CONTRIBUIÇÃO A GEOLOGIA DO GRUPO PINHAL (SP E MG)

EBERHARD WERNICK, FAUSTINO PENALVA

Resumo


The Pinhal Group is an imporlant lithostratigraphic unit, in the NE of the State of São Paulo and SSW of the State of Minas Gerais, Southern Brazil. The Pinhal and Socorro Complexes are the main granitic-migmatitic areas of this unit, including "migmatites forming granites", migmatites, "normal granites" and several medium to high grade metamorphic country rocks. The granitic suite include various equigranular and porphyroid types and porphyritic, pegmatitic and aplitic dikes. Geochronologic age determinations indicate their relation with "Brazilian Cycle" (Baikalian Cycle, of Upper Precamhrian age): the country rocks belongs to the Amparo Group (Niddle Precambrian), which rocks underwent an intensive remobilization during the Brazilian Cycle. The "migmatites forming granites" are surrounded by an aureole of migmatites composed of 3 zones: the outer zone is formed by layered migmatites, the medium on by agmatitic and "schollen" migmatites; and the inner one by "schlieren", nebuliticand homophanic migmatites. The central granitic core is equigranular or porphyroid. The migmatitic aureole is frequently disturbed by faults, by "normal granite" intrusions or by the interference of adjacent aureoles; these disturbances lead to very complex (polyphasic) migmatites. Strong metassomatic phenomena affected the rocks of the Pinhal Group, at two distinc phases. The first one, of potassic nature is characterized by a intensive development of microcline megacrystals; and the second one, of sodic nature, by the formation of pure albite. In each phase, distinct metassomatic pulsations have been observed, linked with the several intrusion and tectonic events which have affected the complexes which form megadiapirs of polydiapiric nature. The big granite plutons have arise from the base of the sialic crust along "thermal channles" related to zones of tectonic weakness. During this slow ascension these bodies have 'undergone deep modifications (remobilizations), subsolidus recrystalization, metasomatism, assimilation and fractional crystalization. From the regional point of view, the Pinhal Group is correlated to the Serra dos Orgaos Group, in the Rio de Janeiro State. Economic geology in the investigated region is mainly related to pegmatites exploitation.

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