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

The Sierras de San Luis, which are part of the Sierras Pampeanas, are locat ed in Central Argentina. The crystalline basement of the Sierras de San Lui s is built up of three main blocks (western block, central block, and easte rn block), which are separated by mylonite zones. The western and the easte rn block are dominated by migmatites, whereas the central block is mostly l ower in metamorphic grade ranging from greensehist facies to amphibolite fa cies, and locally to granulite facies in the vicinity of numerous mafic bod ies. Most parts of the central block is built up of amphibolite facies rock s. These were formed during a first metamorphic event (M1-A) which is chara cterized by a mineral assemblage of staurolite-garnet-biotite-muscovite-pla gioclase-quartz-ilmenite+/-fibrolite+/-chlorite. The PT conditions of M1-A are about 570 degreesC to 600 degreesC and 5 to 5.7 kbar. A mafic intrusion , now seen as numerous mafic lenses included in the basement rocks caused l ocal granulite facies metamorphism. The observed mineral assemblage consist s of garnet-cordierite-sillimanite-biotite-K-feldspar-plagioclase-quartz-ru tile-ilmenite+/-orthopyroxene (M2-G). The PT estimates For granulite facies conditions are 740 degreesC to 790 degreesC and 5.7 to 6.4 kbar. During co oling a mylonite zone developed within the central block retrograding most of the granulite facies rocks to amphibolite facies conditions. The newly f ormed mineral assemblage consists of garnet-biotite-sillimanite-plagioclase -muscovite-quartz-rutile+/-K-feldspar (M3-A). The PT estimates of the local ly overprinting second amphibolite facies event (M3-A) are about 590 degree sC to 650 degreesC and 5.4 to 6.0 kbar. The deduced PT path shows a near is obaric heating from M1-A to M2-G. The mylonite mineral assemblage M3-A equi librated at pressures close to M2-G. The PT path can be explained best by h eating of an amphibolite facies middle crust by a mafic intrusion. During n ear-isobaric cooling tectonic activity in discrete parts of the basement ca used mylonitization at amphibolite facies conditions.