兴蒙造山带南缘东段中二叠世末—早三叠世镁铁质岩浆作用及其构造意义——来自锆石U-Pb年龄与地球化学的证据

Middle Permian-Early Triassic mafic magmatism and its tectonic implication in the eastern section of the southern margin of the Xing'an-Mongolian Orogenic Belt,NE China:Constraints from zircon UPb geochronology and geochemistry

根据兴蒙造山带南缘东段的3个镁铁质侵入岩体的锆石LA-ICP-MS U-Pb定年与岩石地球化学分析结果,揭示研究区晚古生代晚期—早三叠世的构造属性。3个岩体中锆石的CL图像显示,它们主要呈条痕状吸收,并具有较高的Th/U比值(0.22-1.99),暗示其为岩浆成因。岩浆锆石的定年结果表明,它们形成于中二叠世末—早三叠世(263-246Ma),而非前人认为的燕山早期。岩相学研究表明这些岩体主要由辉长岩、角闪辉长岩和辉长闪长岩构成,部分岩石显示堆晶结构。3个岩体的SiO2含量介于46.10%-54.29%之间,MgO的含量在6.49%-13.38%之间,Mg#值介于56-72之间,Al2O3含量为10.23%-22.37%,Na2O的含量为1.22%-3.36%,K2O为0.43%-1.35%,Na2O/K2O为1.22-2.89。稀土元素球粒陨石标准化曲线显示右倾型、平坦型或上凸型,δEu=0.78-2.69;在微量元素蛛网图上,大离子亲石元素相对富集,高场强元素相对亏损。锆石的εHf(t)值介于+9.5--3.8。上述结果表明,该区中二叠世末—早三叠世镁铁质岩浆主要来源于受俯冲物质改造的亏损岩石圈地幔的部分熔融,部分岩浆源区具有少量古老陆壳物质的涉入。结合研究区同时代花岗岩的存在,表明研究区晚古生代晚期—早三叠世岩浆作用显示双峰式火成岩组合,揭示该时期研究区处于一种伸展环境。

LA-ICP-MS zircon U-Pb dating and geochemical data are reported for three representative mafic intrusions in the eastern section of the southern margin of the Xing＇an-Mongolian Orogenic Belt （XMOB）,with the aim of constraining the Late Paleozoic tectonic evolution in the region. The selected zircons exhibit striped absorption in cathodoluminescence （CL） images and have high Th/U ratios （0.22-1.99）, indicating their magmatic origin. The zircon U-Pb dating results indicate that these mafic intrusions formed in the Middle Permian-Early Triassic （263-246Ma）, rather than previously believed Early Yanshanian period. The three mafic plutons are composed of gabbro, hornblende-gabbro, and gabbro-diorite. Some of them display cumulate textures. These mafic igneous rocks have SiO2=46.10%-54.29%, MGO=6.49%-13.38%, Mg^#[= 100Mg/（Mg＋TFe^2＋）] =56-72, Al2O3=10.23%-22.37%, Na2O = 1.22%-3.36% ,K20= 0.43%-1.35%, Na2O/K2O=l.22%-2.89%, and 8Eu=0.78-2.69. These mafic rocks display three types of rare earth ele= ment （REE） distribution patterns, i.e., right-slipped, flat, and dome-like patterns. Meanwhile, they are enriched in large ion lithophile elements （LILEs, such as Ba, K and Sr）, and depleted in high field strength elements （HFSEs, such as Nb, Ta, Zr and Hf）. The εHf （t） values of zircons from these mafic rocks range from ＋9.5 to-3.8. Taken together, it is suggested that the primary magmas for the Middle Permian-Early Triassic mafic rocks could be mainly derived from partial melting of a depleted mantle metasomatized by subduction-related fluid, and that the ancient continental crust material could be involved in some magma source. Combined with the existence of coeval granitoids in the study area, we can conclude that the Middle Permian-Early Triassic bimodal igneous rock association formed under an extensional setting.