Geochemical characteristics and zircon U-Pb geochronology of the Baijiafen augamite granite in Anshan of the North China platform and its significance

The genesis and chronology of Archaean TTG rocks is the key to understanding the problems concerning the beginning of the pre-plate tectonics and plate tectonics and their evolution process. It is an important sign of continental crust accretion and one of the hotspots and difficulties in the study of Archaean geologic bodies. Geochemical characteristics and zircon U-Pb dating of Baijiafen intrusion were studied in Anshan area. The data show that the rock mass has the characteristics of orthogneiss, with high Si, Na content, low TFeO, MgO, CaO content, low light and heavy REE content and some changes. The REE content of rocks is low, REE is 12.6×10^-6~119.34×10^-6 and LREE/HREE ratio is 8.98~13.11. Trace elements of rocks are characterized by enrichment of K, Rb and Ba and depletion of Nb, Ce, Zr, Sm, Ti, Y and Yb; K/Rb mean ratio is 229.37, Rb/Sr mean ratio is 0.58, Ba/Sr mean ratio is 3.28, and Y/Nb mean ratio is 3.36, suggesting that the rock mass is the product of magmatism, with a part being the product of intracrustal recycling or being affected by continental crustal materials. The weighted average age of ²⁰⁷Pb/²⁰⁶Pb is 3816±14Ma. The zircon has clear oscillating zonal structure. Th/U ratios of all core zircon analytical points range from 0.05 to 0.54. It has the characteristics of magmatic zircon and represents the crystallization age of Baijiafen trondhjemite. The age data of 3613~3651Ma and 3331Ma represent a magmatic event in the early Archaean and a thermal event in the important period of continental crust accretion.