Geochemistry And Geochronology Of Ophiolite And Related Volcanic Rocks In Sanchazi-hengxianhe Area, Mianlue Tectonic Zone And Regionial Tectonic Evolution

Mianlue tectonic zone is a main plate junction of the final converging between North andSouth China continental blocks. In recent years, many new views on the tectonic propertiesand evolution of Mianlue belt appear with a group of Proterozoic ages reported. Thefollowing progress has been made in this thesis on the ophiolite and related volcanic rocksthrough system research such as field geology, petrology, rock geochemistry, isotopicchronology and Quartz crystal lattice preferred orientation (LOP) via electron backscatterdiffraction (EBSD) in Sanchazi-Hengxianhe area，Mianlue tectonic zone.1. The zircon age of807±13Ma (MSWD=3.6, n=17) from the arc andesite in Heigouxiaindicates the subduction of the Proterozoic ocean in Mianlue area lasted until about800Ma.Sanchazi supra-subduction zone type ophiolite was the product of the oceanic subduction. Thegeochemical differences in the tholeiitic basic volcanic rocks from Zhuangke and Heigouxiawere the results of inter-arc oceanic evolution at different stages, as some samples stillretained the fingerprints of arc and others had been typical mid-ocean ridge basalt. The lowerNb/U, Nb/Ta, Nb/La and Ce/Pb ratios from the studied samples than typical mid-ocean ridgebasalt suggest the mantle source region had been transformed by continental crust material.Combined with previous research and some other acquired results by our team, this thesisdeems the inter-arc ocean represented by the Heigouxia and Zhuangke basalts was the productof the cracking of the arc that existed from840Ma to800Ma. Zhangyangou bimodal volcanicrocks were the product of cracking of continental crust, and the basic rocks were similar tooceanic island basalts. Intermediate-acid rocks were produced by the partial melting ofcontinental crust material. The zircon age of728±10Ma (MSWD=0.14, n=4) from the rhyoliteindicates the existing of regional cracking in Minalue area.2. Through detailed field observation and indoor study, the deformation sequence ofMianlue belt is divided. The first stage was mainly folding and solid bedding rheologicaldeformations at deep-seated tectonic level with the closing of trough rift in the Early－MiddleTriassic. The deformations such as residual homoclinal tight fold, rootless fold, penetrativestructural surface and structural transposition in penetrative schistosity could be found in thestrongly deformed domain, and the deformations such as homoclinal tight fold and rootlessfold can be found in the feeble deformed domain with the deformation of S0or syntectoniclens. The second atage was mostly sinistral strike slip ductile shear deformations at middletectonic level which belonged to the stage of post-collision in the Middle－Late Triassic, andthe sinistral strike slip ductile shear deformations included asymmetric shear len, asymmetric fold, asymmetric pudding structure, S-C fabric, subhorizontal stretching lineation, nearly erecthub type shear fold and asymmetrical rotational structure. The third stage was mainly thrustdeformations at shallow tectonic level which belonged to the stage of intracontinentalorogenesis after the Late Triassic, and the thrust deformations mainly included asymmetricfold structure, crenulation cleavage and B-type lineation with the deformation of penetrativefoliation (S1). The fourth stage was mostly late brittle deformations.3. On the basis of the studies in this thesis and the latest research results, the regionaltectonic evolution is discussed. Large-scale oceanic subduction in Mianlue area lasted untilabout800Ma, then extensive cracking followed. Nanhua System in Mianlue belt was mainlyneritic facies turbidity current deposition and transitory ice raft deposition. Sinian System wasmainly stable shallow water platform facies sedimentation, and the regional crackingtemporarily came to an end. Lower Paleozoic was mostly sedimentation of chasmic trough.Upper Paleozoic was mainly sedimentation on passive continental margin, and Qinlingmicroplate collided with Yangtze plate in early-middle Triassic.