Stratigraphical reinterpretation and petrology of the Cerro Chuscho Formation. Precordillera of La Rioja

Abstract

In the region of Precordillera of Jagüe, in La Rioja province, the Palaeozoic volcanic rocks, known as the Punta del Agua, Cerro Chuscho and Las Casitas formations, have been grouped into one unit, the "Andesita Cerro Chuscho". These rocks have been considered to be comagmatic and to form a subvolcanic complex of autoclastic breccias and calc-alkaline lava flows of the precordilleran orogenic cycle. In our study, the Cerro Chuscho Formation is reinterpreted as a mafic volcanic complex. Of Ordovician, it includes dykes, sills and pillow-lavas of basaltic composition, representing an ophiolitic facies interbedded in the sedimentary pile. The Cerro Chuscho Formation belongs to the "Famatinian Ophiolitic Belt", which is related to the opening of a marginal basin during Chilenia accretion in Cambro-Ordovician times. The geochemical signature of the basaltic rocks changes along the rift, from N-MORB in the Cordillera Frontal to E-MORB in the Precordillera of San Juan and La Rioja. This variation probably reflects the changing altitude of the rift relative to the mantle. By contrast, the "Andesitas Punta del Agua", of Late Carboniferous to Early Permian age, are considered to be subduction-related andesites according to their geochemical characteristics.