Preliminary Report on the Paleoproterozoic Metasediments Genesis and Features of the Banfora Belt Western Burkina Faso (West Africa Craton)

Metasedimentary rocks of the Banfora belt are examined for their petrography geochemistry and genesis. The main formations are pelite metapelites sandstone, and metagraywackes. The first ones show a fine interlocking layering with unclear graded bedding in the sandstone compared to the metapelites, while the metagraywackes show a clearly discordant graded bedding of light grey to light pink color. The low SiO2/Al2O3 ratios (3.67 to 6.60) suggest their low sedimentary maturity. The metapelites and sandstone-pelites show moderate Na2O/K2O values (10.13 - 22.69), indicating moderate chemical maturity; in contrast, the metagraywackes show low Na2O/K2O values (1.94 - 5.80) suggesting low chemical maturity. The metapelites, sandstone-pelites and metagraywackes have Rb/Sr ratios of 0.866 - 0.004 and 0.173 - 0.607 respectively, indicating a moderate to low degree of chemical alteration. The chemical alteration index (CIA = 58 - 83) and the plagioclase alteration index (PIA = 53 - 81) suggest weak to moderate alteration of the source rocks. This alteration is different from simple alteration as the sole control of chemical composition, but is associated with metasomatism. The metapelites sandstone shows a mixture of mafic and intermediate igneous sources, which indicates that the protoliths could be basalts and andesites, but the metagraywackes would be derived from the erosion of a mixture of andesitic to granitic rocks. The studied rocks were generated from young undifferentiated to differentiated arcs. They are of low to moderate sedimentary and chemical maturity.