RE-OS GEOCHRONOLOGY OF BLACK SHALES FROM THE DOUSHANTUO FORMATION, YANGTZE PLATFORM, SOUTH CHINA

The Ediacaran period of the late Neoproterozoic was characterized by plate tectonic reorganization, and dramatic changes in the chemical composition of the atmosphere and oceans and biological evolution. In South China, the Doushantuo formation represents 90% of the time interval of the Ediacaran period. However, subdivision and correlation of strata in this formation is limited by sporadic occurrences of fossils of uncertain age and scarcity of radiometric time constraints. Re-Os geochronology offers an excellent tool for dating organic rich sedimentary rocks like black shales. In this study, we carried out Re-Os isotopic analysis of basal black shale samples from the fossil-bearing Miaohe member (Doushantuo IV) of the upper Doushantuo formation. The samples were collected from the Jiulongwan section, Three Gorges area, South China. A Re-Os isochron date of 593 ± 17 Ma (MSWD=1.0) with initial ¹⁸⁷Os/¹⁸⁸Os of 0.88±0.13 (Fig.1)was obtained by using a CrO₃-H₂SO₄ dissolution technique, which minimizes the detrital component by preferentially dissolving the organic matter (Selby and Creaser, 2003). This age is consistent with precise U-Pb zircon ages of 551 Ma from the top, and 635 Ma near the base of the Doushantuo formation, respectively (Condon et al., 2005). The minimum age difference of 25 Ma between the base of the Miaohe member and the 551 Ma old tuff on the top of the Miaohe member means either, very low sedimentation rates during black shale deposition (< 0.5 m/Ma), or may be explained by the existence of hiatus in the strata. Samples from both base and top of Miaohe member yield a age of 589±26 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os of 0.89±0.21. Larger uncertainties on this age calculation reflect variable initial ¹⁸⁷Os/¹⁸⁸Os between base and top.

References:

CONDON, D., ZHU, M., BOWRING, S., WANG, W., YANG,A. & JIN, Y. 2005. U–Pb ages from the Neoproterozoic Doushantuo Formation, China. Science, 308, 95–98

SELBY, D. & CREASER, R. A. 2003. Re–Os geochronology of organic-rich sediments: an evaluation of organic matter analysis methods. Chemical Geology, 200, 225–240.