Paleoenvironment, sediment provenance and tectonic setting of Tonian basal deposits of the Macaúbas basin system, Araçuaí orogen, southeast Brazil.

Abstract

The Neoproterozoic Macaúbas Group represents the main precursor basin system of the Araçuaí orogen, including a Tonian volcano-sedimentary succession, at the base, overlain by a Cryogenian sequence. Belonging to the Tonian succession, the Matão – Duas Barras Formation, exposed at the southern border of the Porteirinha basement block in the proximal (western) Araçuaí orogen, represents the very first stage of rifting ever recorded in the Macaúbas basin system. Sedimentological analysis on that formation has revealed two distinct and consecutive facies associations, both indicating half-graben sedimentary filling: i) fault-sourced alluvial fans; and ii) fluvial deposits associated with braided rivers. U-Pb ages of detrital zircon grains from the Matão – Duas Barras Formation and its underlain unit (ascribed to the Espinhaço Supergroup) uniformly show a dominant Rhyacian to Orosirian peak at ca. 2182 Ma – 1863 Ma, as well as several minor Archean age peaks from ca. 3186 Ma to ca. 2655 Ma. The Matão – Duas Barras fluvial deposits also show two Mesoproterozoic age clusters at ca. 1520 Ma and ca. 1156 Ma. Together with sedimentological features, the Archean and Paleoproterozoic zircon ages indicate sediment provenance from the granite-gneiss sources located in the neighboring Porteirinha block, which would have acted as a basement high (a large horst) at least during the initial filling of the Early Tonian Macaúbas rift. The ca. 1520 Ma and ca. 1156 Ma peaks are in good agreement with the Calymmian and Stenian development stages (II and III) of the Espinhaço Supergroup, which acted as an important secondary sediment sources for the basal Macaúbas Group. This also suggests that the Mesoproterozoic sources were exposed by the Porteirinha block uplift coeval with the Matão – Duas Barras basin filling. The maximum sedimentary ages given by the youngest zircon grains, together with a thorough literature compilation, bracket a depositional interval for the basal Macaúbas deposits from 1000 Ma to 933 Ma. Changes in sedimentary style and geochronological spectra suggest variations in the tectonic activity and subsidence rates of the Early Tonian Macaúbas basin, marking its first stage as an entirely siliciclastic sedimentation.