Abstract
Ultramafic xenoliths in Pliocene-Quaternary basalts from Bir Ali, the Shabwa Governorate, southern Yemen, were studied to examine upper mantle processes related to continental rifting. They show a great variety of rock types: spinel lherzolites, harzburgites, dunites, clinopyroxene (= cpx)-rich lherzolites, wehrlites and pyroxenites. The cpx-rich lherzolites and wehrlites with high clinopyroxene/orthopyroxene ratios (> 1), which are characteristic of the Bir Ali xenolith suite, contain lower Fo contents of olivine than ordinary lherzolites at a given Cr# of spinel. Their clinopyroxenes are higher in TiO2 and REE contents than those in the ordinary lherzolites, which are restite. These features strongly indicate metasomatic addition of clinopyroxene from alkali basalts at the expense of orthopyroxene to depleted peridotites for genesis of the cpx-rich lherzolites. The cpx-rich lherzolites are one of common constituents of lithospheric mantle at rifted continental margins.
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