新疆哈密四顶黑山地区镁铁质-超镁铁岩特征及其构造意义

Characteristics and Tectonic Significances of the Mafic-Ultramafic Rocks from Sidingheishan Area in Hami,Xinjiang

采用LA-ICP-MS法获得新疆四顶黑山地区镁铁质-超镁铁岩中辉长岩的锆石U-Pb年龄为(351.5±1.9)Ma,与采用39Ar/40Ar法和Sm-Nd等时线法获得的年龄值存在明显的差异,同时也与同一构造带上的早二叠世黄山东及图拉尔根等含矿杂岩体的形成时间明显不同。岩石学及地球化学特征显示,四顶黑山杂岩体以辉长岩为主,含强分异的斜长岩和闪长岩,以高SiO2、Al2O3、CaO,低TiO2、m/f、ΣREE及不相容元素,具明显的Nb、Ta负异常为特征,并以此区别于相邻的图拉尔根杂岩体。结合该区花岗质岩石的定年结果,认为该杂岩体形成于早泥盆世的岛弧花岗岩和晚泥盆世的碰撞花岗岩之后,与(357.7±3.5)Ma的A型花岗岩共生,应是碰撞后伸展背景的产物。

An age of （351.5 ± 1.9）Ma was obtained from the gabbro of Sidingheishan in the Eastern Tianshan Massif using LA-ICP-MS U-Pb method. It is different from the ages previously obtained using ^39Ar/ ^40Ar or Sm-Nd method, and also apparently different from the ages of Early Permian ore complex rocks from the Eastern Huangshan and Tulaergen in the same tectonic belt. The petrologic and geochemical characteristics show that the main component of the complex is gabbro consisting of highly differentiated plagioclase and diorite. The complex is characterized by high SiO2, Al2O3 and CaO values and low TiO2, m/f, REE and incompatible elements and obviously negative Nb and Ta anomaly which can be distinguished from the nearby Tulaergen complex. Combined with the dating results of the granitic rocks in this area,it is suggested that the intergrowth of the complex with the A-type granite of （357.7±3.5）Ma after the Early Devonian island-arc granite and the Late Devonian eollisional granite may have resulted from the post-collisional extension setting.