大兴安岭北段免渡河地区早二叠世二长花岗岩地球化学特征及构造意义

Geochemical characteristics and tectonic implication of Early Permian monzogranite in Mianduhe region of northern Great Xing'an Range

对大兴安岭北段免渡河地区早二叠世二长花岗岩进行了全岩地球化学、锆石U-Pb年代学和Lu-Hf同位素研究。锆石U-Pb测年结果显示,免渡河地区早二叠世二长花岗岩形成于297±2 Ma,为早二叠世岩浆活动的产物。全岩地球化学研究表明,这些二长花岗具有高硅(Si O_2=67.45%~73.54%)、低Mg~#值(25~42)的地球化学特征,属于高钾钙碱性I型花岗岩,富集Rb、Th、U等大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素。Lu-Hf同位素研究显示,免渡河地区二长花岗岩的锆石ε_(Hf)(t)=+7.5^+11.9,t_(DM2)=554~838 Ma,暗示其岩浆源于新元古代增生的地壳物质的部分熔融。结合前人的研究成果,认为大兴安岭北段免渡河地区早二叠世二长花岗岩形成于后碰撞的伸展环境。

The authors studied the whole-rock geochemical data,zircon U-Pb geochronology and Lu-Hf isotopic compositions for Early Permian monzogranite in the Mianduhe area of the northern Great Xing＇an Range,NE China.The dating results show that the monzogranite in the Mianduhe area was formed in Early Permian,with age of 297±2 Ma.Geochemically,these monzogranite are characterized by high silica（SiO2=67.45%～73.54%）and low Mg^# values（25～42）.They are depleted in high field elements（HFSEs,i.e.Nb,Ta,Ti,and P）and enriched in large-ion lithophile elements（LILEs,i.e.Rb,Th and U）,suggesting that they belong to high K calc-alkaline I-type granites.Primary zircons from the Early Permian monzogranite haveε（Hf）（t）values of＋7.5 to＋11.9,and Hf two-stage model ages（t（DM2））of 554 Ma to 838 Ma,indicating that the parental magma was generated by the partial melting of a Neoproterozioc accreted crustal source.Combining with previous studies,the authors suggested that the Early Permian monzogranite in the Mianduhe area was formed in a post-collision extensional setting.