The South China Craton or South China Block is one of the Precambrian continental blocks in China. It is traditionally divided into the Yangtze Block in the NW and the Cathaysia Block in the SE. The Jiangshan–Shaoxing Fault represents the suture boundary between the two sub-blocks. Recent study suggests that the South China Block possibly has one more sub-block which is named the Tolo Terrane. The oldest rocks in the South China Block occur within the Kongling Complex, which yields zircon U–Pb ages of 3.3–2.9 Ga.
There are three important reasons to study the South China Block. First, South China hosts a great deal of rare-earth element (REE) ores. Second, the South China Block is a key component of the Rodinia supercontinent. Therefore, such study helps us understand more about the supercontinent cycle. Third, almost all major known clades of Triassic marine reptiles have been recovered from the South China sedimentary sequences. They are important to understand the marine recovery after the Permian-Triassic mass extinction.
The South China Block was formed by collision between the Yangtze Block and Cathaysia Block in the Neoproterozoic. On the one hand, the central and eastern part of the South China Block experienced three important Phanerozoic tectonic events. In the Chinese literature, they are named the Wuyi-Yunkai Movement (Early Paleozoic), the Indosinian Movement (Triassic) and the Yanshanian Movement (Jurassic-Cretaceous). They led to extensive deformation and magmatism.
On the other hand, the Late Paleozoic Emeishan flood basalt magmatism is an important event in the western part of the block.
Geology
South China Block is formed by the assembly of both the Yangtze and Cathaysia blocks along the northeasterly trending Jiang-Shao Fault. However, the southwestern extension of this suture is poorly understood due to poor exposure.
Yangtze Block contains several Archean—Paleoproterozoic crystalline basements (e.g. Kongling Complex). The igneous rocks are unconformably overlain by weakly metamorphosed Neoproterozoic sequences (e.g., Banxi Group) and unmetamorphosed Sinian units. In contrast, the Cathaysia block does not contain any Archean basement. Instead, it is composed of mainly Neoproterozoic basement rocks. Rare occurrence of Paleoproterozoic rocks and Mesoproterozoic rocks are reported in southwest Zhejiang and Hainan Island respectively.
Paleozoic magmatism is not common in the South China Block. However, a late Permian Emeishan large igneous province is reported in the western margin of the Yangtze Block.
Mesozoic magmatism is very extensive, especially in the Cathaysia block.
Components This section focuses on how the components of the South China Block were formed.
The South China Block is traditionally divided into the Yangtze Block in the northwest and the Cathaysia Block in the southeast. The northeast-trending Jiangshan-Shaoxing Fault represents the boundary (i.e. suture). It starts from Jiangshan through Shaoxing to Pingxiang. However, the southern extension of the boundary remains unclear. Before they collided together to form the South China block in the Neoproterozic, both of them were part of the Columbia supercontinent.
Recent studies have proposed that the South China Block is possibly divided into three instead of two units. The newly defined unit is termed the Tolo Terrane, which is next to the eastern margin of the Cathaysia Block. The northeast-trending Zhenghe–Dapu fault is thought to be the suture between the Cathaysia Block and the Tolo Terrane. The Tolo Channel Fault in Hong Kong possibly represents a trace of the suture. Therefore, the newly defined unit is named the Tolo Terrane.
Yangtze Block The study of the formation of the Yangtze Block is challenging due to rare Archean outcrops. It is believed that it was formed at around 3.8 – 3.2 Ga. The timing is earlier than the establishment of the Columbia supercontinent. This is supported by the preserved ancient crustal remnant (i.e. 3.8 Ga detrital zircon derived from the South China Block).
Yangtze Block later became part of the Columbia, but its position has only been constrained poorly. The U-Pb crystallization age distribution of 7000 detrital zircons is characterized by several peaks over the history of Earth spans. Those peaks coincide with the age of supercontinent assembly. The Columbia assembled through a global collision event during 2.1-1.8 Ga. Therefore, the constituent continental blocks of the Columbia should record a larger population of 2.1-1.8 Ga detrital zircon. In fact, The Kunyang Group in the Yangtze Block shows this pattern. However, the position of the block is poorly known. It possibly connected with North China, western Australia and/or northwestern Laurentia.
Cathaysia Block
There is only fragmentary study on the formation of the Cathaysia Block due to sparse Precambrian outcrop.
Unlike the Yangtze Block, no Archean outcrop and basement are identified in the Cathaysia Block. However, the finding of Late Archean detrital zircons led scientists to speculate the existence of an unexposed Archean basement. This idea is challenged by the fact that the zircons are oval in shape. They were possibly transported a long distance from another block that was once close to the Cathaysia Block.
Another competing idea suggests that the Cathaysia Block was formed during the assembly of the Columbia supercontinent in the Paleoproterozoic. There are two pieces of evidence.
Sedimentary rocks show a greater population of 2.1-1.8 Ga detrital zircons. The age of the oldest igneous rock coincides with the time of final assembly of the Columbia supercontinent (e.g. 1.89–1.86 Ga S-type granitoids in Badu Complex).
