Application of in situ black shale Rb–Sr geochronology to constrain deposition/post-depositional hydrothermal alteration and assess the fidelity of geochemical proxies

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Black shales are key geological records for reconstructing past biogeochemical processes, the timing of which requires direct and accurate geochronological solutions. Recent developments of in situ Rb-Sr geochronology via tandem plasma-mass spectrometry allow for rapid assessment of Rb-Sr ages in black shales without either losing textural context or consuming much in the way of sample volumes. Meso- to Neoproterozoic, and Phanerozoic black shales from the South China and North China cratons preserve information on the biogeodynamic evolution. Yet, existing relatively poor time constraints undermine our ability to make sense of the timing and effect of depositional/post-depositional events that potentially blurred or otherwise altered primary biogeochemical information locked in these rocks. Here, we report a new Rb-Sr age of 1059 ± 53 Ma for a Mesoproterozoic black shale of the Xiamaling Formation (North China Block), which we interpret to represent a metasomatic alteration. A plausible cause is the intrusive magmatism of 1100–900 Ma in response to continental collision. Two new Rb-Sr ages of 332 ± 47 Ma and 363 ± 50 Ma for Ediacaran black shales of the Lantian Formation (South China Block), are interpreted to represent alteration (metasomatic) events coincident with the Hercynian/Variscan Orogeny. A Phanerozoic Rb-Sr age of 229 ± 12 Ma for a Mesozoic black shale (Ordos Basin, North China Craton), is consistent with its chronostratigraphic framework and thus is interpreted by us as an accurate deposition age. Precambrian black shales commonly show evidence of post-depositional hydrothermal alteration, which has cast doubt on some geochemical data used to deduce ancient atmospheric and hydrosphere conditions. Furthermore, K and Rb content in black shales tends to homogenize in response to post-depositional hydrothermal alterations. This study demonstrates that in situ black shale Rb-Sr geochronology can be adopted as an alternative dating technique to constrain the depositional age of Phanerozoic strata when datable volcanic interbeds are absent. Meanwhile, in situ black shale Rb-Sr dating detected post-depositional hydrothermal alteration in the Meso- to Neoproterozoic strata of South China and North China, thus questioning the reliability of geochemical proxies that has been used in these black shale to deduce ancient atmospheric and hydrosphere conditions. In situ black shale Rb-Sr dating is recommended in the future to test the primary or reworked geochemical compositions before applying them to trace ancient atmospheric and hydrosphere conditions.