Trace element and REE geochemistry of the Ediacaran Doushantuo Formation from Fanjingshan area, northeast Guizhou province, China

摘要

Trace and rare earth elements (REEs) geochemistry of carbonate rocks of the Ediacaran Doushantuo Formation, northeast Guizhou province were studied to present spatio-temporal variations of trace elements and REEs from two suites of different sedimentary facies, so as to discuss simultaneously differing palaeo-environmental conditions during the deposition of Ediacaran carbonate formation. Results demonstrate that the depositional seawater of the Nongjing section is more oxygen-depleted than those of the Tongluo section during the early Doushantuo period. For the whole Doushantuo Formation, the sedimentary water column of the lower Doushantuo Formation is much more oxygen deficient than those of the upper, and is the most anoxic in the middle formation, which is consistent with sedimentary model of the Doushantuo Formation in this area. Relative to average marine carbonate, the depletion and enrichment of certain trace elements indicate that the sedimentary condition of the Doushantuo carbonates deposition was anoxic (the early)-oxic (the middle)-anoxic (the later), with more intensive volcanic and hydrothermal accompanying with lower palaeoproductivity. The North American shale composite-normalized REE + Y patterns of Doushantuo carbonates from two sections are both characteristic of a slightly negative Ce anomaly, positive Eu anomaly and superchondritic Y/Ho ratio. Geochemical characteristics of trace elements and REEs reveal that these carbonates of the Doushantuo Formation formed generally in an alternant anoxic-oxic environment and a moderately stratified ocean with influx of high-temperature hydrothermal fluids and volcanic material. Different from the stepwise and protracted oxidation, an anoxic-oxic switching process of sedimentary water column with two episodic anoxic events had taken place, and would have contributed to a stimulus for increased evolution at that time.