Ordovician Metamorphism of the Sierras Pampeanas, Sistema de Famatina and Cordillera Oriental, Northwestern Argentina

Juana N. ROSSI1, Arne P. WILLNER2 and Alejandro J. TOSELLI1

Abstract: ORDOVICIAN METAMORPHISM OF THE SIERRAS PAMPEANAS, SISTEMA DE FAMATINA AND CORDILLERA ORIENTAL, NORTHWESTERN ARGENTINA. In NW Argentina, the composition, age and relationships of metamorphic country rock with granitoids allow the characterization of four metamorphic belts: 1) The Eastern Belt, with predominantly Pampean metamorphism (Upper Precambrian- Lower Cambrian) to the east, and Famatinian (Ordovician) to the west in deep tectono-metamorphic levels. 2) The Central Belt, with prevalence of ordovician granitoids intruded in supracrustal turbidites of Pampean age, with Famatinian thermal metamorphism overlapping. 3) The Sistema de Famatina Belt is divided in three zones. The eastern and intermediate zones, with granitoids intruded in metaturbidites with low-metamorphic grade are assumed as equivalent to the Puncoviscana Formation, with overlapping Ordovician metamorphism. In the western zone, the granitoids intruded in a basement of high-metamorphic grade, considered with age Grenville/Sunsas. 4) The Western Belt is composed by high grade metamorphic rocks, and it is considered an allochthonous terrain, with partially Proterozoic ages, suffering Ordovician metamorphism. This event is interpreted as the collision of an allochthonous/parauchtothonous terrain against to the western border of Gondwana.

Resumen: METAMORFISMO ORDOVÍCICO DE LAS SIERRAS PAMPEANAS, SISTEMA DE FAMATINA Y CORDILLERA ORIENTAL, NOROESTE DE ARGENTINA. Cuatro cinturones metamórficos fueron propuestos para describir el metamorfismo Ordovícico Famatiniano en el NW de Argentina:1) El Cinturón Oriental, cuyo metamorfismo es mayormente Pampeano (Precámbrico superior- Cámbrico inferior) al Este y Famatiniano en los niveles tectonometamórficos más profundos al Oeste. 2) El Cinturón Central, con predominio de granitoides ordovícicos intruídos en turbiditas supracorticales, de metamorfismo Pampeano, con sobreimposición de metamorfismo térmico Famatiniano. 3) El Cinturón del Sistema de Famatina, con las fajas central y oriental con granitoides intruídos en meturbiditas de bajo grado metamórficos asumidas como equivalentes a la Formación Puncoviscana, con metamorfismo ordovícico sobreimpuesto y la faja occidental de granitoides intruídos en basamento de alto grado metamórfico y edades consideradas de Grenville/Sunsas. 4) El Cinturón Occidental, compuesto por rocas metamórficas de alto grado, es considerado un terreno alóctono, de edades en parte proterozoicas, que tienen sobreimpuesto metamorfismo ordovícico, evento interpretado como la colisión de un terreno alóctono/parautóctono contra el borde occidental de Gondwana.

Keywords: Northwest Argentina. Metamorphic belts. Pampean metamorphism. Famatinian metamorphism.

Palabras clave: Noroeste Argentina. Cinturón metamórfico. Metamorfismo Pampeano. Metamorfismo Famatiniano.

Introduction

In the Northwest of Argentina, the Ordovician metamorphism (495 Ma to 443 Ma), widely overprint the basement rocks from Proterozoic-Neoproterozoic up to Cambrian ages of the Sierras Pampeanas. The oldest basement of Grenville/Sunsas age (1100-1000 Ma) is up to date poorly constrained, and it was recognized in the Sierra Pié Palo, which belongs to the Sierras Pampeanas Occidentales, (Fig.1) and considered by some workers an allochthonous with Laurentian affinity, and for others a Gondwanic parautochthonous terrain that would be collided against the southwestern Gondwana in Ordovician up to Devonian times. The remaining Sierras Pampeanas Occidentales: Valle Fértil-La Huerta, Maz and Umango ranges, were all also considered as exotic terrane to Gondwana, but lately, the different ages of magmatic and metamorphic events in the Sierra de Umango basement (Grenvillian, Cambrian, Ordovician and Devonian) would been interpreted as juxtaposed sheets of allochthonous and autochthonous Gondwanic together (Sato et al., this volume).

In easterly of northwest Argentina, the oldest autochthonous Gondwanic terrain is the Puncoviscana Formation and equivalents whose metamorphic ages of 580–540 Ma, is known as the Pampean metamorphism.

The Ordovician metamorphism (490-470 Ma), overprint the Pampean metamorphism in those supracrustal levels intruded by the huge Ordovician batholiths of the Sistema de Famatina and those of the Batholithic Central Zone encompassing the Sierras de Zapata, Vínquis, Belén-Capillitas, Velasco-Mazán, and Los Llanos-Chepes-Ulapes (Fig. 1).

To the east of the Sierras Pampeanas, the outcrop of basement in the Quilmes, Cumbres Calchaquíes, Aconquija, Ambato, Ancasti ranges, Cordillera Oriental and Salta and Tucumán ranges.

The metamorphic rocks areally predominate on the intrusive rocks and show different metamorphic levels, with different deformation episodes and polimetamorphism, which were isotopic dated in areas of Salta, Tucumán and Ancasti (Miller et al., 1994).

For a better understanding the distribution of the Ordovician metamorphism in northwestern Argentina we will use the division of the basement in the igneous-metamorphic belts proposed by Toselli et al. (2001), see Table 1.

