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181Revista de la Asociación Geológica Argentina 63 (2): 181 - 195 (2008)

MINERAL DEFORMATION MECHANISMS IN GRANULITEFACIES, SIERRA DE VALLE FÉRTIL, SAN JUAN PROVINCE:EVELOPMENT CONDITIONS CONSTRAINED BY THE P-TMETAMORPHIC PATH

Sergio DELPINO1, Ernesto BJERG1,2, Aberra MOGESSIE3, Isabella SCHNEIDER3, Florian GALLIEN3, Brígida CASTRO DEMACHUCA2,4, Lorena PREVILEY2, Estela MEISSL4, Sandra PONTORIERO4 y José KOSTADINOFF1,2

1 INGEOSUR, Departamento de Geología, Universidad Nacional del Sur, San Juan 670, B8000ICN Bahía Blanca, Argentina. E-mail:[email protected] CONICET3 Institute of Earth Sciences, Department of Mineralogy and Petrology, University of Graz, Austria.4 Instituto de Geología, FCEFN, Universidad Nacional de San Juan, Argentina.

ABSTRACTIn the Sierra de Valle Fértil, evidence of granulite facies metamorphism have been preserved either in the constitutive associationsas in deformation mechanisms in minerals from biotite-garnet and cordierite-sillimanite gneisses, cordierite and garnet-cordieritemigmatites, metagabbros, metatonalites-metadiorites and mafic dikes. The main recognized deformation mechanisms are: 1)quartz: a) dynamic recrystallisation of quartz-feldspar boundaries, b) combination of basal <a> and prism [c] slip; 2) K-feldspar:grain boundary migration recrystallisation; 3) plagioclase: combination of grain boundary migration recrystallisation and subgrainrotation recrystallisation; 4) cordierite: subgrain rotation recrystallisation; 5) hornblende: grain boundary migration recrystallisation.Preliminary geothermometry on gabbroic rocks and the construction of an appropriated petrogenetic grid, allow us to establishtemperatures in the range 800-850 C and pressures under 5 Kb for the metamorphic climax. Estimated metamorphic peak con-ditions, preliminary geothermobarometry on specific lithologic types and textural relationships, together indicate an counter-clock-wise P-T path for the metamorphic evolution of the rocks of the area. Ductile deformation of phases resulting from anatexis lin-ked to the metamorphic climax indicates that the higher-temperature ductile event recognized in the study area took place afterthe metamorphic peak. Evidence of ductile deformation of cordierite within its stability field and presence of chessboard extinc-tion in quartz (only possible above the Qtzα/Qtzß transformation curve), both indicate temperatures above 700 C consideringpressures greater than 5 Kb. Based on the established P-T trajectory and the characteristics described above, it can be concludedthat deformation mechanisms affecting the Sierra de Valle Fértil rocks were developed entirely within the granulite facies field.

Keywords: Sierra de Valle Fértil, Counter-clockwise P-T path, Deformation mechanisms, Granulite facies.

RESUMEN: Mecanismos de deformación en minerales en facies granulita, Sierra de Valle Fértil, provincia de San Juan: condiciones de desarro llo acotadas por la trayectoria P-T. En la sierra de Valle Fértil han quedado preservadas evidencias de metamorfismo en facies granuli-ta tanto en las asociaciones constitutivas, como en los mecanismos de deformación en minerales de gneises biotítico-granatífe-ros y cordierítico-sillimaníticos, migmatitas cordieríticas y granatífero-cordieríticas, metagabros, metatonalitas-metadiorites ydiques máficos. Los principales mecanismos de deformación observados son los siguientes: 1) cuarzo: a) recristalización dinámi-ca de los bordes de cuarzo-feldespato, b) combinación de deslizamiento basal y prismático; 2) feldespato potásico: recristaliza-ción por migración de borde de grano; 3) plagioclasa: recristalización por combinación de los mecanismos de migración de bordede grano y rotación de subgranos; 4) cordierita: recristalización por rotación de subgranos; 5) hornblenda: recristalización pormigración de borde de grano. La geotermometría preliminar sobre rocas gábricas y la construcción de una grilla petrogenéticaapropiada, sugieren temperaturas en el rango 800-850 C y presiones inferiores a los 5 Kb para el climax metamórfico. La estima-ción de las condiciones del pico metamórfico, la geotermobarometría preliminar ensayada sobre tipos litológicos específicos y lasrelaciones texturales, indican en conjunto una trayectoria P-T antihoraria para la evolución metamórfica de las rocas del sector.La deformación dúctil de las fases producto de la anatexis asociada al climax metamórfico, indica que el evento dúctil de mayortemperatura reconocido en el área tuvo lugar con posterioridad al pico metamórfico. Las evidencias de deformación dúctil de lacordierita dentro de su campo de estabilidad y la presencia de texturas tipo "tablero de ajedrez" en cuarzo (sólo posibles por enci-ma de la curva de transformación Qtzα/Qtzß), indican temperaturas superiores a los 700 C a presiones superiores a los 5 Kb.Considerando la trayectoria P-T obtenida y las características enumeradas previamente, puede establecerse que los mecanismosde deformación descriptos para las rocas de la Sierra de Valle Fértil se desarrollaron enteramente dentro del campo correspon-diente a la facies granulita.

Palabras clave: Fault rocks, Sierras de Buenos Aires, Cohesive breccias, Microbreccia.

S. DELPINO, E . BJERG, A . MOGESSIE , I . SCHNEIDER, F. GALLIEN, B. CASTRO DE MACHUCA,L . PREVILEY, E . MEISSL , S. PONTORIERO Y J. KOSTADINOFF

