Résumé
The North Tanzanian Divergence (NTD) corresponds to early stage rifting of the eastern branch of the East African Rift. In the southern part, quaternary volcanoes of the Manyara-Balangida rift have erupted primary melilitites (34.9–42.1 wt% SiO2 and Mg# = 79–65) with deep mantle xenoliths. Melilitites are olivine-rich and contain up to 4 vol% magmatic phlogopite as a liquidus phase suggesting that primary melts were K2O-rich and contain H2O (4.63–5.48wt% H2O, 66-117 ppm Cl in phlogopite, from SIMS measurements). Lavas have high incompatible element contents, LREE/HREE fractionation, high Rb/Sr ratio and negative anomaly in K and Zr-Hf. Geochemical modelling indicates that the melilitite magmas resulted from deep and low partial melting of a carbonate-rich (0.3–0.5%) garnet peridotite containing ~2 vol% phlogopite. The depth of partial melting is estimated close to or below the lithosphereasthenosphere boundary (>130 km).Mantle xenoliths include phlogopite-bearing peridotite and phlogopitite (100% phlogopite + rare spinel). Mantle phlogopites have high Al2O3 and MgO content (12.7-15 wt.% Al2O3, Mg#=83-93) and high water and Cl content (2.6-5.3 wt% H2O, 100-200 ppm Cl) with major element composition close to magmatic phlogopites. They have very low incompatible element contents compared to phlogopite in melilitite and differs significantly from phlogopite in phlogopite-rich PIC and MARID xenoliths as deep-seated segregations from melts genetically linked to kimberlitic magmas. Percolation of deep asthenospheric CO2-H2O alkaline magmas during their ascent may have produced the strong heterogeneities in the thick sub-continental lithospheric mantle beaneath the East part of the Tanzanian craton by inducing metasomatism and phlogopite crystallization in spinel lherzolite and phlogopitite lithologies