大兴安岭北段诺敏大山地区早白垩世侵入岩地球化学特征及其构造意义

Geochemical characteristics and tectonic significance of Early Cretaceous intrusions in the Mount Nuomin area,northern Great Hinggan Mountains

笔者等选择大兴安岭北段诺敏大山地区早白垩世侵入岩进行了锆石U-Pb年代学和地球化学研究,探讨该区域侵入岩成因类型、岩浆来源及构造环境。该区侵入岩岩性主要为正长花岗岩、正长花岗斑岩和似斑状正长花岗岩,对其中正长花岗岩样品进行LA-ICP-MS锆石U-Pb测年表明,正长花岗岩侵位年龄为129.5±0.4 Ma,应为早白垩世岩浆活动的产物,结合区内侵入岩与地层相互接触关系,本区侵入岩形成时代为早白垩世。区内侵入岩具有富硅(SiO_(2)=67.36%~74.09%)、富碱(K_(2)O+Na_(2)O=8.88%~9.34%)、高铝(Al_(2)O_(3)=12.56%~16.15%),低MgO、TiO_(2)、CaO的特点,属于高钾钙碱性岩石系列;铝饱和指数(A/CNK)为0.94~1.31,为准铝质—过铝质岩石。微量元素富集Rb、U、Th、K等大离子亲石元素,强烈亏损Ti、Nb、Sr、P等高场强元素,具有明显的Eu负异常,属于高分异I型花岗岩。岩石Rb/Sr为0.9~2.0,Sr/Y为4.2~7.2,显示出高Sr、低Y的特点,指示岩浆源区为地壳物质的部分熔融。结合区域研究成果,蒙古—鄂霍茨克构造域在早白垩世之前已结束碰撞,诺敏大山地区早白垩世岩浆活动可能发生在蒙古—鄂霍茨克造山后的伸展环境。

This paper selects the Early Cretaceous intrusive rocks in the Mount Nuomin area in the northern Great Hinggan Mountains to study the zircon U-Pb geochronology and geochemistry,and discuss the genetic types,magma sources and tectonic environment of the intrusive rocks in this area.The lithology of intrusive rocks in this area is mainly syenite,syenite granite porphyry and porphyritic syenite.LA-ICP-MS zircon U-Pb dating of syenite samples shows that the emplacement age of syenite is 129.5±0.4 Ma,which should be the product of early Cretaceous magmatic activity,and combined with the contact relationship between intrusive rocks and strata in the area,the formation age of intrusive rocks in this area is Early Cretaceous.The Intrusive rocks are characterized by high silica(SiO_(2)=67.36%~74.09%),alkali(K_(2)O+Na_(2)O=8.88%~9.34%)and alumina(Al_(2)O_(3)=12.56%~16.15%),with low MgO,TiO_(2) and CaO.They are of high potassium calcium alkaline rock series and Quasialuminum-peraluminum as indicated by A/CNK ratio of 0.94~1.31.The trace elements are enriched in large ion lithophile elements such as Rb,U,Th,K,and strongly depleted of high field strength elements such as Ti,Nb,Sr,P,and have obvious negative Eu anomalies,which belong to highly differentiated type Ⅰ granites.The rock Rb/Sr is 0.9~2.0,Sr/Y is 4.2~7.2,showing the characteristics of high Sr and low Y,indicating that the magma source area is the partial melting of crustal materials.Based on regional research results,the Mongolia-Okhotsk tectonic domain has ended its collision before the Early Cretaceous,and the early Cretaceous magmatic activity in the Mount Nuomin area may have occurred in the extensional environment after the Mongolia-Okhotsk orogen.