The Cathaysia Block was possibly contiguous with East Antarctica, Laurentia and Australia. It is suggested that the Late Archean oval-shaped detrital zircons were brought from those blocks.
Tolo Terrane The study of the Tolo Terrane is at the initial stage. Most of the evidence comes from Hong Kong. The Tolo Terrane possibly represents a fragment of the Qiangtang Terrane. When the South China Block collided with the India Craton in the Cambrian, the Qiangtang Terrane was sandwiched between those two blocks. During the collision, a fragment (i.e. Tolo Terrane) was calved off from the Qiangtang Terrane.
Formation This section focuses on how the South China Block was formed. Traditionally, the South China Block was formed by the collision between the Yangtze Block and the Cathaysia Block in the Neoproterozoic. They collided to form the Jiangnan Orogen. If the Tolo Terrane does exist, the final formation time should be pushed forward to the Jurassic.
Amalgamation of the Yangtze block and Cathaysia block
There are four major controversies about the amalgamation process.
The timing of amalgamation is unclear. The process of amalgamation is uncertain. The genesis of the Neoproterozoic (830—740 Ma) post-collision magmatism is unclear. The position in the Rodinia supercontinent is controversial.
Timing of amalgamation There are two schools of thought.
They were separated by an early or late Paleozoic ocean. Closure of the ocean by subduction led to amalgamation in the Silurian or Triassic. However, no Silurian or Triassic arc magmatism is found along the Jiangnan Orogen. Therefore, more and more researchers discarded this hypothesis. They assembled in the Neoproterozoic.
Process of Amalgamation A great deal of single-sided subduction systems have been proposed. The diversity arises from different subduction manner including orthogonal subduction, oblique subduction or change in subduction polarity. There is also disagreement on the tectonic setting of the rocks. (e.g. intra-oceanic arc versus continental arc, backarc versus forearc).
Despite so, only divergent double subduction system can provide plausible explanation on two key observations in the Jiangnan Orogen.
Magmatic arcs developed on the margin of the two blocks during the early Neoproterozoic. This indicates that the oceanic plate subducted on two opposite directions simultaneously. Most rocks merely experienced greenschist facies metamorphism (i.e. no high-grade metamorphism). In single-sided subduction system, the subducted oceanic slab would drag the continental crust along the subduction zone, leading to crustal thickening and high-grade metamorphism. This is referred to as deep continental subduction. In divergent double subduction system, no deep continental subduction occurs.
The Neoproterozoic post-collision magmatism
After the amalgamation, it is widely recognized that continental rifting and widespread 800—760 Ma bimodal magmatism occurred in the South China Block. Two models have been proposed.
The magmatism was caused by slab-break off. When the subducted oceanic slab sank into the mantle, this induced mantle upwelling and subsequent decompression melting. The mantle was melted to generate mafic magma. The mafic magma either intruded or underplated the overlying continental crust to form felsic magma. Therefore, mafic and felsic igneous rocks coexisted. The magmatism was due to the Rodinia giant mantle plume. Previous study advocated for a so-called "SWEAT" configuration (i.e. Southwest Laurentia–East Antarctic) in the Rodinia supercontinent. However, the age and geographical position of giant plume-related radiating dyke swarm argue against this model. First, the age difference is too large to be considered as the same dyke swarm. Second, the dyke swarm in Laurentia suggest a mantle plume centre to its west but there is no such evidence in eastern Australia.
The South China Block possibly serves as this missing link (i.e. the "missing link" hypothesis). They suggest the head of the mantle plume, which was sited beneath the South China Block, led to rifting and bimodal magmatism since 825 Ma. The discovery of 825 Ma komatiitic basalts in Yiyang, which is indicative of a hot mantle source, provides an indisputable evidence for the presence of a mantle plume. However, there is alternative genesis of komatiites like hydrous melting in the subduction zone. Moreover, no Neoproterozoic Large Igneous Province has been identified in the South China Block.
Position in Rodinia
There is no consensus about the position of the South China Block in the Rodinia supercontinent. The main controversy is whether it was located in the interior or at the margin of Rodinia.
On the one hand, the South China Block is proposed to be located between eastern Australia and western Laurentia in the interior of Rodinia (i.e. "Missing-link" hypothesis). Several lines of evidence support this hypothesis.
Superplume record: A block, sitting above the head of a mantle plume, between eastern Australia and the western Laurentia is required. The South China Block is a suitable candidate. Igneous rock record: Felsic granitic and volcanic rocks in Hainan Island are similar to the trans-continental granite-rhyolite province in southern Laurentia in terms of age and isotopic features. This implies a close geographical proximity between the South China Block and Laurentia.
Rodinia assembled through global collision events from 1300 Ma to 900 Ma. It is expected that the central part of Rodinia should not record any later collision event since it had already amalgamated. However, there is clear evidence that the final time of the amalgamation of the South China Block is much later than 900 Ma. Therefore, it was not located in the central part of Rodinia. The evidence comes from lithological and structural records.
The Shuangxiwu arc sequence, which lasted at least to 850 Ma, represents an intra-oceanic arc. This indicates the Yangtze block and the Cathaysia block were still separated by an ocean later than 900 Ma.