaeozoic

Permian plutons, and thrust to the west over the TMC toproduce a shear zone referred to as the El Portillo mylonites.

The metamorphic grade of the Pampa Gneiss reachesmigmatitic conditions, and the mineral paragenesis suggestsa high Tlow P metamorphic environment. Ribba (1985) andRibba et al. (1988) presented a RbSr whole-rock isochron of415P14 Ma on the PG, as well as a RbSr mineral isochronof 246P18 Ma. These ages were interpreted as indicative ofa Silurian metamorphic event and a Permo-Triassic resettingby a thermal event, further supported by 236P6 Ma (bt) and239P10 Ma (ms) KAr mineral ages. The Portillo myloniteswere dated at 250P26 Ma (RbSr, wr), a further indication ofthe Permo-Triassic tectonothermal event.

The Trnsito Metamorphic ComplexAlong El Trnsito Valley, east of Vallenar, large exposures ofquartz micaschists, metabasites, quartzites and marbles, whichcrop out below or are in tectonic contact with the Mesozoiccover, constitute the Trnsito Metamorphic Complex (TMC;Ribba 1985; Ribba et al. 1988). It is intruded by Late Carbonif-erous tonalites and is unconformably covered by sedimentaryand volcanic rocks of Middle Triassic age (Reutter 1974).

The metamorphic facies of the TMC is transitional betweengreenschist and amphibolite facies. Herv (1982) used amphib-ole mineral compositions to suggest that the TMC had beenmetamorphosed under an intermediate PT regime, with ametamorphic peak at about 5 kbar and 500P50C (Fig. 2.3).Whole-rock RbSr errorchrons of 303P40, 303P35 and335P20 Ma suggest a Late Carboniferous metamorphic event,with this isotope system having been perturbated by later


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thermal episodes related to plutonic intrusions into the TMC.KAr mineral ages of 238P10, 229P6 and 231P6 Ma onmuscovite testify to Triassic isotopic resetting or cooling.

El Teniente Metamorphic Complex

Discontinuous outcrops of metamorphic rocks occur in theCoastal Cordillera between Huasco (27S) and Los Vilos(32S). At around 31S, Irwin et al. (1988) described the pres-ence of a complex composed of ultrabasic and basalticmetabasites, metacherts, metasandstones and metaconglo-

merates which bear witness to a prolonged history of deforma-tion and metamorphism in Late Palaeozoic through EarlyMesozoic times. As used here, the El Teniente MetamorphicComplex (ETMC) encompasses the Cerro Negro and LaTotora complexes of Thiele and Herv (1984).

On the basis of the presence of staurolite, garnet,clinopyroxene, calcic plagioclase and albite, the F1 episodeof metamorphism is assigned to the upper greenschistamphibolite facies transition. It is possible that an earlier highergrade metamorphism was partially overprinted by this event.Herv (1982) used amphibole compositions to suggest that themetamorphic pressure was lower than in the ETMC and did notreach 5 kbar. Calculation of metamorphic temperature fromplagioclasehornblende pairs from Irwin et al. (1988) suggestit evolved from 700800C in the amphibole cores (pre-F1?) to500550C in the amphibole rims (F1) (Fig. 2.3).

The metabasites have yielded a 311P89 Ma RbSr error-chron (Irwin et al. 1988) interpreted as representing the time ofextrusion or an early metamorphic episode. The metabasitesand metasediments were assembled before the 220200 Mametamorphic episode F1, as suggested by a 201P61 Ma RbSrwhole-rock isochron in the metaconglomerates and radiometricages of the plutons emplaced into the metamorphic complex.RbSr whole-rock isochrons of 220P20 Ma (gabbro) and200P10 Ma (monzogranite) are comparable to six KAr ages(Hb) from the metabasites which range from 220P20 Ma to188P17 Ma. Further deformational episodes F

2(160150 Ma)

and F3 (140121 Ma) are recorded by the ETMC and in theassociated intrusive bodies.

The Choapa Metamorphic Complex

This complex comprises intensely deformed quartz micaschists,phyllites and amphibolitic schists (Rebolledo & Charrier 1994)affected by a polyphase deformation in which up to sevendeformational events have been identified. They have beeninterpreted by Rebolledo & Charrier (1994) as the metamor-phosed equivalent of the Late DevonianEarly CarboniferousPuerto Manso Formation.

The metamorphic conditions of the Choapa MetamorphicComplex (ChMC) were considered by Rebolledo & Charrier(1994) as belonging to the greenschist facies in a low to mediumPT environment, although peak conditions of metamorphism

Fig. 2.3. A compilation of PTt trajectories for the different units of the metamorphic basement complexes of the Norte Chico area. Sources ofdata: Trnsito Metamorphic Complex (TMC), Ribba et al(1988); El Teniente Metamorphic Complex (ETMC), Irwin et al(1988); ChoapaMetamorphic Complex (ChMC), Godoy & Charrier (1991).


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obtained from zoned actinolitic hornblende (Godoy & Charrier1991) suggest a pressure not lower than 5 kbar in thegreenschistamphibolite facies transition (Fig. 2.3). Except fora KAr age of 359P36 Ma (Am), the KAr ages obtained in thecomplex yield Jurassic ages and were probably reset by nearbyJurassic plutons.

