THE LATE JURASSIC I-TYPE GRANITE FROM BAOHETUN OF TUQUAN REGION IN EASTERN INNER MONGOLIA: Geochemistry, Petrogenesis and Geologic Implication
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摘要: 内蒙古东部突泉地区宝合屯侵入体出露于大兴安岭中段东坡，锆石U-Pb测年结果显示花岗斑岩年龄为145.9±3 Ma，形成于晚侏罗世.岩石地球化学数据显示该侵入体高硅（Si2O含量为69.62%~77.43%，平均值为74.14%），富碱（K2O+Na2O含量为6.93%~8.94%，平均含量为8.27%），富铝（Al2O3含量为12.27%~14.46%，平均值为13.35%），贫Ti、Fe、Mg，A/CNK值大于1.1，属于高钾钙碱性系列；微量元素富集Th、K、Hf、Rb，而亏损Ba、Nb、Ta、Sr、P、Ti，稀土元素富集LREE （（La/Yb）N=2.2-13.8），并具明显的负铕异常（δEu=0.1~0.63）.这些特征表明突泉盆地花岗斑岩为过铝质高分异的I型花岗岩.在构造判别图解上位于造山期后区，说明该区花岗岩属造山后花岗岩类.结合前人资料认为：宝合屯花岗斑岩与蒙古-鄂霍次克洋闭合后的伸展构造环境有关.Abstract: The Baohetun intrusive pluton in Tuquan region of eastern Inner Mongolia is outcropped in the eastern slope of the middle section of Daxinganling Mountains. The granite porphyry shows a zircon U-Pb age of 145.9±3 Ma, formed in Late Jurassic. The lithogeochemical data reveal that the intrusion is characterized by high Si (69.62%-77.43% of SiO2, averagely 74.14%), rich alkali (6.93%-8.94% of K2O+Na2O, averagely 8.27%), rich Al (12.27%-14.46% of K2O+Na2O, averagely 13.35%) and poor Ti, Fe and Mg, with A/CNK>1.1, belonging to high-K calc-alkaline series. The trace elements feature the enrichment of Th, K, Hf and Rb, and depletion of Ba, Nb, Ta, Sr, P and Ti, while the REEs show enriched LREEs with (La/Yb)N=2.2-13.8 and obvious negative Eu anomaly (δEu=0.1-0.63). These features above indicate that the granite porphyry belongs to peraluminous high fractioned I-type granitoids. The samples are plotted into the post-orogeny field in the tectonic discrimination diagram, which illustrates the Baohetun granite is classified as postorogenic granitoids. Combined with previous study, it is believed that the Baohetun granite porphyry is probably related with the extensional tectonics after the closure of Mongolia-Okhotsk Ocean.
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