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Snowball Earth - Evidence of Low 18O Magmatism During Rifting of a Supercontinent in South China

The authors1 suggest the key to demonstrating that high-T interactions of water and rock and low 18O magmatism in tectonic zones of rifts can be an efficient interface of heat and material transport from the interior of the Earth to its exterior at the onset of a Snowball Earth event, is the identification of low-18O igneous rocks of Neoproterozoic age. Values of low δ18O ranging from -10.9 to +10.4 permil were observed, that are characteristic of rocks of meteorite origin, with an aereal extent of more than 30,000 sq km were observed, for metaigneous zircons, of mid-Neoproterozoic age, from the ultrahigh-pressure eclogite facies metamorphic belt, of Triassic age, along the Dabie-Sulu Orogenic Belt in China. The abnormal depletion of 18O is confirmed by the presence of δ18O zircons in low grade metaigneous rocks, of Neoproterozoic age, in the same belt, though the oxygen isotope system could be reset by exchange with matrix minerals during "wet" eclogite-facies metamorphism. Zircons obtained from low grade metagranitoid and metavolcanic rocks imaged by cathodoluminescence reveals preservation of the internal structures of oscillatory zoning that is typical of magmatic origin. The suggestion that Phanerozoic metamorphism affected mineral oxygen isotope values has been ruled out by Ar-Ar plateau ages of 747 ± 6 Ma and 776 ± 12 Ma from hornblende obtained from 2 granites of low δ18O. The authors1 measured 2 groups of U-Pb ages at 756 ± 6 5 Ma and 782 ± 6 5 Ma, respectively for the low δ18O zircons that crystallised during 2 episodes of low δ18O magmatism as a response to tectonic evolution from the rifting of a supercontinent at about 780 Ma until the breakup at about 750 Ma. The second episode of low-18O magmatism coincided with the continental Kaigas Ice Age. To account for the tectonic connection between the  low δ18O magmatism, the continental glaciation and the rifting of the supercontinent during the Cryogenian a plate-rift model is proposed. Before the low-δ18O magmatism in rift tectonic zones, high-T meteoric water-rock interaction occurred, and during the rifting of the South China Block from Rodinia, caldera collapse melted hydrothermally altered  low δ18O rocks.

Sources & Further reading

  1. Zheng, Y. F., B. Gong, Z. F. Zhao, Y. B. Wu, and F. K. Chen. "Zircon U-Pb Age and O Isotope Evidence for Neoproterozoic Low-18o Magmatism During Supercontinental Rifting in South China: Implications for the Snowball Earth Event." American Journal of Science 308, no. 4 (April 2008 2008): 484-516.


Author: M. H. Monroe
Last updated 30/05/2013

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