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Baogui Niu, Institute of Geology, Chinese Academy of Geological Science (China)
Zhengjun He, Institute of Geology, Chinese Academy of Geological Science (China)
Jun Wang, Institute of Geology, Chinese Academy of Geological Science (China)
Liwei Xiao, Institute of Geology, Chinese Academy of Geological Science (China)
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The Qinling orogen is a polycyclic tectonic juncture between the Sino-Korean and Yangtze massifs.
Historically, the evolution of the Qinlings could be divided into two stages, i.e., the pre-Nanhuaan stage and the Phanerozoic stage. Within the area, there are widely distributed the Early Precambrian hypometamorphic complexes and pre-Nanhuaan volcaniclastic sedimentary rocks, with the S-type syncollisional granites of 980 Ma ∼ 870 Ma developed. The late Middle Proterozoic ophiolites have been discovered in the North Qinling Mts. and on the northern margin of the Sujiao belt. All this suggests the Qinling orogen was an early Late Proterozoic collisional belt before the Phanerozoic.
During the Phanerozoic, with the Tancheng-Lujiang transform fault taken as a boundary, the Qinlings could be separated from east to west into the Qinling-Dabie and Sujiao-Imjingang belts, and from north to south into the North, Middle and South Qinling belts.
1) North Qinling located north of the Shangdan fault is mainly composed of the Nanhuaan-Lower Paleozoic volcano-sedimentary rocks, with the Ordovician ophiolites and S-type syncollisional granite of 450 Ma occurring. This is a collisional belt formed in the Caledonian cycle.
2) South Qinling is characterized by the Indosinian cycle as its principal one. The strata distributed west of the Nanyang basin are best preserved. They are divided into two sequences by the Caledonian unconformity: the lower sequence consists of passive marginal volcano-sedimentary rocks of the Nanhuaan system and Early Paleozoic, while the upper consists of littoral carbonatites-terrigenous clastics of the Devonian-Early-Middle Triassic. The strata east of the Nanyang basin have experienced strong denudation and the basement rocks of the orogen are widely exposed. This belt is characterized by occurrence not only of high pressure-superhigh pressure metamorphics of about 230 Ma, but also of 487 Ma °C 424 Ma superhigh pressure metamorphics, indicating importance of the Caledonian cycle here.
3) Middle Qinling situated between the South and North Qinlings is composed of post-Caledonian climbing littoral aulacogen sedimentary rocks of the Middle-Late Devonian and Early Carboniferous. It was folded and uplifted by the late Early Carboniferous Variscan orogeny.
To sum up, the Qinling orogen is not a simple Indosinian collisional belt, but a polycyclic composite orogen. Its evolution went through two orogenic processes of the Middle Proterozoic and Early Paleozoic Wilson cycles. In the Qinling area there was no oceanic basin after the Caledonian orogeny. Therefore, for the Qinlings, the Variscan-Yanshanian cycle orogeneses should be considered to be crustal superimposed orogenies.
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