ARCHEAN ACCRETION IN THE SÃO JOSÉ DO CAMPESTRE MASSIF, BORBOREMA PROVINCE, NORTHEAST BRAZIL

Abstract

The granitic-gneissic terranes of the Gaviao Block in the central-southern Sao Francisco Craton are a key area to the understanding of the Archaean evolution of the South American Platform. The Archaean granitic-gneissic rocks are intimately associated with the Umburanas and Contendas Mirante greenstone belts. The metamorphic grades vary from greenschist to amphibolite facies. These rocks were intensely deformed and intruded by Paleoproterozoic granites. The main evolution of the Gaviao Block is marked by the formation of granitoid nuclei during various episodes of TTG plutonism between 3.2 and 3.4 Ga. The T (sub DM) Sm-Nd model ages for these granitoids range from 3.2 and 3.6 Ga, indicating involvement of sialic crust in their genesis, in agreement with the epsilon (sub Nd(t)) values between -4.0 and -1.3. Between 3.1 and 2.5 Ga, the Gaviao Block was affected by volcanic and sedimentary activity associated with the formation of the Umburanas and Contendas Mirante greenstone belts, intruded by granites about 2.75 and 2.5 Ga ago. During the Paleoproterozoic, the Gaviao Block was regionally deformed and metamorphosed, and intruded by granites between 2.1 and 1.9 Ga. During the Mesoproterozoic, tectonic and metamorphic activity occurred between 1.2 and 1.0 Ga as suggested by resetting of Rb-Sr and K-Ar systematics. Finally, regional tectono-thermal overprints in the areas occurred during the Neoproterozoic. These episodes are accompanied by the intrusion of mafic dikes at about 0.9 Ga. K-Ar ages given by biotites fall within the range 0.5-0.7 Ga and mark the end of the cratonization stage.