Tectonic significance of the Meso- to Neoarchean complexes in the basement of the southern Brasília Orogen

The late Archean was a period of important changes in geodynamic processes and magmatism style. This period seems to mark the time when crustal reworking processes start to predominate over new continental crust generation and is most likely related to the beginning of continental collision and “modern-style” plate tectonics. This study reports a new dataset of zircon U-Pb and Hf isotopes, whole-rock geochemistry and Nd isotopes from Meso- to Neoarchean complexes within the basement of the Neoproterozoic southern Brasília Orogen, SE Brazil. The data provide important insights into the petrogenesis of Mesoarchean TTG suites and their implications for tectonic settings of Archean continental crust generation. Isotopic and geochemical data constrain the timing and nature of the Neoarchean transition from TTG-type to high-K granitic magmatism in the studied area and we discuss the involvement of these complexes in the southern São Francisco paleocontinent assembly. A well-defined period of TTG-type magmatism between 3.00 and 2.96 Ga is identified, with mostly suprachondritic εHf(t) values, between 0 and +5.1, associated with average two-stage model ages between 3.2 and 3.3 Ga. Whole-rock Nd analyses yield TDM ages between 3.2 and 3.4 Ga. These TTGs are interpreted as juvenile additions to the continental crust most likely generated by partial melting of a hydrous mafic crust. The variable trace element compositions of the analyzed TTG samples indicate partial melting at different depths at the same time. These results strongly support the idea of a non-unique tectonic setting of Archean continental crust generation. An additional period of Neoarchean high-K granitic magmatism at ca. 2.76 Ga is interpreted to record the transition from TTG-type to high-K granitoid magmatism in the studied area. This Neoarchean magmatism is associated with less radiogenic isotopic signatures with mostly negative εHf(t) values. The isotopic and geochemical signatures suggest that reworking of the Mesoarchean TTG crust, most likely in a collisional setting, was the main mechanism for the generation of the Neoarchean granitic suite. This supports the idea that the Neoarchean transition from TTGs to granites was most likely a result of the beginning of continental collision processes.