Deformation And Microcontinents' Evolution In The West Segement Of The Northern Proto-Tethys Tectonic Domain

The northwest boundary of the Proto-Tethys Ocean, located in the northern Tibetan Plateau of NW China, is the northern margin of the Qilian Orogenic Belt (west segment of theCentral Orogenic Belt of China). It is an important Caledonian suture zone that records the detailed subduction-collision history generated by convergence between the Alax and Qilian-Qaidam blocks in the early Paleozoic. The Proto-Tethys Ocean around the Qilian-Qaidam Block is called the Paleo-Qilian Ocean in this paper. The Paleo-Qilian Ocean contains many microcontinents and blocks, such as the Qilian Microcontinent, the Alax Microcontinent, the Qaidam Microcontinent, the Yangtze Block and the North China Block, which were developed in a complex multi-islands ocean environment. The Qilian area experienced a complex subduction-collision process in the early Paleozoic.This paper discusses the tectonic attribute of the units in the Qilian Orogenic Belt. It is suggested that the southern Alax is a passive continental margin, while the northern Central Qilian is an active continental marginin the early Paleozoic. The Longshoushan tectonic unit, resulted from the subduction-collision process, is an accretionary wedge parallel with the southern Alax. The North Qilian tectonic unit is a subduction accretionary complex composed mostly of high-pressure metamorphic belt, ophiolitic, arc magmatite and some oceanic crusts. It records the evolution of the Paleo-Qilian Ocean from continental rifting, oceanic crust formatting, subduction and collision.Based on deformation analysis of geological outcrops and sections crossing regional boundary faults, we suggest the southward subduction polarity of the Paleo-Qilian Ocean in the early Paleozoic. The Paleo-Qilian Ocean may began to form due to the splitting of Rodinia before 0.8 Ga. The subduction may have begun around 540 Ma based on the deformation in the active continental margin of the northern Central Qilian. The subduction formed a series of MOR type ophiolite belt, such as the Yushigou, the Aoyougou, the Dongcaohe and others ophiolite, accompanied by Kekeli and Yemazui intermediate acid magmatic activities. The subduction zone of the Paleo-Qilian Ocean retreat northward due to the northward subduction of the Qaidam at about 495 Ma, forming the Dachadaban arc. There was a subduction event occurring in the fore-arc of the Dachadaban, forming SSZ type ophiolite belt and a second magmatic event.The subduction ended at ca.460 Ma due to the collision between the Alax and the Central Qilian, and the Paleo-Qilian Ocean began to close. The collision process did not occur at the same time. The "scissors-like" splitting mode from west to east is more reasonable, accompanied by "S" type collision magmatite with the age younging from west to east. The closure of the Paleo-Qilian Ocean terminated at ca. 440 Ma. Then it began the intracontinental orogenic stage. During the subduction process, the HP eclogites and HP-LT blueschists formed. The first episode of deformation included the regional penetrative schistosity, gneissosity and mylonitic foliation etc., which are resulted from the strong shearing from south to north in the subduction-accretion complex.In a tensile extensional environment after the collision, the magmatite formed in the Jinfosi, Niuxinshan, Laohushan and other place after 440 Ma. The retrograde metamorphism of HP eclogites and HP-LT blueschists occured as a result of the exhumation of the subducted crustal materials during 422-406 Ma, which led to the second episodes of deformation characterized by tight asymmetric fold.According to the analysis and comparison of the detrital zircon age spectra of different blocks and microcontinents, it could be concluded that the basement of the Central Qilian is different form the North China Block. Before the convergence of the Rodinia supercontinent, the Central Qilian, as an independent microcontinent, is far away from the North China Block and the Yangtze Block. After the formation of Rodinia during 1.0-0.8 Ga, the Central Qilian and the Yangtze Block aggregated together, still far away from the North China Block. After the breakup of Rodinia, the Central Qilian drifted away from the Yangtze Block and gradually approached the North China Block. The Proto-Tethys Ocean closed during 0.5-0.4 Ga, the Central Qilian, the Alax and the North China Block aggregated ultimately together.