Nd AND Pb ISOTOPE STUDIES BEARING ON THE CRUSTAL EVOLUTION OF SOUTHEASTERN BRAZIL

Abstract

Nd- and Pb-isotopes have been determined on composite samples of selected granites. gneisses, and metasediments from SE Brazil, which range in age from Archaean lo Late Proterozoic (Brasiliano). A brief introduction to Sm-Nd systematics is followed by discussion of the Sm-Nd isotope data on the composite samples which yield model Nd ages of 3.3- 1.3 Ga. Pb isotope compositions are consistent with those model ages, and Nd- and Pb-Isotopes together indicate Ihat, as in southern Africa, lhe main period of crustal growth was in the Late Archaean and Early to Middle proterozoic. The Brasiliano event appears to have been characterized by relatively little new continental crust.