Mantle processes in the sub-continental lithosphere; the case study of the rifted sp-lherzolites from Zabargad (Red Sea)

New petrological and geochemical data (including new data on REE bulk rock compositions of pyroxenites and trace element values for amphiboles from both peridotites and pyroxenites) are presented for the Zabargad ultramafic rocks. The dominant rock type is a four-phase, granular, spinel lherzolite characterized by: 1) relative abundance of clinopyroxene (15-20 vol.%), 2) high contents of fusible components in the constituent minerals, 3) high bulk-rock REE with C1[chondrite?]-normalized pattern almost flat but with slight negative LREE fractionation. During decompression, Zabargad peridotites were intruded by hydrous melts which generated amphibole-bearing websterite dykes, characterized by dominant subhedral pyroxenes with interstitial Ti-pargasite and Ti-phlogopite. Both clinopyroxenes and Ti-pargasites are strongly LREE enriched, suggesting that these rocks crystallized from alkaline melts. Intrusion was probably accompanied by circulation of hydrous fluids which produced widespread cryptic and local modal metasomatism (i.e. crystallization of Ti-pargasite + Ti- phlogopite +- apatite and opaque minerals) in the surrounding lherzolite.