PETROGRAPHY AND GEOCHEMISTRY OF SOME PALEOPROTEROZOIC GRANITOIDS AT THE NORTH-EASTERN MARGIN OF THE KUMASI BASIN IN GHANA

Abstract

This study investigates basin-type granitoid samples from the north-eastern margin of the Kumasi Basin in Ghana to establish their source and geodynamic setting. Petrographic analysis, TAS and A/NK-A/CNK plots classify the granitoids as metaluminous quartz diorite, metaluminous granodiorite, and peraluminous monzogranite; and exhibiting I-type signatures. These rocks are formed by magma differentiation and/or partial melting at various stages. Distribution patterns of incompatible elements and the positive Eu/Eu* anomalies of 1.15 and 1.47 exhibited by quartz diorite and granodiorite, respectively, the values suggest the rocks crystallized from melts formed in a water-saturated environment. The negative Eu/Eu* anomaly exhibited by monzogranite indicate fractionation of plagioclase in the final stages of the magma evolution. The water-rich environment is probably due to dewatering of the basin’s foreland volcaniclastic sediments during regional subsidence, burial and metamorphism. K2O enrichments and wide variations suggest that the granodiorite and monzogranite are formed from fractional crystallization and/or crustal assimilation of the continental crust by under-plating dioritic magma. The higher Al2O3/TiO2 enrichment and the shift from metaluminous to peraluminous in the monzogranite suggest a longer residence time within the continental crust, during which fractional crystallization and the assimilation of pre-existing crustal components into the dioritic magma that resulted in the formation of the monzogranite. The study requires replication at other areas within the basin to generate enough data to enhance metallogenic studies in the terrain.

Keywords

• magma differentiation
• fractional crystallization
• crustal assimilation
• peraluminous
• geotectonic setting.