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Mantle xenoliths in kamafugites of Bunyaruguru (East African Rift Zone): the mineral composition and conditions of formation

Muravyeva N.S., Senin V.G.



There are different types of mantle xenoliths in kamafugite lavas of Bunyaruguru volcanic field. We have studied three samples of the craters Kasimiro, Kyambo and Nyungu, located at a distance of 1 3  km apart. All xenoliths are metasomatized mantle material, but they are significantly different in structure and mineral composition: pyroxenite carbonated in situ from mafurite (Nyungu), phlogopite pyroxenite in mafurite (Kyambo) and zonal glimmerite-verlite xenolith in ugandite (Kasimiro).

Pyroxene is present in all three samples. In one of the xenoliths, zonal, in the central part of the verlite detected primary olivine (with inclusions of chrome spinel), not previously described for this region. Later, olivine, secondary to the primary minerals is also present in the phlogopite pyroxenite. Phlogopite found in two of the three xenoliths. Perovskite occurs in all xenoliths and enclosing volcanic rocks. The distinction is apparent in the chemical composition of individual minerals from xenoliths and mineral-phenocrysts from their host rocks.

One may suggest that verlite-glimmerite xenolith was formed during wall-rock alteration by ascending magma. The phlogopite pyroxenite with equigranular structure rather was generated by percolation of volatile rich low viscosity melts/fluids. Pyroxenite xenolith before

capturing kamafugite magma was influenced by carbonate-sulfate solutions (melts?), as indicated by dissecting its perovskite-calcite veins. The type of the metasomatic alteration reflects differences in depth and the composition metasomatic agent (melt-fluid), influenced on the mantle source.