| The Yarlung-Zangbo Suture Zone, extending more than 2000 km across the southern Tibet and into Burma, marks the boundary between the Indian and Asian plates. Ophiolites that crop out discontinuously along this zone are remnants of Neo-Tethyan Mesozoic ocean lithosphere emplaced during subduction of the Indian plate beneath Eurasia. Recently, less than two significant findings have been made in the Yarlung Zangbo Ophiolites, one is that diamonds and other UHP minerals have been found by Yang Jingsui’s group in ophiolitic peridotites and chromitites along the YZSZ, the diamond is called as ophiolite-type diamond, which derived from much deep mantle sources; another one is that more and more researchers have reached a consensus that the Yarlung Zangbo ophiolites were formed in a supra-subduction zone environment. Unfortunately, the tectonic setting related to ophiolite-type diamond and the dynamic evolution of the YZSZ are still not clear. There are many differences between west and east parts of the YZSZ, e.g., the spatial distribution and rock assemblages of ophiolites, the evolution history and emplacement mechanism. Moreover, previous studies about the west were poor, due to harsh natural environment and traffic inconvenience. Therefore, it is necessary to find out the characteristics of ophiolitic rock sequences, identify the tectonic setting and genesis of the western part of the Yarlung Zangbo ophiolites.This paper shows some new achievements and findings as bellow, based on detailed field survey and researches inside on Dongbo and Purang ophiolites located in the southern sub-belt of the western part of the YZSZ.(1) Establishing petrographic sequences of Dongbo and Purang ophiolites in the first time.Dongbo and Purang ophiolites show similar petrographic sequences, they comprise fresh peridotites and submarine volcanic-sedimentary rocks, which are distinctly absent of the typical cumulates, layered gabbro, isotropic gabbro, sheeted dikes and pillow lava units.The peridotites consist dominantly of harzburgite, minor lherzolite and dunite in lenses, associated with massive chromite. The gabbro-diabase and olivine pyroxenite dikes crosscut the mantle peridotites with several centimeters to over one meter in width. The submarine volcanic- sedimentary units show slightly different rock assemblages between the north and south margins of the two ophiolites. The south is composed, from up downward, of massive limestone with minor oolite, siliceous limestone, radiolarian chert, mudstone and silty shale, basaltic hyaloclastic rocks and massive basalts. The north consists, from bottom to up, of massive limestone, siliceous limestone, radiolarian chert, silty shale interbedded with sandstone, silicic tuff. The silicic tuff usually covered the hornblende gabbro, or thrust over the serpentinized peridotites.(2) The mantle peridotite experienced partial melted and modified later by enriched fluid.From lherzolite, harzbergite to dunite, the depleted degree shows much higher with increasing Fo value of olivine, Mg# value of pyroxene, Cr# value of chromite, and decreasing whole composition of rare earth element (REE), which is accordant with obviously depletion of Nb, Ti, Pb. However, peridotites altered by fluids with enrichment of LREE, LILE (Rb, Ba) and HFSE (Zr, Hf, Ta). In a word, mantle peridotites are the relicts of depleted mantle and altered by enriched fluids later.(3) Mafic dykes intruded into peridotites probably formed in back arc affinity.Mafic rock dykes including gabbro, diabase and pyroxenite yield zircons U-Pb dating of 128-130Ma (Liu et al.,2011; Xiong et al.,2011). They show N-MORB type REE patterns with a LILE (K, Sr, Rb) enrichment and noticeable Nb, Th and U negative anomalies, which illustrate the back arc basin signature. Moreover, the age of magmatic events is later than the eastern part of the YZSZ (about middle Jurassic).(4) Ascertainment of OIB-type and E-MORB type basalt in the first time.OIB-type basalts yielded the zircons U-Pb dating of 140Ma, which located in the bottom of the volcanic-sedimentary sequences. They derived from 1% to 5% partial melting of spinel-riched garnet lherzolite. E-MORB-type basalts interbedded into radiolarian cherts, showed zircons U-Pb dating of 137Ma, they were the result of interaction with middle ocean ridge and plume due to tectonic diagrams, and derived from 10% to 20% partial melting of spinel-garnet lherzolite.(5) Cherts deposited in a continental margin environment and were of biogenic origin, their materials came from terrigenous component and seamount or island arc basalts.The geochemical datas of siliceous rocks indicate a continental margin environment, and show biogenic and volcanic origins. Their materials originated fromterrigenous ingredient and seamount or island arc basalts. The radiolarian fossils suggest that the cherts formed ranged from Tithonian to Valangiaian (J3-K1), which is coincided with the whole rock K-Ar mean dating of 133.7±9.3Ma (sample X71-8).(6) The volcanic- sedimentary sequences are consistent with those of seamountsThe volcanic- sedimentary rocks show typical seamount signatures, on the basis of the rock assemblages, associated with geochemical characteristics. Dongbo and Purang ophiolites probably produced by plume hotspot, on the other hand, both of the ophiolites are the plume-type ophiolites.(7) Preliminary dynamic model was set up for Dongbo and Purang ophiolitesDuring J3-K1 (about 140Ma), a plume hotspot underplated and interacted with the subcontinental lithospheric mantle of the Gondwana northern margin, then promoted to rift continental margin, formed a small oceanic basin and built seamounts; About 130Ma, the initial ocean basin continued to spread, produced mafic magmatic event with back arc affinity.In conclusion, this study is of significance to rebuild the evolution of Neo-Tethys Ocean, the genesis of Dongbo and Purang ophiolites plays a crucial role in the interpretation of the geological setting, magmatic and tectonic processes of the YZSZ. |
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