The Geochronology of the Tanzanian Shield

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Abstract
New whole-rock Rb-Sr isochron dates from the Tanzanian Shield of East Africa fall in the range Myr and outline a major period of granitoid magmatism that has not, until now, been documented from this part of the African continent. Although at least two events can be delineated, one at 2740 Myr, the other at 2540 Myr, the younger is more widespread and reflects extensive granitoid magmatism throughout large areas of Tanzania, southeastern Uganda and western Kenya. Lacking ages greater than 2800 Myr, the Tanzanian Shield is unique among the Archaean cratons of Africa. Initial 87Sr/86Sr ratios lie typically between 0.7010 and 0.7020, similar to late Archaean igneous rocks in other cratons, and provide little evidence to support the origin of the 2500 Myr Tanzanian crustal segment by simple reworking of ancient continental crust The chemical composition of the granitoid rocks ranges from granite (sensu stricto) to granodiorite, but none of the analyses falls within the tonalite field. Comparisons with radiometric dates from adjacent continental nuclei and from elsewhere in Africa suggest diachronous evolution of the African continent. Magmatism in the Kaapvaal craton to the south started about 1000 Myr earlier than the oldest intrusive activity yet confirmed in East Africa. The new Rb-Sr data from Tanzania extend the well-documented worldwide ca. 2500 Myr activity to East Africa; this event probably marked a final period of major crustal incursions to shield areas prior to their stabilization during the Proterozoic.