The Ordovician of Sierra de San Luis: Famatinian Magmatic Arc and Low to High-Grade Metamorphism

Ana María SATO1, Pablo D. GONZÁLEZ1 and Eduardo J. LLAMBÍAS1

Abstract : THE ORDOVICIAN OF SIERRA DE SAN LUIS: FAMATINIAN MAGMATIC ARC AND LOW TO HIGH-GRADE METAMORPHISM. The main features of the Sierra de San Luis basement were delineated during the Ordovician Famatinian orogeny, that involved the active SW Gondwana margin. The metamorphic and sedimentary rocks formed during previous processes (Pampean orogeny?) were affected by arc magmatism, NNE-SSW penetrative deformation, and low to high-grade regional metamorphism. Available isotopic data constrain the arc magmatism (pre- and synorogenic granitoids) between Middle Cambrian and late Ordovician.

Regional metamorphism occurred at variable crustal levels, associated with compressive deformation. The highgrade metamorphism was of Barrovian-type, with P-T conditions mostly in the range 5 - 7.5 Kb and 518° - 770°C. The duration of metamorphism was apparently longer for the high-grade (early to late Ordovician) than for the low-grade rocks (Middle to late Ordovician). After the main Ordovician events, the outlasting compression produced ductile shear zones and a thickened crust, that favored the production of postorogenic granitoids.

Along the collision-related Famatinian orogen of Sierras Pampeanas, the analysis of metamorphic conditions suggests that the compressive regime associated with terrane accretion was variable.

Resumen : EL ORDOVÍCICO DE LA SIERRA DE SAN LUIS: ARCO MAGMÁTICO FAMATINIANO Y METAMORFISMO DE BAJO A ALTO GRADO.- Las características más sobresalientes del basamento de la Sierra de San Luis fueron delineadas durante la orogenia Famatiniana del Ordovícico, que involucró al margen SO activo de Gondwana. Las rocas metamórficas y sedimentarias formadas durante los procesos previos (orogenia Pampeana?) fueron afectadas por un magmatismo de arco, una penetrativa deformación NNE-SSO y un metamorfismo regional variable de bajo a alto grado. Los datos isotópicos disponibles sugieren la actividad del arco magmático (granitoides pre- y sinorogénicos) entre el Cámbrico Medio y el Ordovícico tardío. El metamorfismo regional asociado a la deformación compresiva se produjo en niveles corticales variables. El metamorfismo de alto grado fue de tipo Barroviano, y sus condiciones P-T estuvieron mayormente en el rango 5 - 7.5 Kb y 518° - 770°C. El tiempo de duración del metamorfismo fue aparentemente mayor para las rocas de alto grado (Ordovícico Temprano a Tardío), que para las de bajo grado (Ordovícico Medio a tardío). Con posterioridad a los eventos mayores del Ordovícico, el efecto compresivo residual dio origen a zonas de cizalla dúctil y a una corteza engrosada, que facilitó la producción de granitoides posorogénicos. A lo largo del orógeno colisional Famatiniano de las Sierras Pampeanas, el análisis de las condiciones metamórficas sugieren que fue variable el régimen compresivo asociado a la acreción de terreno.

Key words: Famatinian orogeny. Sierras Pampeanas. Ordovician. Regional metamorphism.

Palabras clave: Orogenia Famatiniana. Sierras Pampeanas. Ordovícico. metamorfismo regional.

Introduction

Different proposals concerning Early Paleozoic geodynamic evolution of the Sierras Pampeanas and surrounding regions coincide that the Famatinian orogeny folows a collision model.

The most important tectono-magmatic processes related to this collision (e.g. Dalla Salda et al., 1992, 1998; Ramos et al., 1998; Astini, 1998; Pankhurst et al., 2000; Casquet et al., 2001) occurred during the Ordovician times, giving rise to the N-S belt of the Famatinian orogen. Within this collisional scheme, a Laurentian-derived terrane was accreted to the southwestern margin of Gondwana, and the Sierra de San Luis represents the locus of the magmatic arc, emplaced within a metamorphic environment that reached upper amphibolite facies. These features are consistent with the inner position of the Sierra de San Luis respect to the suture zone, where higher pressure metamorphism and deeper levels of arc magmatism (Baldo et al., 2001; Casquet et al., 2001; Castro de Machuca et al, 1996 ; Vujovich et al., 1996) are exposed.

The Sierra de San Luis is located in the southern part of the Eastern Sierras Pampeanas (Caminos, 1979), and the basement rocks evolved prior to the Upper Carboniferous – Permian sedimentary cover (Hünicken et al., 1981). The penetrative NNE-SSW Ordovician deformation and metamorphism (Barrovian type) overprinted the older geological features, making difficult the interpretation of previous histories. These old processes were attributed to the late Precambrian – early Cambrian Pampean cycle (Criado Roqué et al., 1981; Kilmurray and Dalla Salda, 1977; von Gosen et al., 2002) or even older events, as suggested by Sato et al. (2001b). After the main Ordovician orogeneny, the Sierra de San Luis was only subjected to localized shear zone deformation (and metamorphism), and post-orogenic (transitional to anorogenic) magmatism, that took place mainly during Devonian to early Carboniferous.