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INTRODUCCIÓN

Evidence of granulite facies metamor-phism are preserved in the constitutingmineral associations and deformationmechanisms recognized in biotite-garnetand cordierite-sillimanite gneisses, cor-dierite and garnet-cordierite migmatites,metatonalites-metadiorites, metagabbrosand mafic dikes, outcropping at the west-southwest of San Agustín del Valle Fértillocality (rectangle, Fig. 1a).Numerous previous contributions dea-ling with the petrology, geochemistry, geo-chronology and structural-tectonics ofthe Sierras de Valle Fértil and La Huertahave been published, among others byMirré (1971, 1976), Rabbia (1996), Vujo-vich et al. (1998), Pankhurst et al. (2000),Rapela et al. (2001), Castro de Machuca etal. (2002, 2005, 2007), Casquet et al.(2003), Murra (2004), Murra and Baldo(2004, 2006), Martino et al. (2004), Roes-ke et al. (2005). Nevertheless, specificgeological information and microstruc-tural-petrologic studies concerning defor-mational events and deformation me-chanisms in the Sierra de Valle Fértil(Schneider et al. 2006, Delpino et al. 2006,Otamendi et al., 2007), are very scarce.This contribution focus mainly in theanalysis of deformation mechanisms de-veloped at high temperatures, due to thefollowing reasons: 1) it provides an ap-proximation to the metamorphic peakconditions attained by the rocks. This isone of the main problems to confrontwith when studying an area affected byseveral superimposed tectonometamor-phic events, which very often lead tomasking of progressive paragenesis dueto retrograde processes; 2) most naturalexamples and experimental studies regar-ding deformation mechanisms in mine-rals, are related to deformation at low tomedium grade metamorphism, thoseconcerning deformation at high and veryhigh grade metamorphic conditionsbeing very scarce; 3) it constitutes anaccessible and cheap tool, which in com-bination with textural and parageneticanalyses, allows to estimate (at least semi-

quantitatively) the physical conditionsprevailing during a deformation event (orevents) affecting a given region.The physical conditions leading to deve-lopment of high-temperature deforma-tion mechanisms are semi-quantitativelyestablished through the analysis of themetamorphic evolution (P-T path) deter-mined on the basis of paragenetic, textu-ral and geothermobarometric studies. Wealso provide evidences and present abrief analysis of at least two overprintedretrograde events.

REGIONAL GEOLOGICAL SETTING

The Sierras Pampeanas tectono-stratigra-phical province includes the Neoprote-rozoic and Lower Paleozoic metamor-phic and igneous rocks of central Ar-gentina. Despite the differences in litho-logy and age of igneous and metamor-phic events within individual mountainblocks, a distinctive feature of the entireSierras Pampeanas province is its exhu-mation prior to the deposition of theUpper Carboniferous to Permian conti-nental sediments, collectively named thePaganzo Group (Bodenbender 19121922, Azcuy and Morelli 1970).Based on the distribution of metamor-phic and igneous rocks, the Sierras Pam-peanas province was divided by Caminos(1979) into the western and eastern Sie-rras Pampeanas. Most authors have sug-gested Late Precambrian-Early Paleo-zoic ages for the Sierras Pampeanas base-ment and associated granitoids. Themost recent geochronological studies(Sims et al. 1998, Rapela et al. 1998, Pank-hurst et al. 1998, von Gosen et al. 2002,Sato et al. 2002, 2003), indicate that themajority of the geological events tookplace during the named Pampean andFamatinian Orogenic Cycles.According to Rapela et al. (1998), EarlyCambrian (∼530 Ma) reconstruction ofthe proto-Andean margin of SouthAmerica is characterized by the forma-tion of a passive margin sedimentarybasin at the time oceanic crust subduc-tion started, with development of an ac-

cretionary prism and a calcalkaline volca-nic arc along the eastern edge of the Sie-rras Pampeanas. The subsequent closureof this ocean due to collision of a micro-continent (Pampean terrane) marked theonset of the Pampean Orogeny (Aceño-laza and Toselli 1976) during early Mid-dle Cambrian times, followed by exten-sional collapse in the Late Cambrian atthe end of the Pampean orogenic cy-cle.The Famatinian orogenic cycle, a majoraccretion and orogenic episode, startedwith subduction along the new Cambrianproto-Pacific margin (~490 Ma; Pnk-hurst and Rapela 1998, Dalla Salda et al.1992). The subsequent and prolongedsequence of events, were grouped intothe so called Famatinian Orogeny (Ace-ñolaza and Toselli 1976). The Famatinianbelt is constituted by an Eopaleozoiccontinental magmatic arc associated toeast-directed subduction linked to theapproaching of a Laurentia-derived te-rrane [Cuyania (Ramos et al. 1998, Ramos2004) and/or Precordillera terrain (Astiniet al. 1995)]. This continent approachstarted in Mid-Cambrian times (DallaSalda et al. 1992, Pankhurst et al. 1998)and ended with a continent-continentcollision in Mid-Ordovician times (DallaSalda et al. 1992, Ramos et al. 1998). Themain Famatinian orogenic phase wasfollowed by a period of extensional co-llapse and the emplacement of fracture-controlled, undeformed Devonian-EarlyCarboniferous granitoids (Brogioni 1993,Lira and Kirschbaum 1990, Pinotti et al.1996, López de Luchi 1996, Llam-bías etal. 1998).A new subduction episode developed tothe west of the Precordillera, related tothe collision of another terrane (Chi-lenia: Ramos et al. 1984, see also Astini1996), was associated with the broadlycoeval intrusion of within-plate plutonsin the Gondwana foreland (Pankhurstand Rapela 1998).

GEOLOGY OF THE SIERRADE VALLE FÉRTIL AREA

The Sierras de Valle Fértil and Sierra de

~

~

La Huerta (Fig. 1a), located between 3011´ and 31 28´ S and 67 15´ and 67 55´W, are part of the Western Sierras Pam-peanas morphostructural unit (Fig. 1b).The limit between Sierra de Valle Fértiland Sierra de la Huerta is approximatelylocated at the central inflexion of themountain range (Fig. 1a).The different litho-stratigraphic unitsthat compose the igneous-metamorphicbasement of the Sierra de Valle Fértil,were first outlined in the pioneer worksof Villar Fabre (1962) and Mirré (1971,1976) through detailed mapping of thearea. The Valle Fértil Complex was defi-ned by Cuerda et al. (1984). According toRapela et al. (2001), the geology of the

central and eastern sectors of the Sierrade Valle Fértil is dominated by a metalu-minous sequence of hornblende-biotitediorites, tonalites and granodiorites, andsuites of noritic and hornblende meta-gabbros emplaced in high-grade parag-neisses, migmatites, amphibolites andmarbles. The magmatic rocks are domi-nantly present in the eastern flank of therange. Schneider et al. (2006) indicatesthat a pre-Ordovician basement compo-sed mainly of gneisses, amphibolites andmarbles was intruded by tonalites-grano-diorites to granite intrusives and meta-gabbro to metadioritic rocks in the EarlyMiddle Ordovician (Famatinian Orogeny).Geochronological data obtained in the

Sierras de Valle Fértil and La Huerta(Pankhurst et al. 1998, 2000, Pontorieroand Castro de Machuca 1999, Roeske etal. 2005) constrain the active magmatismto the interval between 500 and 460 Ma.U-Pb SHRIMP geochronological studieson migmatites from central-easternSierra of Valle Fértil provided ages bet-ween 466.5 ± 7.7 and 465.9 ± 4.4 Ma forthe peak of the metamorphism, whichoccurred after the main intrusive periodof the Famati-nian arc. Therefore, peakmetamorphic temperatures and theattainment of anatectic conditions(Rapela et al. 2001) were approximatelycontemporaneous with plutonism (seealso Otamendi et al. 2007).

