The petrogenesis of plagiogranite of the Erenhot Ophiolite in the eastern Central Asian Orogenic Belt: A zircon Hf-O isotopic and whole-rock geochemical perspective

Abstract

Plagiogranite in ophiolite can provide invaluable insights into its origin, magma evolution, and the evolution of oceanic crust. In this study, we conducted a systematic investigation of the zircon U–Pb–Hf–O isotopes, whole-rock geochemistry, and REE modelling of the Early Carboniferous plagiogranites in the Erenhot Ophiolite, the eastern part of the Hegenshan–Erenhot Ophiolite Belt. The newly obtained zircon U–Pb dating results indicate that the Erenhot plagiogranite was formed between 351 Ma and 348 Ma, akin to the Early Carboniferous plagiogranite in Hegenshan Ophiolite. The 348–390 Ma zircons in the plagiogranites are predominantly zircons of an oceanic crust origin, exhibiting radioactive Hf isotopes (εHf(t) of 15.2 to 19.9) and low δ¹⁸O values (+2.43 ‰ to +5.00 ‰), indicating that the Hegenshan Ocean likely opened prior to 390 Ma. The Erenhot plagiogranites exhibit low TiO₂ (0.09–0.29 wt%), K₂O (0.01–0.31 wt%) contents, along with relatively constant La with varying SiO₂ contents. Combined with REE modelling, zircon Hf–O isotopes, and ΔFMQ (>0), these features suggest that the plagiogranite originated from the remelting of high-temperature hydrothermally altered gabbro in the younger lower oceanic crust with a highly depleted mantle source, under a relatively oxidizing environment. Significant variations in many major elements and their systematic changes with SiO₂, along with diverse Eu and Sr anomalies and REE modelling results, suggest that the parental magma of the plagiogranite underwent two stages of mineral fractional crystallization: first involving clinopyroxene, amphibole, and Ti- magnetite crystallization, followed by plagioclase accumulation and crystallization of amphibole and Ti- magnetite. The enrichment in LREEs, Rb, Ba, Th, and U, depletion in Nb, Ta, and Ti of plagiogranites and gabbros in the ophiolite, and the similar Ordovician–Silurian arc magmatic rocks in the two sides of the Hegenshan–Erenhot Ophiolite Belt, indicate that Hegenshan Ocean likely opened at a pre-existing back-arc basin, with deep mantle upwelling promoting oceanic crust extension.