Ophiolites And Thrust System Of Middle Lhasa Block

Ophiolite and thrust system are two types of geological record in orogenic belt andimportant contents of tectonics and continental geodynamics. The west Namco ophiolite slices andYongzhuzangbu-Guomanco ophiolites in central Lhasa block, Dongqiao-Ando ophiolites in northLhasa block, Zedang-Luobusha ophiolites in south Lhasa block and west Namco thrust, southDamxung thrust of central Lhasa block are studied in detail by field profiling, geochemical testing,isotopic dating and palaeo-structure analyses based on geological survey at scale 1:250000 forreconstruction of paleo-tectonics in Early-Meso-Jurassic, Late Jurassic-Early Cretaceous, LateCretaceous and understanding tectonic evolution of the Tibetan Plateau, which lead toaccumulation of new data and several key progresses.1. Analyses on geochemistry and tectonic setting of ophiolitesGeological profiling, microscope observation, geochemical testing and isotopic dating havefinished systematically for studying ophiolites of west Namco Lake, Yongzhuzangbu-Guomanco,Dongqiao-Ando and Zedang-Luobusha. Pillow lava and cherts were discovered in west Namcoophiolite mélange. Lava and serpentinite peridote from ophiolite mélanges of west Namco Lake,Yongzhuzangbu-Guomancuo and Dongqiao-Ando in central and north Lhasa block exist similargeochemical features of MORB characterized by rich LREE and slightly right-dipping pattern ofrare-earth elements without Eu anomaly. Rb-Sr isochronic ages of serpentinite peridote andmetamorphic gabbra of west Namco ophiolite mélange are 166±26Ma and 173±10Ma respectively,giving time constraints for ophiolites of central Lhasa block. Ophiolites in west Namco Lake andYongzhuzangbu-Guomanco of central Lhasa block formed in spreading back- or intra-island arcbasin, and ophiolites in Dongqiao-Ando of north Lhasa block and Zedang-Luobusha of southLhasa block formed in smaller ocean and slow-spreading ocean respectively according topetrological geochemistry.2. Study on west Namco thrustThree ductile shear zones were discovered in Gayalong, Elongnubu andShenjue-Gechangchayu, and geological features, deformation, motion direction and structuralpattern are studied in detail for major thrust faults, ductile shear zones and folds in west bank ofNamco Lake, leading to identification of west Namco thrust (WNT). And west Namco thrust andophiolite mélanges outcropped from west Namco Lake to Yongzhuzangbu coexist together,indicating another tectonic suture in the Tibetan Plateau termed as the Namco-Yongzhu suture orNYS with its east extension to Jiali fault. Milonite from ductile Shenjue-Gechangchayu ductileshear zone formed in 490-625℃averaged 547℃and 0.20-0.76GPa averaged 0.42GPa,corresponding to 12-15km deep in crust. And ³⁹Ar-⁴⁰Ar dating of hornblende and plagioclase ofmilonite gave plateau ages of 173.97±0.50Ma and 109.45±0.50Ma respectively, indicatingthrusting of WNT in Early-Meso Jurassic and Late Cretaceous. While fission track dating of apatefrom milonite yielded an age of 43.6±3.8Ma, corresponding to Eocene thrusting of WNT.3. Study on south Damxung thrustThe first discovered south Damxung thrust (SDT) in east the Nyainqentanglha Mts. aredivided into 4 sections vertical to its extension including south section of detachment, middlesection of pilled thrust, north section of high-angle fault and back section of thrust slices accompanied by tight folding in south, overturn folding in middle and open wide folds in north.Major faults seems to merge into a deep detachment extended from the south frontier of SDT. Andgeological features, deformation, movement history of the Hamu frontier thrust fault, the Xialathrust fault, the Ria-Linbuchong tectonic belt consisted by high-angle thrust in west and ductileshear zone in east and the Bangzhong thrust slices were studied systematically by field profiling.And The Ria-Linbuchong thrust shows dextral strike-slipping evidences in many localities. TheSDT is inferred to be formed in 49-18.3Ma after isotopic dating of faulted complex and graniteemplaced after thrusting.4. Analyses on Jurassic-Cretaceous palaeo-tectonicsPaleo-tectonics in Early-Meso Jurassic, Late Jurassic-Early Cretaceous and Late Cretaceousare analyzed based on data of petrologic geochemistry of ophiolite mélange, sedimentaryformation, volcanic activity, magma emplacement, metamorphism and deformation. It is inferredthat an intra-island oceanic basin termed as the Namco-Yongzhu Ocean existed inEarly-Meso-Jurassic until Early Cretaceous in central Lhasa block. And Bangoin-Nujiang Ocean,Bange-Sangxiong Island-Arc, Namco-Yongzhu intra-island Ocean, Gangdese Island-Arc andYaluzangbu Ocean formed in Jurassic-Cretaceous in the Lhasa block, formed active continentalmargin in south of the Asia Plate. The Bangoin-Nujiang Oceanic Plate began southwardsubduction while spreading in Early-Meso Jurassic and closed in Early Cretaceous, lead toformation of the Bangoin-Nujiang suture (BNS). The Namco-Yongzhu intra-island Ocean openedand widened in Early-Meso Jurassic by spreading, subducted northward beneath theBange-Sangxiong Island-Arc in Late Jurassic-Early Cretaceous and closed at the end of LateCretaceous, lead to formation of the Namco-Yongzhu suture (NYS). The Yaluzangbu Ocean beganspreading since Triassic via Early-Meso Jurassic until Late-Jurassic-Early Cretaceous whennorthward subduction began, and closed in Paleocene-Early Eocene, lead to formation of theYaluzangbu suture (YZS).5. Analyses of thrust association and tectonic evolution of north Lhasa blockThe Dongqiao-Lunpola thrust (DLT) in north Lhasa block, west Namco thrust (WNT) incentral Lhasa block and south Damxung thrust (SDT) in south Lhasa block show same thrustingdirection, similar composition and texture, ordered spatial pattern and related geodynamic cause.Major thrust faults dip to north, pilled in cross section and merge into a detachment in deep crust(≥12-15km), consisting a large-scale thrusting system in north Lhasa block termed as north Lhasathrust (NLT) covering 250km range vertical to extension. Thermo-chronological diagram ofminerals from milonite shows 4 phases of thrusting in 191-174Ma, 174-144Ma, 109Ma and 44Maof WNT, lead to 0.4mm/a, 0.22mm/a and 0.2mm/a uplifting respectively. It is concluded thatthrusting in 174-173Ma, 109Ma and 44Ma of WNT were caused by southward subduction ofBangoin-Nujiang Oceanic Plate in Early-Meso Jurassic, northward subductions ofNamco-Yongzhu intra-island arc Oceanic Plate and Yaluzangbu Oceanic Plate in Late cretaceousand close of Yaluzangbu Ocean and its following continental collision in Early Eocenerespectively. And Eocene-Oligocene northward subduction of India continent caused southDamxung thrusting and detachment of NLT and huge shortening and thickening of crust and rapiduplift of the Tibetan Plateau before 18.3-13.5Ma.