Mineral deformation mechanisms in granulite facies, Sierra de Valle Fértil, ... 183

Figure 1: a) Location and geologic sketch map of the Valle Fértil-La Huerta ranges, San Juan province, Argentina (modified after Ragona et al. 1995).The rectangle delimits the working area; b) Partial regional view of the Sierras Pampeanas of central-west Argentina showing the location of theValle Fértil-La Huerta ranges.

PETROGRAPHY OF THE STUDIED ROCKS

Biotite-garnet gneisses

Bt-Grt gneisses are composed by the as-sociation Qtz-Pl-Bt-Grt-Ilm-Mag (ab-breviations after Bucher and Frey 1994).Mesoscopically they appear as stronglyfoliated rocks showing dark biotite-bea-ring bands that anastomose around lightquartz-feldspar-garnet microlithons (Fig.2a). Under the microscope, quartz appe-ars as large lenticular monocrystallinegrains with lobate contacts with plagio-clase and biotite. Very often, these coarsecrystals enclose partially or completelyfeldspar or biotite grains (Fig. 3a). Inter-nally, quartz crystals show developmentof large subgrains with square or rectan-gular shapes with chessboard extinctionsindicating high-temperature ductile de-formations (Fig. 3c). Quartz porphyro-clasts also show, by sectors, developmentof low-amplitude bulging and recrystalli-sation at their margins and internal mi-crofractures, which evidence overprin-ting of a lower-temperature event (Fig.3a). Plagioclase appears as equidimensio-nal porphyroclasts with irregular or ellip-tical shapes, surrounded by coarse-re-crystallised aggregates of polygonal new-grains forming triple junctions at 120º(Fig. 3f). Relicts of former bigger por-phyroclasts included in quartz, occur aslenses elongated parallel to foliation andwith smooth boundaries with host quartzgrains (Fig. 3a). Unrestricted quartzgrowth included also recrystallised poly-gonal aggregates of plagioclase (Fig. 3a,bottom right corner). Garnet appears asirregular or elliptical porphyroclasts. For-mer poikiloblastic growth of garnet fromQtz-Pl Bt Ilm/Mag is documented by agreat number of preserved inclusions.Garnet shows fractures filled with bioti-te, which also crystallised and form theirpressure shadows. Biotite appear as coar-se crystals arranged in sub-parallel andanastomosing layers that wrap aroundgarnet, plagioclase and quartz single por-phyroclasts or polymineralic lenses defi-ning foliation (Fig. 3c). Biotite crystals


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Figure 2: a-f) Field appearance of representative rock types used in this study, Sierra de ValleFértil. Abbreviations after Bucher and Frey (1994).


Figure 3: a, b) Dynamicrecrystallisation ofquartz-feldspar bounda-ries. Note the unrestric-ted growth of quartz(Qtz) not pinned by grainboundaries of the coexis-ting phases to whichincludes partially ortotally. Observe the smo-othly lobate contacts bet-ween quartz and feldspars(mainly plagioclase, Pl)and the lenticular or sig-moidal shapes of "felds-par fishes" (Plff) ; c, d)Chessboard extinction inquartz. Note the develop-ment of rectangular andsquare subgrains, due toactivation of both basal<a> and prismatic <c>slip; e) Evidences ofGBMR in perthitic k-feldspar. Observe equidi-mensional shapes ofrelict crystals (Kfs) withscarce intracrystallinestrain and absence ofsubgrains.Pseudopolygonal recrys-tallised new-grains (Kfsr)with dissimilar sizes andtriple junctions at 120are conspicuous; f, g)Evidences of GBMR inplagioclase. Observe in f,the irregular shape ofrelict crystals (Pl) withscarce internal strain,absence of subgrains andrecrystallised grains (Plr)contacting with the relictsthrough high-angle boun-daries without mediationof subgrains (whitearrow). In g, the disparityin size and the highmobility of the grainboundaries, is remarkablealso in the recrystallisedgrains; h) Evidences ofSRR in plagioclase. Notethe development of poly-gonal subgrains (Pls,twinned crystals-centre)with shape and size simi-lar to those of the new-grains (Plr) adjacent tothe relict crystal bounda-ries. All photomicro-graphs with crossed-pola-rized light (XPL).Abbreviations afterBucher and Frey (1994).

show bending, interfingering (parallel tofoliation) with the other relict phases andis one of the main phases forming theirpressure shadows.

Cordierite-sillimanite gneisses

Field appearance of these rocks differsfrom that of the garnet-biotite gneissesdue to the partial loss of the compositio-nal foliation. Light irregular patches orribbons elongated parallel to foliationstand out on a dark bottom (Fig. 2b). Mi-croscopically, the following mineral asso-ciation was identified: Qtz-Pl-Bt-Grt-Sil-Crd-Spl-Ilm-Mag-Ti-Mag-Rt. Light por-tions are composed of coarse-grainedquartz, plagioclase and cordierite, plusvery scarce biotite and sillimanite crys-tals. Dark portions are constituted by pla-gioclase, cordierite and garnet porphyro-clasts, surrounded by coarse grained bio-tite and sillimanite crystals defining folia-tion. Quartz, plagioclase and biotiteshow the same characteristics as thoseobserved in the previous sample. Withinthe light patches, cordierite forms anhe-dral to subhedral grains entirely surroun-ded by large quartz or quartz+plagiocla-se grains. These crystals show sometimespolygonal subgrains and, rarely, somenew-grains. Very often it is pseudomor-phically replaced by garnet in these sec-tors (Figs. 4f, 5b and 5c). In the dark por-tions, cordierite forms irregular or sub-hedral porphyroclasts of varied sizes,which do not show evidence of ductileinternal deformation, except undulatoryextinctions. Cordierite crystals show onlyincipient retrogression to green biotiteand pinnitization at their margins. In thedark portions, sillimanite appear as coar-se and thin prismatic crystals that wraparound garnet, plagioclase and cordieriteporphyroclasts. In the light portions, fi-brolite appear as replacement of cordie-rite crystals in association with the pseu-domorphic replacement of this mineralby garnet (Fig. 5b). Coarse spinel is onlyrecognized as inclusions within large cor-dierite crystals. Inclusions are isolated orassociated to Ti-magnetite/ilmenite. Ten

