内蒙古架子山地区晚石炭世二长花岗岩成因及其地质意义

Petrogenesis and Geological Significance of Late Carboniferous Monzogranite in Jiazishan Area, Inner Mongolia, China

内蒙古架子山地区二长花岗岩位于大兴安岭中段,为了确定其侵位时代,探讨大地构造环境,本文对该区二长花岗岩进行了系统的年代学和岩石地球化学研究。研究结果表明:二长花岗岩锆石U-Pb年龄为(302.9±1.2)Ma,为晚石炭世岩浆作用的产物;二长花岗岩具有高硅(w(SiO_(2))=74.73%~76.60%)、富碱(w(Na_(2)O+K_(2)O)=6.20%~7.03%)、高铝饱和指数(A/CNK=1.43~1.58)的特征,属于高钾钙碱性系列岩石;Rb、K、Th、U富集,Ba、Sr、Ti、Nb、P呈明显V字型亏损,稀土元素配分曲线呈右倾的轻稀土元素富集型,具有负Eu异常(δEu=0.46~0.67)、高分异指数(I_(D)=88.89~91.55)和低固结指数(I_(S)=1.68~4.78)的特征,表明岩石经历了高分异演化作用,为高分异I型花岗岩,具有向A型花岗岩演化的趋势;^(176)Hf/^(177)Hf值为0.282850~0.282951,ε_(Hf)(t)值为9.2~12.5,二阶段模式年龄(T_(DM2))变化范围为910~603 Ma,表明二长花岗岩岩浆可能是新元古代新生地壳部分熔融的产物。内蒙古架子山地区二长花岗岩形成于后造山阶段,其形成可能与古亚洲洋闭合后构造环境从挤压转换成碰撞后伸展有关。

The monzogranite in the Jiazishan of Inner Mongolia is located in the middle part of the Daxing’anling Mountains.To determine its intrusive age and tectonic environment,the geochronology and petrogeochemistry of the monzogranite have been systematically studied.The results show that the zircon U-Pb age of the monzogranite is(302.9±1.2)Ma,which is the product of Late Carboniferous magmatism.The monzogranite is characterized by high silica(w(SiO_(2))=74.73%-76.60%),rich alkali(w(Na_(2)O+K_(2)O)=6.20%-7.03%)and high aluminum saturation index(A/CNK=1.43-1.58),belonging to the high-K calc-alkaline series.Rb,K,Th and U are enriched,while Ba,Sr,Ti,Nb and P are obviously V-shaped depleted.The REE pattern shows a right-leaning enrichment type of LREE,with negative Eu anomaly(δEu=0.46-0.67).The high differentiation index(I_(D))(88.89-91.55)and low solidification index(I_(S))(1.68-4.78)indicate that the monzogranite has undergone the evolution of high differentiation and is a type highly differentiation I-type granite with a tendency to evolve to A-type granite.The ^(176)Hf/^(177)Hf value is 0.282850-0.282951,theεHf(t)value is 9.2-12.5,and the two-stage mode age(T_(DM2))is 910-603 Ma,suggesting that the monzogranite may be the product of partial melting of juvenile crust in Neoproterozoic.The monzogranite in the Jiazishan area of Inner Mongolia was formed in the post-orogenic stage,and its formation may be related to the transformation of the tectonic setting from extrusion to post-collisional extension after the closure of the Paleo-Asian Ocean.