Idades U-Pb e Lu-Hf das Rochas do Complexo Caparaó, porção Meridional do Orógeno Araçuaí, Espírito Santo-Minas Gerais, Brasil

Abstract

The study was carried at the Serra do Caparaó region, located between the border of Minas Gerais and Espirito Santo states. The lithological associations found in this area were correlated to the Juiz de Fora Complex and may represent part of the Rhyacian (ca 2.19 Ga) ortho-derived basement, occurring as a inlier tectonic in the passive-margin-derived paragneiss, in the eastern portion of the Araçuaí Orogen. Therefore, the systematic study of U-P and Lu-Hf (LA-ICP-MS) isotopic ages in Zircons aim to investigate the tectonic implications related to the nature of Caparaó Complex granulitic rocks focusing on the better understanding and characterization of the southern Paleoproterozoic evolution of the São Francisco Craton. U-Pb and Lu-Hf isotopic analysis were carried in fourteen samples from different stratigraphic units of the Serra do Caparaó and one from the enclosing rocks which is represented by the Neoproterozoic-Cambrian granitoids (Galileia Pluton). The Zircon Imaging Technique from cathodoluminescence was used for the U-Pb isotopic analysis in order to observe the grains internal and external structure, as well as variations in the rates of luminescence. This method allowed the identification of zoning patterns, which are both magmatic and metamorphic, and enabled for the select specific areas for U-Pb dating and thus realize the distinction between magmatic crystallization ages and metamorphism. The ages found in the majority samples indicates that at least two thermal events occurred in the area. The former and oldest is related to the Transamazônico Paleoproterozoic magmatism whereas the second event age may represent the isotopic homogenization of the magmatic protoliths during the Brasiliano event. The age of the oldest crystallization of these rocks was 2,209 ± 22 Ga and is found within the Granulitic Ortognaiss Unit of the Complex. Since the age of 1.775 ± 13 found in the upper intercept of a sample of Unit Orthogneiss migmatitic indicated that the crystallization period lasted until this period. In the samples that registered a number of ages Neoproterozoic the geochronological interval was 633 ± 22 to 584 ± 29 Ma. Finally the Lu-Hf data suggested εHf values indicating juvenile magmatism (mantle) with involvement of continental crust in the genesis of these rocks, soon occur any contamination which would explain the variability of the values of this parameter. From all acquired results became possible to correlate the Caparaó Complex to Land Juiz de Fora and determine four evolutionary stages for the area in question