Petrogenesis of mantle-hosted granitoids from the Samail ophiolite

Abstract

The Samail ophiolite in Oman and the United Arab Emirates comprises Cretaceous oceanic lithosphere, from mantle peridotite below the petrologic Moho to crustal gabbro, sheeted dykes, and basalt pillows, with intercalated sediments. Obducted onto the passive margin of the Arabian plate, the Samail ophiolite is interpreted as a supra-subduction zone (SSZ) ophiolite. This study focuses on minor but important components of the Samail ophiolite: granitoids that intrude both its crustal and mantle sections, referred as crust- (CHG) and mantle-hosted granitoids (MHG), respectively. Through major and trace element compositions, stable and radiogenic isotopes, and fluid inclusion studies, this work explores their petrogenesis and implications in SSZ environments. Geochemical and isotopic evidence reveals distinct formation processes for CHG and MHG. CHG are metaluminous tonalites and trondhjemites, low-K, with flat to light rare earth element-depleted patterns. Positive εNd and low δ18O values suggest a mid-ocean ridge basalts-like affinity. They are interpreted to reflect fractionation of basalt/gabbro or anatexis of oceanic crust. In contrast, MHG range from granite to tonalite and are mostly peraluminous, similar to peraluminous sediment-derived granitoids. However, they show scattered K2O, Na2O, CaO, Rb/Sr, higher SiO2, and lower Al2O3+FeO+MgO+TiO2 wt.% compared to typical sediment-derived granitoids. MHG likely formed from the interaction of sediment (pelitic to siliceous) derived melts with basaltic melts from an underthrust oceanic plate within the mantle wedge, without involvement of the overriding oceanic crust. This mixing is supported by low whole-rock εNd values and Sr and Pb isotopic signatures. However, variable δ18O (3-28‰) and εHf values, along with elevated δ7Li in muscovite from MHG, suggest the involvement of deep-sea ferromanganese sediments as an alternative source. Fluid inclusion studies on the MHG provide further insight into fluid evolution and migration within ophiolite systems. Low δ13Ctot (-27‰) values from quartz-hosted fluid inclusions are hydrocarbon-bearing, probably derived from carbonate-bearing rocks below the base of the ophiolite during obduction. The granitoids studied here offer insights into petrogenetic processes in SSZ where CHG represent products of processes operating within the ophiolite, while MHG reflect inputs derived externally to the ophiolite. The MHG record crustal material transport into the mantle, contributing to mantle heterogeneity over time.