Monday 29 August 2016

Using zircon uranium-lead geochronology to understand the origins of the Miocene Sanya Formation beneath the South China Sea.

The Miocene Shanya Formation forms part of the geological sequence in the Yinggehai Basin, which underlies part of the northwestern South China Sea. The formation is 2950 m thick at its maximum, and is known to host a number of hydrocarbon reserves, making it of great interest to exploration geologists. The formation comprises shallow marine sedimentary deposits formed by the deposition of material washed from nearby terrestrial environments, however, like many offshore deposits, the processes that led to the formation of the Shanya Formation are poorly understood, making it hard to make predictions about where hydrocarbons might be found within these beds.

In a paper published in the journal Acta Geologica Sinica in February 2016, Wang Ce of the State Key Laboratory of Isotope Geochemistry at the Guangzhou Institute of Geochemistry and the University of the Chinese Academy of Sciences, Liang Xinquan, also of the State Key Laboratory of Isotope Geochemistry at the Guangzhou Institute of Geochemistry, and Fu Jiangang, Jiang Ying and Dong Chaoge, again of the State Key Laboratory of Isotope Geochemistry at the Guangzhou Institute of Geochemistry and the University of the Chinese Academy of Sciences, describe the results of a study in which zircon uranium-lead geochronology was used to develop an understanding of the origins of sedimentary material in the Shanya Formation.

Zircon is a mineral formed by the crystallization of cooling lavas. When it forms it often contains trace amounts of uranium, which decays into (amongst other things) lead at a known rate. Since lead (which has a much lower melting point) will not have been present in the original lava, it is possible to calculate the age of a zircon crystal from the ratio between these elements.

 A ziron crystal, about 250 µm in length. Wikipedia.

This has obvious uses for the dating of igneous rocks, but can also be useful in sedimentary geology, since it enables geochemists to link zircons found in sedimentary rocks to their sources (i.e. the igneous rocks from which they were eroded before being deposited in the sedimentary strata).

Wang et al. collected zircons from sandstones attributed to the Sanya Formation from drill cores in the northern, northeaster and eastern parts of the Yinggehai Basin. The first sample (from the north) yielded zircon uranium-lead dates with a very wide range of dates, though the most abundant were 274, 432 and 957 million years old, with smaller peaks in abundance at 793, 1966 and 2481. The second site (from the northeast) yielded a similar range of dates, with the commonest dates being 156 and 428 million years old and smaller peaks at 41, 239 and 733 million years old. The third site (the one in the east) produced zircons with a smaller range of dates, being much younger, with peaks at 99 and 238 million years old.

The youngest zircons present, the 41 million-year-old crystals in the second sample, are approximately the right age to be associated with melting and exhumation during a period of motion on the Red River Fault Zone, which runs through Yunnan Province and Vietnam, and which is a fault within the Yangtze Block that accommodates movement by the Indo-Australian Plate.

The zircons aged 99 and 156 million years old, found in the second and third drill cores, are consistent with the formation of the Yanshanian Granites on Hainan Island in the Late Jurassic-Cretaceous.

Zircons with dates of 230-250 million years old, again found in the second and third samples, are likely to have originated during the collision of the Indochina and Yangtze Blocks, and could have come from a wide range of sources in South China, Vietnam or Hainan Island.

Samples dating to about 430 million years ago, found in the first and second samples, are thought likely to be associated with the Caledonian Orogen, when the ancient continents of Laurentia, Baltica and Avalonia collided during the closure of the Iapetus Ocean; grains of this age are common in South China, but very rare on Hainan.

Grains 700-1000 million years old, again found in the first and second samples only, are likely to have formed during the Jinningian Movement, part of the breakup of the ancient supercontinent of Rodinia, and originated on the Yangtze Block.

Zircons 1966 and 2481 million years old, found only in the first sample, are likely to have formed during the origin of the Yangtze block.

Thus zircons in the sediments from the first and second drill cores are dominated by grains that originated from the Yangtze Block to the north (i.e. mainland China), while the third sample appears to contain mostly zircons that originated on Hainan Island, a source which also provided a significant minority of the material in the second sample.

Possible provenance directions of the Lower Miocene Sanya Formation in the Yinngehai Basin. Wang et al. (2016).

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