PETROGRAPHIC AND GEOCHEMICAL CONSTRAINTS ON TECTONIC SETTINGS OF THE BIRIMIAN SUPERGROUP VOLCANIC ROCKS, EVIDENCE FROM NEW DROBO ENVIRONS SOUTH OF JAMAN DISTRICT IN THE BONO REGION OF GHANA

Abstract

The petrographic and geochemical studies of Birimian Supergroup meta-volcanic rocks in the New Drobo environs in Ghana help to decipher the tectonic settings at the study area. Twenty thin sections were prepared with rock samples from the field at the KNUST Geological Engineering Laboratory and petrographic microscope used to determine the different types of minerals in the samples and also the rock type. Whole rock geochemical analysis was done, using both X-Ray Fluorescence and Inductively Coupled Plasma Mass Spectrometer methods at Australian Laboratory Services in Canada. The main volcanic rock types at the study area comprise of basalt, andesite, dacite and rhyo-dacite with porphyritic mineralogical composition such as plagioclase feldspar, augite, olivine, hornblende, biotite, quartz, rutile, chlorite. The average concentrations of Zr < 150 ppm, TiO2 < 1.5 % and P2O5 < 0.25 % and the ratios of Nb/Y < 1.2 and Y/Nb > 1 reveal the magma type in the study area to be continental tholeiitic basalt in nature. The mafic volcanic basalt is from a plate margin tectonic setting and thus of normal MORB and volcanic arc basalts. Also, the low basaltic values of the ratios of Ti/Y and Nb/Y confirm the tectonic setting of the area to be of plate margin. Zr is highly incompatible with respect to Ti and renders the ratio of Zr/TiO2 to be influenced by partial melting and causes heterogeneity in the mantle. The heterogeneity in the source of the mantle can be confirmed with the high value of the ratio of Zr/Y.