Mamil Choique Granitoids, southwestern North Patagonian Massif,Argentina: magmatism and metamorphism associated with a

polyphasic evolution

M.E. CERREDO and M.G. LOÂ PEZ DE LUCHI

CONICET-CIRGEO, Ramirez de Velasco 847, (1414) Buenos Aires, Argentina

AbstractÐ Petrological and structural features of the metamorphic Cushamen Formation (CF), igneous units Tunnel

Tonalites (TT) and Mamil Choique Granitoids (MCG) displaying polyphasic tectonic events in the RõÂ o Chico-Mamil

Choique area, southwestern corner of North Patagonian Massif (NPM), Argentina are studied. The metamorphic series

of the CF displays a medium-pressure regional metamorphism, which is overprinted by the contact metamorphism re-

lated to the emplacement of the igneous units. The TT-MCG are calc-alkalic epidote-bearing plutons, with a metalumi-

nous-peraluminous character. MCG include two magmatic series controlled by di�erences in oxygen fugacity, ¯uid

interaction and crystallization dynamics. Four deformational phases have been recognized in the area. Regional relation-

ships among metamorphism-deformation-magmatism agree with what is to be expected from a collisional setting, with

peak metamorphic conditions being synchronous with the second deformation, and with magmatism postdating regional

metamorphism. This would be consistent with the deep-seated magmatism interpreted from crystallization sequences in

the TT and MCG. The TT and MCG could be ascribed to the same tectonothermal event since their primary magmatic

foliations are overprinted by D3 subsolidus deformation. Rb-Sr isochrons for the MCG are dubious, indicating either

Ordovician/Silurian or Carboniferous ages. This datum would set a minimum age for D3, and could bracket a crustal

thickening event. # 1998 Elsevier Science Ltd. All rights reserved

ResumenÐEste estudio comprende la caracterizacio n petrolo gica y estructural de las rocas metamo r®cas de la Foma-

cio n Cushamen y de las rocas õ gneas de las unidades Tonalitas del Tu nel y Granitoides Mamil Choique (en el a rea Rõ o

Chico-Mamil Choique, en el sector SW del Macizo Nordpatago nico) que en conjunto presentan una evolucio n estruc-

tural polifa sica. La Formacio n Cushamen muestra un metamor®smo regional de presio n media que resulta luego afec-

tado por los efectos de contacto relacionados al emplazamiento de las dos unidades õÂ gneas de tipo calcoalcalino con una

tendencia metaluminosa-peraluminosa. Los Granitoides Mamil Choique incluyen dos series magma ticas controladas por

diferencias en la fugacidad de oxõ geno, interaccio n con ¯uidos y dina mica de cristalizacio n. Se han reconocido cuatro

fases deformativas. Las relaciones regionales entre metamor®smo-deformacio n-magmatismo son las esperables en un

a mbito colisional, donde el climax del metamor®smo regional se alcanza sincro nicamente o poco ma s tarde de la segunda

deformacio n, sucedido luego por el magmatismo. La hipo tesis colisional estarõ a apoyada tambie n por el cara cter pro-

fundo del magmatismo inferido a partir de las secuencias de cristalizacio n en las unidades õ gneas. Las Tonalitas de Tunel

y los Granitoides Mamil Choique podrõ an, de manera preliminar, adscribirse al mismo evento tectonomagma tico ya que

presentan las mismas relaciones magmatismo/metamor®smo y magmatismo/deformacio n, y ambas esta n afectadas por la

tercera deformacio n en condiciones subso lidas. La edad Rb/Sr de los Granitoides Mamil Choique, sea odovõ cico±silu rica

o carbonõ fera, re¯ejarõ a el tiempo de emplazamiento, establecerõ a un lõÂmite cronolo gico inferior para la tercera deforma-

cio n y, en consecuencia, estarõ a situando temporalmente el proceso de engrosamiento cortical. # 1998 Elsevier Science

Ltd. All rights reserved

INTRODUCTION

Any attempt to unravel the tectonic evolution of anorogenic belt must rely on a careful study of meta-morphic evolution, the relation to granitoid emplace-ment, together with the micro-, meso- and large-scalestructural features, supported by petrological, geo-chemical and radiometric studies.

This paper focuses on the study of meta-morphic and igneous units displaying polyphasictectonics in the RõÂ o Chico-Mamil Choique area,located between 41840' and 41855'S and 70800'and 70833'W (Fig. 1), in RõÂ o Negro Province,Argentina. This area is located in the southwes-tern corner of North Patagonian Massif (NPM)which has been regarded as the southern segmentof Famatinian Orogenic Belt (Dalla Salda et al.,

1992a,b). The aim of this paper is to present themain events of the metamorphic, magmatic anddeformational history recognized for the polyphasi-

cally-deformed basement of the area. This is com-posed of the Cushamen Metamorphites (Dalla

Salda, 1989) or Cushamen Formation (Ravazzoliand Sesana, 1977), a part of the Mamil Choique

Formation (Ravazzoli and Sesana, 1977) orMamil Choique Granitoids (Dalla Salda et al.,1994), and the Tunnel Tonalites (Lo pez de Luchi

and Cerredo, 1996; Cerredo and Lo pez de Luchi,1997). This basement is intruded by younger

undeformed Paleozoic granitic rocks (Viuda deGallo and La Pintada granites, Dalla Salda et

al., 1994). The three older units are extensivelycovered by Cenozoic volcanic and sedimentary

rocks and occur as isolated outcrops.

Journal of South American Earth Sciences, Vol. 11, No. 5, pp. 499±515, 1998# 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0895-9811/98 $ - see front matterPII: S0895-9811(98)00025-X

499

Previous studies concerning the relationshipsbetween the Cushamen Formation (CF) and theMamil Choique Granitoids (MCG) consider that theMCG intruded the CF and generated injection mig-matites (Ravazzoli and Sesana, 1977). In contrast,Dalla Salda et al., 1994 suggest the synchronismbetween peak metamorphic conditions in the CF andthe emplacement of MCG. A di�erent evolution wasrecently proposed. A medium-pressure regional meta-morphism was described for metapelites of the CF(Cerredo, 1997), which were later intruded by the twomagmatic units, comprising the areally restrictedTunnel Tonalites (TT) and the widespread MCG(Lo pez de Luchi and Cerredo, 1997a,b).

Even though the peak metamorphic conditions, plu-tonism and ages have been presented, there are manyuncertainties in the previous studies concerning thetiming of these events and the real meaning of the cal-culated ages. Published radiometric data yieldedOrdovician (Dalla Salda et al., 1994) andCarboniferous (Linares et al., 1997) ages for theMCG.

