摘要： 中国华南新元古代成冰纪斯图特(Sturtian)冰期“前夜”的古气候(温室抑或冰室)环境一直未得到一致的认识。新近在扬子北缘神农架成冰系底部层位莲沱组上部发现一系列软沉积物变形构造,表现为紫红色基质内发育灰绿色砂质脉体群。依据这些脉体宏观—微观形态、结构构造以及地球化学特征等分析认为,它们可能是寒冷气候下的沉积物冻融构造,还或许代表永久冻土的存在。根据形态,将砂质脉划分为2组4种类型,即刚性破裂的网状、枝状脉体与塑性变形的条带状、斑块状脉体。研究结果表明,脉体具有颗粒结构明显、杂基含量低、脉壁边缘颗粒弱定向性和Fe³⁺的亏损特征,脉体曾发生液化流动;脉体与基质间Bi、Pr、W、La等微量元素存在差异,2种不同的稀土元素富集类型否定了成岩后期蚀变成因;河流二元结构、Sr/Ba值介于0.05~0.25之间、V/Ni值介于0.05~0.5之间,均表明它们发育在曲流河的洪泛平原与滨岸环境;脉体与基质的碳氧同位素负异常(δ¹³C_PDB 为-17‰~-14‰,δ¹⁸O_PDB 为-15‰~-13‰)与低CIA值(55~65)揭示了可能的寒冷气候背景。上述一系列地质和地球化学证据都支持扬子北缘神农架南华纪(成冰纪)斯图特冰期开始前寒冷气候的存在,为莲沱组沉积晚期寒冷气候条件的识别和认定,特别是为研究扬子地块的“雪球地球”事件的响应,开辟了新的研究思路与途径,同时也为华南成冰纪冰期的划分及其国内外对比提供了新的沉积地质学标识和科学证据。

关键词: 冻融构造, 古气候, 莲沱组, 南华系, 神农架, 扬子克拉通北缘

Abstract: The palaeoclimates(i.e.,greenhouse or icehouse)in South China before the Sturtian period of the Neoproterozoic Cryogenian are still controversial. Recently,a series of soft sedimentary deformation structures(SSDSs)were recognized in the upper part of the Liantuo Formation of the Shennongjia in the northern margin of the Yangtze Craton. They are represented by gray-green sandy veins host in purple mudstone or sandstone matrix. Based on their marco-mirco morphology,texture and structure,and geochemistry characteristics,we interpret them to be a type of freezing and thawing structures developed under cold climates,or perhaps being suggestive of the existence of permafrost. According to their morphologies,the documented Liantuo SSDSs are categorized into four main types,namely reticulate,dendritic veins bearing rigidity fracture,along with banded,mottled veins bear plastic deformation. our results suggest that the characteristics of the veins were more matrix debris supported less clay minerals,weak particles directionality lining the wall,loss of Fe(Ⅲ),indicating that the veins had been liquefied. Moreover the veins are more depleted in trace element such as Bi,Pr,W,La than those in the matrix. However,different REE distribution patterns of veins and matrices also indicate that there is no effect on the alteration during the later diagenesis. The dual structures in debris rocks(i.e.,low Sr/Ba ratio of 0.05 to 0.25 versus low V/Ni ratio of 0.05 to 0.5)suggest that the structures were developed in microenvironments like floodplains of meandering rivers. The low CIA values of 55-65and negative carbon and oxygen isotope negative excursion(i.e.,δ¹³C_PDB of-17‰~-14‰,δ¹⁸O_PDB of-15‰~-13‰) in the veins and matrices,in contrast,indicate that they would probably have formed in cold climates. All of these observations suggest that the Shennongjia area in the northern margin of the Yangtze Craton was located in the cold periglacial environments before the start of Stuartian Period in the Neoproterozoic Cryogenian. Furthermore,the paper opens up a new research idea and approach for the recognition of the cold climatic conditions in late sedimentary period of the Liantuo Formation,particularly the response on the “Snowball Earth”event in Yangtze Craton. Meanwhile,it also provides a new scientific evidence and sedimentary index for the division of glacial period in Yangtze Craton and correlation at home and abroad.

Key words: freezing and thawing structures, palaeoclimate, Liantuo Formation, Cryogenian, Shennongjia, northern margin of Yangtze Craton

中图分类号: 

• P512