Zonación composicional en granates de la sierra de Pie de Palo, San Juan, Argentina: Implicancias en la historia metamórfica

Abstract

Compositional zoning of garnet from two domains of the Sierra de Pie de Palo (Western Sierras Pampeanas) evidence contrasting tectono-metamorphic histories. Garnets from Grt-amphibolites of the underlaying Grenville-age mafic-ultramafic belt (Complejo de Pie de Palo) on the western side of the Sierra show two discontinuous zones, one internal (core) and one external (rim). These zones are separated by inclusions of quartz and plagioclase arranged concentrically. Each zone shows different composition and also different patterns of variation for each end-member. In contrast, garnets from the Neoproterozoic Ca-metapelites in the overlaying southeastern domain of the Sierra de Pie de Palo show a continuous compositional zoning, characteristic of a single stage of growth during the Famatinian orogeny (Ordovician). Alm, Prp and Grs increase regularly outwards while Sps shows a reverse pattern. Garnet chemistry is consistent with the existence of a polymetamorphic basement (Grenvillian + Famatinian metamorphism) on the western side and a younger monometamorphic sedimentary cover.