College of Earth Sciences, Jilin University, Changchun 130061, China

Taking the mudstone cap rocks in the 2nd and 3rd Members of Qingshankou Formation in Hada Mountain of Songliao Basin as an example, the authors studied the major elements and trace elements of rare earths in mudstone samples by combining the petrological and geochemical methods. The results show that: ��Mudstone has high amount of rare earth elements (104.99×10^-6��281.88×10^-6), high ∑ LREE/HREE ratio(6.50��9.96),and REE distribution patterns show enrichment in light rare earth elements and depletion in heavy rare earth element, with negative Eu anomaly(0.88��1.16); ��Source rock types are mainly sedimentary rocks and intermediate-basic volcanic rocks, the origin of potassium metasomatism of source rocks is likely related to the conversion of clay minerals from mixed-layer illite/smectite to authigenic illite; ��The revised CIA value(44.75~72.24)of provenance mudstone is low, suggesting that the provenance experienced weak weathering, and Sr/Cu value indicates that provenance was controlled by dry and hot climate for a long time.