西秦岭天水地区太白花岗岩体LA-ICP-MS锆石U-Pb年龄、地球化学特征及其地质意义

LA-ICP-MS zircon U-Pb dating and geochemistry of the Taibai intrusion in Tianshui areas of western Qinling Mountains and their geological significance

秦岭造山带南缘勉略构造带北侧发育有近东西向展布的印支期花岗岩带,位于甘肃徽县地区的太白岩体是该花岗岩类的一部分。LA-ICP-MS锆石U-Pb同位素年龄表明,太白岩体的形成时代为221.8Ma±1.5Ma,属于印支期晚期。主要岩石类型为灰白色—肉红色中细粒二长花岗岩和花岗闪长岩。地球化学特征表明,太白岩体属于准铝质—弱过铝质的高钾钙碱性系列;稀土元素总量ΣREE为79.0×10-6～194×10-6,稀土元素配分模式呈轻稀土元素强烈富集、重稀土元素相对亏损的右倾型,Eu负异常明显;高场强元素（HFSE）Nb、P、Ti、Y、Yb和大离子亲石元素Ba、Sr强烈亏损。太白岩体是以中基性岩石为主的壳源物质部分熔融形成的岩浆上升侵位形成的,且处于同碰撞（挤压环境）向后碰撞（伸展环境）转化阶段,为后造山期花岗岩,是扬子板块和华北板块全面碰撞导致增厚下地壳物质部分熔融的产物。

An EW-trending Indosinian granitoide belt extends near northern Mianlue suture zone on the south margin of Qinling orogenic belt. The Taibai intrusion is a part of the Indosinian granitoids located in the Huixian County of Gansu Province. LA-ICPMS zircon U-Pb dating of the Taibai intrusion indicates that the emplacement age of the Taibai intrusion is 221.8Ma±1.5Ma, suggesting Late Indosinian. The Taibai intrusion is mainly composed of gray-pale red, medium to fine grained monzonitic granite and granodiorite. According to the geochemical characteristics, it is suggested that Taibai intrusion belongs to the metaluminous to weakly peraluminous high-potassic calc-alkaline series and is characterized by strong depletion of high field strength elements（HFSE） Nb,Ti, P Y, Yb and large ion elements Ba, Sr with obvious negative Eu anomalies. The total amount of rare earth elements ΣREE=79.0×10-6～194×10-6,the rare earth elements（REE） exhibit right-oblique patterns with strong enrichment of light rare earth elements（LREE） and the relative depletion of heavy rare earth elements（HREE）. The rocks of the Taibai intrusion was formed by rising and emplacement of partially melted granitic magma resulting from partial melting of the shell material, belonging mainly to intermediate-basic rocks. The Taibai intrusion, formed at the transitional stage from syn-collision（compression environment） to postcollision（extentional environment）, is of late-orogenic granitoids and also a product formed by partial melting of thickened lower crust resulting from the collision between the North China plate and the Yangtze plate.