U-PB GEOCHRONOLOGY OF DETRITAL ZIRCONS FROM THE RUKWA RIFT BASIN, TANZANIA: NEW DATA ON THE PRE-NEOGENE TECONTIC AND SEDIMENTARY EVOLUTION OF THE WESTERN BRANCH OF THE EAST AFRICAN RIFT SYSTEM

The Red Sandstone Group (RSG), in the Rukwa Rift Basin (RRB) of SW Tanzania, represents a poorly understood continental rift-fill sequence. Faunal data indicate three temporally distinct units of Cretaceous (Unit I), Paleogene (Unit II) and Neogene ages. To clarify the tectonic and depositional history for this part of the East African Rift System (EARS) and to provide age constraints on vertebrate faunas, U-Pb age analyses of five detrital-zircon samples from Units I & II, as well as one modern river sand, were performed. Four dominant age populations clustering around 2040-1790 Ma, 1180-950 Ma, 790-710 Ma, and 640-450 Ma were identified. These grains are linked to the following sources, respectively: (1) the Ubendian Belt along the E-W flanks of the RRB; (2) the Irumide Belt along the S boundary of the basin (Malawi); and (3, 4) alkaline volcanics and granites associated with several stages of Pan-African reactivation of the Ubendian Belt. Maximum age constraining 169-156 Ma grains were identified in several Unit I samples, yet faunal data indicate a mid-Cretaceous age for the mid-upper portion of Unit I. In Unit II, 27.8-24.5 Ma grains provide maximum depositional age constraints, confirming a faunally based late Oligocene age for the top of the unit. Both young zircon populations represent previously unknown tectonic/magmatic events in the RRB. Combined with paleocurrent information, these data indicate that the RRB has been continuously fed (Cretaceous-Recent) by NW flowing axial fluvial systems. Two major provenance shifts occur between the Cretaceous and Oligocene/Recent. First, Cretaceous streams record multiple provenance sources, compared to Oligocene/Recent streams, which record few. Second, there is a major shift from Irumide and Pan-African dominated sources in the Cretaceous to almost exclusively Ubendian sources (and 28-25 Ma volcanic grains) in Oligocene/Recent samples. This suggests larger drainage basins/rivers sourced in the uplands in N. Malawi and E. Zambia for Unit I. Beginning in the Oligocene, rift flank uplift, presumably associated with initiation of the modern EARS exposed extensive Ubendian sources. The build-up of volcanics during the Oligocene and Neogene (Rungwe Volcanics) at the southern end of the RRB also effectively cut off sediment input from Pan-African and Irumide sources.