THE IGWISI HILLS EXTRUSIVE 'KIMBERLITES'

The Igwisi Hills are a group of volcanic hills in Tanzania where a unique series of eruptions has produced volcanic rocks that have many of the features of kimberlites. The rocks are “igneous conglomerates” that are characterized by the presence of abundant ellipsoids of olivine in a fine grained matrix. The matrix contains major amounts of carbonate, serpentine, and complexly zoned Mg–Al–Cr spinels, minor perovskite, apatite and limonite. Large magnesian olivines are highly rounded and some have partially recrystallized to more Fe-rich and Ni-poor subhedral to euhedral grains. Several olivine ellipsoids are partly or wholly rimmed by a black coating consisting of perovskite, and Mg–Al spinel. Mineral inclusions within the olivines include chrome pyrope, similar in composition to garnets in kimberlites, low Al enstatite, low Al magnesian chrome diopside, Mg–Al chromite, and a highly magnesian phlogopite (Mg/Mg + Fe ~ .94). All of these are apparently primary phases in equilibrium with olivine. The total assemblage is similar in many respects to that found in garnet peridotite xenoliths such as those from the Lashaine volcano in northern Tanzania. The Igwisi irruptives apparently contain material derived from phlogopite-bearing, garnet peridotites with a primary mineral assemblage (assuming that all these phases coexist at depth) indicative of equilibrium at upper mantle temperatures and pressures. This primary assemblage was disrupted and brought rapidly to the surface in a gas-charged, carbonate-rich fluid. Rapid upward transport, extrusion, and rapid cooling have preserved inclusions of upper mantle ultramafics in a carbonate-rich matrix and have tended to prevent reaction between inclusions and matrix that might otherwise have yielded a more typical kimberlite.