Eastern belt

In the Sierra de Cachi (Cordillera Oriental) crops out the oldest basement, known as the Puncoviscana Formation, (Zone I of Willner, 1990) constituted by supracrustal turbidites whose earliest U-Pb age of sedimentation was obtained in detrital zircons of silicic volcanic provenance, averaged in 550 Ma (Lork et al., 1990). Open to tight subvertical chevron folds characterize F1; a slaty cleavage S1 is only found in the pelitic layers, with a synkinematic metamorphism of very low grade (Willner, 1990). Towards to the west in the Sierra de Cachi, a series of tonalite – trondhjemite to granodiorite, small plutons of mainly Ordovician age, intruded in the Puncoviscana Formation (Galliski et al., 1990). A low P/T ratio metamorphism, has developed and increased in short distance from the biotite-cordierite zone to the sillimanite-cordierite- potassic feldspar zone close to the plutons in a nearly isobaric P-T path (Rossi de Toselli et al., 1992). The 481 to 462 Ma U-Pb monazite

ages of the plutons (Lork et al., 1989) are interpreted as the oldest ages of the thermal Ordovician metamorphism of this area. The granitoids and their thermal aureoles were strongly sheared, obtaining a penetrative subvertical foliation S2 and rotation of the cordierite metablasts (Willner, 1990).

Toward the south, in the most eastern zone of this belt, the Puncoviscana Formation continue in the sierras of southeast Salta, northeast of Tucumán, and west of Tucumán city (Zone II of Willner, 1990). This zone is characterized by a penetrative tectonic banding in metapsammitic rocks as an axial plane cleavage S1 of tight to subvertical F1 folds. Strike is very constant, NE-SW in the Sierra de San Javier and eastwards, southern in the Cumbres Calchaquíes and to the south. Coaxial F2 drag folds of dm to m scale are also penetrative. A gently dipping S2 occurs as crenulation cleavage

in metapelites and as fracture cleavage in metapsammites. This progressive change in the mesoscopic tectonic fabric from zone I to II, is also accompanied by an increase in synkinematic metamorphism from very low, to low grade. Chlorite and white mica have grown parallel to the cleavage planes.

There is a strong pressure solution of quartz and also of plagioclase between the mica festoons essentially in the dark micaceous bands. Dissolved quartz is partially concentrated in pressure solution shadows, partially it leaves the rocks to form quartz segregations parallel to S1 or oblique veins.

The area that exhibiting the strongest shear strain, is located in the Sierra del Nogalito. The mesoscopic fabric is characterized by monotonous subhorizontal planes with extremely harmonic intrafoliaceous folds of cm to dm scale. Banding is very thin; microscopically one observes extreme pressure solution of quartz and feldspar clasts, strong polygonization as well as synkinematic growth of biotite, which are indicate an increase of temperature in this area, and continued in a diaphtoretic phase with chloritization. Adams et al., (1990) obtained K/Ar whole rock ages of 535- 540 Ma, for the metamorphism, in the less deformed and very low metamorphic grade rocks of southeast Salta; and 560-570 Ma in the more deformed (higher P/T) rocks in the San Javier and northeast of Tucumán ranges. Later Ordovician events were not detected in these regions.

In Cumbres Calchaquíes, at the Tafí del Valle latitude, crop out deeper tectono-metamorphic levels (Zone III of Willner, 1990): during F1 a transposition structure was developed and a second static metamorphism M2, post F2 began, in which the metapsammites and metapelites underwent a gradation to banded schists characteristic of the Cumbres Calchaquíes, and the Aconquija and Ancasti ranges. The F1 fold closures disappear by transposition of the banding subparallel to bedding up to an angle of 15º as a result of extreme flattening and gliding on the banding planes.

This effect is more pronounced by stronger pressure solution. The regional metamorphism is characterized by the appearances of isogrades of synkinematic biotite and static growth of garnet. It follows a deformation D2, with dominant subvertical NW-SE folds and also more local horizontal drag folds, and a M2 metamorphism (Zone IV) with the static growth of staurolite, biotite and andalusite porphyroblasts (Toselli and Rossi de Toselli, 1973, 1984; Willner, 1990).

The M1 and M2 metamorphic events, were dated by Rb-Sr (Bachmann and Grauert 1987a,b) who used thin whole rocks slabs and garnet, that provided two metamorphic events. The oldest, between 580 and 540 Ma, in which the main banding was formed by metamorphic differentiation and almandine crystallization took place. At 470 to 435 Ma, the rocks were again metamorphosed and the already existing almandine in samples from higher metamorphic terrain re-equilibrated their 87Sr/86Sr ratios with that of the whole-rock systems. The Ordovician oldest metamorphic age of 470 Ma, is supported by a whole rock Rb/Sr isochron in the Loma Pelada granite in Tafí del Valle, obtained by Sales de López et al., (1997).

In the Sierra de Ancasti predominate banded schists and gneisses of middle metamorphic grade known as the Ancasti Formation. On the western edge and partially in the south of the sierra, the banded schists of the Ancasti Formation pass to migmatic gneisses of the Portezuelo Formation.

This area of high metamorphic grade and migmatites, extend further to the west in the sierras of Graciana, Colorado, Fariñango and eastern edge of the Sierra de Ambato (Toselli, 1983, 1984).

The conspicuous banding is the dominant element of deformation. It can be interpreted as a second schistosity, which originated by extreme transposition of older structures into a N-S direction.

