Ordovician Magmatism of the Sierras Pampeanas, Sistema de Famatina and Cordillera Oriental, NW of Argentina

Alejandro J. TOSELLI1, Alcides J. SIAL2, and Juana N. ROSSI1

1 INSUGEO, Miguel Lillo 205, S.M. Tucuman, 4000, Argentina. E-mail: atoselli@cpsarg.com

2 NEG-LABISE, Dept. of Geology, UFPE, C.P. 7852, Recife, PE. 50,7432-970. E-mail: ans@ufpe.br

Abstract: ORDOVICIAN MAGMATISM OF THE SIERRAS PAMPEANAS, SYSTEMA DE FAMATINA AND CORDILLERA ORIENTAL. NW. OF ARGENTINA. In NW Argentina, composition, age and relationships of granitoids with country rocks allow the characterization of four groups of granitoids and belts: (1) Western, (2) Famatinian, (3) Central, and (4) Eastern. The Western belt granitoids evolved from protoliths that differ from rocks which constitute the three other belts. Each group of granitoids presents typological patterns that allow their classification according to the tectonic setting in which granitic plutons were formed.

The Western belt is formed by schists, gneisses, amphibolites and marbles at the amphibolite to granulite facies, with mineral associations that indicate intermediate P/T and constitute the country rocks for granitoids with different ages. This belt extends westward up to the Famatina system where a change in basement composition is observed.

The Famatinian belt (includes the Famatina System, the Puna Western Eruptive Zone, and Los Llanos ranges) encompasses metaluminosos to slightly peraluminous granitoids, basic-ultrabasic plutonic complexes and volcanics, related to an active continental margin, associated to a continental hybrid arc, whose basement is formed by sedimentary and low-grade metamorphic rocks, with low P/T.

The Central belt granitoids (including the Central Batholithic Zone and the Eastern Eruptive Zone of the La Puna) are late orogenic, porphyritic to equigranular, peraluminous to calc-alkalic, generated and emplaced as a response to a collision. Granitoids intruded low-grade metamorphic rocks and have produced contact aureoles that indicate low P/T.

The Eastern belt is characterized by metasedimentary sequences with synsedimentary alkalic volcanism, and greenschist-facies metacarbonates of the Puncoviscana Formation that grades up to granulite facies rocks. In these rocks, calc-alkalic to peraluminous magmatic epidote-bearing granitoids have been emplaced, controlled by deep-seated faults in response to continental relaxation phenomena.

The different P/T ratios allow to interpret the tectonic evolution within a scheme of diachronic paired metamorphic belts, under the influence of an active continental margin and strike-slip movements that controlled the magmatic and metamorphic events and whose current configuration results from the Andean orogenesis.

Resumen: MAGMATISMO ORDOVÍCICO DE SIERRAS PAMPEANAS, SISTEMA DE FAMATIMAS Y CORDILLERA ORIENTAL, NOROESTE ARGENTINO. Las composiciones y edad de los granitoides y sus relaciones con las rocas metamórficas, en el Noroeste Argentino, permite caracterizar cuatro cinturones: 1) Occidental; 2) Famatiniano; 3) Central; 4) Oriental. De los cuales el Occidental, evolucionó a partir de protolitos diferentes a los que constituyen los otros cinturones. Cada uno presenta patrones tipológicos que permiten su clasificación con relación al ambiente tectónico en el cual se han formado.

El Cinturón Occidental está formado por esquistos, gneises, anfibolitas y mármoles, en facies de anfibolita a granulitas, con asociaciones, que indican relaciones P/T intermedias y constituyen el entorno de rocas básicasultrabásicas y granitos, con diferentes edades. Se extiende hasta el oeste del Sistema de Famatina, donde se produce el cambio a un basamento diferente, debido a una zona de strike-slip.

