PETROGRAPHY AND MINERAL CHEMISTRY OF THE SÃO GERALDO GRANITE: EVIDENCE OF PERALUMINOUS MAGMATISM IN THE PORTO ALEGRE REGION.

Abstract

The present work describes the first occurrence of a peraluminous leucogranite in the Porto Alegre city region, named São Geraldo Granite. Thin sections from drill cores were studied through petrography (optical and electron microscopy) and mineral chemistry (electron probe microanalysis). The granite shows a medium to fine grained, equigranular hypidiomorphic texture, and a composition ranging from sieno- to monzogranitic. Its mineralogy comprises K-feldspar (37-48 %), quartz (17-33 %), plagioclase (23-28 %), muscovite + biotite (7 to 11%), and subordinate amounts (< 1%) of garnet, zircon, monazite, apatite, ilmenite, and magnetite. The biotite, interpreted as magmatic in origin, shows major element concentrations (e.g. Al, Fe, and Mg) compatible with those generated in peraluminous granites. The muscovite is predominantly magmatic, with a composition compatible with those of the strongly peraluminous granites. The Or contents of the K-feldspar (orthoclase) range from 86 to 97 %. The plagioclase is mostly oligoclase, showing secondary albitic composition. The spessartine contents of the garnet (26-32 %) suggest a magmatic origin. The apatite (fluorapatite) shows F concentration between 3.0-4.5 %. The peraluminous character of the studied granite differs from all granites described in the Porto Alegre region. Its composition is similar to those of the Cordilheira Suite (Pelotas Batholith), which encompasses most of the peraluminous S-type granites syncinematic emplaced in ductile shear zones found in the eastern domain of the Dom Feliciano Belt.