华南二叠纪末侵蚀不整合面上陆相碳酸盐岩沉积组构岩石学和地球化学特征研究

Terrestrial carbonates, which include calcareous crusts, terra rossa, terrestrial oncoids and void-filling deposits, have developed on the erosional unconformity, which truncated the skeletal grain/mudstone in the upper Permian Changxing Formation exposed at Permian-Triassic boundary (PTB) sections in southern China. These terrestrial carbonates indicate the occurrence of sea-level fall during late Permian, but also contain subtle but critical information about past climate and environment. There are, however, relatively few detailed studies of terrestrial carbonates that associated with the erosional unconformity in PTB sections. This study will decipher the provenance, genesis and the processes involved in development of various terrestrial carbonates, by careful analysis of their petrography and geochemical signatures in both macro and micro-scale. Critically, this study would show the variations of petrography and geochemical attributes in terrestrial carbonates take place in response to interactions between environment, climate and biological influences that operated during Paleozoic lowstand regimes, and provide environmental and climatic information during late Permian. These results could improve the understanding of terrestrial carbonates associated with erosional unconformity, but may also contain implications on the impact of late-Permian sea-level fall on P-T mass extinction.

我国华南地区二叠系-三叠系界限剖面（PTB剖面）长兴组海相碳酸盐岩地层顶部广泛发育有一个侵蚀不整合面。在这个不整合面上存在多种陆相碳酸盐岩沉积组构，如钙结壳、红土、陆相核形石和岩溶充填物。它们不仅为证明晚二叠世海平面下降事件的存在提供了证据，也为当时的古环境和古气候恢复提供了重要线索。然而，目前却鲜有关于PTB剖面陆相沉积组构内部精细结构及其蕴含的地质信息的研究。本项目拟通过宏微观岩石学和高分辨率地球化学分析技术，探讨中国华南地区PTB剖面中各陆相沉积组构的母源、成因和形成过程。在此基础上，构建古生代陆相碳酸盐岩沉积组构的微观岩石学和地球化学对海退期间古环境、古气候和古生态的响应机制，并获取关于发生在晚二叠世的极端古环境、古气候和古生态状况的信息。这些成果的取得不仅可以深化对陆相碳酸盐岩沉积组构成因环境的理解，也可能蕴含着关于海退事件对二叠纪-三叠纪之交生物灭绝事件影响的线索。

二叠纪末的生物大灭绝与海退事件和火山喷发在时间上的一致性使得海退事件古环境恢复成为二叠纪-三叠纪之交研究的热点话题。在华南地区（包括紫云、慈利、沫阳、边阳、打讲、打鸟坪、大文、关刀、老龙洞、二龙口和上寺）二叠系-三叠系界限剖面（PTB剖面）中，二叠纪末海退事件常常伴生的陆相碳酸盐岩沉积组构（如钙结壳、红土、陆相核形石和岩溶充填物），并被随后的微生物岩和/或巨鲕所覆盖。这些陆相碳酸盐岩沉积组构，可为海退期间的古环境和古气候恢复提供了重要线索。.本研究以二叠系-三叠系界线附近的大气暴露面和其上下临近沉积物作为研究对象，通过薄片鉴定、X射线衍射分析、微区碳氧稳定同位素分析和微区元素地球化学等方法，系统分析了岩石学、古生物学、矿物学、同位素和元素地球化学特征的时空变化规律。研究结果表明二叠纪-三叠纪之交发生了至少两次海退事件，第一次发生在二叠纪末，第二次发生在三叠纪伊始。第一次海退事件伴随着大规模的火山爆发，产生的火山灰是造成研究区红土沉积物和风化壳上粘土矿物的主要原因，其火山喷发的中心可能更靠近浙江煤山。这为二叠纪末的生物灭绝事件与火山喷发之间的关系提供了新证据。