| 扬子西缘奥陶纪—志留纪之交斑脱岩锆石U-Pb年龄、Hf同位素特征及其地质意义 |
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| 引用本文：袁永盛,唐烽,王万能,张耀堂,李文辉,张宏辉,张沥元,娄元林.2026.扬子西缘奥陶纪—志留纪之交斑脱岩锆石U-Pb年龄、Hf同位素特征及其地质意义[J].地球学报,47(4):733-749. |
| DOI：10.3975/cagsb.2025.110811 |
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| 基金项目:本文由中国地质调查局地质调查项目(编号: DD202601103305; DD20220987; DD202308404)和国家自然科学基金项目(编号: 42172035)联合资助。 |
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| 中文摘要:本次工作在扬子板块西缘昭通地区奥陶纪—志留纪之交临湘组—新滩组发现斑脱岩共10层, 其中临湘组底部的第1层斑脱岩和新滩组第10层斑脱岩均为首次发现。为研究斑脱岩的年代学、原岩类型和大地构造背景, 对临湘组底部和新滩组顶部的斑脱岩进行锆石U-Pb测年和锆石微量元素和Hf同位素分析。其中临湘组底部斑脱岩获得其锆石U-Pb年龄为(447.9±1.7) Ma, εHf(t)=1.3~4.9, TDM2=1 119~1 330 Ma; 新滩组顶部斑脱岩获得其锆石U-Pb年龄为(436.1±2.2) Ma, εHf(t)= –9.4~ –14.2, TDM2=2 017~2 316 Ma。斑脱岩测年数据较好地限定了奥陶纪—志留纪之交火山活动在扬子区的沉积记录, 为扬子板块西缘昭通地区奥陶纪—志留纪之交沉积环境演化提供了年代学参考。于~447.8 Ma扬子板块西缘昭通地区开始了由宝塔组碳酸盐台地向深水陆棚转变, 于~436.1 Ma结束新滩组浅水陆棚沉积进入黄葛溪组碳酸盐岩台地相沉积。斑脱岩的锆石微量元素特征和Hf同位素特征显示: 扬子板块西缘地区奥陶纪—志留纪之交的斑脱岩存在两种形成于不同的源区和大地构造背景的原岩类型, 早期(449~444.7 Ma)岩浆主要为来自新生下地壳的偏基性岩浆, 形成于拉张环境; 晚期(443~438 Ma)岩浆物质以古老成熟地壳为主、形成于弧环境的偏酸性岩浆。通过与扬子周缘同时期火山活动记录、岩浆类型和大地构造背景对比, 斑脱岩可能来源于北秦岭地区奥陶纪—志留纪之交的弧-弧后火山活动。 |
| 中文关键词:扬子西缘 奥陶纪—志留纪 斑脱岩 锆石U-Pb年龄 Hf同位素 |
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| Zircon U-Pb Ages and Lu-Hf Isotope of the Bentonite during the Ordovician–Silurian Transition in the Western Yangtze Block and Their Geological Significance |
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| Abstract:In this study, 10 layers of bentonite were identified in the Linxiang Formation–Xintan Formation during the Ordovician–Silurian transition in the Western Yangtze Block. The first layer of bentonite was at the bottom of the Linxiang Formation, and the tenth layer of bentonite in the Xintan Formation was discovered for the first time. To study the geochronology, protolith type and tectonic setting of the bentonite, zircon U-Pb dating, zircon trace element, and Hf isotope analysis were conducted on the bentonite at the bottom of the Linxiang Formation and the bentonite at the top of the Xintan Formation. The Zircon U-Pb results of the bentonite from the bottom of the Linxiang Formation were (447.9±1.7) Ma, εHf(t) = 1.3–4.9, TDM2=1 119–1 330 Ma. The Zircon U-Pb results of the bentonite at the top of the Xintan Formation were (436.1±2.2) Ma, εHf(t) = –9.4– –14.2, TDM2=2 017–2 316 Ma. The sedimentary record of volcanic activity during the Ordovician–Silurian transition in the Yangtze region is well-defined. The age provides a chronological reference for the sedimentary environment evolution of the Ordovician–Silurian in the Zhaotong area of the Upper Yangtze. The transformation from the platform carbonate facies of the Baota Formation to deep-water shelf facies began at approximately 447.8 Ma. The shallow-water shelf facies of the Xintan Formation entered the platform carbonate deposition facies of the Huanggexi Formation at about 436.1 Ma. The characteristics of zircon trace elements and Hf isotopes of the bentonite indicate that two types of protoliths were generated in different source areas and tectonic backgrounds existing in the Yangtze area: the early (449–444.7 Ma) magma was mainly partial basic magma from the new lower crust, which formed in an extensional environment. In the late stage (443–438 Ma), the magmatic material was mainly derived from the ancient mature crust, which formed in an arc environment of acidic magma. The transformation of tectonic environment occurred near the boundary between the Linxiang Formation and Wufeng Formation. By comparing with the volcanic activity records, magma types, and tectonic background of the same period around the Yangtze, the bentonite may be derived from the arc-backarc volcanism in the North Qinling area during the Ordovician–Silurian period. |
| keywords:western margin of the Yangtze block Ordovician–Silurian bentonite zircon U-Pb age Hf isotope |
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