Geochemistry of chert rocks from the Sikhote-Alin Taukha terrane, Russia far east: significance for determination of their depositional environment

摘要

The first geochemical data of the Benevka section chert rocks belonging to the Taukha terrane of the Sikhote-Alin Late Jurassic-Early Cretaceous accretionary prism, Russia Far East are discussed in this work. In particular, the geochemical data obtained show that the content of Ce increases progressively from the low horizons of stratigraphic succession represented by bedded cherts to the upper horizons composed of clayey cherts, which gradually pass to siliceous mudstones at the uppermost part. Correspondingly, the value of the negative Ce anomaly (Ce/Ce~*) decreases progressively up the Benevka section. The Ce anomaly value of bedded cherts ranges 0.34-0.69 (well-developed negative Ce anomalies), whereas those of upper horizons of bedded cherts and lower horizons of clayey cherts vary from 0.75 to 0.87 (slightly less, but also well-developed negative Ce anomalies). Slight negative Ce anomalies (0.91-0.94) were revealed in the upper horizons of clayey cherts and in siliceous mudstones. In addition, the NASC-normalized La/Ce ratio (i.e., La_n/Ce_n) has been calculated, the value of which also decreases progressively upward of the measured stratigraphic section from 2.87 to 1.02. The totality of all geochemical data indicates that the Benevka section chert rocks were formed in an oceanic depositional environment, but in different paleo-ocean locations. The lowermost horizons of bedded cherts were deposited to near spreading ridge-proximal depositional environment. The open-ocean and transition zone between open-ocean and near continental margin depositional environments are reconstructed for the upper horizons of bedded cherts and low horizons of clayey cherts. The upper horizons of clayey cherts and siliceous mudstones were accumulated at near continental margin depositional environment.