Batholithic intrusions

The Norte Chico region is characterized both by the presence

of the huge composite ElquiLimari Batholith (ELB), whichextends from 2830p to 31Sp in the High Andes, and by aCoastal Belt of intrusions. Following Mpodozis & Kay (1990,1992) the ELB is viewed as comprising the subduction-relatedLate CarboniferousEarly Permian Elqui Superunit (ES) andthe Permo-Triassic Ingaguas Superunit (IS), which resultedfrom crustal anatexis in a thickened continental crust generatedduring the San Rafael orogenic phase (Llambas & Sato 1990).Pankhurst et al. (1996) presented a zircon UPb age of285.7P1.5 Ma and a 256P10 Ma RbSr wr isochron for two ofthe main plutons of the ES and a map showing all the previousage determinations by different authors. Radiometric ages inthe IS are generally in the range 200230 Ma (Brooket al. 1986;Rex 1987; Parada et al. 1988) although some older KAr ageshave been obtained as well. This allowed Parada et al. (1991) to

define a TriassicJurassic plutonic event, the products of whichare distributed in the High Andes Belt (HAB), the IngaguasSuperunit of the Elqui Limari Batholith, and in the CoastalBelt (CB). The Triassic to Lower Jurassic magmas wouldhave formed on an extensional setting, which was replacedby subduction-related magmatism from the Middle Jurassiconwards.

Late Palaeozoic accretionary complexes of the Coastal

Cordillera of central Chile

Between latitudes 32S and 42S, continuous exposures of thebasement occur in the Coastal Cordillera of central Chile(Fig. 2.1). This basement comprises a metamorphic complexwhich is exposed continuously south of 34S, flanked to the eastby the Coastal Batholith, which reaches the coast between 32Sand 34S, and turns east south of 38S to reappear in the MainAndean Range at 40S and then into Argentina (Fig. 2.1).

Gonzlez-Bonorino (1970, 1971) and Gonzlez-Bonorino &Aguirre (1970) distinguished three metamorphic belts or series

the Curepto, Nirivilo and Pichilemu series which differ inmetamorphic grade and direction of increasing metamorphism.Godoy (1970) and Aguirre et al. (1972) modified this subdivi-sion into a Western and an Eastern series which is still widelyused. The two series have been interpreted as representing apaired metamorphic belt, in the sense of Miyashiro (1961),with the Western Series being the higher PT unit. Herv (1977)suggested that the Western Series included accreted oceaniclithologies, and the whole was interpreted as a subduction

complex by Herv et al. (1976b, 1981a) and Forsythe (1982),and an accretionary wedge dominated by basal accretion byGlodny et al. (2005) and Willner et al. (2005). Herv (1988) pre-sented a synthesis of the knowledge of this subduction complex,which has increased greatly in recent years, particularly withregard to geochronology and PT regimes of metamorphism.

The Western Series

The Western Series (WS) comprises highly deformedmetagreywackes with intercalations of metabasite (sometimeswith relict pillow structures), meta-exhalites (spessartinequartzite, stilpnomelane quartzite, massive sulphide, tour-malinite) and serpentinites; that is, it represents a mixture ofcontinent-derived siliciclastics and slices of dismembered upperoceanic crust. A flat-lying east-dipping transposition foliationpredominates. The WS shows a transitional contact with the

Eastern Series at 3530pS, and is in fault contact in other locali-ties. These faults are associated with later destruction ofthe subduction complex. The fault contact at Pichilemu (35S)was interpreted by Ernst (1975) as a coastal suture zone,but newly interpreted by Willner et al. (2005) to represent aCretaceous brittle fault.

The occasional occurrence of glaucophanic amphibole alongthe belt and of lawsonite in Chiloe (Saliot 1968) suggested toAguirre et al. (1972) that the Western Series constituted thehigh Plow T belt of the paired Late Palaeozoic metamorphicbelts in central Chile, an interpretation confirmed by laterstudies. Munizaga et al. (1973) roughly limited the metamor-phism of the basement of central Chile to the interval between273 and 342 Ma (266334 Ma with new decay constants) on thebasis of RbSr whole-rock systematics over a wide area. Morespecific data for exposures at different latitudes follow.

3436SWillner (2005) determined the metamorphic peak at 350P50Cand 711 kbar, followed by static recrystallization during apressure release of 34 kbar and slight cooling. Herv et al.(1974) presented a 329P22 Ma KAr age on glaucophane fromPichilemu, thus establishing a Late Carboniferous age for thehigh PT event in this area. Willner et al. (2005) dated the meta-morphic peak (ArAr plateau ages, phengite) in the interval

319P1 to 292P1 Ma, whereas Ar/Ar laser ablation ages inphengite in the range 322P2 to 257P3 Ma indicate mineralgrowth during retrograde pressure release. The further cooling/exhumation of the WS was monitored through fission trackzircon (206P11 to 232P14 Ma) and apatite (80P4 to 113P8Ma) ages. The end of accretion is marked by a late intrusioninto the WS at Constitucin at 224P1 Ma (UPb, zircon).Average exhumation velocities were c. 0.20.6 mm/a indicatingthat erosion most probably was the prime exhumation factor.

3843SIn this section of the WS, which has also been called the BahaMansa Metamorphic Complex, Massonne et al. (1998b)recorded the second known occurrence worldwide of the highpressure mineral zussmanite in an outcrop of the WS at Ninhue.

The sulphide compositions in rare massive sulphide lenses indi-cate metamorphic pressures of 57 kbar (Collao et al. 1986).Willner et al. (2001) used multivariant equilibria calculationsto establish a metamorphic peak at 270370C and 68 kbar atBaha Mansa. Glodny et al. (2005) determined 420C, 89 kbarin the Valdivia area. Exotic tectonic blocks within the WSat Los Pabilos (Willner et al. 2005) exhibit a counterclockwisePTt path with a metamorphic culmination at 1116.5 kbarand 600760C, overprinted by an epidoteblueschist re-equilibration event at 350400C, 1014 kbar, not recordedin the country rocks, and a further common re-equilibrationfor the blocks and the country rocks at c. 300C and 5 kbar(Fig. 2.4). These rocks are considered to represent the earliestaccreted rocks beneath a still-hot mantle. Maximum PT condi-tions of 710 kbar, 350450C were derived from metapelitic

rocks of the Western Series in Chiloe (Massonne et al. 1999;Hufmann 2002). This range matches those conditions abun-dantly derived from phengites in greenschists and metapelites incentral Chile (Massonne et al. 1998a).

