| 碎屑锆石U-Pb年龄谱系对兴安地块落马湖群原岩形成时代的约束 |
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| 引用本文： | 赵院冬,车继英,许逢明,魏明辉.碎屑锆石U-Pb年龄谱系对兴安地块落马湖群原岩形成时代的约束[J].地球科学,2020,45(2):489-502. |
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| 作者姓名： | 赵院冬 车继英 许逢明 魏明辉 |
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| 作者单位： | 中国地质调查局沈阳地质调查中心, 辽宁沈阳 110034 |
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| 基金项目： | 国家重点研发计划深地资源勘查开采专项项目2017YFC0601305国家自然科学基金项目41502063中国地质调查局地质调查项目12120114055701 |
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| 摘 要： | 发育于大兴安岭地区如兴华渡口群、落马湖群等系列变质岩系一直被认为是中亚造山带东段各地块的变质基底，但其形成时代目前存在较大争议，制约对于区域大地构造演化的研究.为了查明兴安地块东北部落马湖群原岩形成时代，选取不同层位二云母片岩开展碎屑锆石LA-ICP-MS U-Pb年龄谱系研究，分析原岩蚀源区特征和最大沉积上限.分析结果表明，两件样品碎屑锆石均为岩浆成因锆石，获得一致的锆石U-Pb年龄谱系，主要集中在400~430 Ma、430~465 Ma、470~520Ma、630~715 Ma、750~830 Ma、880~940 Ma、1 720~1 870 Ma等7个区间.结合区域地质组成，碎屑锆石年龄谱特征反映原岩蚀源区比较复杂，时空范围较大，但以近源的早古生代岩浆岩为主，而来自额尔古纳地块元古代的岩浆岩和变质岩系可能是重要的物源.根据碎屑锆石最小年龄（406~411 Ma），确定原岩沉积时代的上限为早泥盆世早期，而不是原认为的形成于新元古代-早寒武世.结合原岩沉积建造对比，推断落马湖群可能与上志留统-下泥盆统系列地层（如卧都河组、泥鳅河组）为同时代同环境沉积建造，现有差异可能源于后期经受不同的变质作用.其变形变质作用可能发生在中生代，与蒙古鄂霍茨克洋的闭合作用相关.
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| 关 键 词： | 兴安地块 落马湖群 碎屑锆石 形成时代 地球化学 |
| 收稿时间： | 2018-12-29 |
Constraints on the Forming Age of the Luomahu Group in the Xing'an Block from the Detrital Zircon Population |
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| Abstract: | The series of metamorphic rocks outcropped in the Great Xing'an Range area such as Xinghuadukou Group and Luomahu Group have been considered as the metamorphic basement of the Erguna massif and Xing'an block in the eastern part of the Central Asian Orogenic Belt, but there is a great controversy over the age of its formation, which restricts the study of the regional tectonic evolution. In order to ascertain the protolith age of the Luomahu Group in the northeast of Xing' an block, this paper selects two mica schists to carry out the study of LA-ICP-MS U-Pb age of detrital zircon, and analyzes the characteristics of the provenances and the maximum sedimentary limit. The results show that the dated detrital zircons from two samples are the magmatic origin zircons, and have similar age population, which concentrated in 7 intervals as 400-430 Ma, 430-465 Ma, 470-520 Ma, 630-715 Ma, 750-830 Ma, 880-940 Ma and 1 720-1 870 Ma mainly. Combined the regional geological composition, the detrital zircon population reflects that the provenance was complex and had large space time range, but the early Paleozoic magmatic rocks from adjacent area were the main provenance, and the Proterozoic magmatic rocks and metamorphic rocks from the Erguna massif maybe another important source. Based on the minimum age of detrital zircon, it could be determined that the upper limit of sedimentary age of the protolith was Early Devonian, rather than from the Neoproterozoic to Early Cambrian as originally thought. According to the comparison of the sedimentary formation, it is inferred that the Luomahu Group may be deposited contemporaneously with the Upper Silurian-Lower Devonian series (such as the Woduhe Formation and the Niqiuhe Formation) at the same tectonic environment, and the existing differences may be caused by experiencing different metamorphisms at the later stages. The deformation and metamorphism might be related to the closure of the Mongol-Okhotsk Ocean during the Mesozoic. |
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