The Siderian-Orosirian magmatism in the Gavião Paleoplate, Brazil: U–Pb geochronology, geochemistry and tectonic implications

Abstract

The southern portion of the Gavião Paleoplate is composed by Archean orthogneisses, Archean-Paleoproterozoic metavolcano-sedimentary rocks and Siderian-Rhyacian-Orosirian granitoids. Petrographic, geochemical, U–Pb (Laser Ablation, ICPMS) and Sm-Nd data are presented for five Paleoproterozoic granitoids that were recently mapped: Jussiape II, Lagoa das Almas, Humaitá, Belo Campo and Broco granitoids. These granitoids present U–Pb zircon (LA-ICPMS) ages of 2052 ± 43, 2114 ± 24, 2140 ± 9, 2049 ± 23 and 2038 ± 8 Ma, respectively. In addition to these granitoids, another twenty-five ones were identified and studied by several authors, resulting in a total of twenty-nine plutons. Despite the previous petrography, geochemistry and geochronology studies that have been performed, no model had been proposed to explain the tectonic setting of this extensive granitogenesis. Integration of the new data and the literature has been done and corresponds to the second part of the article. Based on U–Pb dating and geochemical data, Siderian-Rhyacian-Orosirian granitoids of the southern Gavião Paleoplate were classified into five groups, or five suites: 1 (2324 ± 6 to 2091 ± 6.6 Ma), 2a (2054 −6/+8 to 2041 ± 23 Ma), 2b (2066 ± 37 to 2019 ± 32 Ma), 2c (2058 ± 8 to 1852 ± 50 Ma) and 2d (2049 ± 12 to 1929 ± 16 Ma). The granitoids of Group 1 present heterogeneous deformation, while the granitoids of groups 2a to 2d are generally not deformed. Usually the rocks are potassic, but sodic granitic rocks can be found in samples of groups 1, 2c and 2d. Several chemical classification parameters are presented and discussed herein, but it is noteworthy that the granitoids of Group 1 are mainly classified as calcic to calc-alkalic, while the rocks of the second group are mostly classified as alkalic ones. In the remaining groups, the samples vary between calc-alkalic and alkali-calcic. The εNd values range between 4.0 and −15.4 and suggest an important and varied share of the continental crust in the formation of these rocks. The Humaitá granitoid probably presents the lowest contribution from the continental crust in its genesis. The rocks from Group 1 were generated as the product of an active continental margin arc situated eastwards from the southern portion of the Gavião Paleoplate. The spatial distribution between the granitoids of groups 1 and 2a suggests westwards subduction and led to a cordilleran model for the Western Bahia Magmatic Arc. Continental collision between the Gavião and Jequié paleoplates occurred around 2.09 Ga and was followed by the setting of late-collision granitoids of groups 2a to 2d. The comparison between the tectonic model presented in this study and other existing models that explain the Siderian-Rhyacian-Orosirian granitogenesis of the Mineiro Belt and Mantiqueira Complex suggests a continuation of the Western Bahia Magmatic Arc and of the collisional orogen that followed, towards the south. The Western Bahia Magmatic Arc emplacement occurred before the Paleoproterozoic granitogenesis of the Serrinha (Bahia-Brazil) and Congo (Africa) paleoplates.