扬子地台西缘早-中三叠世界线碳同位素异常与火山事件

The inorganic carbon isotopes have a rapid transition from negative to positive at the early-middle Triassic boundary, forming a significant positive drift. The green-bean rocks of volcanic ash deposits are widely distributed at this boundary in southern China, which indicated that there was a strong volcanic activity at the turn of the early-middle Triassic. The two events have a high degree of coupling relationship. Taking Emeishan Longmendong section and Guangyuan Shangsi section of Sichuan province which lie in the western margin of Yangtze platform as case study, this research will investigate the 20m strata above and below the lower-middle Triassic boundary (green-bean rocks). On the basis of high-density sampling and sample diagenetic alteration detection, the evolving curves of high-resolution inorganic carbon isotope and organic carbon isotope will be established by applying multiple methods, such as sedimentology, petrology, chronostratigraphy and geochemistry, as well as the carbon isotope of marine carbonate rocks and kerogen carbon isotope study will be carried out simultaneously. Moreover, a regional and global comparison will be conducted by using the exact isotopic dating of green-bean rocks. Combined with organic carbon content and microelement, we further will discuss the relationship between carbon isotope excursion at the boundary of the early-middle Triassic and volcanic events and its influence mechanism.

无机碳同位素在早-中三叠世界线处发生了由负到正的快速转变，形成了显著的正漂移，而中国南方在该界线处广泛分布火山灰沉积-绿豆岩，表明了在早-中三叠世之交存在强烈的火山活动事件，两者具有高度的耦合关系。本项目选取扬子地台西缘四川峨眉山龙门硐和广元上寺剖面，以下-中三叠统界线（绿豆岩）上下20m地层为研究对象，在高密度样品采集和成岩蚀变检测的基础上，采用沉积学、岩石学、年代地层学和地球化学等手段，同步开展海相碳酸盐岩碳同位素和干酪根碳同位素研究，建立高分辨率的无机碳同位素和有机碳同位素演化曲线，并利用绿豆岩精确的同位素定年，进行区域性和全球性对比。结合有机碳含量、微量元素等分析，进一步探讨早-中三叠世界线碳同位素异常与火山事件的关系及其影响机制。

早-中三叠世界线是继二叠纪生物大灭绝后生物复苏及海洋生态系统全面重建的重要时间限定。而在该界线处，无机碳同位素发生了由负到正的快速转变，形成了显著的正漂移，同时，中国南方在该界线处广泛分布火山灰沉积-绿豆岩，两者具有高度的耦合关系。本项目针对扬子地台西缘早-中三叠世沉积的碳酸盐岩及绿豆岩，重点开展早-中三叠世火山事件及碳同位素演化研究。本项目主要研究内容包括：（1）绿豆岩矿物学、地球化学及沉积环境研究；（2）绿豆岩年代学研究；（3）绿豆岩来源及火山事件；（4）碳同位素演化。通过三年的研究，取得如下重要成果：（1）绿豆岩为一种富含豆粒状石英颗粒的富钾的水云母黏土岩，是中酸性岩浆成分的火山喷发形成的火山灰落入亚还原-还原、微咸水-咸水的水体中，在干燥炎热的沉积环境下形成的（沉）凝灰岩。（2）首次获得扬子地台西缘早-中三叠世界线绿豆岩绝对年龄值为247.49 ± 0.68 Ma，该套绿豆岩可与南盘江盆地发育的该时期凝灰岩进行对比，可作为扬子地台西缘早-中三叠世Olenekian-Anisian界线的对比标志。（3）明确了扬子地台西缘早-中三叠世界线绿豆岩来源于三江造山带，与古特提斯洋在早-中三叠世碰撞造山伴随大量的酸性火山喷发有关。限制了古特提斯洋开始俯冲时间为254.8Ma，即晚二叠世中期。（4）建立了扬子地台西缘早-中三叠世高分辨率碳同位素曲线，Spatnian-Anisian期界线以碳同位素由负变正的显著正漂移为标志，并可与全球对比。