Provenance And Depositional Setting Of The Buem Structural Unit (Ghana): Implications For The Paleogeographic Reconstruction Of The West African And Amazonian Cratons In Rodinia.

Abstract

We present new field, petrological and geochemical data, combined with U-Pb zircon ages and Lu-Hf isotope compositions for the sandstones of the Neoproterozoic Buem structural unit (BSU) of the Dahomeyide belt, and whole-rock geochemical data of BSU shale to investigate their provenance and depositional setting. The BSU contains siliciclastic sequences and fragments of oceanic lithosphere (pillow lavas, gabbro, and peridotite) that have archived the entire evolution of the Ediacaran West Gondwana Orogen (WGO), from early accretion to final amalgamation of the West African Craton (WAC) with the Benino-Nigerian Shield. Three broad groups of samples exist within the BSU in Ghana: those with dominantly older age fractions of 2300 – 1800 Ma, represented by samples from the base of the BSU; those with prominent 1700 – 1100 Ma zircons, occupying the middle part of the BSU; and those with significant 1000 – 970 Ma age fractions, forming the uppermost part of the BSU. Results from this study, together with published data on the BSU, and adjacent Togo structural unit and Voltaian Supergroup, reveal two main sedimentary sequences in the Dahomeyide belt, i.e., passive margin and foreland basin sequences with three potential provenances: Amazonian Craton, Benino-Nigerian Shield, ± WAC. This interpretation resembles that for the sedimentary rocks of the Borborema Province, NE Brazil, which implies similar evolution along the WGO. Thus, a long, >2500 km passive mar gin basin developed at 1000 – 700 Ma, which was subsequently inverted during the Braziliano/Pan African plate convergence and collision, and the concomitant formation of the foreland basin during the assembly of the supercontinent Gondwana. Because a greater portion of the detritus in the BSU is probably from the Amazonian Craton, we propose that the WAC and the Amazonian Craton adjoined each other from the Paleoproterozoic onward within the supercontinents Rodinia and Gondwana, until the opening of the Atlantic Ocean.