Mineralogical Characterization of Lithospheric Mantle Xenoliths from Katwe-Kikorongo, Uganda

*Captions are available!*

Plate tectonics has shaped the face of the Earth throughout its history. Continental rifting and assembly drive the formation of oceans and the configuration of continents on the planet’s surface; however, fundamental questions remain about how these processes occur. The focus of this research was on the inception of continental rifting in the archetypical setting of the East African Rift System. This topic was addressed by exploring xenoliths from the lithospheric mantle collected from the Katwe-Kikorongo portion of the Toro Ankole volcanic field in the Western branch of the East African Rift. Microscopic investigation of 30 µm-thick slices of the rocks revealed areas of the samples that were invaded by a melt or fluid, presumably associated with rift-related metasomatism and/or magmatism. These rocks were found to be mostly comprised of clinopyroxene, phlogopite, and amphibole, with smaller amounts of spinel, apatite, titanite, perovskite, and olivine. Key mineral phases representing both lithosphere and invasive fluid were analyzed using an electron probe microanalyzer (EPMA). Calculations were carried out to determine the minerals’ specific compositions to evaluate heterogeneity within and between samples and thus to determine if the invading melt or fluid was extrinsic or intrinsic. It was determined that the crystals which grew from the invading melt are chemically similar to the xenolith crystals, indicating that the melt or fluid was derived locally. After comparing this data to published data sets, this information suggests that the processes that formed these rocks occurred recently and involve the weakening of the lithosphere through local melting and widespread heating, which both mark the early stages of continental rifting. The knowledge gained from this research can be used to better society’s understanding of the evolution of the Earth and other planets in the solar system and beyond.