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2020 Geological Society of China The Qilian orogenic belt is the northernmost orogen of the Tethyan domain and connects the Altaids to the north. It contains an assembly of Precambrian micro-continental fragments, early Paleozoic island arcs, accretionary complexes, ophiolites, forearc and backarc basins, and high-pressure (HP) metamorphic rocks, indicating a long history of accretionary processes. Spatially, this orogen is adjacent to the Tarim, Qaidam, and North China blocks, which also extends into accretionary orogenic belts to the east and SW such as the Qinling and Kunlun belts. Abundant ophiolites in this orogen record the closure of an early Tethyan Ocean and amalgamations between micro-continents of North China, Qaidam, and Tarim. Thus, the ages and tectonic settings of these ophiolites within this belt provide important information regarding evolution of the Proto-Tethys Ocean and assembly of micro-continental blocks, which aids understanding of the spatial and temporal relationship of this orogen within the Tethyan realm. Dismembered ophiolites sporadically crop out along the northern margin of the South Qilian belt, and, from east to west, are locally referred to as the Lajishan, Gangcha, Muli, and Dadaoerji ophiolites. Much attention had been paid to these ophiolites, and several competing models for the tectonic evolution of this belt have been suggested. Considerable disagreement remains in respect of the temporal and spatial framework of the Qilian Orogen and details such as timing of subduction (s) and associated polarities, early collision events, and final closure of oceanic basins. In particular, the formation age and tectonic setting of Muli arc-ophiolite complex remains unknown, which limits understanding of the tectonics of the South Qilian belt and the history of the Proto-Tethys Ocean. The Muli arc-ophiolite complex is distributed over 20 km2 west of the township of Muli in the western segment of the South Qilian Belt and consists of serpent

Lower to Middle Triassic clastic rocks in the West Qinling Mountains along NE margin of the Qinghai-Tibet Plateau are generally regarded as part of the Songpan-Ganzi flysch Basin. However relatively little attention has been paid to the age and provenance of these units. New petrological and geochemical results demonstrate that these sediments accumulated along an active continental margin and are dominated by feldspathic litharenite and lithic arkose with low mineral and compositional maturity. They were derived primarily from a continental arc source dominated by intermediate to felsic igneous rocks, with a minor contribution from older metamorphosed and sedimentary sources. Turbidite samples yielded two primary detrital zircon U-Pb age populations of ca. 273 Ma and ca. 435 Ma, which is different from the Early Triassic (ca. 252 Ma) and Middle Silurian (ca. 427 Ma) age populations that dominate the Songpan-Ganzi Basin. These data together with paleocurrent results indicate that the South Qilian Belt was the primary source origin because this belt contains both early Paleozoic Andean-type igneous and magmatic rocks and has a basement of Precambrian metamorphosed rocks. Regionally, an Andean-type arc traverses the South Qilian belts and extends into the West Qinling Mountains and Kunlun Orogen, which formed by north-directed subduction of the Paleo-Tethyan Ocean during the Early to Middle Triassic. Voluminous detritus that originated from the Qilian and Kunlun orogens as they were uplifted and eroded was transported to the south and deposited in the forearc area in front of the A'nimaqen-Mianlue suture.

2019. American Geophysical Union. All Rights Reserved. The compositions and ages of the sediments within retro or foreland basins that are formed and preserved adjacent to collisional orogens can reflect the nature of colliding tectonic elements. The nonmarine Yaoshuiquan and Huabaoshan formations in the South Qilian belt on the NE Tibetan Plateau deposited within a retro-foreland basin setting during latest Ordovician to Late Silurian time in response to arc-continent collision. Detritus derivation from a Cambro-Ordovician arc-ophiolite complex contains mixed 530-480 Ma oceanic-crust together with contributions from a 479-450 Ma continental-arc early in development of the basin. The Cambrian arc-accretionary system and Central Qilian block united to form the basement of a continental arc at ~450 Ma, and both then contributed sediments to the Lianhuashan-Huabaoshan basin. After the Hualong complex accreted to the north, a broad Andean-type margin developed along the southern margin of the Central Qilian block from 450 to 440 Ma. These processes generated a wider basin that received detritus from both the south and the north. Consumption of the Proto-Tethyan Ocean ended with collision between the Qaidam and Hualong blocks, which led to mass wasting of detritus from the Andean-type igneous rocks and both blocks with the basin from 440 to 420 Ma.

The Qilian Orogen in the northern margin of the Tibetan Plateau is the northernmost of the Tethyan domain. Abundant ophiolites record the closure of an early Tethyan ocean and amalgamations between micro-continents of North China, Qaidam, and Tarim. The Muli arc-ophiolite complex in the western segment of the South Qilian belt represents remnants of the Proto-Tethyan oceanic lithosphere. It comprises serpentinite, dunite, cumulate gabbro, basalt, plagiogranite, and chert, which are in tectonic contact with Upper Ordovician turbidites. Basalts have typical subduction-related calc-alkaline geochemical affinity, representing portions of an island arc. Geochemical results for plagiogranites and spinels from serpentinite indicate that the Muli arc-ophiolite complex represents a super-subduction zone (SSZ)-type ophiolite. U-Pb zircon data indicate formation associated with southward subduction of the Proto-Tethys Ocean during a short interval between 539 and 522 Ma. Results of petrology, geochemistry, and zircon U-Pb dating demonstrate that granitoids intruded into this complex are Middle to Late Ordovician (470-450 Ma) products of subduction-related arc magmatism. Voluminous Late Ordovician-Early Silurian rocks include deep-water marine siliciclastic and volcaniclastic turbidites and abundant volcanic arc rocks located to the south of the Muli arc-ophiolite complex, whereas fluvial coarse-grained sandstones and conglomerates unconformably overlie the Cambrian-Middle Ordovician ophiolite-arc systems in the eastern South Qilian belt. This indicates that closure of the Proto-Tethys Ocean was diachronous during the early Paleozoic.

The Lajishan ophiolite complex in the Qilian Orogen is one of several ophiolites situated between the Qaidam and North China blocks that record episodic closure of the Proto-Tethyan Ocean. Detailed field relations and geochemical and geochronological studies are critical to unraveling the tectonic processes responsible for an extensive period of intraoceanic subduction that produced juvenile ophiolite/island arc terranes, which were obducted onto continental margins during ocean closure. The Lajishankou ophiolite complex crops out along the northern margin of the South Qilian belt and was thrust over a Neoproterozoic-Ordovician passive margin sequence that was deposited upon the Proterozoic Central Qilian block. The mafic rocks in Lajishankou ophiolite complex are the most abundant slices and can be categorized into three distinct groups based on petrological, geochemical, and geochronological characteristics: massive island arc tholeiites, 509-Ma back-arc dolerite dykes, and 491-Ma pillow basaltic and dolerite slices that are of seamount origin in a back-arc basin. These results, together with spatial relationships, indicate that the Cambrian island arc rocks, ophiolite complex, and accretionary complex developed between 530 and 480 Ma as a single, intraoceanic arc-basin system as a result of south directed subduction of the Proto-Tethyan Ocean prior to Early Ordovician obduction of this system onto the Central Qilian block. Final continental amalgamation involved continental collision of the Central Qilian block with the Qaidam block during the Late Ordovician. This model solves the long-lasting discussion on the emplacement of the Lajishan ophiolite and contributes to an improved understanding of multiple accretionary and collisional processes in the Qilian Orogen.