Age and magmatic evolution of the Famatinian granitic rocks of Sierra de Ancasti, Sierras Pampeanas, NW Argentina

摘要

The granitic rocks of the Sierra de Ancasti represent one of the easternmost outcrops of the Famatinian arc of the Sierras Pampeanas. We report here new U-Pb SHRIMP and LA-ICP-MS Ordovician ages and a petrological and geochemical study of the Las Canadas I-type granitic complex in the central sector of Sierra de Ancasti. Numerical modelling indicates that calc-alkaline and metaluminous monzogranites (SiO_2 = 69.8-73.0 wt.%) were produced by fractional crystallization of plagiodase-rich tonalite (～ 60% plagioclase and SiO_2 = 60.9 -62.5 wt.%) from an inferred parental magma of granodioritic composition (SiO_2 = 67.4 wt.%). Sm-Nd isotopic data on the dated rocks indicate a dominant continental crustal input, as reported for other Ordovician granitic rocks (Famatinian magmatism) farther west in the Sierras Pampeanas. Combined ages and isotopic data are consistent with previous studies indicating that Famatinian magmatism was short-lived (～20 Ma) and lacked a significant asthenospheric contribution. The occurrence of inherited zircon and the petrological and geochemical data from the Las Canadas complex indicate that the parental magma was derived from mixed igneous and sedimentary material in the source or by partial melting of igneous rocks of different ages. Systematic dating of inherited zircon cores is required to clarify this issue. Zircon saturation geothermometry combined with geochemical data indicates that the onset of zircon crystallization, together with variable amounts of other minerals such as plagioclase, quartz, biotite, oxides and apatite, occurred when the parental magma reached 67 wt.% SiO_2 with ～ 440 ppm of Zr at ～ 878 ℃.