Metamorphic evolution of the Sierras de San Luis, Argentina: granulite facies metamorphism related to mafic intrusions

Abstract

The Sierras de San Luis, which are part of the Sierras Pampeanas, are located in Central Argentina. The crystalline basement of the Sierras de San Luis is built up of three main blocks (western block, central block, and eastern block), which are separated by mylonite zones. The western and the eastern block are dominated by migmatites, whereas the central block is mostly lower in metamorphic grade ranging from greenschist facies to amphibolite facies, and locally to granulite facies in the vicinity of numerous mafic bodies. Most parts of the central block is built up of amphibolite facies rocks. These were formed during a first metamorphic event (M1-A) which is characterized by a mineral assemblage of staurolite—garnet—biotite—muscovite—plagioclase—quartz—ilmenite±fibrolite±chlorite. The PT conditions of M1-A are about 570 °C to 600 °C and 5 to 5.7 kbar. A mafic intrusion, now seen as numerous mafic lenses included in the basement rocks caused local granulite facies metamorphism. The observed mineral assemblage consists of garnet—cordierite—sillimanite—biotite—K-feldspar—plagioclase—quartz—rutile—ilmenite±orthopyroxene (M2-G). The PT estimates for granulite facies conditions are 740 °C to 790 °C and 5.7 to 6.4 kbar. During cooling a mylonite zone developed within the central block retrograding most of the granulite facies rocks to amphibolite facies conditions. The newly formed mineral assemblage consists of garnet—biotite—sillimanite—plagioclase—muscovite—quartz—rutile±K-feldspar (M3-A). The PT estimates of the locally overprinting second amphibolite facies event (M3-A) are about 590 °C to 650 °C and 5.4 to 6.0 kbar. The deduced PT path shows a near isobaric heating from M1-A to M2-G. The mylonite mineral assemblage M3-A equilibrated at pressures close to M2-G. The PT path can be explained best by heating of an amphibolite facies middle crust by a mafic intrusion. During near-isobaric cooling tectonic activity in discrete parts of the basement caused mylonitization at amphibolite facies conditions.