SM/ND DATA OF METASEDIMENTARY ROCKS FROM THE CENTRAL SEGMENT OF RIBEIRA BELT, SOUTHEASTERN BRAZIL

Abstract

The central segment of Ribeira Belt, southeastern Brazil, comprises several tectonic domains where, during the Proterozoic, extensive detritic material was deposited over pre-1.8 Ga basement rocks. This paper presents Sm-Nd isotopic data on the high-grade metasedimentary rocks of this belt and discusses some possible implications on their sedimentary provenance. The pre-1.8 Ga basement rocks display different Nd isotopic evolution and thus can be promptly discriminated as sources of different Nd model ages (T DM ) and isotopic characteristics such as: Mantiqueira Complex, 3.0 and 2.6 Ga T DM isochrons ages; Juiz de Fora Complex, 2.2 Ga T DM isochron age and Quirino Complex, 2.0 Ga and 3.2 Ga T DM ages. At the Juiz de Fora Domain, metapelites from the Andrelândia Depositional Cycle display a very narrow range of Sm/Nd values (0.15 to 0.17) and T DM ages between 1.8 and 2.0 Ga. The corresponding measured ε Nd values [ε Nd (0)] vary between – 20.8 and –29.1. The metapsammopelites of the Jardim Glória Unit, in the same geologic domain, display very different isotopic characteristics: higher Sm/Nd ratios (0.22 and 0.24), lower ε Nd (0) values (–15.3 and –18.3) and Neoarchean T DM ages (2.5 and 2.6 Ga). At the Paraíba do Sul Klippe, the analyzed metapelites from the middle unit of Paraíba do Sul Group show T DM ages close to 1.7 Ga with e Nd (0) values of -14.10 and -17.34. The metapsammopelites from the basal and uppermost units plot along the 2.0 Ga T DM isochron and display ε Nd (0) values varying between -21 and –32. An isolated T DM of an amphibolitic body, probably intrusive in the metasedimentary rocks of Paraíba do Sul Group, yields 1.0 Ga T DM age, with a 147 Sm/ 144 Nd value of 0.1428.