Nephelinite–melilitite–carbonatite relationships: Evidence from Pleistocene–recent volcanism in northern Tanzania

The younger (< 1.2 Ma) nephelinite–melilitite–carbonatite-volcanoes in the Gregory Rift Valley of northern Tanzania (termed the Younger Extrusives) comprise one of the most concentrated areas of carbonatite magmatism on Earth. The larger strato-volcanoes, characterised by strongly evolved magmatism, are surrounded by volumetrically much-smaller features (scoria and tuff-cones, explosion craters, lava flows) of carbonate-rich olivine-bearing nephelinites or melilitites that, in their relative primitiveness, are potential parents to the much-largerstrato-volcanoes. The bimodal size distribution is mirrored in bulk magma chemistry, and in their Sr, Nd and Pb isotopes, with the more primitive magmas lying close to a HIMU source; the isotope data for the larger strato-volcanoes cover a wider, linear array interpreted as due to variable degrees of interaction between HIMU magmas and enriched lithosphere, evidence for which is found in Tanzanian mantle xenoliths. An integrated model involves carbonated olivine–nephelinite magma generated in the asthenosphere by melting of carbonate-bearing peridotite. Small amounts of the melt migrate rapidly and are extruded as the olivine–nephelinites and –melilitites. Larger amounts of magma pond in the lithosphere with which they react and, during variable periods of stagnation, varying degrees of olivine–pyroxene–spinel fractionation take place before upward migration of now-differing magma batches. Variable residence time within, and reaction with, variably enriched lithosphere accounts for the array between the HIMU and enriched mantle end-members in the isotope mixing model.