The Wenquan ultramafic rocks in the Central East Kunlun Fault zone, Qinghai-Tibet Plateau—crustal relics of the Paleo-Tethys ocean

The Wenquan ultramafic rocks, located in the East Kunlun Orogenic belt in the northeastern part of the Qinghai-Tibet Plateau, consist of dunite, wehrlite, olivine-clinopyroxenite and clinopyroxenite, and exhibit cumulate textures. Olivine from dunite has high Fo (forsterite, 90.0–91.8 wt%) and NiO content (0.15–0.42 wt%). Cr-spinels from all of the rocks in this suite are characterized by high Cr# (100×[Cr/(Cr + Al)], 67–91), low Mg# (100×[Mg/(Mg + Fe 2+ )], 17–35) and low TiO 2 contents (mostly < 0.5 wt%). Clinopyroxene displays high Mg# (92–98) and low TiO 2 content (0.002–0.099 wt%), similar to those in ophiolitic cumulates. Geochemically, the Wenquan ultramafic rocks show enrichment of LILE, Sr, and Ba, and depletion of Nb and Th. High-Mg# (mostly > 80) and low-CaO (< 0.08 wt%) olivine, high-Cr# (up to 91) spinel, and low Ti contents of clinopyroxene and Cr-spinel indicate that the Wenquan cumulates were generated by high-degree partial melting of a depleted oceanic lithosphere mantle. The ultramafic intrusion most likely evolved from high-Mg basaltic magmas (Mg# = 77.5) that underwent fractional crystallization and crustal contamination. Zircon grains from clinopyroxenites yield a U–Pb weighted mean age of 331 ± 2 Ma, which is nearly coeval with the formation age of the A’nyemaqen ophiolites. The Wenquan Carboniferous ophiolites are confirmed to exist in the Central East Kunlun Fault zone, whereas previous studies have considered them to be the Proterozoic ophiolites. The Wenquan ophiolite might be a relict of the Paleotethyan ocean, indicating that there were two cycles of oceanic–continental evolution along the Central East Kunlun Fault zone.