times smaller sized spinel inclusions werealso recognized within poikiloblastic gar-net, which also contains Ti-magnetite/ il-menite inclusions and very small cordie-rite relicts.

Cordierite migmatites

These stromatic migmatites appear in thefield as banded rocks with great amountsof leucosome segregates, arranged inthin lenses and veins defining a promi-nent layering. A subparallel foliation tothis layering is recognized in the mesoso-me portions between segregated leuco-somes. Mafic inclusions are surroundedby leucosome segregates, which extendparallel to foliation conforming tails (Fig.2c). In thin sections the association Qtz-Pl-Bt-Sil-Crd-Kfs-Spl-Mag-Zr, was re-cognized. Quartz forms large ameboidalgrains or monocrystalline ribbons thatusually enclose -partially or entirely- cor-dierite, plagioclase and/or biotite crys-tals. Chessboard extinctions are com-monly observed. Cordierite shows largeequidimensional crystals with inclusionsof quartz, biotite, spinel, Ilm/Ti-Magand Zr. These crystals show, at their mar-gins, subgrains and recrystallised polygo-nal grains meeting at 120º triple junctions(Figs. 4a y 4b). Cordierite grains are bysectors altered to fibrolitic sillimanite,which also appear at the margins of re-crystallised grains. Very incipient pinniti-zation can also be observed. Biotite ap-pears as irregular interstitial crystals ofvaried sizes or as inclusions in quartz,plagioclase, cordierite and k-feldspar. Si-llimanite forms prismatic to acicular crys-tals, usually associated to biotite defininga rough foliation. As mentioned above,fibrolitic sillimanite partially replace cor-dierite crystals. Plagioclase grains formequidimensional porphyroclasts, surroun-ded by aggregates of subhedral to poly-gonal recrystallised grains meeting eachother at triple junctions. Large K-felds-par crystals are dominant in the leucoso-me sectors. Coarse perthitic k-feldspargrains are more or less equidimensionalor elongated parallel to the layering/ fo-

liation and have ameboidal contours. K-feldspar crystals enclose relicts of plagio-clase, biotite and quartz. Spinel has beenrecognized only as inclusions in cordierite.

Garnet-cordierite migmatites

At outcrop scale, these rocks show a gre-yish tonality and a well marked foliationenhanced by leucosomes forming thinribbons or stretched lenses alternatingwith dark mesosome portions (Fig. 2d).The association Qtz-Pl-Bt-Sil-Crd-Kfs-Grt-Ilm is recognized in thin sections.This association do no differ from thatof the stromatic migmatites, except forthe presence of garnet, which is absentin the later rocks. However, there aresome significant differences: 1) Stromaticmigmatites and garnet migmatites bothshow evidence of a very high-temperatu-re ductile event. However, relict migmati-tic and ductile high-temperature eventtextures are better preserved in the for-mer, whereas garnet migmatites havebeen stronger affected by a myloniticmedium-temperature ductile event thatsensibly obliterate these previous textu-res; 2) Coarse sillimanite and biotite aremuch more abundant than in stromaticmigmatites. Sillimanite forms coarse pris-matic crystals that act as porphyroclasts,and thinner prismatic crystals whichtogether with biotite, quartz and felds-pars form folia that wrap around indivi-dual porphyroclasts or polymineralic len-ses preserving evidence of the migmati-tic (K-feldspar rich leucosomes) and hig-her ductile temperature event (largemonocrystalline quartz with chessboardextinctions, feldspar surrounded by ag-gregates of large polygonal recrystallisedgrains) (Figs. 3e and 4e); 3) Cordierite ap-pear as universally pinnitized porphyro-clasts and partially replaced by biotiteand sillimanite medium-sized crystals.

Metagabbros, metatonalites-metadio-rites and mafic dikes

Metagabbros and metatonalites-metadio-rites appear as irregular, medium sized

186 S. DELPINO, E . BJERG, A . MOGESSIE , I . SCHNEIDER, F. GALLIEN, B. CASTRO DE MACHUCA,L . PREVILEY, E . MEISSL , S. PONTORIERO Y J. KOSTADINOFF


Figure 4: a, b) Evidences ofSRR ductile deformation incordierite. In b, as well as ina (photomicrograph withplane-polarized light (PPL)and gypsum plate to enhan-ce the texture), note the pre-sence of subgrains (sg)whose low-angle boundariesare invisible in PPL. In con-trast, the high-angle boun-daries of recrystallisedgrains (rg) are clearly visible.Also observe the similarshape and size of recrystalli-sed grains and neighbouringsubgrains. In b subgrains arealso indicated by oblitera-tion of twin planes in relictcordierite crystals (RCrd), atypical feature of this phase;c, d) Evidences of ductiledeformation in hornblendeby GBMR. Note the irregu-lar shape of relict crystals(RHbl), the grain boundarymobility evidenced by lobatecontacts amongst Hbl-Hblcrystals and the absence ofsubgrains. Development ofpolygonal recrystallisedgrains with dissimilar sizesand triple joints at 120° (rg)is also seen; e, f) Evidencesof the intermediate tempe-rature ductile deformationevent. Folia composed byQtz-Sil-Bt Kfs Pl crosscutand obliterate textures ofthe higher temperature duc-tile event (rPl: polygonal re-crystallised plagioclase andcoarse quartz ribbons). In fobserve superposition ofthe intermediate temperatu-re event on big sized quartzgrains, typical of the highertemperature event. Despitethe intense obliteration, it isstill possible to recognizethe previously developedchessboard extinction(arrows); g, h) Evidences ofthe low temperature brittledeformation event. In g,intergranular mi-crofracturecuts trough both relict andrecrystallised crys-tals. Themicrofracture is fi-lled bycalcite. In h a microfractureis filled by calcite, epidoteand chlorite. Close to thefracture, feldspars are stron-gly sericitized (Plser).Photomicrographs a, c andh: PPL; b: XPL and gypsumplate; d, e, f and g: XPL.


