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

Abstract

SummaryThe 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 ma®c bodies. Most parts of the central block is built up of amphibolite facies rocks. These were formed during a ®rst metamorphic event (M1-A) which is characterized by a mineral assemblage of stauroliteÀgarnetÀbiotiteÀmuscoviteÀ plagioclaseÀquartzÀilmeniteAE®broliteAEchlorite. The PT conditions of M1-A are about 570 C to 600 C and 5 to 5.7 kbar. A ma®c intrusion, now seen as numerous ma®c lenses included in the basement rocks caused local granulite facies metamorphism. The observed mineral assemblage consists of garnetÀcordieriteÀsillimaniteÀbiotiteÀK-feldsparÀplagioclaseÀquartzÀrutileÀilmeniteAEorthopyroxene (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 garnetÀbiotiteÀsillimaniteÀplagioclaseÀmuscoviteÀquartzÀ rutileAEK-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 ma®c intrusion. During near-isobaric cooling tectonic activity in discrete parts of the basement caused mylonitization at amphibolite facies conditions.

The tectonic significance of K/Ar illite fine-fraction ages from the San Luis Formation (Eastern Sierras Pampeanas, Argentina)

“…Within the western belt, the ''east-side-up'' displacement is consistent with the vertical displacement that is recorded by the La Arenilla shear zone, which is located in the eastern part of the Pringles Metamorphic Complex (Ortíz Suárez et al 1992;Hauzenberger et al 2001). …”

Geodynamic evolution of the Eastern Sierras Pampeanas (Central Argentina) based on geochemical, Sm–Nd, Pb–Pb and SHRIMP data

“…The extensional back-arc setting could have lead to mafic intrusions (Fig. 14b) as described by Hauzenberger et al (2001). These intrusions would be the heat sources that lead to amphibolite and local granulite facies metamorphism in the Pringles Metamorphic Complex.…”

“…Late Cambrian to Ordovician and Devonian intrusives invaded the metamorphic units. Late Cambrian to Early Ordovician (Famatinian) back-arc mafic to ultramafic rocks and islandarc amphibolites are located in the Pringles Metamorphic Complex associated with granulite facies metasedimentary rocks (Sims et al 1997;Hauzenberger et al 2001). During the last three decades, a large number of studies have focused on the structural evolution, metamorphism and magmatism of the basement complexes in the Sierra de San Luis (Kilmurray and Dalla Salda 1977;López de Luchi 1986, 1987Ortiz Suárez et al 1992;Llambías et al 1998;Sims et al 1998;von Gosen and Prozzi 1998;Delpino et al 2001Delpino et al , 2007González et al 2002;López de Luchi et al 2003Steenken et al 2004Steenken et al , 2006Steenken et al , 2008.…”

Early stage evolution of the mafic-ultramafic belt at La Melada, Sierra de San Luis, Argentina: P–T constraints from metapyroxenite pseudosection modelling

“…This observation suggests that, although the metapyroxenite and metagabbros shared a common PeT metamorphic history, their pervasive reequilibration occurred at different PeT conditions along their retrograde PeT path. Hauzenberger et al (2001) proposed that mafic intrusions of the Sierras de San Luis are responsible for granulite facies metamorphism developed at 740e790 C and 0.57e0.64 GPa. According to Zaffarana et al (2011) the mafic intrusions alone could not have caused the thermal input responsible for the increase of the metamorphic grade from amphibolite to granulite facies conditions in neighbouring rocks.…”

“…10, pag. 171) mylonites of the Pringles Metamorphic Complex adjacent to mafic rocks (Hauzenberger et al, 2001) at Las Aguilas-El Fierro underwent four (T 1 , T 2 , T 3 , T 4 ) tectonometamorphic events and a counterclockwise PeTed path. According to Delpino et al (2007), the T 1 and T 2 events developed with increasing temperature and pressure conditions under extensional and compressional regimes, respectively.…”