Age, source, and formation mechanism of Cryogenian cap Mn‑carbonates in South China

The Sturtian cap carbonates in South China contain an appreciable number of Mn deposits that are closely associated with the Sturtian glaciation (717–660 Ma). However, depositional age, source, and formation mechanism of these Cryogenian cap Mn‑carbonates has long been disputed because of a lack of reliable age constraints coupled with inadequate petrographic and geochemical characterization, which undoubtedly prevent us from ascertaining the termination age of Sturtian glaciation, depositional age span of the Cryogenian cap Mn‑carbonates and their formation process. Herein, we present new Usingle bondPb ID-TIMS geochronological, petrographic, and geochemical data on the Sturtian cap Mn‑carbonates from South China. Our zircon Usingle bondPb age data in combination with previous ages suggest the Sturtian cap carbonates were precipitated during a time interval of 661.7 ± 0.6 Ma ∼ 658.1 ± 1.2 Ma. A Monte Carlo simulation suggests the Sturtian cap Mn‑carbonates in South China was precipitated more slowly, with a depositional rate of 0.1 cm/kyr that is consistent with a microbially induced formation process. Reproducibility of the TIMS Usingle bondPb age of ca. 661 Ma at two stratigraphic sections in South China attests to synchronous Sturtian deglaciation and precipitation of the Sturtian cap carbonates. Micro-XRF imaging analyses show that alternating mineralized microbial mats have contributed to the formation of Mn‑carbonates, which are demonstrated by the alternating occurrence of Mn, Ca, Fe, and P with Si, Al, Ti, and K. For chert/calcite vein-bearing Mn‑carbonates, Mn is positively correlated with Ca, Fe and P, and Si is positively correlated with Al, indicating the influence of Si-bearing hydrothermal on earlier formed Mn‑carbonates and terrestrial detritus. The differential duration of Sturtian and Marinoan glaciation could have provided different amounts of hydrothermal Mn (II) to the ocean because of volcanism-related hydrothermal activities, which may explain why the former have produced Mn‑carbonates of ore grade level. Finally, monazite Usingle bondPb ages reveal a post-depositional Indosinian tectonothermal event that could have induced the circulation of hydrothermal fluids to partially dissolve early formed rhodochrosite and dolomite. Such a process resulted in the mobilization of Ca2+, Mg2+ and Mn2+ and precipitation of cyptic Mn/Mg-calcite veins in the Sturtian cap carbonates. Hydrothermal activity also induced the mobility of REE within the cap Mn‑carbonates, as demonstrated by the occurrence of authigenic monazite that dominantly incorporated LREE upon their crystallization. Considering the extensive whole rock geochemical analyses on the Sturtian cap Mn‑carbonates, care should be taken when using geochemical proxies from these secondary cryptic vein-bearing carbonates to recover paleoceanographic chemical conditions. In situ geochemical analyses guided with high resolution petrography is recommended in future so as to extract the primary geochemical information from these Sturtian cap Mn‑carbonates.