Temporal compositional variation of syn-rift rhyolites along the western margin of the southern Red Sea and northern Main Ethiopian Rift

Structural and geochronological relations indicate that the felsic rocks at the top of the Oligocene flood basalt sequences in the Afar volcanic province were erupted coevally with the initial rifting in the Red Sea and Gulf of Aden. In this study, we use the newly established volcanic-tectonic history to examine the geochemical evolution with time of felsic volcanics as rifting has progressed to seafloor spreading in the southern Red Sea and northern Main Ethiopian Rift. Geochemical analyses (major and trace elements; Sr, Nd and 0 isotopic compositions) of syn-rift rhyolites ranging in age from 28 to 2.5 Ma indicate that the rhyolites can be derived from mantle-sourced basaltic magma through fractional crystallization accompanied variable amounts of crustal contamination (e.g. Sr-87/Sr-86 = 0.70489-0.70651; Nd-143/Nd-144 = 0.51254-0.51283; delta O-18 = +4.5 to +6.4 parts per thousand). The input of crust tends to increase with time, which suggests the weakening and heating of the crust in response to lithospheric thinning and magma injection in the past c. 30 Ma. These results support earlier structural and thermomechanical models for rift formation in the southern Red Sea rift and the younger, less-evolved northern Main Ethiopian Rift system.