思茅地块西缘大凹子组地层序列及地质时代

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.

The Stratigraphic Sequence and Geological Time of Dawazi Formation in Western Simao Block

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.