North China craton

The North China craton is one of the oldest cratonic blocks in the world. It covers an area of ~1.7 million km² across most of northern China, the southern part of northeastern China, Inner Mongolia, the Bohai Bay and the northern part of the Yellow Sea. The craton is bounded by the early Paleozoic Qilianshan orogen and the late Paleozoic to Mesozoic Central Asian orogenic belt to the west and north, respectively, and the Mesozoic Qinling-Dabie and Su-Lu ultrahigh-pressure metamorphic belts to the south and east, respectively. The basement of the craton can be divided into Eastern and Western blocks, separated by the Paleoproterozoic Trans-North China Orogen (TNCO). The TNCO is characterized by fragments of ancient oceanic crust, mélanges, high-pressure granulites and retrograded eclogites, crustal-scale ductile shear zones and linear fold belts with sheath folds. These lithotectonic elements contrast with the dominant Archaean tonalite-trondhjemite-granodiorite (TTG) gneiss domes surrounded by minor supracrustal rocks in the Eastern and Western blocks. In addition, petrographic and thermobarometric data have revealed a remarkable difference in metamorphic evolution between the TNCO and the Eastern and Western blocks. The former underwent a major metamorphic event about ~1.85 billion years (Ga) ago with clockwise P-T paths involving isothermal decompression, suggesting a collisional environment, whereas the latter experienced a major metamorphic event at ~2.5 Ga, with anticlockwise P-T paths involving isobaric cooling related to the underplating of mantle-derived magmas. These differences led some researchers to propose that the TNCO was a continent-continent collisional belt along which the Eastern and Western Blocks amalgamated to form a coherent craton at ~1.85 Ga. The North China craton underwent a long-lived period of sedimentation with minor volcanism in the Yanshan-Taihangshan aulacogen (a fault-controlled basin) during the period 1.8-0.8 Ga, forming the Paleoproterozoic Changcheng Group, Mesoproterozoic Gaoyuzhuang Group and Jixian Group, and Neoproterozoic Qingbaikou Group. Meanwhile, the craton also experienced widespread rifting and anorogenic magmatism of the late Paleoproterozoic along the southern and northern margins, forming anorthosite-mangerite-charnockite-granite suites.^{[citation needed]}

About 300-250 million years (Ma) ago the combined North China and Tarim cratons collided with Siberia to comprise the last stage in the formation of Pangaea. At 220-240 Ma, the North China craton collided with the Yangtze block along the Qinling-Dabie-Sulu orogenic belt. The Eastern Block of the North China craton underwent crustal thinning that began in the Mesozoic and is known to have reduced the thickness of the crust from 200 km to as little as 80 km.^{[citation needed]}

The area of the North China craton (see: [1] and [2]) is also referred to as the Sino-Korean craton or block (see: [3]) in some references.

Geology of the Western Block

The basement rocks of the Western Block are mainly exposed in the northern part of the block, especially in the Jining, Daqingshan-Wulashan, Guyang-Wuchuan, Sheerteng, Helanshan-Qianlishan, and Alashan areas, whereas the southern part of the block is covered by the Mesozoic to Cenozoic strata of the Ordos Basin. Data from several boreholes reveal the existence of granulite-facies metamorphosed basement beneath the Ordos Basin, and aeromagnetic data also imply the existence of granulite-facies basement beneath the basin. The exposed basement can be further subdivided into two distinct lithotectonic units: the late Archean (3.0-2.5 billion years old) tonalitic-trondhjemitic-granodioritic (TTG) + supracrustal rocks and the Paleoproterozoic Khondalite Belt. The former crops out as granite-greenstone or high-grade metamorphic terrains in the Guyang, Wuchuan, Sheerteng and Alashan areas in the northern part of the block, whereas the latter is exposed as a linear structural belt along the Jining-Daqingshan-Wulashan-Qianlishan-Helanshan zone. One school of thought considers that the Khondalite Belt is a Paleoproterozoic collision belt, along which the Yinshan Block represented by the late Archean basement in the north, and the Ordos Block covered by the Ordos Basin in the south collided to form the Western Block about 1.93 billion years (Ga) ago.

The late Archean basement of the Western Block has a lithological assemblage, structural style and metamorphic history similar to those of the Eastern Block. It consists of low-grade granite-greenstone and high-grade TTG gneiss and granulite terrains, which underwent a greenschist to granulite facies metamorphism at ~2.5 Ga, characterized by anticlockwise P-T paths involving near-isobaric cooling. The Paleoproterozoic Khondalite Belt in the Western Block is dominated by graphite-bearing sillimanite-garnet gneiss, garnet quartzite, felsic paragneiss, calc-silicate rock and marble, which have previously been referred to as “khondalite series” in the Chinese literature and are considered to represent stable continental margin deposits. Associated with the khondalites are minor TTG gneisses, mafic granulites, syntectonic charnockites and S-type granites. It has long been considered that the khondalites were deposited and metamorphosed in the Archean. However, recent isotopic data suggest that the khondalites were deposited and metamorphosed in the Paleoproterozoic, with depositional ages ranging from 2.30 to 1.98 Ga and a metamorphic age of ~1.93 Ga.

References

• A second look at the geologic map of China: the "Sloss approach"

• Evolution of the lithosphere beneath the Sino-Korean Craton

• Zhao, G. C., Wilde, S. A., Cawood, P. A., Sun M., Archean blocks and their boundaries in the North China Craton: lithological, geochemical, structural and P-T path constraints and tectonic evolution Abstract

• Timothy M. Kuskya, Jianghai Li, Paleoproterozoic tectonic evolution of the North China Craton, 2003, Journal of Asian Earth Sciences

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