Northeast Yunnan Neoproterozoic Granitic Magmatism And Its Tectonic Significance

The Northeast area in Yunnan province, known as the’kangdian axis’, is located in the southwest Yangtze massif and characterized by a large number of Precambrian basement strata. A lot of neoproterozoic magmatic rocks were found in various types including basic rocks, acidic extrusive rocks, and intrusive rocks. The Yangtze massif has experienced stable development platform because of no magmatic activity during late Sinian to Permian. However, based on the Dongchuan geological map at the scale of1:20as well as the survey report of regional geology, Xiatianba granite marked as73, is found located in the north of Dongchuan City. Since the Xiatianba granite was identified as the Sinian System Dengying formation, the formation time and its genesis need to be furthered studied. This study is purposed to investigate the genesis of Xiatianba granite and the formation of the environment based on petrography, geochemistry and isotope chronology. Several main conclusions can be obtained from this research:1. In the study area and the adjacent regions the geological formation and tectonic environment of the analysis show that there is relatively strong tectonic activity in the neoproterozoic in the condition of the magmatism. Early Paleozoic in the study area is mainly stable platform facies sedimentary formation without occurrence of magmatism tectonic environment.2. The Xiatianba granite is saturated with silicon and characterized with aluminum-weakly peraluminous identified for the high potassium calc-alkaline series. The granite has a strong negative Eu anomaly, enrichment of LILE (Rb、Ba、Pb、U), and depletion of HFSE (Nb、Ta、Sr、Ti), showing that the granite is partial melting of the crust falling the category of A type granite.3. Zircon from Xiatianba granite is used in LA-ICP-MS U-Pb dating, yielding a weighted mean206Pb/238U age of769±4.4Ma, not Early Paleozoic as recognized before. In addition, this thesis also obtaiend the weighted average age of the Liugu granite (797.7±15) in Huidong County, Sichuan province. The Xiatianba granite and the Liugu granite are very close (60km), so these two rock masses may be as a result of the same one tectono-magmatic event. 4. The Zircon saturation temperature of Xiatianba granite is between711.83℃and818.16℃(778.91℃on average) with778.91℃representing the granite’s crystallization temperature and818.16℃representing the primitive magma minimum temperature of the magma source area. It means that the Zircon in Xiatianba granite has the Zircon saturation temperature of A type granite.5. Combining with the analysis of regional geological, the Xiatianba granite was formed in the extensional setting, caused by the mantle plume activity in the breakup of Rodinia supercontinent during the continental rift environment. This is confirmed by the southwestern margin of Yangtze Block in the Neoproterozoic rift environment which affects Dongchuan area of northeast Yunnan province.