El Cinturón Famatiniano (que incluye Sistema de Famatina, Faja Eruptiva Occidental de la Puna y Sierras de Los Llanos) está constituido por granitoides, metaluminosos a moderadamente peraluminosos, con complejos plutónicos básicos-ultrabásicos y efusivas relacionadas a un margen continental activo, asociado a un arco híbrido continental y cuyo basamento está formado por rocas sedimentarias y metamórficas de bajo grado, con bajas relaciones P/T.

El Cinturón Central, que incluye la Faja Eruptiva Oriental de la Puna, muestra caracteres, tardío orogénico, con granitoides porfíroides y equigranulares, de caracter peraluminoso calcoalcalino, emplazados durante el levantamiento post-colisional, en respuesta a la colisión. Los plutones intruyen en metamorfitas de bajo grado en las que producen fenómenos de contacto que indican bajas relaciones P/T.

El Cinturón Oriental, está caracterizado por secuencias metasedimentarias, con volcanismo alcalino sinsedimentario y carbonatos, en facies de esquistos verdes, de la Formación Puncoviscana que en forma gradacional llegan hasta facies de granulitas, en las que se emplazan plutones calcoalcalinos y peraluminosos, con epidoto magmático, controlados por fallas profundas en respuesta a fenómenos de relajación post-colisionales. L a s diferentes relaciones P/T permiten interpretar la evolución geotectónica dentro de un esquema de cinturones apareados diacrónicos, bajo la influencia de un margen continental activo y movimientos de strike-slip, que habrían controlado los eventos de magmatismo y metamorfismo.

Keywords: Ordovician Magmatism. Sierras Pampeanas. Geochronology.

Palabras clave: Magmatismo Ordovicico. Sierras Pampeanas. Geocronología.


The diversity of granitic magmatism, with variable petrographic and geochemical characteristics, took place mainly during the Ordovician in the basement of the Eastern Sierras Pampeanas, Famatina System, Puna and Cordillera Oriental. This basement has been formed of psamo-pelitic sediments, partly carbonatic and partly volcanic, during the Late Precambrian-early Cambrian. On the other hand, in the basement of the Western Sierras Pampeanas (Caminos, 1979) with a very restrict Ordovician granitic magmatism, metamorphic rocks developed from psamo-pelitic sedimentary protoliths, carbonates and basic-ultrabasic intrusions, of Sunsas/Grenvillean age, and correspond to a different geotectonic environment.

The scope of this study is to present a synthesis of the Ordovician granitic magmatism and to interpret the related geologic processes, under a scheme of paired metamorphic belts (Miyashiro, 1994). The particular characteristics of the magmatism in each region, together with the polymetamorphic mineral association, allow the interpretation of the development of the partially superposed belts, in response to tectonic processes that have taken place.

Regional geological characters

The Ordovician granoitoids play an important role in the Late Precambrian-early Paleozoic interpretation of the tectonic phenomena that were active in the different terranes that integrate the territory (Dalla Salda et al., 1992; Miller, 1999; Rapela et al., 1992, 1999; Pankhurst and Rapela, 1998; Ramos, 2000; Toselli et al., 2001; Aceñolaza y Toselli, 2000).

Protoliths for the metamorphic rocks permit to distinguish two different regions. An eastern region formed by clastic metasedimentary rocks and rare carbonatic levels, that extends up to the western flank of the Sistema de Famatina (Rossi et al., 1997), in which the Ordovician (Famatinian, Central and Eastern) granitoid belts developed and that Becchio et al. (1999) regard as parauthocton terrane and Pampean arc with Nd(tDM) ages between 1.36 and 2.2 Ga., that permit interpreting them as a Gondwanan authocton. The western region represented by he Western Sierras Pampeanas (Caminos, 1979) is constituted by metasedimentary rocks, carbonates and basic-ultrabasic rocks, that Becchio (2000) and Lucassen et al. (2000) denominated as Precordillera Exotic Terrane. Details for each of the mentioned belts are (Fig. 1) discussed below.

Famatinian Belt

In this belt, granitoids can be grouped in three zones: Western, Intermediate and Eastern zones.

