GEOCHRONOLOGY AND GEOCHEMISTRY OF THE EARLY CARBONIFEROUS GRANODIORITES IN HALADAOKOU AREA OF CHIFENG CITY: Geological Implication

The geochronology of the granodiorites in Haladaokou area of Chifeng City, Inner Mongolia, is studied by zircon LA-ICP-MS U-Pb dating. The test result of 352.4-355.9 Ma indicates that the rocks were formed in the early stage of Early Carboniferous, which provides new data for the research of Early Carboniferous magmatic activity on the northern margin of North China Plate. Geochemically, the granodiorites are generally poor in Si and Al, with high total alkali content and slight Eu depletion, belonging to the Al-unsaturated, high-K calc-alkaline series. Moreover, the granodiorites have high Sr content and low MgO, Y and Yb contents, enrichment of LREEs and depletion of HREEs and HFSEs, with the Sr/Y ratio of 34.62-48.43 and (La/Yb)_N ratio of 15.31-17.14, showing the characteristics of C-type adakite. The high K₂O, Cr and Ni contents and low Mg^# value reveal they cannot be formed by the melting of delamination of lower crust materials, but by the partial melting of thickened lower crust mafic rocks. The characteristics of trace elements indicate that the granodiorites were formed in the active continental margin arc environment. Therefore, the study area was in the geotectonic setting of southward subduction of Paleo-Asian Ocean plate during the early stage of Early Carboniferous.