Tectonic setting of the western Argentine Precordillera during the Early PalaeozoicInferences from mafic volcanic, subvolcanic and sedimentary associated rocks

Resumen

The Argentine Precordillera is part of the Cuyania terrane, accreted to Gondwana during the Early Palaeozoic. The western edge of the Precordillera contains N-S volcanic/subvolcanic mafic rock belts interpreted as possible ophiolites and suture markers, related to the accretion of Chilenia in Late Devonian–Lower Carboniferous times (Chanic Orogeny). We study one of these outcrops of mafic rocks, bounded by a thrust fault, named Sierra del Tigre. Its basalts and gabbros are not internally deformed and are composed by clinopyroxene + plagioclase + ilmenite ± olivine pseudomorphs. Amphibole ± biotite occurs in some rocks. A secondary mineralogy was developed in response to low/moderate hydrothermal alteration. Estimated chlorite temperatures and its spatial distribution within the mafic pile rocks indicate a high paleo-geothermal gradient (~38ºC/km) characteristic of intraplate rift settings. The geochemistry indicates a transitional-tholeiitic nature for these rocks plus an extended continental within-plate tectonic setting (extensional or rifted margin) and put into question its ophiolitic nature. The comparison with other mafic rocks of the western precordillera indicates that the within-plate signature is strong in the Sierra del Tigre rocks whereas towards the south there is a slight increase of the MORB-related signatures.