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.[1] It is located in northeast China, Inner Mongolia, the Yellow Sea, and North Korea.[1] The term craton designates this as a piece of continent that is stable, buoyant and rigid.[1][3][4] Basic properties of the cratonic crust include being thick (around 200 km), relatively cold when compared to other regions, and low density.[1][3][4] The North China Craton is an ancient craton, which experienced a long period of stability and fitted the definition of a craton well.[1] However, the North China Craton later experienced destruction of some of its deeper parts (decratonization), which means that this piece of continent is no longer as stable.[3][4]
The North China Craton was at first some discrete, separate blocks of continents with independent tectonic activities.[5] In the Paleoproterozoic (2.5-1.8 billion years ago) the continents collided and amalgamated and interacted with the supercontinent, creating belts of metamorphic rocks between the formerly separate parts.[5] 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 (480 million years ago).[4] 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 (4.6–1.6 billion years ago) 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.[6]
Geology[edit]
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.[5] The Precambrian rocks were then overlain by Phanerozoic (539 million years ago to present) sedimentary rocks or igneous rocks.[9] The Phanerozoic rocks are largely not metamorphosed.[9] 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.[9] These are overlain by Paleoproterozoic rocks which were formed in rift basins.[9] 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.[9] 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.[9] 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.[4] Apart from sedimentation, there were six major stages of magmatism after the Phanerozoic decratonization.[4] In Jurassic to Cretaceous (100-65 million years ago) sedimentary rocks were often mixed with volcanic rocks due to volcanic activities.[4]