In this review we will focus the descriptions on the tectonic, metamorphic and magmatic events that occurred mainly during the Ordovician, in order to make regional interpretations. The timing of the events are constrained by isotopic ages of the igneous rocks, and less accurately by those of the metamorphic rocks. Former geological studies on Sierra de San Luis include those of Kilmurray and Villar (1981), Dalla Salda (1987), Ortiz Suarez et al. (1992), von Gosen and Prozzi (1998), Llambías et al. (1998), Sims et al. (1997, 1998); Ortiz Suárez (1999), Hauzenberger et al. (2001), among others. The compilation of the map of the Sierra de San Luis (Fig. 1) is based on the previous literature and the observations of the authors, and the metamorphic units are defined according to characteristics on lithology, structure, metamorphic grade and protolith. In order to differentiate the penetrative “Famatinian” (NNE-SSW) deformation from the “pre-Famatinian” (NW-SE) relict deformation, the subscripts “F” and “pF” will be used for the fabric descriptions.

Metamorphic complexes

Low and high-grade metamorphic complexes are in contact mainly by tectonic relationships, with development of N-S to NNE-SSW trending shear zones. These zones consist of protomylonites, mylonites, ultramylonites and phyllonites of low to high grades, that develop through several kilometers of length and tens of meters in width. Less frequently, the limits between the metamorphic units are transitional, especially among those of the lowest metamorphic grades.

The San Luis Formation - SLF - (Prozzi and Ramos, 1988; Prozzi, 1990) or Phyllite Group (von Gosen and Prozzi, 1996) is exposed in two belts of the central and western parts of the Sierra de San Luis. It consists of alternating phyllites and meta-quartzites with minor meta-conglomerates (Conglomerado Cañada Honda, Prozzi, 1990) and acid meta-magmatic rocks, interpreted as extrusive (Brodtkorb et al., 1984; Fernández et al., 1991) or intrusive (von Gosen and Prozzi, 1996; von Gosen, 1998a) rocks. The succession was characterized as a turbiditic sequence, and compared with the Upper Proterozoic to Lower Cambrian Puncoviscana Formation of the Northwestern Argenti-

Figura 1. Geological map of the Sierra de San Luis, compiled and adapted after Ortiz Suárez et al. (1992), Sims et al.(1997), Llambías et al.(1998), von Gosen and Prozzi (1998), and own observations.

na (Prozzi, 1990; von Gosen and Prozzi, 1998). This regional comparison was later supported by the U-Pb crystallization age of 529 ± 12 Ma obtained from two meta-magmatic layers (Söllner et al., 2000). The SLF is affected by a regional compressive D1 F deformation, resulting in a tight to isoclinal, upright to inclined F1 F folds with NNE-SSW trending axes (Sims et al., 1997; von Gosen and Prozzi, 1996, 1998; von Gosen 1998a, 1998b), associated with a penetrative axial-plane S1 F slaty cleavage. The metamorphic grade related to this deformation is low to middle greenschist facies. A second deformation event, refolding the F1 F folds, and associated with S2 F crenulation cleavage, is registered only at few localities (Sims et al., 1997; von Gosen and Prozzi, 1996, 1998; von Gosen 1998a, 1998b). Low pressure - high temperature metamorphic aureoles surround the pre-orogenic intrusive plutons and are both deformed together.

Although there is no direct isotopic dating for the regional metamorphism, the U-Pb ages of the pre-orogenic plutons give a maximum limit at early Ordovician for the metamorphism of the SLF.

The Micaschist Group - MG - (von Gosen and Prozzi, 1996) accompanies the SLF along the western and eastern borders of the central belt of this unit (Fig. 1), and a continuous structural and metamorphic gradation is mentioned among them (von Gosen and Prozzi, 1998). However, in the contact zone between the SLF and the eastern strip of the MG, a broad (up to 3 km wide) ductile shear zone was identified (Río Guzmán shear zone, Sims et al., 1997). To the west, two additional strips of MG are exposed: along the eastern margin of the La Escalerilla pluton and to the east of the Nogolí Metamorphic Complex. The protolith of this meta-clastic succession was characterized as a flysch-like sequence - comparable to that of the SLF (and also to the Puncoviscana Formation) - and hence as a higher metamorphic equivalent of the SLF (von Gosen, 1998a, 1998b). In the eastern margin of La Escalerilla Pluton, the MG is composed of an alternating succession of biotite-muscovite schists and metaquartzites, with pegmatites and few layers of amphibolites (von Gosen and Prozzi, 1996, 1998; von Gosen, 1998a, 1998b). Local migmatites are mentioned by von Gosen (1998a). The MG is affected by at least two deformational events (see details in von Gosen, 1998a). D1 F deformation is related to the folding of the clastic sequence into various types of F1 F folds (associated with a penetrative NNE-SSW trending S1 F foliation), equivalent to those of the SLF. D2 F deformation here is more widespread than in the SLF. S2 F axial plane cleavage developed in relation to small to large scale F2 F open anticline – syncline structures (with local crenulation and kink folds of m-scale). The metamorphic conditions during D1 F deformation reached the middle greenschist facies with local amphibolite facies, and continued through D2 F deformation (von Gosen, 1998a).

The MG strip along the eastern margin of the Nogolí Metamorphic Complex consists of biotite-muscovite-garnet schists and biotite-muscovite-staurolite-garnet (± kyanite) schists, with minor meta-quartzites (González, 2000a). Fabrics are comparable to the D1 F - D2 F deformations of the MG of the central part of the Sierra de San Luis, and the metamorphic overprint is of the highest grade reported for the MG within the entire Sierra, increasing from middle greenschist facies during D1 F up to middle amphibolite facies at D2 F.

Ar-Ar muscovite data from the Río Guzmán shear zone yielded a rising age pattern between 351 and 362 Ma (Sims et al., 1998).

