O GRANITO DA RAMADA, PORÇÃO OESTE DO ESCUDO SUL-RIO-GRANDENSE: GEOLOGIA, PETROLOGIA E GEOQUÍMICA

Abstract

The Ramada Granite is a post-OTogenic, epizonal intrusion, associated to alkaline magmatism which postdates die Brasiliano orogenesis. It is composed of aeno and mon- zogranites containing calcic-amphibofes (magnesium hornblende and feno-edenitic hornblende) and biotite, besides magnetite, ilmenite, titanite, zircon, allanite, and fluorite. Their textures vary from hypidiomorphic equigranular to porphyritic. As far as chemical composition is concerned they are not typically alcaline, although such affinity is pointed out by FeO(tyFeO(t)+MgO ratio higher than 0.9, by high agpaitic index, and by R1R2 plotting. The trace-element contents are not suggestive of alkaline affinity, except for rare earth element geochemistry. According to mineral chemistry data, mafic phases were crystallized under exceptionally high Poj conditions considering alkaline magmas. Rb-Sr isotopic data indicate an age of 463 ± 12.5 Ma an initial 87 Sr/86 Sr ratio of 0.710 for these granitic rocks. The available data suggest the Ramada Granite is part of the alkaline Saibro Intrusive Suite, its origin is still controversial since the available data are consistent with partial melting of quartz-feklspathic high-grade metamorphic rocks or fiactionation of alkali feldspar plus dinopyroxene from trachytic liquids of mantle sources. Late hydrothermal activity may have modified the 87 Sr/86 Sr initial ratio and developed significant mineralization.