Zircon U-Pb Geochronology, Geochemistry and Geological Significance of the Santaishan-Yingjiang Ultramafic Rocks in Western Yunnan, China

The study of ultramafic rocks in Western Yunnan is of great significance for an understanding of the tectonic evolution of the Neo-Tethys Ocean. The zircon U-Pb data indicated that the Santaishan serpentinized pyroxene peridotite (SSPP) was formed 186-190 Ma, and the Yingjiang hornblende pyroxenite (YHP) was formed 182-183 Ma. The content of MgO in the SSPP is relatively high, but the SiO2, Al2O3, CaO and TiO2 content and sigma REE are low, while the YHP has opposite characteristics. The samples from the SSPP and YHP have similar distribution patterns of trace elements, both being enriched in large ion lithophile elements (LILEs) such as Rb, Ba and Th and depleted in high field strength elements (HFSEs) such as Ti, P and Nb. These characteristics are consistent with the supra-subduction zone (SSZ) type and mid-ocean ridge basalt (MORB) type of ophiolite in the Bangong-Nujiang suture zone. The SSPP rocks have relatively high (Sr-87/Sr-86)(i) ratios (0.7091-0.7131) and positive Hf(t) values (11.2-13.8), with epsilon Nd(t) values varying from -1.1 to 9.4. The YHP has relatively low epsilon Hf(t) values (3.5 to 6.9), with the Nd-Hf isotopic model ages ranging from 610 to 942 Ma. The signatures of Sr-Nd and Lu-Hf isotopes indicate that the SSPP and YHP were derived from the depleted mantle, and the crustal material in the magma source may have originated from the Neoproterozoic Rodinia supercontinent. In the early Middle Jurassic (190 Ma), the Tengchong Block was in the setting of an active continental margin induced by the subduction of the Bangong-Nujiang Ocean, where the SSZ-type SSPP with ophiolite characteristics was formed. With the continuous subduction of the Bangong-Nujiang Ocean, the slab retreated and induced mantle convection, which resulted in the gradual thinning of the continental crust. Meanwhile, the Yingjiang back-arc basin was formed 183 Ma. Under the influence of the upwelling of the asthenosphere and the mixture of crustal materials, the MORB-type YHP was formed.