Detrital zircon age patterns and provenance assessment for pre-glacial to post-glacial successions of the Neoproterozoic Macaúbas Group, Araçuaí orogen, Brazil

Abstract

The Neoproterozoic Macaúbas Group represents the precursor basin system of the Araçuaí–West Congo orogen (SE Brazil – SW Africa), located between the São Francisco and Congo cratons. The Macaúbas basin evolved from continental rift settings to an inland-sea basin (a gulf) partially floored by oceanic crust and connected to aulacogens. Its evolution was coeval with at least one of the major Neoproterozoic glaciations. In this paper, we present more than 450 new U–Pb ages on detrital zircon grains from four units of the Macaúbas Group, covering glacial-related to post-glacial sedimentation stages. The results suggest that the main sedimentary supplies of the Macaúbas basin should have come from secondary sources, located in the São Francisco and Congo cratons. The pre-glacial units of the Macaúbas Group are the only ones that show important contribution from 1100 to 1200Ma sources, and lack Archaean zircons, which are common in younger rocks of the group. These features indicate an important change in the source areas, coherent with the climate change to glacial conditions and with a regional unconformity previously mapped. Sources between 1500–1600Ma are more significant in the continental rift stages, decreasing in the passive margin stage. This suggests changes driven by the end of a glaciation and/or the breakup of the continental lithosphere. Tonian zircons occur throughout all the basin stages, and significantly increase in the late rift and passive margin successions, recording the most abundant rift-related magmatism yet found in the Araçuaí–West Congo orogen. Detrital zircon grains as young as 750Ma reveal contributions from Cryogenian sources to passive margin successions, suggesting sedimentary supplies from the South Bahia Alkaline Province and/or Rio Negro magmatic arc, in Brazil, and/or from rocks found in Africa (e.g., La Louila volcanic rocks). The long lasting evolution of the Macaúbas basin (c. 930Ma to c. 600Ma) would have involved more than one continental rifting event before the onset of an ocean-floor spreading phase around 660Ma.