| Wanheyong volcanic basin is located in the southernmost of the south of the Great Hinggan range, and northern area of the north margin accretion zone of North China Plate, and it is near by the Xar Moron fault zone. The basement of the basin is obviously controlled by the near EW Paleo-Asian Ocean tectonic regime, and the cap rock is modified by the NNE Pacific tectonic regimes, so it is inevitably influenced by the Paleo-Asian Ocean tectonic regime and the Pacific tectonic regimes in the history of its tectonic activity. Volcanic rocks which are main alkaline and high-K calc- alkaline rhyolites in Late Mesozoic widely develop in this district, and it extends in NE direction.Based on the predecessors’ data, Xiamajia-Liujiayingzi rhyolites in Wanheyong volcanic basin are the main object of study. By using the method of SHRIMP zircon U-Pb age, we determine its rock-forming age; by the actual measurements of field geological section and the investigation of rock association, we confirm its geological age; finally, from the analysis of the petrology and lithogeochemistry and combining the existing data, we discuss its origin, material source and tectonic setting.There is a combination of intermediate-felsic volcanic rocks and pyroclastic rock in the research area. Combining the sign of purple clastic rock combination and the age data of Xiamajia-Liujiayingzi rhyolite, we put the purple clastic rock combination which belonged to Mankeebo group in the earlier days into Tuchengzi group. The upper part belongs to Xinmin group, and the decender belongs to Xingan group.SHRIMP zircon U-Pb age of the Xiamajia-Liujiayingzi rhyolite is 159.6±1.5Ma, and it is indicated that it existed volcanic-magmatic events in Late Mesozoic in Wanheyong volcanic basin, and the time of magmatic activities is Late Jurassic.The data of rock geochemistry shows that the alkaline rhyolites are characterized by high in silicon, rich in alkaline, poor in calcium and magnesium, and they belong to alkaline series rovks in the TAS diagram. The DI value of differentiation index is between 89.83 and 96.32, and it reflects that during the process of magma evolution, the magma went through the high degree fractional crystallization. The total amount of REE is relatively high, and the ∑REE value is between 264.44×10-6 and 540.98×10-6. REEs fractionate evidently. The(La/Yb)N ratios are between 5.34 and 14.46, and the ∑LREE/∑HREE ratio is varying range from 7.83 to 10.33. The HREE is flat, and the chondrite normalized REE fractionation patterns are right declined, with strong negative Eu anomaly and less obvious positive Ce anomaly. The trace elements show that alkaline rhyolites enrich Rb, Zr, Th, Ga, Y obviously, and deplete Ba, Sr, Ti, P, and they are consistent with low Ba-Sr rhyolites in Great Hinggan Range.On the K2O+Na2O, K2O/MgO, Zr, Nb-10000Ga/Al diagrams, and Fe OT/MgO,(K2O+Na2O)/Ca O-Zr+Nb+Ce+Y diagrams, all the samples of Xiamajia-Liujiayingzi rhyolites belong to A-type granite area. The values of Rb/Sr, Ti/Zr, Ti/Y and Th/La are 1.52~10.00, 0.01~0.03,0.28~0.39 and 0.18~1.45, and they are all within the scope of the shell source magma; On the Ce/Nb-Th/Nb diagrams, due to located in the continental crust area, their source came from crust; On the R1-R2, Nb-Y-3Ga and Nb-Y-Ce diagrams, all the samples belong to post-orogenic granites area; the rhyolites may form in the tectonic environment of plate internal post-tectonic stretching tension because all the samples are lacated in the WPG area on the Y-Nb and(Y+Nb)-Rb diagrams.In conclusion, Xiamajia-Liujiayingzi rhyolites belong to Xinmin group, and they formed in Late Jurassic. The source may be partial melting of crust material, and may form in the tectonic environment of plate internal post-tectonic stretching tension. The form of the Xiamajia-Liujiayingzi may be related to the evolution of Mongol-Okhotsk Ocean. |
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