The Nogolí Metamorphic Complex - NMC - (Sims et al., 1997) is exposed in the westernmost part of the Sierra de San Luis (Fig. 1), and was also referred as Western Basement Complex (von Gosen and Prozzi, 1998). It is juxtaposed to the SLF and the MG along a regional scale ductile shear zone (El Realito - Río de la Quebrada shear zone), where granitoid plutons are emplaced. The NMC is composed of micaschists, meta-quartzites, paragneisses and migmatites, with minor orthoamphibolites, komatiites to high-Fe tholeiitic basalts, marbles, calcsilicates and banded iron formation (Ortiz Suárez, 1999; González, 2000a; González et al., 2002a). Two sets of structural orientations were identified (González and Llambías, 1998; von Gosen and Prozzi, 1998; González et al., 2002a): (1) Remnant NW-SE trending structures (S0 pF to S3 pF) attributed to pre-Famatinian events. (2) Penetrative NNE-SSW trending structures ascribed to the Famatinian events. The pre- Famatinian deformational events are complex and their timing is not yet well defined. These older fabrics do not appear in the SLF and MG, but are recognized in the Pringles and Conlara Metamorphic Complexes. Within the NMC, the multiply deformed relics of the NW-SE fabric were re-folded and re-orientated to the NNE-SSW Famatinian trend of strike, and the high-grade metamorphic rocks (amphibolite plus local granulite facies) were re-metamorphosed to a new amphibolite facies. The PT conditions of Famatinian metamorphism were assessed using several geothermometers and geobarometers (González, unpublished data) and were determined in various rock types. The obtained data are 518° - 612°C and 5 - 7 Kb for a sillimanite-garnet paragneiss, 636° - 760°C and ~5 Kb for biotite amphibolites, 589° - 633°C and 6.8 - 7.2 ± 0.4 Kb for garnet amphibolites and 652° - 688°C and 7.3 - 7.5 ± 0.4 Kb in garnet-clinopyroxene amphibolites. The Famatinian evolution of the NMC was accompanied with several NNE-SSW trending, E or W dipping low to high grade shear zone development, that continued up to Devonian times.

The deposition of the original volcanic-sedimentary sequence and its structural and metamorphic evolution started at some pre-Famatinian times and continued through the Famatinian events (Sims et al., 1997; González and Llambías, 1998; von Gosen and Prozzi, 1998; González et al., 2002a). The presence of BIF and komatiites whitin the same sequence, and the Early Mesoproterozoic Sm-Nd isochron date of the mafic to ultramafic rocks (Sato et al., 2001b) might be evidences supporting the existence of Precambrian protoliths in the Sierras de San Luis.

Scarce age constraints on the Ordovician metamorphism include monazite datings by conventional U-Pb (458 ± 3 Ma) and chemical Th-U-Pb (470 ± 15 Ma) methods for the same sillimanite-garnet paragneiss used for P-T calculations, Sm-Nd whole rock and mineral isochron (445 ± 21 Ma) and Ar-Ar plateau ages (476 to 457 Ma, amphiboles) from amphibolites (González et al., 2002a). Another K-Ar amphibole date comes from an amphibolite of this western region, with 452 ± 23 Ma (Ortiz Suárez, 1999).

Ductile shear zones are particularly conspicuous within this complex (Fig. 1), and their relationships with the post-orogenic plutons suggest their long-lasting activity accompanied with retrogression, through Ordovician to Devonian times. From the area of Nogolí, K-Ar dates between 414 and 364 Ma were obtained from mylonite zone biotites (Sato et al., 2001a, and unpublished data of the authors).

The Pringles Metamorphic Complex - PMC - (Sims et al., 1997) is partly equivalent to the Eastern Basement Complex of von Gosen and Prozzi (1998). It is exposed in the central part of the Sierra de San Luis between two belts of the MG (Fig. 1). To the west, the PMC is juxtaposed to the MG along a mylonite zone steeply inclined to the east, whereas the eastern boundary is masked by granite and pegmatite intrusions (von Gosen and Prozzi, 1998). The PMC is composed of pelitic and psammitic schists and gneisses with sillimanite-garnet-biotite assemblage (± cordierite ± spinel), orthogneisses and minor meta-quartzites, amphibolites and calcsilicates. All these rocks are weakly to densely injected by granitic, pegmatitic and aplitic veins and dykes on different scales (Sims et al., 1997; von Gosen and Prozzi, 1998). The main structures are tight F2 F folds associated with N-S to NNE-SSW trending and steeply inclined to subvertical S2 F foliation overprinted to an older fabric probably comparable to the pre-Famatinian structures of NMC (von Gosen and Prozzi, 1998).

According to these authors, prior to and after the compresive D2 F event, the rocks were migmatized by granitic melts, and then affected by a F3 F folding. Prograde amphibolite facies metamorphism accompanied the D2 F - D3 F events (Sims et al., 1997, 1998; von Gosen and Prozzi, 1998). Peak granulite metamorphic conditions are recorded within the thermal aureole of the mafic to ultramafic intrusions (see below, Igneous Activity), particularly in the Virorco and Las Aguilas areas (González Bonorino, 1961; Sims et al., 1997; Hauzenberger et al., 2001). Ductile shearing affected the metamorphic and mafic to ultramafic rocks at retrograde amphibolite to greenschist facies conditions (Hauzenberger et al., 2001; Brogioni and Ribot, 1994). Three metamorphic stages were quantified by Hauzenberger et al. (2001), at 570° – 600°C and 5 – 5.7 Kb, 740° – 790°C and 5.7 – 6.4 Kb, and 590° – 650°C and 5.4 – 6.0 Kb, respectively for regional metamorphism, local aureole metamorphism, and metamorphism associated with shearing. The regional N-S trending ductile shear zone, affecting the belt of the mafic to ultramafic rocks and extending along more than 100 km, was identified as the La Arenilla mylonite zone (Ortiz Suárez et al., 1992), and described by von Gosen and Prozzi (1998).

