The Preservation of Meso- Archean Refractory Lithospheric Mantle Underneath the Eastern Margin of the Tanzania Craton

Numerous studies on the petrology and geochemistry of peridotite xenoliths from the Tanzanian Craton and its rifted margins have investigated the origin, chemical change and thermal state of the cratonic roots from its core area (Nzega and Mwadui), its Northern (Marsabit) and Eastern margin Labait and Lashaine area (e.g. Dawson, 1964; Henjes-Kunst and Altherr, 1991; Lee & Rudnick, 1999; Chesley et al., 1999; Gibson et al., 2013). These studies suggest that the Tanzanian cratonic mantle formed via high degrees of melt extraction in the Archean (oldest Re-depletion age T_RD = 3.4 Ga, Burton et al., 2000) and sev­eral episodes of refertilization. In order to gain further temporal and chemical understanding on the effects of tectonic processes on cratonic roots, we carried out a Re-Os isotopic study on peridotites (n = 11) from Lashaine, which will be followed by Lu-Hf, Sm-Nd and Sr isotope investigations of the constituent minerals of the same samples.

The preliminary whole-rock Os isotope data from Lashaine peridotites show a large range of ¹⁸⁷Os/¹⁸⁸Os (0.1061 - 0.1261), with T_RD ages from Meso-Archean to very young (3.1 Ga to 0.3 Ga). There is a negative correlation between T_RD and bulk alumina contents. One sample with the lowest Al₂O₃ yields the oldest age of 3.1 Ga. Five samples range from 2.5 to 2.8 Ga, three give ages close to 2 Ga, and one sample with a high Al₂O₃ has a T_RD at 0.3 Ga. The positive Al₂O₃-¹⁸⁷Os/¹⁸⁸Os correlation trend passes above the PM composition may reflect ancient metasomatism by high Re/Os melts or recent metasomatism by very radiogenic Os plume-derived melts. These processes could be related to the evolution of the peripheral Proterozoic mobile belts, or Cenozoic rifting on the Eastern margin. Collectively, our new Os isotope data demonstrate that Meso-Archean (at least 3.1 Ga old) mantle portions are still retained underneath the rifted Eastern margin of the Craton. This is in line with previous results indicating that Archean cratonic mantle extended to the far eastern margin of the Craton, and also that the ages of the mantle root of the Tanzanian Craton are comparable to the other Cratons (e.g., Kaapvaal and Zimbabwe).