Study On The Geological Characteristics And Tectonic Of Buqingshan Melanges Belt, The South Margin Of East Kunlun Mountains

The Buqingshan tectonic melange belt (BTMB) in south margin of the East Kunlun Mountains is the suture between the East Kunlun block and the Bayan Har block, stretching nearly along E-W in the whole. In the thesis, BTMB was chosen for researching, the modern plate tectonics theory was for guiding ideology, and through detailed and systematically geological mapping, structural geology, petrology, geochemistry and zircon LA-ICP-MS U-Pb isotopic chronology methods were applied to study the structure, material composition and tectonic evolution of BTMB synthetically, which provided crucial significance to study of the Central Orogenic System and the plate tectonic pattern of the Eastern Tethyan tectonic domain, as well as reconstruction of its evolution process since the early Paleozoic. At the same time, some new advances and preliminary cognitions are proposed as follows.1. BTMB in the southern margin of the East Kunlun Mountains, stretching nearly along E-W in strike was tectonically thrusted from north to south with strike-slip moving in the whole framework, and was superimposed on the nappe structure types. The whole deformation process is composed of orogenic substrate of subduction and collision between Bayan Har block and East Kunlun massif in Caledonian, the superposition of the subduction orogenesis from south to north during late Hercynian-early Indosinian, comprehensive collisional orogenesis during late Indosinian and early Yanshan period, nappe structure in Yanshan period and extensional graben formation in Himalaya period. The structural deformation sequence can be divided into4stages:fold deformation in late Permian-middle Triassic, thrust with strike-slip deformation by the end of middle Triassic, nappe and strike-slip deformation and extensional graben deformation in late Triassic-early Jurassic.2. BTMB consists of tectonic melange and exotic limestone nappe. The former is composed of matrix, including sediment assemblage of turbidites and molasse rock, and melange blocks, which can be divided into metamorphic basement rocks, ocean-crust type mixed rock and island-arc type hybrid block. Ocean-crust type mixed rock is subdivided into Early Paleozoic ophiolite, Late Paleozoic ophiolite and seamounts rock assemblage, and island-arc type hybrid block can also be snbdivided into late Caledonian granodiorite and intermediate-acid volcano rock.3. The proofs of formation, expansion, subduction of BTMB in proto-Tethyan period are confirmed.Delistan ophiolite is composed of serpentinite, serpentinized lherzolite, gabbro, diabase, meta-basalt, siliceous argillite, and is tectonically dismembered for many times but with clear sequence and integrated assemblage. The zircon U-Pb age of gabbro from the ophiolite is516.4±6.3Ma, and the basalt from which shows N-MORB geochemical characteristics, indicating the MOR-type crust of Delistan ophiolite, which is the product of spreading of Proto-Tethys in the Early Cambrian.The zircon U-Pb ages of Yikehalaer granodiorite block and first-discovered Bairiqiete granodiorite block and intermediate-acidic volcanic rock block are437.5±6.6Ma,441.1±6.3Ma and437.7±2.8Ma respectively. The stripped granodiorite block distributed in the north of the early Paleozoic ophiolite, with gneissic structure, the petrological and geochemical characteristics of which rocks display island-arc type calc-alkaline series, similar to adakite. The intermediate-acidic volcanic rocks are composed of rhyolite porphyry, dacite porphyry and some andesite, geochemically indicating of island-arc type calc-alkaline volcano rock series. These island-arc type igneous rocks represent the volcano-arc products oceanic crust subduction of proto-Tethyan in the Early Silurian.4. The rock assemblage of BTMB from the late Paleozoic oceanic crust includes Haerguole ophiolite and Buqingshan seamount basalt/limestone. Haerguole ophiolite, consisting of serpentinite, strongly serpentinized peridotite, gabbro, diabase, basalt and silicon argillaceous rocks is N-MORB type, which is geochemically proved by its basalts, and is formed in oceanic spreading during the early Carboniferous, as the zircon U-Pb age of the gabbro from the rocks is332.8±3.1Ma. Buqingshan seamount is composed of basalt basement and reef limestone overlays, which basalt is N-MORB type in geochemistry, and which limestone overlays is formed in the early-middle Permian identified by the fossil genus. Thereby, Buqingshan Seamount rock and Harguole ophiolite are the products of the same oceanic basin in Late Paleozoic.5. BTMB from the early Paleozoic ophiolite and island-arc intermediate-acid magmatic rocks and the late Paleozoic ophiolite and seamounts rock assemblages represent the products of ocean formation and evolution of the Proto-Tethys Ocean and the Paleo-Tethys Ocean respectively. Considering the structure, material composition and regional geological data of BTMB, the tectonic framework and regional tectonic evolution of BTMB since the early Paleozoic are established:Proto-Tethys Ocean evolution and the main orogenic phase,1) initial disintegration stage of the unified continent formed in Proterozoic at the end of Sinian,2) the oceanic crust formed stage in early Cambrian,3) the oceanic crust expansion stage during Cambrian-Ordovician,4) subduction of the oceanic crust stage from early Silurian to middle Silurian,5) the oceanic crust withering away and collide-to-orogenesis stage from late Silurian to early Devonian. Paleo-Tethys Ocean evolution and composite orogenesis stage from Late Paleozoic to Early Mesozoic,1) Continental stable development stage in middle and late Devonian2)oceanic crust formation stage in early Carboniferous,3) oceanic spreading stage from late Carboniferous to middle Permian,4) the oceanic crust subduction stage from late Permian to middle Triassic,5) demise of the oceanic basin and collide-to-orogenesis stage at the end of Triassic. Intra-continent orogenesis and superimposed redeformation stage form Mesozoic,1) the Shuweimenke formation limestone nappe to BTMB, in late Triassic to early Jurassic,2) extrusion deformation in the early Jurassic and Cretaceous,3) uplift of the Tibetan Plateau remote effect, make East Kunlun South Fracture zone occurred indentations formed in rift basin in the Cenozoic.