Pb-Pb isotope dating of black shales from the lower Cambrian Niutitang formation, Guizhou province, south China

The Lower Cambrian black shales of the Niutitang Formation and their lateral equivalents occur widely on the Yangtze Platform, South China. In this study, we examine two sections of the Niutitang Formation in Guizhou Province. We employ a stepwise acid-leaching technique to obtain Pb-isotope compositions of the black shales. At the Zhongnan section, the black shales display large Ph isotope variations, with Pb-206/Pb-204 of 20.5396 similar to 33.5709, and Pb-207/Pb-204 of 15.8407 similar to 16.5782. In contrast, Pb isotope data show a relatively limited range in Pb-206/Pb-204 (18.2797 similar to 18.5972) and Pb-207/Pb-204 (15.6813 similar to 15.7159) at the Zhijin section. Data for all acid-leached black shale samples from the two sections yield a Pb-Pb isochron age of 531 +/- 24 Ma. This age may represent the sedimentary age of the black shales. The initial Pb-isotope composition of the black shales falls near the upper-crust evolution curve in a Zartman Pb-Pb evolution diagram. Hence, it is likely that the Ph in the black shales may have been derived from an upper continental crustal source.