Pb-Pb isotope dating of black shales from the Lower Cambrian Niutitang Formation, Guizhou Province, South China

摘要

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 Pb isotope variations, with 206Pb/204Pb of 20.5396～33.5709, and 207Pb/204Pb of 15.8407～16.5782. In contrast, Pb isotope data show a relatively limited range in 206Pb/204Pb (18.2797～18.5972) and 207Pb/204Pb (15.6813～15.7159) 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 Pb in the black shales may have been derived from an upper continental crustal source.