首页|Melting geodynamics reveals a subduction origin for the Purang ophiolite,Tibet,China
Melting geodynamics reveals a subduction origin for the Purang ophiolite,Tibet,China
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Melting geodynamics reveals a subduction origin for the Purang ophiolite,Tibet,China
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-sub-duction 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 west-ern 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 TiO2 contents(＜0.1wt％)in spinel,high Al2O3(2.9wt％-4.4wt％)and CaO(1.9wt％-3.7wt％)contents in orthopyroxene and LREE-depletion in clinopy-roxene.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 Al2O3(＜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 har-zburgites 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 lher-zolites 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.
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