Excess Ar in Biotites from the Broderick Falls (webuye) Area, Western Kenya: Implications for the Tectonothermal History of the Mozambique Belt and Its Archaean Foreland

Rb-Sr whole rock and K-Ar mineral age determinations were made on rocks from the Broderick Falls (Webuye) area, western Kenya. Granitic rocks yielded a Rb-Sr whole rock isochron age of 2555 +/- 101 Ma with an initial Sr-87/Sr-86 ratio of 0.70121 +/- 0.00038. This age represents the time of granitoid emplacement. K-Ar mineral ages range from 574 to 3420 Ma, which is very variable with respect to mineral type and locality. Mylonitic granodiorite very close to the Nandi Escarpment gave a K-Ar age of 916 Ma from biotite, suggesting the time of the activity of the Nandi Fault, which may be an earlier phase of the Pan-African Orogeny. Ages of biotites in a zone between 4 and 6 km northeast of the Nandi Fault are anomalously high compared to those of coexisting hornblende and the Rb-Sr isochron age, confirming the existence of excess Ar-40 in biotite. Excess Ar-40 was probably introduced into biotite under the appropriate temperature conditions prevailing near the Nandi Fault. Taramite, a rare sodic calcic amphibole, was found in a cordierite-biotite gneiss of the Kavirondian Supergroup and gave a typical Pan-African K-Ar age of 574 Ma. The last Pan-African metamorphism occurred in the terrane east of the Surongai Thrust. Copyright (C) 1997 Published by Elsevier Science Ltd.