U-Pb sensitive high-resolution ion microprobe ages from the Doushantuo Formation in south China: Constraints on late Neoproterozoic glaciations
- Shihong Zhang1,
- Ganqing Jiang*2,
- Junming Zhang3,
- Biao Song4,
- Martin J. Kennedy5 and
- Nicholas Christie-Blick6
- 1School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China, and Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
- 2Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154, USA
- 3Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China
- 4Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
- 5Department of Earth Sciences, University of California, Riverside, California 92521, USA
- 6Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA
Abstract
Two distinctive volcanic ash beds were found in the terminal Proterozoic Doushantuo Formation in south China. The lower ash bed, ∼2.5 m above the cap carbonate at the base of the Doushantuo, yields a U-Pb zircon age of 621 ± 7 Ma, providing the closest upper limit for a correlative of the Marinoan glaciation. The upper ash bed, near the Doushantuo-Dengying boundary, yields a U-Pb zircon age of 555.2 ± 6.1 Ma, providing for the first time a direct age determination for a prominent negative δ13C excursion (≤−5‰) above the Marinoan glacial level. This excursion, if interpreted to be of glacial origin, is much younger than the Gaskiers Formation (ca. 580 Ma) in Newfoundland, and perhaps the fifth or sixth such level in the Neoproterozoic. That interpretation, however, is not supported by the proliferation of organisms within strata encompassing the negative δ13C excursion in south China and globally, by the lack of a ca. 555 Ma glacial record, and by the absence of stratigraphic evidence for sea-level change. The data call for alternative paleoceanographic models for the origin of Neoproterozoic δ13C excursions not clearly related to glaciation.
Footnotes
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↵*Corresponding author jianggunlv.nevada.edu
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↵GSA Data Repository item 2005089, Tables DR 1–DR3, carbon and oxygen isotope data and SHRIMP U-Pb zircon data, and Figures DR1–DR4, photomicrographs of dated zircons and plots of age data with accompanying text, is available online at www.geosociety.org/pubs/ft2005.htm, or on request from editinggeosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.
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- Accepted 11 February 2005.
- Received 24 November 2004.
- Revision received 9 February 2005.
- The Geological Society of America, Inc.
