Discovery of Precambrian deep-water turbidites and submarine volcanism in the Brasília Belt, Central Brazil: The Quilombo Formation

Abstract

A deep-water volcano-sedimentary sequence exposed along the Brasília Orogenic Belt was discovered and is herein presented and ascribed to a new stratigraphic unit. This sequence was previously unknown to contain volcanic rocks and to represent a turbidite record. It has been earlier loosely assigned to the Vazante Group based only on map continuity and interpreted as part of the Neoproterozoic passive margin of the western São Francisco Craton, but comprehensive studies have not been conducted for this specific sequence. Based on recent detailed fieldwork integrated with petrology, stratigraphy, geochronology and geochemistry, we describe a previously ignored succession of volcanic rocks interleaved in deep-water turbidites in the Brasilia Belt, and named as the Quilombo Formation. This formation comprises two sub-units, the Ribeirão da Porteira Member (lithoarenite, greywacke, siltstone, shale, and minor conglomerate) and the Ribeirão do Franco Member (fine-grained greywacke, siltstone, shale, and minor intermediate volcanic layers interpreted as subaqueous lavas). The lavas are strongly altered by SiO2 enrichment and depletion of Ca, K and Na. Based on immobile and rare earth elements analysis results, the volcanic rocks are classified as andesite to basaltic-andesite with a calk-alkaline to shoshonitic signature. U–Pb zircon dating of crystals extracted from lava samples yielded an age of 1394.46 ± 19.4 Ma, considered as the maximum depositional age of the sequence. Nonetheless, given the scarcity of zircon grains in the volcanic sample and the possible presence of xenocrysts, the exact age of the volcanic rocks still demands confirmation. The Quilombo Formation is for sure stratigraphically positioned between the Traíras Group (maximum depositional age of ca. 1543 ± 31 Ma) and the Bambuí Group (Ediacaran age). However, it is not yet possible to establish a confident correlation with either the Vazante, Paranoá, Canastra or (Upper) Traíras groups. Nonetheless, the volcanic rocks found represent the first tangible chance to obtain an absolute age for one of these groups, up to now only dated via detrital zircon ages.