High-MgO lavas associated to CFB as indicators of plume-related thermochemical effects: The case of ultra-titaniferous picrite–basalt from the Northern Ethiopian–Yemeni Plateau

Abstract

A comprehensive petrological and geochemical dataset is reported in order to define the thermo-compositional characteristics of Ti (Fe)-enriched picrite–basalt lavas (HT2, TiO2 3–7 wt%), erupted close to the axial zone of the inferred Afar mantle plume, at the centre of the originally continuous Ethiopian–Yemeni CFB plateau (ca. 30Ma) which is zonally arranged with progressively lower Ti basalts (HT1, TiO2 2–4 wt%; LT, TiO2 1–3 wt%) toward the periphery. Integrated petrogenetic modelling based on major and trace element analyses of bulk rocks, minerals, and melt inclusions in olivines, as well as Sr–Nd–Pb–He–O isotope compositional variations enables us to make several conclusions. 1) The phase equilibria constraints indicate that HT2 primary picrites were generated at ca. 1570°C mantle potential temperatures (Tp) in the pressure range 4–5GPa whereas the HT1 and LT primary melts formed at shallower level (<2–3GPa, Tp 1530°C for HT1 and 1430°C for LT). Thus, the Afar plume head was a thermally and compositionally zoned melting region with maximum excess temperatures of 300–350°C with respect to the ambient mantle. 2) The HT2 primary melts upwelled nearly adiabatically to the base of the continental crust (ca. 1GPa) where fractionation of olivine, followed by clinopyroxene, led to variably differentiated picritic and basaltic magmas. 3) Trace element modelling requires that the primary HT2 melts were generated—either by fractional or batch melting (F 9–10%)—from a mixed garnet peridotite source (85%) with 15% eclogite (derived from transitional MORB protoliths included in Panafrican terranes) that has to be considered a specific Ti–Fe and incompatible element enriched component entrained by the Afar plume. 4) The LT, HT1, and HT2 lavas have 143Nd/144Nd=0.5131–0.5128, whereas Sr–Pb isotopes are positively correlated with TiO2, varying from 87 Sr/86Sr 0.7032 and 206Pb/204Pb 18.2 in LT basalts to 87Sr/86Sr 0.7044 and 206Pb/204Pb 19.4 in HT2 picrite–basalts. High 3He/4He (15–20RA) ratios are exclusively observed in HT2 lavas, confirming earlier evidence that these magmas require a component of deep mantle in addition to eclogite, while the LT basalts may more effectively reflect the signature of the pre-existing mantle domains. The comparison between high-MgO (13–22%) lavas from several Phanerozoic CFB provinces (Karoo, Paranà–Etendeka, Emeishan, Siberia, Deccan, North Atlantic Province) shows that they share extremely high mantle potential temperatures (Tp 1550–1700°C) supporting the view that hot mantle plumes are favoured candidates for triggering many LIPs. However, the high incompatible element and isotopic variability of these high-MgO lavas (and associated CFB) suggest that plume thermal anomalies are not necessarily accompanied by significant and specific chemical effects, which depend on the nature of mantle materials recycled during the plume rise, as well as by the extent of related mantle enrichments (if any) on the pre-existing lithospheric section.