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

作者：Dahlquist Juan A^*; Rapela Carlos W; Pankhurst Robert J; Fanning C Mark; Vervoort Jeffrey D; Hart Garret; Baldo Edgardo G; Murra Juan A; Alasino Pablo H; Colombo Fernando

来源：Journal of South American Earth Sciences, 2012, 34: 10-25.

DOI：10.1016/j.jsames.2011.07.006

• U-Pb SHRIMP and LA-ICP-MS ages
• I-type granite
• Fractional crystallization

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 (SiO2 = 69.8-73.0 wt.%) were produced by fractional crystallization of plagioclase-rich tonalite (similar to 60% plagioclase and SiO2 = 60.9 -62.5 wt.%) from an inferred parental magma of granodioritic composition (SiO2 = 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 (similar to 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.% SiO2 with similar to 440 ppm of Zr at similar to 878 degrees C.

• 出版日期2012-3