Herve et al. (1990) deduced the possible presence of agesin the range 300330 Ma, based on RbSr wr dating on rocksfrom five localities which bear massive sulphide mineralization.Sllner et al. (2000a) presented a 293P23 Ma UPb zircon ageon a meta-ignimbrite at Caleta Parga (4130pS), and Duhartet al. (2001) a 396P1 Ma age on a trachyte body emplacedin mafic schists, a similar age range to the previously reportedUPb conventional ages obtained on detrital zircon (between388 and 278 Ma) in different metasedimentary rocks. Thesedata suggest that the protoliths of the WS accumulatedfrom Early Devonian to Early Permian times and are in partcontemporaneous with the cooling of other parts of thecomplex.


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Phengites of the oldest accreted rocks at Los Pabilos weredated by Kato & Godoy (1995: 304P9 Ma) and Kato et al.(1997: 323P2 Ma), whereas Willner et al. (2004b) producedRbSr mineral isochrons of 305P3 and 297P5 Ma intendingto date a second retrograde stage of re-equilibration. Duhart

et al. (2001) produced Ar/Ar ages of white mica in the entirearea in the range 260220 Ma believed to be cooling ages,and Glodny et al. (2002) obtained a RbSr mineral isochronof 285 Ma at Quidico. Younger RbSr mineral isochronages (c. 250245 Ma) have been presented by Glodny et al.(2005) for the Valdivia area that are interpreted as dating thedeformation associated with basal accretion. RbSr mineralisochrons dating younger deformations during exhumation atc. 235 Ma and c. 210 Ma limit the mean exhumation rate to0.6+/0.2 mm/a. Zircon fission track ages are in the range176P49 to 212P46 in the Valdivia area (Glodny et al. 2005),whereas apatite fission track ages are between 53P7 Ma and65P8 Ma.

In summary, these data allow us to deduce that basal accre-tion in the WS of central Chile was active for c. 100 Ma fromLate Carboniferous to Early Triassic times.

The Eastern Series

The Eastern Series (ES) is mainly composed of metagreywackesof turbiditic origin accompanied by minor but ubiquitouscalcsilicate pods and lenses. They show stratigraphic continuity

at outcrop scale as well as a non-transposed first folding ofbedding planes, and represent a weakly deformed retro-wedgearea (Herv et al. 1988; Willner et al. 2001). The predominantdeformation style with upright tight chevron folds folding thebedding planes points to frontal accretion of sediments of aformer stable continental margin as inferred by Glodny et al.(2005). In central Chile the ES is mostly overprinted by apost-kinematic high T low P metamorphic event. Mappednorthsouth trending metamorphic zones display an increasingmetamorphic grade towards the batholith of the LatePalaeozoic arc which intrudes the Eastern Series in its easternpart (Gonzlez Bonorino 1971; Herv 1977). The ES has beenconsidered the low P high T component of the pairedmetamorphic belts of central Chile, and its metamorphic gradelocally attains the amphibolitegranulite facies transition. PeakPT conditions of this high T metamorphism range from

Fig. 2.4. A compilation of PTt trajectories for the different units of the metamorphic basement complexes of central Chile. Sources of data:Western Series 1 (WS1=exotic blocks), Willner et al. (2004b); Western Series 2 (WS2=greenschist) and WS3 (blueschist), Willner et al. (2005);Eastern Series 1 (ES1), Willner et al. (2005); Eastern Series 2 (ES2), Herv (1977).


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c. 400C to 720C at 3P0.5 kbar, indicating a regional meta-morphic event causing a thermal dome with the batholith in itscore (Willner 2005). Herv (1977) suggested a PT culminationat 45 kbar and 600C for the metamorphic series in theNahuelbuta Mountains.

Herv et al. (1984) presented RbSr whole-rock isochrons of347P32 Ma (andalusite zone) and of 368P42 Ma (sillimanitezone) with corresponding KAr ages of 299P10 Ma and278P7 Ma. Muscovite Ar/Ar plateau ages between 301P1 Ma

and 296P2 Ma were obtained by Willner et al. (2005) on abroader area in the region, indicating a short-term thermaloverprint during the emplacement of the 305 Ma old coastalbatholith (Fig. 2.4). The cooling/exhumation of the ES wasmonitored through zircon fission track ages of 221P12 Ma and215P14 Ma and apatite fission track ages of 98P6 Ma to113P8 Ma.

The Coastal Batholith

The Coastal Batholith (CB) of central Chile (Fig. 2.1) is mainlycomposed of calcalkaline granitoids of Late Carboniferous toPermian ages. The granitoids were intruded into the EasternSeries of the metamorphic basement, which was subject tocontemporaneous deformation and metamorphism. Triassic

high level plutons of limited areal extent occur as post-tectonicbodies in the high PT Western Series north of 38S, and similarCretaceous plutons occur around 40S. Jurassic plutons, inmany places difficult to distinguish from the Palaeozoic ones,increase in volume towards the northern limit of the area andbeyond it, and were emplaced when the tectonic and metamor-phic activity had ceased in the accretionary complex to the west.Later Mesozoic and Cenozoic plutonism migrated eastward tothe Main Range cross-cutting the NWSE trend of the southernend of the Late Palaeozoic plutonic belt at 40S.

