内蒙古四子王旗头号岩体锆石U-Pb同位素年代、岩石成因及构造意义

ZIRCON U-PB DATING ISOTOPE CHRONOLOGY, GENETIC AND TECTONIC SIGNIFICANCE OF THE TOUHAO PLUTON IN SIZIWANGQI, INNER MONGOLIA

头号岩体位于华北板块北缘，西拉木伦河缝合带以南，侵位于新太古代变质花岗岩中，被晚侏罗世钾长花岗岩所侵入。通过LA—ICP—MS锆石IJ-Pb测年获得了（236．2±9．6）Ma（MSWD=3．6）的206Pb／238U加权平均年龄，应代表其侵位年龄。岩石地球化学分析结果显示，ω（Si02）为73．09％～76．96％，ω（Na20＋K20）为8．14％～8．95％，A／CNK为1．03％～1．13，标准矿物计算中各样品均出现刚玉（C），σ值为1．95～2．54，为过铝质岩石、高钾钙碱性岩石系列。稀土元素蛛网图中，轻重稀土元素分馏较强，Eu为明显的负异常，大离子亲石元素（LILE）富集，高场强元素亏损（HFSE），Zr／Hf比值偏离较大，为S型花岗岩，并且岩浆作用后期可能与高温含水流体发生了相互作用。头号岩体形成于造山晚期向造山期后转变的构造体制下。结合前人区域研究资料，认为华北板块与西伯利亚板块已沿西拉木伦河缝合带闭合。

Touhao pluton, which is located in the northen margin of the North China Plate and the south of Xar Moron suture belt, emplaced in Archaean Metamorphic granite and was intruded by late Jurassic K-feldspar granite. One sample of zircons has been run for U-Pb geochronological dating using LA-ICP-MS techniques, through which the mean age of （236.2±9.6） Ma （MSWD=3.6） have been achived , and should be on behalf of its emplaced age. The result of rock petrochemical analysis shows that it belongs to peraluminous rock and high-K calc-alkaline series with higher concent of SiO2（ω （SiO2） =73.09%-76.96%, ω （Na20±K20） = 8.14-8.95%, A/CNK=1.03-1.13, σ =1.95-2.54）. It is rich in LILE and lack of HFSE, and the ratios of Zr/Hf deviate from （38±2）. The fractionation of LREE and HREE is obvious, and Eu is obviously negtive abnormity. So it belongs to granit of S-type, and interactes with high temperature aqueous fluids in the later stage of diagenetic process. It formed in tectonic regime when the area tectonic regime was changing from the late orogenic period to the post-orogenic. Combined with the previous studies on regional geology, it is concluded that the North china plate and Siberian plate were sutured along the Xar Moron suture belt at that time.