Composition of Amphiboles from Toro Dioritic Complex, Northcentral Nigeria: A Potential Petrogenetic and Geothermobarometric Indicator

Abstract

The Toro symmetrical complex is localized within the Precambrian Basement Complex of the north central Nigeria. The complex is composed of three granites types surrounding an orthoenstantite dioritic core. Amphiboles samples from the diorite have higher Mg/ (Mg+Fe) values and are richer in Si, Ti and poorer in Al and Na+K than those in the granites. The calculated P-T results show that diorite crystallized at pressures between 4.7 kbar and 5.4 kbar and temperatures of 796°C to 821°C. The hornblende-biotite granite crystallized at pressures between 5.8 kbar to 8.5 kbar and temperatures between 633°C to 771°C. The average pressure (5.1 kbar ) for dioritic amphibole corresponds approximately to 19 km in depth of emplacement for the diorite and 24km in depth of emplacement for both hornblende-biotite (average pressure = 6.5 kbar) and porphyritic hornblende-biotite granites (average pressure = 6.4 kbar). Nature of dioritic amphibole evolution, coupled with its P-T suggests that the Toro diorite may have originated earlier with non consanguineous magma source different from the granites, which corresponds to a model of origin involving melting in the lower crust under granulite facies conditions.