The characteristic banding is deformed by numerous closed minor folds of N-S strike and few megastructures, which are the effects of a third deformational phase. A fourth deformational phase is documented by a weak schistosity as well as by open NW-SE-trending minor folds. The fifth deformational phase is a wide spread cataclasis affecting all rocks to some extent, which culminates in some narrow zones with mylonitization and subsequent cataclasite formation with pseudotachylytes. Under a sixth deformational phase all faults and differential uplift stages are summarized, which already started in the Carboniferous and culminated in the older Quaternary giving the actual shape to the sierra.

Parallel to the deformational phases a number of phases of metamorphic overprint can be distinguished. No mineral relics were found that are related to a supposed first phase. The second phase of metamorphism probably overprinted the whole area in equal intensity in greenschists facies, but did not reach the range of garnet. The third metamorphic phase was by far the strongest one (5 – 5.8 Kbar, 500º - 670ºC) reaching the anatexis on the western edge of the sierra. Typical isogrades run N – S; Willner (1983b) recognizes three prograde metamorphic zones: the almandine zone; the cordierite, staurolite and andalusite zone; and a cordierite, potassium feldspar, sillimanite, associated with migmatization zone.

A fourth metamorphic phase led to recrystallization of micas and growth of chlorites. A fifth metamorphism is entirely dynamic coming up to the formation of pseudotachylytes locally.

Summarizing, the metamorphism was more controlled by temperature than by pressure.

The absolute ages data can clearly be correlated with the relative dating of the metamorphic deformational and intrusive events. The oldest data derived from an uncertain Rb-Sr isochrone of metamorphic rocks with values of 535.7±42.4 Ma for the metamorphism M1-M2 which produced the banding of the schists (Knüver, 1983). They document Middle to Upper Cambrian metamorphic events, which can be correlated with the deformational phases D1 and D2. An homogenization age of 472±26 Ma is quite certain and document the climax of M3 metamorphism and migmatization after the third deformational phase (Knüver, 1983). Numerous biotite ages (K-Ar and Rb-Sr) between 400 and 420 Ma in metamorphic and granitic rocks, document the end of the fourth metamorphic phase. All pre-Carboniferous granites were weakly affected by the fourth deformational phase. It can be clearly seen that deformation, metamorphism and magmatism in the Sierra de Ancasti are of a Paleozoic age with a climax in the Ordovician.

Other early Ordovician regional tectonic events are so called megastructures. The Tafí Megafracture defined by Baldis et al. (1975), is developed in a NNW direction and forms the boundary between the Cumbres Calchaquíes in the northeast and the Sierra de Aconquija in the southwest. It continues in the Sierra de Quilmes to the west of Santa María valley as the Chusca fault. The Tafí has been an active fracture since early Paleozoic times (Willner, 1990). Along this lineament, a series of calcalkaline granitoids was emplaced in low-to medium-grade metamorphic rocks (Toselli et al., 1989).

Another Megafracture crosses the south of the Sierra de Ancasti (NW-SE). Along this lineament the granitoid series of La Majada was intruded (Toselli et al., 1983).

The eastern and western wedges of the Sierras de Ambato and Aconquija show deeper tectonicmetamorphic levels respectively, with the gradation from banded schists, to gneisses and migmatites (González Bonorino, 1951a, b). In the future, when the knowledge of the tectonic-metamorphic evolution of these regions were improved and as more isotopic data obtained, the Ordovician metamorphism, documented in Cumbres Calchaquíes, and in the western center region of the Sierra de Ancasti, will be regionally more important.

In the Sierra de Quilmes the outcrops of crystalline basement predominate on the magmatic intrusions. The basement was lithologic described by Toselli et al. (1978), but up to day very little is known over its tectonic-metamorphic evolution and ages. To the south of Colalao del Valle, schists and gneisses with medium metamorphic grade crop out, while to the NE, between Colalao del Valle and Cafayate rises up a block from deeper levels (the Chusca Cenozoic fault) constituted by granulitic gneisses and migmatites (Rossi de Toselli et al., 1976), Rapela, 1976, Toselli et al., 1978, Rossi et al., 1987) whose ages are possibly Proterozoic, but the current data of Sm/Nd give exhumation and/ or cooling ages between 442 to 412 Ma (Becchio et al.,1999; Lucassen et al., 2000).

The Cafayate granite (Rapela, 1976) is partially intrusive in this high grade basement, but the cupola contains roof pendants of schists of supracrustal levels, which underwent thermal metamorphism, with development of cordierite megablasts. Rapela et al. (1982) obtained an Rb/Sr isochrone of 475 Ma. This granite age also corresponds to the contact metamorphism.

The Central belt

In the Central Belt, the granitic batholiths represent more than 80% of the granitoids outcrops that intruded in metaturbidites of low to medium metamorphic grade. These geological relationships are characteristic of the Faja Eruptiva Oriental de la Puna, and the Batholithic Central Zone of the Sierras Pampeanas (Toselli et al., this volume). The host rocks of most of these granitoids are scarce and are exposed frequently as roof-pendants. The regional metamorphism varies from low- to medium grade, but hornfels are frequent as xenoliths in granites or as roof-pendants. Here will be considered the better know examples.

In the Sierra de Velasco, the host rock is exposed in discontinuous outcrops to the east margin of the sierra. It consists of strong deformed, supracrustal turbidites, of low metamorphic grade, mainly phyllites, in tectonic contact relationship with the granites. The host rocks are not yet dated, but they are compared with the Upper Proterozoic-Lower Cambrian, Puncoviscana Formation.

