Geochemistry and provenance of siliciclastic rocks from the Mesoproterozoic Upper Vazante Sequence, Brazil: Insights on the evolution of the southwestern margin of the São Francisco Craton and the Columbia Supercontinent

Abstract

Studies of provenance and tectonic settings of Proterozoic sedimentary basins on the margins of the São Francisco Craton (SFC) can enhance understanding of its evolution. The Mesoproterozoic Upper Vazante Sequence is made up of siliciclastic and carbonate rocks (mainly dolomites) that were deposited on the southwestern margin of the SFC between 1300 and 1100 Ma and then thrust eastward on top of the SFC during the Neoproterozoic Brasiliano Orogeny (650–550 Ma). The Serra do Garrote Formation, stratigraphically below the main carbonate sequence, is made up of phyllites, carbonaceous phyllites (>1% TOC), as well as minor meta-arenitic rocks. Serra do Poço Verde Formation meta-siliciclastic rocks, which occur interbedded with dolomitic carbonates, are made up of phyllites, carbonaceous phyllites, dolomitic phyllites, carbonaceous dolomitic phyllites, minor meta-arenitic rocks and silty dolomites. Preserved sedimentary features in the meta-siliciclastic rocks include cross-bedding, ripples and mud drapes that are consistent with a shallow, tidally-influenced marine depositional setting. Three subunits (SG1, SG2 and SG3) were identified within the Serra do Garrote Formation based on molar Al/Ti ratios. Samples from the Serra do Poço Verde Formation have Al/Ti ratios similar to Serra do Garrote samples, indicating they shared similar sediment sources. Geochemical indicators of source rock composition and tectonic discrimination plots (Zr/Sc versus Th/Sc, Th – Sc – Zr/10, Hf versus La/Th, and rare earth element) revealed that andesitic to felsic continental arc rocks were the major source of detritus.The majority of concordant detrital zircons are Paleoproterozoic in age, with 207Pb/206Pb dates between 2350 and 1850 Ma, which overlap with the early and late stages of the Trans-Amazonian orogeny (≈2.3–1.9 Ga) and are coeval with construction of the Paleo-Mesoproterozoic Columbia Supercontinent. Detrital zircon provenance patterns suggest a rift-related tectonic setting during deposition. The youngest concordant U – Pb zircon 207Pb/206Pb dates (≈1340 Ma) are coeval with the formation of regional bimodal volcano – sedimentary sequences and layered that are interpreted to have formed during continental rifting. These events are synchronous with increased tectonic and magmatic activity (Mackenzie dyke swarm, North America; mafic dyke swarms, North China Craton) related to the break-up of the Columbia Supercontinent.