Geochemistry Of Neoproterozoic Phyllites From The Akuapim Range, Southeastern Ghana

Abstract

Neoproterozoic phyllites from the Akuapim range, a prominent part of the Togo Structural Unit forms part of the Pan-African Dahoeyide orogen exposed at the southeastern part of Ghana. The area of study exhibit three varieties of phyllites which are carbonaceous, ironstained and micaceous phyllites. These phyllites have been analyzed for their mineralogical and geochemical characteristics to infer their source -area weathering, source rock composition and tectonic setting. The mineralogical content of ten (10) samples were determined using Xray diffraction method. Twenty-eight (28) samples were analyzed for their major, trace and rare earth element compositions using ICP-AES and ICP-MS. From the mineralogy, the phyllites are composed of quartz, feldspar, muscovite, chlorite, illite with some siderite, graphite and calcium carbonate. Compared to post-Archean Australian average Shale (PAAS), the phyllites are depleted in major elements with the exception of SiO2 and MnO which are enriched relative to PAAS. The phyllites show relative depletions in transition metals such as Sc, V and Cr whereas high field strength elements such as Zr, Hf, Y and Th are at an average to above-average concentrations as compared to PAAS. The phyllites show a single trend of LREE enrichment (average LaN/SmN = 3.78), nearly flat HREE and negative Eu anomaly (average Eu/Eu* = 0.61) similar to the trend of post-Archean Australian average shale. From geochemistry, the phyllites can be classified mostly as quartz-rich with few quartzintermediate. Al2O3 shows positive correlations with Sc, V, Cr, Rb, Sr, Cs, Ba and Y suggesting that these elements are primarily controlled by the dominant clay minerals. High values of K2O/Na2O (average = 54.31) and SiO2/Al2O3 (average = 8.17) of the phyllites indicates compositional maturity of the sediments. The phyllites reveal high concentrations of Zr (average = 408.08 ppm) which suggests sedimentary sorting. This is supported by the plot of Th/Sc against Zr/Sc. High Chemical Index of Alteration (average = 74.73%), Plagioclase Index of Alteration (average = 87.68%), Chemical Index of Weathering (average = 98.36%), Rb/Sr (average = 4.22), Th/U (average = 4.09) values and low Index of Compositional Variability (average = 0.64) values of the analyzed samples indicate intense chemical weathering in the source areas. On the A-CN-K ternary diagram, data for the Neoproterozoic phyllites from the Akuapim range plot in a trend (along A-K line) indicating intense chemical weathering. The phyllites show geochemical characteristics typical of sediments derived from old Upper Continental Crust. Modeling of the average phyllites using the relatively immobile REEs and Th/Sc shows that the sediments are composed of a mixture of 80% Upper Continental Crust and 20% granite. The phyllites show mineralogical and geochemical characteristics indicative of sediments deposited within a passive continental margin and/or intra-cratonic terrane. The phyllites exhibit provenance characteristics similar to that of the Kwahu/Bombouaka Group of the Voltaian Basin and the Buem Structural Unit which are believed to be lateral equivalents and are probably derived from the same source in the Amazonian Craton.