Birrimian metamorphic and associated granitic rocks (Precambrian), south-central Ghana, West Africa
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The Birrimian System forms most of the exposed Precambrian shield of West Africa. About half of Ghana is underlain by Birrimian metamorphic and granitic rocks. In the Dunkwa S. E. area (Field Sheet 48) of south-central Ghana, interlayered sedimentary and volcanic rocks were metamorphosed to greenschist and amphibolite facies. These sheared and tightly folded metamorphic rocks have incipient retrograde metamorphism of biotite and garnet to chlorite or hornblende to biotite. Two large intrusive bodies in the metamorphic terrain may be partly the result of granitization: (1) the Pra River complex, an albitized laccolith or sill, consisting of quartz monzonite, quartz diorite, and granodiorite; (2) the Twifu Praso complex of albite granite and granodiorite gneiss. All of the above rocks were folded along northeast-trending fold axes. Then a third granitic body, the Wuwu River albitized quartz monzonite, intruded and locally re-folded the older rocks.