Lithospheric Control of Melt Generation Beneath the Rungwe Volcanic Province, East Africa: Implications for a Plume Source

The Rungwe Volcanic Province (RVP) is a volcanic center in an anomalous region of magma-assisted rifting positioned within the magma-poor Western Branch of the East African Rift (EAR). The source of sublithospheric melt for the RVP is enigmatic, particularly since the volcanism is highly localized, unlike the Eastern Branch of the EAR. Some studies suggest the source of sublithospheric melt beneath the RVP arises from thermal perturbations in the upper mantle associated with an offshoot of the African superplume flowing from the SW, while others propose a similar mechanism, but from the Kenyan plume diverted around the Tanzania Craton from the NE. Another possibility is decompression melting from upwelling sublithospheric mantle due to lithospheric modulated convection (LMC) where the lithosphere is thin. The authors test the hypothesis that sublithospheric melt feeding the RVP can be generated from LMC. We develop a 3D thermomechanical model of LMC beneath the RVP and the Malawi Rift and constrain parameters for sublithospheric melt generation due to LMC. We assume a rigid lithosphere and use non-Newtonian, temperature-, pressure-, and porosity-dependent creep laws of anhydrous peridotite for the sublithospheric mantle. We find a pattern of upwelling from LMC beneath the RVP. The upwelling generates melt only for elevated mantle potential temperatures (T-p), which suggests a heat source possibly from plume material. At elevated T-p, LMC associated decompression melts occurs at a maximum depth of similar to 150 km beneath the RVP. We suggest upwelling due to LMC entrains plume materials resulting in melt generation beneath the RVP.