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

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

2.33～2.06Ga期间发生了全球性大气圈充氧作用及其相关的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_(carh)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,大石桥菱铁矿及其围岩白云岩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铁矿(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。

The worldwide 2.33～2.06Ga positiveδ^(13) C_(carb) 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 positiveδ^(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 d...