Abstract

The evolution and consolidation of the Birrimian part of the West African shield covered a much longer time span than is generally believed. K-Ar ages of seven new samples reported here range from 1,206 to 2,487 m.y. old. The Eburnean (Birrimian) orogeny at the end of early Proterozoic time (2,150 to 1,950 m.y. ago) was only one of several events which have shaped the region through Proterozoic time, and western Niger does not even seem to have been affected by it.

In the Aïr Mountains of the central portion of northern Niger, no dates older than 660 m.y. have yet been found, although a complex Precambrian history is suspected from geologic studies; the Pan-African event (about 600 m.y. ago) has strongly overprinted earlier features.

While the “Younger Granites” of the Jos plateau in northern Nigeria were intruded in Jurassic time, the similar granites of the “young” ring complexes in the Aïr Mountains are not younger than early Carboniferous in age.

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