Eruptive and magmatic evolution of North Chamo Volcanic Field (southern Ethiopia)

A group of pyroclastic cones is dispersed in the North Chamo Volcanic Field, i.e. in the northern surroundings of the Chamo Lake and over neighbouring part of the Nech Sar plains (southern termination of the Main Ethiopian Rift). The activity of scattered cinder cones was partly coeval with that of Tosa Sucha Volcano (Calabrian), but continued also after Tosa Sucha's extinction until Middle Pleistocene (c. 0.5 Ma). Whereas scoria cones on the Nech Sar plains displayed a rather simple Strombolian eruptive style, the cones located within the northern part of Chamo Lake were characterized by more complex evolution. Ganjulle scoria cone, with a uniform olivine basalt composition, started with a Surtseyan-style eruption, which turned into Strombolian as the volcano grew above the water level. An even more complex history was documented for the Ganta cone. Compositional zoning of pyroclastic rocks is explained by zoned-chamber exhaustion. The transition from magmatic to phreatomagmatic style of the eruption was then most likely linked to syn-eruptive subsidence of the area on the Chamo Lake banks. Subsequent transition back to Strombolian style reflected the growth of the cone above water level. The Sr-Nd-Pb isotopes, together with major-element-based thermodynamic modelling, demonstrate that magmas parental to the North Chamo alkaline volcanic rocks (alkali basalt, through trachybasalt and trachyandesite to trachyte) evolved initially by closed-system fractionation of olivine, later joined by clinopyroxene, spinel and calcic plagioclase. The subsequent stage was characterized by a substantial (c. 25% by mass) assimilation of country-rock felsic igneous material, perhaps corresponding to the Paleogene ignimbrites.