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

摘要

Cenozoic alkaline lavas from the Al Ashkharah area, facing the Indian ocean along the North-East Oman coastline, contain numerous small (<2 cm) mantle xenoliths. They provide a unique opportunity to investigate the nature and evolution of the upper mantle beneath the Oman passive margin, bordering the Owen Basin. All studied xenoliths are porphyroclastic to equigranular spinel lherzolites and harzburgites. They are all devoid of amphibole and phlogopite. The composition of their clinopyroxenes, orthopyroxenes, olivines and spinels indicate that these samples are derived from a typical sub continental lithospheric upper mantle and are quite distinct from the peridotites cropping out in the nearby Oman ophiolite. 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 (La_N/Yb_N: 2.5-30; La_N/Sm_N: 3.2-24; Sm_N/Yb_N: 0.25-4.6; Ho_N/Lu_N: 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.