Further SHRIMP geochronology on the early Cambrian of south China

Zircon Pb-206/U-238 ages are reported for bentonites from two Cambrian sections in South China, the Meishucun and the Fandian. The analyzed zircons vary widely in age because of the presence of detrital and altered grains. Spot ages were corrected for calibration drift during analysis, and the age of reference zircon SL13 was calibrated as 580 Ma against other standards using SIMS. Use of mixture modeling was essential to resolve the different age components and to identify the volcanic ages. Two age groups occur within the Meishucun Bed 5 bentonite, of which the larger at 518.7 +/- 2.5 Ma (sigma) is interpreted as a post depositional zircon growth event and the other at 539.4 +/- 2.9 Ma as the volcanic age. Bentonites from the Meishucun Bed 9 and its Fandian equivalent have two age groups, 526.5 +/- 1.1 Ma and 540.2 +/- 1.0 Ma. We interpret the younger of these as the tuff magmatic age and the older as detrital. Small shelly fossils indicate Meischucunian Bed 5 is an equivalent of the late Nemakit Daldynian, and its 539.4 +/- 2.9 Ma age both closely times the base of the Cambrian and is a maximum for the basal Tommotian. The 526.5 +/- 1.1 Ma for Bed 9 and its equivalents approximates the close of the Tommotian.