THE NATURE, DISTRIBUTION AND RELATIONSHIP OF ARCHAEAN TERRANES IN EASTERN BRAZIL

Abstract

This is a study of the general geological characteristics, distribution and relationship of the major Archaean terranes of eastern Brazil. It also íncludes a concise descriptíon of several líthostratigraphic sequences of typical greenstone signature and associated granite-gneisses belonging the São Francisco Craton, which is interpreted as the only Archaean cratonic area within the region under consideration. The greenstone successions, however, present a poorly preserved stratigraphy, due mainly to the emplacement of gneissic tonalite, and granite plutons, and to the proportion of volcanics, particularly of komatiític composition, ís low, while aquagene tuffs and associated sediments form the most important lithological components. The high-grade metamorphic terranes, subjected to granulite facies of regional metamorphism most1y during the late Archaean (~ 2.7 Ga), are represented by the Jequié and Goiás mobile belts, and the Santa Catarina and Santa Maria Chico complexos. In the first two belts, graníte-greenstone nucleus display relatively well preserved stratigraphic sequences exist within the high-grade metamorphites as isolated remnants, while adjaceut areas have been reworked by a intense tectono-thermal evento. In comparison to Archaean terranes in other parts of the world, the Brazilian counterparts are rather different in features such as transitional boundaries, fragmentary patterns and poorly preserved greenstone sequences.