Petrogenesis of silicic peralkaline rocks in the Ethiopian rift: geochemical evidence and volcanological implications

Major, trace element and isotopic data for mafic to peralkaline silicic volcanic rocks from the northern sector of the main Ethiopian rift are discussed with the aim of placing constraints on processes of magma genesis and evolution and to present models for magma plumbing systems of rift volcanoes. Basalts straddle the subalkaline-alkaline boundary and exhibit important variations of incompatible element abundances and ratios. Silicic rocks consist of dominant pantellerites and minor comendites and trachytes, although some volcanoes along the rift shoulders consist entirely or predominantly of trachytes. Rocks with intermediate compositions are very scarce. Mafic and silicic rocks exhibit similar values as some basalts for many incompatible element and radiogenic isotopic ratios. Geochemical and petrological modelling shows that the most likely petrogenetic process for rift magmatism is a derivation of rhyolites from basalts by dominant fractional crystallisation occurring at shallow depths. Variations of incompatible element ratios and radiogenic isotopes in the basalts suggest heterogeneous sources and significant interaction with the crust. In contrast, the role of crustal assimilation during evolution of silicic magmas is negligible. It is suggested that large amounts of basalts were emplaced both into the lower continental crust, and at shallow depths. Shallow level fractional crystallisation generated zoned magma chambers with rhyolitic melts accumulating at the top, and mafic magmas ponding at the bottom. Volcanic activity was fed preferentially by the upper rhyolitic layer, whereas mafic magmas were erupted only accidentally, when extensional faults intersected the bottom of shallow reservoirs or tapped directly the deep magma chambers. The presence of trachytic volcanoes along the rift shoulders could result from clinopyroxene-dominated high-pressure basalt fractionation, which did not allow melts to reach rhyolitic compositions. Satellite imagery and field studies reveal the occurrence of a large number of caldera collapses in the main Ethiopian rift, suggesting that several magma chambers have been formed at shallow depths, possibly favoured by block tilting and strike-slip faulting. This explains the huge amounts of silicic rocks along the northern Ethiopian rift. The occurrence of huge magma reservoirs is also supported by positive gravity anomalies detected by previous studies beneath several silicic volcanic centres.