GRUPO ARAXÁ EM SUA ÁREA TIPO: UM FRAGMENTO DE CROSTA OCEÂNICA NEOPROTEROZÓICA NA FAIXA DE DOBRAMENTOS BRASÍLIA

Abstract

ARAXÁ GROUP IN THE TYPE-AREA: A FRAGMENT OF NEOPROTEROZOIC OCEANIC CRUST IN THE BRASÍLIA FOLD BELT This study reviews the geological characteristics and puts forward a new evolution model for the Araxá Group in its type-area, the southern segment of the Neoproterozoic Brasilia Belt, Minas Gerais, Brazil. The Araxá Group is confined within a thrust sheet belonging to a synformal regional fold, the Araxá Synform, overlying two other thrust sheets made of the Ibiá and Canastra Groups. The Araxá Group is described as a tectonostratigraphic terrane in the sense of Howell (1993). It comprises an igneous mafic sequence, with fine and coarse grained amphibolites, associated with pelitic metasedimentary rocks, and subordinate psanmites. All rocks were metamorphosed to amphibolite facies at ca. 630 Ma ago and were intruded by collisional granites. The amphibolites represent original basaltic and gabbroic rocks, with minor ultramafics (serpentinite/ amphibole-talc schist). The basalts are similar to high FeO tholeiites, with REE signatures that resemble E-MORB and ε Nd(t) =+ 1.1. The metasedimentary rocks are interpreted as the result of a marine deep-water sedimentation. They have Sm-Nd model ages of 1,9 Ga, and ε Nd(t) =-10.21. The amphibolites and metasediments could represent a fragment of back-arc oceanic crust. The data presented here differ significantly from the original definition of Barbosa et al. (1970) who describe the Araxá Group as a pelitic/psanmitic sequence and the collisional granites as a basement complex.