Neoproterozoic Strata And Tectonic Evolution In Lushan Area, Jiangxi Province

The "Jiangnan Orogen" is a large-scale tectonic belt, which was formed when the Yangtze Block collided with the Cathaysia Block during Proterozoic. The Lushan area is one of the most complex regions in the "Jiangnan Orogen", and is crucial for understanding the tectonic evolution of whole "Jiangnan Orogen". In the framework of the isotope chronology, the sedimentology, sequence stratigraphy, petrology, geochemistry and isotopic dating were employed to study the Neoproterozoic geological issues of Lushan area and "Jiangnan Orogen" in this thesis. Combined the field investigation and the experimental measurement with the results of other researchers, a few of geological puzzles on the Neoproterozoic strata and tectonic evolution of the Lushan area was systematacially resolved.A lot of original understandings were obtained in this thesis, and the main conclusions and innovative results were listed as follows:The LA-ICP-MS zircon U-Pb dating was employed to constraint the lower depositional time limit of the protolith of the Xingzi complex to be806Ma, and the Xingzi complex is located above Wuling Movement (820±Ma) tectonic transformation plane. Therefore, the Xingzi complex belongs to the "Banxi period" stratum. The latest research result of this thesis rejected the conclusion that the Xingzi complex was the ancient metamorphic crystalline basement of the Yangtze Block proposed by the previous researchers, and denied the theoretical premise that the Xingzi complex was the old metamorphic core of the "Lushan metamorphic core complex" as well. This result verified that the relationship between the Guanyinqiao gneissic granite and the Xingzi complex group need be reexamined.The diagenetic age of the Hanyangfeng Formation was accurately determined to be838±4Ma by SHRIMP zircon U-Pb dating. The latest research result positioned the Hanyangfeng Formation to the lower part of the Shuangqiaoshan group under Wuling Movement (820±Ma) tectonic transformation plane. Combining the dating results and geochemical composition with the formational tectonic setting of the Shaojiwa Formation and the Hanyangfeng Formation, this study classified the Shaojiwa Formation and the Hanyangfeng Formation into a unified rift-type early Neoproterozoic multi-cycles eruptional coastal-continental facies basalt-andesite-rhyolite assemblage. This assemblage may correlate with the lower Xikou Group in Southern Anhui and the Lower Lengjiaxi Group in Eastern Hunan. From the tectonic setting to the diagenetic age, the spilite-keratophyre assemblage of the Shaojiwa Formation is similar with the Fuchuan ophiolite in Southern Anhui, the ultrabasic rocks in Zhangyuan and the Wenjiashi basic rock in Hunan, therefore, they may correlate each other.For the first time, according to the field observation and the geochemical analysis, the Lushan area was confirmed to be a volcanic crater during-840Ma. At that time this volcanic was located in the continental marginal back-arc rift, and belonged to the coastal to continental setting. The zone ought to be a transitional zone between the volcanic arc and a little back-arc ocean basin. The volcanic eruption is mainly an intermittent fissure one accompanied a strong outbreak.The Hengyong Formation and Jilin Formation of the Shuangqiaoshan Group outcrop in Lushan area. The protolith of the strata above the hengyong formation probably deposited during from832to823Ma. The primary sedimentary structural feature of the Hengyong Formation indicates that its sedimentary environment is a relatively fluctuant ocean basin. The Jilin Formation was a stratum which deposited in the transgression system to the early high water system. In Lushan area. The Nanhua System is not complete, and only reserves the Liantuo Formation and the Nantuo Formation. The basic features of the sequence of the Liantuo Formation display the transgression facies, and the Nantuo Formation performs the features of the alternant glacial retreat-progress.For the first time, this thesis intensively researched the Yangtianping Formation, which only locally outcrops in Lushan area. The Yangtianping Formation is the Neoproterozoic silty mudstone. According to a few of the isotope dating results and the field bedding relation, the Yangtianping Formation was preliminarily classified as one between the Hengyong Formation and the Liantuo Formation. Through analyzing the field features of the Yangtianping Formation and the tectonic setting discrimination of the geochemical compositions, the sedimentary environment of the Yangtianping Formation was inferred as the later Qingbaikou period deep back-arc sea basin.Through summarizing the studying results of other researchers on the "Jiangnan Orogen" and the latest high precise isotope chronology data attained by this thesis, the chronostratigraphy framework was reestablished. The Neoproterozoic tectonic evolution pattern of the middle and eastern segment of "Jiangnan Orogen" including Lushan area was built as well.