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, produced a chemically and isotopically diverse volcanic field. The predominantly mafic samples include alkali basalt, transitional basalt, basanite and trachybasalt; rarer evolved compositions are typically trachyandesite. No samples represent primary magmas, all having undergone fractionation of olivine+clinopyroxene ± plagioclase±Fe-Ti oxides±apatite. Sr-Nd-Pb-O isotopic variatian (87Sr/86Sr= 0·7032-0·7046; 143Nd/144Nd= 0·51298-0·51278; 206Pb/204Pb= 18·93-18·05; δ18O=+5·2 to + 5·5‰) is largely the result of mixing between a mantle component, with a composition intermediate between that of mid-ocean ridge basalt (MORB) and the Afar plume, and 0-20% of an Early Proterozoic or Late Archaean silicic lower-crustal component which produced little change in δ18O values. Ratios of very to moderately incompatible trace elements vary widely (e.g. Ce/Y= 0·9-3·7) and independently of isotopic composition. Semi-quantitative modelling of REE ratios requires this heterogeneity to be the product of mixing between small melt fractions (<1%) from recently incompatible-trace-element-enriched garnet peridotite facies mantle and relatively larger melt fractions (∼5%) from spinel peridotite facies mantle, with samples containing 40-90% spinel-facies melt. Substantial variations in Zr/Sm and Nb/La ratios also suggest that the spinel-fecies mantle may be amphibole bearing. Intraplate volcanism in western Yemen appears to be the result of melting shallow mantle, perhaps in response to small amounts of lithospheric extension, that was metasomatized and hydrated by the Afar plume during, or shortly after, Oligocene flood volcanism.