Research On The Tectonic Setting And Fluid Origin For Serpentinization Of The Hegenshan Ophiolite, Inner Mongolia

The Hegenshan ophiolite suite, which is located in the Hinghan-Mongolian Orogenic belt, is the largest fragment of the paleo-Asian Oceanic lithosphere in this belt, which can provide important keys to understanding the tectonic evolution of the paleo-Asian Ocean. Serpentinite in the ophiolite suite is formed by hydration of the ultramafic component, which can be applied to constrain the tectonic setting of the ophiolite, fluid origin and temperature of serpentinization. In this dissertation, we conduct a comprehensive study(including petrology, X-ray diffraction, whole-rock major and trace elemental and H-O-B isotope geochemistry) on the serpentinites sampled in three localities(Xiaobaliang, the eastern Hegenshan and the southern Hegenshan) of the Hegenshan ophiolite,. The main conclusions are summarized below.1. The Hegenshan serpentinites, which are characterized by low Al2O3(0.2–1.3wt%) and high Mg#(89 – 92), were the highly refractory mantle peridotites after high-degree basalt extraction. Their U-type REE patterns, enrichment in large ion lithophile elements(LILE) and depletions in heavy rare earth elements(HREE), reflecting their derivation from a super-subduction zone(SSZ). The significant correlations of LREE(e.g. La, Ce, Pr, Nd) with immobile high field strength elements(HFSE, e.g., Zr) suggest that enrichments of both LREE and HFSE resulted from a common magmatic process.2. The Hegenshan serpentinites have extremely low Y and Yb content. The melting degree of ultramafic rocks in southern Hegenshan is about 26-31% while Xiaobaliang and the eastern Hegenshan is about 17-25%.3. The Hegenshan serpentinites have relatively uniform δD values(-120 ~-133‰), and mass balance consideration suggests that these serpentinites may re-equilibrated with local meteoric water after exposure on the surface.4. The δ18O values of Hegenshan serpentinites range from 4.3‰ to 9.8‰, which reflects different alteration temperature among three localities. The serpentinites from the eastern and southern Hegenshan and Xiaobaliang have δ18O values of 7.7- 9.8‰, 6.0- 9.7‰ and 4.3- 5.0‰, respectively, corresponding to alternation temperatures of 90- 130°C, 90- 170°C and 205-235°C. Also, the occurrence of carbonate minerals in some serpentinites from the eastern Hegenshan indicates the near-seafloor serpentinization.5. The serpentinites from the eastern and southern Hegenshan and Xiaobaliang also show variable δ11B values(δ11B= 9.1- 14.7‰), suggesting that serpentinization was likely caused by fluids dehydrated from the subducted oceanic crust and pelagic sediments with a δ11B range of 15-25‰.