The geochronology of some Precambrian rocks of southern West Africa

Abstract

Age determinations completed during the last two years indicate a structural complexity previously undetected in the southern part of West Africa. Holmes and Cahen identified very ancient metamorphic rocks in Sierra Leone, in the western part of the Nigerian Shield. Their date of 2900 m.y. for monazite by the U–Pb method is comparable to a date of 2540 m.y. for biotite, recently determined by the K–Ar method.A Lower Proterozoic geosyncline, developed on the Archean basement, was deformed and metamorphosed 1800 to 2000 m.y. ago. The metamorphic complexes of the Ivory Coast and Ghana belong mainly to this period. The dominant trend of foliation is north to northeast.On the north slope of the Guinean crystalline complex younger granites are dated at 980 m.y. Associated extrusive rhyolites give an age of 1000 m.y.In eastern Ghana and Togo a thick sequency of conglomerates, sandstones, and pelitic sedimentary rocks contain glauconites dated at 620 m.y. These sedimentary rocks are the miogeosynclinal portion of an Upper Proterozoic geosyncline. The eugeosynclinal portion of this geosyncline was metamorphosed about 600 m.y. ago to form the Nigerian gneissic complex. The younger granites in Nigeria were intruded 170 m.y. ago.