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

Most rocks of the southern Ashanti belt have experienced a pervasive greens chist-facies overprint in accordance with observations made in other studie s of Birimian rocks. A detailed mineral-chemical and textural examination o f mineral relies and mineral cores in combination with geothermobarometry o f amphibolites and metagranitoid rocks from a profile across the entire sou thern Ashanti belt suggests minimum pc:ak-metamorphic conditions of amphibo lite-facies grade at 500-650 degrees C and 5-6 kbar for the Eburnean orogen ic event. Zoned amphibole and plagioclase supply information on certain sta ges during the metamorphic evolution of Birimian rocks. In order to obtain information on the shape of the retrograde P-T path, baricentric projection s of a reduced amphibole composition space were used. This qualitative desc riptive method along with conventional geothermobarometry as well as compos itional zoning of minerals reveals a clockwise P-T path with a moderate slo pe, thus indicating simultaneous cooling and decompression during exhumatio n. The entire Ashanti belt seems to represent the same crustal level and ap pears to have undergone a similar P-T evolution. (C) 1999 Elsevier Science B.V. All rights reserved.