This study includes lithological and structuralsampling, meso- and microscopic analysis and somenew major element geochemistry. Previous workers inthe area (Dalla Salda et al., 1994) kindly provided uswith thin sections of chemically and isotopically ana-lyzed samples, which greatly enlarged our databaseand allowed a critical review and analysis of the stu-died units. These, coupled with previously publisheddata (Dalla Salda et al., 1994; Linares et al., 1997)form the background of this paper.

CUSHAMEN FORMATION

Cushamen Formation crops out principally alongtwo E±W ravines: La Angostura and ChacayHuarruca (Fig. 2). The metamorphic series is mainly

composed of metapelitic, metagreywackes and minorquartz-rich sandstones, with some thin acidic andbasic metavolcanic layers. The regional metamorph-ism ranges from low-greenschist facies to upper-amphibolite facies.

Two deformational phases (D1 and D2), representedby foliations S1 and S2 have been identi®ed. Thesephases are followed by two fold phases a�ecting theregional foliation (D3 and D4). The second defor-mation is characterized by the development of a mainfoliation (S2) in the area, produced during the climaxof the regional metamorphism. An earlier S1 foliationis recognized as folded inclusion trails in porphyro-blasts only at the microscopic scale. Folds associatedwith the D3 deformation (F3) are very tight and duc-tile and have a general N±S trending axial plane,somewhat modi®ed by later D4 deformation (Fig. 3a).

Fig. 1. Geological sketch of Paleozoic units in RõÂ o Chico-Mamil Choique area, North Patagonia, Argentina. Square indicatesthe area covered by Fig. 2.

Fig. 2. Geological sketch of Cushamen Formation in RõÂ oChico area.

M.E. CERREDO and M.G. LOÂ PEZ DE LUCHI500

Fig.3.(a)D

3/D

4fold

interference

®gure.Note

theductilestyle

ofF3folds(m

arked

withacontinuousline)

andtheopen,fragileF4(m

arked

withdotted

line);(b)fragileD

4fold

inmetapelites

from

biotite±garnet

zone;

(c)biotite±garnet

zoneschists

within

shearzone;

(d)tectonic

contact

betweenstromaticmigmatite

andmylonitized

biotite±garnet

zoneschists.

Twofoldingphasesare

recognized

inthemigmatite,theolder

(F3)beingparallel

toshearplane.

Mamil Choique Granitoids: Magmatism and Metamorphism Associated with a Polyphasic Evolution 501

In the eastern part of the studied area, shear zones

and mylonitic rocks are developed parallel to the F3

axial planes. The folds associated with the last defor-

mational phase (D4) have a NE trend, their axial

plane has a fairly constant direction throughout the

area, producing mesoscopic fragile folds (Fig. 3b) and

crenulation cleavage in some places. The D4 overprint

on D3 folds produces interference ®gures (Fig. 3a;

Cerredo and Lo pez de Luchi, 1995; Cerredo, 1997).

The evolution of mineral assemblages in the meta-

pelitic rocks has been reported elsewhere (Cerredo

and Lo pez de Luchi, 1995; Cerredo, 1997). It is linked

to an earlier regional metamorphism and a later

dynamic metamorphism restricted to D3 shear zones.

Five regional metamorphic zones have been described:

chlorite zone, chlorite±garnet zone, muscovite±garnet

zone, biotite±garnet zone and K-feldspar zone in mig-

matitic rocks. The prograde regional metamorphism

of the CF has been regarded as of medium pressure

type given the presence of garnet in low-grade rocks

and of sillimanite as the ®rst aluminum silicate phase

formed.

Three principal west dipping shear zones are

observed in the eastern half of La Angostura ravine

(Fig. 2) where they bring in tectonic contact rocks of

di�erent zones (Fig. 3d). These shear zones are geo-

metrically and genetically related to D3 deformation.

They represent the locus of both high shear strain

and dynamic metamorphism. Mylonitic rocks are

present in these shear zones (Fig. 3c). Two crystalliza-

tion stages are associated with this dynamic meta-

morphism. The earlier stage is characterized by clearly

retrograde mineral paragenesis: chlorite±muscovite

(sericite)±quartz±opaque minerals. The later crystalli-

zation stage is more restricted than the ®rst one, being

characterized by andalusite±plagioclase assemblage of

late mylonitic growth. This association results from a

localized dehydration reaction, restricted to very

narrow microscopic bands parallel to C planes. A gen-

eral clockwise PT path has been proposed for the

metamorphic evolution of CF (Cerredo, 1997).

Leucomonzogranitic sheets and veins occur within

the metamorphic series, being especially profuse in the

biotite±garnet zone (Fig. 2). They are peraluminous,

medium-K leucomonzogranites (Fig. 6) composed of

quartz, orthoclase and oligoclase as the main phases,

together with biotite, rare garnet and sillimanite, inter-

preted as metamorphic relicts. Mylonitic textures were

developed during the D3 deformation and the shear-

ing event was accompanied by the localized develop-

ment of retrograde assemblages: an early muscovite±

chlorite and a late andalusite±biotite±quartz assem-

blage along microshear zones. Leucomonzogranitic

rocks have been regarded as emplaced roughly syn-

chronous with the metamorphic climax and probably

resulted from the partial melting of CF below the pre-

sent exposure level (Cerredo, 1997).

TUNNEL TONALITES

The mesocratic tonalites that crop out to the north

of RõÂ o Chico (Fig. 1) have been called Tunnel

Tonalites (Lo pez de Luchi and Cerredo, 1996;

Cerredo and Lo pez de Luchi, 1997). Minor outcrops

are seen to the north covered by Cenozoic volcanic

rocks. Contacts between host rock and tonalites have

not been found except for some small satellite bodies

or apophysis concordantly intruded into the biotite±

garnet zone of CF in La Angostura area. At outcrop

and sample scales, mylonitic textures with unevenly

de®ned SC surfaces are common. A former magmatic

planar fabric characterizes the medium- to ®ne-

grained facies of the TT, which in turn is a�ected by

later shearing related to D3 as well as D4 regional de-

formations. Although an igneous origin could be

assigned to the tonalites, on the basis of their textures

and composition, their present features are mainly

controlled by both the mylonitic overprint and the D4

folding phase.

TT are coarse-grained (5 to 10-mm average size),

dark green rocks (M index spans from 18 to 55) with

some medium- to ®ne-grained facies. Modal compo-

sitions of these rocks indicate a predominance of

tonalites and subordinate quartz dioritic facies (Fig. 6).

