ABSTRACT

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.

摘要

对扬子地块北部神农架地区间冰期过渡期间南环系统样品进行了主微量元素、粘土矿物学和硫同位素分析，以推断物源、烃源岩古风化强度、构造背景和海相。化学。岩浆成因沉积岩是典型的酸性火成岩源，而其余样品则显示出沉积趋势。被动大陆边缘的构造区分是通过主元图确定的。显示俯冲亲和地球化学的样品可能代表几乎完全由旧回收材料组成的出处。沉积物的化学蚀变值低，伊利石含量高，表明该盆地在构造运动和长期干冷气候条件的共同作用下，具有物理侵蚀的特征。极端δ大塘坡组³⁴ S 富集表明该地层为受限裂谷盆地，与公海交换有限。δ ³⁴ S_硫化物和伊利石值在斯图尔特冰期结束时迅速增加，随后在间冰期粉砂沉积结束时急剧下降。低海洋硫酸盐浓度和硫酸盐的强烈细菌还原可能是这个受限制的深水盆地中³⁴ S 富集的原因。