Petrogenesis of Quaternary Intraplate Volcanism, Sana'a, Yemen: Implications for Plume-Lithosphere Interaction and Polybaric Melt Hybridization

Abstract

Quaternary intraplate volcanism near Sana’a, western Yemen, INTRODUCTION produced a chemically and isotopically diverse volcanic field. The The ultimate source and reason(s) why continental inpredominantly mafic samples include alkali basalt, transitional traplate volcanism occurs remain unclear, despite combasalt, basanite and trachybasalt; rarer evolved compositions are mon links with continental rifting and regions of mantle typically trachyandesite. No samples represent primary magmas, upwelling. The extensive belt of late Cenozoic (<15 Ma) all having undergone fractionation of olivine+clinopyroxene± intraplate volcanic fields that crops out along the length plagioclase±Fe–Ti oxides±apatite. Sr–Nd–Pb–O isotopic variof the western margin of the Arabian peninsula in Jordan, ation ( Sr/Sr=0·7032–0·7046; Nd/Nd=0·51298– Israel, Saudi Arabia and Yemen forms one of the largest 0·51278; Pb/Pb=18·93–18·05; dO=+5·2 to intraplate volcanic provinces on Earth. These volcanic +5·5‰) is largely the result of mixing between a mantle component, rocks are generally more alkaline than older Eocene to with a composition intermediate between that of mid-ocean ridge Miocene flood volcanism erupted at the Afro-Arabian basalt (MORB) and the Afar plume, and 0–20% of an Early triple junction in Yemen, Ethiopia and Eritrea (Camp & Proterozoic or Late Archaean silicic lower-crustal component which Roobol, 1992). Intraplate volcanism in Yemen is clearly produced little change in dO values. Ratios of very to moderately incompatible trace elements vary widely (e.g. Ce/Y=0·9–3·7) and associated with both continental rifting, which produced independently of isotopic composition. Semi-quantitative modelling of the Red Sea and Gulf of Aden, and also a mantle plume, REE ratios requires this heterogeneity to be the product of mixing which produced Oligocene flood volcanism (Baker et al., between small melt fractions (<1%) from recently incompatible1996). Given the well-characterized crustal and mantle trace-element-enriched garnet peridotite facies mantle and relatively (asthenospheric, plume and lithospheric mantle) comlarger melt fractions (~5%) from spinel peridotite facies mantle, positions in this region, Yemen is an ideal setting in with samples containing 40–90% spinel-facies melt. Substantial which to attempt to identify the source of intraplate variations in Zr/Sm and Nb/La ratios also suggest that the spinelvolcanism and to speculate on why it occurred during facies mantle may be amphibole bearing. Intraplate volcanism in the Late Cenozoic. western Yemen appears to be the result of melting shallow mantle, Detailed chemical, but not isotopic, studies of some of perhaps in response to small amounts of lithospheric extension, that the Saudi Arabian volcanic fields have been published was metasomatized and hydrated by the Afar plume during, or (Camp & Roobol, 1989; Camp et al., 1991, 1992), highshortly after, Oligocene flood volcanism. lighting the role of fractional crystallization and variable degrees of partial melting in generating the diversity of predominantly mafic rock types present in these fields.