| Abstract: | The plate tectonic model constructed by R.H. Sillitoe for the origin of porphyry copper deposits said that porphyry copper deposits were associated with convergent plate boundaries and compressional tectonic settings. They were formed along destructive plate margins above subduction zones of oceanic crust. Although this model is suitable for most porphyry copper deposits because the main porphyry copper deposit belts accord with the main Mesozoic or Cenozoic plate subduction zones in space and time in the world, it is difficult to use Sillitoe's model for explaining the formation of some intracontinental porphyry deposits such as Yulong, the largest one in East Tibet.Having no relation with a subduction zone of plates, Yulong porphyry copper deposit was formed in an intracontinental extensional tectonic setting. Qamdo block, in which Yulong lies, was a small massif in East Tibet. With its Proterozoic crystalline basement and Lower Paleozoic folded basement, Qamdo block had experienced relative stable geologic evolution and a cover more than 20000m in thickness mainly composed of clastic rock and carbonate formed from Devonian to Cretaceous. During this time, although two small oceanic basins(Jinshajiang basin and Lancangjiang basin) between which Qamdo block lay developed and subducted and relevant island arc belts occured along the east and west margins of the block, all of them were closed before Later Triassic. In Cenozoic, mainly from 52 to 35Ma, influenced by the intracontinental convergence of the three large plates(India to the southwest, Tarim to the northwest,and Yangtze to the east), Qamdo block and its surrounding areas (East Tibet) which were just to the west of Yangtze plate were in an intracontinental pull\|apart situation induced by the sinistral strike\|slip fault system while most Himalaya—Kunlun—Tibet areas between India and Tarim plates were in a compression state by the SN\|trending tectonic stress. Controlled by the regional strike\|slip fault systems, the calc\|alkaline magma deriving from the upper mantle or lower crust ascended, occurring porphyry copper mineralization in the relative uplift areas. At the same time, in the relative depression areas, also controlled by the regional deep strike\|slip faults which might cut the lithosphere, several linear or rhombic pull\|apart red molasse basins such as Gongjoe, Nangqen, parallel to the porphyry belt, were formed in this region. |
