内蒙古莫尔道嘎地区晚三叠世-早侏罗世花岗岩的年代学、地球化学及其地质意义

本文对内蒙古莫尔道嘎地区(额尔古纳地块)花岗质岩石行了锆石LA-ICP-MS U-Pb年代学和岩石地球化学研究, 确定其形成时代、岩石成因, 从而揭示区域构造背景。莫尔道嘎地区花岗岩中的锆石均呈半自形-自形, 震荡生长环带明显, 暗示其岩浆成因。测年结果显示, 研究区花岗岩可划分为两个期次: 晚三叠世(~216Ma)和早侏罗世(201~195Ma)。莫尔道嘎地区晚三叠世-早侏罗世花岗岩的地球化学属性相似, 它们具有高硅(SiO₂=67.09%~77.05%)、富铝(Al₂O₃=12.18%~16.83%)、贫铁(Fe₂O₃^T=1.30%~3.48%)等特征, 属于准铝质-过铝质(A/CNK=0.97~1.13)、高钾钙碱性系列, 具有I型花岗岩的地球化学属性。此外, 晚三叠世-早侏罗世花岗岩相对富集大离子亲石元素(LILEs)和轻稀土元素(LREEs), 亏损重稀土元素(HREEs)和高场强元素(HFSEs, Nb、Ta、P、Ti等), 具有与俯冲带环境下形成的火成岩类似的特征。额尔古纳地块上同时期的钙碱性系列岩石组合的发现, 进一步表明莫尔道嘎地区晚三叠世-早侏罗世花岗岩形成于活动大陆边缘环境。结合前人对东北地区中生代火成岩时空变异分布特征的研究, 本文莫尔道嘎地区晚三叠世-早侏罗世花岗质岩体形成与蒙古-鄂霍茨克大洋板片南向俯冲作用密切相关。

Geochronology and geochemistry of Late Triassic-Early Jurassic granites in Moerdaoga area,NE China and its tectonic implications

In this paper, we present new zircon U-Pb dating and whole-rock geochemical data for the granites in Moerdaoga area, NE China, with the aim of constraining its formation time, petrogenesis and the regional tectonic setting. Zircon grains from the granites in the study area are euhedral-subeuhedral in shape, and display fine-scale oscillatory growth zoning, indicating a magmatic origin. Zircon U-Pb dating indicates that two stages of Mesozoic magmatism are recorded in the Erguna Massif, namely during the Late Triassic (~216Ma) and Early Jurassic (201~195Ma). The Late Triassic and Early Jurassic granites in the study area exhibit similar geochemical features. They contain high concentrations of SiO2 (67.09%~77.05%) and Al2O3 (12.18%~16.83%), and low concentrations of Fe2O3T (0.11%~0.30%). They are classified as subalkaline, high-K calc-alkaline, metaluminous to peraluminous series, and exhibit chemical affinities to the Ⅰ-type granites. In addition, the granites in the Moerdaoga area are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs). These features are similar to the igneous rocks that formed in the subduction zone. Additionally, the formation of coeval calc-alkaline rock associations in the Erguna Massif further indicated that the Late Triassic to Early Jurassic granites formed at an active continental margin setting. Combined with the temporal-spatial distributions of Mesozoic igneous in NE China, we concluded that the formation of the Late Triassic to Early Jurassic magmatism was related to the southward's subduction of the Mongol-Okhotsk Oceanic Plate.