西藏桑日县地区中生代火山岩地层层序——基于锆石U-Pb 年龄及地球化学数据
A discussion on Mesozoic volcanic stratigraphic sequence in Sangri County, Tibet: evidence from zircon U-Pb ages and geochemical compositions
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摘要: 青藏高原中生代时期新特提斯洋演化及其相应的岩浆作用一直是高原基础研究的热点之一，其系统研究对新特提斯洋及拉萨地块的地质演化具有重要的意义。以桑日县地区出露的叶巴组、桑日群比马组、桑日群麻木下组、旦师庭组火山-沉积岩系为研究对象，在详细的野外调查及前人研究基础上，选择麻木下组安山岩和旦师庭组火山凝灰岩样品进行锆石U-Pb测年，对叶巴组和桑日群比马组火山岩进行地球化学对比研究。结果表明，桑日群上部比马组火山岩（189.0±3.0~195.0±3.0Ma）与叶巴组火山岩（174.2±3.6~192.7±1.3Ma）均形成于早侏罗世，且两者具有相似的地球化学特征，均为钙碱性岛弧火山岩，表明二者可能形成于相同的构造背景。因此，将比马组从原桑日群中解体出来与叶巴组合并为同一套火山岩地层，而桑日群下部麻木下组（99.9±0.7~136.5±1.7Ma）的形成时间明显晚于比马组，属早白垩世火山岩地层，且具有埃达克岩的地球化学特征，应单独建组；旦师庭组形成于93.7±1.2Ma，属晚白垩世地层，也单独成为一组。Abstract: The evolution of Neo-Tethys and associated magmatism during Mesozoic has become a topic of much debate in the basic study of the Tibetan Plateau. Voluminous Mesozoic volcano-sedimentary sequences corresponding to the subduction of the Neo-Tethys crop out on the southern margin of Lhasa Block. The systematic study of these volcano-sedimentary sequences is of great importance in understanding the evolution of the Neo-Tethys and the Lhasa Block. With the volcano-sedimentary sequences of the Yeba Formation, the Sangri Group and the Danshiting Formation exposed in Sangri County as the study object, the authors carried out zircon LA-ICP-MS and SHRIMP dating and geochemical analysis for the volcanic rocks. In combination with the data available, the authors hold that both the Bima Formation (189.0±3.0~195.0±3.0Ma) in the upper part of the Sangri Group and the Yeba Formation (174.2±3.6~192.7±1.3Ma) were formed in the early Jurassic. Furthermore, the volcanic rocks of the Bima and Yeba formations display similar geochemical characteristics, and thus might have been formed in the same tectonic setting. It is suggested that they are arc-related calc-alkaline volcanic rocks. Therefore, the Bima Formation could be separated from the Sangri Group, and correlated with the Yeba Formation. In contrast, the Mamuxia Formation (99.9±0.7~136.5±1.7Ma) in the upper part of the Sangri Group was much younger than the Bima Formation, and belongs to Lower Cretaceous strata. The Mamuxia Formation has geochemical characteristics of adakite, and should be regarded as a single independent formation. The Danshiting Formation was formed at 93.7±1.2Ma, belonging to Upper Cretaceous, and should also be classified as an independent formation.
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Keywords:
- Tibetan Plateau /
- Lhasa Block /
- Mesozoic volcanic /
- zircon U-Pb dating /
- Yeba Formation /
- Sangri Group /
- LA-ICP-MS /
- SHRIMP /
- sedimentary strata
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