Modelling the petrogenesis of the Ethiopian-Yemeni picrite-basalt CFB association: inferences on mantle heterogeneities and plume processes

The Oligocene Northern Ethiopian-Yemeni LIP, represented by a CFB plateau extending ca. 700 km in diameter, is characterized by a well-defined zonal arrangement with increasing plume-related physico-chemical features of erupted magmas, such as thermal regime, incompatible element enrichment and specific Sr-Nd-Pb-He isotopic fingerprint, from the periphery to the central plateau area (Beccaluva et al., 2009). Two CFB volcanic piles in the Lalibela district (Northern Ethiopia, ca. 2 km thick) and in the Manakhah section (Northern Yemeni plateau, ca. 1 km thick) which erupted close to the Oligocene Afar plume axis, are similarly characterized by very high-Ti transitional basalts and picrites (HT2, Beccaluva et al., 2009; 2011) that account for ca. 13% (40,000 km3) of the total Ethiopian-Yemeni CFB lavas. These magmas are characterized, in addition to the extremely high TiO2 content (3-6 wt.%) by a high MgO content (mostly between 8 and 18 wt.%), and show striking compositional analogies with those from the Karoo province and the Siberian meimechites (Ellam & Cox, 1991; Heinonen et al., 2014). Petrological modelling based on whole rock FeO-MgO and Ol composition (Herzberg et al., 2007) indicates that some of picrites (MgO 16-17 wt.%) are near-primary magmas with olivine phenoscrysts up to Fo 90.4. Calculation shows that the primary melts have picrite composition MgO 19.8-20.7 wt.% and were generated by polybaric melting in the pressure range 3-4 GPa at a potential temperature of 1570 °C. Together with high-MgO lavas from Hawaii and Gorg ona, these are the highest temperatures of any OIB and LIP lavas. The available data suggest that HT2 magma sources necessarily require the involvement of specific high-Ti (and Fe) deep-seated sublithospheric components which were entrained and remobilized by the rising plume. References: Beccaluva L., Bianchini G., Natali C. & Siena F. 2009. Continental Flood Basalts and Mantle Plumes: a Case Study of the Northern Ethiopian Plateau. J. Petrol. 50, 1377-1403. Beccaluva L., Bianchini G., Ellam R.M., Natali C., Santato A., Siena F. & Stuart M.F. 2011. Peridotite xenoliths from Ethiopia: inferences on mantle processes from Plume to Rift settings. In: Beccaluva L., Bianchini G., Wilson M. Eds., Volcanism and evolution of the African Lithosphere. Geol. Soc. Am. Sp. Paper, 478, 77-104. Ellam R.M. & Cox K.G. 1991. An interpretation of Karoo picrite basalts in terms of interaction between asthenospheric magmas and the mantle lithosphere. Earth Planet. Sci. Lett. 105, 330-342. Heinonen J.S., Carlson R.W., Riley T.R., Luttinen A.V. & Horan M.F. 2014. Subduction-modified oceanic crust mixed with a depleted mantle reservoir in the sources of the Karoo continental flood basalt province. Earth Planet. Sci. Lett. 394, 229-241. Herzberg C., Asimow P.D., Arndt N., Niu Y., Lesher C.M., Fitton J.G., Cheadle M.J. & Saunders A.D. 2007. Temperatures in ambient mantle and plumes: Constraints from basalts, picrites and komatiites. Geochem. Geophys. Geosys., 8, doi:10.1029GC001390.