Petrological characteristics of the Neoproterozoic Ess ophiolite mantle section, Arabian Shield, Saudi Arabia: a mineral chemistry perspective

The Ess ophiolite, one of the most important ophiolitic massifs of the Arabian Shield, consists of a lower succession of serpentinized mantle rocks overlain by an ultramafic cumulate sequence, layered and isotropic gabbros, sheeted dykes, pillow lavas and pelagic sediments. The Ess mantle section is composed mainly of serpentinized peridotites with chromitite pods, dunite, wehrlite and pyroxenite. Extensive metasomatism and alteration has transformed the ultramafic rocks to talc-carbonates, magnesite deposits and listvenite, especially along shear zones and fault planes. Nevertheless, relics of primary chromian spinel, olivine and pyroxenes are observed. Both primary and metamorphic olivines can be recognized in dunite; the latter is marked by very high forsterite content (97-98), low NiO content (< 0.2 wt%) and wide variations in MnO content (0.03-1.3 wt%). The mesh and bastite textures of the serpentine suggest that protoliths were mainly harzburgite with minor dunite. Some fresh cores of Cr-spinel are rimmed by ferritchromite and Cr-magnetite, indicating prograde metamorphism at lower amphibolite facies under oxidizing conditions. The high Cr# (> 0.6) and low TiO2 content (< 0.14 wt%) of fresh Cr-spinel and the high forsterite (0.90-0.93) and NiO contents (0.4-0.5 wt%) of fresh olivine are all consistent with residual mantle rocks that experienced high degrees of partial melt extraction. Orthopyroxene and clinopyroxene in the Ess peridotites have low CaO, Al2O3 and TiO2 contents resembling those typically found in depleted harzburgites from fore-arcs. Consequently, we propose that the Ess mantle peridotites formed in a forearc setting during subduction initiation that developed as a result of northwest subduction due to the convergence between East and West Gondwana, leading eventually to closure of the Mozambique Ocean during the Pan-African orogeny.