High metamorphic grade hornfels, with cordierite-andalusite-sillimanite-potasssium feldspar paragenesis had been found in three localities; one in the north tip of the sierra (Rossi of Toselli et al., 1997); another in the southwestern margin of the sierra, in the Quebrada de la Puerta (Bellos et al., 2002), and toward the south, in the Bolsón de Paluqui, Coira et al. (1968) described low grade metamorphic rocks and high grade hornfels. The central part of the sierra consists of undeformed, mainly porphyritic, medium to coarse grained, two mica-bearing granitoids. The west margin of the sierra contains several NNW-trend belts of medium to coarse grain deformed orthogneiss. The UPb SHRIMP data of 479 and 481.4 Ma from zircon cores in two granitoids, were obtained by Rapela et al. (1999, 2001).

Many granitoids in the Sierra de Velasco are variably deformed with development of NNW-SSE trending shear zones especially the so called “Faja Milonítica TIPA”, defined by López and Toselli (1993) which spreads further runs for its western flank, NNW-SSE and other, parallel to its that runs along the Quebrada de La Rioja-Cerro La Cruz, which are part of the so called. The western block is formed by a porphyritic orthogneiss, with fabric elements of non coaxial deformation, with S/C foliations, well developed stretching lineations and asymmetric porphyroclasts that evidence thrust toward the west as kinematic indicative (López et al., 1996). They contain microcline megacrysts from 5 to 10 cm, in a plagioclase, microcline, quartz, biotite matrix in higher proportion that muscovite, garnet, zircon and apatite. In the layers of white mica, it is developed kyanite and sillimanite. The deformation occurred under conditions of almandine amphibolite facies (Rossi et al. 1999). This deformation zone extends toward Antinaco and great part of the southwest of the mountain.

The zones of deformation in the Sierra de Velasco cannot be correlated temporarily, at least, with those of the mountains of Copacabana and Fiambalá for not being had opposing milonites of sufficiently fine grain, for the reequilibration of the system Rb/Sr or Sm/Nd closure (Höckenreiner et al., 2001, Söllner et al., 2001); it is only had the uncertain measured K/Ar that give variable ages between 460 and 340 Ma; and lately, with an age U-Pb SHRIMP of 481.4 Ma in zircons cores, interpreted as the crystallization age, and another in the borders of growth, of 469 Ma interpreted as age of the deformation, determined in a milonitized cordierite-bearing porphyritic granite (Rapela et al., 2001). On the other hand, the continuation of the Faja Milonítica TIPA in the Copacabana and Fiambalá ranges, where they meet milonites with enough fine grain, it allowed to Höckenreiner et al. (2001), to carry out isotopic analysis in the shears area. The conventional data U-Pb in zircons they give ages of 495 Ma, for strongly milonitized granites of the Sierra de Copacabana that are interpreted as intrusion ages, while data of Sm-Nd in whole rock- garnet they give values of 420–409 Ma, corresponding to the ages of the shears. In the Sierra de Fiambalá, three systems of ductile shears are recognized by Neugebauer (1996) and Neugebauer and Miller (1996), dated by Rb/Sr in minerals and whole rock, they gave ages among 392±8.7 Ma and 357.3±8.1 Ma (Söllner et al., 2001) with that the milonitization in the Faja Milonítica TIPA was developed between the Lower- and Upper- Devonian. It is considered that the maximum age of deformation for the granitoids of the Sierra de Velasco, would be of 469 Ma, since there is not other datations of shear zones, but it is only a punctual data In the Sierra de Capillitas, the relationships between the cordierite granite and their country rock are visible in the summit of the Cerro Negro, called this way by the color of the hornfels that they form partitions and roof pendants. The felsic phases of the granite is in contact with their country rock in the Quebrada de Villavil. Along the same one it appears the basement constituted by biotitemuscovite schists, partly gneissic, with low- to medium metamorphic grade, whose tectonometamorphic evolution, so much local as regional, it is not still known. The age of crystallization of the porphyritic granite, was determined by Pankhurst et al. (2000) in 469.6 Ma.

The frequency with which appear similar banded schists in pelitic-psammitic rocks, in Cumbres Calchaquíes, Ancasti and Aconquija it makes reasonable to assume that the one banded would have been developed during the Pampean metamorphism and the thermal metamorphism overlapping there would be been, in the Famatinian cycle.

In the sierras of Los Llanos-Malanzán-Chepes, the metamorphic basement is formed by supracrustals metaturbidites of Olta Formation (Caminos, 1979), and was described by Dahlquist and Baldo (1996) in a profile E-W through the Sierra de Chepes. This authors recognize a progressive metamorphism that culminates in the highest sector in the mountain, being reflected in the textural and mineralogical changes of the rocks that begin as spotted lines, they pass to schists, and with the increase of the size of the grain to gneisses that show textures with migmatitic aspect. The authors recognize a relictic schistosity S1 conserved in the biotite and cordierite porphyroblasts, and another dominant schistosity S2 with metamorphism M2, in which they distinguish a progressive zonation with index minerals: 1) biotite zone, 2) cordierite zone, and 3) andalusite/sillimanite, cordierite, potassium feldspar zone. The maximum age of the metamorphism M2 of Dahlquist and Baldo (1996) it was defined by crystallization ages U-Pb SHRIMP, in zircons in the range of 495–470 Ma, by the igneous Chepes Complex (Sims et al., 1998). Pankhurst et al. (1998) obtained similar ages.

