华南江口冰期沉积年代的厘定：成岩磷钇矿 U-Pb 年龄证据

The Jiangkou glaciation in South China is correlative with the worldwide Sturtian glaciation. The onset age of Jiangkou glaciation and its occurrence frequency remains disputative because previous research mainly depends on zircon U-Pb ages obtained from volcanic tuff overlying or underlying the glacial strata. It is not perfect to constrain the age of glaciation by dating volcanic tuff. Rather, the best method would be to find datable diagenetic minerals directly from the glacial strata. By dating diagenetic xenotime to constrain the depositional age of sedimentary rocks has been successful in recent years, the method of which has been widely adopted. This program aims to find diagnenetic xenotime from inside the glacial rocks well exposed around the Liping and Congjiang Counties in Guiyang, by dating the age of diagenetic xenotime using Cameca IMS 1280 to constrain the onset age of Jiangkou and Sturtian glaciations. In doing so, reliable time constraints will be put on the timing, occurrence frequency of Neoproterozoic glaciations and their global as well as local Neoproterozoic glacial stratal correlations. Meanwhile, new time constraints will become chronostratigraphic evidence for establishing Cryogenian.

中国华南的江口冰期和国际上的 Sturtian 冰期相对应，其发生时间和发生期次一直存在争议。原因是以往的研究只从冰期地层下伏岩层或者上覆岩层中的火山凝灰岩中测得锆石 U-Pb 年龄。用火山凝灰岩进行沉积岩定年并非权宜之计。要想对冰期事件进行精确定年，必须从代表冰期事件的岩层（混杂陆缘沉积岩）内部找出可用于定年的成岩矿物。近年来发展起来的用成岩磷钇矿矿物定年来厘定沉积岩的沉积年龄的方法取得了一定的成效并且得到了广泛的应用。本文以成岩作用时形成的磷钇矿矿物为切入点，试图从贵阳黎平县和从江县一带出露的代表江口冰期的混杂陆缘沉积岩中寻找成岩磷钇矿，通过用Cameca IMS 1280 仪器测得成岩磷钇矿的年龄来厘定江口冰期的形成时代，从而为解决新元古代晚期冰期的发生时间、发生期次和冰期地层单元对比以及国际成冰系建系等问题提供可靠的年龄制约。

新元古代晚期发生了全球规模的冰川事件。由于缺少可靠的同位素年龄制约，这些冰川事件发生的期次及同时性等问题一直存在争议。我国华南的南华系对这些事件有着很好的记录，但南华系区域地层对比多年来一直存在争议，争议的焦点就是长安组、渫水河组、莲沱组和板溪群之间的对比问题。以往的研究大部分是以岩石地层对比为主，前人曾尝试过对地层进行定年，但得到的年龄都为碎屑锆石的年龄，不能用来限定地层的沉积时代。我们通过测试夹层火山灰/凝灰质粉砂岩中锆石 U-Pb 年龄和碎屑岩中成岩磷钇矿年龄，对这些重要地层单元进行直接定年，不但有效地解决了区域地层对比问题，对于全球冰川事件的发生期次和发生时间等问题也会提供了可靠的年龄制约。