Irregular shaped, very coarse grained, lighter plagio-

clase-rich patches are evenly distributed throughout

the pluton. These patches contain scattered amphibole

crystals, up to 3 cm long. The satellite bodies are ®ne-

grained greenish grey tonalites.

TT are hypidiomorphic and slightly porphyric with

subhedral labradorite±Ca±andesine, green hornblende,

2biotite and sphene crystals with anhedral and

mainly interstitial quartz and epidote, as magmatic

minerals and the association chlorite±prehnite2 tre-

molite±actinolite, opaque minerals and microgranular

sphene as the retrograde metamorphic phases.

Primary magmatic foliation is inferred from the paral-

lelism of clinoamphibole crystals in the ®ne-grained

types. There is textural evidence that magmatic

epidote precipitated after resorption of hornblende

and plagioclase. Rounded and mutually isolated,

hornblende and plagioclase are enclosed in epidote

(up to 3.5 mm) that occasionally shows allanitic

cores (Fig. 4a). Epidote is also present as an intersti-

tial phase among plagioclase. The crystallization

sequence, as interpreted from microscopic observation

is: amphibolec plagioclasec epidotec biotite. The

early crystallizing phases (clinoamphibole and plagio-

clase) constitute the textural framework of the rock,

in which later phases are located.

The textures described above as well as the inter-

preted magmatic crystallization sequence agree with

the proposed reaction in the CFMASHKNa multi-

system in tonalite melts with excess H2O (Schmidt

and Thompson, 1996): hornblende + (K-feldspar)L+

M.E. CERREDO and M.G. LOÂ PEZ DE LUCHI502

Fig.4.(a)Magmaticepidote

porphyroclast

inTT

bearingplagioclase

andamphibole

inclusions.

Plagioclase

andbiotite

are

inpressure

shadow

areas;

(b)rounded

magmaticepidote

porphyroclast

showingsomebendingin

cleavagetraces.Epidote

ispartiallyenveloped

bysynkinem

aticchlorite±prehniteassem

blageandrelict

biotite;(c)mylonitized

tonalite

ofTT

inshearzoneofLaAngostura

ravine.

Asigmoidalplagioclase

porphyroclast

isseen

surrounded

by®ne-grained

¯ow

bandcomposedofbiotite,amphibole,quartzandplagioclase;(d)

magmaticepidote

intonalite

ofMCG

displayingverywelldeveloped

euhedralallanitecoresbeingsurrounded

andpartiallyem

bayed

bybiotite

andplagioclase.Scale

baris0.5

mm.

Mamil Choique Granitoids: Magmatism and Metamorphism Associated with a Polyphasic Evolution 503

plagioclase + magnetite + Lc epidote + bio-

tite + (quartz)L+H2O.

The progress of this reaction is also supported by

modal compositions, which indicate a decrease in

amphibole and plagioclase contents as modal epidote

increases.

Even though chemical data are scarce as to allow a

detailed characterization (Table 1), available data in-

dicate that the tonalites (SiO2: 54±56%) are calc-alka-

lic, metaluminous with medium- to high-potassium

contents. They have low Mg] (42±46), and show FeO,

MgO, CaO, FeOt/MgO decreasing as SiO2 increases,

which suggest oxidizing conditions during melt evol-

ution. Al2O3 increasing with SiO2 could be related to

amphibole fractionation. Rocks plot in the pre-plate

collisional ®eld and are orogenic in the MgO±FeOt±

Al2O3 diagram (Lo pez de Luchi and Cerredo, 1997b).

The mylonitic D3 event partially retrogrades the

primary igneous association to greenschist facies.

Protomylonite microstructures are recognized with

plagioclase, clinoamphibole and epidote porphyro-

clasts enveloped by a matrix with chlorite±prehnite

discontinuous ¯ow bands. The interstitial magmatic

epidote displays patchy extinction, subgrains and

bending of cleavage traces. Some rounded epidote

porphyroclasts are seen (Fig. 4a and b).

The ®ne-grained matrix is composed of both relict

of igneous phases and syntectonic minerals developed

during shearing. Chlorite±prehnite (2sphene) associ-

ations enveloping porphyroclasts are seen as the pro-

duct of biotite breakdown, which in some cases

survives only as small brown rafts. Some amphibole±

epidote±chlorite equilibria are seen locally, where clin-

oamphibole displays a thin light-colored rim against

chlorite, which is regarded as the product of: horn-

blende2epidotec actinolite + chlorite.

The ®ne-grained TT apophysis interlayered in the

biotite±garnet zone schists present well developed

mylonitic foliation but no retrogression is associated

with the ¯ow bands (Fig. 4c).

Summarizing, a pre-D3 emplacement of the Tunnel

Tonalites is supported by the development of myloni-

tic textures. The primary magmatic foliation recog-

nized in ®ne-grained facies could be assigned to D2 or

be considered as being due to a primary magmatic

¯ow independent from some regional stress ®eld.

Therefore, the D2±D3 span roughly brackets the

emplacement timing.

Table 1. Geochemical analyses of TT and MCG

1ÐNew analyses performed in LAQUIGE (analytical methods: SiO2 gravimetry and paper chromatography; Al2O3 atomic absorption andpaper chromatography; CaO, MgO atomic absorption; Fe2O3, MnO, P2O5, chromatography; FeO, H2O + , H2O-gravimetry; Na2O, K2O ®rephotometry).2ÐData from Dalla Salda et al. (1994)

M.E. CERREDO and M.G. LOÂ PEZ DE LUCHI504

MAMIL CHOIQUE GRANITOIDS

The Sierra de Mamil Choique is mainly composedof granitoids, which were ®rst embodied within asingle unit named Mamil Choique Formation(Ravazzoli and Sesana, 1977) areally divided in a wes-tern granodiorite member and an eastern graniticmember. Recently the term Mamil ChoiqueGranitoids was employed in a more restricted sense,referring only to the granitoid rocks outcropping inthe western half of the Sierra, where granodiorites,monzogranites and some transitional migmatitic facieshave been described (Dalla Salda et al., 1994).

MCG are de®ned here as the granitoids a�ected byD3 that cover an area of about 230 km2 and make upthe most extensive outcrops of the Sierra (Fig. 1). Nocontacts have been recognized between MCG andboth CF and TT except for screens and small enclavesof CF, where overprinting of regional metamorphismby contact e�ects indicate that MCG emplacementpostdated peak metamorphic conditions (Lo pez deLuchi and Cerredo, 1997b).

