REE geochemistry of carbonates from the Guanmenshan Formation, Liaohe Group, NE Sino-Korean Craton: Implications for seawater compositional change during the Great Oxidation Event

摘要

The worldwide 2.33-2.06Ga positive δ~(13)C_(carb) excursion has been correlated with the Great Oxidation Event (GOE) and termed as the Lomagundi Event. The 2.3-1.85Ga Guanmenshan Formation in the Liaohe Group of the northeastern Sino-Korean Craton is characterized by positive δ~(13)C_(carb) excursion and is a potential candidate to evaluate the Lomagundi Event using REY (rare earth element and yttrium, REE+Y) chemical fingerprints. Here we present major and trace element analysis of 42 samples from the Guanmenshan Formation which are pure marine chemical sediments and use the data to trace the seawater composition during 2.3-1.85Ga. 15 least altered dolomicrite samples (>600m strata) have ∑REE values of 0.739-4.175ppm (2.414±1.184ppm) and the Y/Ho ratios of 34.5-56.6 (44.1±5.7). They show uniform positive La_(SN)/La_(SN)* (1.04±0.27) and Gd_(SN)/Gd_(SN)* (1.64±0.40) anomalies, and notable LREE depletions indicated by Nd_(SN)/Yb_(SN) values of 0.24-0.92 (average 0.56±0.19). These features are consistent with the geochemistry of well-oxygenated, shallow ambient seawater, and suggest that these samples provide a robust record of the primary REY signature of seawater during the Lomagundi Event. The REY patterns of 15 silicified dolomites/marbles (locally with veinlets) from the Pb-Zn mining camps in the region, with average Eu_(CN)/Eu_(CN)*=1.56±0.95, are identical to those of high-temperature hydrothermal fluids (>250°C), characterized by a flat pattern and marked positive Eu anomalies, indicating that these rocks were subjected to metasomatism by hydrothermal fluids. The Guanmenshan Formation shows average Ce_(SN)/Ce_(SN)* of 0.93±0.09 and Sm_(CN)/Yb_(CN) of >1 which are higher than those of the Archean (>2.33Ga) chemical sediments (generally <1), suggesting that the REY geochemical characteristics of the carbonates from our study area were dominantly controlled by the nature of atmosphere-hydrosphere system, such as fO_2 and pCO_2. The REY in the dolomicrite were mainly sourced from fluxes of solutes from terrestrial weathering, and also from seafloor hydrothermal processes on a subordinate scale. The Guanmenshan dolomicrites have Eu_(SN)/Eu_(SN)* values of 1.34-2.55, i.e. around 1.53, indicating that they were deposited during 2.33-2.06Ga, as the Eu_(SN)/Eu_(SN)*≈1.53 can be used as a proxy for the 2.33-2.06Ga marine chemical sediments. Our study shows that the Guanmenshan Formation was formed at a critical turning point in Earth history when the global atmosphere-hydrosphere system witnessed a dramatic change from reducing to oxidizing conditions.