东昆仑造山带东段牦牛山组英安岩年代学和地球化学研究

Chronology and Geochemistry of the Dacite in the Maoniushan Formation in Eastern Part of the Eastern Kunlun Orogen

为探讨东昆仑东段牦牛山组英安岩的年代及其成因,对克合特一带牦牛山组底部英安岩进行了年代学和地球化学研究。LA-ICP-MS锆石U-Pb定年结果为（412.5±8.6）Ma,表明英安岩及牦牛山组形成时代为早泥盆世;岩石地球化学与柴北缘阿木尼克山牦牛山组流纹岩相似,具高SiO2、K2O及K2O/Na2O,富集Cs、Rb、Ba及Th、U、LREE等特征,负铕异常明显,可能为地壳中角闪岩经部分熔融及分离结晶形成。结合区域资料,认为晚志留世-早泥盆世,东昆仑构造体制由挤压向伸展转化,幔源岩浆的底侵作用,致使下地壳物质部分熔融、分离结晶,形成本区及柴北缘牦牛山组酸性岩浆,同时也意味着东昆仑同柴北缘地区已进入海西-印支期构造旋回。

In order to probe the timing and genesis of the dacite in the Maoniushan Formation in eastern part of the Eastern Kunlun orogeny,the geochronology and geochemistry of the dacite occurred in the bottom part of the Maoniushan Formation in the Kehete area have been undertaken in this paper. The LA-ICP-MS Zircon U-Pb dating of zircon grains dacite yields a Concordia age of（ 412. 5±8. 6） Ma,indicating that the dacite of the Maoniushan Formation was formed in early Devonian. The dacite has similar geochemical characteristics of the rhyolite in the Maoniushan Formation in the Animuke area in the northern Qaidam terrain. It is characterized with high contents of SiO2 and K2O,high K2O/Na2O ratios,enriched Cs,Rb,Ba,Th,U,and LREE,and obvious negative Eu anomalies. It could be formed by fractional crystallization of the magma derived from partial melting of the amphibolite in the crust. Combining with regional geological data,it is believed that the tectonic framework of the Eastern Kunlun was transformed from compressional to extensional environments during period from the late Silurian to early Devonian. The acidic magma of dacite in the Maoniushan Formation could be formed through fractional crystallization of the original magma generated by partial melting of the lower crust due to underplating of the mantle-derived magma. This means that the Hercynian-Indosinian tectonic cycle of the eastern Kunlun and northern Qaidam areas probably started at the early Devonian.