MCG are comprised of medium to coarse-grainedgrey to greenish grey banded and partially foliatedtonalites (including minor quartz±diorite types) andgranodiorites and pinkish grey, partially foliated,slightly porphyric monzogranites.

The tonalite±granodiorite facies rocks crop outmainly in the northwestern half of the Sierra. Themost impressive outcrop feature is a banding de®nedby alternating mesocratic and leucocratic layers ran-ging from several cm to 2 m. This structure is a�ectedby two later folding phases. An earlier folding eventwith isoclinal and recumbent folds, is best recognizedwhen pegmatitic veins are present displaying some-times rootless intrafolial fold hooks (Fig. 5a). In themost strained domains S±C planes are recognized,with S parallel to banding and C parallel to axial foldplane. C planes are drawn by thin mica-rich folia,which generally lie at 30±508 of banding. This ductiledeformation has been regarded as resulting from D3.A younger folding, D4, is characterized by uprightfragile folds of fairly constant NE strike (Fig. 5b).Mesoscopic fold interference ®gures are recognizedemerging from D4 overprinting D3 (Fig. 5c).

The monzogranite facies constitutes extensive out-crops of typical rounded shapes being especially pro-fuse in the southeastern half of the Sierra.Monzogranites are intrusive in tonalite/granodioritefacies. Inter®ngering of the two facies is seen in somecontacts. Some tonalite enclaves have been found inmonzogranites. Two textural types of monzogranitesare identi®ed: an equigranular medium- to coarse-grained type, which in some localities is intruded by aporphyroid type with pink, sometimes oriented K-feldspar megacrysts (up to 4 cm). An uncommon par-allel alignment of microcline megacrysts could be con-sidered as a primary magmatic feature together with a

banding of alternating mica-rich and feldspar-richlayers up to 3 cm which is seen a�ected by D3 and D4

folding phases (Fig. 5d). S/C textures are recognizedin most strained rocks displaying the same geometricand genetic relationship described above for the tona-lite/granodiorite facies.

Petrographic compositional features in both faciesallow a distinction between two magmatic groups,which are also re¯ected in geochemical diagrams (seebelow). One series (Series 1 or Reduced Series) is rep-resented by biotite2epidote-bearing tonalite±grano-diorites and two mica monzogranites. The other(Series 2 or Oxidized Series) is composed of biotite2-epidote2clinoamphibole-bearing tonalite±granodior-ites and two mica monzogranites; the presence ofprimary iron-oxide phases characterizes this series.

Series 1

The tonalite±granodiorite facies of this series com-prises hypidiomorphic, equigranular, medium-grainedrocks, with an M index ranging from 5 to 25 (Fig. 6).Essential mineralogy comprises subhedral oligoclase±andesine (An22±30 in granodiorites and An35±45 intonalites) with poorly de®ned zoning, quartz, micro-cline, brown biotite and varying amounts of musco-vite, epidote, allanite, apatite and rare sphene.Magmatic fabric is de®ned by a parallel alignment ofsubhedral zoned plagioclase, sometimes combinedwith tiling. Microcline is anhedral to slightly subhe-dral, either as interstitial or in individual perthiticcrystals with plagioclase inclusions. Brown biotite isthe widespread ma®c phase. Magmatic epidoteappears as irregular masses (up to 2.5 mm) withzoned and euhedral allanite-rich cores (Fig. 4d and7a). Some rounded hornblendes are enclosed (Fig. 7b).Irregular outlines result from biotite and plagioclaseembayment and resorption, although some euhedralepidote boundaries are preserved against biotite(Fig. 7a and b). Relict blue-green hornblende is some-times present, always as tiny inclusions inside plagio-clase and epidote. Rarely an euhedral magmaticsphene is observed; apatite is often conspicuous asrobust prisms either associated with ma®c minerals oras inclusions; scarce zircons are present.

Rocks of monzogranite facies are hypidiomorphicwith M spanning from 2 to 10 (Fig. 6) and are com-posed of microcline, plagioclase (An13±25), quartz, red-dish-brown biotite, magmatic muscovite and apatiteand zircon as accessory phases. Microscopic featuresof the listed minerals are similar to the abovedescribed facies, except for the presence of anhedralK-feldspar megacrysts irregularly distributed in por-phyroid types. The most evolved types show a slightpredominance of plagioclase over K-feldspar, withsome albite-rich facies. Some monzogranites containsmall relict magmatic epidote with allanite cores andlarge skeletal epidote inclusions within plagioclasethat are still in optical continuity even though they

Mamil Choique Granitoids: Magmatism and Metamorphism Associated with a Polyphasic Evolution 505

Fig.5.(a)TightrecumbentD

3foldsdisplayingrootlessintrafolialfold

hooksin

granodiorite

ofMCG;(b)brittle

uprightD

4fold

trendingN

608E

ingranodiorite;(c)interference

emergingfrom

D3/D

4overprintingin

granodiorite;(d)veryductileD

3fold

(clearlydrawnbypegmatiticvein)refolded

byD

4.Monzogranitefacies.

M.E. CERREDO and M.G. LOÂ PEZ DE LUCHI506

extend through adjacent synneusis-joined plagioclasecrystals.

Series 2

The tonalites/granodiorites share the same primarymagmatic features described for the same facies inSeries 1. Mineralogical composition is similar but pla-gioclase is sodic andesine (An30±37), biotite is greenand microcline is, on average, more abundant even intonalitic terms. Mean values of modal quartz arelower (Fig. 6). Amphibole appears as individual crys-tals with wormy and resorbed boundaries against pla-gioclase, epidote, biotite and later minerals. Magmaticsphene occurs as large euhedral crystals (up to3.5 mm) bearing some opaque mineral cores. It crys-tallized early, as it is found sometimes as inclusions inepidote. Opaque minerals, mainly magnetite, appearas subhedral crystals (from 0.4±1.4 mm) forming therock aggregate or as inclusions in later crystallizingphases. The presence of primary non-silicate iron andtitanium-bearing phases strongly in¯uences biotitecolor, which is green (or greenish-brown) in thisseries. Even in the same thin section, the presence ofbiotite of di�erent compositions may be inferred onthe basis of its color; biotite in equilibrium with epi-dote is more greenish than biotite surrounded by feld-spar and quartz. When in contact with opaqueminerals, biotite color goes from brownish to yellow-ish or nearly colorless, indicating compositions richerin Mg. Apatite and zircon are present in smallamounts.

