Petrology of the La Florida stock, Sierra de San Luis.

Abstract

La Florida pluton is located in the central part of the Sierra de San Luis. It is a nearly semicircular-shape intrusion, slightly elongated perpendicularly to the main NNE-SSW country rocks foliation, which deforms at the pluton margins. The body is emplaced in medium-grade metamorphosed subgreywakes and sandstones-derived rocks of the Micaschist Group. Intrusive rocks are mostly greyish medium to coarse-grained peraluminous monzogranites, composed of quartz, microcline, plagioclase, muscovite, biotite and minor garnet. Biotite content decreases from the core to the margin of the pluton while muscovite, usually forming up to 4 cm-long porphyroblasts, and garnet amounts increase in such direction. This leads to a remarkable variation of the colour index across the body, grading from 12 % to 0 % at the contact with the country rocks. Abundants dykes and irregular bodies of pegmatites intrude the outer parts of the pluton as well as the adjacent metasedimentary country rocks. The mineralogical zonation and the increase of pegmatization to the margins is interpreted here as the result of the enrichment in water-rich late-magmatic fluids which caused subsolid reactions with the minerals cristallized during the orthomagmatic stage. The magmatism is related to the Famatinian subduction, showing a syn-collisional affinity and S-type evolutionary trend. However, several differences have been found between La Florida and others syn-kinematic S-type granitoids from the Sierra de San Luis.