Isotopic geochemistry of granitoids in the Sierra de Chepe: a geotectonic and thermal model, implications for the Famatinian Orogeny

Abstract

Isotopic data obtained from the Sierras de Chepes and Los Llanos (Famatinian Orogen, Early Ordovician) have demonstrated that the parental magma of intermediate (SiO2= 60-62%) and metaluminous composition was derived mainlyfrom the melting of old continental lithosphere, with possible partial contribution from the subcontinental lithospheric mantle. Other parental magma was also generated at the same time by the melting of a metasedimentary sequence by means of heat liberated from metaluminous magma during its emplacement. Although the petrogenetic processe which explain the origin for the granitoidsof the Famatinian Orogeny have been considered previously, a general consensus still not does exist for the thermal and geotectonic regimes that led to the production of these magmas. The combined isotopic, petrological and geochemical data from the Sierra de Chepes and Los Llanos suggest that the thermal demands of magma genesis are compatible with an extensional regime model. The Papudo-Quintero granitic complex in the Coastal Batholith of Central Chile is representative of the Andino-type magmatism. Studies in that granitic complex indicate that the parental magma was mainly of mantle origin (i.e. asthenospheric mantle). The generation of the parental magma follows the classical setting for granitoids produced in convergent plate environments with subduction of an oceanic plate beneath a continental margin with partial melting of an asthenospheric edge. This process contrasts with the extensional model proposed for the granitoids of the Sierras de Chepes - Los Llanos, which suggests that the Andino-type model is not applicable to those Famatinian granitoids. The geochronological and petrological data indicate that the Famatinian magmatism was brief, and that an initial extensional regime became compressive during magma emplacement and it reaches their peak with the cessation of the magmatism during the Middle to Late Ordovician.