K-Ar Retention Ages from the Pan-African of Northern Nigeria

Abstract

Twenty-four mineral and whole-rock K-Ar ages have been determined from twenty-two samples collected during recent mapping in northern Nigeria. With three exceptions, the samples are from narrow synclinorial belts of relatively low-grade metasedimentary rocks within the crystalline basement complex. The whole sequence is believed to have been extensively folded and metamorphosed during the Pan-African thermo-tectonic episode. No K-Ar ages older than 700 m.y. were found in the metasedimentary rocks, suggesting they may belong to the Katangan depositional cycle. The majority of metamorphic mica and whole-rock K-Ar ages range from 550 to 490 m.y. and are indicative of a period of post-metamorphic, epeirogenic uplift and cooling initiated in Middle Cambrian times. Late Cambrian deposition of molasse-type sediments in southern Ghana is believed to be the result of this uplift. A single K-Ar age of 656 m.y., obtained from a pegmatitic muscovite, suggests that metamorphic recrystallization was completed during late Precambrian times. K-Ar hornblende ages (690 to 650 m.y.) would appear to support this conclusion, but the presence of excess 40 Ar in the analyzed hornblende samples is indicated by an isochron plot.