SHRIMP ZIRCON U-Pb GEOCHRONOLOGY AND GEOCHEMISTRY OF CHAQIN MAFIC-ULTRAMAFIC INTRUSION,INNER MONGOLIA

The Chaqin mafic-ultramafic complex is located along the northern margin of the North China Craton,composed of pyroxenites,gabbros,and with extensive hydrothermal alteration and Cu-Pt-Pd mineralization.Hydrothermal alteration is spatially related to the mineralization,near to the Cu-Pt-Pd ore bodies.According to major elements analysis,the altered pyroxenites contain a higher level of K,Na,H2 O,fO2 and a lower level of Ca,P than those of the fresh ones.Thus,it is concluded that Cu-Pt-Pd mineralization of the complex is likely to be the result of post-magmatic hydrothermal fluids action with rich K,Na,H2 O and high fO2.In addition, SHRIMP zircon U-Pb dating indicated that the emplacement age of Chaqin complex is～415Ma, equivalent to early Devonian.Based on the latest regional geological data,it is suggested that the northern margin of North China Craton has been strongly modified as early as the Devonian,and the Chaqin complex provides new evidence for this tectonic-magmatic event.