Petrogenesis of Xiwanggou olivine gabbro in East Kunlun Mountains: Constraints from geochemistry, zircon U-Pb dating and Hf isotopes

The mafic-ultramafic rocks in Paleo-Tethys domain of East Kunlun Mountains are not well documented. In this paper, the authors present petrographical, geochronological, lithogeochemical and Hf isotopic data for the Xiwanggou olivine gabbro located in the eastern section of East Kunlun Mountains, Qinghai Province, with the purpose of constraining its formation age, petrogenesis and tectonic setting and providing new constraints for the Late Paleozoic-Early Mesozoic tectono-magmatic evolution in East Kunlun Mountains. Lithogeochemical studies show that the olivine gabbro, which is of sub-alkaline series, is characterized by low SiO₂ (40.91%-42.14%), low TiO₂ (0.29%-0.39%) and alkali content (K₂O+Na₂O=1.09%-1.36%) but high MgO content (28.18%-30.66%). The m/f ratios range from 5.03 to 5.39, falling into the field of ferrous-ultrabasic rocks. This suite of rocks are enriched in LILE (such as Rb, Th, U and K) and Pb, and relatively depleted in HFSE (such as Nb, P and Ti). The rocks have low REE content, with LREE-rich pattern and slightly negative Eu anomaly (ΣREE=28.1710^-6-30.9510^-6, (La/Yb)_N=3.77-4.98, δEu=0.80-0.95). LA-ICP-MS zircon U-Pb dating indicates that the weighted mean age of olivine gabbro is (264.9±1.2)Ma(n=26, MSWD=0.71), suggesting Middle Permian. Zircon ¹⁷⁶Hf/¹⁷⁷Hf values are in the range of 0.282709-0.283152 with corresponding ε_Hf(t) values of 3.7-19.3. and Lu-Hf single-stage modal ages (T_DM) vary from 135 to 753 Ma with mean age being 414Ma, older than U-Pb age. The authors hold that the parental magma of olivine gabbro was likely derived from the depleted mantle, with the probable addition of fluid-modified lithospheric mantle components, and was contaminated by crustal material. Combined with evolutionary characteristics of regional structures in East Kunlun Mountains, the authors consider that Xiwanggou olivine gabbro was formed during the subduction of Anyemaqen-Paleo-Tethys Ocean, and the northward subduction of Anyemaqen-Paleo-Tethys Ocean started at least in Middle Permian.