The most evolved terms of monzogranite facies inseries 2 are more K-feldspar rich than those of series1 (Fig. 6) and bear primary opaque minerals.Plagioclase composition is relatively Ca-rich (An25±30);M index varies from 2 to 10. Whereas a reddish-brown biotite is observed in iron-oxide free monzo-granites, a brown biotite is present together with pri-mary iron oxides. Epidote is recognized in some

examples as resorbed skeletal crystals similar to thosedescribed for monzogranites of series 1. Apatite isalso a common accessory phase.

The crystallization sequence in tonalites and grano-diorites of both series, as interpreted from microscopicanalysis would be: amphibolec epidotec plagiocla-sec biotite.

Subsolidus imprint

A subsolidus history is common for both magmaticseries. Ductile deformation related to D3 is character-ized in the tonalite rocks by an unevenly distributedprotomylonitic texture with plagioclase porphyroclastspartially enveloped by biotite, muscovite, epidote,sphene and partially recrystallized and reorientedquartz, plagioclase and biotite which de®ne discon-tinuous ¯ow bands. Granodiorite rocks are alsodeformed with moderately rounded plagioclase por-phyroclasts partially enveloped by muscovite, epidote,reoriented biotite and quartz-feldspar ®ne-grainedaggregates. In granodiorites, as soon as microclineappears as a modal representative phase quartz-feld-spar ®ne-grained aggregates and myrmekite are con-spicuous. The former are generally located in the tailsof K-feldspar porphyroclasts and the latter growingon sides facing shearing.

In both tonalites and granodiorites, plagioclase por-phyroclasts (Fig. 7c) are moderately to highly strainedwith comb and curved twins, subgrains and internalmicrocracks welded by ®ne-grained recrystallizedaggregates. Microcline in the granodiorites showsundulate extinction. Quartz is variably strained fromBoÈ hm lamellae, subgrains of serrated boundaries torecrystallized new grains; locally quartz ribbons wrap-ping around plagioclase porphyroclasts are developed.Primary igneous muscovite, in the granodiorite facies,displays ®sh shapes partially surrounded by synkine-matic smaller crystals of muscovite and recrystallizedbiotite. Primary low aspect ratio biotite grains arebent and drawn out at their ends into the discontinu-ous foliation; locally some biotite plates have under-gone partial recrystallization along the foliation to anaggregate of new biotite + muscovite + sphene.Rarely larger epidote crystals behave as porphyro-clasts.

In monzogranites muscovite2recrystallized biotitemica folia wrap around plagioclase and microclineporphyroclasts; scarce muscovite ®shes are seen(Fig. 7d). Mylonitic bands are restricted to thin mica-rich layer (up to 1 cm), leucocratic layers are lessstrained since feldspar generally retains its magmaticshape. Perthitic microcline mantled by myrmekite ispartially enveloped by a feldspar±quartz aggregatebut is almost undeformed. Quartz appears both as oldsomewhat deformed grains in Q domains, whereas inthe highly strained mica folia subgrains, new recrystal-lized grains and quartz ribbons are common.

Fig. 6. QAP plot of leucomonzogranitic rocks emplaced inCF (solid triangles), Tunnel Tonalites (asterisks) and MCG

(open squares: Series 1 rocks; ®lled circles: Series 2 rocks).

Mamil Choique Granitoids: Magmatism and Metamorphism Associated with a Polyphasic Evolution 507

Fig.7.(a)Magmaticepidote

withallanitecore

displayingirregularboundaries

withplagioclase

andcoherentinterfacesagainst

biotite;granodiorite

from

MCG;(b)magmaticepidote

withallanitecore

enclosingamphibole

inclusionsandpartiallyresorbed

byplagioclase;(c)rounded

plagioclase

porphyroclast

enveloped

by®ne-grained

biotite,quartzandepidote.

Granodiorite

from

MCG;(d)muscovite®sh

withtailsof®ne-grained

quartzandfeldspar.Monzogranitefrom

MCG.Scale

baris0.1

mm

forA

andB,and0.5

mm

forcandd.

M.E. CERREDO and M.G. LOÂ PEZ DE LUCHI508

In summary, mesoscopic and microscopic features

developed during D3 event in the MCG are of ductile

type and indicative of overall subsolidus conditions

for this rock. The general lack of biotite retrogression

denotes a relatively high temperature for this subsoli-

dus imprint, which agrees with the localized and

somewhat restricted presence of plagioclase recrystal-

lized to a polygonal mosaic, indicative of steady-state

dislocation-creep processes which operate at tem-

peratures above 450±5008C (Hanner, 1982; Tullis,1983).

Enclaves in Mamil Choique Granitoids

Two main types of enclaves have been found in theMCG: xenoliths of metamorphic rocks and igneousmicrogranular enclaves.Xenoliths. The xenoliths are considered as remnants

of CF and are mainly located in the southwestern cor-ner of the hill where screens of schistose rocks (up to10 m) inter®ngering tonalite/granodiorite and monzo-granite facies of MCG (Fig. 1) are also found. Con-tacts between metamorphic and igneous lithologiesare sharp and well de®ned. An incipient contact aur-eole is suggested by the lack of the schistosity andrecrystallization, which partially obliterated S2 foli-ation in the metamorphic rocks.

Xenoliths are ®ne to medium-grained schistosepsammopelitic to pelitic types with minor acid meta-volcanic rocks. The pelitic lithologies are composed ofplagioclase±quartz±biotite2muscovite; a well-devel-oped foliation is de®ned by oriented biotite plateswith minor muscovite. Large muscovite porphyro-blasts randomly oriented in the foliation plane butoverprinting S2 biotites are regarded as contact re-lated, probably due to K-metasomatism. It di�ersfrom the prograde regional muscovite by its loweraspect ratio and larger size. Both regional and contactmetamorphic assemblages have undergone the D3

ductile deformation, as is evident from large plates ofcontact related muscovite displaying ®sh shapes.

CF occurring both as enclaves inside or as hostrocks of MCG are characterized by multivariantKFMASH assemblages, with biotite as the only AFMphase present. These associations do not provide pre-cise constraints about metamorphic conditions, whichmay be generally regarded as above the biotite iso-grade, which coupled with the absence of progradechlorite, would locate these rocks in the amphibolitefacies. This broad constraint combined with the ®neto medium-grained nature of metasedimentary rockscould allow a preliminary correlation with metapelitesof muscovite±garnet zone of west Chico River.

