Nature and scale of lithospheric heterogeneity: xenoliths and lavas at Toro Ankole, Uganda

Abstract:

Determining the three-dimensional distribution of metasomatic fluids in the lithosphere is an essential first step to understanding how magma-assisted mechanical failure promotes continental rifting. We document the distribution of two geochemically distinct fluids (one silicate and one carbonatitic) across the Toro-Ankole Volcanic Province (SW Uganda). By integrating over 90 years of published geochemical and mineralogical data tied to specific sampling locations, we observe the spatial distribution of major, minor and trace element abundances in lavas (e.g., LILE, LREE, HFSE) and key metasomatic phases in mantle xenoliths (titanite, phlogopite, perovskite) throughout the volcanic province. The data show the distribution of alkaline silicate and carbonatitic lavas to be strongly correlated with faults, lineations, and monogenetic craters, indicating a relationship between structurally controlled metasomatic fluid infiltration and lithospheric fracture. There are also temporal relationships found in the Katwe-Kikorongo Volcanic Field. The carbonatitic and mafic alkaline silicate lavas align with faults of differing ages, suggesting first-order control on the timing of the two metasomatic events or sequential tapping of melts from the lithospheric mantle during progressive rifting.