• 责任者：Wang Zijin (College of Earth Sciences ; Jilin Univercity ; Changchun 130061 ; China) ; Xu Wenliang
• 刊名：Geological Bulletin of China
• 出版年：2013
• 卷期：32(2/3)
• 关键词：镁铁质岩浆 ; mafic magmas ; 铀—铅计时 ; U-Pb dating ; 内蒙古 ; Inner Mongolia
• 页码：p.374-387
• 图表：illus., 3 tables, 50 refs.
• 分类号：0801
• issnNo：ISSN1671-2552
• isbnNo：CN11-4648/P

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.