内蒙古苏尼特右旗白垩纪A型花岗岩锆石U-Pb年龄、地球化学特征及其构造意义

兴蒙造山带中段苏尼特右旗北部敖包吐岩体主要由二长花岗岩和正长花岗岩组成,LAICP-MS锆石U-Pb年龄分别为143.37±0.83Ma和142.58±0.61Ma,时代为早白垩世,代表着区域上燕山期最广泛的一期岩浆活动。地球化学分析表明,敖包岩体具高硅(71.68%~76.65%)、高碱(7.51%~9.01%))、低铝(12.01%~14.10%)、富铁、贫钙、贫镁特点,属于高钾钙碱性-碱性过渡系列花岗岩;稀土元素丰度中等,呈轻稀土富集、重稀土亏损的右倾模式。微量元素特征表现为大离子亲石元素(LILE)Rb、Th、U、K相对较为富集,高场强元素(HFSE)Nd、Ta、Zr和Hf相对富集,而Ba、Sr、P、Ti强烈亏损。上述特征与典型的A1型板内花岗岩相似,其成因可能为在岩石圈伸展环境下,大陆拉张减薄导致软流圈物质上涌,岩浆与上地壳发生混染而形成。

Zircon U-Pb ages, geochemical characteristics and tectonic significance of the Cretaceous A-type granites in Sunite Youqi, Inner Mongolia

The granitic rocks of Sunite Youqi Aobaotu, located in the middle of the Xing'an Mongolia orogenic belt, are composed of monzonitic gran-ite and syenogranite. The LA-ICP-MS zircon U-Pb ages of monzonitic granite and syenogranite are 143. 37 ± 0. 83Ma and 142. 58 ± 0. 61Ma, re-spectively, belonging to Early Cretaceous, which represent the most extensive magmatic activity in Yanshanian period. All samples are characterized by high silicon (71. 68% ~ 76. 65%), high alkali (7. 51% ~ 9. 01%), low aluminum (12. 01% ~ 14. 10%), high iron, low calcium and magnesi-um. They are thus classified as high-K calcium alkaline - alkaline transition series. These samples exhibit medium REE abundance and LREE enrich-ment. They also show LILE enrichment ( Ta,Zr,Hf) ,HFSE depletion ( Ba,Sr,P,Ti) and enrichment of ( Nd,Ta,Zr,Hf) . These features are similar to the typical A1 -type intraplate granites, which may be attributed to the upwelling of the asthenosphere in the extensional environment of the lithosphere, and the magma contaminated by the upper crust.