藏北荣玛乡亚丹地区下二叠统曲地组的厘定及大地构造意义

Redefinition of the Lower Permian Qudi Formation in Yadan area, Rongma Town, northern Tibet, and its tectonic significance

西藏羌塘中部增生杂岩的不断解体对于该区古生代—三叠纪地层格架的厘定及特提斯洋演化研究具有重要的指示意义。对西藏荣玛乡亚丹地区的增生杂岩进行实测剖面时,发现一段早二叠世大陆斜坡相深水重力流形成的海底扇细碎屑岩沉积,北侧被新生代康托组红层覆盖,南侧与玄武岩等基性岩块呈断层接触,沉积序列不完整,主体为一套海底扇中扇-外扇厚层粉砂岩夹水道砂岩透镜体。碎屑岩层中垮塌沉积的生屑灰岩夹层含Parafusulina,Pseudofusulina,Schwagerina,Chalaroschwagerina parampla共4属10种■类化石,指示地层时代为早二叠世亚丁斯克期。1∶25万日干配错幅区域地质调查将该地层归为吞龙共巴组,主要为混积陆棚相沉积。依据岩石地层及化石组合时代,厘定本套海底扇沉积为下二叠统曲地组。结合前人报道的早—中二叠世含■灰岩建造,以及羌塘中部晚石炭世—早二叠世基性岩的大陆裂谷背景,认为羌塘中部早二叠世沉积代表了中特提斯洋演化的初始阶段,并推测在空谷阶之后发育小规模洋盆。

The continuous separation of accretionary complex in central Qiangtang of Tibet is significant for defining the framework of strata from Paleozoic to Triassic and studying the evolution of Tethys. The authors measured a section of the accretionary complex in Yadan area, Rongma Town, Nyima County, Tibet, and discovered a sequence of submarine fan clasolites formed by deep-water gravity flow on the continental slope in Early Permian. The north of the strata is covered by Cenozoic red Kangtuo Formation, and the south is in faulted contact with basalt blocks. It is intercalated with sandstone lenses from the outer part to the middle fan. The intercalated layer of bioclastic limestone formed by collapse deposition contains a fusuline fauna and consists of ten species in four genera such as Parafusulina, Pseudofusulina, Schwagerina, Chalaroschwagerina, suggesting Artinskian period. The 1∶250000 geological map of Rigan Peicuo defined the strata as Tunlong Gongba Formation which was mainly formed by diamictite continental shelf deposits. Based on the lithostratigraphy and the time of the fossil assemblage, The authors redefined the submarine fan strata as Qudi Formation. In combination with previous widely reported limestone formation containing fusuline and the continental rift background of basic dikes in central Qiangtrang from Late Carboniferous to Early Permian, the authors hold that the early Permian sedimentary formation represented the initial evolution of the Meso-Tethys Ocean and there existed a small ocean basin after the Kungurian Stage.