CHEMICAL COMPOSITION AND CHARACTERIZATION OF CHEMICAL WEATHERING OF LATE TERTIARY RED CLAY IN XIFENG,GANSU PROVINCE

Geochemical studies have been conducted on the red clay samples from Xifeng,Gansu Province,China.Results show that these samples are enriched in carbonate and depleted in other elements such as Si,Al,and Fe.To eliminate the effect of the carbonate fraction,five samples were leached by 1mol/L acetic acid.It can be found that the acid-insoluble fractions of the samples have similar compositions,indicating same sources.The distribution patterns of rare earth elements and trace elements indicate the similarity of eolian origin between the red clay and Quaternary loess and paleosols.Further investigations show that the red clay experinece the incipient stage of chemical weathering characterized by removal of Na and Ca.Several geochemical parameters such as CIA values,Na/K and Rb/Sr ratios in the acid insoluble fractions reveal that the red clay has undergone more intense chemical weathering than loess and paleosols,suggesting that the paleoclimate on the Loess Plateau has evolved from warm and moist conditions during Late Tertiary to dry and cold conditions during Quaternary.