The detrital zircons measurations in the Olta Formation, give concentrations among 560-600 Ma, correlated with the Pampean metamorphism of the Puncoviscana Formation, in the Eastern Belt (Sims et al., 1998). This would be confirmed by the existence of relictic D1 and S1, in the porfiroblasts (Dahlquist and Baldo, 1996). In schists and phyllites of Malanzán, Pankhurst et al. (1998) they obtained an errorchrone Rb-Sr 513±31 Ma that one cannot interpret as Cambrian metamorphism affected by the magmatic event.

The Famatinian belt

This belt, corresponds to the metamorphites and granites of the Sistema de Famatina that for its complexity, lithology, lie and geologic relationships, we have divided it in three zones: Western, Intermediate, and Eastern zones.

The Western Zone: the Espinal Formation, represented the country rock of the Cerro Toro and San Agustín plutons; while the Cerro Blanco Granite, doesn’t present metamorphic country rock, and the group is covered in its biggest part for silts of the Upper Paleozoic (Cisterna and Toselli, 1996; Cisterna, 2000).

The tonalite-granodiorite Cerro Toro is a late-tectonic intrusive in the Sierra del Espinal one to the west, while to the east, by effect of the differential exhumation during the Carboniferous age, and added then to the Andean tectonic, it is in structural discontinuity with volcaniclastic sedimentary rocks, of the Lower-Ordovician (Toselli et al., 1988, Saavedra et al., 1992, 1996.) Here they are very visible remarkable roof-pendants and xenoliths of country rock of the Espinal Formation (amphibolites, gneisses and migmatites) that they attain extensions of hundred of meters that were incorporate to the tonalitic magma and they show different degree of assimilation. The tonalite crystallization pressure, using the hornblende geobarometer (Hollister et al., 1987) it was determined in 6 Kbar by Rossi de Toselli et al. (1991). It doesn’t register here, Ordovician metamorphism.

Rapela (2000) determined an age U-Pb SHRIMP in zircon, of 529 Ma for a migmatite of the Espinal Formation, while the intrusives gives younger ages. The Cerro Toro tonalite for Rb/Sr gives an age in total rock in 456±14 Ma (Saavedra et al., 1992, 1996) and a gabbro of the same intrusive complex gave for U-Pb SHRIMP, 468±3 Ma (Rapela et al., 1999.)

The Intermediate Zone: It is formed by metasediments and metavolcanites corresponding to Gondwana land, in which intruded different plutons that producing contact metamorphism.

The Narváez pluton, the most northern area of the Sistema de Famatina, are intruded in volcanites and volcaniclastic sediments, with fossils of the Lower Ordovician, from the Arenig to the Caradoc. Rubiolo et al., (2002) obtained an age U-Pb in zircon of 485±7 Ma that would correspond to the age of the contact metamorphism.

In the summit of the Sierra of Famatina and their continuation to the south, in the mountains of Sañogasta and Vilgo, the plutons show epizone character and they are intrusives in metaturbidites of the Upper Precambrian – Lower Cambrian of the Negro Peinado/La Aguadita formation, of low metamorphic grade whose geochronologic age is not still known, and likewise they are scarce the data of its internal structure. In the summit of the range, like in the Cerro Negro Overo, and Cumbre Baya hills, the granite presents roof pendants, transformed in hornfels of fine grain, massive with cordierite- potassium feldspar- andalusite-sillimanite (Toselli, 1978; Rossi et al., 1997a and b). The Ordovician volcaniclastic sediments with fossils, are reduced outcrops in the Sierra de Famatina, with ages from the Lower Tremadocian to the Llanvirn.

Rapela et al. (1999) obtained in circones of a biotite-granodiorite of the Cerro Ñuñorco, an U-Pb age SHRIMP 484±5 Ma, considered as the maximum age of the contact metamorphism. The discontinuous deformation zones, always has cataclastic textures.

Eastern zone: In the Sierra de Paganzo, at southeasternmost of the Sistema de Famatina, the metamorphic rocks crop out as the roof cover of the plutons (Saal, 1993, Saal et al., 1996). Schists and gneisses predominate at de summit of the sierra and pass to schists in its eastern flank. An older S1 relict foliation was recognized, but it was overprinted by the strong thermal metamorphism. The septa and roof pendant consist of cordierite-biotite-potassium feldspar-plagioclase-sillimanite, high temperature-low pressure paragenesis, estimated in 650ºC±50ºC and 3.5±0.5 Kbar (Saal, 1993).

The Rb-Sr whole rock isotopic data allowed to estimate an age of 522±47 Ma for the metamorphic basement prior the thermal metamorphism, and another Rb-Sr biotite-whole rock yielded a 450.9 Ma which was interpreted as the thermal metamorphism or the cooling age (Saal, 1993).

The Sierra de Paimán is the easternmost outcrop of the Sistema de Famatina. It consists of mainly porphyritic coarse grain two mica granites. Scantily remainder of supracrustal metawackes with low metamorphic grade of host rock, are in tectonic contact to the granite.

Dark fine-grained hornfels appear in roof pendants and consist of cordierite, andalusite, potassium feldspar and sillimanite. (Rossi et al., 1997b). No isotopic data are available therefore, the age of the contact metamorphism is estimated equivalent to the age of the Ñuñorco granite of 484±5 Ma (Rapela et al., 1999).

The Sierra de Fiambalá located to the northeast of the Sierra de Famatina is partially correlated with the Famatinian Belt. It consists of metawackes, metapelites and carbonates considered as the equivalents of the Puncoviscana Formation (Neugebauer, 1996), in which the metamorphic grade increases from the west to the east from greenschists to amphibolite facies and migmatites. Pampean age is estimated for these sequences (Neugebauer, 1996; Neugebauer and Miller, 1996; Grissom et al., 1998). The last authors have recognized in the basement ages from Proterozoic to Pampean, and an Ordovician deformation event of 464±2 Ma which originated a NNW-SSE trending strong foliation. The thermobarometric data were obtained in calc-silicate paragenesis of the contact aureole of a metagabbro-norite intrusion with a peak of T and P of 800ºC and 7.5 kbar, respectively (Grissom et al., 1998).