The Western Zone (Toselli et al., 1996a, b). This zone is represented by the Cerro Toro, Cerro Blanco and San Agustín plutons, constituted by acid and basic rocks, with marked interaction between them. The equigranular, fine to coarse-grained, dominant tonalites, are associated to gabbros, granodiorites and granites, with hornblende, biotite and epidote. The granitoids are of calc-alkalic, meta- to peraluminous tendency (ASI = 0.8 -1.15) , while diorites and gabbros are tholeitic. A Rb- Sr isochron yielded an age of 456 ± 14 Ma for the Cerro Toro granite with r0= 0.70967. Rapela et al. (1999, 2001) determined for a hornblende-biotite gabbro, a U-Pb SHRIMP age (zircon) of 468 ± 3 Ma. and in a hornblende gabbro from the Sierra de Valle Fértil, an age of 486 ± 8 Ma. In this zone, we include the El Peñón pink granite of the Sierra de Umango (Varela et al., 2000), finely foliated with biotite, quartz, potassic feldspar and plagioclase, and with a Rb-Sr age of 469 ± 9 Ma. (r0=0.7110); as well as the Arenoso granite (Pontoriero et al., 2001), from Sierra de la Huerta, constituted by small monzogranite outcrops, with microcline, quartz and oligoclase, rare biotite and muscovite, besides garnet, apatite, allanite, opaques and epidote. They present calc-alkalic characteristics with ASI = 1- 1.08, which pose them as of volcanic arc. They have a wall rock with an age of 488 ±2.2 Ma.

The Intermediate Zone. This band is integrated by the Narváez, Ñuñorco, Sañogasta and Vilgo plutons with granodioritic, granitic and tonalitic compositions, meta- to peraluminous (ASI = 0.9 –1.2) (without primary muscovite, with biotite, hornblende and allanite in epidote), and intrusions of lamprophyric dikes. The epizonal emplacement is evidenced from the contact metamorphism produced in the Negro Peinado/La Aguadita Formations (Toselli, 1975; Rossi et al., 1997). Rapela et al. (1999) obtained in zircons from the Cerro Ñuñorco biotite granite, a U-Pb SHRIMP age of 484 ± 5 Ma and Loske and Miller (1996) determined and age of 459 Ma by U-Pb.

Rubiolo et al. (2002) determined, by U-Pb in the Narváez granite, an age of 485 ± 7.

The Eastern Zone. In this zone, the Fiambalá (west), Copacabana, Paimán, Paganzo, southwest of Velasco and Los Llanos ranges, in association with basic rocks, with meta- to peraluminous character (ASI = 0.9-1.4) and wall rocks vary from gneisses to metapelites. In the Sierra de Paganzo, Saal (1993) describes synkinematic granitoids, with age of 450 ± 7 - 456 ± 9 Ma and r0 of 0.709– 0.706. Rapela et al. (1999) obtained in a biotite granite from the Sierra de Chepes, U-Pb SHRIMP zircon age of 483 ± 5 Ma and in a two-mica leucogranite, 479±4 Ma.

The Puna Western Eruptive Zone (Palma et al., 1986), it is constituted by granitoids, besides volcanic associations of basic rocks (Coira and Koukharsky, in this volume) and basic-ultrabasic plutonic complexes. The granitoids plutons correspond to the Macón, Chiquivar, Taca-Taca, Chuquilaqui, Arita, Archibarca, Antofalla, south of Petaquilla, Batin and Campo Negro, and the continuation in Chile in the Cordón de Lila and Almeida range, with the Choschas and Tucúcaro granitoids, all them are intrusives in sedimentary or metamorphic basement with ordovician ages or older.

