Early Paleozoic Geological Characteristics And Tethys Tectonic Evolution Of Baoshan-zhenkang Terrane And Adjacent Areas

Baoshan-Zhenkang terrane locates in the southwest of Sanjiang Tethys orogenic belt. It is sandwiched between Luxi-Santaishan suture zone and Changning-Menglian suture zone, which is key to study the evolution of the Tethys. This area is also the well-known lead-zinc-copper and gold ore-concentration area, which was considered as one of the national integrated exploration area. The temporal-spatial texture and evolution of the Baoshan-Zhenkang terrane and its adjacent areas are very important to study tectonic division of China, polymetallic prospecting and Tethysevolution. Previous researches of this area mainly focus on the Late Paleozoic paleontology and paleogeography, while the Early Paleozoic geology is overlooked because of little geological records and intensively metamorphism and deformation. In this dissertation, the Baoshan-Zhenkang terrane and adjacent areas are the main research area and their geological features and tectonic evolution during the early Paleozoic are well-studied based on the systemic results of sedimention, magmatism, tectonics and geochronology.Under the guidance of theories of plate tectonics and continental dynamics in combination with using the methods of basin analysis and structural analysis, Early Paleozoic geological characteristics and tectonic evolution of the Baoshan-Zhenkang terrane and its adjacent areas are systematically studied based on the geological survey projects “Basic geological survey of Dali to Ruili, Yunnan province” and the data of recent accomplished 24 sheets 1:50000 geological maps. According to systematically study of sedimentary strata, we propose that the Baoshan-Zhenkang terrane is one part of Gandwana continental passive margin basin in Early Paleozoic, which developed into the palaeogeographic pattern of “horst in the middle and grabens” on both sides in the late Neoproterozoic to middle Cambrian. Then, the discrepancy of palaeogeographic pattern “horst and graben” was becoming more obvious during late Cambrian to early Ordovician, while in Middle Ordovician- Silurian it evolved into “platform in the west and graben in the east”. Geochemical and isotopic geochronological data of the newly discovered ophiolite suggest that Changning-Menglian Proto-Tethys Ocean began to develop at least in the Early Paleozoic. According to the similarities and differences between the Ordovician cumulates in Longmu Co-Shuanghu suture zone and the ophiolite in this study area, we propose that Longmu Co-Shuanghu-Nujiang-Changning-Menglian junction belt represents the final consuming location of the Paleo-Tethys Ocean and this ocean can last from Early Paleozoic to middle Triassic. According to geochemical and geochronological data of magmatic rocks, this research firstly established the tectonic framework of the Southwest Sanjiang arc-basin system which was influenced by the eastward subduction of the Tethys Ocean in late Early Paleozoic. Regional tectonic evolution of the study area is also discussed in this paper. The model of three tectonic evolution stages of the Baoshan-Zhenkang continental margin basin and the Proto-Tethys Ocean are proposed as foolows:(1) in late Neoproterozoic to Middle Cambrian, the Changning-Menglian Proto-Tethys Ocean expanded and a passive continental margin basin developed in the northern margin of Gondwana continent;(2) in Late Cambrian to Early Ordovician, the Changning-Menglian Ocean expanded quickly, and a passive continental margin rift basin also developed in the northern margin of Gondwana continent;(3) in Middle Ordovician to Silurian, the Changning-Menglian Ocean subducted eastward, the tectonic framework of the Southwest Sanjiang continental margin arc-basin system was formed, and the northern part of Gandwana continent transformed into passive margin basin.