Insight into the origin of gabbro-dioritic cumulophyric aggregates from silicic ignimbrites: Sr and Ba zoning profiles of plagioclase phenocrysts from Oligocene Ethiopian Plateau rhyolites

Abstract

The Were Ilu ignimbrites are unlike other Oligocene rhyolites from the Ethiopian continental flood basalt province, in that they consist of plagioclase (An19–54), augite, pigeonite and Ti-magnetite, instead of anorthoclase, sodic sanidine, aegirine-augite and ilmenite. The minerals occur as (micro-)phenocrysts isolated within a glassy matrix or forming gabbroic and dioritic cumulophyric clots. Plagioclase is partially re-melted (sieve-textures with infilling glass). It is zoned with sudden changes in composition. However, the bulk zoning is normal with An-rich core (An45–54) and more sodic rim (An19–28). Ba and Sr concentration profiles of two plagioclase phenocrysts show a bulk rimward increase with compositions ranging from 250 ppm to 1,060 ppm and from 400 ppm to 1,590 ppm, respectively. The matrix glass has low CaO content (0.1–0.5 wt.%), a peralkalinity index of 0.79–1.04 and average Sr and Ba contents of 48±22 and 525±129 ppm, respectively. Geochemical modelling of Ba and Sr zoning profiles of plagioclase, based on experimental constraints, suggests that the cumulophyric clots can be derived from fractional crystallisation associated with limited assimilation (8 wt.%) from melts slightly less evolved than their rhyolitic matrix glass. These clots are not witnesses of intermediate magmas allowing the Daly Gap to be filled, but are cumulates differentiated from rhyodacitic melt. This indicates that parental magmas were stored in crustal magma chambers where they differentiated before being erupted at the surface.