North China Craton

to the north, the Qilianshan Orogen to the west, Qinling Dabie Orogen to the south and Su-Lu Orogen to the east. The Eastern Block covers areas including southern Anshan-Benxi, eastern Hebei, southern Jilin, northern Liaoning, Miyun-Chengdu and western Shandong. Tectonic activities, such as earthquakes, increased since craton root destruction started in the Phanerozoic. The Eastern Block is defined by high heat flow, thin lithosphere and a lot of earthquakes. == Geology ==

The rocks in the North China craton consist of Precambrian (4.6 billion years ago to 539 million years ago) basement rocks, with the oldest zircon dated 4.1 billion years ago and the oldest rock dated 3.8 billion years ago. The Phanerozoic rocks are largely not metamorphosed. The Eastern Block is made up of early to late Archean (3.8–3.0 billion years ago) tonalite-trondhjemite-granodiorite gneisses, granitic gneisses, some ultramafic to felsic volcanic rocks and metasediments with some granitoids which formed in some tectonic events 2.5 billion years ago. These are overlain by Paleoproterozoic rocks which were formed in rift basins. The Western Block consists of an Archean (2.6–2.5 billion years ago) basement which comprises tonalite-trondhjemite-granodiorite, mafic igneous rock, and metamorphosed sedimentary rocks. The Archean basement is overlain unconformably by Paleoproterozoic khondalite belts, which consist of different types of metamorphic rocks, such as graphite-bearing sillimanite garnet gneiss. Sediments were widely deposited in the Phanerozoic with various properties, for example, carbonate and coal bearing rocks were formed in the late Carboniferous to early Permian (307–270 million years ago), when purple sand-bearing mudstones were formed in a shallow lake environment in the Early to Middle Triassic. Apart from sedimentation, there were six major stages of magmatism after the Phanerozoic decratonization. In Jurassic to Cretaceous (100–65 million years ago) sedimentary rocks were often mixed with volcanic rocks due to volcanic activities. == Tectonic evolution ==

