Architecture Of The Magmatic System In The Main Ethiopian Rift

The Main Ethiopian Rift represents a young continental rift with an associated large volume of magmatism that forms one of the major Large Igneous Provinces. Plio-Quaternary magmatic activity is located in the Main Ethiopian Rift. Transitional to peralkaline mafic and felsic magmatic products dominate the volcanism which is related to complex variation in the crustal structure across the rift. Clinopyroxene, olivine and plagioclase are liquidus phases in the transitional and alkaline mafic magmas. Here we used the ability of clinopyroxene to record variations of P, T and fO2 to reconstruct the host-rocks geological history. Transitional and alkaline basalts and basaltic trachyandesite (with MgO>7 wt.%) from across the MER have augitic phenocrysts as liquidus phase. Euhedral centrimetric megacrysts of similar composition are sometime associated with alkaline rift basalts. Clinopyroxene geobarometry performed combining X-ray diffraction with mineral chemistry points to a complex polybaric plumbing system active since 7.5-3.7 My. MER clinopyroxene crystallize in a wide pressure range from the lower crust (ca. 10 kbar) up to middle crust (ca. 4 kbar) in the plumbing system beneath Debre Zeyit Fault Zone and the Wonji Fault Belt. The continuous polybaric MER clinopyroxene crystallization from lower to middle crust can be explained by a plumbing system composed of a complex of dykes and sills where magmas rise, stall and crystallize.