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bodies, included in basement rocks beingthe present contacts tectonics. Most gab-bros are strongly altered. However, de-formation affected the bodies essentiallyat their margins and along narrow inter-nal shear bands. Thus, magmatic layeringand high-temperature subsolidus meta-morphic textures (coronas and simplecticintergrowths), are preserved in the lessdeformed sectors of the bigger bodies.Grain size varies from coarse-grained inthe central portions of the bodies tofine-grained towards to the contact withthe basement the rocks. The metagab-bros are composed by the association Ol-Pl-Opx-Cpx-Amp-Spl-Ilm-Mag. Thesecoarse-grained rocks very often show al-teration of olivine and pyroxenes toiddingsite, serpentine and chlorite, rela-ted to a low-temperature fragile event.Pyroxenes show sometimes small pseu-dopolygonal crystals indicating incipientrecrystallisation. Clinopyroxene showsfrequently exsolution lamellae of spineland magnetite. Plagioclase appears as ve-ry large crystals, sometimes partially alte-red to sericite. Spinel constitutes largegrains in the groundmass or forms sim-plectic intergrowth with magnetite oramphibole in the coronas developedaround former magmatic phases. Meta-tonalite-metadiorite essentially differsfrom the metagabbros described aboveby its mineral constituents (Qtz-Pl±OpxHbl±Bt±Cpx-Mag-Ilm). Abundance offelsic components like plagioclase andquartz increase significantly, whereas themain mafic components are representedby orthopyroxene and amphibole, withscarce biotite and -locally- clinopyroxene.Furthermore, coronas and simplectitesare not present in these rocks. Anothersignificant difference between theserocks and metagabbros, is the presencein the former rocks of clear evidences ofhigh and medium-temperature ductiledeformation events. Quartz and plagio-clase show the same deformational textu-res like those previously described for thesame phases in the basement rocks (com-pare quartz and feldspar deformationtextures in figures 3b, 3d and 3g corres-

ponding to metatonalites, with texturesdeveloped by the same phases in base-ment rocks showed in the rest of thisfigure). Mafic dikes are fine-grained(Figs. 2e and 2f) and cut across magmaticbodies in different directions. They arecomposed by the association Pl-Hbl±Opx±Bt±Cpx-Mag-Ilm-Ap-Py). Plagio-clase and hornblende are largely the do-minant constituents, being orthopyroxe-ne, biotite or clinopyroxene subordinatedand their occurrence usually related totheir presence in the neighbouring hostrock. Plagioclase and hornblende showpor-phyroclasts surrounded by aggrega-tes of polygonal recrystallised grains me-eting at triple junctions denoting ductilerecrystallisation of these phases (Figs.3h, 4c, 4d and 4g).

EVIDENCE OF HIGH TEM-PERATURE DEFORMATIONMECHANISMS IN MINERALS

Quartz: The most frequently used refe-rence for the estimation of the physicalconditions prevailing during ductile de-formation, are the textures developed inexperimental studies on quartz. Texturalmodifications reflect changes in the ope-rating deformation mechanism as a func-tion mainly of variations in temperature,differential stress, strain, presence offluids, etc. As a result, three different de-formation regimes have been defined(Hirth and Tullis 1992). These deforma-tion regimes in quartz, with some modi-fications, were extrapolated to the naturalenvironment and semi-quantified by se-veral workers (Dunlap et al. 1997, Stöck-hert et al. 1999, Zulauf 2001, Stipp et al.2002). The ductile deformation characte-ristics of this mineral at temperaturesexceeding those of the regime 3 (the hig-hest temperature regime, Hirth and Tullis1992), were also considered by otherauthors (Blumenfeld et al. 1986, Main-price et al. 1986, Kruhl 1996, Stipp et al.2002).Based on the preceding considerations,attention will be paid at first to the beha-viour of quartz that presents two types

of textures in the studied rocks:1) Development of large crystals (fre-quently forming monocrystalline ribbonselongated parallel to foliation), by growththrough grain boundary migration not-controlled (pinned) by other phases pre-sent. Well-equilibrated boundaries betwe-en quartz-quartz and quartz-feldspargrains, show smooth, curved or lobateshapes (Fig. 3a). Feldspar grains isolatedwithin quartz crystals, show lenticular orsigmoidal shapes (Figs. 3a and 3b). Thesetextures are characteristic of rocks defor-med under granulite facies conditions,being migration assisted by grain boun-dary diffusional creep, the mechanismconsidered as responsible for their for-mation (Simpson and De Paor 1991,Martelat et al. 1999). This mechanism hasbeen called "dynamic recrystallisation ofquartz-feldspar boundaries" by Gowerand Simpson (1992). Grain boundary mi-gration results from diffusive mass trans-fer along phase boundaries, a processonly possible at very high homologoustemperatures (T/Tmelting).2) Chessboard extinction due to the pre-sence of square or rectangular subgrains(Figs. 3c and 3d), which has been attribu-ted to a combination of basal <a> andprism <c> slip (Blumenfeld et al. 1986,Mainprice et al. 1986). The operation ofthis slip systems combination, have beenconsidered to occur at temperatures ex-ceeding the Qtzα/Qtzβ transition (Kruhl1996, Stipp et al. 2002). As a reference,this transition takes place at around 660ºC at 4Kb and 730º C at 7 Kb.Feldspars: Regarding plastic deformationof feldspars, only the beginning of itsductile deformation has been constrai-ned, although not accurately, to the range400-500 ºC (see Passchier and Trouw1996 and references therein).Concerning deformation mechanisms,textural evidences suggest a general be-haviour similar to quartz, although withregimes displaced towards higher tempe-ratures. However, little is known aboutthe limits of such regimes for feldspars.Therefore, it is interesting to evaluate theobserved textures and consider the pos-

