内蒙古中东部蘑菇气地区玛尼吐组火山岩构造背景

Volcanic rocks tectonic setting of the Manitu Formation in the Moguqi area,Mid-Eastern Inner Mongolia

内蒙古中东部蘑菇气地区位于大兴安岭火山岩带中部,通过对玛尼吐组火山岩进行系统的岩相学研究和地球化学特征分析可知,玛尼吐组火山岩的岩石类型主要为粗面岩、安山岩和玄武安山岩,属于亚碱性、高钾钙碱性火山岩系列。SiO2与TiO2,Al2O3,Fe2O3T,MgO,CaO和P2O5呈较好的负相关,与Na2O和K2O呈较好的正相关,暗示了岩浆演化过程中发生了分离结晶作用。稀土配分曲线呈“右倾”型式,轻重稀土元素分馏明显,La N/Yb N=7.76~15.7,∑REE=126~317 ppm,富集轻稀土元素(LREE)(LREE=114~292 ppm)和大离子亲石元素(LILE),亏损高场强元素(HFSE),具明显负Eu异常(δEu=0.72~0.97)。对其地球化学特征综合研究显示,玛尼吐组火山岩岩浆来源于下地壳,有部分幔源物质的加入,为壳-幔混合型岩浆。该组火成岩形成于岩石圈伸展减薄的背景下,与蒙古-鄂霍茨克洋闭合,碰撞造山后的伸展环境有关。

The Mogoqi area of mid-eastern Inner Mongolia is located in the central volcanic belt of the Daxing’anling.A detailed study on petrology and geochemistry has been carefully taken.Volcanic rocks from Manitu fomation are mainly trachyte,andesite and basaltic andesite.They belong to sub-alkaline and high potassic calc-alkaline series.SiO2 and TiO2,Al2O3,Fe2O3T,MgO,CaO,P2O5 have positive correlations while SiO 2 and Na 2 O,K 2 O have negative correlations respectively.These reflect a fractional crystallization process of magma.The REE patterns show a right-dipping distribution and significant fractionation of LREE(Light Rare Earth Element)from HREE(Heavy Rare Earth Element),La N/Yb N=7.76~15.7,∑REE=126~317 ppm.They contain high LREEs(LREE=114~292 ppm)and LILEs and low HFSEs,while they are characterized by negative Eu anomalies(δEu=0.72~0.97).To conclude,the geochemical data suggest that the studied volcanic rocks derive from the lower crust and manlte-derived material is added.The magma is of crust-mantle mixed type.Volcanic rocks from Manitu formation formed in the tectonic setting of lithosperic extended attenuation and are related to the tectonic evolution of the Menggu-Okhostk Ocean.