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 "folded systems" and "massifs"; these rocks belong to the ..Brasiìiaho Cycle" (corresponding to Baykalian Cycle, Upper Precambrian). Granites are of cata-, meso- or epiiectonic type, formingsimple or polydiapiric intrusions. The big polydiapiric intrusiont (.utu- o. *é*t..tonic) ãre cont.ollêd úy fuUm, which separate massifs from folded systems; mesotectonic simple intrusions are controlled by fol- ding structures or faults, and the epitectonic (related to the molassic phase) are fracture- controlÌed. Also the alkali granites are inrrusions controlled by faulting. Frequency of granitic bodies is variable in the different massifs and folded sysrems; composition varies from diorites to true granites, calco- alkalic to aikalines. 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. Sorne late- to post-tectonic bodies are migmatite- forming granites. -fhe sequence sylì-, late- and post-tectonic granites is continuous, without sharp breaks or time intervals. The granitic magmatism of the Ribeira Foldrd Belt is very rypical of geosynclinal areas with weak or absent basic volcanism during subsiderrce phase, with weak acicl magmatism at the differentiation phase and strong granitic magmatism (intrusive ancl extrusive) during the "closure" phase (a typical behavior ofgeosynclinal areas with sialic profile).