The hornfelsic textures overprinting S2 foliation inCF xenoliths and screens indicate that MCG emplace-ment occurred after the climax of regional meta-morphism; contact related muscovite porphyroblastsdisplaying lozenge shapes, in turn, suggest a pre-D3MCG intrusion.Igneous microgranular enclaves. Ma®c microgranular

enclaves are small (generally <0.2 m) rounded orelliptical hosted in granodiorite and porphyritic mon-zogranite. They exhibit no obvious zoning and nosharp internal boundaries. However some examples ofsubtabular enclaves with some di�use margins arealso found. These enclaves may be further separated

Fig. 8. (a) TAS diagram indicating the chemical classi®cationand nomenclature of MCG. (b) Q±P diagram showing the

trends depicted by the two MCG series. Note that Series 2display a trajectory more akin to that of the calc-alkalicseries. (c) K2O vs. silica diagram with the subdivisions for

the low-, medium- and high-K series. Open squares: Series 1;®lled circles: Series 2.

Mamil Choique Granitoids: Magmatism and Metamorphism Associated with a Polyphasic Evolution 509

in three types: melanocratic biotite-cumulates, dioriticand tonalitic enclaves.

The melanocratic biotitic enclaves (M>80) arecomposed of brown biotite (>70%) displaying euhe-dral plates up to 5 mm in length in an interlocked dis-position; interstitial spaces are occupied by minorplagioclase (Ca-oligoclase), euhedral subhexagonalopaque minerals (magnetite?), apatite (up to 4%).Minor white mica is present along with biotite.

The dioritic enclaves (M = 40), display granoblastictexture with polygonal zoned plagioclase (An35/40),brown biotite, epidote with euhedral allanite cores,relict bluish-green amphibole enclosed in plagioclaseand primary opaque minerals; muscovite is rare.Textural relationships suggest the same order of crys-tallization that has been described for the tonalite±granodiorite facies.

The tonalite enclaves are found as thin strings(<5 cm wide) or as small rounded (0.1±0.3 cm) pieces,hosted both in granodiorites and monzogranites. Theyare ®ne-grained, quartz-rich tonalites with green andbrown biotite and plagioclase. In some cases scarcepre D3 muscovite of low aspect ratio is found display-ing lozenge shapes. These igneous enclaves are inter-preted as fragments of early-crystallized melt thatwere disrupted and entrained by the fractionatingmagma.

Geochemical characterization

MCG de®ne oversaturated medium- to high-K calc-alkalic series (SiO2:63 to 75%), Fig. 8a,b and c). Boththe granodiorite-tonalites and the monzogranites aremainly mildly peraluminous to peraluminous (ASI:1±1.3) with a trend towards increasing peraluminosityfor the monzogranites of series 2 (Fig. 9a and b).

Series 1 rocks vary from medium-K for the tona-lites to high-K for the more acidic facies (Fig. 8c). Inthe tonalite±granodiorite facies there is a gradualincrease in total alkalis, with average contents slightlylower than in the series 2. CaO, MgO, Al2O3, Fe2O3t

and P2O5 are higher than in series 2 and show nega-tive slopes against SiO2. NaO2 shows no variationagainst SiO2. Increase in K2O appears in the SiO2-richsamples. TiO2 is lower than in series 2 and show anegative slope against SiO2 (Fig. 10). Whereas scatterin Rb could be assigned to ¯uid interaction, thesmooth negative slope for Sr could be assigned to pla-gioclase fractionation; scattering could imply plagio-clase cumulates. Zr and Y concentrations are higherthan in the series 2 and indicate a negative slopeagainst SiO2, that could be correlated with biotite, zir-con and apatite fractionation (Fig. 11). Rb/Sr versusSiO2 have a slightly positive slope, indicating ratherlow enrichment in Rb, with Rb/Sr values varyingfrom 0.2 to 0.5 except for an anomalous sample with0.94 (Fig. 11).

Series 2 comprises medium- to high-potassium gran-itoids with some metaluminous tonalites (Fig. 8a,b,cand 9a). ASI shows a positive slope against SiO2

(Fig. 9b), and negative slopes are observed for Al2O3,MgO, CaO, P2O5, Fe2O3t, TiO2. Na2O almost remainsconstant up to the highest silica content where itdecreases and a strongly scattered positive pattern isshown by K2O (Fig. 10). Zr and Y absolute valuesare lower than in series 1 and even though both seriesshow negative slopes, those from series 2 are less pro-nounced. Sr shows no correlation with silica and Rbincrease albeit certainly scattered. Rb/Sr vs. SiO2

shows a slightly positive slope with values varyingfrom 0.2 to 0.6 (Fig. 11).

The high Sr values and their lack of correlationwith silica could be related with Ca-rich plagioclase ofthe monzogranites of the series 2. Convection couldprevent previously crystallized plagioclase fromsettling out and then monzogranite facies wouldentrain ``tonalitic to granodioritic'' plagioclase.

Even though Fe2O3 has negative correlation withsilica in both series, series 1 shows a more scatteredpattern. Decreasing Fe2O3 in series 2 could beassigned to magnetite, epidote and biotite fraction-ation. FeO/MgO ratio has poor correlation with silicain the Series 1 and de®nes two trends in the Series 2;

Fig. 9. (a) Shand's index diagram showing peraluminousindex variation for the two series; (b) ASI (Alumina ShandIndex) vs. silica diagram. A slightly positive slope is de®nedby Series 2 samples. Symbols as in Fig. 8.

M.E. CERREDO and M.G. LOÂ PEZ DE LUCHI510

a negative trend in the 64±69% SiO2 range, and apositive one for higher SiO2 values. These trendscould be explained by fractionation of magnetite,implying increasing in the oxygen fugacity conditionsin the last stages of crystallization (Fig. 10). In thisconnection it is interesting to compare with REE datafrom Sierra del Medio (Rapela et al., 1992) as no pro-nounced Eu anomaly is recognized implying high oxy-gen fugacity.

Data in the R1±R2 diagram (Fig. 12a) plot in thepre-plate collision ®eld for both the tonalites andgranodiorites and in the syncollisional ®eld for themonzogranites and de®ne a trend of silica enrichmentwith a steeper negative slope for R2 parameter for theseries 1 and a gentle one for the series 2. In the Rb/(Y + Nb) plot data are mainly located in the VAG®eld close to the syncollisional ®eld; one sample fromSeries 2 is located inside the postcollisional granitoid®eld (Pearce 1996; Fig. 12b). In the Al2O3/Na2O + K2O and Na2O/K2O diagrams (Maniar andPiccoli, 1989; Fig. 12c and d), Series 1 monzogranitesplot in the continental collisional granites (CCG) ®eldand values for the Series 2 monzogranites plottowards the postorogenic granitoids (POG) ®eld(Fig. 12c). If Na2O/K2O ratio vs. silica is analyzed, allthe monzogranites plot in the CCG ®eld but thosefrom Series 1 concentrate in the POG ®eld (which inthis diagram is completely enclosed in the CCG ®eld).

