The Longjiang Formation in northern Da Hinggan Mountains is composed mainly of dacite and rhyolite with minor andesite.the LA-ICP-MS zircon U-Pb dating of these rocks yielded ages of 122.2～126.5Ma,corresponding to Early Cretaceous.These volcanic rocks are of high potassium calc-alkaline series possessing such geochemical characteristics as high ∑REE,remarkable (La/Yb)N values and negative Eu anomalies.In the spidergram,the rocks show pronounced enrichment of Rb,Ba,Th,U,K and depletion of Nb,Ta,P,Ti,much identical with the characteristics of the upper crust.Most of these rocks have typical adakitic characteristics,as shown by the data 300＜Sr＜700,Yb=2.24 and Y=18.36 (on average); the values of Y/Yb (9.15 on average),(Ho/Yb)N (0.98 on average) and δEu(0.71 on average) suggest that the magmas should have been derived from partial melting of the granulite facies of the thickened lower crust under a high pressure.According to regional simultaneous volcanic geochemical features,the volcanic rocks were probably derived from a collision setting of the Mongol-Okhotsk orogenic belt.%大兴安岭北段龙江盆地中生代火山岩主要为一套中酸性岩石,岩石化学成分显示其主要为英安岩与流纹岩,少量安山岩.各类岩石的LA-ICP-MS锆石U-Pb定年结果显示,其形成于122.2～126.5Ma之间的早白垩世.火山岩为高钾钙碱性系列岩石,稀土元素总量较高(∑REE=103.22×10-6～249.29×10-6),轻重稀土元素分馏明显((La/Yb)N=6.29～25.89),负Eu异常(Eu/Eu* =0.48～0.84).富集大离子亲石元素Rb、Ba、Th、U、K和LREE,亏损高场强元素Nb、Ta、P、Ti;火山岩稀土元素与微量元素标准化配分曲线与地壳配分曲线近乎一致,为一套壳源岩浆系列火山岩；大部分岩石300＜Sr＜700；Yb平均为2.24,Y平均为18.36,符合埃这克岩型岩石标准;岩石Y/Yb平均为9.15,(Ho/Yb)N=0.98,而弱负Eu异常(平均值δEu=0.71).地球化学特征说明,岩浆主要是增厚(高压)基性麻粒岩下地壳部分熔融的结果.结合区域上同期火山岩的资料,龙江盆地中生代火山岩可能是蒙古-鄂霍茨克洋闭合碰撞造山构造背景下加厚下地壳部分熔融的产物.
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