Petrogenesis and geodynamic setting of high-Cr chromitites in fore-arc peridotites: A case study from the Halaban ophiolite, Eastern Arabian Shield, Saudi Arabia

Abstract

The present study deals with chromitite pods and the associated mantle section of the Neoproterozoic Halaban ophiolite, Eastern Arabian shield, Saudi Arabia. The NNW–SSE-trending Halaban ophiolite represents a moderately dismembered Penrose-type ophiolite section that is tectonically emplaced over island arc assemblages. It comprises a mantle section below metagabbros upward to the pelagic sedimentary cover that overlies the mafic volcanics. The Halaban mantle section includes chromitite-bearing serpentinized ultramafics, as well as metaperidotites, listvenites and talc‑carbonates along shear zones and faults with variable degrees of alteration and metamorphism. Preserved textures include bastite and mesh indicating respectively harzburgite and dunite protoliths of the studied serpentinites. The chromitite pods are scarce being restricted to the uppermost mantle section and the overlying Moho transition zone (MTZ) dunite. They are commonly massive with interstitial serpentines, carbonates and Cr-rich chlorite (kämmererite). Cr-spinel of the chromitite pods hosts inclusions of isolated sulfides and silicates (serpentine, olivine and kämmererite). The Cr-spinel of the massive chromitites has higher Cr# (0.77–0.83) and Mg# (0.60–70) than the disseminated fresh Cr-spinel cores in the serpentinites (Cr# = 0.60–0.70 and Mg# = 0.43–0.60). High-Cr# combined with low-TiO2 character (<0.12 wt%) of the disseminated Cr-spinel consistent with residual mantle rocks that experienced high degrees of partial melt extraction. The mineralogical features of the Halaban serpentinites and the associated chromitite pods are similar to the mantle section that most probably formed in forearc peridotites of Supra-subduction zone (SSZ) environments. High-Cr# in Cr-spinel of chromitite pods suggesting possible boninitic parent magma. The parent magma had undergone melt–melt and melt–rock interactions with the precipitation of high-Cr chromitite from Al- and Ti-poor, and Cr-rich boninitic melts. Notably, the Cr# and Mg# of Cr-spinel increase from the harzburgite through the dunite to the chromitite due to increase intensity of melt-rock reaction during formation of the chromitite.