Stratigraphic evidence for the Late Palaeozoic age of the CBis scarce, although Carnian to Rhaetian sedimentary succes-sions lie unconformably over the CB and the ES (but not theWS) in several localities in the Coast Ranges between 34and 37S, and also in the Main Range exposures at 40S. This

stratigraphical relationship limits the age of the unroofing ofthe batholith.Near the northern end of the Coastal Batholith, Gana &

Tosdal (1996) determined a UPb zircon age of 299P10 Mafor the Mirasol unit, and 214P1 Ma for a gneissic tonalite atCartagena. The late Carboniferous age is similar to previouslyreported RbSr, UPb and KAr ages in the same area, andthe Late Triassic pluton has equivalents north and south of it.Further dating of the CB in this area by Gana & Tosdal (1996)shows that Middle Jurassic plutons in the age range of156161 Ma are widespread in this northern portion of the CB.Finally, for the Nahuelbuta Central Pluton RbSr whole-rockisochrons were presented by Herv et al. (1988) (294P24 Ma)and Lucassen et al. (2004) (306P5 Ma), which are concordantwith the 305P1 Ma UPb age on zircon in the Pichilemu area

(Willner et al. 2005). Similar UPb zircon ages in the range300P2 Ma to 305P2 Ma were obtained by Martin et al. (1999)at 40S.

The metamorphic complexes of the Patagonian and

Fuegan Andes

In the Patagonian and Fuegan Andes, metamorphic rock unitscrop out quite extensively. They have been referred to asa metamorphic basement to the Mesozoic and Cenozoicsedimentary and volcanic units. In the latter, it is possible toinvestigate the age and geological evolution by means of classicstratigraphic and palaeontological methods. The metamorphicbasement, on the contrary, is for the most part composedof polydeformed rocks, where no stratigraphic controls can be

established, and which contain very little or no fossil evidencefor their depositional age.

The application of new methods, such as SHRIMP UPbdetermination of the detrital zircon age spectra, geochemicalprovenance analysis, and determination of metamorphic PTconditions, have allowed researchers over the past decadeto acquire new insights on the geological evolution of themetamorphic basement of the Patagonian Andes. As a conse-quence, units differing in depositional and metamorphic ages,

geodynamic setting and metamorphic characteristics have beenidentified. A description of their lithologies, metamorphiccharacteristics and geodynamic significance is given below.

The Patagonian Andes

The Patagonian Andes consist of a rather topographicallysubdued mountain belt that has had a prolonged evolutiongoing back to Late Palaeozoic times. The backbone of thesemountains is provided by the Mesozoic to CenozoicPatagonian Batholith, whose earliest (c. 150 Ma) componentsintrude low grade metamorphic complexes, which at presentcrop out both west and east of the continuous batholithic belt.These complexes were classically considered to be time equiva-lents, and are represented as such on the 1:1 000 000 Geological

Map of Chile (Escobar et al. 1980). Research work in thelast decade, however, has modified this view and allowed asubdivision of these units, the whereabouts and extent of whichare presented in Figures 2.1 and 2.5. A summary of PTttrajectories is given in Figure 2.6.

Eastern Andes Metamorphic ComplexThis unit consists mainly of polydeformed turbidite succes-sions, with minor bodies of limestones and metabasites. Itincludes the previously defined Cochrane and Lago GeneralCarrera units (Lagally 1975), Bahia de la Lancha and RioLacteo formations, as well as the Staines Complex (Allen 1982).The regional metamorphic grade is in the greenschist facies orlower, with higher grade rocks appearing only in the contactaureoles of Mesozoic to Cenozoic intrusions (Caldern 2000).Herv et al. (2003a) concluded that this unit has sedimentarycomponents deposited during Late DevonianEarly Carbonif-erous times, as well as younger deposits in their western outcropareas, ranging in age up into the Permian period. Herv et al.(1998), Fandez et al. (2002), Ramrez (2002), Augustsson &Bahlburg (2003) and Lacassie (2003) have suggested that theturbidites represent deposition in a passive continental marginenvironment. This interpretation was based mainly on pro-venance considerations from petrographic and geochemicaldata. The turbidites were derived from a cratonic source, whichpossibly had undergone a complex and extended sedimentaryrecycling history. A combination of UPb detrital zirconages and fission track age data on the same zircons allowedThomson & Herv (2002) to conclude that these sediments

were metamorphosed before Late Permian times under lowerPT metamorphic conditions than those that are typical ofaccretionary complexes (Ramirez 2002), as shown in Table 2.1.

Puerto Edn Igneous and Metamorphic ComplexThis complex consists of medium to high grade metamorphicrocks, migmatites and plutonic rocks, which crop out east ofthe South Patagonian batholith (49S). Geothermobarometricconstraints indicate a nearly isobaric high T low P meta-morphic and partial melting event superimposed on earliergreenschist-facies metamorphic rocks (Caldern 2000). Meta-morphic overgrowths on zircons in sillimanite paragneissesrecord a Late Jurassic (c. 150 Ma) age taken as evidence of localgneiss formation under in situ anatectic conditions during theemplacement of the Jurassic components of the batholith(Herv et al. 2003a).


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14 F. HERV ET AL.

Coastal accretionary complexesThese comprise, from north to south, the Chonos MetamorphicComplex (CMC), the Madre de Dios Accretionary Complex(MDAC) and the Diego de Almagro Metamorphic Complex(DAMC), all of which crop out west of the PatagonianBatholith (PB) (Fig. 2.5).

