A geochemical study of volcanism associated with the early stages of continental rifting in northern Tanzania.

Abstract

The volcanic province of northern Tanzania lies at the southern end of the eastern branch of the East African Rift. In Kenya the underlying lithosphere is Proterozoic in age, while in northern Tanzania the rift intersects the Archean Tanzanian Craton. A petrological, isotopic and chronological study was undertaken to determine the source of the volcanic rocks and the relationship between rifting and volcanism. The isotopic compositions of the volcanic rocks indicate that the source is located in the chemically heterogeneous continental lithospheric mantle. Trace element concentrations in the northern Tanzanian lavas suggest that the continental lithospheric mantle in this area has been metasomatized by small degree partial melts from the asthenospheric mantle. Many of the mineral grains present in the samples were not in isotopic or chemical equilibrium with the magma represented by the whole rock chemical and isotopic compositions. This complex petrogenetic history is consistent with the small volume of lava produced in northern Tanzania. It is probable that many mantle-derived melts freeze before they reach the surface and these solidified magmas are the source of the xenocrysts in the erupted lavas. If the volcanic rocks in northern Tanzania are being generated in a rising mantle plume, the direction of the initiation of volcanism should be consistent with the movement of the African plate over this time. Ages obtained for the volcanoes in northern Tanzania show no consistent geographic trend; there is no evidence of a plume track. If the lavas are being generated in the continental lithospheric mantle, the heat source may be a mantle plume but geographic location of volcanism may be controlled by the location of low melting temperature material in the lithospheric mantle. Mantle xenoliths from the Labait cone display a range of isotopic compositions similar to those measured in the northern Tanzanian lavas. While melt from an individual xenolith has isotopic compositions that match those of the lavas, the xenoliths represent a potential heterogeneous source for the northern Tanzanian lavas.