Sedimentary Characteristics And Tectonic Palaeogeography Research On“zhanjin” Formation In The Central Uplift Zone Of Qiangtang Basin

The formation mechanism and geological revolution of the Qiangtang central uplift zone are involved with important scientific issues, such as Paleo-Tethys Ocean reconstruction and Gondwanaland dispersion. Therefore, the central uplift zone is always a focus area of tectonic research in Qinghai-Tibet Plateau. The Early Permian is a key period of its evolvement. Hence, the sedimentary records within this stage can provide significance evidence for regional stratigraphical correlation and tectonic palaeogeography rebuild. On the basic of detailed geological survey, The Lower Permian “Zhanjin Formation” sedimentary cores samples were collected from Qiangzi 5 well in Jiaomuchaka Area for this research. Petrological and geochemical characteristics of those rocks were analyzed for sedimentary association study, palaeoenvironment discussion and tectonic palaeogeography rebuild:Through core investigation and indoor rock thin-section observation, 14 types of sedimentary rocks of “Zhanjin Fm.” were identified, bottom-up, it can be described as 3 intervals: carbonate with thin siliceous rocks-volcaniclastic rock- terrigenous clastic rocks. According to the characteristic of rocks and combination type the restricted platform facies and platform-marginal slope facies can be recognized, which indicates weak activity-activity enhancement-activity weaken environment. The rock original composition indicates its provenance was related to a subduction tectonic background. In the tectonic environment discrimination diagrams, “Zhanjin Fm.” siliceous rocks suggests the characteristics of the continental margin, while the terrigenous clastic source area shows obviously island arc tectonic environment features. By means of comprehensive analysis, Jiaomuchaka area probably was a carbonate platform or slope that located in the fore-arc environment in the Early Permian.For purpose of investigating the character of Qiangtang central uplift zone, this paper selected 3 Lower Permian sedimentary strata sections in both north and south Qiangtang areas. Qiangzi 5 well reveals that the “Zhanjin Fm.” mainly developed carbonate tableland sediments with high micrite contents in Jiaomuchaka Area, its lithological association similar with the sedimentary formation characteristics of Northern Qiangtang. In spite of the same formation name, its lithology is entirely different from the northern margin of Gondwana sedimentary system. This difference shows that the central uplift zone was a significant plate boundary, witch divided Northern and Southern Qiangtang into 2 different continental margin system and suggests they have different evolution history in the Paleozoic period. Volcanic debris-terrigenous clastic suite reveals the basic characteristics of the Northern activities continental margin. Combing with the radiolarian siliceous rocks, suggested that during the “Zhanjin Fm.” deposited period, there most probably is archipelago ocean basin in central Qiangtang. Through the comparison between “Zhanjin Fm.” radiolarian siliceous rocks and Changning-Menglian Paleo-Tethys records, support the existence of Palaeo-Tethys Ocean in central Qiangtang within the Permian period.