THE U-Pb CHRONOLOGICAL EVIDENCE FOR THE VOLCANIC ROCKS OF TANGJIATUN FORMATION IN EAST HARBIN, HEILONGJIANG PROVINCE

The Tangjiatun Formation, dominated by strong schisteous acidic and acidic-intermediate volcanic rocks, with minor intermediate volcanic rocks and metamorphosed normal sedimentary rocks, are widely exposed in East Harbin, Heilongjiang province. The volcanic rocks contain SiO₂ of 61.50%-68.96%, TiO₂ of 0.61%-0.88%, Fe₂O₃ of 1.21%-4.24%, MgO of 0.30%-2.08% and CaO of 0.77%-4.57%, with Mg^# of 4-25, Al₂O₃=15.12%-16.99%, K₂O=2.68%-5.2%. The alkaline content is rich, with Na₂O/K₂O=0.94-2.18, Na₂O + K₂O=7.05%-9.99%. The total REE contents are 140.81×10^-6-276.36×10^-6, with LREE/HREE ratio of 7.72-9.70. The rocks are enriched in LREE but depleted in HREE, with δEu=0.77-1.03, i.e. negative anomaly to slightly positive anomaly. The mode of REE patterns shows right-inclined curves. The trace elements of the rocks are rich with LILEs (Zr and Ba) and depleted with HFSEs (Nb, Ta and Ti), Sr and P, showing the characteristics of bimodal volcanic rocks. The LA-ICP-MS U-Pb dating result of the volcanic rocks yields the weighted average age of 300±1 Ma, indicating the forming age of Late Carboniferous Epoch, which was an extensional environment.