Temperature-pressure distribution patterns in metamorphosed rocks of the Nigerian basement complex – a preliminary analysis

Abstract

The Nigerian Basement Complex has undergone polyphase deformation and polycyclic metamorphism during the Proterozoic and early Phanerozoic periods. Andalusite-sillimanite type, kyanite-sillimanite type and metamorphic pressure types between these two have been recognized in the basement. The metamorphic grade ranges from middle greenschist facies (400°C) through medium (530–560°C) to upper amphibolite facies (680–750°C). Pressure, during the metamorphism, varied from low (2 to 3 Kbar) for the andalusite-sillimanite type to high (7 to > 9 Kbar) for the kyanite-sillimanite type. Low pressure minerals such as cordierite and andalusite were developed in some localities due to contact metamorphic effects caused by the emplacement of Older Granite batholiths in already regionally metamorphosed terrains. The association of sillimanite zone rocks with migmatites in most parts of the Nigerian basement indicates that the region experienced partial melting owing to high-grade amphibolite facies regional metamorphism (680–750°C) during the Pan-African and possibly earlier tectonothermal events.