Close to the eastern border of the northern segment of this mylonite zone, P-T conditions of 525° - 774°C and 3.7 - 7.6 Kb (garnet amphibolites) and 650° - 774°C and 5.0 - 9.5 Kb (biotite-garnet gneiss) were computed within the PMC by Ortiz Suárez (1999), and related to peak stages of Famatinian metamorphism.

The U-Pb SHRIMP data of 460 – 450 Ma from zircon rims and monazites from the garnet sillimanite gneiss of the PMC were interpreted as the timing of the regional metamorphism (Sims et al., 1998). These dates are perhaps documenting the retrograde rather than the prograde stage of metamorphism, because of the difficulty in dating the prograde high-grade metamorphism (Foster et al., 2000). Zircon core dates with peak distribution through Neoproterozoic to early Cambrian were considered as representing the Pampean processes in the source region. Another K-Ar amphibole date from an amphibolite of the PMC is 466 ± 23 Ma (Ortiz Suárez, 1999). The rocks affected by the La Arenilla mylonite zone yielded moscovite Ar-Ar dates of 366 ± 2 Ma in the central part and 375 ± 1 Ma in the southern part of the Sierra (Sims et al., 1998).

The Conlara Metamorphic Complex - CMC - (Sims et al., 1997) is exposed in the eastern part of the Sierra de San Luis (Fig. 1), and includes the Las Aguadas Metamorphic Complex described by Ortiz Suárez (1988) in the northeastern region. von Gosen and Prozzi (1998) suggested a continuous structural and metamorphic gradation from the MG. The CMC consists of pelitic and psammitic biotite-muscovite-garnet-sillimanite (± tourmaline ± chlorite) schists and pelitic and psammitic biotite (± garnet ± sillimanite) gneisses. These rocks show a metamorphic differentiation layering and various generations of granitic to pegmatitic injections, allowing local migmatite formation.

Minor amphibolites, marbles and calcsilicates are also mentioned (Llambías and Malvicini, 1982; Delakowitz et al., 1991, Brodtkorb and Pezzutti, 1991). At least two deformational events were identified within the CMC (Kilmurray, 1981, 1982; Kilmurray and Dalla Salda, 1977; Ortiz Suárez, 1988; Ortiz Suárez and Sosa, 1991; Sims et al., 1997; Llaneza and Ortiz Suárez, 2000). D1 deformation is related to the symmetric to asymmetric F1 folding with E-W to NW-SE trending axis, associated with a non-penetrative S1 schistosity variably dipping to the north or south (Kilmurray, 1981, 1982; Kilmurray and Dalla Salda, 1977). This orientation of D1 fabric elements is comparable to the S0 pF - S3 pF of the NMC in the western region. The penetrative D2 F deformation refolded the F1 folds around tight F2 F folds with N-S to NNE-SSW trending axis, whereas S1 was crenulated by NNESSW trending, steeply E or W dipping S2 F foliation (Kilmurray, 1981, 1982; Kilmurray and Dalla Salda, 1977; Ortiz Suárez, 1988; Llaneza and Ortiz Suárez, 2000). Open F3 F folds with NE-SW trending axis is then restricted to localized areas near fractures (Ortiz Suárez, 1988). Peak metamorphic overprints reached amphibolite facies conditions during the D1 deformational event, whereas D2 F was accompanied with retrogression at greenschist facies (Kilmurray, 1981, 1982; Sims et al., 1997).

Available K-Ar Ordovician and Siluro-Devonian dates from this complex (455-410 Ma, Llambías and Malvicini, 1982; 430-397 Ma, López de Luchi et al., 2002) are difficult to interpret in relation to peak metamorphism and subsequent shearing or cooling processes.

Igneous activity

The metamorphic complexes of the Sierra de San Luis were affected by felsic and mafic to ultramafic plutonism, as well as by mafic to ultramafic volcanism. Felsic magmatic rocks emplaced at different stages during the Famatinian orogenic cycle were defined as pre-, syn-, and post-orogenic granitoids, with respect to the main Ordovician deformation event (Llambías et al., 1998; see also Llambías et al., 1991 and Ortiz Suárez et al., 1992). The ages of the pre- and syn-orogenic granitoids range between Late Cambrian and Middle Ordovician, whereas the post-orogenic group is mainly Devonian to Early Carboniferous.

Felsic Magmatic Rocks

The pre-orogenic granitoids crop out mainly in the western region of the Sierra, within the NMC, the SLF, in the transition between SLF and MG, and along the NE-SW trending ductile shear zones that separate the NMC from the SLF and MG (Fig. 1). The plutons share the penetrative deformation of the country rocks, having intruded the already multiply deformed rocks of the NMC (Llambías et al., 1996a) and also the still undeformed sedimentary rocks of the SLF that were later transformed into phyllites (Sato et al., 1996; von Gosen, 1998). According to the most abundant rock types, they were classified into Tonalite and Granite Groups, and represent the arc magmatism emplaced prior to the main Ordovician deformation (Llambías et al., 1998 and references there in). The Tonalite Group includes Bemberg, Las Verbenas, Gasparillo, El Realito and Tamboreo plutons (Fig. 1). Medium-grained tonalites, with diorite, quartz gabbro, granodiorite and monzogranite facies, have a magmatic arc calc-alkaline signature (Sato et al., 1996). The primary shapes of the plutons and their contact aureoles were modified by the subsequent deformation and greenschist facies metamorphism, resulting in highly sheared borders and heterogeneously foliated inner zones. U-Pb SHRIMP zircon ages from El Tamboreo and Bemberg plutons indicates crystallization at 470 ± 5 Ma and 468 ± 6 Ma respectively (Sims et al., 1998; Stuart-Smith et al., 1999).

