Melting geodynamics reveals a subduction origin for the Purang ophiolite, Tibet, China

Abstract

The debate regarding whether the Yarlung–Zangbo ophiolite (YZO) on the south of the Qinghai-Tibet Plateau, formed in a mid-ocean ridge (MOR) or a supra-subduction zone (SSZ) setting has remained unresolved. Here we present petrological, mineralogical, and geochemical data associated with modeling melting geodynamics of the mantle peridotites from the Purang ophiolite in the western segment of the Yarlung–Zangbo Suture Zone (YZSZ) to explore its tectonic environment. The Purang lherzolites are characterized by the protogranular texture and have abyssal-peridotite-like mineral compositions, including low Cr^# (20–30) and TiO₂ contents (<0.1wt%) in spinel, high Al₂O₃ (2.9wt% – 4.4wt%) and CaO (1.9wt% – 3.7wt%) contents in orthopyroxene and LREE-depletion in clinopyroxene. Compositions of these lherzolites can be modeled by ~11% dynamic melting of the DMM source with a small fraction of melt (~0.5%) entrapped within the source, a similar melting process to typical abyssal peridotites. The Purang harzburgites are characterized by the porphyroclastic texture and exhibit highly refractory mineral compositions such as high spinel Cr^# (40–68), low orthopyroxene Al₂O₃ (<2.2wt%) and CaO (<1.1wt%) contents. Clinopyroxenes in these harzburgites are enriched in Sr (up to 6.0 ppm) and LREE [(Ce)_N = 0.02–0.4], but depleted in Ti (200 ppm, on average) and HREE [(Yb)_N < 2]. Importantly, the more depleted samples tend to have higher clinopyroxene Sr and LREE contents. These observations indicate an open-system hydrous melting with a continuous influx of slab fluid at a subduction zone. The modeled results show that these harzburgites could be formed by 19% – 23% hydrous melting with the supply rate of slab fluid at 0.1%–1%. The lower clinopyroxene V/Sc ratios in harzburgites than those in lherzolites suggest a high oxidation stage of the melting system of harzburgites, which is consistent with a hydrous melting environment for these harzburgites. It is therefore concluded that the Purang ophiolite has experienced a transformation of tectonic setting from MOR to SSZ.