内蒙古林西朝阳沟早白垩世花岗岩锆石U-Pb年代学和地球化学及其意义

林西县朝阳沟地区位于华北板块与西伯利亚板块之间的兴蒙造山带的东部.朝阳沟花岗岩体的LA-ICP-MS锆石U-Pb测年结果表明,岩体的侵位时间为早白垩世(126.2±2.5 Ma).地球化学分析显示,该花岗岩体具有富硅(SiO_2=68.5%~72.1%)、铝(Al_2O_3=14.05%~14.95%)、碱(K_2O+Na_2O=8.34%~10.03%)和Fe_2O_3(2.27%~4.15%)的特点,属于准铝质碱性花岗岩系列;A/CNK=0.97~1.09,Mg#值较高,为12.66~15.02;稀土配分曲线呈明显的右倾型,轻、重稀土元素分馏明显;具有显著的Eu负异常和Ba、Ta、Nb、Sr、P、Ti亏损,富集Rb、Th、U、K等大离子亲石元素.在(Yb+Nb)-Rb判别图解和R1-R2判别图解中,样品全部落于后碰撞花岗岩区和造山晚期花岗岩区.结合区域地质背景,认为该岩体形成于后碰撞伸展构造环境中.

ZIRCON U-Pb DATING,GEOCHEMISTRY AND THEIR SIGNIFICANCE OF THE EARLY CRETACEOUS CHAOYANGGOU GRANITES IN LINXI,INNER MONGOLIA

The Chaoyanggou pluton in Linxi area, Inner Mongolia, is tectonically located in the east of Xingan-Mongolia orogenic belt between the North China plate and Siberian plate. The LA-ICP-MS zircon U-Pb dating for the Chaoyanggou rock body indicates that the granite was emplaced in Early Cretaceous (126.2±2.5 Ma). The intrusive rock is geochemically characterized by high SiO₂(68.5%-72.1%), Al₂O₃(14.05%-14.95%), alkali(K₂O+Na₂O=8.34%-10.03%) and Fe₂O₃ (2.2 7%-4.15%), belonging to quasi aluminum and calc-alkaline granite, with A/CNK=0.97-1.09 and high Mg^# values (12.66-15.02). The pluton shows relatively more enriched LREE(124×10^-6-199×10^-6) in comparison with HREE(15×10^-6-21×10^-6). It has evident negative Eu anomaly, with distinct negative Ba, Ta, Nb, Sr, P and Ti anomalies and enrichment of LILE such as Rb, Th, U and K. Based on the (Yb+Nb)-Y and R1-R2 diagrams, all samples fall in postcollisional granite area and late orogenic granite area. Considering the regional geological setting, it is concluded that the Chaoyanggou granite pluton is the product of extensive background after collision.