DISCUSSION

Previous ideas concerning the evolution of the

NPM consider that CF and MCG constitute a unique

complex evolved during a prograde syncollisional

metamorphic event related to the Laurentia±

Gondwana collision in Ordovician times (Dalla Salda

et al., 1994).

The present study points to diachronic peak meta-

morphic conditions and magmatic events developed in

a continuously evolving dynamic setting. In the CF,

the regional metamorphic climax postdates a ®rst

deformational event (here represented by relict S1)

and develops in the late stages of the second most

penetrative foliation (S2), which is consistent with

models proposed for collisional settings (Thompson

and Ridley, 1987). Autochthonous or slightly paraau-

tochthonous melts may be represented to the east of

La Angostura pro®le by granodioritic leucosome of

the migmatites and by leucomonzogranitic veins and

sheets. Both TT and MCG display primary magmatic

structures, evidenced by relict plagioclase and amphi-

bole parallel orientation. A D2 synkinematic emplace-

ment is supported by the parallelism between S2foliation, contacts and primary magmatic banding. In

mid- to deep-crustal plutons such features often re¯ect

regional deformation rather than emplacement pro-

Fig. 10. Variation diagrams showing the fractionation trends for MCG symbols as in Fig. 8.

Mamil Choique Granitoids: Magmatism and Metamorphism Associated with a Polyphasic Evolution 511

cesses (Patterson et al., 1996). In screens and enclaves

of CF inside MCG, slight recrystallization blurred S2regional climax related foliation. Randomly oriented

muscovite porphyroblasts in S2 surfaces, considered tobe the result of contact related metasomatism, mayimply that MCG emplacement took place during thewaning stages of D2 deformation.

A pre-D3 TT and MCG emplacement is indicatedby overprinting of magmatic planar fabrics by subsoli-dus D3-deformation, although D3-related paragenesisare of lower-grade in TT than in MCG (suggestingdi�erent thermal conditions for both units by the timeof the D3 event). Although there exists a compo-sitional gap between the less evolved, almost inter-mediate facies of MCG and TT, a working hypothesiscould link TT to MCG as the more basic facies ofthis intrusive.

Recent experimental work carried out on epidotestability in water saturated tonalite composition(Schmidt and Thompson, 1996) reveals that the ®rstappearance of epidote during the crystallization his-tory of a cooling magma may provide a ®rst ordergeobarometer. For TT, the interpreted crystallizationsequence (amphibole±plagioclase±epidote±biotite) in-dicates that the main stages of magma crystallizationtook place at pressures around 8.5±9.5 kb. The inter-stitial location of magmatic epidote in TT suggeststhat it crystallized at the emplacement site, and so theinterpreted pressure would correspond to the intrusionlevel. For MCG tonalites and granodiorites of bothseries, the interpreted crystallization sequence (amphi-bole±epidote±plagioclase±biotite) indicates that the®rst stages of magma crystallization took place atpressures >9±10 kb. The absence of garnet providesan upper pressure constraint around 12 kb (Figs 13and 14).

It has been shown for both subsolidus and abovesolidus regions, that epidote stability is extendedabout 2 kbar toward lower pressures, as ¦O2 isincreased from NNO to HM bu�ers (Holdaway,1972; Liou, 1973; Schmidt and Thompson, 1996).Both in TT and MCG, water saturation could nothave been attained at the early stages of the magmaticcrystallization. Although no appropriate grids atwater activities <1 are available at present for thetonalite and granodiorite systems, progressive lower-ing of aH2O will displace the epidote-out equilibriumup in both P and T (Thompson, oral comm.). It fol-lows that the simultaneous e�ects of lowering wateractivity and increasing ¦O2 would produce oppositedisplacements of the epidote-out reaction and, there-fore the grids presented in the Figs 13 and 14 may beregarded as good approximations.

Di�erences in oxygen fugacity could be deducedfrom both petrographic and geochemical data. The 6±8 kbar range would bracket the emplacement pressurefor TT, whereas in MCG the two series may be pro-duced due to di�erent pressure conditions during epi-dote crystallization. Although precise geobarometricconstraints are needed in order to con®rm the empla-cement depth, the deduced pressure indicates that TT

Fig. 11. Trace element variation diagrams for selected

samples of MCG symbols as in Fig. 8.

M.E. CERREDO and M.G. LOÂ PEZ DE LUCHI512

could have resulted either from a deep-seated magma-tism in a normal crust or from middle levels of athickened crust. Pressure constraints of both epidote-bearing MCG and its host rocks are needed in orderto establish whether 7 to 10 kb re¯ects the sourcedepth or the emplacement level.

Two magmatic series have been separated in MCGon the basis of ¦O2. Series 1 is magnetite free withfractionation trends dominated by plagioclase and

biotite (2apatite and zircon) and series 2 bears pri-mary magnetite as an early crystallizing phase andamphibole and magnetite control the fractionationtrend. Crystal±liquid equilibrium, in series 1 would becontrolled by ¯uid interaction as shown by the scatterpattern in LIL elements whereas the trends depictedby series 2 would indicate convection that allowedentrainment of previously crystallized phases. LowerY content could rise from amphibole being an import-

Fig. 13. Pressure±temperature diagram for tonalite melting at

water-saturated conditions with ¦O2 bu�ered by NNO, fromSchmidt and Thompson (1996). Arrows indicate inferredcrystallization paths for TT and MCG.

Fig. 14. Pressure±temperature diagram for granodiorite melt-

ing at water-saturated conditions with ¦O2 bu�ered by NNO,from Schmidt Thompson (1996). Arrow indicates inferredcrystallization path for MCG.

Fig. 12. Major and trace-element tectonic diagrams. (a) R1±R2; (b) Rb vs (Y + Nb) with the postcollisional granitoids ®eld

as described in Pearce (1996); (c) and (d) tectonic diagrams for the monzogranite facies of MCG. Outlined ®elds are for conti-nental collisional granites (CCG) and postorogenic granites (POG) as de®ned by Maniar and Piccoli (1989).

