Early Carboniferous sub- to mid-alkaline magmatism in the Eastern Sierras Pampeanas, NW Argentina: A record of crustal growth by the incorporation of mantle-derived material in an extensional setting

摘要

A recently discovered granitic intrusion at Cerro La Gloria in western Sierra de Famatina(NW Argentina) is representative of sub- to mid-alkaline Carboniferous magmatism in the region. The main rock type consists of microcline, quartz and plagioclase, with amphibole, magnetite, ilmenite, biotite, epidote, zircon, allanite and sphene as accessory minerals. We report a U-Pb zircon SHRIMP age for the pluton of 349±3Ma(MSWD=1.1), i.e., Tournaisian. Whole-rock chemical composition and Nd isotope analyses are compatible with an origin by melting of older mafic material in the lower crust(εNdt between -0.58 and +0.46 and T DM values of about 1.1Ga). The pluton is intruded by penecontemporaneous to late alkaline mafic dykes that are classified as back-arc basalts. Coeval, Early Carboniferous A-type granites occur farther east in the Sierras Pampeanas, probably generated during lithospheric stretching. Overall, the Early Carboniferous granitic rocks show a west-to-east mineralogical and isotopic zonation indicating that magma genesis involved a greater contribution of juvenile material of mantle character to the west. Based on the observed patterns of geochronology, geochemistry and field relationships we suggest that A-type magma genesis in the Eastern Sierras Pampeanas was linked to an Andean-type margin where the lithospheric mantle played a role in its generation.