The Western Block of the North China Craton is an ancient micro-continental block mainly composed of Neoarchean and Paleoproterozoic rock basement, with some parts overlain by Cambrian to Cenozoic volcanic and sedimentary rocks. It is one of two sub-blocks (the Eastern Block and the Western Block) within the North China Craton, located in east-central China. The boundaries of the Western Block are slightly different among distinct models, but the shapes and areas are similar. There is a broad consensus that the Western Block covers a large part of the east-central China.
Igneous, sedimentary and metamorphic rocks are present in the Western Block. The oldest geological records in the Western Block are 2.7 billion-year-old intrusive igneous rock found in Xiwulanbulang in Inner Mongolia.Dong, X.J.; Xu, Z.Y.; Liu, Z.H.; Sha, Q. (2012). Discovery of 2.7 Ga granitic gneiss in the northern Daqingshan area, Inner Mongolia and its geological significance. Earth Sci. J. China Univ. Geosci. 37: 20–27. The youngest rocks are 23-million-year-old extrusive igneous rocks, which are located in Sanyitang (in Hebei) in the orogenic belt across the block. The sedimentary rocks distribute predominantly in the Ordos Basin located in the south of the Western Block. Exposures of metamorphic rocks are mostly scattered in the north of the block.
The tectonic setting and evolution of the Western Block are debated. There are various models hypothesizing the subdivisions and tectonic history of the block and they are generally mutually exclusive. However, most models agree with the presence of a Paleoproterozoic orogenic belt cutting across the Western Block in the east-west direction, despite having various names for it.
Since the geological events of the Western Block started in Precambrian time, when more than 80% of the present volume of continental crust formed, complex geological evolution and early tectonic history can be studied through the geological records in the Western Block.
Lithologies Precambrian basement (4600–539 million years ago)
Archean rocks (4.0–2.5 billion years ago) The earliest geological units found in the Western Block are formed in Neoarchean , when the major crustal accretion and reworking took place. The Neoarchean rocks in the Precambrian basement are mainly composed of greenstones, high-grade metamorphic rocks and granitoids. Mineral examinations show an isobaric cooling-type anticlockwise pressure-temperature (P-T) path, indicating there were intrusions and underplating events during the crustal growth in Neoarchean.Jin, W., Li, S.X., Liu, X.S. (1991). "The Metamorphic dynamics of Early Precambrian high-grade metamorphic rocks series in Daqing-Ulashan area, Inner Mongolia." Acta Petrol. Sin. 7: 27-35.
Guyang granite–greenstone terrane
The Guyang granite-greenstone terrane is located in the northern part of the Western Block, spreading from Mount Serteng to Donghongsheng in the east-west direction. The terrane is dominated by metamorphic and granitoid rocks. It is suggested that the terrane may represent ancient metamorphism in the upper crust.Li, S.X., Sun, D.Y., Yu, H.F., Jin, W., Liu, X.S., Cao, L., 1995. Distribution of Ductile Shear Zones and Metallogenic Prediction of the Related Gold Deposits in the Early Precambrian Metamorphic Rocks, Middle-Western Inner Mongolia. Jilin Science and Technology Press, Changchun, pp. 1-111.
Greenstones are sequences of Precambrian metamorphosed ultramafic to mafic rocks and sedimentary rocks. They are well exposed in Mount Serteng in Guyang granite-greenstone terrane. There are three sub-units in the greenstone sequence in this terrane. The lower layer is dominated by metamorphosed mafic and ultramafic volcanic rocks, with interlayered banded iron formation.Chen, L. (2007). Geochronology and geochemistry of the Guyang Greenstone Belt. Post-Doctorate Report. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing. pp. 1-40 (in Chinese with English abstract). The middle rock assemblage consists of a series of metamorphosed volcanic rocks with compositions varying from acidic to mafic. The top layer is made by meta-sedimentary rocks, for example quartzite and marble. According to the data of zircon dating, the bottom layer of the greenstone sequence formed in about 2.54 billion years ago, while the middle and upper layers have the age later than 2.51 billion years.
Granitoids are intrusive igneous rocks that primarily made by quartz, feldspars and micas. The granitoid rocks found in Guyang granite-greenstone terrane are mostly tonalite-trondhjemite-granodiorite (TTG) and sanukitoid. There were two phases forming TTGs, with the first stage at about 2.53 billion years agoRen, Y.W. (2010). The Study of Granite-Greenstone Belt in Xihongshan Area, Inner Mongolia. Doctor’s thesis: Jinlin University, pp. 1–69 (in Chinese with English abstract). and the second one at between 2.52-2.48 billion years ago. The sanukitoid in this terrane formed between the two phases of TTG formation, at about 2.53-2.52 billion years ago.
Wuchuan high-grade complex Wuchuan high-grade complex extends from Zhulagou in the west to Xiwulanbulang in the east. The complex includes granitoids, granulites and charnockite. The granitoid rocks are predominantly medium (550-650 °C)- to high-grade (650-900 °C) metamorphosed diorite and the granulites are high-grade metamorphosed TTG rocks. Similar to Guyang granite-greenstone terrane, the rocks in Wuchuan have the age of 2.55-2.50 billion years. The high-grade complex is interpreted to be the metamorphosed lower crust in 2.55-2.50 billion years ago.
