Conventional potassium - Argon age of the Shai Hills, Southeastern Ghana

Abstract

KAr ages of samples from the Dahomeyan orogen have been determined. Hornblendes of three plagioclase-garnet-hornblende-gneiss samples from the Shai Hills area, southeastern Ghana give KAr ages of 649 ± 4, 549 ± 8, 562 ± 6 M.a. respectively while hornblendes from two plagioclase-garnet-hornblende-gneisses from about one kilometer east of these samples give ages of 547 ± 2 M.a. and 667 ± 9 M.a.. The average age of 595 ± 29 M.a. is in agreement with recently reported measurements on hornblende of a single rock sample from the Shai Hills. This result supports the belief that the Shai Hills were highly affected by the Pan-African orogeny of the late Precambrian to Cambrian (500 – 700 M.a.) and the Hills did not preserve any record of the Birimian during the Pan-African orogeny. The possible explanation of the 120 M.a. range in ages is briefly discussed.