RbSr and KAr geochronometry of southeastern Ghana

The Cape Coast granite complex of the West African Craton in southern Ghana has been dated as 2051 ±45 Ma by whole rock RbSr;. Mineral dates are ∼ 20006 for hornblende by KAr, 1974 ± 28 Ma for biotite by RbSr and ∼ 1900 Ma for biotite by KAr. A few biotite dates are somewhat younger. The low whole-rock ⁸⁷Sr/⁸⁶Sr initial ratio, 0.7015 ± 0.0002, indicates that the granites were created shortly before 2050 Ma. The successively younger mineral dates result from uplift, erosion, and cooling following the Eburnian cratonization event.

Dahomeyan metamorphic rocks of southeastern Ghana give biotite RbAr dates of 330–542 Ma, mostly 508–542 Ma, due to the Pan-African orogeny. Hornblende KAr dates are somewhat older, at 561–611 Ma.

Gneisses from Teshie, Numgua, and Tema Beach are parts of a reworked older basement at least 2841 ± 468 Ma old with an initial ratio of 0.7041 ± 0.0014. Gneisses from Ho and Dzigakope also appear to be pre-Eburnian with RbSr dates of 2176 ± 44 Ma and about 2800 Ma, respectively, but this is not as well established. Nine whole-rock samples of Dahomeyan gneiss scatter about an isochron of 515 ± 16 Ma with an initial ratio of 0.7056 ± 0.0002.

An intrusive carbonatite from Kpong gives concordant whole-rock RbSr, biotite RbSr, and biotite KAr dates of 593 ± 250, 545 ± 11 and 572 ± 15 Ma, respectively, but it also gives discordant and geologically contradictory plagioclase KAr and Rb dates of 665 ± 20 and 975 ± 167 Ma, respectively. Excess Ar and radiogenic Sr in xenocrysts unrelated to the groundmass are suspected.