内蒙古赤峰敖汉地区酸性火山岩的形成时代、地球化学特征及其意义

Geochronology and Geochemical Characteristics of Acid Volcanic Rocks in Aohan Area, Inner Mongolia

为对内蒙古敖汉地区酸性火山岩进行划分对比,对其进行了锆石U-Pb测年、地球化学分析.流纹岩和流纹质晶屑凝灰岩年龄分别为122.8±2.3 Ma、122.1±1.9 Ma,属于早白垩世义县组.晚侏罗世流纹岩SiO2和全碱含量较高,Al2O3含量较低;稀土总量较低、具有负铕异常;富集Rb、Th、U、K,亏损Ba、Sr、P、Ti.早白垩世流纹岩SiO2、全碱、Al2O3含量较高;稀土总量较高、稀土分馏明显、弱负铕异常;富集K、Rb、Ba、Th、U,亏损Nb、Ta、Sr、P、Ti.将敖汉地区晚侏罗世酸性火山岩进行了解体,首次将其大部分划为早白垩世义县组.晚侏罗世流纹岩可能为高分异流纹岩,形成于相对低的压力环境下,可能起源于中上地壳物质部分熔融.早白垩世流纹岩显示出埃达克质岩石的地球化学特征,可能形成于加厚的下地壳环境.

Zircon U-Pb dating and geochemical analysis were carried out to classify and compare the acidic volcanic rocks in Aohan,Inner Mongolia.Zircon U-Pb ages of rhyolite and rhyolitic crystal tuff are 122.8±2.3 Ma and 122.1±1.9 Ma,belonging to the Yixian Formation of Early Cretaceous.Late Jurassic rhyolite contains of high SiO2 and alkali(Na2O+K2O),low Al2O3 and low total REE,with negative Eu anomaly,indicating REE“tetrad effect”.It is enriched in large ion lithophile elements Rb,Th,U,K and and depleted in high field strength elements Ba,Sr,P and Ti.Early Cretaceous rhyolite has high contents of SiO2,alkali(Na2O+K2O)and Al2O3;it is characterized by high total REE,enrichment of LREE,clear fractionation of light and heavy rare earth,weak negative Eu anomaly;enrichment of large ion lithophile elements K,Rb,Ba,Th,U,and depletion of Nb,Ta,Sr,P and Ti.In this paper,the Late Jurassic acid volcanic rocks in Aohan area are disintegrated and most of them are classified as Early Cretaceous Yixian Formation for the first time.Late Jurassic rhyolite may be highly differentiated rhyolite.Geochemical characteristics show that it was formed under relatively low pressure environment and may have been originated from partial melting of upper and middle crustal materials.Early Cretaceous rhyolite shows the geochemical characteristics of adakitic rocks,which may formed in the thicker lower crustal environment.