Potassium-argon ages of gneisses from the basin granitoid in South-Eastern Ghana

Biotite and hornblende from gneiss samples from the basin granitoid in South-eastern Ghana were separated and analyzed for potassiumargon (K / Ar) ages. The results indicated that the gneisses have average biotite and hornblende ages of 1942 ± 11 Ma and 2044 ± 17 Ma, respectively. The highest single hornblende age of 2092 Ma was recorded for the amphibolite gneiss, which is a protolith of tholeiitic basalt. These ages fall within the main phase of the Eburnean thermo-tectonic event. The defined ages correspond to the metamorphic event, and are expectedly lower than the average ages of 2270 Ma (Pb/Pb), 2116 Ma (U/Pb), 2216 Ma (Rb/Sr, WR Isochron), and 2240-2290 Ma (Sm/Nd, Model) determined for the basin granitoid in other parts of Ghana and the reported Birimian age (2000- 2300 Ma). This confirms that the basin granitoid was derived from older basement rocks, and that its emplacement was contemporaneous with the activities that resulted in the emplacement of the Birimian volcanic rocks and sediments. A period of 70 to 250 Ma separates the period of emplacement of the basin granitoid and subsequent metamorphic activities. However, the period of metamorphism of the basin granitoid (1922-2045 Ma) was contemporaneous with the intrusion by the Nsawam microgranite (1996 ± 43 Ma), and that of the Tarkwaian sediments by the K-rich granitoid (1968 ± 49 Ma). Ghana Journal of Science Vol. 45, 2005: 27-3