乌兰浩特地区早白垩世晚期花岗斑岩年代学、地球化学特征及其构造背景

Geochronological and geochemical characteristics of the late Early Cretaceous granitic porphyry in the Ulanhot area and its tectonic setting

内蒙古乌兰浩特地区出露的2条花岗斑岩带分布在与嫩江—八里罕断裂相关的左行走滑剪切带内。通过LA-ICP-MS锆石U-Pb年代学研究,确定该花岗斑岩的形成时代为124.99±0.99 Ma,属于早白垩世晚期。地球化学特征表明,该花岗斑岩SiO 2含量为70.56%~78.34%,(Na 2O+K 2O)为7.10%~8.99%,铝饱和指数A/CNK为0.94~1.20,富硅、碱,属于准铝质-过铝质花岗岩。花岗斑岩稀土元素总量中等,轻、重稀土元素分馏明显((La/Yb)N=9.49~14.71),Eu负异常较强(δEu=0.33~0.73),相对富集大离子亲石元素Rb、Ba和高场强元素Th、U,Sr、Nb、Ti相对亏损,呈明显负异常,属于高分异I型花岗岩。嫩江—八里罕断裂在早白垩世晚期曾发生短暂的伸展作用。

Two granitic porphyry belts outcropped in the Ulanhot area,Inner Mongolia,occur mainly in the sinistral strike-slip shear zone related to the Nenjiang-Balihan fault.LA-ICP-MS zircon U-Pb dating shows that granitic porphyry formed at 124.99±0.99 Ma,which belongs to the late Early Cretaceous.Geochemical characteristics show that the granitic porphyry has SiO 2content of 70.56%~78.34%and(Na 2O+K 2O)content of 7.10%~8.99%,with A/CNK ratio of 0.94~1.20,and enrichment in Si and alkaline suggests that it belongs to subluminous-peraluminous granite.Granitic porphyry is characterized by medium REE contents,distinct fractionation of LREE and HREE((La/Yb)N=9.49~14.71),strong negative Eu anomaly(δEu=0.33~0.73),relative enrichment of large ion lithophile elements(Rb and Ba)and of high field strength-elements(Th and U),but relative depletion of Sr,Nb and Ti,with distinct negative anomaly,suggesting that it belongs to highly fractionated I-type granite.It can be concluded in this study that the Nenjiang-Balihan fault in the Ulanhot area experienced a short-period extension in the late Early Cretaceous.