sible operating deformation mechanismsin areas where it is possible to constrainthe metamorphic physical conditions byother means (for example, through de-formation mechanisms developed inother phases during the same deforma-tion event, determination of stabilityfields of specific mineral associations inequilibrium during this event, throughgeothermobarometry, etc). This couldcontribute to the establishment of suchlimits.Perthitic k-feldspar shows more or lessequidimensional relict grains with undu-latory extinction, absence of subgrains,lobate boundaries and presence of bul-ging. It evidences recrystallisation withdevelopment of polygonal shaped new-grains with variable sizes, showing triplejunctions at 120° (Fig. 3e). These featuressuggest recrystallisation by means ofgrain boundary migration (GBMR, seePasschier and Trouw 2005 and referencestherein).Relict plagioclase grains are irregular andshow embayments and recrystallisationto aggregates of polygonal new-grainscontacting each other through rectilinearor lobate boundaries and forming 120°triple junctions (Fig. 3f).The high mobility of grain boundariesevidenced by both relict (Fig. 3f) andrecrystallised grains (Fig. 3g), as well asthe recrystallisation characteristics itself,indicate that the dominant deformationmechanism was GBMR. However, in so-me sectors plagioclase also shows poly-gonal subgrains with sizes and appearan-ce similar to contiguous recrystallisedgrains (Fig. 3h). Such features indicatethat subgrain rotation recrystallisationmechanism (SRR), also contributed tothe ductile deformation of this mineral.Although, as were previously mentionedthe transition between GBMR and SRR(see Passchier and Trouw 2005 and refe-rences therein) mechanisms has not beenwell established up to the present, Tullisand Yund (1985) noticed that feldspars inrocks belonging to the high amphiboliteand granulite facies frequently show sub-grains and, therefore, the SRR mecha-

nism would be active under these condi-tions.The observed differences in the rheolo-gical behaviour of k-feldspar and plagio-clase (the first one evidencing onlyGBMR and the second a combination ofGBMR and SRR), are consistent with thefact that the activation energy required bydynamic recrystallisation and dislocationglide is lower for plagioclase than for k-feldspar, at medium to high homologoustemperatures (Fitz Gerald and Stunitz1993, Schulman et al. 1996).Cordierite: This mineral occurs as more orless equidimensional relict grains, sub-rounded or irregular, with developmentof subgrains and recrystallised grains atthe margins. Recrystallisation gave placeto aggregates of polygonal new-grains ofsimilar sizes to those of the subgrains inthe neighbouring relict crystals, formingtriple junctions at 120° (Figs. 4a and 4b).These characteristics indicate recrystalli-sation by means of the SRR mechanism.It is very interesting the possibility to ob-serve evidences of cordierite recrystalli-sation and be able to establish the proba-ble operating deformation mechanism,since references about the ductile beha-viour of this phase are very scarce in theliterature.Hornblende: This mineral shows evidenceof recrystallisation and absence of brittlefracturing, indicating that it was also duc-tile deformed. Hornblende occurs asrelict crystals with irregular shapes, with-out significant flattening or presence ofsubgrains. Recrystallisation has given pla-ce to arrangements of polygonal new-grains with varied sizes forming triplejunctions at 120° (Figs. 4c and 4d). Suchtextural arrangement strongly suggeststhat GBMR was the dominant deforma-tion mechanism.

LOWER TEMPERATUREOVERPRINTED EVENTS

Although the previous detailed analysisof deformation mechanisms documentthe occurrence of a ductile deformationevent developed at high to very high tem-

peratures, the studied rocks show alsoevidences of other superimposed eventsdeveloped at lower temperatures, whichwill be briefly described in the next para-graphs.At least two events have been documen-ted, one ductile at intermediate tempera-tures and the other brittle at low tempe-ratures.

Intermediate temperature event

It is characterized by development of adefined foliation that cut and obliteratestextures resulting from the highest tem-perature event (Figs. 4e and 4f). The mi-nimum temperatures of this event weredetermined by the fields of stability-ins-tability of some phases and by the textu-res developed in quartz. At intermediatepressures the absence of muscovite andpresence of stable sillimanite+k-feldspar(Fig. 4e), are typical of conditions excee-ding the second sillimanite isograd (Ms+Qtz Sil+Kfs+H2O, Fig. 5). Quartz tex-tures (amoeboid grains with large ampli-tude sutures and development of dissec-tion microstructures, Fig. 4f) indicatethat deformation conditions exceededthe transition between SRR and GBMRregimes (Stipp et al. 2002). Both characte-ristics are indicative of minimum tempe-ratures in the order of 600 °C. This eventcould be correlated with the myloniticevent recognized by Castro de Machucaet al. (2007) on metagabbros from theSLH, which indicates temperaturesaround 650-700 ºC and pressures betwe-en 6 and 7 Kb. According to Murra andBaldo (2001), generalized mylonitizationin the area would have begun at ca. 452-459 Ma and could be related to the laterstages of the Famatinian orogeny.

Low temperature event

The brittle low temperature event is cha-racterized by the presence of thin conti-nuous intergranular microfractures, whichcut through both relict and new-grains ofrecrystallised phases (Figs. 4g y 4h).Depending on the lithology they are


affecting, microfractures can be filledwith calcite (Fig. 4g), Fe-oxides, chloriteand/or epidote (Fig. 4h). This retrogradeevent is often associated to pervasive re-placement of feldspars by sericite in theproximity of the microfractures (Fig. 4h).The described mineral association is indi-cative of deformation conditions withinthe field of low greenschist facies, attemperatures not exceeding 400°C.

PHYSICAL CONDITIONSOF DEVELOPMENT OFTHE HIGH-TEMPERATUREDUCTILE EVENT

To analyze the physical conditions invol-ved in the development of deformationmechanisms at high temperature, a dia-gram was constructed (Fig. 5) using thePerplex software (Connolly 1990, version2006) and the thermodynamic databaseof Holland y Powell (1998, actualizedversion 2002). Based on the petrographicstudy, the following phases were conside-red for the system TKFMAS-HC: Rt-Ilm-Bt-Grt-hCrd-Opx-Spl-Kfs-Ms-Als-Qtzα−β-fluid (H2O+CO2). Isopleths ofconstant composition for garnet and bio-tite (Fig. 5) were calculated using the acti-vity models of Holland and Powell(1998) and Powell and Holland (1999),respectively.The most likely attained metamorphicpeak conditions were established basedon the following: 1) Stromatic migmati-tes in the region (Fig. 2c) comprise themineral association Crd-Spl-Qtz, garnetabsent. This association is typical of highgrade metamorphism at intermediate tolow pressures and is stable below theGrt+Sil+H2O hCrd+Spl+Qtz reaction(Fig. 5); 2) Preliminary geothermometriccalculations on coronitic gabbros of theSVF (Cpx-Opx equilibrium) suggest ma-ximum temperatures of the order of 850°C (Schneider et al. 2006, Castro de Ma-chuca et al. 2007). Considering this tem-perature as the thermal maximum rea-ched by the rocks in the area of SVF, theassociation Crd-Spl-Qtz is stable onlybelow pressures of around 4.7 Kb (Fig.