Proterozoic rocks (2500–539 million years ago) Many researchers proposed that the Western Block of the North China Craton was assembled in the Paleoproterozoic (2.5-1.6 billion years ago), creating a linear structure composed of khondalite that cuts across the Western Block. The khondalite belt stretches from Helanshan in the west to Jining Complex in the east. Pelitic granulites, quartzite, felsic paragneiss and marble, which are the members of the "khondalite series", are exposed along this belt. The khondalite series is metamorphosed from sedimentary protoliths formed under stable continental margin condition. By zircon dating, the sedimentary protoliths were deposited at 2.3-2.0 billion years ago and then experienced metamorphism at 1.95-1.87 billion years ago. The minerals in the khondalite exhibit isothermal decompression-type clockwise P-T paths, implying the sedimentary protoliths were metamorphosed in a collisional setting.
Phanerozoic stratigraphy (539 million year ago–present) The Western Block became much more stable after Precambrian. Sedimentary rocks were deposited, covering part of the Precambrian basement. There were also magmatism in Phanerozoic.During Cambrian to Middle Ordovician, extensive carbonates were formed in the Western Block.Chen, S.Y., Liu, H.J., 1997. Carboniferous-Permian lithofacies and paleogeography in the eastern part of the North China Platform. Regional Geology of China 16, 379–386 (in Chinese with English abstract). Deposits were generally absent from Late Ordovician to Early Carboniferous. However, carbonates with some coal-bearing rocks started to deposit again between Late Carboniferous and Early Permian. In Late Permian, conglomerates as well as red iron-bearing sandstone, siltstone and mudstone (red beds) were formed.Meng, X.H., Ge, M., 2002. Research on cyclic sequence, events and formational evolution of the Sino-Korean Plate. Earth Science Frontiers 9, 125–140 (in Chinese with English abstract). In Triassic and Jurassic, the stratigraphy was dominated by sandstone and mudstone.Peng, Z.M., Wu, Z.P., 2006. Development features of Triassic strata and analysis of original sedimentary pattern in North China. Geological Journal of China Universities 12, 343–352 (in Chinese with English abstract).Wu, Z.P., Hou, X.B., Li, W., 2007. Discussion on Mesozoic basin patterns and evolution in the eastern North China Block. Geotectonica et Metallogenia 31, 385–399 (in Chinese with English abstract). Sandstone deposition, together with magmatism happened in the Early Cretaceous, forming extrusive igneous rocks in the Western Block, for example rhyolite, andesite, basalt and dacite. From Late Cretaceous onwards, igneous rocks became minor. Late Cretaceous to Cenozoic sediments with Cenozoic basalt overlay the pre-existing strata. + Summary table of lithologiesGeological periodTime of rock formationLithologiesLocationNeoarchean2.7 GaTonalite-trondhjemite-granodioriteXiwulanbulang2.55–2.50 GaGreenschist, amphibolite, banded iron formation, hornblende-plagioclase gneiss, paragneiss, mica schist, quartzite, marble, tonalite-trondhjemite-granodiorite,
quartz diorite, adakite, sanukitoids, granitoids, granulites and charnockiteGuyang granite–greenstone terrane
and Wuchuan high-grade complexPaleoproterozoic1.95–1.87 GaPelitic granulites, quartzite, felsic paragneiss and marbleKhondalite BeltCambrian539–488 MaCarbonate rocksMainly in Ordos BasinOrdovician488–460 Ma460–443 MaGenerally absent/Silurian443–416 MaDevonian416–359 MaCarboniferous359–318 Ma318–299 MaCarbonate rocks and coal-bearing rocksMainly in Ordos BasinPermian299–270 Ma270–251 MaRed beds and conglomeratesTriassic251–228 MaSand-bearing mudstone, medium- to fine-grained sandstone with grey mudstone layers228–199 MaFine-grained sandstone and mudstone with coal layersJurassic199–145 MaMedium- to fine-grained sandstones, siltstones, conglomerates, argillites and coalCretaceous145–65 MaBasalt, andesite, dacite, rhyolite and fossilized sedimentary rocksCenozoic65–presentSediments and basalts
Tectonic subdivision The tectonic subdivision of the Western Block is still under intense discussion among geologists. There are several models illustrating the tectonic subdivision of the Western Block and these models assign different names to the components and structures of the Western Blocks. The shapes and areas of Western Block in these models are similar but they may not completely concordant with each other.
Zhao and his colleagues proposed that the Western Block of the North China Craton can be subdivided into two sub-blocks: Yinshan Block and Ordos Block. In between the sub-blocks is a Paleoproterozoic continent–continent collisional belt defined as Khondalite Belt. The Khondalite Belt extends across the whole block in east-northeast (ENE) and west-southwest (WSW) direction. Kusky and his co-workers divided the Western Block into three elements, including the Inner Mongolia-Northern Hebei Orogen, Hengshan Plateau and a micro-continental block. The southern boundary of the Hengshan Plateau is marked by the normal faults, which have a general trend of east-northeast (ENE) and west-southwest (WSW) direction. There is a northeast-southwest trending fault called Datong-Wuqi Fault, cutting across the Western Block. Similar to Zhao et al.'s model, Santosh split the Western Block into Yinshan Block and Ordos Block in the north and south respectively. However, instead of Khondalite Belt, he named the collision region of the two sub-blocks Inner Mongolia Suture Zone. Discontinuous Khondalite Belt exposures lie inside south of the Inner Mongolia Suture Zone.
Tectonic evolution Precambrian history There are various evolutionary models of the Western Block proposed by different geologists. Three most popular models explaining the tectonic evolution of the Precambrian basement are discussed below.
Zhao et al's model