华南长安冰期下界年龄及成冰纪生物地层学研究

In the border area of Hunan, Guizhou and Guangxi Provinces，the thickness of Neoproterozoic glacial and interglacial strata is very large and succession, which contains multiple volcanic tuffs can be worked isotope dating studies. So, this area is one of the important areas to study the Earth system change during the Neoproterozoic Ero. However, due to transportation and other reasons in the past, the research degree of the Nanhuan system in there is very low, especially the disputed glacial attributes and stratigraphic correlation of the Chang'an Formation. In this project, we will select the typical sections, where the Chang'an and underlying formation is continuous. By serial volcanic tuff zircon dating research, determines the lower age of the Chang'an Formation. We will use stable isotope and element geochemistry to study the environmental background of the Chang'an Formation. The biostratigraphic study will reveal relations ancient organisms with environment changes. This study will be solved the lower boundary age of Nanhuan system of China and provides evidence for comparison with the international Cryogenian.

湘黔桂三省交界地区新元古代冰期和间冰期地层厚度大，沉积连续，并含有多层位火山凝灰岩可供同位素定年研究。因此，该区是研究新元古代地球系统变化的重要地区之一。但是，过去由于交通不便等原因，该区南华系地层研究程度较底，特别是长安组的冰期属性和地层对比存在争议。本项目拟在该区选择长安组与下伏地层连续的典型剖面。通过系统火山凝灰岩锆石定年研究，确定长安组的下界年龄。利用同位素和元素地球化学等研究手段对长安组的环境背景进行探讨。生物地层学研究将揭示古生物与环境变化的关系。该项研究将为解决我国南华系底界年龄和与国际成冰系的对比提供依据。

通过系统的火山凝灰岩锆石定年研究,在湘西铁山乡剖面长安组底部获得凝灰岩锆石年龄758.6±5.4 Ma；怀化石宝-袁家剖面长安组距底界10余米处获得锆石年龄743.8±4.1 Ma：黔桂交界三江地区拱洞组底部获得锆石U-Pb年龄799.8±5.5 Ma；江西湖南交界的广寒寨长安组底部获得锆石U-Pb年龄770±10 Ma。依据目前的同位素年龄和化学地层资料，南华系的底界年龄被限定为780 Ma,并被划分为下、中、上三统。下统时限为780～725 Ma，中统时限为725～660 Ma，上统时限为660～635 Ma。三统分别相当全球新元古代Kaigas冰期（≈770～735Ma），Sturtian冰期（≈715～680Ma），Marinoan冰期（≈660～635Ma）。