CONTRIBUIÇÃO AO CONHECIMENTO DAS ROCHAS GRANITÓIDES DO SUL DO BRASIL

Abstract

In this paper, the authors make an attempt to presenr a synthesis on the problem of granitic rocks of Southern Brazil related to the Ribeira Folded Belt, tectonically compartmented in systems and massifs; these rocks belong to the Brasiliano Cycle (corresponding to Baykalian Cycle, Upper Precambrian). Granites are of cata-, meso- or epiiectonic type, forming simple or polydiapiric intrusions. The big polydiapiric intrusions (cata- or mesotectonic) are controlled by faults, which separate massifs from folded systems; mesotectonic simple intrusions are controlled by folding structures or faults, and the epitectonic (related to the molassic phase) are fracture-controlled. Also the alkali granites are inrrusions controlled by faulting. Frequency of granitic bodies is variable in the different massifs and folded systems; composition varies from diorites to true granites, calco- alkalic to alkalines. Mineral deposits, always of small importance, are of varied distribution, including occurrences of Sn, W, Cu, Pb, and Au. Late tectonic bodies are the most common; the syntectonic are restricted to the massifs and the post-tectonic ones are rare and of small size. Some late- to post-tectonic bodies are migmatite- forming granites. The sequence syn-, late- and post-tectonic granites is continuous, without sharp breaks or time intervals. The granitic magmatism of the Ribeira Folded Belt is very typical of geosynclinal areas with weak or absent basic volcanism during subsidence phase, with weak acid magmatism at the differentiation phase and strong granitic magmatism (intrusive and extrusive) during the closure phase (a typical behavior of geosynclinal areas with sialic profile).