PLATE CONVERGENT MARGINS AND ISLAND-ARC OROGENESIS IN EASTERN TETHYS

The spatio-temporal distribution of the ophiolitic melanges and island-arc zones is emphasized in combination with the trench-arc-basin systems in the eastern Tethyan tectonic domain,for which three evolutionary stages of Eo-Tethys,Mid-Tethys and Neo-Tethys have been distinguished.The geological records of the ophiolitic melanges,mid-ocean ridge basalts,oceanic sediments and island-arc zones,which are recognized,have provided essential evidences for plate movement in the early oceanic lithosphere.The plate convergent margins in orogenic zones are characterized by all kinds of fore-arc,inter-arc and back-arc basins which are related with island arcs and formed in varying times and stages.Moreover,the formation and evolution of the distinctive basins are sujected to the effects and controls of different geological settings.It is obvious that most of the basins have experienced three stages of evolution,including rifting and extensional,steady sinking,and compressional and uplifting processes.The first stage is characterized by intense volcanic activities and well-developed fan-deltaic sediments;the second stage by mixed and low-density turbidity current deposits of carbonates and siliciclastics,and the early time of the third stage by the development of submarine fans.It is important that the features of recycling orogenesis in many basins are indicated by the terrigenous clastic sediments of quartzitic mylonite.There is a typical example for arc-arc collision between the Yidun island arc and Shalulishan magmatic arc,or even throughout eastern Tethys.The growth of the Yidun island arc records the transition from immature to mature arc on the basis of the early rift, whereas the Shalulishan magmatic arc was developed on an ancient landmass.The subduction and collision of the two arcs stimulated the uplifting of the Korlondo-Yajin zone.The ovogenesis not only brought westwards about a vast amount of mylonite clasts,but also resulted in the formation of the lag-type granitoids in the Cuojiaoma-Cuomolong zone and of Sn polymetatlic metallogenic zones in the west of Yidun.Besides,the arc-continent collision may be served as an additional model for orogenesis.Finally,the common features are also described for the basins and island arcs on the plate convergent margins in the eastern Tethyan tectonic domain.