The mafic-ultramafic complex, Ojo de los Colorados, it is described by Zappettini et al. (1994), to the south of Pocitos saline. In this one they recognize sills of stratified gabbros of until 200 m of thickness, with alternation of clear and dark bands of 2 to 10 cm of thickness, made up of plagioclase and clinopiroxene or for piroxenites. The basal cumulates is represented for wehrlites constituted by olivine, with frequent clinopiroxene intercumulus and serpentine. These rocks present intrusives or tectonic relationships with the Tolillar formation (Tremadocian-Arenig). The chemical characters frames them inside the arc associations, with intermediate characteristics among tholeitic and calc-alkaline.

In the Macón range outcrop granodiorites, with variations to tonalites and granites with biotite and hornblende, together allanite, apatite, zircon and titanit. They are of thick grain inequigranular and gray to pink colors. The pluton is of shallow emplacement and it contains microgranular mafic enclaves, as likewise metavolcanic and hornfels roof-pendants (Koukharsky, 1988; Koukharsky et al., 2002). They are situated to the east of Tolar Grande, and its intrusive in Ordovician sediments with trilobites (Méndez, 1974; Turner and Mendez, 1979). To the north of this mountain they outcrop monzogranites in the Batin area that they produce biotitic hornfels in the Tremadocian (Koukharsky, 1988). Similar relations they have been observed in the southern end of the south plutón of Petaquilla, on Arizaro saline. The chemical analyses carried out by Damm et al. (1990) and Koukharsky et al. (2002) they indicate calc-alkalic characters with ASI = 0.97 to 1.15. In the diagram Rb vs. Y+Nb of Pearce et al. (1984) they are projected in the field of the Volcanic Arc Granitoids. The age Ar/Ar in hornblende is 482,7±7.8 Ma. (Koukharsky et al. 2002).

The diorite of the Pocitos Igneous Complex, defined by Zappettini et al. (1994) it was dated by Blasco et al. (1996) for K/Ar on amphibole and biotite in 494±20 Ma. and 470±17 Ma.

The Llullaillaco granite, has 12 for 4 Km and it is situated to the southeast of the Salines of Llullaillaco. It is a granodiorite constituted by quartz, plagioclase and potassium feldspar, with biotite and hornblende. It is of medium grain, of yellowish cream color and alkaline-calcic characters.

The Chuculaqui granite, has 12 for 2 Km and it is located to the south of the Llullaillaco granite. It is a granite constituted by quartz, plagioclase and potassium feldspar, with biotite. It is of medium grain, of pink color and alkaline-calcic characters.

The Taca-Taca pluton is constituted by two-mica granite, of medium grain to thick, in contact with a granite with biotite and hornblende, of medium grain to thick, that presents enclaves of tonalites and diorites of fine grain, and whole is crossed by aplite dikes and pegmatites (Koukharsky and Lanés, 1994). The granite has values ASI=1.04-1.07. An isochron Rb/Sr gave 469±4 Ma (Llambías and Caminos, 1986) and (419±16 Ma, Koukharsky and Lanés, unpublished).

From of the Cordillera de Calalaste the magmatic registrations are prolonged toward the south with a plutons of: Navarro formation 429±36 Ma (Blasco et al., 1996); Arita, Antofalla and Campo Negro, 419-418 Ma (Voss et al., 1996). Becchio (2000) he gives to know ages K-Ar or of exhumation, of the Ordovician granitoid plutons of the Southern Puna, corresponding to: the Salines de Hombre Muerto – Cerro Blanco: 452 Ma (hornblende) and El Jote–El Peñón: 446 Ma (hornblende).

The Arita granitoid is situated to the south of Arizaro salar. It has 18 for 5 Km. It is an intrusive complex constituted by a two-mica granite, and that is intruded by dikes of piroxene-granodiorite. It is of medium grain to thick, with similar chemical characters to Taca-Taca granite (Damm et al. 1990).

The Archibarca granite, outcrop to the west of the one on the way of Tolar Grande to Antofalla.

It is formed by a biotite monzogranite, pink of thick grain. An age K/Ar in biotite gave 485±15 Ma. (Palma et al., 1986).

