摘要：

地球演化过程中共经历了6次主要的大型冰期或冰川事件,其中最早的冰川事件被认为是发生在2.9 Ga前的南非Pongola冰川,但目前对其知之甚少。为了厘清太古宙究竟有无冰川作用以及如何识别此次冰川作用,笔者通过辨析南非Kaapvaal克拉通Pongola超群Mozaan群(2.9 Ga)中杂砾岩的特征,总结了冰川沉积的识别标志,并讨论了太古宙Pongola超群的杂砾岩是否为冰碛岩。中太古界Pongola超群主要分布于南非Kaapvaal克拉通东南部,可进一步分为Nsuze群和Mozaan群。对Mozaan群杂砾岩进行岩石学和沉积学研究以及主量和微量元素地球化学分析,结果支持对Mozaan群杂砾岩的冰川成因解释。Mozaan群杂砾岩总体特征为: (1)厚度较小(<50 m);(2)块状混杂堆积; (3)砾石分选、磨圆差; (4)不同粒径、不同成分的砾石漂浮在细粒基质中; (5)具有多组方向擦痕和光面的砾石; (6)薄片中,石英颗粒有拉长或变形特征,细小的粉砂级颗粒环绕砂级颗粒分布(星云构造);(7)纹泥中发育坠石。元素地球化学研究表明,Mozaan群杂砾岩的投点在SiO₂/Al₂O₃图上分布局限,明显不同于经历了显著风化和分选的现代和古代沉积物的典型线性负相关关系; TiO₂/Al₂O₃图上的分布范围也很局限,主量元素组成类似,并且杂砾岩基质中富含Fe₂O₃(T),且明显亏损Ca和Na。依据杂砾岩和下伏页岩的三氧同位素(Δ¹⁷O)反演得出风化水的δ¹⁸O值与现代极地大气水同位素特征一致,指示寒冷气候。另外,CIA等指标表明Mozaan群杂砾岩几乎没有遭受成岩蚀变或化学风化,可代表物源区特征。依据上述结果构建出的冰川沉积模式为: 在低海平面时,冰川沿山谷向大陆架区域推进,携载的冰碛物中除了底碛、侧碛和冰川刨蚀基底产生的岩粉外,还包含了从陆架砂中混入的圆形石英颗粒以及一些来自铁质泥的细粒风化物质。有些冰碛岩来自冰缘地带冰川沉积物的直接卸载以及后期的改造,表现为冰碛岩与其基底截然接触,且在部分冰碛岩层下方存在变形层,而另一部分由厚层冰碛岩上部的坠石指示,属于被冰筏搬运到距海岸较远的环境沉积形成。

关键词: Pongola冰川, 冰川识别标志, 太古宙, Mozaan群, 南非

Abstract:

The Earth’s evolutionary history has witnessed six major glaciations or glacial events,among which the earliest is believed to be the Pongola glaciation in South Africa,occurring approximately 2.9 billion years ago. A number of papers have been published on the last five major glaciations,yet understanding of the Pongola glacier remains limited. This raises key questions: did glaciation exist in the Archean,and how to identify it?By analyzing the characteristics of diamictites in the Mozaan Group(2.9 Ga)of the Pongola Supergroup in the Kaapvaal Craton,South Africa,this study summarizes the diagnostic markers of glacial deposits,and discusses whether the diamictites of the Archean Pongola Supergroup are glacial origin. Meanwhile,it aims to draw more attention from peers to the establishment and development of glacial sedimentology:a subdiscipline of event sedimentology in China. The Mesoarchean Pongola Supergroup is primarily distributed in the southeastern part of the Kaapvaal Craton and can be further divided into two groups: the Nsuze Group and the Mozaan Group. Petrological,sedimentological,and major/trace element geochemical analyses of the Mozaan Group diamictites support the interpretation of glacial origin. The Mozaan Group diamictites generally exhibit the following characteristics: (1)relatively thin thickness(<50 m);(2)massive mixed accumulation;(3)poor sorting and roundness of gravels;(4)gravels of varying particle sizes and compositions “floating” in a fine-grained matrix;(5)gravels with multiple sets of striations and polished surfaces;(6)quartz grains show elongation or deformation with silt-sized grains distributed around sandy grains in thin sections(referred to as a “nebular structure”);(7)dropstones developed in varves. Geochemical studies of elements reveal the following: the data points of Mozaan Group diamictites show a restricted distribution in the SiO₂/Al₂O₃ diagram,which is significantly different from the typical linear negative correlation observed in modern and ancient sediments that have undergone significant weathering and sorting;The distribution range of TiO₂/Al₂O₃ is also narrow,and the major element compositions are similar;The matrix of the diamictites is rich in total iron(Fe₂O₃(T))and shows a marked depletion of calcium(Ca)and sodium(Na). The mathematical inversion of measured δ¹⁸O and δ¹⁷O values in the diamictite and the underlying shale yields δ¹⁸O values of weathering water consistent with the isotopic characteristics of modern polar meteoric water,indicating cold climatic conditions. In addition,indicators such as the Chemical Index of Alteration(CIA)suggest that the Mozaan Group diamictites have barely experienced diagenetic alteration or chemical weathering,thus well representing the characteristics of their provenance. Therefore,the following glacial depositional model is proposed based on the above findings: during periods of low sea level,glaciers advanced along valleys toward the continental shelf. The till(glacial sediment)consisted of not only basal till,lateral till,and rock flour produced by glacial abrasion of the bedrock,but also rounded quartz grains mixed from shelf sands and fine-grained weathered materials derived from ferruginous mud. Some diamictites originated from the direct deposition of glacial sediments in periglacial zones and subsequent modification,characterized by a sharp contact between the diamictites and the underlying bedrock,with deformed layers present beneath parts of the diamictites. Another part of the thick diamictite layers,indicated by dropstones in the upper part,was transported by ice rafts to ice-distal glaciomarine environments.

Key words: Pongola glaciation, identification marks of glaciation, Archean, Mozaan Group, South Africa

中图分类号: 

• P578.61