Alkalic Pyroxenite Xenoliths from the Lashaine Volcano, Northern Tanzania

Abstract

The ankaramitic scoria and carbonatite tuffs of the Lashaine volcano, northern Tanzania, contain a suite of alkalic pyroxenite xenoliths, in addition to the previously investigated magnesian lherzolite types. The rocks of the pyroxenite suite, which include mica-dunite and iron-rich lherzolite, consist of varying combinations of olivine (Fo₈₆₋₇₂), sodic diopside, Ti-pargasite, Ti-phlogopite, ilmenite, chromite, and magnetite. The over-all assemblages are poorer in alumina than those from other alkalic pyroxenite localities. Comparison with the products of experimentally investigated systems is difficult because of low alumina, and emphasizes the need for experimental syntheses on rocks of this type.