A later Devonian ductile shear deformation affected de basement. Neugebauer (1996) recognized three NNW-SSE trending shear systems. The Rb/Sr mineral-whole rock data of 392±8.7 Ma and 357.3±8.1 Ma were obtained by Söllner et al., (2001) for these ductile shear zones.

The Western belt

This belt includes the Sierras Pampeanas Occidentales (from Caminos 1979). From north to south the following ranges belong to this belt: the western flank of the Sierra de Fiambalá, Sierra de Toro Negro, de Umango, Las Ramaditas, Maz, Valle Fértil - La Huerta and Pié de Palo. This crystalline basement was developed from psammopelitic sediments and carbonates, with important basic and ultrabasic intrusives and minor granites. The strong deformation and metamorphism overprinted the older structures and no allows to distinguish the different tectonic-magmatic and metamorphic events. Over this basement, rests of Cambrian to Devonian aged carbonatic platform of La Rioja, San Juan and Mendoza Precordillera outcrop. The geological interpretation of the spatial and contemporary carbonatic platform of the Precordillera and the magmatic Famatinian arc has been, and it is today a controversial subject. While a hypothesis, based mainly in biostratigraphic evidences (Vaccari, 1995, Benedetto, 1998) and limited data that suggest the Precordillera terrain is underlain by grenvillian-age basement rocks holds up a Laurentian affinity of the Precordillera (Dalla Salda et al., 1992; Varela y Dalla Salda, 1992; Mc Donough et al., 1993; Kay et al., 1996; Varela et al., 1996; Ramos et al., 1998; Baldo et al., 2001; Casquet et al., 2001); another much earlier model (Baldis et al, 1975) in which the Precordillera terrain was always part of Gondwana has been recently considered (Aceñolaza and Toselli, 1999; Finney et al. in press).

The first detailed studies of the structure and metamorphism of these sierras were accomplished by Kilmurray and Dalla Salda (1971a, b); Dalla Salda and Varela (1982, 1984) and Dalla Salda (1987).

The authors distinguished in the Sierras de Maz, Cerro Valdivia and the half- south of the Sierra de Pié de Palo three relative aged structural domains. The D1 older domain developed during a strong folding event, especially in the Sierra de Pié de Palo and the Cerro Barbosa, with plastic flow, tight and overturned folds. The WSW-ENE the fold axis dips vary between 30º and 60º and their axial planes strike NNW-SSE. Also steep faults and low-angle thrusting planes trend in a similar nearly E-W direction. The same old-trending structures were recognized in the Sierras de Córdoba and San Luis. Dalla Salda (1987) attributed this event to the Upper Cambrian.

The D2 domain, the most conspicuous one, has NNW-SSE trending, tight and west overturned folds. They are frequently associated with low-angle faults parallel to the fold axial planes and to the main shear belts (Dalla Salda, 1987). This domain is well developed in the Sierras de Valle Fértil (Mirre, 1971) and Pié de Palo. A metamorphic event syntectonic with D2, is the main metamorphic episode of the region. Synkinematic garnets, garnets grown over previous garnets, intersections of D2 schistosity on D1, and microfolding of D1 schistosity by D2, have been observed in the Pié de Palo, Barbosa, Valdivia and de Maz ranges. The D2 domain was considered related to the Famatinian (Ordovician) events (Dalla Salda, 1987).

The D3 structural domain is characterized by NNE-SSW trends. The structures are the product of a less intense tectonism than D2, and only 15% of the folds are tight flowing-types. The great majority are open folds locally accompanied by kink band sets and crenulation cleavage. In the Sierra de Pié de Palo this phase was characterized by a compressive event marked by tight folding and thrusting. Several pegmatitic stripes, narrow chloritic shear zones, mylonitic belts and crenulation cleavage planes cutting a prevoius D2 schistosity follow the D3 direction (Dalla Salda, 1987).

In the sierras de Maz and Valle Fértil, the peak of the metamorphism was reached during the D2 phase and estimated in the pelitic mineral paragenesis garnet-sillimanite-kyanite as belonging to the amphibolite facies - barrowian type - of intermediate pressures. In the Sierra de Pié de Palo, the western area grades from greenschists to amphibolite metamorphic facies, while in the eastern sector the metamorphism increases up to upper amphibolite and pass locally to granulite facies (Dalla Salda and Varela, 1984). Lately, the basement of the Sierra de Pié de Palo was divided in two metamorphic units: the Pié de Palo Complex (high metamorphic grade) and the Caucete Group (low metamorphic grade), separated by a regional structure known as the Pirquitas Fault (Ramos and Vujovich, 1995). The Pié de Palo Complex consists in part of an ophiolitic assemblage, represented by peridotites, meta-gabbros, greenschists, talc, serpentinites and amphibolites. East of the ophiolites, the central and eastern parts of the Sierra de Pié de Palo is composed of feldspar-biotitegarnet schists, gneisses, amphibolites and marbles. The Caucete Group consists of low grade metamorphic rocks. These were carbonate and clastic shelf sequences, now represented by marbles, quartzites and quartzose schists.