The CMC consists predominantly of metaturbidites(Pimpirev et al. 1999) with restricted occurrences of maficschists and metacherts, and broken formations are conspicu-ous. It has a Late Triassic depositional age, as indicated byfossil fauna (Fang et al. 1998) and by UPb age determinationson detrital zircon using SHRIMP (Herv & Fanning 2001). Thecomplex has been divided into two belts by Herv et al. (1981b)

similar to central Chile, with an Eastern Belt having wellpreserved primary sedimentary and volcanic structures thatbecome progressively obliterated when passing into the morepervasively deformed and recrystallized rocks of the WesternBelt. They were metamorphosed under high PT metamorphicconditions (Willner et al. 2002) as shown in Table 2.1, before orduring Early Jurassic times (Thomson & Herv 2002).

The MDAC is composed of three tectonically interleavedlithostratigraphic units: the Tarlton limestone, the Denaro(DC) and the Duque de York (DYC) complexes (Forsythe &Mpodozis 1979, 1983). The Tarlton limestone (TL), a massivepelagic limestone body, was deposited in an intra-oceaniccarbonate platform during Late CarboniferousEarly Permian

Fig. 2.5. Distribution of the metamorphic complexes in the Patagonian and Fuegan Andes (Herv et al, 2003a). The bold numbers indicate theyoungest detrital zircon UPb SHRIMP ages in metasedimentary rocks. The white italic numbers indicate UPb crystallization ages of igneous rockswhich were later involved in the metamorphism. The ages from the North Patagonian Massif and the Deseado Massif are from Pankhurst et al.(2003), at from Tierra del Fuego is from Sllner et al. (2000b). Key: darker grey=coastal accretionary complexes and Cordillera Darwinmetamorphic complex; intermediate grey=Eastern Andes Metamorphic Complex; lightest grey=ice and sea. Segmented line indicates the supposedeastern limit of the Diego de Almagro Complex drawn to include the Diego Ramrez Islands where Mesozoic blueschists were described by Wilson

et al. (1989).


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times (Douglass & Nestell 1976), over a penecontemporaneous(Ling et al. 1985) oceanic substrate (the DC) composed ofpillow basalts, metalliferous and radiolarian cherts, probably inan oceanic ridge environment away from the continental influ-ence of Gondwana. This exotic terrane was later accreted to thecontinental margin. Large upright chevron folds of beddingplanes combined with steep reverse thrusts resemble accretion-ary prisms formed by frontal accretion (Forsythe & Mpodozis1979, 1983). The DYC is a turbiditic succession which wasdeposited uncomformably over the TL and the DC, when theyreached the vicinity of the continental margin (Forsythe &Mpodozis 1979, 1983). The DYC has radiolarian cherts atDesolacin Island, which indicate an Early Permian age ofdeposition (Yoshiaki, written communication, 2002), anddetrital zircons of late Early Permian age in al l main outcrops ofthe unit. The Duque de York Complex, and probably theunderlying TL and DC, were metamorphosed before or within

the earliest Jurassic (Thomson & Herv 2002). The very lowgrade metamorphism (pumpellyiteactinolite facies) of thethree units of the MDAC has been sparsely studied: only themetamorphic characteristics of the Denaro Complex are shownin Table 2.1, and these suggest that the MDAC was metamor-phosed under a relatively high geotherm of 1520C/km, simi-lar to the Eastern Belt of the CMC. Combined petrographicand geochemical analyses (Lacassie 2003) indicate that thegreywackes of the DYC were derived from an intermediate(granodioritic) composition igneous source within a dissectedmagmatic arc tectonic setting, where erosion had enough timeto expose its plutonic roots. The DYC basin was probably adja-cent to the continental crust of Gondwana, in an active margintectonic setting (Fandez et al. 2002; Lacassie 2003).

Palaeo-magnetic data on the Tarlton limestone and theDenaro complex (Rapalini et al. 2001) indicate that these

Fig. 2.6. A compilation of PTt trajectories for the different units of the metamorphic basement complexes of the Patagonian and Fuegan Andes.Sources of data Chonos Metamorphic Complex (CMC), Willner et al(2001); Eastern Andes Metamorphic Complex (EAMC), Ramirez (2002);Puerto Edn Igneous and Metamorphic Complex (PEIMC), Calderon et al. 2007; Denaro Complex (DC), Seplveda (2004); Diego de AlmagroMetamorphic Complex (DAMC), Willner et al(2004b); Cordillera Darwin Metamorphic Complex (DMC), Kohn et al. (1995). The curves GA, BSand OG refer to different units within the Diego de Almagro Metamorphic Complex. Aluminium silicate invariant point, minimum melting ofgranite (MMG) and muscovite dehydration-melting reactions are taken from Spear et al(1999). The MMG is displaced to lower temperatures due toinvolvement boron released during the prograde breakdown of tourmaline.


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16 F. HERV ET AL.

units have undergone a very large counterclockwise rotation(117P29.9), with negligible palaeo-latitude anomaly, afterEarly Cretaceous remagnetization by the thermal influence ofthe South Patagonian Batholith. This evidence allowed theabove-cited authors to conclude that the rock units involvedhad been accreted to the Gondwana margin from the NWrather than from the SW as had been previously suggested byForsythe & Mpodozis (1979, 1983) on the basis of structuralstudies.

The DAMC is composed of two subunits of differingmetamorphic imprint, one composed of garnet amphibolitesand blueschists, the other of quartzmica schists and anorthogneiss. The contact between them has not been observedin the field. SHRIMP UPb ages in zircons in both theorthogneiss and a quartz-rich spessartine-bearing schist inter-leaved in the blueschists (Herve & Fanning 2003), have yieldedMiddle Jurassic ages interpreted as the age of crystallization oftheir igneous protoliths: a muscovitegarnet-bearing granite,

and a rhyolitic rock respectively contemporaneous with thegeneration of the silicic Large Igneous Province (LIP) on main-land Patagonia (Pankhurst & Rapela 1995). The subsequenthigh PT metamorphism occurred during Cretaceous times(Herv et al. 1999). This complex is in tectonic contact(Forsythe 1981, 1982) with the DYC along the mid-crustalsinistral strike-slip Seno Arcabuz shear zone (Olivares et al.2003).

