Stratigraphy, depositional environments and zircon U-Pb (LA-ICP-MS) ages of the Statherian volcano-sedimentary Beneficente Group: Implications for tectonics and gold mineralization in SW of the Amazon Craton

摘要

The Amazon Craton exhibits preserved volcano-sedimentary paleo-basins with age intervals of 2.0-1.96 Ga, 1.90-1.87 Ga and 1.80-1.60 Ga. Recent works in this craton have shown the efficacy of the description and organization of volcanic and sedimentary lithofacies as a tool to postulate stratigraphic frameworks. In the present work we propose the first formal stratigraphy for the Beneficente Group (BG), a Statherian (similar to 1.74 Ga) volcano-sedimentary unit outcropping in the southwestern portion of the Amazon Craton. The BG is divided into three formations: (1) Pedro Sara Formation (PSF), composed of felsic volcanic rocks represented by thick lava-like and densely welded tuffs, normal-graded tuffs, both from pyroclastic density currents, and laminated ash-fall tuffs; (2) Camaiii Formation (CF), comprising sheetfloods, low-energy alluvial channels, non-cohesive sediment-gravity flows, rubbly pahoehoe basaltic flows, stream-channel deposits, and deposits reworked in foreshore/superior shoreface settings, and; (3) Vila do Carmo Formation (VCF), formed by delta front and pro-delta lithofacies associations. The sequence of sedimentary rocks contains clasts exclusively from the related volcanic rocks (PSF), with no clasts from other volcanic sequences nearby. Geochronological analyses evidence a Statherian age for the sequence. A pheno-latite from PSF yielded a crystallization age of 1743.6 +/- 7.5 Ma, the youngest zircons of a sandstone from CF have ages between 1746 and 1732 Ma and the youngest zircons of a sandstone from VCF have 1760 Ma to 1601 Ma. The Beneficente paleo-basin is preserved as a NW-SE rift in the margin of the Tapajos-Parima Province. This structure remained as an active depositional locus during the Paleozoic and is the location of the main courses of modern rivers. The development of BG is akin to Statherian taphrogenic systems which were widespread in the Amazon, Sao Francisco-Congo and North China cratons, and are a key tectonic feature before the Columbia (Nuna) supercontinent break-up.