Datos preliminares sobre el metamorfismo de la Sierra de Pie de Palo, Sierras Pampeanas Occidentales (Argentina)

Abstract

The Sierra de Pie de Palo belongs to the Western Sierras Pampeanas. Although still poorly known in detail, the sierra consists of a relatively autochthonous sequence of shelf sediments metamorphosed to low grade (the Caucete Group) and an allochthonous complex of polymetamorphic medium-to-high grade igneous and sedimentary rocks thrust towards the west and northwest. Mylonitization is widespread. Radiometric dating by different methods (U-Pb, 4°Ar/39Ar and K-Ar) suggests that at least two tectonothermal events are present in the allochthonous: Grenville (ca. 1 Ga) and Famatinian (480-312Ma). Metamorphism has been studied in a Ca-rich pelite with a complex mineralogical composition (Qtz, Ms, Pg, Bt, Grt, Ky, Hbl, St, Rt, 111, PI, Ep). This rock is a mylonite. Three metamorphic stages are recognized: the first two correspond to a prograde M, metamorphism under relatively high P/T conditions (peak at 13kb and about 600ºC). The second is related to mylonitization (Mm 1) with T - 575ºC and P < 1 Okb. We suggest that this rock is polymetamorphic, M, metamorphism beirig probably related to the Grenville orogeny and Mmyl to the Famatinian orogeny.