However, a secondary peak at 496 ± 8 Ma in the SHRIMP data from Bemberg may be interpreted as an alternative crystallization age, with varying degrees of Pb loss at younger times (Stuart-Smith et al., 1999). This alternative can be consistent with the Rb-Sr isochron age of 512 ± 16 Ma for the same pluton (Sato et al., 1999).

The Granite Group consists of a few granitic bodies that share the same structural features as the Tonalite Group. Brogioni et al. (1994) described a NE-SW elongated garnet-bearing monzogranite pluton intruded into the El Realito tonalite and deformed together. The Río Quinto muscovitebiotite monzogranite is composed of a few lens-like sheets, parallel to the country rock foliation, and is cut by shear zones (Carugno Durán et al., 1992). The Río Claro granodiorite comprises at least three N-S trending lens-shaped plutons, elongated parallel to the foliation of the country rocks (Ortiz Suárez, 1999). It truncates the multiply deformed rocks of the NMC, and both are then heterogeneously sheared. The composition is mainly two mica and garnet-bearing leucogranodiorite with minor transitions to monzogabbros and gabbros (González, 2000a). The Pantanos Negros granitoids share the same composition and structural features as the El Realito and Río Claro plutons, and are also intruded into the multiply deformed NMC, suggesting that they belong to the Granite Group (González, 2000a). The 52 km long, monzogranitic to granodioritic La Escalerilla pluton is the largest intrusive body, and its eastern margin is dominated by a shear zone that juxtaposes it to the MG. At the western side, the pluton intruded the still undeformed sedimentary rocks of SLF, the Las Verbenas Tonalite and the multiply deformed NMC. The pluton was later deformed together with all these country rocks (Llambías et al., 1998; von Gosen and Prozzi, 1996, 1998; von Gosen, 1998b). The conventional U-Pb zircon ages of Río Claro (490 ± 15 Ma) and La Escalerilla (507 ± 24 Ma) granites (von Gosen et al., 2002) indicates that the Ordovician arc magmatism had already started since late Cambrian times. For the Devonian (403 ± 6 Ma) U-Pb SHRIMP age obtained from a porphyritic phase of the southernmost part of La Escalerilla pluton (Sims et al., 1998; Stuart-Smith et al., 1999), von Gosen et al (2002) suggest the possibility of representing a post-orogenic emplacement within the pluton.

Another metamorphosed small felsic intrusive, spatially related to the ultramafic Las Aguilas intrusion (PMC) was dated at 484 ± 7 Ma (U-Pb SHRIMP, Sims et al., 1998).

From the Rodeo Viejo tonalite pluton, Ordovician K-Ar amphibole and biotite dates were obtained (466 ± 23 Ma y 452 ± 23 Ma, Ulacco, 1997; Ortiz Suárez and Ulacco, 1999). However, the non-deformed and discordant features of these plutons (Ortiz Suárez, 1996) might suggest a postorogenic emplacement (Ortiz Suárez and Ulacco, 1999).

The syn-orogenic granitoids are located in the central part of the Sierra de San Luis and are emplaced within PMC, MG and CMC (Fig. 1). The plutons included in this group are Paso del Rey, Cruz de Caña, Río de la Carpa, Cerros Largos, La Ciénaga, La Represa, La Tapera and La Florida (see synthesis in Llambías et al., 1998). These plutons are composed of garnet-biotite-muscovite granodiorites and granites, with minor biotite tonalites, and are closely related in space and time with pegmatite dyke swarms. The contacts are sharp and parallel to the country rock foliation, and the plutons are often folded or boudinaged together. The edges are highly sheared in most of them, with a strong mylonitic foliation that decreases in intensity towards the inner parts of the bodies.

The Rb-Sr isochron age of 454 ± 21 Ma from Paso del Rey to Río de la Carpa area (Llambías et al., 1991) was taken as the best approximation to the crystallization age, closely associated with the regional deformation. Other Rb-Sr dates (485 ± 30 Ma and 460 ± 39 Ma) were mentioned by López de Luchi (1987) and López de Luchi and Cerredo (2001) for the Tapera pluton. The U-Pb conventional zircon data of 608 +26/-25 Ma recently obtained by von Gosen et al. (2002) for the Paso del Rey granitoid must be analyzed with care, because of the peraluminous and anatectic character of the granitoid (Llambías et al., 1996b) and the possibility of inherited processes in it(e.g. Mezger and Krogstad, 1997). The K-Ar biotite dates between 391 and 372 Ma from Paso del Rey and Río de la Carpa granodiorites (Varela et al., 1994) may reflect reset ages by ductile shear zone activity, rather than a very slow cooling rate.

The post-orogenic granitoids (see synthesis in Llambias et al., 1998) are emplaced throughout the Sierra, cutting the already juxtaposed different metamorphic complexes and also some of the ductile shear zones. The circular shapes and ring dykes, as well as the very high-K character of many of them indicate transitions to an anorogenic environment. The El Molle and Barroso plutons emplaced in the NMC (González and Sato, 2000; Sato et al., 2001a) are the oldest ones (with 417 +6/ -7 Ma, conventional U-Pb unpublished data) of this goup. These plutons cut previously developed shear zones and also are cut by other later zones dated by K-Ar method on biotite at 364 ± 7 Ma. For other plutons, Rb-Sr, U-Pb and K-Ar ages are between 408 and 320 Ma (Brogioni, 1987, 1993; Sims et al., 1998; Lema, 1980; Varela et al., 1994), excluding some dates with very large errors.

