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冲绳海槽中部沉积物中火山物质的地球化学研究
方雪
Subtype博士
Thesis Advisor曾志刚
2020-05-19
Degree Grantor中国科学院大学
Place of Conferral中国科学院海洋研究所
Degree Name理学博士
Degree Discipline海洋地质
Keyword冲绳海槽中部 火山活动 地球化学 来源 岩石成因
Abstract

  冲绳海槽是位于东海东缘的一个尚处于早期扩张阶段的新生弧后盆地。海槽中火山活动频繁,火山岩广泛发育。晚第四纪以来的连续沉积作用,使得冲绳海槽被一层巨厚沉积物所覆盖,沉积物中的火山物质为研究地质历史时期弧后盆地的岩浆体系提供了宝贵材料。本学位论文以冲绳海槽中部S9站位获取的沉积物岩心中的火山物质为主要研究对象,开展了较为系统的地球化学研究。主要包括对沉积物中浮岩碎屑进行了全岩主量元素、微量元素和Sr-Nd-Pb同位素组成分析;沉积物中火山玻璃和沉积物中浮岩碎屑的单斜辉石进行了原位主量元素和微量元素分析;沉积物浮游有孔虫放射性碳定年分析。重点探讨了沉积物中灰白色浮岩的成因及岩浆演化过程;对比分析了不同颜色浮岩的物理性质及化学成分差异;讨论了浮岩中单斜辉石的结晶温度和压力;揭示了火山玻璃的来源及其蕴含的沉积学意义。取得的主要认识包括以下几点:

   岩心S9中的灰白色浮岩,岩性上属于英安岩,是1311.2 cal. ka BP期间冲绳海槽中部长英质火山活动的产物。浮岩的母岩浆具有与俯冲相关岩浆的特征,是受俯冲沉积物交代的印度洋型地幔上升至位于下地壳的岩浆房,在岩浆房中与少量下地壳组分发生同化混染后经过充分的分离结晶作用形成的。冲绳海槽中部下地壳可能存在一个千年尺度的岩浆房,在1311.2 cal. ka BP期间进行了长英质岩浆供给。

  冲绳海槽中部的长英质火山活动产生了白色、灰白色及黑色三种颜色的浮岩。这三种浮岩具有同源性,但经历的岩浆演化程度存在差异。白色浮岩岩浆演化程度最高,灰白色浮岩次之,黑色浮岩最低。浮岩颜色上的差异与富Fe斑晶矿物的含量有关,含量越高颜色越深。浮岩气孔构造的差异可能与岩浆的物理性质有关。黏度大、连续减压的岩浆易于形成具有流动构造和密集气孔的浮岩,黏度小、阶段性减压的岩浆易于形成气孔大而疏松的浮岩。

  沉积物中灰白色浮岩中的单斜辉石是拉斑玄武质岩浆演化过程中结晶分异作用的产物,其结晶温度为10791108℃,平均值为1096℃。岩心S9不同层位浮岩中的单斜辉石结晶温度相近。单斜辉石的地球化学特征表明岩浆体系在演化的过程中发生了斜长石、单斜辉石、斜方辉石、钛铁矿/钛磁铁矿和磷灰石等矿物的分离结晶。

    岩心S9中沉积的火山玻璃是在12.26.5 cal. ka BP期间喷发的。冲绳海槽中存在一个潜在的长英质火山玻璃源,在12.27.3 cal. ka BP期间对研究区的沉积作用进行了持续的供给。在7.36.5 cal. ka BP期间,源自日本九州岛(Kikai火山)的火山玻璃对岩心S9中火山玻璃可能存在较大贡献。火山玻璃对冲绳海槽中部陆源沉积物的地球化学组成存在长期的影响;因此,在使用研究区沉积物硅酸盐组分地球化学组成建立物源判别和区域硅酸盐风化历史重建替代指标时,要充分考虑和评估火山物质带来的影响。研究区的古环境变化可能对岩心S9中火山玻璃的沉积存在较大影响。在12.27.3 cal. ka BP期间,由于气候干冷且海平面处于低水位期,冲绳海槽中火山活动的产物是岩心S9中火山玻璃重要的源所在,对海槽中部的沉积作用起到了积极的贡献。在7.36.5 cal. ka BP期间,由于海平面上升,从日本九州岛向冲绳海槽中部输送的火山物质通量显著增大,使得沉积速率加快和沉积物平均粒径增大。

