Geochemistry and petrogenesis of high-MgO ultramafic tertiary volcanic rocks from Bagah area, NW Ad Dhala Province, Yemen

Abstract

The studied high-MgO ultramafic Tertiary volcanic rocks are recorded in Bagah Village at the NW Ad Dhala province within the southeastern exposures of the volcanic rocks of the late Oligocene-early Miocene Yemen Trap Series (YTS) that belong to the Yemen Volcanic Group (YVG) which constitute part of the Afro-Arabian rift-related continental large igneous province. These rocks are strongly porphyritic composed of olivine phenocrysts set in hypocrystalline groundmass composed of olivine, pyroxene, opaque, rarely plagioclase and glass materials. The Bagah high-MgO volcanic rocks characterized by their high MgO (23.98–25.47 wt.%) and TiO2 (1.32–1.48 wt.%) with low Na2O + K2O (1.15–1.60 wt.%) and SiO2 (43.71–44.26 wt.%) and are classified as meimechite, and this is the first reported meimechite rocks throughout the volcanic rocks of the western Yemen continental volcanic province and, upon our knowledge, throughout the Afro-Arabian continental flood province. These rocks are classified to be of the high-Ti type and assigned to HT1 type corresponding to the Ethiopian-Yemeni CFB zonal arrangement reported in the literatures. They have high Mg number (79.4–80.0) and high concentrations of Cr (up to 2976 ppm), Ni (up to 1115 ppm) and other compatible elements with overall enrichment in an incompatible and LREEs and depletion in Y and other HREEs with variable anomalies of LIL and HFS elements. These characteristics are similar to meimechite rocks in many publications particularly those from the Vestfjella area of the Karoo LIP and Yanbian Terrane of the Emeishan LIP, but slightly differ from those of Meimecha-Kotui region of the Siberian LIP. The genesis of these rocks implies the incorporation of four mantle sources closely related to the interaction of the Arabian lithospheric mantle with the upwelling thermal Afar plume beneath study area and include OIB, N-MORB and E-MORB sources with participation of HIMU source as an additional component. The OIB-like mantle source derived directly from Afar plume. The N-MORB and E-MORB sources are formed by partial melting of normal and metasomatized Arabian lithospheric mantle due to the continuous raising of Afar mantle plume, and these mantle lithosphere-derived magmas had introduced significant contributions to the genesis of Bagah ultramafic rocks than those derived from Afar plume. Accordingly, the geochemical characteristics of these rocks can be interpreted in terms of mixing between these mantle-derived magma sources and the conditions that were operated during the creation of such magmas. The Bagah ultramafic volcanic (supposed meimechite) rocks were formed by small degrees of partial melting of garnet-bearing peridotite mantle at great depths under high melting pressure and temperatures above the amphibole stability due to the thermal effects of Afar plume. Such small degrees of partial melting, under similar melting conditions of either normal and/or metasomatized mantle peridotite are suggested for the formation of meimechites rocks from the Meimecha-Kotui province of the Siberian LIP, the Vestfjella area of the Karoo LIP and the Yanbian Terrane of the Emeishan LIP. Finally, these meimechite-forming magmas are rapidly ascent through the Arabian lithosphere with no or negligible crustal assimilation in rout to the surface.