Mafic to ultramafic magmatic rocks

Two distinctive belts of mafic to ultramafic (meta-) magmatic rocks were identified within the Sierra de San Luis, one being an intrusive complex and the other mainly extrusive.

The La Jovita - Las Aguilas belt (Kilmurray and Villar, 1981; equivalent to the Las Aguilas Group of Sims et al., 1997) extends along ~ 80 km of length and 2 km of width in the central part of the sierra, within the PMC (Fig. 1). This belt comprises a NNE-SSW trending group of intrusive lenses and complexes (Peñón Colorado, La Gruta, La Melada, La Bolsa, Los Manantiales-El Fierro, Las Pircas, Virorco and Las Aguilas, from north to south) of pyroxenites, peridotites, dunites, gabbros and hornblendites, with a contact aureole associated with many of them (González Bonorino, 1961; Cucchi, 1964; Brogioni, 1994, 2001a and b; Brogioni and Ribot, 1994; Malvicini and Brogioni, 1993; Sims et al., 1997). Their margins are extensively recrystallized to high grade metamorphic assemblages, and share the same foliation of the country rocks. The less deformed cores of some bodies (Virorco, La Melada and La Gruta) still preserve the primary cumulate layering (González Bonorino, 1961; Brogioni y Ribot, 1994; Sims et al., 1997; Brogioni, 2001a). About the timing of their emplacement, pre- to syn-tectonic relation respect to the main Famatinian metamorphism and deformation (Sims et al., 1997, 1998; Brogioni y Ribot, 1994; von Gosen y Prozzi, 1998), as well as post-metamorphic emplacement (Hauzenberger et al., 2001) models were proposed. However, and Hauzenberger et al. (2001) Brogioni (2001a) coincide that the intrusion upgraded the country rock metamorphism from amphibolite to granulite facies conditions. The crystallization age (478 ± 6 Ma, U-Pb SHRIMP, Sims et al., 1998) is older than the zircon rims and monazite metamorphism ages (460-450 Ma) obtained from the country rock of PMC by the same authors. This situation might be related to the long lasting high-grade conditions common in many orogenies (e.g. Mezger et al., 1991) and the difficulty in dating the peak metamorphism (Kröner and Williams, 1991). There is a coincidence in the post-intrusive and post-S2F character of the ductile shearing produced by La Arenilla mylonite zone. Back arc or marginal basin settings were proposed for these tholeiitic rocks (Brogioni and Ribot, 1994; Heuzenberger et al., 2001).

The San Francisco del Monte de Oro - Villa de la Quebrada belt (González et al., 2002b; Ortiz Suárez, 1999; Merodio et al., 1978) extends through 45 km in NNE-SSW direction within the NMC. This belt comprises a group of strongly folded and boudinaged pod-like mafic to ultramafic bodies. To the west of this main belt, scattered and minor outcrops are exposed as small lenses and tabular layers. The bodies consist of amphibolites with relics of komatiites, komatiitic basalts and high-Fe tholeiite basalts, whose possible crystallization age is as old as early Mesoproterozoic (c. 1.5 Ga, Sato et al., 2001b). These mafic to ultramafic volcanic protoliths were affected by a first metamorphism and deformation event at some pre-Famatinian time, and then by the early Paleozoic Famatinian events (Sato et al., 2001b; González et al., 2002 b).

Within the CMC of the eastern part of the Sierra, ultramafic rocks have not been reported yet. However, tholeiitic basalts extruded in a back-arc setting are interpreted as protoliths of the interlayered amphibolites. These amphibolites are distributed in a N-S belt through Sierra del Morro, San Felipe and Villa de Praga, and many tungsten deposits are genetically related to them (Llambías and Malvicini, 1982; Delakowitz et al., 1991; Brodtkorb and Brodtkorb, 1999; Brodtkorb and Ortiz Suárez, 1999).

Discussion

On the basis of structural and metamorphic characteristics, two groups of metamorphic complexes are recognized in the Sierra de San Luis: (1) Multiply deformed and metamorphosed NMC, PMC and CMC, in which older NNW-SSE fabrics are found only as relicts surviving the penetrative NNE-SSW deformation. (2) Simply deformed and lower grade metamorphic complexes of SLF and MG, sharing only the dominant NNE-SSW structures (see synthesis in Fig.2).

Both groups of metamorphic complexes are characterized by a penetrative deformation that produced NNE-SSW trending foliations. Complexes covering a wide range of metamorphic grades and representing different crustal levels share these penetrative structures, and are juxtaposed to the

present level along ductile shear zones or transitional contacts. Hence, we consider that these structures were originated by the same compressive stress field of the Famatinian orogeny. Foliations associated to these structures are the first one (S1 F) found in the lower grade complexes (SLF and MG), the second one (S2 F) for the PMC and CMC, and the fourth one (S4 F) for the NMC. The high strain compressive regime that accompanied these penetrative structures is evidenced by isoclinal to tight folding of the original S0 in the lower grade complexes, and the tight refolding or reactivation of older foliations in the higher grade complexes.

