西秦岭宕昌-舟曲地区晚三叠世埃达克质花岗岩年代学、地球化学特征及其构造意义

Geochronology,Geochemistry and Tectonic Significance of Late Triassic Adakite Granites in Tanchang-Zhouqu area of West Qinling

西秦岭造山带位于秦岭-大别造山带西段,其印支期花岗岩研究对认识中央造山带的构造演化具有十分重要的意义。笔者选择西秦岭宕昌-舟曲地区的燕麦层与憨班花岗岩体进行了岩相学、LA-ICP-MS锆石U-Pb年代学、地球化学的研究。结果表明,燕麦层的二长闪长岩、憨班的花岗闪长岩,具有高Al_(2)O_(3)、MgO,Mg^(#)<45,富钠贫钾的特征,主体为亚碱性-碱性系列,准铝质-弱过铝质I型花岗岩,轻稀土高度富集,弱Eu负异常,富集Th、K、U等LILE;HFSE中强烈亏损Nb、Ta、P和Ti元素。锆石U-Pb同位素测年结果显示燕麦层与憨班岩体分别形成于(219.4±1.5)Ma和(222.1±1.9)Ma,其侵位时间为晚三叠世中期。综合地质、地球化学和年代学特征,研究区花岗岩体为后碰撞环境,可能形成于增厚地壳下部岩石圈地幔拆沉、软流圈上涌加热下地壳部分熔融的环境下。

The west Qinling orogenic belt is located in the west of Qinling-Dabie orogenic belt.The study of granites is of great significance for understanding the tectonic evolution of the central orogenic belt.In this paper,Petrography,LA-ICP-MS zircon U-Pb chronology and geochemistry of the Yanmaiceng and Hanban granitoids in the Tanchang-Zhouqu area of the Western Qinling Mountains were studied.The results show that the Yanmaiceng monzodiorite and HanBan granodiorite are characterized by high Al_(2)O_(3),MgO,Mg^(#)<45,rich in Na and poor in K.The main–granites are subalkaline–alkaline series,quasi-aluminous~weakly peraluminous Itype granites,which are highly enriched in LREE,weak negative Eu anomaly,and enriched in LILE such as Th,K,U.The elements of Nb,Ta,P and Ti in HFSE were strongly depleted.Zircon U-Pb isotopic dating shows that the Yanmaiceng and Hanban granitoid were formed at(219.4±1.5)Ma and(222.1±1.9)Ma,respectively,and emplaced in the middle of Late Triassic.According to the geological,geochemical and chronological characteristics,the granites in the study area are in a post-collision environment,which may have been formed in the crust thickening,and the lower lithospheric mantle detachment occurred,resulting in the upwelling and thickening of the asthenosphere and the partial melting of the lower crust.