IMPLICATIONS OF MANTLE XENOLITHS IN THE PLIOCENE-QUATERNARY BASALTS FROM THE SHAWRAN CRATER, BIR ALI AREA, YEMEN

Abstract

Mantle xenoliths in basalt and tuffaceous volcanoes at the Shawran Crater, Bir Ali area, along the southeastern coast of Yemen are very useful to elucidate the nature of the lithospheric mantle beneath the area, which is characterised by rift tectonic activity and volcanism during the Pliocene–Quaternary period. The studied mantle xenoliths are mostly fresh showing igneous textures and composed mainly of spinel harzburgite and lherzolite, in addition to, less prominently, dunite. Spinel harzburgite and lherzolite are holocrystalline with abundant granular spinel that occurs interstitially to olivine, orthopyroxene, and clinopyroxene. The forsterite content of olivine is high (Fo_89–91) attesting its mantle residue origin. Orthopyroxene is mostly enstatite (En_89–92), and clinopyroxene is diopsidic in composition (Wo_24–47En_49–52 Fs_2–4). Spinel Cr# (Cr/(Cr + Al) is distinguishable between lherzolites (Cr# = 0.12–0.15) and harzburgites (Cr# = 0.20–0.31). The calculated oxygen fugacity (logfO₂) of the studied lherzolites (−1.76 to −0.20) and harzburgite (−2.7 to −0.98) xenoliths, and their equilibration temperature (~ 820–1,145°C) at about 1.5 GPa pressure, along with their mineralogical and petrological characteristics, are all support their evolution by mixed enrichment from a fertile and depleted asthenospheric mantle beneath the Bir Ali area.