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

24 25 Cenozoic alkaline lavas from the Al Ashkharah area, facing the Indian ocean along 26 the North-East Oman coastline, contain numerous small (‹ 2cm) mantle xenoliths. 27 They provide a unique opportunity to investigate the nature and evolution of the 28 upper mantle beneath the Oman passive margin, bordering the Owen Basin. All 29 studied xenoliths are porphyroclastic to equigranular spinel lherzolites and 30 harzburgites They are all devoid of amphibole and phlogopite. The composition of 31 their clinopyroxenes, orthopyroxenes, olivines and spinels indicate that these 32 samples are witnesses of a typical subcontinental lithospheric upper mantle and are 33 quite distinct from the peridotites cropping out in the nearby Oman ophiolite. 34 The clinopyroxene major element composition record an evolution from fertile 35 lherzolites (Mg#: 89 and Al2O3: 7.5 wt%) to refractory harzburgites (Mg#: 93.5 and 36 Al2O3: 2.5 wt%). The clinopyroxene of most samples are characterised by REE 37 patterns evolving continuously from spoon-shaped to LREE-enriched with almost flat 38 HREE spectra (LaN/YbN: 2.5-30; LaN/SmN: 3.2-24; SmN/YbN: 0.25-4.6; HoN/LuN : 0.8839 1.15) and strong negative Ba, Nb, Zr, Hf and Ti anomalies. We propose that these 40 geochemical fingerprints can be accounted for in the frame of a two stages; (1) a 41 likely ancient decompression melting event characterised by a degree ranging from 42 1 to a maximum of 19 % and unrelated to the recent tectonic evolution of the Oman 43 margin, followed by (2) metasomatic transformation possibly related to the circulation 44 of carbonate-rich silicate melt during the Cenozoic rifting event that led to the 45 opening of the Owen basin. 46