5); 3) Given that stromatic migmatitesare the result of "in situ" partial melting,the lower temperature limit for the meta-morphic peak, should be above the gra-nite solidus. Curves corresponding to thesolidus of an aplogranitic composition(Ebadi and Johannes 1991) and to ter-nary alkaline feldspars (Ab25-33) in equili-brium with plagioclase An30 (Bohlen etal. 1995), both for low water activity con-ditions (XH2O=0.25), are shown in Fig. 5.The curve of minimum melting tempera-ture for the rocks of the study areashould be located between these two ex-tremes, most probably closer to the curveto the right because it takes into accountthe calcium content in plagioclase. Thethree formerly described characteristicsfix a limit to the metamorphic peak tem-perature very probably in the range 800to 850 °C and pressures below 5 Kb(shaded grey area, Fig. 5).The preliminary geothermobarometricresults applying both TWEEQU (Ber-man 1991, version 2001) and Perplex(Connolly 1991, version 2006) softwaresto garnet-bearing gneisses and migmati-tes from the study area, indicate higherpressures and lower temperatures (circlesand triangles, Fig. 5a) than the estimatedfor the metamorphic peak, and a CO2-rich fluid phase composition (XCO2=

0.75, the best fit between both formula-tions) (Fig. 5). Composition of the pha-ses used in TWEEQU calculation, areshowed in Table 1. These determinationswould indicate that the P-T establishedvalues based on garnet-bearing rockswould represent the physical conditionsprevailing during some stage of retro-gression, but not those of the metamor-phic peak. This interpretation is suppor-ted by the textural relationships. Cord-ierite and spinel-bearing stromatic mig-matites are garnet free or, if present, gar-net is the result of cordierite retrograda-tion. The pseudomorphic replacement ofcordierite by garnet is accompanied bythe formation of fibrolitic sillimanite andthe replacement of ilmenite by rutile(Figs. 5a, 5b and 5c). These textural rela-tionships indicate that the Grt+Sil+H2O hCrd+Spl+Qtz and Rt+Grt+Qtz+Sil+H2O Ilm+hCrd reactions pro-ceeded from the right to the left. The-refore, the metamorphic trajectory mustbe such that, beginning at the determinedmetamorphic peak conditions, it shouldevolve towards the stability field of theassociations representative of higherpressures and lower temperatures locatedto the left side of both reactions (Fig. 5).The estimated peak metamorphic condi-tions, the preliminary geothermobaro-


190

Mineral Garnet Plagioclase BiotiteSample MIJ0805 MIJ1505 MIJ1705 MIJ0805 MIJ1505 MIJ1705 MIJ0805 MIJ1505 MIJ1705

TABLE 1: Representative phase chemical compositions used in TWEEQU calcula-tion. MIJ0805: Garnet-cordierite migmatite. MIJ1505: Cordierite-sillimanite gneiss.MIJ1705: Garnet-biotite gneiss.

SiO2 36,86 37,34 36,97 60,48 56,71 58,76 35,16 35,31 34,62

TiO2 0,04 0,05 0,04 0,02 0,02 0,03 3,64 3,27 4,42

Al2O3 20,04 21,20 20,17 22,95 26,49 25,21 16,63 17,06 15,99

Cr2O3 0,09 0,06 0,06 0,02 0,02 0,01 0,07 0,03 0,04

Fe2O3 1,17 2,32 1,22 0,05 0,01 0,00 2,57 0,00 0,00

FeO 29,66 26,43 26,13 0,00 0,07 0,07 13,10 13,76 15,97MnO 3,34 1,45 5,84 0,04 0,03 0,02 0,12 0,09 0,09ZnO 0,00 0,00 0,00 0,07 0,04 0,00 0,00 0,00 0,00MgO 6,29 8,40 5,89 0,00 0,00 0,01 12,65 14,33 12,50CaO 0,72 1,35 1,70 4,21 8,25 7,06 0,07 0,09 0,06Na2O 0,00 0,00 0,00 9,79 6,46 6,82 0,00 0,13 0,08

K2O 0,00 0,03 0,00 0,18 0,12 0,27 9,78 9,61 9,63

H2O 0,00 0,00 0,00 0,00 0,00 0,00 3,83 3,90 3,81

Total 98,20 98,64 98,02 97,81 98,21 98,27 97,61 97,58 97,19

metry on garnet-bearing rocks and thetextural relationships, are characteristicsthat together suggest a counter-clockwiseP-T path as the one shown in Fig. 5.Both cordierite and k-feldspar (and pro-bably the albitic plagioclase present inthese rocks), are closely related to mig-matization and were affected by ductiledeformation after crystallisation. There-fore, the development of the observeddeformation mechanisms must have oc-curred after the metamorphic peak (ther-mal maximum-anatexis). On the otherhand, the lower temperature limit should

not exceed the Qtzα/Qtzß curve (givenpresence of chessboard extinction inquartz) and the curve corresponding tothe reaction Bt+Qtz+Sil hCrd+Kfs+H2O that defines the lower temperatu-re stability limit for cordierite (this phasereflects ductile deformation within itsstability field) (Fig. 5). This segment ofthe counter-clockwise trajectory is loca-ted above 700 °C at the considered pres-sures, i.e., entirely within the granulitefacies field (Fig. 5).