Other Abstract

       The Okinawa Trough is a nascent back-arc basin located in the eastern margin of the East China Sea, which is still in the early stage of expansion. Frequent volcanic activity has given rise to numerous volcanic rocks in the Okinawa Trough. Because of the continuous sedimentation since the late Quaternary, the Okinawa trough is covered by a thick layer of sediments. The volcanic materials in the sediments provide valuable materials for the study of the magma system in the back-arc basin during the geological history. In this dissertation, the volcanic materials in the sediment core obtained from S9 site in the middle of Okinawa trough is taken as the main research object, and a systematic geochemical study has been carried out. The pumice clasts selected from the sediments were analyzed for major, trace element and Sr-Nd-Pb isotope compositions. The volcanic glass collected from the sediments and the clinopyroxene in pumice clasts were performed for in situ major and trace element analysis. In addition, the planktonic foraminifera in the sediments were separated for radiocarbon dating. The petrogenesis and magmatic evolution of the grayish-white pumice, the physical properties and chemical composition of the pumice with different colors, the crystallization temperature and pressure of clinopyroxene in the pumice, and the source of the volcanic glass and its sedimentary significance were discussed on the basis of the analysis results. The major findings are as follows:
       The grayish-white pumice clasts in Core S9 belongs to dacite in lithology, which are the products of felsic volcanic activity in the middle Okinawa trough during 13~11.2 cal. ka BP. The parent magma of pumice has the characteristics of subduction related magma, which was generated from hybrid magma through extensive fractional crystallization. The Indian MORB type mantle was first metasomatized by the subducted Philippine Sea sediments to form the primitive magma, and then contaminated with a small amount of lower crust material in the lower crust to generate the hybrid magma. The geochemical characteristics of grayish-white pumice indicated that a millennial scale magma chamber might exist in the lower crust of the middle Okinawa Trough, and provided magma supply during 13~11.2 cal. ka BP.
       The felsic volcanism in the middle of Okinawa Trough has produced white, gray and black pumice. The three kinds of pumice were derived from similar primitive magma but with different degree of magma evolution. The white pumice has the highest magma evolution degree, followed by grayish-white pumice and black pumice. The variations in pumice color might be related to the content of Fe-enriched phenocryst mineral. Pumice with higher content of such mineral tend to have darker color. The characteristics of pumice vesicles might be associated with the physical properties of the magma. The magma with high viscosity and continuous decompression is easier to form pumice with dense vesicular and flowing structure, while the magma with low viscosity and staged decompression are more likely to generate pumice with large and loose pores.
       The clinopyroxene in grayish-white pumice of Core S9 were originated from the crystallization differentiation process of tholeiitic magma. The crystallization temperature for clinopyroxene is 1079~1108℃ with the average value of 1096 ℃. The geochemical characteristics of clinopyroxene indicate that minerals such as plagioclase, clinopyroxene, orthopyroxene, magnetite/titanomagnetite, and apatite were crystallized during the evolution of magmatic system.
       The volcanic glass deposited in the core site were erupted during 12.2~6.5 cal. ka BP. There was a potential felsic volcanic glass source in the Okinawa Trough, which provided continuous volcanic glass supply during the period of 12.2~7.3 cal. ka BP. The volcanic glass from Kyushu Island (Kikai volcano) might contribute significantly to the volcanic glass in Core S9 during the period of 7.3~6.5 cal. ka BP. Volcanic glass has a long-term impact on the geochemical composition of terrigenous sediments in the middle of Okinawa trough. Therefore, the impact of volcanic materials should be fully considered and evaluated when using the geochemical composition of silicate fractions of sediments in the study area to establish proxies for source discrimination and regional silicate weathering history reconstruction. The variabilities of paleoenvironment in the study area may have a great influence on the sedimentation of volcanic glass in Core S9. During the period of 12.2~7.3 cal. ka BP, because the climate was dry and cold and the sea level was in a lowstand period, the products of volcanic activity in Okinawa Trough were the dominant sources of volcanic glass in Core S9, which has played a positive role in the sedimentation of the middle Okinawa Trough. Due to the rise of sea level during the period of 7.3~6.5 cal. ka BP, the volcanic material flux from Kyushu Island to the middle of Okinawa Trough has enhanced significantly, which led to the acceleration of sedimentation rate and the increase of mean grain size.
 

Subject Area海洋地质学
MOST Discipline Catalogue理学::海洋科学
Language中文
Document Type学位论文
Identifierhttp://ir.qdio.ac.cn/handle/337002/164718
Collection海洋地质与环境重点实验室
Recommended Citation
GB/T 7714
方雪. 冲绳海槽中部沉积物中火山物质的地球化学研究[D]. 中国科学院海洋研究所. 中国科学院大学,2020.
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