The Andrelândia depositional cycle (Minas Gerais/Brazil), a post-transamazonic sequence south of the São Francisco Craton: Evidence from U-Pb dating on zircons of a metasediment

Abstract

U-Pb age determinations were carried out on zircons from a quartz-rich micaschist of the Andrelândia depositional cycle, part of a mobile belt adjacent to the southern border of the São Francisco Craton. The metamorphism suffered by the sampled unit is characterized by the mineral assemblage: almandine + kyanite + rutile + muscovite + quartz. Zircons separated from the sample are globular (95%), transparent to translucent and slightly yellow coloured. Three grain size fractions and an abraded fraction of this zircon type yield discordia intersection ages with the Concordia at 1872 ± 11 Ma and 567 ± 11 Ma. Three grain size fractions with increasing amount of a third, slightly older inherited lead component were omitted from discordia calculation. The 1872 Ma-age, representing a minimum age for the main phase of zircon growth in the source area, limits the maximum age for the onset of sedimentation. According to U and Th contents, the U/ Th ratio and the limited amount of inherited lead of pre-Transamazonic age the detritus is mainly derived from magmatic rocks of the Transamazonic basement, predominantly crystallized within a time span of at most 130 Ma. No indication of a significant thermal event during the subsequent long lasting interval of calm and sedimentation (about 1.3 Ga) was found. The end of sedimentation is still under debate. Because of the large discordance of the data points to the lower intercept in the Concordia diagram (Brasiliano thermo-tectonic event), newgrowth of zircon and/or a distinct loss of radiogenic lead during this phase of thermal overprint can be considered as low. An early high pressure-stage of metamorphism was dated at about 600 Ma by Sm-Nd in garnets versus whole rock (Trouw & Pankhurst, 1993). Because of the difference of about 40 Ma between this age and the age presented above it seems probable that the new- and/or overgrowth of zircon happened predominantly during a second lower pressure and higher temperature stage of the same metamorphic event (M1), associated with the migration of a H 2O-rich fluid phase and limited anatexis.