Differential Fractionation of Rhyolites During the Course of Crustal Extension, Western Afar (Ethiopian Rift)

Abstract

AbstractWe report field observation, age, chemical (major and trace elements), and isotope (Sr‐Nd‐Pb) data for felsic volcanic rocks from Central Afar and adjacent western margin. Investigated volcanic rocks are dominantly rhyolites with minor trachytes, and they are geochemically similar. Their ages range from ~30 Ma (prerift stage), ~20 Ma (early synrift), ~8–4 Ma (main thinning event) to ~2.5–0.1 Ma (late synrift/continental breakup), representing the entire volcanic‐tectonic events that occurred episodically. Major element variations are consistent with fractionation of gabbroic cumulates. Trace element and isotope data preclude an origin by crustal anatexis; the rhyolites are rather genetically linked to the associated basalts and variously contaminated by the crust during differentiation of magmas. Chemical and isotopic data of the rhyolites support an origin by open system differentiation at deep crustal levels (hot wall rock and high r = rate of assimilation/rate of crystallization) and shallow crustal levels (cold wall rock and low r) with a change in the composition of the assimilated material from lower crustal to upper crustal type. Assimilation appears to decrease in recent times with Quaternary rhyolites, emplaced nearby the active magmatic segments in Afar, which exhibit the isotopic compositions closest to original mantle signature. This is compatible with a crust below the active magmatic segments resulting from important addition of juvenile basic magmas. Such results and interpretations provide actual constrains to suggest that the present‐day stage is probably very close to continental breakup, which will be achieved once the continental crust will be entirely replaced by new magmatic crust.