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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±6 Ma,而二长花岗岩(样品GT11-01)的结晶年龄是199±4 Ma,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199 Ma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221～199 Ma。
The Guangtoushan granite body is exposed on the northern side of the Mianlue suture and is mainly composed of diopside diorite and monzogranite. The British diorites show the gneiss structure, and the local British diorites mylonitize to form the granitic mylonites. The monzonitic granite was emplaced after the mylonite formation, with a small amount of garnet and weak gnarled to massive structures. The results of LA-ICPMS zircon U-Pb isotopic dating indicate that the Guangtoushan intrusive rock is in two stages of emplacement. The emplacement age of the mylonitized micritic diorite (sample GT18-01) is 221 ± 6 Ma , Whereas that of the monzogranite (Sample GT11-01) is 199 ± 4 Ma, representing an age of late-stage monzonitic granite. According to the regional tectonic setting and geochemical characteristics of previous studies, the early diagenetic diorites may emplaced during island arc development before closure of the Mianmiao Ocean basin, representing the product of the arc-magmatism subduction of the oceanic crust. Then the Yangtze platform and the Qinling micro-land pieced together to form Mianlue suture zone. The late Paleozoic granodiorite (garnet) was formed during the collision with the magmatic activity of the main Qinling orogenic belt at about 199 Ma. Therefore, the Mianliao ocean basin closure and Mianliao suture formation period is about 221 ~ 199 Ma.