A OROGÊNESE BRASILIANA NO SEGMENTO CENTRAL DA FAIXA RIBEIRA, BRASIL

Abstract

The structure of the central segment of the Ribeira belt is defined by three large thrust sheets (Lower, Intermediate and Upper) which override the autochthonous domain, towards the southern portion of the São Francisco Craton. Four lithotectonic associations are individualized within the domains: basement rocks (pré-1,8 Ga rocks); orthogneisses of unknown age; Post-1,8 Ga metasedimentary cover and Brasiliano granitoid rocks. Continental tholeiitic deformed dykes and disrupted lenses within the cover indicate exten- sional periods. Investigation of the tectono-metamorphic and magmatic aspects, associated with new U/Pb geocronological data support the subdivision of Brasiliano Orogeny within three main tectonic periods: a) the syn-collisional period (590-563 Ma) is represented by the main deformation (D1+2), coeval with the MI metamorphic stage and γl -γ3 granitoid rocks. Crustal shortening took place initially through ductile thrusts and tight recumbent folds. The late stage of this collisional event is characterized by oblique convergence, with pervasive deformation throughout the belt. MI had its thermal peak during D2, and is characterised by interme- diate to high pressures. The spatial distribution of the metamorphic zones defines an inverted gradient. Defor- med I and S type granitoid rocks occur during this period, b) the post-collisional period (535-520 Ma) is characterized by D3 deformation, coeval with M2 metamorphic stage. During DS phase, the oblique convergen- ce continued, but was accomodated by late to post-metamorphic peak strike slip ductile shear zones. Between these shear zones, shortening was achieved by steep folding of S2 foliation. The Ma stage with lower pressure regime is associated with intense melting of the basement and the cover, resulting in the generation of I-type magmatism and subordinated S-type; and c) the post-tectonic period is represented by the Post-D4 γ5 mag- matism registered between 503 to 492 Ma. This magmatism may be related to melting of crustal rocks associated with mantelic contribution related with high temperature conditions and uplift just after the Brasiliano Orogeny.