塔里木陆块西南缘新元古代冰碛岩的年代学与地球化学研究

It’s widely accepted that drastically decreasing atmospheric CO2 level might have triggered the severe glaciations on the Neoproterozoic Earth. Several models invoke the special compositions of the epi-crust to propose different processes which could result in an avalanche of atmospheric CO2. The unstratified tillites have similar geochemical signatures with the average geochemical composition of their provenances. Therefore, it is feasible to probe the generally epi-crustal compositions by means of statistically analyzing geochemical data from the globally synchronous unstratified tillites. Most of the Neoproterozoic glacial tillites all over the world have been profoundly constrained on their ages and geochemical signatures. However, the related researches from the southwestern marginal part of the Tarim Block still need improvement. Since the debates on ages of its glaciations are ongoing and geochemical characteristics of its unstratified tillites are unknown, the Qiakemakelike Group distributed in the southwestern margin of the Tarim Block is the focused object of this application. Systematic integration of the sedimentary, geochronologic and geochemical studies on the Qiakemakelike Group will enable us to construct a chronostratigraphic framework of the Qiakemakelike Group with an applicable accurate, and to know the average geochemical composition of the provenances corresponding to each glaciation event. Finally, the geochemical comparison with the synchronously unstratified tillites all over the world will be carried out to recover the generally epi-crustal compositions of each glaciation epoch, which could cast light on the originating mechanism of the severe glaciations.

多种模式认为，“适宜的地壳表层物质组成”能够以不同形式消耗或减少大气CO2的含量，从而引发新元古代的全球冰期事件。由于块状冰碛岩的地球化学特征近似于冰蚀源区物质的平均化学组成，因此，针对全球不同地区、相同期次的块状冰碛岩，系统开展地球化学特征的对比研究和综合分析，能够有效恢复相应时期地壳表层物质的基本组成。目前全球大部分新元古代冰碛岩的时代和地球化学特征已有较好的资料约束，但相关研究在塔里木陆块的西南缘地区仍需加强。聚焦该区冰期时代争议较大、块状冰碛岩地球化学特征不明的科学问题，本申请拟以塔里木陆块西南缘的新元古代恰克马克力克群为研究对象，开展系统的沉积学、年代学和地球化学研究，以期为该群建立精度较高的年代地层格架，查明其中块状冰碛岩层位的地球化学特征，最终通过与全球同期块状冰碛岩的对比研究和综合分析，恢复相应时期地壳表层物质的基本组成特征，为理解新元古代极端气候事件的成因机制提供依据。

塔里木陆块周缘保存了数层新元古代杂砾岩沉积序列，前人通过区域对比和综合地层学研究将其分别与全球的Kaigas, Sturtian, Marinoan和Gaskiers冰期对比。与东北缘库鲁克塔格地区、西北缘阿克苏—乌什一带的新元古代冰碛岩相比，出露于塔里木陆块西南缘叶城地区的恰克马克力克群仍然存在冰期时代争议较大、块状冰碛岩地球化学特征不明等科学问题。本项目选择叶城地区和阿克苏—乌什一带出露的新元古代块状杂砾岩及其相关沉积体系为研究对象，通过沉积构造和碎屑物结构的系统调查、地球化学分析和年代学研究，阶段性查明了塔里木陆块西部地区的新元古代冰期事件期次和陆表物质组分特征，主要取得以下认识：1）塔里木陆块西缘存在3套冰川冰成因块状杂砾岩沉积序列，分别归属波龙组、巧恩布拉克组和尤尔美那克组；2）波龙组冰期时代由冰碛岩序列之下的凝灰岩、之上的沉凝灰岩共同限定在约770-760 Ma之间，表明塔里木陆块西南缘地区可能存在一次可与Kaigas冰期相对比的古老冰期事件；3）阿克苏群的原岩沉积时代应为820-800 Ma，其蓝片岩相变质时代应于800-760 Ma之间，该群的沉积时代应早于巧恩布拉克组，进一步说明新元古代冰期事件在塔里木陆块不同地区的保存有显著差异；4）由细碎屑成分法处理后的块状杂砾岩样品，其地球化学特征既不同于UCC、也与PAAS存在显著差异，与花岗岩—花岗闪长岩的典型物质组成极为相似，但不同冰期事件对应杂砾岩记录的物质组成之间并无显著差异，表明塔里木陆块在拉伸纪晚期—成冰纪期间，其陆表物质组成并未发生显著变化。