滇西南澜沧江带龙洞河组沉积特征及其构造古地理意义

SEDIMENTARY CHARACTERISTICS AND TECTONIC PALAEOGEOGRAPHYOF LONGDONGHE FORMATION IN LANCAJIANG ZONE,SOUTHWESTERN YUNNAN

龙洞河组位于滇西南澜沧江带东区，为一套以中酸性火山熔岩、火山碎屑岩、火山凝灰岩为主夹硅质岩、灰岩和岩屑砂砾岩的建造组合，代表了弧后裂陷盆地的半深水斜坡－较深水局限海盆－浅水沉积环境。其物源主要来自其西侧的大凹子火山岛弧。其地层序列反映了弧后裂陷盆地从形成到消亡的过程。龙洞河组沉积特征与其构造古地理表明，澜沧江带在思茅地块西缘在晚石炭世一早二叠世已属滇西南古特提斯洋东侧的活动大陆边缘。

The Longdonghe Formation,located in the east of Lancangjiang zone in southwestern Yunnan,is a suite of intermediate-acid lava,pyroclastics,tuffite with siliceous rock,limestone and petromictic sandy conglomerate association.It reflects the deep-water slope-restricted basin-neritic environment of a back-arc rift basin.Its provenance is the Dawazi volcanic island-arc on the west side.Its stratigraphic succession reflects the evolution of the back-arc rift basin from starting to the end.Its sedimentary characteristics and tectonic palaeogeography show that Lancangjiang zone or the west of Simao massif is an active continental margin of the palaeotethys in southwestern Yunnan during Late Carboniferous to Early Permian and represents the earliest eastward subduc-tion of the palaeotethys in southwestern Yun nan.