North China Craton
The North China Craton is a continental crustal block with one of Earth's most complete and complex records of igneous, sedimentary and metamorphic processes. It is located in northeast China, Inner Mongolia, the Yellow Sea, and North Korea. The term craton designates this as a piece of continent that is stable, buoyant and rigid. Basic properties of the cratonic crust include being thick, relatively cold when compared to other regions, and low density. The North China Craton is an ancient craton, which experienced a long period of stability and fitted the definition of a craton well. However, the North China Craton later experienced destruction of some of its deeper parts, which means that this piece of continent is no longer as stable.
The North China Craton was at first some discrete, separate blocks of continents with independent tectonic activities. In the Paleoproterozoic the continents collided and amalgamated and interacted with the supercontinent, creating belts of metamorphic rocks between the formerly separate parts. The exact process of how the craton was formed is still under debate. After the craton was formed, it stayed stable until the middle of the Ordovician period. The roots of the craton were then destabilised in the Eastern Block and entered a period of instability. The rocks formed in the Archean and Paleoproterozoic eons were significantly overprinted during the root destruction.
Apart from the records of tectonic activities, the craton also contains important mineral resources, such as iron ores and rare earth elements, and fossils records of evolutionary development.
Tectonic setting
The North China Craton covers approximately in area and its boundaries are defined by several mountain ranges, the Central Asian Orogenic Belt to the north, the Qilianshan Orogen to the west, Qinling Dabie Orogen to the south and Su-Lu Orogen to the east. The intracontinental orogen Yan Shan belt ranges from east to west in the northern part of the craton.The North China Craton consists of two blocks, the Western Block and the Eastern Block, separated by the wide Trans North China Orogen, which is also called Central Orogenic Belt or Jinyu Belt. 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. It experienced a number of earthquakes with a magnitude of over 8 on the Richter scale, claiming millions of lives. The thin mantle root, which is the lowest part of lithosphere, is the reason for its instability. The thinning of the mantle root caused the craton to destabilize, weakening the seismogenic layer, which then allows earthquakes to happen in the crust. The Eastern Block may once have had a thick mantle root, as shown by xenolith evidence, but this seems to have been thinned during the Mesozoic. The Western Block is located in Helanshan-Qianlishan, Daqing-Ulashan, Guyang-Wuchuan, Sheerteng and Jining. It is stable because of the thick mantle root. Little internal deformation occurred here since Precambrian.
Geology
The rocks in the North China craton consist of Precambrian basement rocks, with the oldest zircon dated 4.1 billion years ago and the oldest rock dated 3.8 billion years ago. The Precambrian rocks were then overlain by Phanerozoic sedimentary rocks or igneous rocks. The Phanerozoic rocks are largely not metamorphosed. The Eastern Block is made up of early to late Archean 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 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, 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 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. In Mesozoic to Cenozoic time, the Precambrian basement rocks were extensively reworked or reactivated.Precambrian Tectonics (4.6 billion years ago to 1.6 billion years ago)
The Precambrian tectonics of the North China Craton is complicated. Different scholars have proposed different models to explain the tectonics of the Craton, with two dominant schools of thought coming from Kusky and Zhao. The major difference in their models is the interpretation of the two most significant Precambrian metamorphic events, occurring 2.5 billion years ago and 1.8 billion years ago respectively, in the North China Craton. Kusky argued that the metamorphic event 2.5 billion years ago corresponded to the amalgamation of the Craton from their ancient blocks, while Zhao argued that the later event was responsible for the amalgamation.Kusky's Model: The 2.5 Ga craton amalgamation model
Kusky's model proposed a sequence of events showing the microblocks amalgamating 2.5 billion years ago. First, in the Archean time, the lithosphere of the craton started to develop. Some ancient micro-blocks amalgamated to form the Eastern and Western Blocks 3.8 to 2.7 billion years ago. The formation time of the blocks is determined based on the age of the rocks found in the craton. Most rocks in the craton were formed at around 2.7 billion years ago, with some small outcrops found to have formed 3.8 billion years ago. Then, the Eastern Block underwent deformation, rifting at the Western Edge of the Block 2.7 to 2.5 billion years ago. Evidences for a rift system have been found in the Central Orogenic Belt and they were dated 2.7 billion years old. These included ophiolite and remnants of a rift system.Collision and amalgamation started to occur in Paleoproterozoic time. From 2.5 to 2.3 billion years ago, the Eastern and Western Blocks collided and amalgamated, forming the North China Craton with the Central Orogenic Belt in between. The boundary of the Central Orogenic Belt is defined by Archean geology which is from west Liaoning to west Henan. Kusky proposed that the tectonic setting of the amalgamation is an island arc, in which a westward dipping subduction zone was formed. The two blocks then combined through a westward subduction of the Eastern Block. The timing of the collision event is determined based on the age of crystallisation of the igneous rocks in the region and the age of metamorphism in the Central Orogenic Belt. Kusky also believed that the collision happened right after the rifting event, as seen from examples from orogens in other parts of the world, deformation events tend to happen closely with each other in terms of timing. After the amalgamation of the North China Craton, Inner Mongolia–Northern Hebei Orogen in the Western Block was formed by the collision of an arc terrane and the northern margin of the craton 2.3 billion years ago. The arc terrane was formed in an ocean developed during post-collisional extension in the amalgamation event 2.5 billion years ago.
