内蒙古大板地区火山岩的地球化学特征

Analysis of geochemical characteristics of volcanic rocks in Daban area,inner Mongolia

大板地区属华北与西伯利亚两大板块之间的兴蒙造山带的一部分,晚古生代-中生代时期,岩浆活动强烈频繁.主要岩石流纹岩经历过典型的钙碱性系列演化过程,主量元素具S型花岗岩的组成特征,指示其岩浆源区可能为沉积岩;微量元素蛛网图显示岩浆经历了充分的分异演化.喷出岩的稀土元素配分模式呈轻稀土相对富集、重稀土相对亏损的右倾型,Eu强烈至中等亏损,Ce无明显富集或亏损,REE为112.4×10-6 ～216.5×10-6,LREE/HREE为2.65～4.24,w（La）/w （Yb）N为7.3 ～13.9,w（La）/w（Sm）N为3.114 ～5.204,δEu为0.24 ～0.74,δCe为0.9～1.1.微量元素特征表明,构造环境属火山弧花岗岩及同碰撞花岗岩.综合认为,大板地区的岩浆岩具火山弧花岗岩的典型特征,其源区可能是岛弧火山岩,流纹岩是岛弧内部物质调整的产物.

Daban is located between the North China Craton and the Siberia Craton,belong to the Xing-meng orogenic belt.Volcanic-magmatic action occurs frequently in Paleozoic-Mesozoic.The major rock type is rhyolite and the volcanic has experienced a typical evolution of calc-alkaline series.The major element has the features of S-type granite,indicating that magma source may be sedimentaryrocks.The cobweb map of trace elements shows that the magma has experienced a complete differentiation and evolution.The REE distribution model of extrusive rocks indicate that LREE show relative enrichment while HREE loss,REE =1 12.4 × 10-6 -216.5 × 10-6,LREE/HREE =2.65 ～4.24,w（La）/w（Yb）N =7.3 ～ 13.9,w（La）/w（Sm）N =3.114 ～5.204,δEu =0.24 ～0.74,δEu =0.9 ～ 1.1.The analysis results of trace elements indicate that the tectonic setting are volcanic arc granite and syn-collisional granite.The comprehensive research presents that the magmatic rocks of Daban has the typical characteristic of volcanic arc granite,implying that the magma source is probably arc volcanic rocks,rhyolite results from the adjustment of internal arc.