内蒙古苏尼特左旗巴音乌拉二叠纪埃达克质花岗闪长岩类地球化学特征及其地质意义

内蒙古苏尼特左旗巴音乌拉二叠纪花岗闪长岩、石英闪长岩分布于二连-贺根山蛇绿岩带的北侧,1∶20万区域地质调查将其归于华力西晚期侵入体(γδ4(3)).经研究,该岩体具高铝(Al2O3=13.69%～16.48%)、富钠(Na2O＝3.53%～4.58%)、贫钾(K2O＝2.12%～4.10%)、高锶(Sr＝425.1×10-6～645.2×10-6)的特点,SiO2=62.13%～72.87%,Y＜18×10-6(7.23×10-6～14.29×10-6),δEu＝0.71～0.83,轻重稀土元素分馏强烈,稀土元素配分曲线为右倾斜型,铕异常不明显,为典型的埃达克质岩.在微量元素原始地幔标准化蛛网图中具明显Sr高峰和Nb低谷,有与埃达克岩一致的曲线,与岛弧安山岩-英安岩-流纹岩及大陆弧安山岩-英安岩-流纹岩有明显差异.花岗闪长岩锆石U-Pb年龄为256.1±0.9 Ma,为早二叠世岩浆活动的产物.Mg#值很高(＞50),表明晚古生代洋壳向北俯冲时在75～85 km深度(角闪岩相与榴辉岩相过渡相)发生部分熔融形成埃达克质熔体,侵入地壳形成埃达克岩.

Geochemical characteristics of Permian adakitic granodiorite in Bayinwula of Sonid Left Banner, Inner Mongolia

Permian adakitic granodiorite and quartz-diorite in Bayinwula of Sonid Left Banner are distributed on the northern side of the Erlian-Hegen Mountain ophiolite zone. The 1∶200 000 regional geological survey has assigned it to Late Variscan intrusive mass (γδ4(3)). Studies show that the rocks have characteristics of high aluminum (Al2O3=13.69%～16.48%), higher sodium(Na2O=3.53%～4.58%), lower potassium(K2O=2.12%～4.10%)and higher strontium(Sr=425.1×10-6～645.2×10-6),SiO2=62.13%～72.87%, Y＜18×10-6(7.23×10-6～14.29×10-6), δEu＝0.71～0.83. The fractionation of light-weight thulium is sharp and the partition curve of thulium forms a right-oblique model. Europium anomaly is not conspicuous, suggesting typical adakitic rock. The spidergram of traced elements relative to the original mantle obviously shows a Sr peak and a Nb valley, and the original earth mantle standard curve is in accordance with that of adakites but obviously different from things of island arc andesite, dacite and rhyolite and continental arc andesite, dacite and rhyolite. The zircon U-Pb age of granodiorite is 256.1±0.9 Ma, implying a product of Early Permian magmatic movement. Mg# is quite high (＞50), indicating that the northward subduction of the Late Paleozoic oceanic crust experienced partial melting and formed adakitic melt at 75～85 km (the transitional point of amphibolite facies and eclogite facies) , which then intruded the earth's crust and eventually formed adakites.