The Central Belt

The granitoids of this belt are microcline phenocryst- and mica-rich, emplaced in low-grade metamorphic rocks, that allow to integrate, genetically, the Puna Eastern Eruptive Zone with the Central Batholitic Zone.

The Puna Eastern Eruptive Zone (Méndez et al., 1973). It is constituted by peraluminous plutons (ASI =1.17-1.20), deformed to augen gneisses, as the monzogranite to granodiorite of the Quebrada de Tajamar and the granodiorite of Salar de Diablillos. The Quepente granodiorite, with gneissic to hypidiomorphic texture, is composed by oligoclase, microcline, quartz and biotite, that

together with the Cobres granodiorite, of similar composition but without deformation, were intruded by the Churcal granite, constituted by a monzogranite with cordierite megacrysts, microcline, oligoclase and biotite and the Las Burras granite, with an age of 428 ± 17 Ma (Zappettini, 1990). The two-mica porphyritic Ochaqui granite does not exhibit deformation (Coira et al., 1999). Quenardelle (1989) described fine to medium-grained tonalites, with plagioclase, quartz, biotite, muscovite and apatite, together with garnet, tourmaline, titanite and epidote. They pass into leucosyenogranites and monzogranites that contain microcline, oligoclase, quartz, biotite, muscovite, apatite and zircon, together with tourmaline, sillimanite, epidote, opaques, titanite, andalusite and garnet (absent in the leucosyenogranites) and myrmekites in the monzogranites. They belong to the same zone of granitoids of Luracatao, Laguna Blanca and Agua de Las Palomas (422 ± 15 Ma). Likewise the foliated diorite of the Galán hill (417 ± 9.8 and 422 ± 9.8 Ma) and the granitoids of the Chango Real Formation (448-445 Ma), all were dated by K-Ar (Linares and González, 1990) and the Tacuil monzogranite of 472 and 473 ± 1 Ma was dated by U-Pb in monazite (Lork and Bahlburg, 1993).

The Central Batholitic Zone is integrated by the Fiambalá E., Vínquis, Zapata, Mazán, Velasco NE, Capillitas, Hualfín, Las Cuevas and Belén batholiths (González Bonorino, 1950; Turner, 1971; Caminos, 1979; Schalamuk et al., 1989; Rapela et al., 1992).

The granitoids are calc-alkalic character and peraluminous (ASI = 1.1-1.7), with K2O>Na2O.

They show porphyritic textures that grade to medium-grained, equigranular textures with compositions from biotite monzogranite to two-mica granodiorite. Felsic muscovite granites (porphyritic, aplites or pegmatites) are abundant. The mineralogy encompasses quartz, microcline and plagioclase with biotite, muscovite, cordierite, sillimanite, andalusite and garnet, besides tourmaline and fluorite (Rossi de Toselli et al., 1985). Toselli et al. (1992, 1996c) determined their conditions of formation (600º-700ºC and 2-4 kbar H2O).

Rb-Sr studies carried out in the Sierra de Velasco by Rapela et al. (1999, 2001) determined zircon U-Pb SHRIMP ages of 479 ± 3 and 481.4 ± 2.4 Ma for a two-mica porphyritic monzogranite and also identified a thermal-ductile event of 469 ± 3.9 Ma. K-Ar ages, in Mazán hill, are between 345 and 475 Ma and Vínquis hill, between 316 and 444 Ma (Linares and González, 1990).

In Capillitas, Linares and González (1990) determined K-Ar ages (biotite and muscovite) of 365 and 471 Ma. Rapela et al. (1999) reported ages of 470 ± 3 Ma by U-Pb SHRIMP in zircon, for a porphyritic two-mica monzogranite.

The Eastern Belt

This belt presents the largest extension of the metamorphic basement, with respect to granitoids and its development and comprenhends the Cordillera Oriental, Cumbres Calchaquíes, and Quilmes, Aconquija, Ancasti and Ambato ranges.

