Petrogenetic evolution of Cryogenian Halaban ophiolite, Saudi Arabia: A fragment of fore-arc oceanic lithosphere mantle

Abstract

In this study, Neoproterozoic Halaban ophiolite situated in the eastern part of the Arabian shield, Saudi Arabia, was investigated. The Halaban ophiolite represents a section of a dismembered ophiolite, tectonically emplaced over a volcano-sedimentary succession of island arc assemblages. It is variably, altered, deformed and metamorphosed to serpentinized rocks. It includes serpentinized peridotite and metagabbro with rare metabasalt, chromitite pods, listvenite and rodingite. The contact between the lower mantle unit (serpentinized peridotite) and the upper crustal unit (metagabbro) is originally magmatic, although it is disrupted by tectonism. Nevertheless, because of the high degree of alteration and metamorphism, few fresh relicts of primary minerals (chromian spinel, olivine and pyroxenes) were recorded. The mineral assemblages in the Halaban ophiolite are comparable to greenschist, to lower amphibolite facies metamorphism. The fresh olivine relics have high Fo and Ni content, while the fresh chromian spinel cores have high Cr# accompanied by a low TiO2. The preserved texture (mesh and bastite), fresh relict minerals, and the whole-rock compositions (Mg# = molar Mg/ (Mg + Fe2+) = 0.91–0.93 with low CaO and Al2O3 content) suggest that the protolith is mainly harzburgite with minor dunite. The chemical composition of Halaban metagabbro is indicative of a tholeiitic affinity. In addition, the very low modal abundance of clinopyroxene and the high Ni, Cr and Co content of the serpentinized peridotite, and high-Cr# (av. 0.66) with low-Ti (mostly ≤0.1 wt%) of chromian spinel, suggest significant partial melting. The serpentinized peridotite (harzburgite and dunite) undergone different degrees of partial melting, ranging from approximately 32% to 37% for harzburgite and approximately 37% to 42% for dunite. The variation of the degree of partial melting between harzburgite and dunite is indicative of mantle heterogeneity beneath the Arabian Shield. The general petrological characteristics of the Halaban ophiolite are substantially similar to those formed by melt-peridotites interaction and subsequent melt mixing in a fore arc setting within a supra-subduction zone system. Halaban chloritite is formed as a secondary product due to the metasomatic interaction of the ophiolitic ultramafic rocks. The content of Cr, Co and Ni of the chloritite sample was consistently close to that of the serpentinites.