The Relationship between Volcanism and Reef Sedimentary Evolution from Late Permian to the Beginning of Early Triassic in Ziyun Area, Guizhou Province
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摘要: 晚二叠世长兴期是华南地区重要的成礁期，也是火山事件频发的一个时期。生物礁是一种对海洋环境十分敏感的生态系，晚二叠世的火山作用势必影响到当时生物礁的生长和沉积相的演化。然而，前人有关晚二叠世火山事件的研究主要集中在二叠系与三叠系界线附近，而对整个长兴期生物礁的演化与火山事件关系的研究则较少。选择贵州紫云的台地边缘礁为研究对象，通过大量磁化率的测试，分析整个晚二叠世生物礁的沉积演化与火山作用的关系。分析结果显示，长兴早期和末期岩石的磁化率值明显偏高，反映较强的火山活动。在这两个时期，造架生物相对不发育。长兴中期，岩石的磁化率值相较于长兴早期和长兴晚期总体偏低，表明火山活动相对较弱，导致生物礁的快速生长。但长兴中期生物礁灰岩中夹有几层生物碎屑灰岩，其磁化率值有小幅度的升高，说明短时期小规模的火山作用虽然导致了生物礁生长的暂时中断，但礁相动荡的环境使得火山灰中细小的铁磁性物质难以沉积下来。研究发现，尽管早三叠世早期泥岩具有高的磁化率值，但泥质岩中多层火山黏土具有异常高的磁化率值，火山灰物质对磁化率大小的影响远远超过陆源物质的贡献。Abstract: Late Permian Changhsingian is an important period of reef buildup as well as a period of frequent volcanic activities in South China. Because reefs are peculiar ecosystems extremely sensitive to environment change, late Permian volcanism would surely have affected the reef growth and evolution of facies. However, most of the researches concerning the late Permian volcanism are concentrated near the Permo-Triassic boundary. Studies appear to be relatively lack on the relations between the evolution of reefs and volcanism throughout the whole late Permian Changhsingian. This paper documented the relationship between late Permian volcanism and the marginal platform reef evolution in Ziyun area of Guizhou province based on the analysis of magnetic susceptibility. The magnetic susceptibility data reflect two pulses of high magnetic susceptibility values respectively in the early Changhsingian and late Changhsingian, indicating two periods of intense volcanic activities. During both periods, reef-building organisms were clearly stressed and reef-building process was interrupted. The magnetic susceptibility values in the middle Changhsingian are generally lower, indicating weak volcanism activities that favored the rapid growth of reef-building organisms. However, the magnetic susceptibility values from the bioclastic limestone interbedded in reef limestone do not increase significantly, implying that turbulent water conditions prevented tiny volcano ash rich in ferromagnetic minerals from depositing in shallow reef environment although periodic volcanic eruption may have resulted in temporary breakdown of reef growth. This study shows that several beds of volcanic clay interbedded in the early Triassic mudstone possess abnormally high magnetic susceptibility values compared with the mudstone itself, indicating that much greater contributions of volcanic ash to the magnetic susceptibility values than the terrigenous material.
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Key words:
- Changhsingian /
- reef /
- magnetic susceptibility /
- sedimentary evolution /
- volcanism
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