The metamorphic evolution of the Paleoproterozoic (Birimian) volcanic Ashanti belt (Ghana, West Africa)

Most rocks of the southern Ashanti belt have experienced a pervasive greenschist-facies overprint in accordance with observations made in other studies of Birimian rocks. A detailed mineral-chemical and textural examination of mineral relics and mineral cores in combination with geothermobarometry of amphibolites and metagranitoid rocks from a profile across the entire southern Ashanti belt suggests minimum peak-metamorphic conditions of amphibolite-facies grade at 500-650°C and 5-6 kbar for the Eburnean orogenic event. Zoned amphibole and plagioclase supply information on certain stages during the metamorphic evolution of Birimian rocks. In order to obtain information on the shape of the retrograde P-T path, baricentric projections of a reduced amphibole composition space were used. This qualitative descriptive method along with conventional geothermobarometry as well as compositional zoning of minerals reveals a clockwise P-T path with a moderate slope, thus indicating simultaneous cooling and decompression during exhumation. The entire Ashanti belt seems to represent the same crustal level and appears to have undergone a similar P-T evolution.