CALC-ALKALINE ARC GRANITOID ACTIVITY IN THE SIERRA-NORTE-AMBARGASTA RANGES, CENTRAL ARGENTINA

A petrological and geochemical study of an area of approximately 750 k m(2) has been carried out on representative lithological units of the extensive Sierra Norte-Ambargasta batholith (SNAB), in the Sierra Nort e-Ambargasta range of the easternmost Sierras Pampeanas system of cent ral Argentina. This batholith has intruded a dominantly metasedimentar y basement of Precambrian age, which is mainly composed of quartzofeld sphatic-biotite or sericite-chlorite schists, and minor quartz-arenite s of local distribution. The most primitive rocks of the magmatic sequ ence are diorite to tonalite microgranular enclaves (ME), which are in cluded in the granodiorite-monzogranite (GM) basement. A large differe ntiated dacite-rhyolite (DR) stock intruded the earlier granitoids. Al l of these units were later intruded by highly evolved subalkaline gra nitoids (HESG) composed of miarolitic monzogranites, granite porphyrie s and large aplite dykes, which emplacement was controlled by old regi onal structural corridors. Geochemical parameters show that these gran itoids series were derived from crystal fractionation of calc-alkaline magmas of metaluminous to weakly peraluminous composition. Discrimina tory diagrams for tectonomagmatic settings, and the petrological and g eochemical signature of the studied lithologies indicate that most gra nitoids of the intrusive sequence are the result of I-Type calc-alkali ne magmatism. These granitoids are typical representatives of a precol lisional volcanic are environment, spatially related to an active cont inental margin. A significant chemical gap along the fractional crysta llization trend separates the most evolved silica-rich granitoids from the less evolved units. This gap is attributed to the absence of repr esentative magmatism of the syncollisional stage within the orogenic c ycle. Silica and K-feldspar rich-rocks of the HESG-group are the resul t of intrusions that occured after the main magmatic activity, in a la te-to post-orogenic tectonic setting. The occurrence of molybdenum, co pper and other base-metal sulfides, associated with alteration assembl ages that are common in porphyry systems, are in line with I-Type daci tic-rhyolitic magmatism in a volcanic are geotectonic regime. Barren q uartz-sericite greisen alteration is characteristic of the late-stage evolution of miarolitic monzogranites of the HESG-group. Radiometric a ges for the SNAB granitoids suggest Upper Precambrian to Lower Paleozo ic magmatic activity. Calc-alkaline are magmatic activity of this age in the Sierra Norte of Cordoba, the easternmost block of the Oriental Sierras Pampeanas, mises the important question whether to assign the main magmatic activity of this region to the Pampean or to the Famatin ian orogenic cycles. Our data suggest that the main igneous basement o f the Sierra Norte-Ambargasta batholith consist of precollisional gran itoids related to an orogenic cycle that most likely overlaps the G1 m agmatic cycle of the Pampean orogen. (C) 1997 Published by Elsevier Sc ience Ltd.