ORIGIN OF CENOZOIC BASALTS FROM JINBOHU, NORTHEAST CHINA AND CHEMICAL CHARACTERISTICS OF THEIR MANTLE SOURCE——Sr-Pb ISOTOPE AND TRACE ELEMENT EVIDENCE
Liu Beiling Chen Yuwei Zhu Bingquan(Institute of Geochemistry, Academia Sinica)
The Cenozoic basalts in the Jinbohu area, Northeast China have been studied systematically for Sr and Pb isotopes, trace elements and major elements. Various lines of evidence show that there are two types of basalts differing in source and genesis in this area. One is Na-alkali ba-salts ranging from Neogene to Hoiocene and formed in a shear environment. These basalts have Sr and Pb isotopic compositions similar to those of the primitive mantle except for a few samples which show a slight deviation in isotopes or/and trace elements because of fractional crystallization or AFC during magma upwelling. The other is quartz-tholeiites emplaced during the Eocene and derived from the lithospheric mantle with higher Sr but lower Pb ratios (87Sr/ 86Sr 0.7050, 206Pb/204Pb-17.0). In addition, it is found that the isotopic characteristics of the Na-alkali basalts in the Jinbohu area are distinct from those in the Shando and other areas.