In the Diego Ramirez islands, pillow basalts and meta-sediments form a crush mlange (Wilson et al. 1989), withglaucophane-bearing metamorphic assemblages. A MiddleJurassic RbSr whole-rock errorchron from the metasediments(Davidson et al. 1989) suggests that these rocks can becorrelated with those of the DAMC. Within this context it isrelevant to note that non-basement rocks, such as recently

studied metarhyolites of the Jurassic Tobfera Formation in theMagellanes fold and thrust belt (Herv et al. 2004), have alsobeen affected by contemporaneous metamorphic events alsocharacterized by high PT conditions.

Extra-Andean PatagoniaThe pre-Mesozoic metasedimentary and plutonic units ofextra-Andean Patagonia which crop out in the Deseadomassif, have been studied recently by Pankhurst et al. (2003)and Rapela et al. (2003a). The latter authors have datedmetasedimentary rocks of probable latest Neoproterozoicdepositional age, and plutonic bodies of Cambrian, Ordovicianand Late Silurian to Early Carboniferous age (Fig. 2.5). Sllneret al. (2000b) dated 530 Ma old orthogneisses recovered fromthe bottom of oil wells in northern Tierra de Fuego, suggestingthat early Palaeozoic rocks may extend over large tracts ofsouthern Patagonia under the MesozoicCenozoic cover.

The Fuegan Andes

The Darwin Cordillera Metamorphic Complex

The basement rocks of the Darwin Cordillera (DCMC) consistof metasedimentary and metavolcanic units, of supposedly latePalaeozoic to early Mesozoic age, which have a Mesozoic meta-morphic imprint peculiar to that area (Kohn et al. 1995).This metamorphism is characterized by the generation ofbiotite, staurolite, kyanite and sillimanite zones, which areunique among the metamorphic basement complexes of thePatagonian and Fuegan Andes. Several authors (Dalziel &Cortes 1972; Nelson et al. 1980; Dalziel 1981, 1986) have sug-gested that their protoliths formed as an accretionary wedgeon the pre-Middle Jurassic Pacific margin of South America.However, it is not known at present if these metamorphic rocksof the Darwin Cordillera were originally part of the EasternAndes Metamorphic Complex, which is probably not a LatePalaeozoic accretionary complex but served as a backstopduring the generation of the accretionary wedge (Augustsson &Bahlburg 2003), or if they were part of the coastal accretionarycomplexes of the Patagonian Andes. An orthogneiss within theDCMC showed a Middle Jurassic RbSr whole-rock isochron(157P7 Ma; Herv et al. 1979b, 1981c) and UPb zircon(164P1 Ma; Mukasa & Dalziel 1996) ages affected by the mainmetamorphic event. Nelson et al. (1980) have suggested thatpart of the protolith of the complex might be the Middle to LateJurassic Tobfera Formation silicic volcanic rocks.

Geodynamic considerations

The study of basement metamorphic rocks in Chile, and related

complexes in neighbouring Argentina, provides indicationsabout the changing tectonic environments that have existedin the area. It is clear that these metamorphic complexes havedifferent ages and that their metamorphic evolution, thoughpoorly known in some areas, varies widely in space and time.

Probably the best known and most readily interpreted eventrecorded by these rocks is the Late Palaeozoic development ofan accretionary prism over an east-dipping subduction zonebelow the southwestern Gondwana margin in central Chile.This process generated an elongated high Plow T metamor-phic belt which includes accreted oceanic lithologies and acontemporaneous parallel magmatic belt in the upper plate,both of which are well recorded and exposed.

The existence of tectonostratigraphic terranes has been iden-tified mainly by the presence of oceanic rocks flanked on bothsides by blocks of older continental crust. This interpretation is

Table 2.1A compilation of depositional ages, metamorphic ages and metamorphic peak PT conditions for the complexes of the Patagonian and FueganAndes

Metamorphic complex Maximum pressure (kbar) Maximum temperature (C) Ages of emplacement and Reference for peak PTmetamorphism (Ma) conditions

Chonos (EB) 4.55.5 250280 213198* Willner et al. 2002(WB) 8.010.0 380500

Denaro 4.15.5 260300 234195* Sepulveda (2004),Willner (unpubl. data)

Diego de Almagro (BS) 9.513.5 380450 157110 Willner et al. (2004a)(GA) 11.213.2 460565(GM) 4.96.5 580690

Eastern Andes 4.0P1.2 320380 364250* Ramrez (2002)Cordillera Darwin 6.57.5 575625 16486 Kohn et al. (1995)

For the Diego de Almagro Complex: BS, blueschists; GA, garnet amphibolites; GM, garnetmica schists. The age column indicates first the deposi-tional/emplacement ages and then the maximum metamorphic age as obtained through fission tracts zircon or Ar/Ar data. Ages marked with an asteriskare from Thomson & Herv (2002). The age indicated for the Denaro Complex is at obtained for the Duque de York Complex. The authors cited in thelast column have determined the peak PT conditions of metamorphism.


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less straightforward, as it depends heavily on the evaluation ofstructural, geochemical and geochronological data, which canbe imprecise and incomplete. Also, the sudden displacement ofmagmatic arcs in time has been considered to imply the dockingof terranes to the continental margin. In contrast, a paucityor absence of magmatic and tectonic activity, together withdeveloping sedimentary provenance analysis, has been used asan indicator of passive margin conditions. The subduction ofupstanding oceanic features such as ridges and plateaux can

bring subduction to a stop, or flatten the subducting slab to pre-vent magmatic activity near the trench. Finally, margin-parallelstrike-slip environments have also been considered in someinterpretations, and the process of tectonic erosion of the lead-ing edge of the overriding plate (Stern 1991b) may destroythe continuity, or even the entire evidence for the generation ofsubduction complexes in a particular place and time.