In the Cordillera Oriental, plutons of the Cachi Formation (Turner, 1961) constitute an intrusive activity axis in the Puncoviscana Formation, producing contact aureoles, constituted by spotted phyllites, schists, gneisses and migmatites of La Paya Formation (Aceñolaza et al., 1975).

The intrusive rocks form two groups. The first one (Galliski, 1983) is composed, from north to south, by the Cerro Bayo, Las Palomas, Aguas Calientes, Tres Tetas, Peñas Blancas, El Morado, Cachi, Libertador, El Brealito and La Angostura plutons. They are equigranular to porphyritic trondhjemites, formed by plagioclase and quartz, with biotite, muscovite, epidote, opaques, zircon, rutile, apatite and titanite. Their emplacement was after the main folding phase and they intruded in migmatites and gneisses with cordierite, microcline and biotite formed at the same time. The Al2O3 contents are greater than 15% at 70% of SiO2, that together with Rb/Sr ratios of 0.08 permit classify them as alumina-rich continental granitoids (Barker, 1979). The monazite U-Pb dating varies from 462 to 481 Ma and, in zircon, is of 453 Ma (Lork et al. 1989; Miller, 1999).

The second group, to the south of the previous group, formed by the Vallecito, La Paya, Las Cabritas, El Alto, El Hueco and Incauca plutons, composed by porphyritic tonalites and trondhjemites, whose wall rocks are gneisses and migmatites.The plagioclase is dominant, followed by quartz, biotite and muscovite, besides perthitic microcline, epidote, cordierite, opaques, tourmaline, garnet, sillimanite and zircon. The monazite U-Pb ages vary from 466 to 468 Ma and zircon age is of 488 Ma (Lork et al. 1989; Miller, 1999). Monazite from migmatites yielded an age of 467 Ma. This group of granitoids show higher SiO2, lower Al2O3, and similar trace element contents in relation to the previous group of granitoids.

Schön and Miller (1990) observed that the trondhjemites and granites differ in the major element contents and display similar rare-earth element (REE) contents. The former are a product of anatexis of pyroxene-plagioclase while the granites are differentiated melts from a volcanic granitic arc.

Galliski and Miller (1989) believe that the behavior of the REE point to anatexis from an amphibolite source from a subducted oceanic crust, while the granite crystallized from an intracrustal melting.

In the Sierra de Quilmes, the Cafayate pluton varies from gray to greenish tonalites and granodiorites to pink monzogranites, with variable amount of plagioclase, quartz and microcline, besides biotite and muscovite (Rapela, 1976). Chemical analyses evidence a calc-alkalic, peraluminous character (ASI =1.0-1.3). This pluton shows sharp contact with phyllites and schists, in its eastern margin, and developed contact metamorphism with cordierite and diffuse contact to west, with migmatites and gneisses. The Rb-Sr age determined by Rapela et al. (1982) is of 475 Ma, with r0 of 0.7051. Miller et al. (1991) obtained a Rb-Sr age of 507 ± 13 Ma with r0 of 0.7043.

The peraluminous, calc-alkalic tonalite of Las Viñas, northwest of Cafayate, is associated with microgranitic and pegmatitic dike-rocks composed of plagioclase, quartz and microcline, with biotite and epidote, that develop notable phenomena of contact metamorphism (Oyarzabal, 1988).

In the Cumbres Calchaquíes, Famatinan plutons were emplaced following the NW-SE lineament known as Tafi megafracture, generated during the D3 deformation (Toselli et al., 1989). The granitoids in this megafracture comprise the Infiernillo, Loma Pelada, Ñuñorco Grande, Angostura and El Indio plutons, that are fine- to medium-grained, equigranular and seldomly porphyritic.

Compositions vary from two-mica tonalites and granodiorites with magmatic epidote and, sometimes, titanite, to monzogranites with muscovite and garnet. The ASI index varies from 0.90 to 1.85. In the Loma Pelada pluton, Sales de López et al. (1997) obtained a Rb-Sr isochron age of 470 Ma with r0 of 0.7063.

