Rb [sbnd]S0r and K [sbnd]Ar age measurements on the Modipe gabbro of Bechuanaland and South Africa

Abstract

Rb Sr total rock measurements are reported for six samples of the Modipe gabbro. Although the rubidium concentration in the samples is low, the points on the isochron diagram lie on a straight line within the experimental error and an age of 2630 ± 470m.y. is obtained with initial ratio 0.7010 ± 0.0010. The gabbro is intruded by the Gaberones granite and total rock measurements on three samples of the granite give an age of 2340 ± 50m.y., which although not significantly different from the gabbro age is consistent with the known geological relationship of them. K Ar measurements on separated mineral fractions from three of the gabbro samples give consistent plagioclase ages at 1980 m.y., which seems to indicate the gabbro suffered a mild reheating at this time, which probably relates to the intrusion of the Bushveld Complex. The pyroxene ages fall in the range 2670 to 3000 m.y., and this may represent another instance of excess argon in pyroxenes. Alternatively the pyroxenes might still record the date of intrusion of the gabbro, and this is consistent with the upper range given by the Rb Sr age.