大同火山阶段性活动历史的古地磁年代学研究

发布时间：2018-05-22 11:45

本文选题：大同火山 + 沉积地层　；参考：《上海师范大学》2017年硕士论文

【摘要】：大同火山群位于山(西)陕(西)地堑系最北端的大同盆地中。第四纪时期,大同火山群火山多次喷发,在盆地内堆积了大量的火山喷发物。自上世纪30年代开始,中外学者对大同火山展开了大量的科研工作,在大同火山的地质构造背景、火山活动类型、岩浆化学成分、火山深度动力学机制及活动年代等方面都取得了众多研究成果。然而,由于野外采样困难,火山活跃期次较多等因素的影响,使得目前的火山活动年代数据无法形成有效的比对,更不能反映大同火山活动的阶段性历史特征,因此人们对于大同火山的活动期次及年代问题一直争论不断。野外调查发现:火山锥体坡麓附近和泥河湾古湖湖相沉积层与湖岸之上黄土—古土壤沉积层中夹有多层火山碎屑层。(1)在肖家窑火山坡麓位置存在有多套倾角不同的火山碎屑层。且位于底部的火山碎屑层倾角大于其上部碎屑层的倾角。(2)陈庄湖相沉积层和黑山黄土—古土壤沉积层中发现有多层疑似火山灰物质沉积的延续良好的薄层灰黑色特征沉积层。这些沉积现象表明:大同火山在地质历史时期存在多个活跃期。区域的湖相沉积和黄土—古土壤沉积都详细的记录了当时的火山活动信息。为从沉积地层中获得更为详细的火山活动信息、建立沉积剖面的年代框架、判断沉积剖面中所夹火山物质沉积层位的沉积年代。本文对火山群周边受火山活动影响的湖相沉积层和黄土—古土壤沉积层进行了仔细的野外物理形态描述和高精度的采样。我们选取陈庄剖面作为湖相沉积的典型剖面、黑山剖面作为黄土沉积的典型剖面。野外考察中都发现这两剖面中存有明显的火山活动记录信息,实验室分析表明这两个剖面都有多个层位在沉积过程中受到了区域火山活动的影响。在湖相沉积剖面中出现四层磁化率、Ti元素含量、Fe元素含量和平均粒度组成同步异常变大的沉积层位;在黄土—古土壤沉积剖面中出现三层磁化率、Ti元素含量或Fe元素含量同步异常变大的沉积层位。这些异常变大的现象表明沉积时受到区域环境突变的影响。在大同盆地中则是受到火山活动导致的区域沉积环境突变。利用古地磁定年法和黄土古土壤定年法建立不同沉积剖面的年代框架,判断湖相沉积层中四个火山物质沉积层的沉积年代分别为:早于0.90MaB.P.、约0.51MaB.P.、约0.32MaB.P.和约0.09MaB.P.;黄土—古土壤沉积层中三个火山物质沉积层的沉积年代分别为约0.32MaB.P.、0.19MaB.P.和约0.09MaB.P.。通过沉积层沉积年代比对认为大同火山群共存在五个活跃期,分别为早于0.90M aB.P.、约0.51MaB.P.、约0.32MaB.P.、0.19MaB.P.和约0.09MaB.P.
[Abstract]:Datong Volcano Group is located in the Datong basin at the northernmost end of the Shanshan (West) Shaanxi (West) graben system. During the Quaternary period, the Datong volcanic group erupted many times and accumulated a lot of volcanic eruptions in the basin. Since the 1930s, Chinese and foreign scholars have carried out a great deal of scientific research on Datong volcano. In the geological tectonic background, volcanic activity type, magmatic chemical composition of Datong volcano, Many achievements have been made in the dynamic mechanism and age of volcanic depth. However, due to the difficulty of sampling in the field and the influence of many factors such as the secondary active period of volcanism, the current dating data of volcanic activity can not form an effective comparison, let alone reflect the historical characteristics of the stages of the volcanic activity in Datong. As a result, there has been a constant debate about the active periods and chronologies of Datong Volcano. Field investigation shows that there are multiple layers of volcanic clastic layer located in the loess paleosol sediment layer near the slope of the volcanic pyramidal body and above the lacustrine facies of the ancient lake in Nihewan) and there are several sets of volcanic clastic layers with different dip angles at the foothills of the Xiaojiayao fire slope. Moreover, the dip angle of the volcanic clastic layer located at the bottom is greater than that of the upper clastic layer. The Chenzhuang lacustrine sedimentary layer and the Montenegrin loess-paleosol sedimentary layer are found to have several layers of thin-layer gray-black sedimentary layers with good continuity of suspected volcanic ash deposition. These sedimentary phenomena indicate that there are many active periods in the geological history of Datong volcano. Regional lacustrine deposits and loess-paleosol deposits both recorded the volcanic activity in detail at that time. In order to obtain more detailed volcanic activity information from sedimentary strata, the chronological framework of sedimentary profiles is established, and the sedimentary ages of sedimentary layers of volcanic material in sedimentary profiles are determined. In this paper, the lacustrine sediments and loess-paleosol sediments around the volcanoes affected by volcanic activity were carefully described in the field and sampled with high accuracy. We select Chenzhuang section as typical section of lacustrine facies and Montenegro section as typical section of loess deposit. During the field investigation, it is found that there are obvious records of volcanic activity in the two profiles, and laboratory analysis shows that there are many layers affected by the regional volcanic activity in the sedimentary process of the two profiles. In the lacustrine sedimentary profile, there are four layers of magnetic susceptibility and Ti content, Fe content and average particle size composition. In the loess paleosol sedimentary profile, the three layers of magnetic susceptibility and the content of Ti element or Fe element become larger and larger synchronously. These abnormal phenomena indicate that the deposition is affected by the abrupt change of regional environment. In the Datong basin, the regional sedimentary environment changes due to volcanic activity. Paleomagnetic dating method and loess paleosol dating method were used to establish the chronological framework of different sedimentary profiles. The sedimentary ages of the four volcanic sediments in the lacustrine sediments were determined to be earlier than 0.90 MaB.P., about 0.51MaB.P.and about 0.32MaB.Prespectively. The sedimentary age of the three volcanic sediments in the Loess paleosol sedimentary layer is about 0.32 MaB.P./ 0.19 MaB.P. And about 0.09 MaB.P. Five active periods of the Datong volcanic group, which are earlier than 0.90 Ma B.P., 0.51MaB.P., 0.32 MaB.P.and 0.19 MaB.Prespectively, are considered to exist in Datong volcanic group by comparison of sedimentary ages. And about 0.09 MaB.P.
【学位授予单位】：上海师范大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P318.44;P317

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