The earliest data of the Pié de Palo Complex are from Varela and Dalla Salda (1992), who obtained a Rb-Sr whole rock isochrone of 1027 ± 59 Ma. U-Pb ages of abraded zircons from magmatic and metamorphic rocks of the Pié de Palo Complex have indicated Middle Proterozoic ages of 1100 - 1000 Ma (Mc Donough et al., 1993). These authors conclude that the data represent an important tectono-metamorphic event of Grenville age.

In a southwestern sector of the Caucete Group, in a calc-pelitic milonitized schist and a migmatitic milonitized paragneiss, Casquet et al. (2001) obtained thermal-barometric data of high P/T, gradient with a pressure peak of 13 kbar, a clockwise P-T path and T of 600º C. U-Pb SHRIMP ages of zircon rims yielded 460 Ma, while the zircon cores provided ages of 1224-1032 Ma. The zircon rims ages were interpreted as the Ordovician collisional event (the Precordillera terrane against the Famatinian belt), while the zircon cores ages were interpreted as a Grenville inheritance (Casquet et al., 2001).

Ramos et al., (1998) obtained 40Ar/39Ar age spectra in hornblende and muscovite from the ductile shear belts mainly the Pirquitas thrust, that yuxtapose rocks of the Caucete Group and the Pié de Palo Complex. The plateau ages of 470-460 Ma for hornblendes is interpreted as the initiation of the collision of the Precordillera platform against Gondwana and the plateau ages of 394 Ma for muscovite indicate that the subsequent cooling lasted until the early Devonian (Ramos et al., 1998).

The structural and metamorphic evolution of the Sierras de Umango, Espinal, Las Ramaditas and Maz were studied recently by Fernandez et al. (2001, 2002), Vujovich et al., (2001), Porcher et al. (2001). These authors suggest that the outcroping rock units share the same deformation histories.

The first known episode occurred in upper amphibolite facies metamorphism and developed a planar foliation with a well defined NNW trending mineral stretching and lineations. The major shear zones indicate a transport to the north. The original fabric was affected by several tight folding events, that ductile fabric control (Fernandez et al., 2001). The two deformation events would be equivalent to the D1 and D2 domains of Dalla Salda (1987). The absolute age of the last deformation

is still uncertain but it could be attributed to Famatinian events, in the original sense of Dalla Salda (1987)

The T-P conditions of metamorphism were obtained in several petrographic different rocks. In metapelites of the Sierra de Maz, the values of T and P resulted 630º C and 5 Kbar, and 771º C and 6.3 Kbar, respectively, while in amphibolites of the Sierra de Umango were obtained 650ºC and 6 Kbar. These metamorphic conditions belong to a Barrovian Type of intermediate pressures that share both sierras in a same baric style (Porcher et al., 2001). The hornblende crystallization pressure in the Cerro Toro Tonalite, intruded in the Espinal Formation, of 6 kbar, similar to that of the host rocks, support these assumptions (Rossi de Toselli et al., 1991).

The Sierra de Umango radiometric data were determine by Varela et al., (1996). A whole rock Rb- Sr isochrone of 1030±30 Ma was obtained from an orthogneiss within an antiform core at the Quebrada de Juchi and a new zircon data by conventional U-Pb in the same rock yielded 1108±13 Ma (Varela et al., in press). The El Peñon granite is a deformed granitoid, intruded in garnet-bearing phyllites and schists of the greenschists facies, located at the west flank of the Cerro El Cordobés, a Rb-Sr isochrone of 469±9 Ma was obtained, which was interpreted as the deformation age (Varela et al., 1996) and a new determined zircon U-Pb age of 534±9 Ma is interpreted as the crystallization age (Varela et al., in press).

The central- east sector of the Sierra de Valle Fértil is composed by a metaluminous sequence of hornblende-biotite tonalites/granodiorites, and suites of troctolitic, noritic and hornblende gabbros intruded in high-grade metapelites, marbles and amphibolites. In two migmatitic samples, Rapela et al. (2001) acquired U-Pb SHRIMP data. The zircon rims provided 465.9±4.4 Ma and 466.5±7.7 Ma, which were interpreted as the anatexis age, and the zircon cores ages range from 500 to 2000 Ma which were interpreted as Gondwanan inheritances. There are not thermal-barometric data in central- east sector of the Sierra de Valle Fértil, but the predominance of cordierite-garnet-sillimantebearing diatexites and hypersthene-pleonast-bearing metagabbros (Mirre, 1971) indicate high temperatures and middle- to low pressures.

The Loma de Las Chacras to the southwestern of Sierra de la Huerta, was studied by Vujovich (1994) who recognized biotite-garnet-bearing gneisses and kyanite-sillimanite-garnet-bearing gneisses, which predominate with a 60% of the outcrop rocks. The T-P conditions of the metamorphism were estimated as intermediate and of barrovian type (Vujovich, 1994).

Baldo et al., (2001) interpreted the pelitic gneisses as migmatitic, and in a potassium feldsparquartz- plagioclase-garnet-biotite-bearing leucosome obtained 12.1 Kbar and 769 ºC and concluded that the Loma de las Chacras metasediments, were migmatized at high pressure.

U-Pb SHRIMP zircon data of the migmatites provided zircon rims ages of 463±2 Ma which were interpreted as the anatexis age, and zircon cores with a range of Mesoproterozoic to Cambrian ages, which were interpreted as Gondwanic inheritance.

Based on the U-Pb SHRIMP data from Casquet et al., (2001) and Baldo et al. (2001) the same authors placed the limit between the Precordillera terrain and autochthonous Gondwana along the present day Bermejo river.

