Rhyolites associated to Ethiopian CFB: Clues for initial rifting at the Afar plume axis

A comprehensive tectono-magmatic model based on new geochemical and field data is discussed in order to highlight the significance of the high-TiO2 bimodal picrite basalt/rhyolite association in the north-eastern sector of the Ethiopian Plateau, which is considered to be the axial zone of the 30 Ma Continental Flood Basalt activity related to the Afar plume (Beccaluva et al., 2009). In this area the volcanic sequence consists of approximately 1700 m of high TiO2 (4–6.5%) picrite basalts, covered by rhyolitic ignimbrites and lavas, with an average thickness of 300 m, which discontinuously extend over an area of nearly 13,500 km2 (ca. 3600 km3). Petrogenetic modelling, using rock and mineral chemical data and phase equilibria calculations by PELE and MELTS, indicates that: 1) picrite basalts could generate rhyolitic, sometimes peralkaline, residual melts with persistently high titanium contents (TiO2 0.4–1.1%; Fluorine 0.2–0.3%; H2O 2–3%; density ca. 2.4) corresponding to liquid fractions 9–16%; 2) closed system fractional crystallisation processes developed at 0.1–0.3 GPa pressure and 1390–750 °C temperature ranges, under QFM fO2 conditions; 3) the highest crystallisation rate – involving 10–13% of Fe–Ti oxide removal – in the temperature range 1070–950 °C, represents a transitory (short-lived) fractionation stage, which results in the absence of erupted silica intermediate products (Daly gap). The eruption of low aspect ratio fluorine-rich rhyolitic ignimbrites and lavas capping the basic volcanics implies a rapid change from open- to closed-system tectono-magmatic conditions, which favoured the trapping of parental picrite basalts and their fractionation in upwardly zoned magma chambers. This evolution resulted from the onset of continental rifting, which was accompanied by normal faulting and block tilting, and the formation of shallow – N–S elongated – fissural chambers parallel to the future Afar Escarpment. The eruption of large volumes of rhyolitic magmas generated from underlying picrite basalts appears to be a common geological process acting in most plume-related LIP, which in turn is related to the inversion of the stress regime from regional extension lithosphere doming to localised continental rifting.

► Rhyolites associated to Northern Ethiopian CFB at the Afar plume axis.
► Petrogenetic modelling of rhyolites using major, trace elements and phase equilibria.
► Inversion of the stress regime and incipient rifting.
► Tectono-magmatic conditions changed from “open” to “closed” system.
► Rhyolites fractionated from picrite basalts in upward zoned fissural magma chambers.