PRESSURE-TEMPERATURE CONSTRAINTS ON THE EVOLUTION OF AN EARLY PROTEROZOIC PLUTONIC SUITE IN SOUTHERN GHANA, WEST-AFRICA

Contrasting structural, textural, fabric and mineralogical features be tween an Early Proterozoic plutonic migmatite suite and granitoids of the Dixcove-type from southern Ghana have been attributed to their con trasting geological histories, despite the fact that they are coeval, cogenetic and have similar geochemical properties. The migmatites occu r as strongly foliated, banded, medium- to coarse-grained rocks and di splay shear and swirled structures. In contrast, the Dixcove granitoid s are weakly foliated, homogeneous, fine- to medium-grained porphyriti c rocks associated with volcanic rocks and show extensive hydrothermal alteration. There is no observable variation in P-T regime within the migmatite terrane exposed in southern Ghana, yet different geothermob arometers applied to constrain their conditions of formation consisten tly indicate that the migmatites were emplaced at relatively deep crus tal levels, in excess of 5 kbars, compared to the Dixcove granitoids w hich crystallised under relatively shallow conditions, generally less than 5 kbars. These results suggest that the differences between the m igmatites and Dixcove granitoids might have been caused by their diffe rent depths of emplacement, rather than deformational or bulk composit ional differences as previously thought. These data provide significan t constraints to any model for the evolution of these rocks and, indee d, the early Proterozoic terrane of the West African craton.