Hybridization between I-type and S-type granites in the ordovician famatinian magmamatic arc, Tafí del Valle, Tucumán, NW Argentina

Abstract

In the Tafí del Valle region, in northwestern Argentina, several intrusive bodies of lower Paleozoic age wereemplaced in the metasedimentary Puncoviscana Formation, belonging to the Sierras Pampeanas. Four intrusivebodies outcrop in the study area: La Ovejería, El Infiernillo, Loma Pelada and Los Cuartos. La Ovejería and ElInfiernillo intrusive bodies represent the I-type magmatism according to their major element contents and showcotectic evolutions similar to those described by Famatinian I-type batholiths. These moderately peraluminousadakitic trondhjemites have characteristic high Sr/Y ratios and low Fe2O3+MgO+MnO+TiO2 contents. They arerelated to high-pressure conditions at the source, where dehydration melting of basaltic rocks may be involvedand garnet is retained in the residue leading to generation of HREE depleted melts. The Loma Pelada granitesshow characteristics of S-type magmatism (low CaO and MgO, and high SiO2 and K2O contents) typical ofgranites segregated in the last stages of magmatic differentiation, or anatectic granites. They show an increasingperaluminosity due to garnet entrainment and they are related to the anatectic melts generated in the PuncoviscanaFm. Both the Loma Pelada and Los Cuartos granitoids include samples with intermediate geochemicalcharacteristics that range between those of the El Infiernillo and La Ovejería and the regional metasedimentaryrocks. These characteristics could be explained by assimilation processes involving the I-type intrusive magmasand the metasedimentary host rocks or by hybridization processes between trondhjemitic I-type magmas as LaOvejería and El Infiernillo and anatectic S-type melts.