The Chaquivil and Cuchiyaco plutons, located out of the Tafi megafracture, are discordant, medium-grained, two-mica granodiorite to tonalite, accompanied by aplites and pegmatites, with calc-alkalic character and index (ASI = 0.95-1.10), that have produced contact metamorphism in the wall rocks. The K-Ar ages for the Chaquivil pluton are between 456 ± 21 and 479 ± 9 Ma (Linares and González, 1990). Rapela et al., (1982) determined a Rb-Sr age for the Cuchiyaco granodiorite, of 446 Ma, with a r0 of 0.7066.

In the Sierra de Ancasti, granitoids of Sauce Guacho, Santa Rosa, Vilisman and Albigasta are two-mica plutons and vary from granodiorites to monzogranites, while El Alto is a differentiated granite, with muscovite and garnet. The La Majada pluton shows tonalitic to granodioritic composition and is associated with gabbros and diorites with magmatic epidote and the emplacement was controlled by NW-SE shearing during the D3 deformation. A whole-rock Rb-Sr isochron yielded ages from 470 to 440 Ma, with (87Sr/86Sr)0 of 0.7052-0.7121 (Knüver, 1983).

The geochemical parameters indicate meta-peraluminous character (ASI= 0.6-1.6) with K2O<Na2O and initial Sr ratio suggest contribution from the mantle or lower mafic continental crust in its genesis, that coupled with the presence of magmatic epidote evidence a rapid ascent, controlled by shearing (Saavedra et al., 1987, Sial et al., 1999).

Discussion and conclusions

The magmatic evolution of the NWArgentina was controlled by several geotectonic events of first magnitude that gave birth to the development of the Western, Famatinian, Central and Eastern belts that we report below.

During Tilcárica orogeny (Moya and Salfity, 1982) that marks the peak of the Pampean cycle (520 Ma.) produced the angular unconformity between the Late - Mid Cambrian Meson Group and the Puncoviscana Formation and a crustal thickening during the D2 deformation and low-grade M2 regional metamorphism. During this event, the Tipayoc, Cañaní and Tastil granitoids were generated, and intruded between 536 and 513 Ma.

The Guandacol orogeny (475 Ma) is related to an activation of the transcurrency phenomena on the western flank of Gondwana, with dextral displacement of the Western region in relation to the Rio de la Plata craton. This orogeny caused the D3 deformation and M3 metamorphism. The Ocloyica orogeny (450 Ma.) corresponds to a new displacement of the Western region, giving rise to the D4 deformation and M4 metamorphism. Both orogenies, Guandacol and Ocloyica, caused a large development of Famatinan granitic magmatism, in the Famatina system, Pampean Ranges, Cordillera Oriental and Puna. A deformational event (D5) took place between 420 and 409 Ma in the Central belt, probably in response to the amalgamation of the western and eastern regions.

The transcurrency phenomena and oblique subduction between the Western and Eastern regions, gave place to the Famatinan belt that generated a continental volcanic arc with tholeitic magmas, related to calc-alkalic and meta- to peraluminous, crustal granites that intruded Ordovician sedimentary and metamorphic rocks of the Espinal Formation, to the west of the Famatina system. In the middle part of this system, intrusive rocks are meta- to peraluminous, calc-alkalic biotite granites/ granodiorites, epizonally emplaced and developed pyroxene-bearing hornfels in the contact zones.

Towards the end of this event, peraluminous, calc-alkalic, granites, gabbros and quartz diorites were generated to the continent, and are interpreted as being of active continental margin (Saavedra et al., 1992). Peacock index (57-61) is similar to values found for circum-Pacific calc-alkalic magmatic arcs (Brown, 1982).

Resemble magmatic characteristics show the Puna Western Eruptive Zone, metaluminous to peraluminous, calc-alkalic to tholeitic, in association with basic-ultrabasic complexes, took place from 418 – 490 Ma.

