甘肃天水地区早古生代黄门川花岗闪长岩体LA-ICP-MS锆石U-Pb定年及构造意义

甘肃天水地区黄门川花岗闪长岩体位于北祁连造山带东端,侵位于晚奥陶世陈家河群中酸性火山岩系中,岩石学和地球化学特征表明其具有壳幔岩浆混合的特点。对黄门川花岗闪长岩体进行了LA-ICP-MS锆石U-Pb定年,结果表明该花岗闪长岩体的年龄为440.5Ma±4.4Ma,形成于早志留世。地球化学特征表明,黄门川花岗闪长岩体属于中钾钙碱性系列,具I型花岗岩的特征。构造环境判别表明其形成于安第斯型大陆边缘弧。综合区域地质背景,认为祁连造山带东端在早古生代期间发育有有限洋盆,洋盆向北俯冲消减产生大量弧岩浆岩。对进一步研究祁连与秦岭造山带构造交接部位早古生代的构造格局、演化等大陆动力学问题具有重要意义。

LA-ICP-MS zircon U-Pb dating of Early Paleozoic Huangmenchuan granodiorite in Tianshui area of Gansu Province and its tectonic significance

Located in the northern Qilian orogenic belt of Tianshui region within Gansu Province, the Huangmenchuan granodiorite intruded into basic volcanic rocks of the Upper Ordovician Chenjiahe Group. The petrologic and geochemical characteristics show that this granodiorite was formed by emplacement of crust-mantle mixed type of magma. The results of LA-ICP-MS zircon U-Pb data of the granodiorite from Huangmenchuan show that the Huangmenchuan granodiorite was emplaced at 440.5Ma±4.4Ma, belong ing to Lower Silurian. The element geochemical characteristics indicate that Huangmenchuan granodiorite belongs to middle-K cal-alkaline series with features of I-type granite. The granodiorite samples are located in Andeans-type continental margin granite area in the environment discrimination diagram. Based on regional geology, the authors hold that there existed a limited ocean basin in Early Paleozoic, and large quantities of arc-type magmatic rocks were produced by the northward subduction of the ocean crust in eastern Qilian orogenic belt. The study of the continental dynamics of the tectonic framework and the evolution history in the area between Qilian and Qinling orogenic belts is of great significance.