GEOCHEMICAL CHARACTERISTICS AND EVOLUTION OF REE IN THE EARLY PRECAMBRIAN SEDIMENTS - EVIDENCE FROM THE SOUTHERN MARGIN OF THE NORTH CHINA CRATON

The REE characteristics of both the chemical and the clastic sediments from six early Precambrian terrains at the southern margin of the Nor th China craton shout that the sedimentary Eu-depletion started front about 2300 Ma and the values of Sigma REE of the sediments less than 2 300 Ma are higher than those of the sediments more than 2300 Ma. These phenomena cart be logically ascribed to considering the changes in su rficial environment and interpreted by using the new HSAB (hard and so ft acids and bases) theory. We argue that the sediments deposited in a noxic environment before 2300 Ma are characterized by positive Eu-anom aly, low Sigma REE and occasionally high La-N/Yb-N; whereas the sedime nts in oxidic condition after 2300 Ma show Eu-depletion, high Sigma RE E and low La-N/Yb-N. Possibly, rite surficial environment (including a tmosphere) turned to be oxidic at about 2300 Ma. Hence, rite REE can b e used to indicate the nature of the environment.