Tonian rift-related, A-type continental plutonism in the Araquai Orogen, eastern Brazil: New evidence for the breakup stage of the Sao Francisco-Congo Paleocontinent

摘要

Zircon U-Pb SHRIMP, petrographical and geochemical data lead to the first characterization of the Tonian plutonism (Salto da Divisa Granite Suite), ascribed to the continental rift stage of the precursor basin of the Aracuai Orogen (Eastern Brazil). The suite includes batholitic plutons and comprises mainly fluorite-bearing, dominantly mesoperthitic hornblende-biotite leucogranites. The presence of mafic (tholeiitic) gabbroic enclaves and syn-plutonic dykes confers to the suite a bimodal character. The plutons were locally deformed and foliated under amphibolite facies conditions, in response to the Neoproterozoic collage of the Aracuai Orogen against the Sao Francisco Cratonic margin. However, undeformed magmatic facies are well preserved at inner portions of the plutons. The granitoids are metaluminous, with high SiO2 and HFSE: Nb, Zr, Y, Ta and REE (except Eu); low CaO, Al2O3, Sc, Ba, Sr; high FeOt/MgO ratios, characterizing a chemical signature akin to the subalkaline, A-2 type granites. U-Pb SHRIMP data obtained on zircons from the main pluton yielded a magmatic crystallization age of 875 +/- 9 Ma. Some inherited xenocrysts revealed ages of ca. 2080 Ma, corresponding to ages of the host rocks, a Paleoproterozoic basement. Nd isotopic evolution studies confirm the Paleoproterozoic influence on magma genesis with a T-DM model age of ca. 1.6 Ga and epsilon(Nd) of -5.58 at 880 Ma. The African counterpart, the West Congo Belt, encompasses thick rift-related alkaline volcanic-sedimentary basin (Zadinian and Mayumbian groups, and associated anorogenic granites), dated in the interval of ca. 1000-900 Ma. The age differences between the Salto da Divisa Suite intrusion and the anorogenic magmatic episode at the West Congo Belt suggests a westward migration (i.e. to the Brazilian side) of the thermal axis of the rift, ca. 30 Ma after the ending of the extensional process in Africa. (C) 2007 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.