The North China Craton experienced complex tectonic events throughout the Earth's history. The most important deformation events are how the micro continental blocks collided and almagamated to form the craton, and different phases of metamorphism during Precambrian time from around 3 to 1.6 billion years ago. and Santosh, 2010. Faure and Trap proposed another model based on the dating and structural evidences they found. They used Ar-Ar and U-Pb dating methods and structural evidences including cleavages, lineation and dip and strike data to analyse the Precambrian history of the craton For the time frame of the deformational events, he generally agreed with Zhao's model based on metamorphic data. A large-scale lithosphere thinning event took place especially in the Eastern Block of the craton, resulting in large-scale deformations and earthquakes in the region. Jurassic subduction of the Paleo-Pacific Plate (200–100 million years ago) and Cretaceous collapse of orogens (130–120 million years ago). As for the destabilisation mechanism, 4 models could be generalised. They are the subduction model, and the lithospheric folding model. • Scientists studied the chemical composition of the rocks to determine their origin and process of formation, • Magmatism was prevalent, and the isotopic studies showed that the mantle composition changed from enriched to depleted, which proved that new materials were replacing the mantle root. xenolith zircon studies, and analysis of the metamorphic rocks. Causes of craton destruction The causes of the craton destruction event and the thinning of the Eastern Block lithosphere are complicated. Four models can be generalized from the different mechanisms proposed by scientists. • Subduction Model • This model explained subduction as the main cause of the craton destruction. It is a very popular model. • Subduction of oceanic plate also causes subduction of water inside the lithosphere. As the fluid encounters high temperature and pressure when being subducted, the fluid is released, weakening the crust and mantle due to the lowered melting point of rocks. • Subduction also causes the thickening of crust on the over-riding plate. Once the over-thickened crust collapses, the lithosphere would be thinned. • Subduction causes the formation of eclogite because rocks are under high temperature and pressure, for example, the subducted plate becomes deeply buried. It would therefore cause slab break-off and slab rollback, thinning the lithosphere. • Subduction was widely occurring in the Phanerozoic, including subduction and closure of Paleo-Asian Ocean in Carboniferous to Middle Jurassic, subduction of the Yang Tze Craton under the North China Craton in Late Triassic, and subduction of Paleo-Pacific Plate in the Jurassic and the Cretaceous as mentioned in the previous part. The subduction model can therefore be used to explain the proposed craton destruction event in different periods. • Extension Model • There are 2 types of lithospheric extension, retreating subduction and collapse of orogens. Both of them have been used to explain lithospheric thinning occurred in the North China Craton. • Retreating subduction system means that the subducting plate moves backward faster than the over-riding plate moves forward. The over-riding plate spreads to fill the gap. With the same volume of lithosphere but being spread to a larger area, the over-riding plate is thinned. This could be applied to different subduction events in Phanerozoic. For example, Zhu proposes that the subduction of Paleo-Pacific Ocean was a retreating subduction system, that caused the lithospheric thinning in the Cretaceous. • Collapse of orogen introduces a series of normal faults (e.g. bookshelf faulting) and thinned the lithosphere. Collapse of orogens is very common in the Cretaceous. • Magma Underplating Model • This models suggests that the young hot magma is very close to the crust. The heat then melts and thins the lithosphere, causing upwelling of young asthenosphere. • Magmatism was prevalent throughout the Phanerozoic due to the extensive deformation events. l This model can therefore be used to explain lithospheric thinning in different periods of time.s were developed in the lower crust. They are later fragmented and sank because of convection of asthenosphere. Edited from Zhang, 2011. • Asthosphere Folding Model • This model is specifically proposed for how the Yang Tze Craton and the North China Craton collided and thinned the lithosphere. • The collision of the 2 cratons first thickened the crust by folding. Eclogite formed in the lower crust, which made the lower crust denser. New shear zones also developed in the lower crust. • The asthenosphere convected and seeped into weak points developed in the lower crust shear zones. The heavy lower crust was then fragmented and sunk into the lithosphere. The lithosphere of the North China Craton was then thinned. == Biostratigraphy ==

The North China Craton is very important in terms of understanding biostratigraphy and evolution. In Cambrian and Ordovician time, the units of limestone and carbonate kept a good record of biostratigraphy and therefore they are important for studying evolution and mass extinction. Biomeres are small extinction events defined by the migration of a group of trilobite, family Olenidae, which had lived in deep sea environment. Olenidae trilobites migrated to shallow sea regions while the other trilobite groups and families died out in certain time periods. This is speculated to be due to a change in ocean conditions, either a drop in ocean temperature, or a drop in oxygen concentration. They affected the circulation and living environment for marine species. The shallow marine environment would change dramatically, resembling a deep sea environment. The deep sea species would thrive, while the other species died out. The trilobite fossils actually records important natural selection processes. The carbonate sequence containing the trilobite fossils hence important to record paleoenvironment and evolution. == Mineral resources in the North China Craton ==

The North China Craton contains abundant mineral resources which are very important economically. With the complex tectonic activities in The North China Craton, the ore deposits are also very rich. Deposition of ore is affected by atmospheric and hydrosphere interaction and the evolution from primitive tectonics to modern plate tectonics. Ore formation is related to supercontinent fragmentation and assembly. They are interlayered with volcanic-sedimentary rocks. The greenstone belt type gold deposits are not commonly found in the craton because most of them were reworked in the Mesozoic, so they appeared to be in some other form. Copper deposits are found in the Zhongtiaoshan area of Shanxi province. Rare earth elements can make high quality permanent magnets, and are therefore irreplaceable in the production of electrical appliances and technologies, including televisions, phones, wind turbines and lasers. Duobaoshan Cu and Bainaimiao Cu-Mo deposits are found in granodiorite. The largest cluster of gold deposits in China is found in the Jiaodong peninsula (east of Shandong Province). ==See also==