Apart from the deformation event in a local scale, the craton also interacted and deformed in a regional scale. It interacted with the Columbia Supercontinent after its formation. The northern margin of the whole craton collided with another continent during the formation of Columbia Supercontinent from 1.92 to 1.85 billion years ago. Lastly, the tectonic setting of the craton became extensional, and therefore began to break out of the Columbia Supercontinent 1.8 billion years ago.
Zhao's Model: the 1.85 Ga craton amalgamation model
Zhao proposed another model suggesting the amalgamation of the Eastern and Western Blocks occurred 1.85 billion years ago instead. The Archean time was a time of major crustal growth.Continents started to grow in volume globally during this period, and so did the North China Craton. Pre-Neoarchean rocks are just a small portion of the basement rocks, but zircon as old as 4.1 billion years old was found in the craton. He suggested that the Neoarchean crust of the North China Craton, which accounts for 85% of the Permian basement, was formed in two distinct periods. First is from 2.8 to 2.7 billion years ago, and later from 2.6 to 2.5 billion years ago, based on zircon age data. Zhao suggested a pluton model to explain the formation of metamorphic rocks 2.5 billion years ago. Neoarchean mantle upwelled and heated up the upper mantle and lower crust, resulting in metamorphism.
In the Paleoproterozoic time, the North China Craton amalgamated in three steps, with the final amalgamation took place 1.85 billion years ago. Based on the metamorphic ages in the Trans North China Orogen, the assembly and the formation process of the North China Craton is determined. Zhao proposed that the North China Craton was formed from 4 blocks, the Yinshan Block, the Ordos Block, the Longgang Block and the Langrim Block. The Yinshan and Ordos Blocks collided and formed the Western Block, creating the Khondalite Belt 1.95 billion years ago. For the Eastern Block, there was a rifting event in the Jiao-Liao-Ji Belt, which separated the Longgang Block and the Langrim Block with an ocean before the block was formed 2.1 to 1.9 billion years ago. A rifting system is proposed because of how the rocks were metamorphosed in the belt and symmetrical rocks have been found on both side of the Belt. Around 1.9 billion years ago, the rift system at the Jiao-Liao-Ji Belt switched to a subductional and collisional system. The Longgang Block and the Langrim Block then combined, forming the Eastern Block. 1.85 billion years ago, the Trans North China Orogen was formed by the collision of the Eastern and Western Blocks in an eastward subduction system, with probably an ocean between the 2 blocks subducted.
Zhao also proposed model about the interaction of the North China Craton with the Columbia Supercontinent. He suggested that the craton's formation event 1.85 billion years ago was part of the formation process of the Columbia Supercontinent. The craton also recorded outward accretion event of the Columbia Supercontinent after it was formed. The Xiong'er Volcanic Belt located in the Southern Margin of the craton recorded the accretion event of the Supercontinent in terms of a subduction zone. The North China Craton broke away from the Supercontinent 1.6 to 1.2 billion years ago via a rift system called Zhaertai Bayan Obo rift zone where mafic sills found is an evident of such event.
| Time | The 2.5 Ga Amalgamation Model | The 1.8 Ga Amalgamation Model |
| 3.8–2.7 Ga | Ancient micro blocks amalgamated to form the Western and Eastern Block | Crust grew and formed, with plutons upwell in the region, causing extensive metamorphism |
| 2.7–2.5 Ga | Eastern Block deformation | Crust grew and formed, with plutons upwell in the region, causing extensive metamorphism |
| 2.5–2.3 Ga | The Western and Eastern Block collided, and formed the N-S trending Central Orogenic Belt between where the 2 blocks are amalgamated | |
| 2.3 Ga | Arc Terrane collision to form Inner Mongolia- Northern Hebei Orogen in the North of the Craton | |
| 2.2–1.9 Ga | Rifting and collision of the Eastern Block along the Jiao-Liao-Ji Belt | |
| 1.95 Ga | Northern margin collided with continents in the Columbia Supercontinent | Yinshan and Ordos Block collided and formed the Western Block and the Khondalite Belt |
| 1.85 Ga | Northern margin collided with continents in the Columbia Supercontinent | Collision of the Eastern and Western Blocks leading to their amalgamation and the formation of Trans North China Orogen |
| 1.8 Ga | The tectonic setting of the craton became extensional where the craton broke out from Columbia Supercontinent |