中甸春都铜矿区岩体成岩时代及地质意义

Ages and Geological Significance of the Porphyries in the Chundu Copper Mining Area in Zhongdian,Yunnan Province

春都斑岩铜矿床位于中甸铜多金属成矿带的西带南段,矿区主要出露由闪长玢岩和花岗闪长斑岩组成的复式岩体,后者呈岩枝或岩脉产于闪长玢岩体中,并与斑岩铜矿有密切成因关系。本文应用LA-ICP-MS U-Pb定年方法,对闪长玢岩以及花岗闪长斑岩的锆石进行年代学研究。结果表明:岩石中的锆石颗粒为浅黄色-无色透明,呈正方双锥状、柱状的自形晶体,内部发育完好的韵律环带,并具较高的Th/U比值,属典型的岩浆成因锆石;主岩体——闪长玢岩体中的锆石LA-ICP-MS U-Pb年龄为(212±3)Ma,成矿花岗闪长斑岩锆石LA-ICP-MS U-Pb年龄分别为(217±2)Ma和(218±2)Ma。锆石LA-ICP-MS U-Pb年龄数据与地质体穿切关系的矛盾,可能与测试数据较少等因素有关。我们认为,矿区斑岩铜矿的成岩成矿时代均为晚三叠世,与义敦岛弧在237～208Ma之间发生大规模的岛弧岩浆活动相吻合。

The Chundu porphyry copper deposit is located at the southern part of the western branch of the Zhongdian eopper-polymetallic metallogenic belt. The copper mineralization is closely related to the Chundu granite which consists of diorte-porphyrite and dykes or veins of granodiorite-porphyrite intruded the former. The diorite-porphyrite and granodiorite-porphyry are chronologically dated using zircon LA-ICP-MS U-Pb method. The zircon grains are light yellow to colorless transparent, tetragonal bipyramid crystals, showing regular rhythmic banding in the cathodoluminescence images which are typical for zircons of magmatic origin. The LA-ICP-MS zircon U-Pb analyses yield ages of （212 ± 3 ） Ma （ sample B-9, n = 13, MSWD = 0.23, Th/U ratios of 0.51 - 0.98 ） for the diorite-porphyrite, （217±2）Ma （sample B-28, n =21, MSWD =0.31, Th/U ratios of 0.54-3.76） and （218±2）Ma （ sample E-9, n = 20, MSWD = 0.31, Th/U ratios of 0.5 - 1.16） for the granodiorite-porphyry respectively. As inadequate of the diorte-porphyrite＇s age dates, the contradiction between the test results and the geological phenom ena is existed. The results show that the Chundu granite was formed in the Late Triassic, a period （237 -208Ma） of massive magmatic activity in the Yidun arc.