大兴安岭牙克石乌奴尔地区晚石炭世正长花岗岩锆石U-Pb年龄、地球化学特征及地质意义

Zircon U-Pb age,geochemical characteristics and geological significance of syenogranite in Wunuer area,Great Hinggan Mountains

内蒙古牙克石市乌奴尔地区位于西伯利亚板块东南缘古生代陆缘增生带,该区发育大量正长花岗岩。本文对该区正长花岗岩进行了LA-ICP-MS锆石U-Pb测年和岩石地球化学测试。结果表明:正长花岗岩锆石U-Pb年龄为310.8±2.5 Ma(MSWD=1.6),属晚石炭世。岩石地球化学数据显示,岩体属高钾钙碱性系列,铝饱和指数A/CNK=1.03～1.08,为过铝质花岗岩。岩石整体富集Rb、Th等大离子亲石元素,亏损Ba、Sr以及高场强元素Nb、Ti,LREE相对富集,HREE相对亏损,具有明显的负Eu异常(δEu=0.36～0.90)。岩石地球化学分析表明,正长花岗岩具有S型花岗岩特征,具有壳源的特征,源岩为泥质岩,形成于中等温度(700～720℃)、低压(0.15～0.2 GPa)环境。综合区域地质特征及本次研究认为,正长花岗岩应为晚石炭世古亚洲洋闭合时的产物,为古亚洲洋的闭合及东北地块构造演化研究提供了新的约束和佐证实例。

Objectives: Wunuer area,Great Hinggan Mountains( Daxing’an Mountains),Inner Mongolia is a part of the accretion zone of the Paleozoic continental margin of the southeast Siberian plate,where the syenogranite is widely found,it is essential to carry out geochronological study and petrogeochemical analysis for better understanding of the syenogranite.Methods: On the basis of detailed field work,the petrogeochemical testing and LA-ICP-MS zircon U-Pb dating age are carried out on the syenogranite Results: LA-ICP-MS zircon U-Pb age 310. 8 ± 2. 5 Ma( MSWD = 1. 6),belongs to Late Carboniferous.Petrogeochemical researches indicate that the granite belongs to high-K calc-alkaline,the granite is peraluminous granite with A/CNK = 1.03 ~ 1.08. The granite is enriched in Rb,Th,depleted in Ba,Sr and Nb,Ti;is enriched in LREE and relatively depleted in HREE, with obvious negative Eu anomaly(δEu = 0. 36 ~ 0. 90).Petrogeochemical analysis shows that the granite has the characteristics of S-type granite. Its source rock is continental crust and it is from partial melting of continental crust mudstone. The initial magma temperature is 700~ 750℃,the initial magma pressure is 0.15 ~ 0.2 GPa.Conclusions: Comprehensive studies indicate that the granite is formed in the closing of the Paleo Asian Ocean,which provided new constraints and a supporting instance on the close time of the Paleo Asian Ocean and tectonic evolution of the northeast China block group.