内蒙古朱日和地区早古生代岩浆岩年代学、地球化学特征及其构造意义

Geochronology and Geochemistry of Early Paleozoic Igneous Rocks in Zhurihe Area,Inner Mongolia and Their Tectonic Significance

中亚造山带早古生代的构造演化一直存在不同的认识,特别是其俯冲转为碰撞的时限.内蒙古朱日和南部的额尔登陶勒盖地区出露的早古生代岩浆岩,对于限定白乃庙岛弧的延伸方向、岛弧演化和拼贴时间具有重要意义.对额尔登地区出露的火山岩和侵入岩进行了精确的锆石SHRIMP和LA-ICP-MS U-Pb定年、锆石Lu-Hf同位素和地球化学分析以确定其时代和成因.研究获得4个岩浆岩年龄,1件英安岩年龄为434.0±4.4Ma,2件花岗岩年龄分别为432.8±2.6Ma和428.1±1.8Ma,此3件样品形成于早中志留世;另有1件流纹岩年龄为411.8±1.0Ma,形成于早泥盆世.早中志留世侵入岩为低钾、过铝质钙碱性中酸性岩石,富集Rb、Th,弱富集Zr、Hf,明显亏损Nb、Ta、P、Ti.在稀土配分曲线上,侵入岩均不存在Eu的负异常.其中2件花岗岩样品的锆石基本为正εHf(t)值(-1.19~11.51,3.32~10.28),TDM2为684~1 493Ma及759~1 202Ma,主要来自新生地壳;早中志留世火山岩具有中-低钾、偏铝质-过铝质、钙碱性酸性岩石特征,富集Rb、Th,弱富集Zr、Hf,明显亏损Nb、Ta、P、Ti.在稀土配分曲线上,多数火山岩不存在Eu负异常.英安岩中的锆石具有正εHf(t)值(3.70~7.94),TDM2为912~1 183Ma,主要来自新生地壳;而早泥盆世流纹岩(411 Ma)存在明显的Eu负异常,其锆石具有负εHf(t)值(-14.95^-7.07),TDM2为1.8~2.3Ga,应为古老地壳再循环的产物,与早中志留世岩浆岩源区明显不同,可能代表构造环境的转变.综合区域地质和前人研究资料表明,早志留世英安岩和花岗岩形成于俯冲岛弧环境,白乃庙岛弧向东可延伸至本区,而早泥盆世流纹岩可能形成于后碰撞环境.

There are some existing debates about the tectonic history of the Central Asian Orogenic Belt,including the time of collision.New zircon SHRIMP and LA-ICP-MS U-Pb isotopic data,zircon Lu-Hf isotopic data and geochemical data of the volcanic rocks and plutons from the Eerdengtaolegai area are presented to study their formation ages and petrogenesis.Zircon U-Pb ages of 432.8±2.6Ma,428.1±1.8Ma of Silurian granites,434.0±4.4Ma of Silurian dacite,411.8±1.0Ma of the Devonian rhyolite were obtained.The Silurian plutons are mostly intermediate-acid rocks,and are geochemically low in K contents,peraluminous,and calc-alkaline with enrichment in Rb and Th and depletion in Nb,Ta,P and Ti.They do not have negative Eu anomalies.Two granites from these plutonic rocks have positiveεHf（t）values（-1.19 to 11.51 and 3.32 to 10.28）with TMD2of684-1 493 Ma and 759-1 202 Ma,suggesting that the granites may come from a juvenile continental crust.Geochemically,theSilurian volcanic rocks,all acid rocks,are low to moderate in K content,metaluminous-peraluminous and calc-alkaline with enrichment in Rb,Th and depletion in Nb,Ta,P and Ti.In addition,they do net have negative Eu anomalies.One Silurian dacite has positiveεHf（t）value（3.70-7.94）with TMD2 of 912-1 183 Ma,suggesting that the rocks may come from the juvenile continental crust.The Devonian rhyolite have clear negative Eu anomalies,exhibiting negativeεHf（t）values（-14.95to-7.07）with Hf model ages（TMD2）of 1.8-2.3Ga,indicating considerable contribution of reworked materials from the ancient crust of the North China Craton.Based on our new data and previous studies,the Silurian granites and dacite were likely formed in an arc environment and the Devonian rhyolite developed in a post-collisional environment.