Abstract: Guangtoushan granitoids are locatedin the north of Mianlue suture zone, which consist of tonalites and monzogranites. Thetonalites display gneissic feature, which are locally mylonitized into granitoid mylonites. The weakly gneissic to massive monzogranites with a little garnet emplaced after the mylonite zone. LA-ICPMS zircon U-Pb isotope dating reveals that Guangtoushan granitoidpluton experiencedtwo stages of emplacement. The earlier gneissic tonalite, sample GT18-01, emplaced at 221±6 Ma and the later monzogranite, sample GT11-01, yields a crystalline age of 199±4 Ma. Integrated with the previous studies in regional tectonic setting and geochemical characteristics, the early tonalites of the Guangtoushan granitoids were most likely to emplace at ～221 Ma, which possibly represents the arc magmatism of oceanic crust subduction before Mianlue ocean basin closed. Then, Mianlue ocean basin closed andthe Yangtze Craton was amalgamazed with Qinling micro-block, which formed Mianlue suture zone. The S-type monzogranitic magmatismprobably occured in the syn-collision tectonic setting during the main orogenic stage of the Qinling orogenic belt at ～199 Ma. The closure event of the Mianlue ocean basin, which formed the Mianlue suture zone, occured between ～221 Ma and ～199 Ma.

Key words: Western Qinling, Guangtoushan granitoids, LA-ICPMS, zircon U-Pb isotopic geochronology, Mianlue suture zone

摘要： 光头山花岗岩体出露于勉略缝合带北侧, 主要由英云闪长岩和二长花岗岩组成。英云闪长岩表现为片麻状构造, 局部英云闪长岩糜棱岩化形成花岗质糜棱岩。而二长花岗岩在糜棱岩带形成之后侵位, 含有少量的石榴石,弱的片麻状到块状构造。LA-ICPMS 锆石原位U-Pb同位素定年结果表明, 光头山岩体为两个阶段侵位, 糜棱岩化英云闪长岩( 样品GT18-01)的侵位结晶年龄是221±6Ma, 而二长花岗岩(样品GT11-01)的结晶年龄是199±4Ma,代表了晚期二长花岗岩形成的时代。结合区域构造背景和前人研究的地球化学特征, 早期的英云闪长岩可能在勉略洋盆闭合前的岛弧发育阶段侵位, 代表了洋壳俯冲的弧岩浆活动的产物。然后扬子地台与秦岭微陆块拼合, 形成勉略缝合带。约199Ma秦岭主造山期同碰撞岩浆活动形成了晚期(石榴石)二长花岗岩。因此, 勉略洋盆闭合和勉略缝合带形成时期大约为221～199Ma 。

关键词: 西秦岭, 光头山花岗岩, LA-ICPMS, 锆石U-Pb同位素年代学, 勉略缝合带