LATE CENOZOIC ALKALINE-ULTRABASIC AND ALKALINE BASANITE MAGMATISM OF THE RUNGWE PROVINCE, TANZANIA

Alkaline-ultrabasic and alkaline basaltic lavas of the Rungwe volcanic field comprise melanephelinite (Mg# = 76-81, Cr = 650-920 ppm, Ni = 3 10-590 ppm), pyroxene melanephelinite, basanite, alkaline olivine basa lt, and olivine tholeiite (Mg# = 55-73, Cr = 15-620 ppm, Ni = 30-260 p pm). Variations in the contents of TiO2 and P2O5 of the lavas and in t heir Ca/Al, Ba/Rb, Zr/Sm, Hf/Sm, La/Yb, and Sr-87/Sr-86 ratios provide evidence for the mixing of three mantle-derived components of the enr iched continental lithosphere. One of the components is hypothesized t o have been apatite-bearing garnet harzburgite (or amphibole-bearing I herzolite metasomatized by aqueous solutions) with anomalously low Zr/ Sm and Hf/Sm ratios (similar to 1.3 and similar to 0.07, respectively) and Sr-87/Sr-86 = 0.7048-0.7051, Another component was most probably recently metasomatized garnet Iherzolite, whose composition approached that of the primitive mantle. The Iherzolite had Zr/Sm = similar to 1 5-20, Hf/Sm = similar to 0.43, and Sr-87/Sr-86 = similar to 0.7045. Th e third component was supposedly ancient garnet Iherzolite enriched in Rb and having high Sr-87/Sr-86 > 0.7055. All the alkaline-ultrabasic and alkaline basaltic rocks were produced by the low-degree (F < 5%) p artial melting of the mantle under pressures of more than 30 kbar. Two models of magma generation are proposed. In one of them, all the thre e components affiliate with the garnet depth facies of the upper mantl e. The other model allows for the involvement of low-depth amphibole-b earing mantle rocks into the melting processes.