内蒙古阿尔哈达地区花岗岩锆石LA-ICP-MSU-Pb年龄、稀土元素、Hf同位素组成及其地质意义

Zircon LA-ICP-MS U-Pb ages and REE and Hf isotope compositions of the granite in the Arehada area,Inner Mongolia,and their geological significance

内蒙古阿尔哈达地区位于西伯利亚板块东南缘查干敖包-奥尤特-朝不楞岩浆岩带东段,该区发育大量晚古生代花岗岩,主要岩石类型为似斑状花岗岩和中细粒花岗岩。对该区1件似斑状花岗岩、2件中细粒花岗岩进行了锆石LA-ICP-MSU-Pb同位素定年,获得成岩年龄分别为319.0±5.0 Ma(MSWD=4.9)、324.0±2.0 Ma(MSWD=1.3)和332.0±8.5 Ma(MSWD=5.7)。定年结果显示所有花岗岩的形成时代均为早石炭世,属于海西中期。锆石的原位微区稀土元素分析表明,所有花岗岩中的锆石具有轻稀土元素亏损、重稀土元素富集、强烈的正Ce异常和负Eu异常特征,与岩浆锆石的特点一致。Lu-Hf同位素测试结果显示,锆石的εHf(t)值为-0.71^+10.21,二阶段模式年龄为709~1 436 Ma,表明其主要起源于中新元古代增生的地壳。根据已获得的同位素年龄和锆石稀土元素数据,结合野外地质调查和区域构造演化,可以推测阿尔哈达地区可能存在时限为310~330 Ma的构造岩浆活动。

Located in the eastern part of the Chagan Obo-Aoyoute-Chaobuleng magmatic belt at the southeastern edge of the Siberian plate, the Arehada area has wide distribution of Late Paleozoic granite, comprising mainly porphyritic and medium-fine grained granite. Zircon LA-ICP-MS U-Pb isotopic dating indicates that the petrogenetic ages are 319.0±5.0 Ma（MSWD=4.9）, 324.0±2.0 Ma（MSWD=1.3） and 332.0±8.5 Ma（MSWD=5.7）. The results show that all the granites are of early Carboniferous, belonging to middle Hercynian period. In situ rare earth elements analyses of zircons show the depletion of LREE and enrichment of HREE as well as the obvious existence of positive Ce anomaly and negative Eu anomaly, suggesting that zircons are of typical magmatic origin. Analyses of zircon Lu-Hf isotopes show that these granites have εHf（t） values of -0.71~＋10.21 with Hf modal ages of 709~1 436 Ma, indicating that the granite in Arehada area was derived from mantle during Meso-Neoproterozoic period. Based on the isotopic ages and zircon rare earth elements data in combination with geological survey and regional tectonic evolution, the authors hold that a tectonic-magmatic activity occurred during 310 to 330 Ma in the Arehada area.