内蒙古西乌旗阿鲁包格山A型花岗岩锆石U-Pb年龄、地球化学特征及地质意义

Zircon U-Pb Age and Geochemical Characteristics of the Alubaoge A-type Granite in Xiwuqi, Inner Mongolia and its Geological Significance

内蒙古西乌旗阿鲁包格山花岗岩位于华北板块北缘中段,岩性以斑状二长花岗岩为主。本次1/5万区域地质调查工作研究发现其具有如下地球化学特征：高硅（SiO2为71.29%-72.12%）、富碱（Na2O＋K2O为8.70%-8.98%）、高钾（K2O为4.93%-5.27%）、富轻稀土元素而亏损重稀土元素（LREE/HREE为7.69-12.18）、明显的负铕异常（δEu为0.08-0.14）、较高的Ga/Al比值（2.96×10^-4-4.00×10^-4）和Zr＋Nb＋Ce＋Y值（548×10^-6-739×10^-6）、富集Rb、Th、K、La、Nd、Hf、Zr和Pb以及亏损Ba、Ta、P和Ti等特征,类似于A型花岗岩。在判别图解上,所有样品均显示了A2型（后造山）花岗岩特征,其形成可能与后造山伸展构造有关。锆石U-Pb年龄为132.19±0.77 Ma（MSWD=4.6）,其形成时代为早白垩世。结合区内分布有大量晚中生代中基性-中酸性火山岩断陷盆地,表明早白垩世测区已进入后造山伸展构造环境。

The Alubaoge granite in Xiwuqi of inner-Mongolia, which is located in the middle segment of the northern margin of North China Craton geologically, consists mainly of porphyritic monzogranite. The Alubaoge granites are characterized by high SiO2（71.29%-72.12%）, high potassium（K2O=4.93%-5.27%, Na2O＋K2O=8.70%-8.98%） content, and enriched in LREE, depleted in HREE（LREE/HREE=7.69-12.18）, with obviously negative Eu anomaly（δEu= 0.08-0.14）. While the trace elements are characterized by Rb, Th, K, La, Nd, Hf, Zr, Pb enriched, and Ba, Ta, P, Ti depleted, as well as with high Ga/Al ratio（2.96×10^-4-4.00×10^-4）, high Zr＋Nb＋Ce＋Y value（548×10^-6-739×10^-6）. The chemical signatures belong to A-type granite. In discrimination diagrams, all the samples show geochemical characteristics of A2-type granitoid, which means that the Alubaoge granite may be formed in post-orogenic stage. The zircon LA-ICP-MS U-Pb dating gives a weighted average age of 132.19±0.77 Ma（MSWD=4.6）. The age suggests that the granite was emplaced in the Early Cretaceous. Considering the wide distribution of the Late Mesozoic basic and intermediate-acid volcanic rocks in depression basins, the authors suggest that this area was in post-orogenic regime in the Early Cretaceous.