摘要:
在剖面实测、薄片鉴定和阴极发光分析的基础上, 结合元素和同位素地球化学分析, 研究黔西北毕节地区中二叠统碳酸盐岩的岩石学和地球化学特征并揭示其地质意义。结果表明: 1)中二叠统栖霞和茅口组主要发育泥晶灰岩和颗粒灰岩, 不具阴极发光或显示弱阴极发光, 而白云石化作用主要发育于栖霞组上部和茅口组一段, 白云石具有暗玫红色阴极发光; 2)栖霞组—茅口组碳酸盐岩相对富集 Ni、V、Cr、U, 贫亲铜元素, 具有稀土总量低(∑REE+Y 为 2.11～8.10 μg/g), 轻稀土相对亏损和重稀土相对富集的左倾型稀土分配特征; 3)易于在陆源组分中富集的 Ti、Al、Zr、Th 等元素含量均较低、具有低 Mn 含量(<50×10–6)和 Mn/Sr比值(<1)、结合阴极发光和 REE 组成特征, 说明研究区中二叠统碳酸盐岩样品受陆源物质影响或改造的程度较小, 而极低的 Re、Cd、Mo、U、Cu 含量以及明显的 Ce 负异常表明, 中二叠世研究区以氧化环境为主;4)碳酸盐岩样品的 δ13CPDB为 1.2‰ ～ 4.6‰(平均值 3.6‰), δ18OPDB为–7.4‰ ～ –10.1‰(平均值–9.08‰), 碳氧同位素之间无明显相关性, 碳同位素的变化与区内的海侵—海退旋回相一致, 而氧同位素的偏负则反映的是渐进埋藏成岩作用的增温效应。
Abstract:
Based on field observations, thin section identification, and cathodoluminescence analysis, combined with elemental and isotope geochemical analysis, the petrological and geochemical characteristics of the Middle Permian carbonate rocks in northwestern Guizhou were studied. The results show that: 1) Grainstone and micritic limestone dominate the Middle Permian strata, and the rock samples exhibit weak to no luminescence.Dolomitization mainly occurs in the upper part of Qixia Formation and the first member of Maokou Formation;the dolomites have dark rose cathodoluminescence. 2) The carbonate rocks of Qixia–Maokou Formation are relatively rich in Ni, V, Cr and U, and poor in chalcophile elements. They generally have a left-leaning REE distribution pattern with low total REE (∑REE 2.11～8.10 μg/g), relative deficit of LREE, and relative enrichment of HREE. 3) The contents of elements such as Ti, Al, Zr, and Th, which are easily enriched in terrigenous components, are low. The low Mn content (<50×10–6) and Mn/Sr ratio (<1), combined with the characteristics of cathodoluminescence and REE composition, indicate that the Middle Permian carbonate samples in the study area are less affected or modified by terrigenous materials. The extremely low Re, Cd, Mo, U, Cu content and obvious negative Ce anomaly indicate that the environment of the study area was dominated by oxidation conditions during the Middle Permian. 4) The δ13CPDBand δ18O(PDB )ranges from 1.2‰ to 4.6‰ (mean 3.6‰), and from–7.4‰ to –10.1‰ (mean –9.08‰), respectively. There is no obvious correlation between δ13CPDBand δ18OPDB.The variation of carbon isotope is consistent with the transgression-regression cycle in the area, while the negative oxygen isotope is mainly the result of burial diagenesis.
下载: