锆石U-Pb定年对大兴安岭东北部“兴华渡口群”形成时代和组成的约束

兴华渡口群等大兴安岭北部前寒武纪变质岩系的组成和演化对于确定额尔古纳和兴安地块的构造属性具有重要意义,是近年大兴安岭北部基础地质研究的热点之一。本次工作通过对黑河北部石灰窑—明智山一带的兴华渡口群二云石英片岩和"混合岩"进行锆石LA-ICP-MS U-Pb定年发现该变质岩系并非前寒武纪变质岩,而是由早古生代碎屑沉积岩(或变质岩)和晚古生代岩浆岩经后期构造岩浆作用改造而形成的构造杂岩。其中二云石英片岩中具有岩浆成因特征的碎屑锆石核部年龄主要存在401~427 Ma、442~448 Ma、473~517 Ma、639~714 Ma、757~818Ma、896~933 Ma和1704~1751 Ma 7个年龄组,其中473~517 Ma段碎屑锆石的峰最明显,与早古生代多宝山组岛弧火山岩等早古生代岩浆作用形成时间相一致,其他年龄组亦在区域上其他地区有报道,这表明该变质岩的原岩物源来源较广泛,不仅有元古宙岩浆岩和变质岩系,还有大量的早古生代岩浆岩,因此其原岩形成时代不应是前寒武纪,而是早古生代。根据碎屑锆石最小峰值年龄,本次工作推断该二云石英片岩原岩的最大沉积年龄应不早于416Ma,另外大量的元古宙碎屑锆石表明区域上可能存在前寒武纪变质基底。对所谓混合岩的调查发现其应为发生动力变质的糜棱岩化二长花岗岩,其中岩浆锆石(304.5±3.1)Ma的206Pb/238U加权平均年龄反映花岗岩形成于晚石炭世晚期,该期花岗岩为晚古生代兴安地块东缘花岗岩带的一部分。

Constraints from the zircon U-Pb dating on the formation age and composition of the Xinhuadukou Group in northeastern Da Hinggan Mountains

The study of the composition and evolution of the Precambrian metamorphic series such as Xinghudukou Group in northeastern Da Hinggan Mountains is critical for determining the tectonic attribute of Ergun massif and Xing''an block, and thus is one of the hotspots of recent studies of northern Da Hinggan Mountains. Based on the U-Pb dating results of the detrital zircons from two-mica-quartz schist and the magmatic zircons from migmatite, the authors hold that the Xinghuadukou Group in the Shihuiyao-Mingzhi Mountain of northern Heihe area is not the Precambrian metamorphic rock, but is a tectonic complex composed of early Paleozoic sedimentary (or metamorphic) rocks and Late Paleozoic magmatic rocks. The dating results of the magmatic core of detrital zircons from two-mica-quartz schist show 7 age groups mainly, i.e., 401-427 Ma, 442-448 Ma, 473-517 Ma, 639-714 Ma, 757-818 Ma, 896-933 Ma and 1704-1751 Ma, in which the 473-517 Ma group exhibits most obvious peaks of detrital zircons and is consistent with the early Paleozoic magma event such as the island arc volcanic rock from the Duobaoshan Formation, and the other age groups have also been reported in many places in northern Da Hinggan Mountains. These characteristics suggest that the provenance was complex, composed of not only Proterozoic rocks but also Paleozoic rocks, with the early Paleozoic magmatic rocks being the main source. So the protolith age of the two-mica-quartz schist is the Paleozoic and might be sfter 416 Ma, as shown by the minimum peak age of detrital zircons. The considerable Proterozoic detrital zircons from the schist suggests that there indeed exists Precambrian basement in Ergun massif or Xing''an blocks. The migmatite outcropped in the Mingzhi Mountain is the mylonitized granite actually caused by the dynamic metamorphism, and it has a ²⁰⁶Pb/²³⁸U weighted average age of (304.5±3.1)Ma, which reflect the granite formed during the Late Carboniferous. The granite was part of the estern margin granite belt in of Xing''an blocks during the late Paleozoic.