义敦岛弧带前岛弧期早—中三叠世地层序列与构造演化

Yidun Arc Belt. The strata had been strongly deformed and slightly metamorphosed, and it is called the.Yidun Group, with a total thickness of 3000 to 6000 meters, and the lower part is called the Dang’en Formation, while the upper part called the Lieyi Formation. Because of the strong tectonic reworking no reliable fossil has been found before..By the operation of this project, we have found out some significant fossils from the interlayered limestone and silica in this group in Rejia of Baiyu county and in Yulong and Gala of Ganzi county,Sichuan province. These fossils include Conodonts: Clarkina carinata, Icriospathodus sp.,Microzarkodina sp. and Neogondolella sp., etc., among which Clarkina carinata and Icriospathodus sp.are the characteristic elements of the Early Triassic and the late Early Triassic separately; Radiolaria:Triassomitra zhangi, also the characteristic early Triassic elements; and a few foraminifera and fish teeth. Therefore we have confirmed the existance of the lower Triassic in the Yidun Group, and filled the gap in biostratigraphy of the Yidun Group.We have got many fossil plants in the Late Triassic Lamaya Formation in Xionglongxi of Xinlong.county. These include 15 genera and 20 Species, and there are 2 new species among them:Cladophlebis xinlongoensis，Marattia ovalis. We have found a very special genus and species,Bernettia phialophora Harris, which is firstly found in china. This fossil had been found in Lower Jurassic in Nurnberg of Germany and in Scoresby Sound of East Greenland, and here may be the.third place of production. This discovery is of important significance in Palaeobiogeography and in.evolutionary-Palaeobiogeography..We have proved that the Jin Shajiang oceanic basin had already closed before Triassic by the analysis of the Early-Middle Triassic rock formation and sedimentary environment of several.representative sections. From then on, the marine transgression and regression in the area of western Sichuan to eastern Tibet were controlled by the Ganzi-Litang oceanic basin opened at the terminal Permian. It also controlled the formation and evolution of the Yidun arc belt.The Yidun arc belt had undergone a multicyclic tectonic evolution characterized by alternation of extension and compression regime，which could be divided into 10 stages：①Permian and pre-Permian stage: being one part of the pan-Yangtze plate. At present, Yidun Island Arc Belt is.far separated from the Yangtze Landmass by the extensive Yajiang Fold System, the marginal tract of the Yangtze Landmass. However, in Permian and pre-Permian time this area was directly connected with the present Yangtze landmass, forming an unified and broad plate, which could be called pan-Yangtze Plate; ②Terminal Permian—early Middle Triassic stage：marginal continental rift→ocean. In terminal Permian the pan-Yangtze Plate cracked and formed some land blocks such as Zhongza and Ruoergai blocks. These blocks drifted westward in Early Triassic—early Middle Triassic and the oceanic crust initiated, forming the Ganzi-Litang ocean, which has its relics representing by a sequence of basic pillow lava, basic-ultrabasic cumulates and radiolarian siliceous rocks exposed intermittently along the Ganzi-Litang fracture zone; ③Late Middle Triassic stage: short-term’s subduction of the Ganzi-Litang ocean and the appearance of the immature island arc in the Yidun area; ④Early Carnian stage: the rifting Yidun area and the spreading Ganzi-Litang ocean; ⑤Late Carnian—Early Norian stage: the thorough subduction of the Ganzi-Litang ocean and the forming of the Yidun mature island arc.;⑥Middle—Late Norian stage: rifting on the basis of the island arc; ⑦Terminal Norrian—Rhaetian stage: collision and the.relict sea; ⑧Jurassic — Early Cretaceous stage: intensive compression and orogeny;⑨Late Cretaceous—Neogene stage: intermontane rift-basin; ⑩Quaternary—today: rapid uprising and.orogeny.

寻找义敦岛弧带前岛弧期早—中三叠世地层的确切化石年代依据，确定不同沉积环境和沉积建造类型，建立能代表义敦岛弧带早—中三叠世地层整体特征的包括多门类化石分带序列、沉积旋回、火山地层对比关系等内容的综合地层序列，最终阐明义敦岛弧带早—中三叠世的地质演化历史,力争成为造山带区域地层研究的成功范例。