EXPLORING THE ORIGIN OF PYROXENITES THROUGH SPINEL COMPOSITIONS OF WESTERN RIFT UGANDAN XENOLITHS

Pyroxenite is recognized increasingly as an important lithological component contributing to mantle-derived volcanism. Pyroxenite is not a primary mantle lithology and, instead, can be generated through a variety of processes (crustal recycling, melt-rock interaction, fractional crystallization, etc.). Mineral compositions can provide insight into the origin of pyroxenites. However, few studies have explored spinel chemistry as a means to understand pyroxenite petrogenesis. This study investigated the composition of spinel minerals contained in pyroxenite and rare peridotite xenoliths from the Western Rift of Uganda in the East African Rift System to understand pyroxenite origins. Petrographically, the pyroxenite xenoliths record a combination of three textures: (1) granular without compositional zoning, (2) granular with subtle zoning and areas of glassy melt pockets or coarsely crystallized channels, (3) coarse-grained cumulate textures. Spinel compositions were measured on grains encased in olivine or, in olivine-free samples, clinopyroxene. Compositionally, spinel ranges between aluminum-rich and chromium-rich endmembers. When viewed by locality and within individual samples, spinel record significant heterogeneity in both Mg# (0.14-0.63) and Cr# (0-0.59). All of the spinel, including those within the peridotite, contain moderate to high TiO₂ contents (0.74-19.64 wt. %); the cumulate-textured pyroxenite contains the highest TiO₂ abundance, consistent with a magmatic origin. The range in Al₂O₃ (3.38-46.09 wt. %) suggests varying degrees of partial melting and fractional crystallization. The TiO₂ contents are consistent with crystallization at elevated temperatures and pressures. The spinel data suggest that the pyroxenite and peridotite xenoliths experienced variable degrees of melt-rock interaction via melt channels. Inferences based on these data provide insight into the complexity and heterogeneity of pyroxenite formation and its role in lithospheric evolution beneath the East African Rift System.