PETROLOGY AND MINERAL CHEMISTRY OF PLUTONIC IGNEOUS XENOLITHS FROM THE CARBONATITE VOLCANO, OLDOINYO-LENGAI, TANZANIA

Magmatic plutonic rocks in the Oldoinyo Lengai pyroclastics are jacupi rangite, pyroxenite, ijolite, nepheline syenite and wollastonitite. Ma inly cumulates they are combinations of nepheline, clinopyroxene, Ti-a ndradite, spinel (sensu lato), apatite, perovskite, titanite, wollasto nite, sulphides, mica, glass and K-rich feldspar, most of which are st rongly zoned Low analytical sums for glasses, vesiculation of intergra nular glasses, and the generally explosive nature of the volcanicity p oint to significant concentration of dissolved volatiles in the parent magma; the absence of hydrous phases suggests that the dominant volat ile is CO2. Cumulate textures, widely variable modes, veining and vari ations in specimen consolidation and metasomatism all indicate derivat ion from a structurally complicated and multiply injected sub-volcanic complex. Complex zoning of phases and mineral disequilibrium is attri buted to convective percolation of fluids through permeable cumulates, possibly complicated by magma replenishment during crystallization of individual magma batches. Olivine, mica and pyroxene megacrysts in so me ijolites indicate polybaric crystallization; mixing of potassic and sodic magmas may be the cause of these megacryst-bearing ijolites, bu t the main parent is highly evolved, of carbonated ijolitic (nephelini tic) composition and with Nd and Sr isotope characteristics slightly m ore depleted than Bulk Earth.