PETROGENESIS OF THE EARLY JURASSIC TRACHYANDESITE IN WUFENG AREA OF YANBIAN, JILIN PROVINCE

Study on the petrology, LA-ICP-MS zircon U-Pb dating and whole rock geochemistry of the trachyandesite in Wufeng area of Yanbian, Jilin Province in Northeast China, shows that the SiO₂ content is 58.33%-61.36%, K₂O content 1.61%-3.28% and total alkali content 6.77%-10%, belonging to the medium potassium-high potassium calc-alkaline series. The weighted mean age of magmatic zircon ²⁰⁶Pb/²³⁸U is 175.52±2.7 Ma, which is consistent with the subduction time of Early Jurassic paleo-Pacific plate. The trachyandesite is characterized by relative enrichment of LILEs(K, Ba and Rb) and strongly incompatible elements (Th and U), and depletion of HFSEs (Nb, Ta and Ti), with high LREE/HREE ratio, and enriched LREEs and depleted HREEs, indicating that the magma may originate from the partial melting of mantle and its source area is metasomatized by the subducted shell source materials. From the above, it is believed that the trachyandesite was formed in the active continental margin of oceanic subduction system and was the magmatic response to the subduction of Early Jurassic paleo-Pacific plate.