DISCUSSION

Several peak metamorphic conditionsand P-T trajectories have been proposedfor different localities of the Famatinianarc. A low-pressure (< 3 Kb) contactaureole representing upper level exposu-res of the arc has been described in theSierra de Chepes, located east of theValle Fértil-La Huerta range (Dahlquistand Baldo 1996) (Fig. 1a). Peak metamor-phic pressures in the range 5-7 Kb wererecorded by Delpino et al. (2007) for thePringles Metamorphic Complex (Sierra


Figure 5: a) P-T diagram for the Sierra de Valle Fértil basement rocks. The grey field represents the most probable conditions attained by the rocksat the metamorphic peak. Empty circles represent the P-T values obtained with the TWEEQU program for rocks with the equilibrium associationQtz+Pl(an)+Kfs+Grt+Bt+Sil+fluid(H2O+CO2). Triangles and filled squares represent the P-T values established for the same rocks by means ofisopleths of constant composition for garnet and biotite, calculated with the Perplex program. The grey arrow indicates the most probable meta-morphic path, based on the analyses of paragenetic associations, textural relationships and geothermobarometry. b, c, d) Examples of the texturalrelationships that allow to constrain part of the metamorphic path. b and c: PPL photomicrographs showing details of cordierite pseudomorphicreplacement by garnet and sillimanite (fibrolite). The replacement is associated to partial transformation of ilmenite into magnetite; d: SEM imageshowing in detail the transformation of ilmenite into rutile and magnetite.

de San Luis) and Otamendi et al. (2007)for the Sierra de Valle Fértil-La Huerta,being backarc and deep arc the most pro-bable geological settings proposed bythese authors, respectively. These threelocalities have in common high to veryhigh peak metamorphic temperatures at-tributable to magmatic heating and firststages of retrogression characterized bynearly isobaric cooling, within the con-text of a counter-clockwise metamorphictrajectory. On the contrary, a clockwiseP-T metamorphic path was deducedfrom the study of metaigneous rocksfrom the Sierras de Las Imanas, locatedsouth of the Sierra de La Huerta (Murraand Baldo 2006). Neither textural norpetrological evidence for decompressionhave been found in the studied rocksfrom Sierra de Valle Fértil area (see alsoOtamendi et al. 2007). On the otherhand, metamorphic pressures appre-ciably more higher have been recorded inthe Loma de Las Chacras (Baldo et al.2001, Vujovich 1994) (Fig. 1a). This dif-ference in the levels of exposure of dif-ferent parts of the Famatinian arc havebeen attributed by Otamendi et al. (2007)to a differential upward movement givingrise to higher rates of exhumation in theLomas de Las Chacras than in the Sierrasde Valle Fértil-La Huerta.Otamendi et al. (2007) carried out a verydetailed study of the metamorphic evo-lution of basement rocks of SVF-SLHand its correlations with other areas ofthe Famatinian orogen. These authors in-tegrated petrographic and mineral che-mical data from a sequence of migmati-zed pelites and quartz-feldspathic grey-wackes at SVF-SLH, in order to identifycoexisting equilibrium assemblages andto constrain their P-T evolution. Theseauthors also provide insights into twoissues related to the evolution of the Fa-matinian supracrustal sedimentary rocks:the crustal levels they originally occupiedand the evolutionary P-T path they follo-wed as they were migmatized by the addi-tion of magmatic heat. This informationwas used to establish the minimum thick-ness for the Famatinian arc crust and to

provide constraints on geodynamic mo-dels accounting for the subduction-to-collision orogenesis that finished the for-mer magmatic arc. Combining their re-sults with those of other studies that ha-ve constrained the tectonic-thermal tra-jectories of metasedimentary and me-taigneous rocks from distinct settingswithin the Famatinian arc, they presentedan integrated view of the metamorphicevolution of the Famatinian magmaticbelt from the backarc (Hauzenberger etal. 2001, Delpino et al. 2007) through thecontact aureole of plutonic batholiths(Dahlquist and Baldo 1996, Murra andBaldo 2006) to the accretionary wedge(Vujovich 1994, Baldo et al. 2001).The present study is a contribution forthe elucidation of the tectonometamor-phic evolution of a sector of the Fa-matinian belt. The proposed P-T meta-morphic path fit very well with that pre-sented by Otamendi et al. (2007), whoalso proposed a counter-clockwise trajec-tory for the basement rocks of the SVFand SLH. There is a total corresponden-ce for the pressure and temperature con-ditions of retrogression related to analmost isobaric cooling following meta-morphic peak. The only -small- differen-ce between both models can be observedin the portion of the trajectory corres-ponding to the progressive stage. Thisdifference is due to the fact that Ota-mendi et al. (2007) found spinel enclosedonly in garnet, while we found coarsespinel inclusions in cordierite of garnet-absent migmatites and evidence of pseu-domorphic replacement of cordierite(plus included spinel) by garnet in cordie-rite-sillimanite gneisses. This is the rea-son why our P-T path start at lower pres-sures so that the metamorphic peak fallswithin the stability field of the associa-tion Crd+Spl+Qtz, as was previouslydescribed.

CONCLUSIONS

1) At the metamorphic peak, temperatu-re in the SVF reached 800 to 850 °C andpressures were below about 5 Kb.

2) Phase relations and geothermobaro-metry point to a counter-clockwise P-Tpath for the metamorphic evolution ofthe rocks in this region.3) On the basis of paragenetic associa-tions and deformation mechanisms inminerals, at least three overprinted defor-mation events can be differentiated in therocks of the area, and their physical con-ditions of development constrained bythe established P-T metamorphic path: a)a ductile event developed entirely withinthe granulite facies field, at temperaturesexceeding 700 °C and in the probablerange of pressures between 5 to 6 Kb; b)a ductile deformation event occurringunder the conditions of medium to highamphibolite facies, at temperatures pro-bably exceeding 600 °C and pressures inthe interval 6-7 Kb; c) a brittle deforma-tion event developed at low greenschistfacies, probably at temperatures below400 °C and pressures not determinedwith certainty until present.

ACKNOWLEDGEMENTS

This work was financed by project FWF-P17350-N10 of the Austrian ResearchFund to AM and the SGCyT of theUniversidad Nacional del Sur, Argentina.This paper was presented in the 13Reunión de Tectónica, San Luis, Argen-tina, 17-21 October 2006. We ackno-wledge to the reviewers Dr. Pablo D.González and Dr. Roberto D. Martinofor their constructive comments thatallowed to improve the final presentationof the manuscript.

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Recibido: 19 de noviembre, 2007 Aceptado: 25 de abril, 2008


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