四川会理地区古近纪雷打树组碎屑锆石LA-ICP-MSU-Pb年龄及其地质意义

Detrital zircon LA-ICP-MS U-Pb ages of the Paleogene Leidashu Formation in Huili, Sichuan Province and their geological significance

采用LA-ICP-MS方法，对四川盆地南部会理地区古近纪雷打树组碎屑锆石进行了U-Th-Pb同位素测定，获得了72组单颗粒锆石的U-Pb年龄，建立了碎屑锆石的U-Pb年龄谱。结果表明，雷打树组碎屑锆石U-Pb年龄区间为2465~204Ma，地质时代为古元古代最早期成铁纪至晚三叠世最晚期瑞替阶，年龄分布具有清晰的幕式分布特征，集中分布于5个区间，出现了5个明显的峰值，物源区主要为扬子陆块西缘及其西侧的“三江”造山带。雷打树组碎屑锆石U-Pb年龄谱显示，扬子陆块西缘经历了古元古代陆壳增生、中元古代Rodinia超大陆汇聚、新元古代晚期Rodinia超大陆裂解、二叠纪玄武岩喷溢及中-晚三叠世印支运动5次重要的构造热事件，与扬子陆块西缘形成演化进程完全吻合。与四川盆地古近纪柳嘉组碎屑锆石的U-Pb年龄谱相比，雷打树组碎屑锆石U-Pb年龄谱缺失侏罗纪、白垩纪信号，增加了早奥陶世和早泥盆世信号，说明四川盆地北部与南部的物源存在一定的区别。碎屑锆石U-Pb年龄谱对比结果显示,雷打树组碎屑锆石U-Pb年龄谱具有较高的精确度,扬子陆块与华夏陆块自1000Ma汇聚以来具有很好的亲缘性,而与华北克拉通之间直至400Ma才开始建立亲缘关系。

Based on LA-ICP-MS U-Pb dating method, the authors made the U-Th-Pb isotopic measurement of the detrital zir-cons from the Paleogene Leidashu Formation in Huili area, the south part of the Sichuan Basin. 72 groups of effective single grained zircon U-Pb ages were obtained, and the U-Pb age spectrum was set up in this paper. The results show that the detrital zircon ages are in the range of 2465~204Ma, suggesting a period between the most early Early Paleoproterozoic Siderian period and the most late Late Triassic Rhaetian stage. The age distribution has a strong episodic style with five concentrated ranges and five distinct peak val-ues, and the main provenances were the western margin of the Yangtze Block and the“Sanjiang”orogenic belt on the western side. The detrital zircon U-Pb age spectrum of the Leidashu Formation reveals that the western margin of the Yangtze Block has experi-enced five important tectonic thermal events, i.e., the crust accretion in the Paleoproterozoic, the Rodinia Supper Continent assem-blage in the Mesoproterozoic, the Rodinia Supper Continent breakup in the late Neoproterozoic, the basalt eruption in the Permian and the Indosinian movement in the Late Triassic, evidently corresponding with the evolution of the western margin of the Yangtze Block. A comparison with the detrital zircon U-Pb age spectrum of the Paleogene Liujia Formation in the north part of the Sichuan Basin shows that the detrital zircon U-Pb age spectrum of the Leidashu Formation is characterized by the lack of Jurassic and Creta-ceous signals and the addition of Early Ordovician and Early Devonian signals, indicating that the provenance of the north part of the Sichuan Basin and that of the south part of the Sichuan Basin show some difference. Detrital zircon U-Pb age spectrum similarity comparison shows that the detrital zircon U-Pb age spectrum of the Leidashu Formation has relatively high precision and that the Yangtze Craton and Cathaysian Continent have had tight affinity since 1000Ma, whereas the affinity between the Yangtze Craton and the North China Craton began to be established at 400Ma.