内蒙古架子山钼(银)矿区花岗岩锆石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 Granite of Jiazishan Mo(Ag)Deposit,Inner Mongolia

通过对内蒙古架子山钼(银)矿区内的花岗岩进行锆石U-Pb年代学和岩石地球化学的研究,获得花岗岩LA-ICP-MS锆石U-Pb年龄为149.19±0.47 Ma,为晚侏罗世岩浆作用的产物;花岗岩具有高硅(78.59%~80.58%)、富碱(5.82%~6.34%)、低P2O5(0.002%~0.018%)、高铝饱和指数A/CNK(1.33~1.60)的特征,属于高钾钙碱性系列岩石;富集微量元素Rb、K、La、Th、U、Hf、Pb,亏损微量元素Ba、Sr、Nb、Ta、Ti、P,稀土元素配分曲线呈右倾的轻稀土元素富集型,具有明显的负Eu异常(0.25~0.49)、高分异指数(DI)(91.87~94.30)、低固结指数(SI)(1.20~2.02),表明岩石经历了高分异演化作用,为高分异I型花岗岩;^(176)Hf/^(177)Hf值为0.28290~0.28296,ε_(Hf)(t)值为7.82~9.86,二阶段模式年龄(t_(DM2))为744~929 Ma,表明花岗岩岩浆是在新元古代亏损地幔物质部分熔融形成新生下地壳之后,经再次熔融形成.内蒙古架子山花岗岩与大兴安岭地区的晚侏罗世花岗岩岩浆基本同期侵位,其形成可能与蒙古-鄂霍茨克洋闭合后构造环境从挤压转换成碰撞后伸展有关,并可能叠加了古太平洋板块俯冲引发的弧后伸展作用.

The zircon LA-ICP-MS U-Pb age of the granite from the Jiazishan Mo(Ag)deposit in Inner Mongolia is 149.19±0.47 Ma,which is the product of Late Jurassic magmatism.The granite is characterized by high silicon(78.59%-80.58%),rich alkali(5.82%-6.34%),low P2O5(0.002%-0.018%)and high Aluminium saturation index A/CNK(1.33-1.60),belongs to the high-K calc-alkaline series.The granite is enriched in trace elements Rb,K,La,Th,U,Hf,Pb,and depleted in trace elements Ba,Sr,Nb,Ta,Ti,P.The REE pattern shows a right-leaning enrichment type of LREE,with obvious negative Eu anomaly(0.25-0.49).The high differentiation index(DI)(91.87-94.30)and low solidification index(SI)(1.20-2.02)indicate that the rock has undergone the evolution of high differentiation and is a type of high differentiation I granite.The ^(176)Hf/^(177)Hf value is 0.28296-0.28296,theε_(Hf)(t)value is 7.82-9.86,and the two stage mode age(T_(DM2))is 744-929 Ma,suggesting that the granitic magma was formed by re-melting after the Neoproterozoic lower crust,which was formed by partial melting of depleted mantle material.The granite magma of Jiazishan and the Late Jurassic granite magma in the Da Hinggan Mountains were basically contemporaneous emplacement.The granite of Jiazishan may be related to the transformation of the tectonic environment from compression to post-collision extension after the closure of the Mongolia-Okhotsk Ocean,and superimposed influence by the subduction of the paleo-Pacific plate,which caused tectonic back-arc extension.