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.