The Stratigraphic Sequence and Geological Time of Dawazi Formation in Western Simao Block
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摘要: 目前关于思茅地块西缘大凹子组的形成时代仍有分歧.在思茅地块西缘大中河剖面采集了硅质岩、砂岩、凝灰岩和玄武岩，通过放射虫组合和锆石U-Pb年龄方法，厘定其地质时代，并结合区域资料恢复地层序列.通过详细剖面实测，发现该剖面由6个地层断片组成：第一、四断片以含放射虫硅质岩为特征，放射虫组合指示其时代为晚泥盆世至早石炭世早期；第二、五断片以火山碎屑岩、具有鲍玛序列沉积特征的火山碎屑沉积岩为主，锆石U-Pb同位素年龄指示其时代为志留纪中期至早泥盆世；第三、六断片以火山岩沉积为特征，锆石U-Pb同位素年龄指示其时代为志留纪早期.结合前人资料认为思茅地块西缘分布的海相火山岩、碎屑岩和含放射虫硅质岩地层层序代表了志留纪到早石炭世早期的岛弧火山-沉积地层序列.Abstract: There are great controversies in the time of Dawazi Formation located at the edges of western Simao Block. Siliceous rocks, sandstones and volcanic rocks in Dazhonghe Section, western Simao Block were selected to research the geological time by the radiolarian assemblage and zircon U-Pb age, and reconstruct the stratigraphic sequence. Dazhonghe Section is a fragment stratum system composed of six different lithologic strata fragments, through the detailed section measurement. The first and forth fragments are characterized by radiolaria-bearing siliceous rocks, and the radiolarian assemblage indicates that the geological age is from Late Devonian to Early Carboniferous. The second and fifth fragments are dominated by pyroclastic rocks and pyroclastic sedimentary rocks with Bouma sequence characteristics. The U-Pb isotope ages of the zircons indicate that the geological age is from the Middle Silurian to the Early Devonian. The third and sixth fragments are characterized by volcanic rock deposition, and the U-Pb isotope age of zircons indicates that the geological age is the Early Silurian. Combined with the previous data, it is believed that the stratigraphic sequences of marine volcanic rocks, clastic rocks and radiolarian-bearing siliceous rocks distributed in the western margin of Simao Block represent the island-arc volcanic-sedimentary stratigraphic sequences from Silurian to Early Carboniferous.
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图 1 滇西南构造略图（a）和思茅地块西缘大中河地区地质简图（b）
图a据刘桂春（2020）修改；前志留纪基底为澜沧群
Fig. 1. Geological map of SW Yunnan(a)and simplified geological map in Dazhonghe area, western Simao Block(b)
图 2 思茅地块西缘大中河大凹子组实测剖面图
Fig. 2. Measured profile of the Dawazi Formation in Dazhonghe, western Simao Block
图 3 野外及显微镜下照片
a.硅质岩野外照片；b.砂岩鲍玛序列；c.砂岩镜下照片；d.凝灰岩野外照片；e.凝灰岩镜下照片；f.玄武岩野外照片；g.玄武岩镜下照片
Fig. 3. Field photographs and photomicrographs
图 4 大凹子硅质岩样品的放射虫
a. Entactinia pantotolma Braun; b. Trilonche palimbola (Foreman); c、d. Entactinia herculeus Foreman; f~h. Astroentactinia sp.; e、i、j. Trilonche foremanae (Ormiston and Lane); k、l. Entactinia sp.; m~p. Belowea variabilis Won
Fig. 4. Radiolarian of siliceous rock of Dawazi Formation
图 5 大凹子组凝灰岩、砂岩中代表性锆石的CL照片
Fig. 5. Cathodoluminescence images (CL) of the represent zircons from tuff and sandstone
图 6 凝灰岩中锆石U-Pb年龄谐和图和加权平均年龄分布图
Fig. 6. Zircon U-Pb age concordia and weighted average plots for the tuff
图 7 砂岩中锆石U-Pb年龄谐和图、加权平均年龄及频率分布图
Fig. 7. Zircon U-Pb age concordia, weighted average and histogram plots for the sandstones
图 8 玄武岩中代表性锆石的CL照片
Fig. 8. Cathodoluminescence images (CL) of the represent zircons for the basalt
图 9 玄武岩中锆石U-Pb年龄谐和图和加权平均年龄分布图
Fig. 9. Zircon U-Pb age concordia and weighted average plots for the basalt
图 10 锆石球粒陨石标准化微量元素蛛网图
Fig. 10. Chondrite-normalized trace element spider diagram of zircon
图 11 大凹子凝灰岩、砂岩及玄武岩锆石Th/U-Nb/Hf（a）和Th/Nb-Hf/Th（b）图解(据Yang et al., 2012)
Fig. 11. Th/U-Nb/Hf (a) and Th/Nb-Hf/Th (b) plots for the zircons of tuff, sandstone and basalt (from Yang et al., 2012)
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