Black shale LA-ICP-MS Rb-Sr and monazite SIMS U-Pb geochronology from the Cryogenian successions in the northern Yangtze Block

Abstract

The late Cryogenian (possibly Marinoan-aged) sedimentary successions from the Shennongjia area in the northern margin of the Yangtze Block host important biological evidence for multicellular eukaryotes that have survived the Neoproterozoic glaciation. However, absolute constraints on the depositional age of these fossiliferous successions are lacking. Herein, we present direct dating of black shale and diagenetic monazite, which yield an Rb-Sr isochron age of 630 ± 27 Ma and a lower intercept U-Pb age of 626.4 ± 56.8 Ma from these fossiliferous successions. These dates are consistent with deposition during the Marinoan glaciation. A Marinoan-aged deposition is also demonstrated by the initial 87Sr/86Sr ratio (0.7084) of the ca. 630 Ma black shale sample, which is consistent with the initial 87Sr/86Sr ratio of contemporaneous seawater. In addition, petrographic observations coupled with REE distribution patterns and Th/Ce-Th and LREE-Y2O3 discrimination plots, facilitate identification of detrital and hydrothermal monazite. The former yields U-Pb dates that are older than the youngest detrital zircon peak at ca. 824 Ma and may have been recycled from igneous precursors. The later, in contrast, yields dates of 585.0 ± 95.1 Ma and 197.3 ± 31.2 Ma and likely records post-depositional hydrothermal events. Further evidence of an old hydrothermal event is recorded in the black shales, which yield Rb-Sr isochron dates of 554 ± 27 Ma and 545 ± 17 Ma that overlap the monazite date. Hydrothermal fluid circulation is associated with precipitation of minerals such as pyrite, sphalerite, and calcite, and potentially has genetic links with the large-scale Bingdongshan Pb-Zn ore deposit around the Shennongjia area, which has an Rb-Sr date of 571 ± 86 Ma.