Prior to this main Famatinian orogeny, an older NW-SE deformation event took place, as recognized locally in the higher grade complexes (NMC, PMC, CMC). An independent high strain sequence was determined in the NMC, with isoclinal, tight and open folding associated with the S1pF to S3 pF foliations, preceding the intrusion of large tonalitic to granitic bodies and local migmatization associated with low-P high-T metamorphism (González et al., 2002a). The timing of these pre- Famatinian deformations and metamorphism, as well as that of the previous deposition of the original sequence is not yet well constrained. It must be at least older than the 490 Ma Río Claro pluton (whose emplacement cuts the S0 pF to S3 pF of the NMC), and covers a possible time interval from Meso- to Neoproterozoic and early Cambrian. This interval is based on Sm-Nd data (Sato et al., 2001b), SHRIMP U-Pb zircon core data (Sims et al., 1998), a complex U-Pb conventional zircon data (von Gosen et al., 2002), and the NW-SE orientation of the old structures, similar to the Pampean deformation in Sierra de Córdoba.

The Barrovian-type metamorphism associated with the penetrative deformation and the successive cooling in the NMC is poorly constrained between 475 and 445 Ma by U-Pb monazite, Sm-Nd, Ar-Ar and K-Ar methods, whereas at PMC zircon and monazite U-Pb data indicate the range of 460-450 Ma. The c.480 Ma (Sims et al., 1998) obtained for the crystallization of a mafic segregation and an orthogneiss emplaced in the PMC, was considered by these authors as constraining the moment of synchronous metamorphism and deformation, and could represent earlier stages of the same metamorphism. For the CMC, only minimum K-Ar ages younger than 455 Ma were obtained. All these scattered data from the higher grade complexes are consistent for a duration of the high-grade metamorphism associated with the main deformation between early and late Ordovician (480 to 445 Ma). The U-Pb crystallization age of 490 ± 15 Ma (von Gosen et al., 2002) from the Río Claro pre-orogenic granitoid is consistent with this timing.

For the lower grade complexes (SLF and MG), the age of metamorphism must be constrained by the ages of the intrusives. U-Pb ages from the pre-orogenic granitoids are between 507 and 468 Ma (von Gosen et al., 2002; Sims et al., 1998). Since these country rocks are affected only by one sequence of regional deformation and metamorphism, this had to occur after the last intrusion registered at 468 Ma. The fact that the oldest La Escalerilla granite (507 Ma) is intruding the Las Verbenas tonalite (Sato, 1993) implies that the Famatinian arc magmatism started at some older time during the Cambrian, with the possibility of linking to the Pampean events, that are fully developed to the east at Sierra de Córdoba (Sims et al., 1998; Rapela et al., 1998). The only reliable Pampean date from Sierra de San Luis is the U-Pb conventional age of 529 ± 12 Ma obtained from metavolcanic rocks emplaced in the SLF (Söllner et al., 2000). However, this age is slightly older than or coetaneous with the peak metamorphism age of around 530-525 Ma of Sierra de Córdoba. The post-468 Ma time constraint for the main Famatinian deformation in the lower grade rocks is consistent with the 454 ± 21 Ma Rb-Sr data for the Paso del Rey - Río de la Carpa syn-orogenic granitoids (Llambías et al., 1991). However, it is apparent that the duration of metamorphism for the higher grade complexes (480 to 445 Ma) was longer than metamorphic and deformation processes that occurred at higher crustal levels.

By the end of Ordovician, arc magmatism and regional metamorphism ceased, and late orogenic shear zones started juxtaposing different crustal levels. Post-orogenic granitoids were then emplaced.

Structural relationships suggest that shear zones were already active since Ordovician times (von Gosen and Prozzi, 1998; González and Sato, 2000), being conspicuous during Devonian. Ar-Ar and K-Ar ages constrain these medium to low grade shear zones between 414 and 351 Ma. Most of the post-orogenic granitoids cut these shear belts, but the oldest El Molle and Barroso plutons (417 +6/-7 Ma) are affected by them. In this western region the shear belts are more abundant compared to the eastern region.

A synthesis of the age constraints for the orogenic evolution of Sierra de San Luis is represented in the Fig. 3, where three stages are recognized. The oldest pre-Famatinian processes (sedimentation, volcanism, high-strain NW-SE deformation and associated metamorphism) are still poorly constrained. Up to now, scarce data support the existence of the Pampean orogeny, comparable to that of the Sierra de Córdoba. The collision-related Famatinian orogeny started with the arc magmatism during at least Middle Cambrian, and was active through the major part of the Ordovician. Penetrative NNE-SSW deformation and metamorphism occurred at different crustal levels, affecting also the plutons of the magmatic arc (pre- to syn-orogenic granitoids). This magmatism ceased after the main deformation, probably due to the end of the east-directed subduction, as a consequence of terrane accretion. The outlasting compressive effects producing ductile shearing and exhumation of the metamorphic complexes are mainly recognized during the Devonian, associated with retrogressive metamorphism and post-orogenic magmatism. These processes led to the final uplift of the basement, and can be interpreted as the last events of the so called Famatinian orogenic cycle (late Cambrian to Devonian), closed with the Chañica or Precordilleranica phase, as mentioned for surrounding regions.

These last events are also equivalent to the Achalian orogeny, proposed by Sims et al. (1998), and were interpreted in relation to accretion of another exotic terrane, farther to the west, with very low to low grade metamorphism associated in Western Precordillera, Southern Frontal Cordillera and San Rafael Block (Ramos and Basei, 1997; Basei et al., 1998; Tickyj et al., 2001).

Regional correlations: The Sierra de San Luis was dominated by a low- to high-grade regional metamorphism that affected the arc magmatism within the Famatinian orogenic axis. The minor calc-alkaline and trondhjemitic intrusives located in the Sierra de Córdoba (Rapela et al., 1998; Gromet and Simpson, 1999) suggest that, although not residing in the main Famatinian orogenic belt, this mountain block was neither remote from the magmatic arc (Gromet and Simpson, 1999). The Ordovician magmatic arc is fully developed to the north of San Luis, through Sierra de Chepes, Valle Fértil-de la Huerta, Velasco and Famatina (Pankhurst et al., 1998; 2000; Stuart-Smith et al., 1999; Loske and Miller, 1996; Saavedra et al., 1992). Further north, coetaneous plutonism extends up to Puna.

