大兴安岭新林区中生代流纹岩年代学、地球化学特征及其地质意义

CHRONOLOGY,GEOCHEMISTRY AND GEOLOGICAL IMPLICATION OF THE MESOZOIC RHYOLITES IN XINLIN AREA,DAXINGANLING MOUNTAINS

大兴安岭北段新林区满克头鄂博组火山岩主要由流纹岩、英安岩及其凝灰岩组成,LA-ICP-MS锆石U-Pb年龄为154.5±0.68 Ma,时代为晚侏罗世.白音高老组火山岩主要由流纹岩及其凝灰岩组成,LA-ICP-MS锆石U-Pb年龄为121.12±0.5 Ma,时代为早白垩世.满克头鄂博组火山岩属于高钾钙碱性系列-钾玄质系列.稀土元素配分模式图呈右倾型,轻重稀土分馏明显,具有中等的Eu负异常.微量元素显示相对富集大离子亲石元素K、Rb、Ba、Sr和轻稀土元素,亏损高场强元素Nb、Ta、Ti、P等.白音高老组火山岩属于高钾钙碱性系列.稀土元素配分模式图呈右倾型,轻重稀土分馏明显,明显Eu负异常.微量元素显示相对富集大离子亲石元素K、Rb和轻稀土元素,Ba、Sr明显亏损,高场强元素Nb、Ta,Ti强烈亏损.结合地球化学特征和前人研究资料,认为新林区满克头鄂博组火山岩来源于下地壳玄武质岩浆的结晶分异作用,与蒙古-鄂霍次克洋闭合造山后的伸展环境有关.白音高老组火山岩源于下地壳斜长角闪岩的非理想熔融,与太平洋板块俯冲影响的伸展作用有关.

The volcanic rocks of Manketouebo Formation in Xinlin area, northern Daxinganling Mountains are mainly composed of rhyolites, dacites and tufts, while those of Baiyingaolao Formation mainly consist of rhyolites and tufts. The zircon U-Pb dating results show they are formed in the Late Jurassic （154.5±0.68 Ma） and Early Cretaceous （121.12±0.5 Ma） respectively. Both are characterized by obvious fractionation of LREEs and HREEs with right-dipping REE patterns. The former, belonging to high-k calc alkaline-shoshonite series, has moderate negative Eu anomaly, with enrichment of LILEs （K, Rb, Ba and St） and LREEs and depletion of HFSEs （Nb, Ta, Ti and P）. The latter, being high-k c alc-alkaline series, features evident negative Eu anomaly, with enrichment of LILEs （K, Rb） and LREEs, obvious depletion of Ba and Sr and severe depletion in HFSEs （Nb, Ta and Ti）. Combined with the geochemical characteristics and previous researches, it is believed that the Manketouebo volcanic rocks derived from crystal fractionation of basaltic magma in lower crust are related to the post-orogenic extensional environment after the closure of Mongolia-Okhotsk Ocean, and the Baiyingaolao volcanic rocks are formed by partial mehing of amphibolites in lower crust and the extension induced by subduction of Pacific Plate.