Re-Os systematics of mantle xenoliths from the East African Rift: age, structure, and history of the Tanzanian craton

Abstract

In order to understand the effects of contractional and extensional tectonics on thick, mantle roots, we have undertaken a systematic study of mantle xenoliths from the Labait volcano, which lies within the East African Rift on the eastern boundary of the Archean Tanzanian craton. The Re-Os systematics of the Labait xenoliths show that ancient, refractory lithosphere is present to depths of ∼140 km. Above this depth, the mantle section consists of harzburgitic xenoliths with whole rock 187Os/ 188Os between 0.1081 and 0.1140, corresponding to Re depletion (T RD) ages of 2.8 to 2.0 Ga. Chromites from these samples are generally less radiogenic than their corresponding whole rocks and have T RD ages between 2.5 to 2.9 Ga, yielding the best estimate for the age of this portion of the lithosphere. Coupled petrographic and isotopic data for some of these samples indicate they have been variably overprinted by recent addition of Re and/or radiogenic Os. Between 4.4 to 4.7 GPa (∼140 to 150 km depth), peridotites are more fertile and yield younger T RD ages (1.0 Ga to future ages). The highest temperature sample has radiogenic 187Os/ 188Os (0.133), overlapping the range measured for metasomatic xenoliths and the Labait host melilitite (0.13 to 0.14). This range is taken to represent asthenospheric mantle beneath the Tanzanian craton, which has plume-like isotopic characteristics. The suite shows a good correlation on 187Os/ 188Os vs. temperature (hence depth) and 187Os/ 188Os vs. wt.% Al 2O 3 or CaO plots. These trends, which pass above primitive mantle compositions, may reflect mixing of recent plume-derived Os with ancient lithospheric Os, formation of the lowest portion of the lithosphere during successive melting events or a combination of both processes. Our data show that complete delamination of the lithospheric mantle has not occurred beneath the Tanzanian craton during its long tectonic history. However, if the Archean lithosphere was originally thicker than the ∼140 km currently beneath Labait, then the lithosphere has been thinned, either by thermal erosion associated with the rift or by partial delamination during Proterozoic collision. Finally, we see no evidence for extensive lithospheric thinning associated with development of the East African Rift, although overprinting of the lithosphere by rift-related magmas has occurred.