Abstract
The Ediacaran-Cambrian (E-C) succession in South China records remarkable oceanic, biological and geochemical variations, but it was not well defined geochronologically, which hinders the interpretation of the spatio-temporal seawater chemical architecture during the time E-C interval. This study presents two Re-Os isochron ages of 520.2 ± 6.1 and 561.7 ± 8.5 Ma for the barite-rich black shales from the top Liuchapo and Doushantuo formations respectively in Tianzhu County, Guizhou Province. In combination with existing age data, the two new Re-Os isochron ages suggest that the Liuchapo Formation was deposited between 550 and 520 Ma. Moreover, like the polymetallic Ni-Mo-PGE layers of shelf margin (or platform) facies and V-rich horizons of transitional (or shelf slope) to deep-water facies, the barite deposits were likely formed due to differential mineralization. The timing offset likely resulted from differential elemental concentration related to certain local factors (i.e., hydrothermal fluids, seawater redox and biological activity). The isochron-derived initial 187Os/188Os ratios of the top Liuchapo Formation (0.902 ± 0.048) and the Doushantuo Formation (0.740 ± 0.042) fall within the range of continental weathering flux (1.54) and oceanic crust (0.126), implying the involvement of marine hydrothermal fluids. Moreover, their difference of initial 187Os/188Os ratios may reflect variations of continental weathering intensity and uplift magnitude.
Highlights
INTRODUCTION The EdiacaranCambrian (E-C) transition, a critical time interval in Earth history, witnessed dramatic changes in global tectonic configuration and oceanic environment, e.g., the as‐ sembly and breakup of Rodinia supercontinent (Zhao et al, 2018), long-lasting ocean anoxia (e.g., Sun et al, 2021; Huang et al, 2019; Feng et al, 2014), and extensive seafloor hydro‐ thermal activities and mineralization along platform margin (e.g., Liu et al, 2021; Han et al, 2017; Chen et al, 2009)
Together with the existing data, the newly obtained ages suggest that the deposition of the Liuchapo Formation in transitional and deep-water facies was most likely diachronous (e.g., Wang et al, 2020; Fu et al, 2016; Chen et al, 2015)
A U-Pb SHRIMP age of 522.7 ± 4.9 Ma was obtained by Wang Y et al (2012) from a tuffa‐ ceous bed of the basal Niutitang Formation in the transitional facies of Taoying (Guizhou Province)
Summary
INTRODUCTION The EdiacaranCambrian (E-C) transition, a critical time interval in Earth history, witnessed dramatic changes in global tectonic configuration and oceanic environment, e.g., the as‐ sembly and breakup of Rodinia supercontinent (Zhao et al, 2018), long-lasting ocean anoxia (e.g., Sun et al, 2021; Huang et al, 2019; Feng et al, 2014), and extensive seafloor hydro‐ thermal activities and mineralization along platform margin (e.g., Liu et al, 2021; Han et al, 2017; Chen et al, 2009). In South China, the EC succes‐ sions were deposited continuously and well preserved notably in the deep-water setting, which could serve as an important win‐. Numerous studies mainly focus on the deep-water shaledominated environments or the correlative shallow-water car‐ bonate platform. In contrast to shallow-water environments, the deep-water envi‐ ronments are characterized by more continuous sedimentation that is fundamental for the construction of age framework. Ad‐ ditionally, the precise chronostratigraphic framework of deepwater environments and correlation between deep-water and shallow-water sedimentary sequences are of significant impor‐ tance to elucidate the coupling relationship between global tec‐ tonic evolution, variations in geological environment, fauna evo‐ lution and development of ore deposits during the E-C transition
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