Constraints of Parent Magma on Altered Clay Minerals: A Case Study on the Ashes near the Permin-Triassic Boundary in Xinmin Section, Guizhou Province

The study on the influence of sedimentary environments on the altered clay mineralogy facilitates accurate stratigraphical correlation using volcanic ashes as marked beds. However, the relationships among the stacking ordering, illite layer contents, parent magma and environmental condition are still poorly known. A case study on altered vocalnic materials near the Permian-Triassic boundary in the Xinmin section, Guizhou province was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), geochemical analysis, and oxygen isotope analysis methods. Results show that all the four volcanic ash layers contain two phases of mixed-layered illite/smectite (I/S) with R3 structure and varied layer contents, and clay mineral aggregates were found having replaced the volcaniclastic particles or have grown on the surfaces of volcaniclastic particles. It can be inferred that clay minerals formed in sedimentary to early diagenesis periods since the materials preserve argillaceous texture when clay minerals were produced. The number of Fe³⁺ atoms are 0.16 and 0.17 respectively in the samples XM-5-1 and XM-5-2, and 0.14 in both samples XM-5-3 and XM-5-4, indicating that parent magmas of the latter ones were more of meta acid compared with the former two. It is indicative of more intense alteration of the latter two volcanic ash layers, together with the apparent Eu negative anomaly and less K₂O contents. Oxygen isotopic composition of the volcanic ashes ranges narrowly from 17.3‰ to 18.1‰, consistent with the equilibrium value between smectite and ocean water at 25 ℃, suggesting that chemical composition of the Xinmin clay minerals is related to conditions of submarine diagenesis alteration since the clay mineralogy of different volcanic ash layer depends on its parent magma and degree of diagenesis alteration.