Calc-alkaline arc granitoid activity in the Sierra Norte-Ambargasta Ranges, central Argentina

A petrological and geochemical study of an area of approximately 750 km2 has been carried out on representative lithological units of the extensive Sierra Norte-Ambargasta batholith (SNAB), in the Sierra Norte-Ambargasta range of the easternmost Sierras Pampeanas system of central Argentina. This batholith has intruded a dominantly metasedimentary basement of Precambrian age, which is mainly composed of quartzofeldsphatic-biotite or sericite-chlorite schists, and minor quartz-arenites of local distribution. The most primitive rocks of the magmatic sequence are diorite to tonalite microgranular enclaves (ME), which are included in the granodiorite-monzogranite (GM) basement. A large differentiated dacite-rhyolite (DR) stock intruded the earlier granitoids. All of these units were later intruded by highly evolved subalkaline granitoids (HESG) composed of miarolitic monzogranites, granite porphyries and large aplite dykes, which emplacement was controlled by old regional structural corridors. Geochemical parameters show that these granitoids series were derived from crystal fractionation of calc-alkaline magmas of metaluminous to weakly peraluminous composition. Discriminatory diagrams for tectonomagmatic settings, and the petrological and geochemical signature of the studied lithologies indicate that most granitoids of the intrusive sequence are the result of I-Type calc-alkaline magmatism. These granitoids are typical representatives of a precollisional volcanic arc environment, spatially related to an active continental margin. A significant chemical gap along the fractional crystallization trend separates the most evolved silica-rich granitoids from the less evolved units. This gap is attributed to the absence of representative magmatism of the syncollisional stage within the orogenic cycle. Silica and K-feldspar rich-rocks of the HESG-group are the result of intrusions that occured after the main magmatic activity, in a late-to post-orogenic tectonic setting. The occurrence of molybdenum, copper and other base-metal sulfides, associated with alteration assemblages that are common in porphyry systems, are in line with I-Type dacitic-rhyolitic magmatism in a volcanic arc geotectonic regime. Barren quartz-seriate greisen alteration is characteristic of the late-stage evolution of miarolitic monzogranites of the HESG-group. Radiometric ages for the SNAB granitoids suggest Upper Precambrian to Lower Paleozoic magmatic activity. Calc-alkaline arc magmatic activity of this age in the Sierra Norte of Cَrdoba, the easternmost block of the Oriental Sierras Pampeanas, raises the important question whether to assign the main magmatic activity of this region to the Pampean or to the Famatinian orogenic cycles. Our data suggest that the main igneous basement of the Sierra Norte-Ambargasta batholith consist of precollisional granitoids related to an orogenic cycle that most likely overlaps the G1 magmatic cycle of the Pampean orogen.