In the Central belt, that encompasses the Eruptive Zone of the Eastern Puna, the granitoids exhibit K2O>Na2O and low Ca contents that allow distinguishing two suites. One of them is peraluminous and formed by two-mica granites with microperthite phenocrysts, and the other one, of the S-type, shows Al-silicates and micas. The granitoids display porphyritic textures and late orogenic characteristics, and have undergone a strong post-collisional uplift that resulted from crustal shortening, folding and regional metamorphism. These granitoids were probably responsible for the high-temperature metamorphism that resulted from geothermal gradients >50ºC.km-1 at low pressure, at depths <20 km. According to Miyashiro (1994), this happens in the crust when the intrusions are voluminous, at shallow depth. The ages of the granitoids vary from 479 to 458 Ma and those from a second pulse of granitic magmatism display ages from 444 to 413 Ma. A deformational event generated ortogneisses and mylonites at 365 to 318 Ma.

The granitoids of the Eruptive Zone of the eastern Puna are intepreted as associated to magmatic arcs related to subduction (Dalziel and Forsythe, 1985; Ramos, 1988; Rapela et al., 1992). Damm et al. (1990) regarded these granites as of the S-type, syn- to tardi-kinematic, of crustal origin and associated to a collisional orogeny. On the contrary, Davidson and Mpodozis (in Coira et al., 1982) assign them to an extensional, ensialic regime developed between the Arequipa massif, to the west, and the Brazilian craton, to the east. Miller (1999) interpreted the granitoids as originated from a transpressional collision. It is probable that they have been generated in the margin of the Arequipa- Antofalla massif, while the tardi to posttectonic granites of the central Batholithic Zone, have been generated from an ensialic, thickened crust for a rapid adiabatic decompression (Rossi et al., 2002). In the Eastern Belt, Puncoviscana Formation sedimentary rocks constitute the wall rocks for the Pampean and Famatinan granitoids, with metamorphism from greenschist to amphibolite and granulite facies, under low P/T conditions, corresponding to the Tilcárica orogeny, passing to medium P/T conditions that prevailed during the Guandacol and Oclóyica orogenies. The genesis and ascent of the granitoids were controlled by movements of crustal blocks, with faulting and shearing, in response to the interaction between the Western and Eastern regions. This is evidenced by the initial Sr ratios of the granitoids of the Tafi megafracture, that point to crustal- as well as mantle-derived magmas and bimodal associations (basic-acidic; e.g. Majada pluton; Miller et al., 1991).

The Chánica orogeny (355 Ma; Moya and Salfity, 1982; Ramos, 1988) marks the culmination of the Famatinan cycle, giving rise to deformation (D6), metamorphism (M5), collapse of the Eastern, and Western regions (Aceñolaza and Toselli, 2000; Ruiz et al., 2001) and the uplift of the Famatinan belt, by means of strike-slip phenomena. This orogeny also allowed the deposition of the sedimentary sequences of the Paganzo group and the generation of the Carboniferous granitoids, recognized in different belts.

The development of magmatic and metamorphic phenomena in the belts under consideration were coeval. In the Famatinan belt, the ages of the igneous rocks vary from 500 to 416 Ma, with an event of 515 to 495 Ma in the Sierra de Fiambalá. In the Central and Eastern belts, the granitic plutonism took place from 507 to 419 Ma, but the Eastern belt show also an older magmatic event represented by Tastil and Cañaní, with ages between 545 and 514 Ma, has been observed.

Acknowledgments. We wish to express our gratitude to the National University of Tucumán, project CIUNT, 2001-2003, CONICET-PIP n. 02573, as well as to F.G. Aceñolaza, for reading an early version of the manuscript and whoses suggestions helped improving it, and D. Holgado for drafting of maps and figures.


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Recibido: 10 de Septiembre de 2002

Aceptado: 7 de Noviembre de 2002