内蒙古扎兰屯北部地区早白垩世酸性火山岩锆石U-Pb年代学和地球化学研究

Zircon U-Pb geochronology and geochemistry of the Lower Cretaceous felsic volcanic rocks in the northern Zhalantun region,Inner Mongolia

本文对大兴安岭扎兰屯北部地区早白垩世酸性火山岩进行了锆石U-Pb年代学、岩石地球化学以及Sr-Nd同位素的研究,以便对其岩石成因和区域动力学背景予以制约。通过锆石LA-ICP-MS U-Pb定年,获得大兴安岭北部地区满克头鄂博组流纹岩(D14208)加权年龄141±3Ma(早白垩世),玛尼吐组粗面英安岩(D14210)加权年龄135±5Ma(早白垩世)。在地球化学研究方面,它们均为钙碱性系列,SiO2分别为76.41%~78.01%和64.71%~65.63%,MgO分别为0.04%~0.22%和1.12%~1.35%,Mg#值分别为12~42和32~38,富集大离子亲石元素、轻稀土元素,亏损高场强元素、重稀土元素,Eu/Eu*分别介于0.40~0.52和0.82~0.95之间。Rb/Sr比值分别变化于2.42~3.21和0.12~0.15之间,K/Rb比值分别变化于284~290和284~306之间,Nb/Th比值分别变化于2.35~3.82和2.60~3.63之间。结合两个样品εNd(t)值分别为1.57~2.22和1.72~2.24,二阶段模式年龄(tDM2)分别为751~804Ma和743~786Ma,认为岩石应来源于地壳。综合东北地区前人的同时代岩浆-构造成果,认为岩石形成于蒙古-鄂霍茨克洋闭合所引起的伸展环境。

This study carried out zircon U-Pb dating, lithogeochemistry and Sr-Nd isotope analysis for the Lower Cretaceous felsic volcanic rocks in the northern Zhalantun region of Great Xing’an Range, NE China, with the aim to restrain the petrogenesis and regional dynamic background. Zircon LA-ICP-MS U-Pb dating yielded an weighted age of 141±3 Ma(Early Cretaceous) for rhyolite rock of the Manketouebo Formation(D14208) and an weighted age of 135±5 Ma(Early Cretaceous) for the trachy dacite rocksof the Manitu Formation(D14210). Geochemcially, two groups of the rocks belong to calc-alkaline series, with SiO2 values of 76.41%~78.01% and 64.71%~65.63%, MgO values of 0.04%~0.22% and 1.12%~1.35%, Mg# numbers of 12~42 and 32~38, enrichment in LILE and LREE, depletion in HFSE and HREE, and Eu/Eu* values of 0.40~0.52 and 0.82~0.95, respectively. In addition, the rocks have Rb/Sr ratios of 2.42~3.21 and 0.12~0.15, the K/Rb ratios of 284~290 and 284~306,the Nb/Th ratios of 2.35~3.82 and 2.60~3.63. εNd(t) values of 1.57~2.22 and 1.72~2.24, and tDM2 values of 751~804 Ma and 743~786 Ma indicate that the primary magma derived from partial melting of the crust. Together with previous studies of contemporaneous magmatism and tectonism in NE China, the present results suggest that the generation of the rocks was related to the closure of the Mongol-Okhotsk Ocean.