Mamil Choique Granitoids: Magmatism and Metamorphism Associated with a Polyphasic Evolution 513

ant fractionation phase. Rb/Sr varies mostly between0.2±0.5, except for one sample in series 1. This inter-val corresponds to values assigned to the middlecrust. 87Sr/86Sri values determined for MCG are0.70555 (Dalla Salda et al., 1994) and 0.70644(Linares et al., 1997). Low 87Sr/86Sri could suggesteither lower crustal sources or a mixed mantle±crustsource. Trends in Rb vs. (Y + Nb) plot (Fig. 12b)display a path typical for fractional crystallization, inthe VAG and post-collisional ®elds, that could evolvefrom a melt produced by any type of mantle±crustalinteraction (Pearce, 1996). An alternative explanationfor the low initial ratio would be that source materialshad their 87Sr/86Sri homogenized, and lowered by in-teraction with aqueous ¯uids prior to melting(Wickham, 1990). Further studies on a regional scaleare needed in order to identify the extent and timingof the processes by which 87Sr/86Sri would be loweredduring metamorphic evolution of the source, which inturn, would control the geochemical evolution of thecrust. Advective heat transfer from the mantle (viamagmatic underplating or crustal delamination pro-cesses) could not be ruled out.

The ductile D3 event originated folding, recrystalli-zation, localized mylonitization and overprinted re-gional assemblages in CF and magmatic fabrics inMCG-TT. Major shear zones with mylonitic rockswest of Chico river are related to this deformationoccurring during a generalized uplift. Although themeaning of this deformational phase is not fullyunderstood a major displacement may have beeninvolved.

D1±D2±D3 evolution is considered to be a singleorogenic event. Therefore CF, TT and MCG re¯ectthe compressive tract of the evolution of an activemargin. D4 deformation is regarded here as the resultof upper crustal processes, related to a distinct geody-namic event that a�ects CF, MCG and TT as awhole, producing fragile upright folds.

The crystallization age of MCG would indicate atime marker for the waning stages of the orogenicevent. Published Rb±Sr isochrons for the MCG(Dalla Salda et al., 1994; Linares et al., 1997) haveyielded Ordovician (439210 Ma) and Carboniferous(313224 Ma) ages, respectively. The study of handsamples and thin sections of rocks included in theOrdovician isochron indicate that they belong toMCG, as far as strain related features and geochem-ical trends are concerned. However, some of thempresent anomalous Rb/Sr ratios, which could a�ect87Rb/86Sr and so the slope of the isochron and thecalculated age. In this connection, a preliminary mag-netic susceptibility survey revealed the existence ofboth ferromagnetic and paramagnetic rocks in MCG(Lo pez de Luchi et al., 1997) indicating the presenceof magma components with di�erent oxidation ratios.Mineralogical and geochemical composition allowedus to distinguish two magmatic series as has been pre-

viously described. Current disagreement between pub-lished ages may be due to the presence ofcomponents, which may not have achieved completeequilibration before emplacement.

In any case the MCG Rb±Sr age, eitherOrdovician/Silurian or Carboniferous, re¯ects anemplacement event and sets a lower chronologicalconstraint for D3. Such an age may bracket a crustalthickening event. The Lower Middle Jurassic age ofthe undeformed dyke swarm that intruded MCG(Lo pez de Luchi and Rapalini, 1997) may set a mini-mum age for D4 deformation.

CONCLUSIONS

The petrological and structural features describedlead to the following proposition for the evolution ofthe polyphasically deformed basement of RõÂ o Chico-Mamil Choique area:

ÐFirst evolutionary stages are related to medium-pressure regional metamorphism of the CF roughlysynchronous with the second deformation recognizedin the area. Metamorphism a�ected a siliciclastic pilewith some acid and basic volcanic thin interlayers.

ÐMCG were emplaced slightly postdating regionalmetamorphism. TT and MCG could be ascribed tothe same tectonothermal event, since their primarymagmatic foliations are overprinted by D3 subsolidusdeformation. The interpreted contemporaneousnessbetween TT and MCG constitutes primary evidenceto support a major magmatic event involving bothunits that requires further study.

ÐRegional relationships among metamorphism±de-formation±magmatism events agree with what is to beexpected from a collisional setting, with peak meta-morphic conditions being synchronous with or shortlyafter the second deformation while magmatism post-dated regional metamorphism. This would be consist-ent with the deep-seated magmatism interpreted fromcrystallization sequences in TT and MCG.

ÐThe magmatism comprising TT±MCG is calc-alkalic with a metaluminous±peraluminous trend.MCG include two magmatic series, which di�er intheir evolutionary trend. These are mainly controlledby di�erences in oxygen fugacity, ¯uid interaction andcrystallization dynamics.

ÐMajor element data for the monzogranites indi-cate that they are mainly collisional to postorogenicplutons whereas tonalites and granodiorites arelocated in the pre-plate collision ®eld. This wouldre¯ect the sources more than the tectonic setting.Trace element pattern locates the whole compositionalspan of the two MCG series in the VAG ®eld exceptfor one granodiorite of the Series 2.

ÐSource cannot be accurately modeled on thebasis of the available data. Either lower crustal

M.E. CERREDO and M.G. LOÂ PEZ DE LUCHI514

sources or a mixed mantle±crustal source mightexplain the low initial Sr ratio and the lack of Rbenrichment. An alternative approach, in accordancewith the synkinematic emplacement, would be to con-sider lowering of the 87Sr/86Sri of the metasedimentarypile by interaction with ¯uids, the circulation of whichwould be enhanced by the development of shearzones.

ÐPost-magmatic D3 ductile deformation overprintsregional assemblages in the CF and magmatic fabricsin MCG±TT. Major shear zones with mylonitic rockswest of Chico river were developed during this defor-mation.

ÐMeso- and microscopic D3-related myloniticstructure is proposed here as a diagnostic criteria todistinguish MCG from younger granitoids.

In summary, the evolution of the area is con®nedto deep- and middle-crustal processes related to a col-lisional setting. New isotopic data would allow toaccurately situate this compressive event in the con-text of the geodynamic evolution of the Patagoniaregion.

AcknowledgementsÐThe authors thank Dr A. Sial for having

invited them to publish this work. This work was partially sup-

ported by a Fundacio n Antorchas Project directed by Dr A.

Rapalini. We want to acknowledge Drs L. H. Dalla Salda, C.

Cingolani, R. Varela for having generously provided us samples and

analytical data and for the comments and discussions that have

resulted in this and other papers. We are grateful to the anonymous

reviewers whose critical suggestions improved the manuscript. A.

Gonza lez and G. Giordanengo helped us with the illustrations.

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