Abstract

Thirty-one new potassium-argon age determinations from the Archean basement gneisses and overlying metasedimentary sequences define more precisely the effects of the Pan-African episode in Uganda. The majority of mica retention ages from the basement complex gneisses (650 to 540 m.y.) reflect a widespread late Precambrian-early Paleozoic thermal event, which partially or completely overprints the isotopic record of the earlier events. Relict K-Ar biotite ages of up to 2,860 m.y., obtained from granite and granite-gneisses near Soroti, within the basement complex in central Uganda, are indicative of a much earlier period of plutonic activity.

Low-grade metasediments of the Buganda-Toro System, overlying the basement gneisses to the south, were not affected by the Pan-African event, and retain whole-rock K-Ar ages of up to 1,860 m.y. Granite gneisses near Iganga in southern Uganda, also unaffected by the Pan-African event, yield concordant K-Ar biotite ages of 1,840 m.y., but low-grade meta-sediments of the Bulugwe Series, near the Kenya border (correlatives of the Nyanzian of Kenya), yield significantly older whole-rock K-Ar ages of up to 2,375 m.y.

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