Discussion and conclusions

The division of the Argentine northwestern basement in several metamorphic belts allows synthesize the distribution of the Ordovician tectonic-metamorphic events in relationship with the contemporary magmatic events, and with the structural level exposed by the Andean tectonics.

The Eastern Belt that encompasses the Cordillera Oriental (Puncoviscana Formation) with a few Middle Cambrian aged granitoids, the Sierra the Cachi, the south of Salta ranges, the North East, Center and North West of Tucumán ranges, that consists mainly of supracrustal turbidites of very low- to low metamorphic grade, which from North to South underwent a little increase of the metamorphic grade and the P/T ratio. They are mainly phyllites, that had developed from a S0 parallel cleavage to a recrystallization schistosity by transposition, and growth of layering with separated white mica and chlorite rich bands and quartz rich bands by pressure-solution mechanism.

The age of M1 metamorphism, range 560-540 Ma and there are not later events recorded.

In the Cumbres Calchaquies, Aconquija and Ancasti ranges, the D2 deformation phase produced a NNW-SSE to NNE-SSW trending strong isoclinal folding with complete transposition of older structures. The Pampean metamorphism M2 developed in deeper tectonic-metamorphic levels and caused the characteristic schists banding of the region whose age range 580 - 540 Ma.

The deformation phase D3 developed NNW-SSE trending tight folding, minor folding, great scale flexures and strong axial plane schistosity. The M3 metamorphism increased from low- to high grade with low-to intermediate pressures. It had developed mineral index zones of garnet, cordierite, andalusite and staurolite. The peak of the metamorphism was reached in the cordierite-potassium feldspar-sillimanite zone accompanied by synkinematic to postkinematic migmatization. The age of this metamorphism was poorly isotopic constrained in 470 Ma.

Shears belts and overthrusts are not significant during the Ordovician and later times in this region, since tectonic discontinuities among the metamorphic unities as those of the Sierra de San Luis (Sato et al., this volume) are not observed.

The development of regional megastructures during the Ordovician controlled several granitoid intrusions.

The Central Belt and central and eastern zones of the Famatinian Belt, included the Ñuñorco granite of the Sierra de Famatina, are characterized by the predominance of granitoids outcrops in relation to the host rocks. The country rocks consist of supracrustal turbidites of low- to middle metamorphic grade, whose ages are Pampean equivalent to the Puncoviscana Formation, supported by whole rock Rb-Sr and U-Pb in detrital zircon rims data. The maximal ages of the Ordovician metamorphism in contact aureoles and hornfels are defined by the crystallization ages of the plutons.

Several ductile shear belts that affect the granitoids and its contact aureoles are also characteristic of the Ordovician and Devonian times.

The western zone of the Famatinian Belt is constituted by biotite-hornblende-bearing tonalites and granodiorites that had intruded in the Espinal Formation. They represent the Ordovician magmatic arc, which continues in the Cerro Blanco at the southernmost of the Sistema de Famatina, in the tonalites and granodiorites at eastern flank of the Sierra de Valle Fértil and continues to the south in the western flank of the Sierra de San Luis.

The Western Belt that include the Sierras Pampeanas Occidentales and partiality the Sierra de Fiambalá was considered an allochthonous terrane known as “Occidentalia” or “Cuyania” of Grenville age, although in several localities were recognized several superimposed events: Pampean magmatism and metamorphism, Famatinian metamorphism and anatexis, Devonian and Carboniferous magmatism.

In the Sierra de Pié de Palo that has been the most dated range of the Sierras Pampeanas Occidentales with Grenville ages, was found a superimposed Ordovician tectonic-metamorphic event interpreted as the beginning of the collisional episode with a high pressure peak of 11 Kbar up to 13 Kbar. The 40Ar/39Ar recrystallization ages of hornblendes and muscovites in the ductile shear zones, show a wide spectrum of 470 Ma to 394 Ma that are interpreted as the beginning of the collision between the Cuyania terrane and Gondwana and lasted up to Devonian times.

In the Sierra de Valle Fértil were dated Ordovician metamorphic and magmatic events in gabbros, granites and migmatites.

The pressures from 11 kbar to 13 kbar obtained for the Ordovician metamorphism in the Sierra de Pié de Palo and Sierra de La Huerta are somewhat unexpected when compared with the pressures in the Sierras de Maz, Umango, and the western flank of the Sierras de San Luis and El Gigante which underwent strong Ordovician deformation although the pressures range from 5 Kbar up to 8 kbar.

An alternative hypothesis holds a Gondwanian origin, against a Laurentian affinity of the Precordillera terrain (Baldis et al., 1975; Aceñolaza and Toselli, 1999). Recently, newly acquired detrital zircon ages show evidences of an early Gondwanian provenance for the Precordillera terrain (Finney et al., in press).

In conclusion: The Eastern Belt records mainly Pampean metamorphism. The Central Belt and part of the Famatinian Belt have Ordovician tectonic-thermal events overprinting the Pampean metamorphism, with low P/T ratio and low to high temperatures. In the Western Belt the Ordovician to Devonian collisional tectonic-metamorphic events with intermediate to high P/T ratio overprinted older terrains, allochthonous or parautochthonous of Grenville-Sunsas age.

Acknowledgments. We wish to express our gratitude to the National University of Tucumán, project CIUNT, 2001-2003, and PICT 07-09686, as well as to F. G. Aceñolaza, for reading an early version of the manuscript and whose suggestions helped to improve it. D. R. Holgado is also aknowledged for the line drawings.

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Recibido: 2 de Octubre de 2002

Aceptado: 6 de Diciembre de 2002