Isotopic constraints on contamination processes in the Tonian Goias Stratiform Complex

The Tonian Goias Stratiform Complex (TGSC, Goias, central Brazil), is one of the largest mafic-ultramafic layered complexes in the world, emplaced during the geotectonic events that led to the Gondwana accretion. In this study, we present trace elements and in-situ U/Pb-Lu-Hf analyses of zircons and Sr-87/Sr-86 ratios of plagioclases from anorthosites and gabbros of the TGSC. Although formed by three isolated bodies (Cana Brava, Niquelandia and Barro Alto), and characterized by a Lower and Upper Sequence (LS and US), our new U/Pb zircon data confirm recent geochemical, geochronological, and structural evidences that the TGSC has originated from a single intrusive body in the Neoproterozoic. New Hf and Sr isotope ratios construe a complex contamination history for the TGSC, with different geochemical signatures in the two sequences. The low Hf and high Sr isotope ratios of the Lower Sequence (epsilon Hf(t) from -4.2 down to -27.5; Sr-87/Sr-86 = 0.706605-0.729226), suggest the presence of a crustal component and are consistent with contamination from meta-pelitic and talc-silicate rocks found as xenoliths within the Sequence. The more radio genic Hf isotope ratios and low Sr isotope composition of the Upper Sequence (epsilon Hf(t) from 11.3 down to -8.4; Sr-87/Sr-86 = 0.702368-0.702452), suggest a contamination from mantle-derived metabasalts in agreement with the occurrences of amphibolite xenoliths in the US stratigraphy. The differential contamination of the two sequences is explained by the intrusion of the TGSC in a stratified crust dominated by metasedimentary rocks in its deeper part and metavolcanics at shallower levels. Moreover, the differential thermal gradient in the two crystallizing sequences might have contributed to the preservation and recrystallization of inherited zircon grains in the US and total dissolution or magmatic overgrowth of the IS zircons via melt/rock reaction processes. (C) 2018 Elsevier B.V. All rights reserved.