内蒙古东乌珠穆沁旗查干敖包花岗岩体时代、成因及地质意义

Ages,Origin and Geological Implications of the Chaganaobao Granite Body in the East Ujimqin Banner,Inner Mongolia

内蒙古东乌珠穆沁旗北部的查干敖包岩体可分解为灰黄色细粒花岗岩单元和灰黄色中细粒似斑状含二云母花岗岩单元。前者锆石U-Pb加权平均年龄为(306.4±2)Ma,为晚石炭世岩浆活动产物,后者锆石U-Pb加权平均年龄为(278.3±2)Ma,为早二叠世岩浆活动产物。两期花岗岩具有相似的地球化学特征,主量元素均具有富Si,贫Ti、Mg、Ca,且有较高的碱/铝(NK/A)比值等特征;均表现出明显的负Eu异常;微量元素富集Rb、Th、U、K,弱富集P、Zr及Hf,强烈亏损Ba、Sr及Ti。通过分析讨论,认为晚石炭世晚期细粒花岗岩单元和早二叠世中细粒似斑状花岗岩单元的成因类型均为I-S过渡型钙碱性花岗岩。岩石化学、岩体产状、分布特点和源岩特征都表明,查干敖包岩体晚石炭世晚期和早二叠世花岗岩构造背景环境一致,均为晚古生代中亚造山大地构造背景下碰撞后转换期构造—岩浆事件产物。

The Chaganaobao pluton in the north of the East Ujimqin Banner, Inner Mongolia can be divided in- to gray-yellow fine-grained granite unit and gray-yellow medium - fine-grained porphyraceous medium - fine- grained two-mica granite unit. The U-Pb weighted average age of the former is （306.4 ± 2） Ma, which is the product of Late Carboniferous magmatic activity. The latter has a U-Pb weighted average age of （278.3 ±2 ） Ma, which is the product of Early Permian magmatic activity. The two kinds of granites have similar geochemi- cal characteristics. The main elements are all rich in Si, but poor in Ti, Mg, Ca and have a high ratio of al- kali/aluminum （NK/A） , all of which show obvious negative Eu anomalies. Trace elements are enrichment of Rb, Th, U, K and are low enrichment of P, Zr, Hf, and have a strong loss of Ba, Sr, Ti. Based on the analysis and discussion, we conclude that the Late Carboniferous granite unit and the Early Permian medium- fine-grained porphyraceous granite unit are all the I-S transitional calc-alkaline granite type. The petrochemis-try, rock formations, distribution and rocks sources all indicate that the late Late Carboniferous and early Per- mian granite body of the Chaganaobao were consistent with the product of the Late Paleozoic Central Asia oro- genic tectonic setting and transformation period tectonic-magmatic event.