The Evolution Of Shiquanhe-yongzhu-jiali Ophiolitic Melange Belt, Tibetan Plateau

Shiquanhe–Yongzhu–Jiali ophiolitic mélange belt (SYJMB) is an important tectoniczone in Tibetan plateau and the relict of Neo–Tethys Ocean in central Tibet as well asBangong–Nujiang suture zone (BNSZ). The study of SYJMB is so limited that its attribute,evolutional time and eastward extend are still controversial, which has constituted an obstaclefor understanding the evolution of Neo–Tethys Ocean in central Tibet.In the thesis, the Guomangco and Zhongcang ophiolitic mélanges in middle and westernparts of SYJMB were investigated by fieldwork, petrology, mineral chemistry, geochemistry,zircon LA–ICP–MS U–Pb geochronology and Lu–Hf isotope. Based on the previous studieson ophiolitic mélanges, strata and magmatic rocks, the relation between SYJMB and BNSZand evolution of SYJMB were investigated.Guomangco ophiolitic mélange develops all subunits of a typical Penrose–type ophiolitepseudostratigraphy, including metamorphic peridotite, mafic–ultramafic cumulate, mafic dike,pillow basalt, siliceous and minor chromitite. Mineral chemical and geochemical analysesshow that Guomangco ophiolitic mélange are probably originated in a back–arc basin withstrong MORB–related characteristics near continental margin. The mafic dikes mostly haveE–MORB major and trace element signatures, and formed in late stage of evolution ofback–arc basin. With the evolution of back–arc basin, subducted slab gradually retreated, andnew mantle convection occurred in mantle wedge. The recycling may make themetasomatized mantle undergo high degrees of partial melting and generate E–MORBs.According to the single zircon U–Pb dating, the evolutional time of Guomangco ophioliticmélange is between Early Jurassic and Early Cretaceous. Suoer clastic rock covers theperidotite of Guomangco ophiolitic mélange in angular unconformity, and is mainly composed of clastic and groundmass. Based on the field and petrologic characteristics, anorigin of Suoer clastic rock is considered: Guomangco peridotites break and peridotitecalstics were metasomated by hydrothermal fluid of carbonate.Zhongcang ophiolitic mélange is new discovered during regional geological surveying inDachagou area. The ophiolitic sequence mainly consists of metamorphic peridotite, cumulategabbro and gabbro dike with minor basalt and siliceous. The cumulate gabbros and gabbrodikes have island arc and mid–ocean ridge basalt affinities, suggesting that they wereoriginated in an oceanic back arc basin. Whole rock geochemistry and high positive εNd(t)values show that these gabbros were derived from~30%partial melting of a spinel lherzolitemantle, which was enriched by interaction with slab–derived fluids and melt from sediment.U–Pb analyses of zircons from cumulate gabbros and gabbro dike yield weighted mean agesof114.3±1.4Ma,116.1±1.8Ma and113.4±1.7Ma, respectively. Zhongcang ophiolitic mélangewere probably formed in an Early Cretaceous oceanic back arc basin.The ages of SYJMB ophiolitic mélanges vary from Late Triassic to Early Cretaceous,and geochemistry and stratigraphic log could be connected to short–lived intra–oceanicback–arc basin origin. The ages from BNSZ ophiolitic mélanges are between Late Permianand Early Cretaceous. The BNSZ mafic rocks show variable mixing of components fromMORB–type to IAT and to OIB end–members, the mantle source of BNSZ is probablymainly composed of DDM and EM1with minor EM2. The BNSZ probably represents themain oceanic basin of Neo–Tethys ocean in central Tibet, which underwent a complexevolution.The SYJMB joins into BNSZ in the east of Jiali, which was closed in late Early Jurassic.The middle and western parts of SYJMB represent a short–lived back–arc basin caused bysouthward subduction of BNSZ, and the back–arc basin is near continental margin from Jialito Yongzhu. The back–arc basin opened earlier in the east and propagated westwards frommiddle Jurassic to early Cretaceous. In Late Jurassic, the back–arc basin subductedsouthward and finally closed in Early Cretaceous.