The Cryogenian Nanhuan System (South China) during the interglacial-glacial transition: geochemistry, sedimentary provenance, and tectonic setting

Samples of the Nanhuan System during the interglacial-glacial transition in the Shennongjia area, northern Yangtze block, were analysed via major and trace elements, clay mineralogy, and sulphur isotopes to infer provenance, paleoweathering intensity of the source rocks, tectonic setting, and marine chemistry. The magmatogenic sedimentary rocks are typical of acidic igneous sources, whereas the remaining samples show a sedimentary trend. The tectonic discrimination of a passive continental margin was determined via major element diagram. The samples showing subduction-affinity geochemistry may represent provenance virtually entirely comprising old recycled material. Low chemical index of alteration values and high illite contents of the sedimentary deposits reveal that the basin is characterized by physical erosion owing to the combined effects of tectonic movement and long-term cool and dry climatic conditions. Extreme δ³⁴S enrichment in the Datangpo Formation indicates that this Cryogenian formation was a restricted rift basin with limited exchange with the open ocean. The δ³⁴S_sulfide and illite values increase rapidly at the end of the Sturtian glaciation followed by a sharp decline at the end of inter-glacial silt deposition. Low oceanic sulphate concentrations and intense bacterial reduction of sulphate may account for the ³⁴S enrichment in this restricted deep-water basin.

针对扬子地块北部（northern Yangtze Block）神农架地区冰期-间冰期转换期的南华系（Nanhuan System）样品，研究者通过主量与微量元素、黏土矿物学及硫同位素分析，对其物源、源岩古风化强度、构造背景及海水化学特征开展了系统性推演。岩浆成因沉积岩具有典型的酸性火成物源特征，其余样品则呈现沉积演化趋势。通过主量元素图解可判定研究区构造背景为被动大陆边缘。表现出俯冲带亲和性地球化学特征的样品，其物源几乎完全由古老再旋回物质构成。沉积岩样较低的化学风化指数（Chemical Index of Alteration, CIA）值与较高的伊利石含量表明，该盆地以物理侵蚀为主要侵蚀方式，这是构造运动与长期冷干气候共同作用的结果。大塘坡组（Datangpo Formation）中δ³⁴S极端富集，指示这套成冰纪（Cryogenian）地层形成于与开阔大洋交换受限的封闭裂谷盆地。斯图特冰期（Sturtian Glaciation）结束时，硫化物硫同位素δ³⁴S与伊利石含量均快速升高，随后在间冰期粉砂沉积末期出现急剧下降。该封闭深水盆地中较低的海水硫酸盐浓度与强烈的硫酸盐细菌还原作用，或是造成δ³⁴S富集的核心成因。