内蒙古西太仆寺破火山碎斑熔岩与流纹岩LA-ICP-MS锆石U-Pb年龄和地球化学特征

LA-ICP-MS zircon U-Pb geochronology and geochemistry of the porphyroclastic lava and rhyolite in west Taipusi Banner area of Inner Mongolia

内蒙古西太仆寺破火山内碎斑熔岩与流纹岩的LA-ICP-MS锆石U-Pb定年结果分别为140.1±0.6Ma和138.3±0.9Ma,为早白垩世初期岩浆活动的产物。2类岩石具有一致的地球化学特征,都表现出高硅,富碱,贫钙、镁、钛、磷的特征,属于高钾钙碱性系列;铝饱和指数A/CNK=0.99~1.13,主要为过铝质岩石。岩石富集Rb、Th、U等大离子亲石元素,强烈亏损Ti、Nb、Ta等高场强元素,强烈负Eu异常(δEu=0.06~0.24),稀土元素配分图呈右倾的海鸥型。Ba、Sr也表现出强烈亏损的特征。此外,岩石还具有高的TFeO/MgO(5.64~43.29)和104×Ga/Al(2.96~4.41)值。地球化学特征表明,碎斑熔岩和流纹岩具有铝质A型花岗岩的特征,可能是在区域岩石圈伸展环境下,深部软流圈的上涌和幔源镁铁质岩浆的底侵作用,对上覆地壳直接的加热作用促使其部分熔融形成的。

LA-ICP-MS zircon U-Pb dating was carried out for two samples of porphyroclastic lava and rhyolite from the west Taipusi Banner,Inner Mongolia.As a result,their ages were obtained,which are 140.1±0.6Ma and 138.3±0.9Ma,respectively.The results show that all the rocks were produced by magmatic activities in Early Cretaceous.Porphyroclastic lava and rhyolite have the same geochemical features,characterized by high content of SiO2,Na2O+K2O and low CaO,MgO,TiO2 and P2O5,and belonging to the high-K calc-alkaline series.A/CNK values of the rocks are from 0.99 to 1.13,suggesting that they are peraluminous.The rocks are characterized by LILE enrichment(Rb,Th,U)and HFSE(Ti,Nb,Ta)strong depletion.They are characterized by enriched LREE patterns with strong negative Eu anomalies(δEu=0.06~0.24),with high TFeO/MgO(5.64~43.29)and 104×Ga/Al(2.96~4.41)ratios,and strong depletion of Ba and Sr.All these geochemical data indicate that they are mainly aluminous A-type granite.Under the regional lithospheric extension,the asthenosphere upwelling and mafic magma underplating provided enhanced heat flux and triggered the partial melting of the overlying crust and then produced the porphyroclastic lava and rhyolite.