豫西小河岩体中脉岩LA-ICP-MS锆石U-Pb年龄及地质意义

LA-ICP-MS Zircon U-Pb Age and Its Geological Significance of Vein Rock in Xiaohe Rock Mass,Western Henan Province

小河岩体位于小秦岭成矿带南部,其形成年龄一直存在争议。利用LA-ICP-MS法对侵入小河岩体中岩脉样品进行锆石U-Pb测年,结果显示,样品测点大多位于或接近207Pb/235U-206Pb/238U一线,变辉绿岩脉年龄为(1853±53)Ma,闪长岩脉年龄为(1833±100)Ma和(2418±70)Ma(后者为俘获的太华群继承锆石年龄),花岗伟晶岩脉年龄为(1219±53)^(1373±30)Ma。说明小河岩体形成之后的岩脉至少为古元古代造山纪末和中元古代延展纪两期。据侵入岩脉与被侵入岩体的穿切制约关系,确定小河岩体侵位年龄不晚于(1853±53)Ma,相当于中条运动末期。该年龄与区内主要金银矿床的形成时间差异巨大,认为小河岩体对区内金银成矿贡献有限。董家埝大型银矿床的形成需巨量成矿物质堆积,矿床深部可能存在更年轻的隐伏岩体。

Xiaohe rock mass is located in the south of Xiaoqinling metallogenic belt,its age has long been controversial.In this study,the LA-ICP-MS method was used for zircon U-Pb dating of vein samples in the intrusive Xiaohe intrusion.The results show that,most of the sample observation points were located at or near the 207Pb/235U-206Pb/238U consistent line,in which the age of the metamorphic diabase was(1853±53)Ma.The age of the diorites were(1833±100)Ma and(2418±70)Ma,the latter being the age of the inherited zircons in the captured Taihua group.The age of granitic pegmatite was ranked at(1219±53)^(1373±30)Ma.It indicates that there are at least two periods of vein rock after the formation of Xiaohe intrusion:the end of Paleoproterozoic orogeny and the extension of Mesoproterozoic.According to the close relationship between vein rock and intrusion,the emplacement age of Xiaohe intrusion was determined to be no later than(1853±53)Ma.Equivalent to the end of the Zhongtiao orogeny.The difference between this age and the formation time of the main gold and silver deposits in this area is huge.It is believed that the contribution of the Xiaohe intrusion to the gold and silver mineralization in this area is limited.The formation of the Dongjianian large-scale silver deposit requires huge amounts of ore-forming materials,and it is speculated that there may be younger hidden intrusion in the deep part of the deposit to provide the source.