Serpentinised Mantle Section of Neoproterozoic Ophiolite at Al-Barramiya District, North Arabian-Nubian Shield: Tectono-Magmatic Evolution and Metamorphism

Abstract

This research focuses on the field observations, petrography, mineral chemistry and geochemistry of the serpentinised peridotite of Al-Barramiya ophiolitic sequence to place constraints on their magmatic history and their geodynamic evolution. Al-Barramiya ophiolitic rocks are a dismembered ophiolite which was strongly deformed and metamorphosed under greenschist to lower amphibolite facies. They comprise a mantle section dominated by highly serpentinised peridotite with less metapyroxenite and chromitite, as well as a crustal portion represented by metagabbros. Along shear zones, the ophiolite sequence was affected by several types of alteration. Extensive carbonate alteration is common in the ultramafic section, resulted in talc carbonates, listvenites and magnesite, while rodingitisation is common in the metagabbro resulted in rodingite. Despite the extensive serpentinisation, some fresh relics of primary mantle minerals such as Cr-spinel, olivine and pyroxenes are preserved sporadically in the serpentinised peridotite. Few Cr-spinel crystals are sometimes surrounded by subhedral flakes of Cr-chlorite (kämmererite) that was formed due to replacement of Cr-spinel during later alteration or regional metamorphism. The serpentinite samples are depleted in the total REE (0.56–1.19 ppm) with slightly negative to slightly positive Eu anomalies (0.89–1.28). The fresh cores of Cr-spinel have Cr# mostly > 60, and the relics of pyroxenes and olivine are Mg-rich suggesting that the Al-Barramiya serpentinites are residual to high degrees of melt extraction. The estimated degrees of partial melting range between 18.2% and 20.7%. All the mineralogical and geochemical characteristics of the ultramafic section of the Al-Barramiya ophiolites are most consistent with its formation in a fore-arc setting.