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

作　　者：Tao Ruan Zhong-Jie Bai Wei-Guang Zhu Shi-Ji Zheng 

机构地区：[1]State Key Laboratory of Ore Deposit Geochemistry,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,China [2]University of Chinese Academy of Sciences,Beijing 100049,China

出　　处：《Acta Geochimica》2024年第4期754-773,共20页地球化学学报（英文）

基　　金：supported by the Second Tibetan Plateau Scientific Expedition and Research(SETP)(2019QZKK0806-02);the National Natural Science Foundation of China(42121003,42122024);CAS“Light of West China”Program(xbzg-zdsys-202310);Guizhou Provincial High level Innovation Talent program(GCC[2023]057);Guizhou Provincial 2021 Science and Technology Subsidies(No.GZ2021SIG)

摘　　要：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 TiO_(2) contents(<0.1wt%)in spinel,high Al_(2)O_(3)(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 Al_(2)O_(3)(<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.

关 键 词：Melting geodynamics SSZ peridotites MOR peridotites CLINOPYROXENE Purang ophiolite 

分 类 号：P541[天文地球—构造地质学] P58[天文地球—地质学]