The regional metamorphism associated with the collisional Famatinian orogeny affected not only the autochthonous Gondwana margin and the magmatic arc, but also the allochthonous Cuyania (Precordillera) terrane. The proposed suture zone position along the present day Bermejo River (Ramos et al., 1998) was confirmed by the high P/T Ordovician metamorphism reaching 10 to 13 Kb at both sides of the zone, and the U-Pb SHRIMP zircon core patterns (Baldo et al., 2001; Casquet et al., 2001, Rapela et al., 2001). Associated with this highly compressive regime, the deep roots of the magmatic arc are exhumed in the Sierra de la Huerta, represented by tonalitic magmatism and mafic to ultramafic intrusives (Castro de Machuca et al., 1996; Vujovich et al., 1996). The eastwest polarity in the P/T gradient observed at this latitude, toward lower-P conditions to the east (Baldo et al., 2001) is generally consistent with the P-T range of the high-grade rocks of the Sierra de San Luis (mostly 5 – 7.5 Kb, 518° – 770° C). However, just east of the suture zone at this latitude, the metamorphism associated with polyphase deformation of Sierra del Gigante (5 – 7 Kb, 400° – 650°C broad estimation, Gardini and Dalla Salda, 1997) is not comparable with the high P/T conditions of Sierra de la Huerta. This situation might reflect a progressive decrease of deformation

Figure 3. Compilation of selected isotopic data, constraining three stages in the basement evolution of Sierra de San Luis. Data source: Nogolí Metmorphic Complex: Ortiz Suárez (1999), Sato et al.(2001), González et al.(2002a), and our unpublished data. Pringles Metamorphic Complex: Sims et al.(1997, 1998), Ortiz Suárez (1999). San Luis Formation and equivalents: Sims et al. (1997, 1998), Söllner et al.(2000). Magmatic rocks: Lema (1980), López de Luchi (1987), Brogioni (1987, 1993), Varela et al.(1994); Llambías et al.(1991;1998), Stuart-Smith et al.(1999), Ortiz Suárez and Ulacco (1999); Sato et al.(1999, 2001a), López de Luchi and Cerredo (2001), von Gosen et al. (2002), and our unpublished data. Time scale after Remane (2000).

and metamorphism toward the south, as it is clearer at latitudes of the Chadileuvú Block. Here, the undeformed granitoids (U-Pb 503 ± 54 (503±54) and 431 ± 12 (431±12) Ma), and the dates of the phyllites (Ar-Ar 523 ± 3 Ma), gneisses (Ar-Ar 461 ± 2 Ma) and amphibolites (Kr-Ar 467 ± 13 Ma) (Tickyj et al., 1999; Tickyj and Llambías, tihs volume), only weakly support the existence of Ordovician regional metamorphism, and might rather suggest the existence of a Pampean metamorphism (Linares et al., 1980). At this latitude, neither the Las Matras basement (Sato et al., 2000) nor the carbonate cover (Melchor et al., 1999) of the Cuyania terrane show evidences of a regional Ordovician metamorphism, suggesting a more passive terrane accretion. This N-S heterogeneity in the compressive regime of the collisional orogen is possibly related to a different accretion geometry, such as irregular terrane shape or different angle of incidence. Only at Sierras Pampeanas, where the more severe compression developed a thickened crust, also favoured the emplacement of postorogenic, very high-K magmatism of mainly Devonian age. Farther south, although the existence of Ordovician metamorphism at Northpatagonian Massif was shown by Pankhurst et al. (2001), up to now the Cuyania terrane is not documented at these latitudes.

The compressive regime that characterizes the Ordovician collision is also noted in the region of Sierra de Umango, where the Famatinian tectonism possibly juxtaposed sheets of allochtonous and autochthonous terranes together, and this situation makes difficult the distinction of the Grenvillian, Pampean and Famatinian metamorphism and magmatism (Varela et al., 2000, and in prep.). These three cycles are also equivalent to those recognized at the area of Fiambalá (Grissom et al., 1998). East of Umango, high grade metamorphic rocks are exposed in the western and southern part of the Sierra de Famatina (Rossi de Toselli, 1996; Saavedra et al., 1998; Saal, 1993), reaching up to 6 Kb and 650°C. To the east, the country rocks of the arc magmatism are of higher crustal levels (Pampean to Famatinian rocks), only reaching higher grades the Famatinian regional ductile shear zones (e.g. Durand and López, 1996; Rapela et al., 2001; Lopez and Toselli, 2002; Báez et al., 2002). Details on the Famatinian metamorphism of the Northwestern Argentina can be seen in Rossi (this volume).

In synthesis, the Ordovician of the Sierra de San Luis is part of the Famatinian collisional orogen that involved the southwestern Gondwana margin and the accreted Cuyania terrane. Variations in deformation degree and metamorphic grade are moticeable along and across the orogen. Along the Gondwana margin, the Famatinian orogen overprinted the western part of the Pampean orogen, and only in few localities the recognition of Pampean rocks is clear. Rocks formed undoubtedly during the Pampean orogeny, and less affected by the Ordovician collisional effects, are exposed in an eastern belt parallel to the Famatinian orogen.

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Recibido: 2 de Octubre de 2002

Aceptado: 13 de Noviembre de 2002