Northern Chile

This is the only portion of the Andean basement in Chile wheresignificant bodies of Early (pre-Devonian) Palaeozoic rocksexist, including latest Proterozoic (?)Early Cambrian igneousand metamorphic suites. Dalziel (1991) and Moores (1991)suggested that the Laurentian craton was located close toAntarctica and South America in Neoproterozoic and EarlyPalaeozoic times. The clockwise movement of Laurentiaaround South America would have resulted in repeated colli-sional tectonic interaction between the two continents, givingrise to the Famatinian orogen in southern South America andto the Taconic orogen in Laurentia (Dalla Salda et al. 1992).The collision included two major events, resulting in the EarlyCambrian Pampean Orogeny, and later the late OrdovicianFamatinian orogeny. A sliver of Laurentia detached to consti-tute the ArequipaAntofalla terrane, with the Beln and Chojasmetamorphic complexes near its eastern margin providinga record of the Pampean orogeny. The Ocloyic orogen couldhave been caused by the collision of this detached sliver the ArequipaAntofalla terrane and not of the whole ofLaurentia, if the Cordn de Lila Complex is interpreted as theproduct of a magmatic arc caused by eastward subductionof oceanic crust (Bahlburg & Herv 1997) from the west(Fig. 2.7a). Alternatively, Forsythe (1982) have suggested thatthe ArequipaAntofalla terrane was a sliver of the Gondwanacontinent, and rotated clockwise to generate the oceanic areain the Ordovician, and then counterclockwise to collide withits continent of origin, producing the Ocloyic orogen. Lucassenet al. (2000) questioned the entire terrane concept and show thatLower Palaeozoic metamorphic crystallization ages, peak PTdata (high temperatures, low to moderate pressures, Fig. 2.2)and whole-rock NdSm systematics rather favour a continuousmobile belt in northern Chile and NW Argentina, i.e. southof the Arequipa Massif at 18S and north of the ArgentinePrecordillera at 28S. This basement would have originated at aconvergent margin at mid-crustal levels under a very high geo-thermal gradient involving intensive magmatic underplatingover wide areas. In the northernmost exposures the Arequipacraton would have been partly reworked within the mobile belt.

After the SilurianDevonian lull in subduction activity, themargin of Gondwana resumed as an active margin facingand consuming an extensive oceanic plate. As a result of this,the metamorphism of the Chaaral Melange (Fig. 2.7b) tookplace during Late Carboniferous times within what Marioth &Bahlburg (2003) describe as a particular type of subductionzone in which the PT regime was not high. Around this time,the Limn Verde high pressure metamorphism occurred. Asthis unit crops out far from the present-day coastline and theChaaral melange, the interpretation of its tectonic settinghas been difficult. Herv et al. (1985) and Bahlburg & Herv(1997) favoured a subduction zone environment whereasLucassen et al. (1999) preferred a strike-slip environment for itsdevelopment.

The way in which the Limn Verde Metamorphic Complexwas exhumed remains questionable, but it was broadly contem-poraneous with the development of mylonitic rocks at Sierra deMoreno and in the Trnsito Metamorphic Complex, north andsouth respectively of Limn Verde, which exhumed the Belnand Sierra de Moreno complexes and the Pampa Gneiss, thenpart of the backstop of the accretionary prism. It is possible thatthis rapid exhumation might be related to tectonic erosion ofthe oceanward portions of the backstop, or by subduction of anoceanic ridge, favouring the exhumation process which broughtthese units to the surface in mid-Triassic times. In Figure 2.1,these units are seen aligned in what has been referred to as thePrecordillera upthrust belt, after Bahlburg & Breitkreuz (1991).

Norte Chico

As a whole this region is characterized by the presence of the(hardly exposed) Chilenia Terrane, which was amalgamatedto Gondwana in Devonian times (Ramos et al. 1986). As seenfurther north, the Late Carboniferous Trnsito MetamorphicComplex crops out in a much more easterly position than theChoapa and El Teniente complexes (Fig. 2.7c), which both havea lower metamorphic grade. The El Teniente rocks additionallyrecord a late Triassic metamorphic event, not recognizedin those of El Trnsito, which at that time had already been

exhumed and exposed at the surface. It is possible that thisexhumation was contemporaneous with that of the LimnVerde rocks mentioned above, thus representing a wide-scaleexhumation of the backstop and of the deeper parts of theaccretionary wedge which continued to develop further west.A westward jump of magmatic activity in this area fromCarboniferous to Triassic times, could be interpreted as causedby a roll-back of the subduction zone by the collision of a smallterrane (Fig. 2.7d) that could have contributed to the exhuma-tion of the early subducted rocks of the El Teniente Metamor-phic Complex (Terrane Equis; Mpodozis & Kay 1990, 1992).Finally, it is interesting to note the existence of the late Triassicmetamorphic event in the coastal El Teniente exposures,contemporaneous with the Chonos event in Patagonia, whichhas no counterpart in central Chile.

