华北克拉通南缘早前寒武纪沉积物稀土地球化学特征及演化

REE GEOCHEMICAL CHARACTERISTICS AND EVOLUTION OF EARLY PRECAMBRIAN SEDIMENTS: EVIDENCE FROM THE SOUTHERN MARGIN OF THE NORTHERN CHINA CRATON

华北克拉通南缘六个早前寒武地体的31件沉积物样品,其稀土特征及演化规律是:2300Ma后沉积物的∑REE高于2300Ma前者,沉积物Eu亏损始于2300Ma左右。此规律可以通过考虑沉积环境的改变,用软硬酸碱理论来合理地加以说明,2300Ma前还原背景的沉积物∑REE低,Eu正异常,2300Ma后氧化环境的沉积物∑REE高,Eu负异常。大气圈在2300Ma时由还原性质变为氧化性质。

The REE characteristics of sediments from six Early Precambrian terrains at the southernmargin of the North China Craton show that the sedimentary Eu-depletion started at about2 300 Ma and the ∑REE values of the sediments older than 2 300 Ma in age are higher than thoseof the sediments younger than 2300 Ma. These phenomena can be interpreted reasonably byconsidering environmental changes and by applying the theory of newly developed HSAB (hardand soft acids and bases). It is suggested that the sediments deposited in the anoxic environmentsbefore 2 300 Ma are characterized by positive Eu-anomaly and low ∑REE; whereas those underoxidic conditions after 2 300 Ma by Eu-depletion and high ∑REE. It is expected that the atmo-sphere became oxidic at about 2 300 Ma ago.