The rhyolitic porphyroclastic lavas from the Heishan Formation, Dongchuan Group,Eshan area,Yunnan: Zircon U-Pb age and its geological significance
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摘要: 滇中地区中元古界"昆阳群"、"东川群"的地层层序是长期争议的重大基础地质问题。本次在峨山地区东川群黑山组中发育的厚近5m厚的流纹质碎斑熔岩采集样品D0023一件,选出120余粒锆石,获得的锆石LA-ICP-MS U-Pb加权平均年龄为(1569.2±4.5) Ma。样品锆石的Th/U值均大于0.5,具有清晰的振荡环带结构,均为岩浆成因的锆石,表明东川群黑山组形成于中元古代早期,属非造山相关的火山岩系,可能是哥伦比亚超大陆裂解作用在扬子陆块的响应,为东川群的地层时代和层序研究增添了新资料。Abstract: The classification of the stratigraphic sequences of the Mesoproterozoic "Kunyang Group" and "Dongchuan Group" in central Yunnan is a major basic geological issue that has been controversial for a long time. In this paper,more than 100 zircon grains are selected for age dating from a piece of the sample D0023 collected from nearly 5m-thick rhyolitic porphyroclastic lavas developed in the Heishan Formation of the Dongchuan Group in the Eshan area,and give a LA-ICP-MS zircon U-Pb weighted average age of 1569.2±4.5Ma. All the selected zircons have the Th/U ratios of more than 0.5 and exhibit clear oscillatory zonal structures,suggesting that these zircons are magmatic in origin. The age determinations for the volcanic rocks in this study indicate that the volcanic rocks in the Etouchang Formation of the Dongchuan Group formed during the early Mesoproterozoic should belong to the an orogenic-associated volcanic rock series,and may represent the responses in the Yangtze continental mass to the breakup of the Colombian supercontinent. The age data of the Heishan Formation in the Dongchuan Group have provided new age data for the delineation of the strata and sequences in the Dongchuan Group,and new information for the approaches to the formation and evolution of the basement on the western margin of the Yangtze continental mass.
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