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| 冲绳海槽中部岩心沉积物中浮岩的物理性质和地球化学特征差异及其对岩浆活动的指示 |
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方雪1,2,3, 曾志刚1,2,3,4, 胡思谊1,2,3, 朱博文1,2,3, 齐海燕1,2, 万世明1,2,5, 徐兆凯1,2,5
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1.中国科学院海洋研究所, 山东 青岛 266071;2.中国科学院海洋地质与环境重点实验室, 山东 青岛 266071;3.中国科学院大学, 北京 100049;4.青岛海洋科学与技术试点国家实验室海洋矿产资源评价与探测技术功能实验室, 山东 青岛 266071;5.青岛海洋科学与技术试点国家实验室海洋地质过程与环境功能实验室, 山东 青岛 266071
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| 摘要: |
| 为进一步了解冲绳海槽浮岩的物理性质和地球化学特征差异，对冲绳海槽中部岩心沉积物S9中的浮岩进行了微观结构和地球化学组成分析。结果显示，冲绳海槽中部存在白色、灰白色及棕色3种浮岩，其中灰白色浮岩又可以根据构造特征分为气孔构造和流动构造浮岩两个亚类。浮岩的地球化学组成表明白色、灰白色及棕色浮岩都是由玄武质岩浆经过充分的分离结晶作用形成的流纹质或流纹英安质火山岩。玄武质岩浆在演化的过程中发生了斜长石、角闪石、辉石、Fe-Ti氧化物、磷灰石等矿物的结晶分离。结合有孔虫14C年龄，认为浮岩是冲绳海槽中部距今13.1 ka左右的长英质火山活动的产物。演化程度相对较低的棕色浮岩具有比白色浮岩高的TiO2，Al2O3，Fe2O3，MgO，CaO含量，且棕色浮岩具有相对低的稀土总量和轻稀土总量。根据浮岩的物理性质及地球化学组成差异推测，岩浆的黏度和压力是影响浮岩构造特征的主要因素。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。 |
| 关键词: 浮岩 微观结构 地球化学 岩浆演化 冲绳海槽 |
| DOI：10.11759/hykx20190626001 |
| 分类号:P736 |
| 基金项目:国家自然科学基金项目（41325021）；全球变化与海气相互作用专项（GASI-GEOGE-02）；国家重点基础研究发展计划（2013CB429700）；中国科学院国际合作局对外合作重点项目（133137KYSB20170003）；泰山学者工程专项（ts201511061）；青岛海洋科学与技术国家实验室鳌山人才计划（2015ASTP-0S17）；创新人才推进计划（2012RA2191）；青岛海洋科学与技术国家实验室鳌山科技创新计划项目（2015ASKJ03，2016ASKJ13） |
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| Physical properties and geochemical characteristics of pumice rocks in core sediments from the middle Okinawa Trough and their indications of magmatism |
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FANG Xue1,2,3, ZENG Zhi-gang1,2,3,4, HU Si-yi1,2,3, ZHU Bo-wen1,2,3, QI Hai-yan1,2, WAN Shi-ming1,2,5, XU Zhao-kai1,2,5
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1.Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;2.Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;3.University of Chinese Academy of Sciences, Beijing 100049, China;4.Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology(Qingdao), Qingdao 266071, China;5.Laboratory for Marine Geology and Environment, Pilot National Laboratory for Marine Science and Technology(Qingdao), Qingdao 266071, China
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| Abstract: |
| To further understand the physical properties and geochemical characteristics of pumice rocks in the Okinawa Trough, we studied the microstructure and geochemical compositions of pumice clasts recovered from the sediment core S9 from the middle Okinawa Trough. The results indicate that white, grayish-white, and brown pumice rocks exist in the middle Okinawa Trough, and the grayish-white pumice rocks can be divided into two subtypes according to their structural characteristics:one with well-developed flow structure and the other with fumarolic structure. The geochemical compositions of pumice clasts suggest that the white, grayish-white, and brown pumice rocks were rhyolite and rhyolitic-dacite that were derived from basaltic magma through extensive fractional crystallization. Plagioclase, amphibole, pyroxene, Fe-Ti oxide, and apatite were crystallized during the evolution of the basaltic magma. Based on radiocarbon dating data and the geochemical compositions of the pumice clasts, we conclude that the white, grayish-white, and brown pumice rocks are the products of a felsic volcanic activity that erupted approximately 13.1 cal. ka BP. The brown pumice rocks feature relatively higher contents of TiO2, Al2O3, Fe2O3, MgO, and CaO, and relatively lower values of REEs and LREEs than those of the white pumice rocks. According to the physical properties and geochemical compositions, we infer that the structures of the pumice rocks were likely associated with the viscosity and pressure of the magma. The magma with high viscosity that undergoes continuous decompression process tends to produce pumice rocks with dense vesicular and well-developed flow structure, whereas the magma with low viscosity that undergoes episodic decompression process is likely to produce pumice rocks with large and loose vesicular structures. |
| Key words: pumice microstructure geochemistry magmatic evolution the Okinawa Trough |
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