Central Chile

The geological development of central Chile in Late Palaeozoictimes can be best described in terms of processes occurringalong a continental margin. These processes can be related tosubduction of oceanic crust leading to an accretionary prismand a magmatic arc (Fig. 2.7e). The Western Series includeslithologies of oceanic parentage, mixed with detrital sedimentsof continental derivation, and exhibits the imprint of a high P low T metamorphic regime. The history of metamorphismand exhumation and the ductile deformation shown by the WScan best be explained if the outcropping rocks were initiallysubjected to basal accretion in the accretionary complex,followed by exhumation from 2540 km depth during ongoing

accretion.The Eastern Series, mainly composed of a metamorphosedturbidite succession, was probably deposited in a forearc settingover the continental shelf. The deposition may have taken placeduring passive margin conditions in Devonian and EarlyCarboniferous times. Thus, the rocks of the ES were probablypart of the uppermost accretionary wedge formed by frontalaccretion as well as part of the retrowedge. Deformation tookplace mainly under very low-grade conditions and intermediatepressure, but the ES was later metamorphosed under lowP high T conditions close to the site where the magmatic arcdeveloped.

Patagonian Andes

The protoliths to the eastern Andes Metamorphic Complexwere deposited in a passive margin environment from Early


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18 F. HERV ET AL.

Devonian to Early(?) Permian times. The detrital zircon agespectra on these rocks have Gondwanan affinities. The sourceareas could have been located in older rocks of the Atlanticmargin of Patagonia, such as the Deseado massif, or in SouthAfrica and Antarctica (Herv et al. 2003a). It is not known if itis in place with respect to the older continental blocks, or if ithas been displaced (c. Mpodozis, personal communication).

The western accretionary complexes, on the contrary, haveevolved in subduction zone environments, where accretion ofocean floor basaltic material is recorded. However, in contrastto previous assumptions, there are no indications of Late

Palaeozoic subduction in the Patagonian Andes. The ChonosMetamorphic Complex reveals a subduction event near theTriassicJurassic boundary (Fig. 2.7f), in which the corre-sponding arc might have been the largely coeval Sub-Cordilleran Batholith (Rapela et al. 2003b) in Argentina. TheMadre de Dios Accretionary Complex appears to involve acomposite exotic terrane, probably frontally accreted to theGondwana margin during the same Late TriassicEarlyJurassic event as the CMC. The provenance of the Duque deYork Complex, characterized by a major Early Permian zirconcomponent, is not easily attached to a contemporaneous

Fig. 2.7. Schematic cross-sections at different latitudes and time slots during the main subduction-related metamorphic and plutonic events asdescribed in the text. Scales are not uniform.


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magmatic arc in Patagonia, where Permian igneous rocks donot crop out extensively. Lacassie (2003) suggests a far-sitedorigin for the MDAC, which would have collided with theGondwana margin in the southeastern Pacific area (presentcoordinates) and then transported along the margin to itspresent position, following a model proposed by Cawood et al.(2002). Alternatively, as the accretion of the MDAC was fromthe NW, a possibility remains that the limestones originated inan oceanic environment at the latitude of northern Chile, wherethick limestone deposits of Early Permian age were depositedover continental crust, mainly in Bolivia (CopacabanaFormation) but also in Chile.

Herv & Fanning (2003) have suggested that the LateJurassicCretaceous evolution of the Diego de Almagro Meta-morphic Complex in a subduction zone along the westernmargin of Gondwana only occurred after the Antarctic Penin-sula, which could have been located outboard of the presentcontinental margin (Lawver et al. 1998), started to drift southallowing subduction to occur near the present-day continentalmargin.

The PT evolution of the metamorphic complexes (Figs 2.2,2.3, 2.4 & 2.6) varied widely in time and space. The differencesbetween the coastal accretionary complexes, which evolved inPT regimes characterized by geothermal gradients between10 and 20C/km, and the Puerto Edn and Cordillera Darwin

metamorphic complexes are evident. Only the former areconsidered to be typical of subduction zone environments dueto the derived metamorphic PT conditions. These conditionssuggest that the subduction was slow, or that the subductingoceanic lithosphere was rather young and, thus, relatively hot.

Concluding remarks

The metamorphic and plutonic basement complexes of Chilethus reveal the following history (sketched in Fig. 2.7) for a partof the southwestern Gondwana margin.

1. Early Palaeozoic metamorphic and plutonic eventsoccurred, which can be assigned to the Pampean andFamatinian orogenic phases. The products of these eventsseem to be restricted to northernmost and southernmostChile as there are no outcrops in between.

2. Conditions of a passive margin prevailed in the regionduring Silurian and Devonian times, when the collision ofChilenia is recorded in western Argentina.

3. A Late Carboniferous metamorphic and plutonic event,

related to subduction of the Palaeopacific or Panthalassianocean, is recorded in areas between latitudes 42S and23S. This event has been called the Toco Orogeny innorthern Chile. A chain of accretionary prisms is preservedwith ages decreasing from north to south

4. Late TriassicEarly Jurassic metamorphic and plutonicactivities are recorded in the Norte Chico (3033S) and inthe northern Patagonian archipelagos (4346S). Plutonicrocks of the same age were emplaced in the Western Series.It is not known if these two outcrop areas represent twoends of a previously continuous accretionary complexlocated west of the present-day continental margin thatwas subducted by tectonic erosion or displaced south alongthe Jurassic to Cretaceous left-lateral strike-slip faults.

5. Evidence for deep seated JurassicCretaceous tectono-metamorphism related to an Andean setting has beenfound only south of 48S. This includes the Diego deAlmagro (subduction) and the Darwin (metamorphic core)complexes.

Fondecyt grants, including 1010412 and the current 1050431, havesupported F.H. in the research of the metamorphic basement of Chile.Collaboration and discussion with most of the cited authors havegreatly contributed to the understanding of the subject. Also theGermanChilean BMBF-CONICYT cooperation projects CHL 01A6A High pressure metamorphic rocks in Chile and follow-ups aregratefully acknowledged.

3. Chapter 2_Metamorphic and Plutonic Basement Complexes

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