Revealing the enigmatic alkaline volcanism of the Rukwa Rift through titanite - a melt inclusion study

The East African Rift System is the archetypal continental rift; it is host to active volcanism, tectonism, landscape evolution and some of the most important hominid fossils sites in the world. Traditionally, the East African Rift System has been described as consisting of a much older (>30Ma), volcanically active and well exposed eastern branch, and a younger (<12Ma), less exposed western branch that is instead characterised by a series of deep rift lakes. To date, much of the focus has been directed towards the eastern branch of the system to better understand the processes associated with early rifting of the African continent. Nevertheless, over the past decade researchers in the Rukwa Rift Basin (western branch) have catalogued a wealth of new and diverse vertebrate fossil discoveries (including hominids) that are well constrained by a series of enigmatic ash units (Nsungwe tuffs) that date to the Oligocene ( 25-26 Ma).

In hand specimen, the devitrified Nsungwe tuffs appear as discrete clay-rich horizons rich in the refractory minerals titanite (sphene), pyrochlore, apatite, andradite, phlogopite, sanidine and calcite. Although these ashes represent the earliest eruptive products associated with rifting of the western branch and demonstrate rift initiation occurred at Rukwa 14 Ma earlier than had been previously recognised, little has been said about their source and proposed carbonatitic nature. Currently, no appropriately aged volcanic vent or source for the Nsungwe tuffs is known. Furthermore, their pervasively weathered state prohibits conventional petrographic approaches.

This study presents the first description of magmatism associated with the 25 Ma initiation of rift development in the western branch of the East African Rift System. Using a variety of in-situ microanalytical techniques (i.e. EMPA, LA-ICPMS, LA-MC-ICPMS), we obtain major element, trace element and Sm/Nd isotope concentrations for a suite of large (30-100μm) silicate glass (melt) inclusions and their pristine euhedral titanite hosts from four discrete ash units across three Oligocene field localities.With these data, we trace the evolution of this previously unknown volcanic system from peralkaline phonolite towards peraluminous dacitic compositions; a magma evolution that is incompatible with pure crystal fractionation or crustal contamination. Instead, we model magma evolution as a combination of carbonatite liquid immiscibility and multi-stage crystal fractionation processes. These glass inclusions and their host titanite provide us with a remarkable record of nature and complex evolution of magmatism in the Rukwa Rift basin, and in turn has implications for understanding discrete explosive volcanism and landscape evolution at the onset of continental rifting.