Pressure-temperature constraints on the evolution of an Early proterozoic plutonic suite in southern Ghana, West Africa

Abstract

Contrasting structural, textural, fabric and mineralogical features between an Early Proterozoic plutonic migmatite suite and granitoids of the Dixcove-type from southern Ghana have been attributed to their contrasting geological histories, despite the fact that they are coeval, cogenetic and have similar geochemical properties. The migmatites occur as strongly foliated, banded, medium- to coarse-grained rocks and display shear and swirled structures. In contrast, the Dixcove granitoids are weakly foliated, homogeneous, fine- to medium-grained porphyritic 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 geothermobarometers applied to constrain their conditions of formation consistently indicate that the migmatites were emplaced at relatively deep crustal levels, in excess of 5 kbars, compared to the Dixcove granitoids which crystallised under relatively shallow conditions, generally less than 5 kbars.

These results suggest that the differences between the migmatites and Dixcove granitoids might have been caused by their different depths of emplacement, rather than deformational or bulk compositional differences as previously thought. These data provide significant constraints to any model for the evolution of these rocks and, indeed, the early Proterozoic terrane of the West African craton.