内蒙古锡林浩特毛登牧场大石寨组细碧-角斑岩系地球化学特征、锆石U-Pb年龄及地质意义

Zircon U-Pb Age of the Spilite-Keratophyre Sequence of the Dashizhai Formation in Maodeng of Xilinhaote,Inner Mongolia and Its Geological Significance

本次1∶5万区域地质调查工作在内蒙古锡林浩特毛登牧场地区大石寨组地层中发现了细碧-角斑岩系,单颗粒锆石LA-ICP-MS U-Pb法获得其年龄为(287.4±1.7)Ma(MSWD=0.63),形成于早二叠世早期。细碧岩具有高SiO2、Al2O3(>16%)、Na2O、TiO2(>1.3%)含量,低CaO、MgO含量的特征;(石英)角斑岩具有高SiO2、Na2O含量,低Al2O3、TiO2、K2O、CaO含量的特征。Zr/TiO2-Nb/Y微量元素分类图解显示细碧岩与(石英)角斑岩呈典型的双峰式岩石组合,在SiO2-FeO/MgO图解中均位于钙碱性岩石系列区域;大离子亲石元素较相邻高场强元素明显富集,稀土配分曲线总体特征相似,细碧岩与(石英)角斑岩微量元素、稀土元素分布曲线显示岛弧环境的特征。综合细碧-角斑岩系岩石组合、岩石系列以及岩石地球化学特征,该套岩系形成于大陆边缘弧局部裂陷环境,细碧岩与(石英)角斑岩为同源岩浆演化的结果,其源区应为流体交代的楔形地幔。

The spilite-keratophyre sequence is formed of spilite and quartz keratophyre interbeddedly in Dashi-zhai Formation in Maodeng district of Xilinhaote,Inner Mongolia.The zircon LA-ICP-MS U-Pb gives a weighted average age of（287.4±1.7） Ma（MSWD=0.63）.These new data suggest that the protolith age of Dashizhai Formation was about Early Permian.The spilite is characterized by high SiO2,Al2O3（16%）,Na2O,TiO2（1.3%） and low CaO,MgO contents.The contents quartz keratophyre is characterized by high SiO2,Na2O and low Al2O3,TiO2,K2O,CaO contents.The spilite-keratophyre sequence belongs to typical bimodal rock series showed in the classification diagram of Zr/TiO2-Nb/Y and calc-alkaline rock series showed in the SiO2-FeO/MgO diagram.Spilite and quartz keratophyre are richer in LILE than in adjacent HFSE and similar in REE curve,which displays the characteristics of island-arc environment.Rock assemblage,rock series and geochemical characteristics prove that the spilite-keratophyre sequence formed in a continental margin-arc and local rifting environment,evoluting from the cognate magma which is from the wedge mantle by fluid metasomatism.