辽东辽河群大石桥组碳酸盐岩稀土元素地球化学及其对Lomagundi事件的指示

2.33～2.06Ga期间发生了全球性大气圈充氧作用及其相关的占δ~(13)C_(carb)正异常,被称为Lomagundi事件.2.2～2.174Ga的辽河群大石桥菱镁矿及其围岩显示了δ~(13)C_(carb)正异常,是运用REY(REE+Y)指纹技术研究Lomagundi事件的良好对象.本文研究表明,6件白云岩围岩样品∑REE为0.988×10-6～2.744×10-6;Y/Ho比值为37.9～49.4(平均42.5±4.7);(La/La~*)_(SN)为1.075±0.317,(Gd/Gd~*)_(SN)为1.390±0.166,均为正异常;HREE富集(Nd_(SN)/Yb(_SN)=0.38～0.78).所有上述特征与现代海水REY配分模式一致,表明这些样品记录了Lomagundi时期海水的REY特征.6件菱镁矿样品∑REE为4.549±2.239,高于围岩白云岩;HREE弱亏损(Nd_(SN)/Yb_(SN)=1.141±0.265),Y/Ho平均值为40.2±3.2;(La/La~*)_(SN)为0.657～1.149(平均0.919±0.203),(Gd/Gd~*)_(SN)=1.036±0.081,正异常程度弱于白云岩,但仍显示以海水来源为主.矿体顶板滑石白云岩∑REE含量最高(10.758);页岩标准化稀土配分模式为平坦型,(Eu/Eu~*)_(SN)正异常高达1.97,Gd和Y正异常最小,Y/Ho比值最低(31.3),(Nd/Yb)_(SN)为0.89,显示受后期热流体交代影响.菱镁矿(Sm/Yb)_(CN)值(2.61±0.45)高于白云岩(1.19±0.27),指示海水由深变浅,大石桥菱镁矿及其围岩白云岩REY主要来自陆源溶解物,洋底热液贡献微弱,制约REY特征的主导因素为大气圈-水圈的性质,如fO_2,pCO-2等.白云岩和菱镁矿(Ce/Ce~*)_(SN)值平均分别为1.11±0.13和1.04±0.08,表明2.2～2.174Ga期间大陆风化作用加剧,海水呈碱性,pH值＞8.2.2.33Ga前的化学沉积物(Eu/Eu~*)_(SN)＞1.53,2.06Ga后的化学沉积物(Eu/Eu~*)_(SN)＜1.53,大石桥组白云岩和菱镁矿(Eu/Eu~*)_(SN)均值分别为1.44±0.11和1.58±0.20.表明2.2～2.174Ga时海相沉积物(Eu/Eu~*)_(SN)≈1.53,2.33～2.06Ga是地球水圈-气圈系统由还原向氧化转变的关键时期.

Rare earth element geochemistry of carbonates of Dashiqiao Formation, Liaohe Group, eastern Liaoning province: Implications for Lomagundi Event

The worldwide 2.33～2.06Ga positive δ~(13) Ccarb excursion is related to the Great Oxidation Event(GOE)of the atmosphere and called as Lomagundi Event.The Dashiqiao magnesite ore-bed and its dolomite hostrocks of the Dashiqiao Fm.,Liaohe Group,are characterized by positive δ~(13) C_(carb) excursion and suitable for the study of the Lomagundi Event using REY(rare earth element and yttrium,REE+Y)chemical fingerprints.Our results show that the PAAS(Post-Archean Average Shale)normalized REY patterns of six dolomite samples,whose ∑ REE range from 0.988×10~(-6)～2.744×10~(-6) and Y/Ho ratios range from 37.9～49.4 with average of 42.5±4.7,show uniform positive(La/La~*)_(SN)(1.075±0.317)and(Gd/Gd~*)_(SN)(1.390±0.166)anomalies and notable HREE enrichment indicated by(Nd/Yb)_(SN) = 0.38～0.78(average 0.49±0.15).These features are consistent with the geochemistry of well-oxygenated,shallow ambient seawater,and suggest that these samples substantially record the primary REY signatures of seawater during the Lomagundi Event era.The ∑ REE of six magnesite samples,in average of 4.549×10-6±2.239×10~(-6),are higher than those of dolomite hostrocks;and the Y/Ho ratios,averaging 40.2±3.2,are slightly lower than the dolomites.The magnesites show slight depletion in HREE(Nd~(SN)/Yb~(SN) = 1.141±0.265),and positive(La/La~*)_(SN)(average 0.919±0.203)and(Gd/Gd~*)_(SN)(average 1.036±0.081)anomalies which are weaker than those of dolomites and still originated from seawater.The mega-cylindrical talc-bearing dolomite in the hanging-wall shows highest ∑ REE(10.758×10~(-6))content and a flat PAASnomalised REY pattern,with positive(Eu/Eu~*)SN anomaly up to 1.97,the lowest positive(Gd/Gd~*)SNand Y anomalies,and the lowest Y/Ho(31.3)and(Nd/Yb)_(SN)(0.89).This unique talc-bearing dolomite was reworked by hydrothermal.The(Sm/Yb)CNvalues of magnesites average 2.61±0.45,higher than those of dolomites(1.19±0.27 in average),which indicates that the seawater depth became shallower during sedimentation of the Dashiqiao Formation,and that the dominant factor controlling the REY geochemical magnesite and dolomite were mainly sourced from fluxes of solutes from terrestrial weathering,whereas the contribution from marine hydrothermal processes was negligible.The(Ce/Ce~*)~(SN) of dolomites and magnesits range 0.95-1.25 and 0.94～1.16,with averages of 1.11±0.13 and 1.04±0.08,respectively.This suggests that weathering of continental crust was strong during 2.2～2.174Ga,and seawater might be alkaline with pH value above 8.2.Considering that the pre-2.3Ga and post-2.06Ga chemical sediments have(Eu/Eu~*)-(SN) values higher and lower than 1.53,respectively,the(Eu/Eu~*)_(SN) values of the Dashiqiao dolomites (1.44±0.11)and magnestes(1.58±0.20)indicate that the(Eu/Eu~*)_(SN)≈1.53 can be used as one of criteria for 2.2～2.174Ga marine chemical sediments;and that the 2.33～2.06Ga period was a critical turnpoint that the Earth's atmosphere-hydrosphere system sharply changed from reduction to oxygenation.