Ordovician S-type granite and related pegmatite, Sierra de San Luis, Argentina: a fluid inclusions study

SOSA, G.; VAN DEN KERKHOF, A.; LÓPEZ DE LUCHI, M. G.; ORTÍZ SUAREZ, A.

Berna, Suiza

Congreso; European Current Research on Fluid Inclusions (ECROFI-XIX); 2007

University of Bern, Switzerland

Ordovician syn-orogenic garnet-biotite-muscovite-granodiorite, granite, and minor biotite-tonalite from the Sierra de San Luis, Argentina, are closely associated with barren internal and external pegmatites. These granitoids form meter- to km-scale bodies within meta-sedimentary schists. The largest pluton (La Tapera) outcrops in an area of ca. 2 km2,in the Conlara Metamorphic Complex whereas the smaller ca.1.7 km2Paso del Rey pluton crops out in the Pringles Metamorphic Complex Pegmatites make out variable volume fractions of the plutonic bodies (internal pegmatite) and also form swarms of external pegmatites around the plutons. Fluid inclusion and cathodoluminescence studies were performed in order to correlate the fluid evolution of the granitoids and pegmatites. Furthermore, the results were compared with Sn-Nb-Ta-Li-bearing pegmatites, which occur in a larger area of the Sierra de San Luis (Sosa et al. 2002).   The fluid inclusions in quartz, plagioclase and apatite from the granitoids and internal pegmatites contain only aqueous low-salinity solutes of ca. 3-4 wt.% NaCl-eq. The fluid inclusions form clusters and intragranular trails. Primary inclusions are found in all the minerals, but best preserved in apatite. Decrepitated fluid inclusions decorated with halos of tiny fluid inclusions indicate implosion-decrepitation and testify isobaric cooling. Melt (glass + vapor) inclusions are sometimes preserved in quartz, garnet, zircon, apatite and rutile.   In contrast the external pegmatites show a much larger compositional variation, including mixtures of H2O, CO2±CH4 and salt: Type I (59%) H2O-CO2±CH4 inclusions with ~7 wt.% NaCl-eq.; Type II (23%) aqueous inclusions with ~7% wt.% NaCl-eq. and no detectable CO2, and Type III (18%) CO2±CH4 inclusions with no detectable water. CH4-concentrations are always << 5 mol%; daughter minerals are absent. The type I inclusions are in part primary and may represent the earliest magmatic fluid. Types II and III fluid inclusions were trapped along healed microfractures (visible as non-luminescent secondary quartz in cathodoluminescence) and may have formed as a result of unmixing of H2O-CO2 towards lower temperatures (<400°C).