Magmatism at the Southern End of the East African Rift System: Origin and Role During Early Stage Rifting
Creators: Mesko, Gary
Publisher: name: Columbia University
Year: 2020
Subjects: subject: Geochemistry
subject: FOS: Earth and related environmental sciences
subject: Magmatism
subject: Melting points
subject: Rifts (Geology)
subject: Volcanism

description: The composition of volcanic products can provide critical information about the source and the conditions of melting. This information is used to highlight differences in melting environments from volcanic regions around the globe. Volcanic lavas and other products from the Rungwe Volcanic Province, in southwest Tanzania (9.13S,33.67E), were collected and studied to test a number of lingering questions about the role of magmatism in a continental rift tectonic environment. The Rungwe Volcanic Province is the only region in this portion of the East African Rift (EAR) system with apparent magmatism. Is magmatism here the product of rifting, like melts generated in oceanic rift tectonic environments (Mid-ocean ridge basalts, MORB), or is melting here facilitated by the upwelling asthenospheric mantle, like melts generated at hotspots or plumes (oceanic intraplate basalts, OIB)? To address this, contributions from the continental lithosphere must also be identified and addressed. Each chapter of this dissertation approaches this fundamental question using different aspects of the comprehensive chemical and isotopic dataset from this study. The second chapter outlines a novel thermobarometer that is then applied to Rungwe samples to estimate the temperatures and depths at which the melts equilibrated. Laboratory melt experiments of garnet peridotite, some containing CO2, create melt with major element characteristics applicable for pressure and temperature estimation of Rungwe samples. The parameterization of Al2O3 and MgO from the experimental melt compositions provides a thermobarometer with a temperature range of 1100-1500C (16C, 1), and a pressure range of 2-5 GPa (0.2 GPa, 1). The maximum potential temperature reached for Rungwe samples is 1372C. Potential temperatures at Rungwe overlap with the ambient asthenospheric mantle, as sampled by the global range of MORB. Potential temperature range for Rungwe is too high for melts to have a derivation from the continental lithosphere, and too low for melts to be derived from the thermally-driven plume. The pressures of melt equilibration for Rungwe span a range from GPa, when converted to depths is 55-101 km. Depth estimates can be compared to the estimated depths of the lithosphere-asthenosphere boundary (LAB) from seismic tomography models. Rungwe melts appear to be derived from the depths at or below the LAB, supporting their derivation from an asthenospheric source. Under the same parameters, other volcanic regions from the Western Branch of the EAR give similar results, while maximum potential temperatures from the Eastern Branch exceed estimates from the ambient asthenospheric mantle, providing more support for a thermally-derived mantle plume there. The third chapter provides a timeline of volcanism at Rungwe including ages from Ar-Ar geochronology performed on samples from this study, as well as dates of two precursor carbonatite bodies in the vicinity of the volcanic province. Most of the Rungwe Volcanic Province was emplaced between present-9Ma, with emerging evidence for eruptions between 9Ma and ~25Ma. A proposed broadening of the age range of each volcanic stage definition helps to include eruptions prior to 9Ma, and encompass eruptions shown to have occurred between the original volcanic stage age ranges. Two carbonatite bodies in the northwest edge of the volcanic province date to 169.0 0.6 Ma and 154.4 0.9 Ma, and show no evidence of Cenozoic reactivation. The emplacement ages of the majority of Rungwe samples coincide with accelerated rifting and basin formation present-9Ma. The updated timeline of Rungwe volcanism suggests that eruptions prior to 9Ma are still tied to tectonic extension, based on comparison to thermochronology cooling ages from the major border faults. The fourth chapter characterizes and provides context about the chemical and isotopic composition of the mantle source of Rungwe melting. Isotopic Sr-Nd-Pb-Hf, as well as major and trace elemental compositions provide a fingerprint for Rungwe melts in which to compare to the range of global OIB and to other EAR melts. The majority of Rungwe melts possess isotopic traits that are consistent with an asthenospheric plume-derived source. Many isotopic and trace element ratio characteristics identified are not shared with any identified OIB-source volcanic region, but are present in other EAR volcanoes. These indicators suggest that some Rungwe melts, together with some EAR volcanoes, share a common source characteristic or melt process that the global OIB does not sample or experience. Homogeneity of plume source or continental lithosphere over the large geographic distances between volcanic provinces in the EAR are not expected. No OIB emplaced on oceanic crust must traverse Archaean or Proterozoic subcontinental lithosphere or crust. The influence of melt interaction with these elements are explored in detail as the main cause of differences between OIB and Rungwe compositions. Metasomatic phases accumulated by melt interaction at the LAB interface over eons create compositions that can influence low-volume melts that traverse them. It appears that no Rungwe melt evaded this overprint from the subcontinental lithospheric mantle, despite large-scale preservation of the plume-derived melt origin.