Av. Ivanov et al., LATE CENOZOIC ALKALINE-ULTRABASIC AND ALKALINE BASANITE MAGMATISM OF THE RUNGWE PROVINCE, TANZANIA, PETROLOGY, 6(3), 1998, pp. 208-229
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.