Nature and evolution of the lithospheric mantle beneath the passive margin of East Oman: Evidence from mantle xenoliths sampled by Cenozoic alkaline lavas

The clinopyroxene major element composition record an evolution from fertile lherzolites (Mg#: 89 and Al2O3: 7.5 wt.%) to refractory harzburgites (Mg#: 93.5 and Al2O3: 2.5 wt.%). The clinopyroxene of most samples are characterised by REE patterns evolving continuously from spoon-shaped to LREE-enriched with almost flat HREE spectra (LaN/YbN: 2.5-30; LaN/SmN: 3.2-24; SmN/YbN: 0.25-4.6; HoN/LuN: 0.88-1.15) and strong negative Ba, Nb, Zr, Hf and Ti anomalies. We propose that these geochemical fingerprints can be accounted for by two processes; (1) a - relatively old (pre-Cenozoic rifting) - decompression melting event characterised by ~ 1 to a maximum of 13% partial melting and unrelated to the recent (Eocene) tectonic evolution of the Oman margin, followed by (2) metasomatic transformation possibly related to the circulation of alkaline mafic silicate melt displaying geochemical similarities with the host basanites during the Cenozoic rifting event that led to the opening of the Owen basin.