U-Pb SHRIMP ages of Famatinian granits: news constraints on the timing, origin and tectonic setting of I- and S-type magmas in an ensialic arc

RAPELA, C.W.; PANKHURST, R.J; DAHLQUIST, J.A.; FANNING, C.M.

Carlos Paz

Simposio; II South American Symposium on Isotope Geology; 1999

Asociación Geológica Argentina

Current views on the early evolution of the proto-Andean margin of southern South America are mostly the result of studies carried out during the last five years, which have radically improved our previous knowledge. Initially, most attention was focused on the Precordillera, but has now also been given to the basement rocks and sedimentary basins located to the east of this terrane (see Rapela et al., 1998b for references). Taking into account the new geochronological and geological studies on the Sierras Pampeanas, different belts or terranes may be distinguished on the basis of the characteristics and ages of deformation and/or magmatism (Rapela et al., 1998b). Thus two belts are recognized in the innermost and central part of the early Palaeozoic orogen: a Pampean Mobile Belt characterized by early-to-middle Cambrian deformation and magmatism, and a Famatinian Mobile BeIt of Ordovician-to-Silurian age (Fig. 1). The reconstruction depicted in Fig. 1 assumes that, during the early Ordovician, the Precordillera terrane was still not sutured to the western border of the Famatinian belt, as it was from at Ieast middle Silurian times. Key issues in unraveling the evolution of the proto-margin and the docking of the Precordillera terrane are the P-T-t paths of different basement blocks and the timing and characteristics of the Famatinian magmatism, which are the main target of this contribution. Precise U-Pb and Rb-Sr ages have recently been reported for granites and metamorphic rocks from the Sierras de Córdoba, Chepes, Ulapes y San Luis (Pankhurst e a!., 1998; Rapela et al., 1998a; Sims el al., 1998). In particular, Pankhurst el al. (.1998) showed that the most voluminous I-type Famatinian granitoids in La Rioja Province were of Early Ordovician age, older than previously supposed. However, most of the largest and most typical granitic units of the Sierras Pampeanas, such as the Capillitas and Velasco batholiths, the Sierra de Famatina, and Sierra de Valle Fertil, lacked comparable data. Fieldwork in March 1998 was designed to sample the typical rocks of some of these units for a U-Pb geochronological, the results of which are presented here.