PETROGENESIS OF QUATERNARY INTRAPLATE VOLCANISM, SANAA, YEMEN - IMPLICATIONS FOR PLUME-LITHOSPHERE INTERACTION AND POLYBARIC MELT HYBRIDIZATION

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 an d trachybasalt; rarer evolved compositions are typically trachyandesit e. No samples represent primary magmas, all having undergone fractiona tion of olive+clinopyroxene+/-plagioclase +/- Fe-Ti oxides +/- apatite . Sr-Nd-Pb-O isotopic variation (Sr-87/Sr-86, = 0.7032-0.7046; Nd-143/ Nd-144 = 0.51298-0 51278; Pb-206/Pb-204=18.93-18.05; delta(18)O = + 5. 2 to + 5.5 parts per thousand is largely the result of mixing between a mantle component, with a composition intermediate between that of mi d-ocean ridge basalt (MORB) and the Afar plume, and 0-20% of an Early Prpterozoic or Late Archaean silicic lower-crustal component which pro duced little change in delta(18)O values. Ratios of very tb moderately incompatible trace elements vary widely (e.g. Ce/Y= 0.9-3.7) and inde pendently of isotopic composition. Semi-quantitative modelling of REE ratios requires this heterogeneity to be the product of mixing between small melt fractions (< %) from recently incompatible-trace-element-e nriched garnet peridotite facies mantle and relatively larger melt fra ctions (similar to 5%) from spinel peridotite facies mantle, with samp les containing 40-90% spinel-facies melt. Substantial variations in Zr /Sm and Nb/La ratios also suggest that the spinel-facies mantle may be amphibole bearing. Intraplate volcanism in western Yemen appears to b e the result of melting shallow mantle, perhaps in response to small a mounts of lithospheric extension, that was metasomatized and hydrated by the Afar plume during, or shortly after, Oligocene flood volcanism.