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| 马努斯海盆东部裂谷岩浆演化过程——火山岩中斜长石斑晶的启示 |
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王雄1,2, 黄朋1,3,4, 黄海军1, 胡宁静3,5
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1.中国科学院海洋研究所 海洋地质与环境重点实验室, 山东 青岛 266071;2.中国科学院大学, 北京 100049;3.青岛海洋科学与技术国家实验室 海洋地质过程与环境功能实验室, 山东 青岛 266071;4.中国科学院 海洋大科学研究中心, 山东 青岛 266071;5.自然资源部第一海洋研究所 海洋沉积与环境地质重点实验室, 山东 青岛 266061
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| 摘要: |
| 岩浆岩中斜长石矿物的结构、成分特征记录了岩浆演化过程的重要信息。本文对马努斯海盆东部裂谷(East Rift, ER)火山岩中的斜长石斑晶进行了详细的矿物学研究, 结果表明: 在玄武质安山岩中, 斜长石斑晶具正环带结构, 核-幔部属高An值(斜长石中钙长石分子百分数)斜长石(An值大于80, 最大为87), 而边缘的An值骤降(最小为63); 而在英安岩中, 大多数斜长石斑晶的成分从核部到边部变化较小, An值逐渐缓慢降低, 呈现正环带结构, 少量为韵律环带。结合火山岩斑晶矿物形成的温压条件计算结果, ER地区岩浆演化过程为: 地幔源区发生部分熔融产生玄武质岩浆(温度~1 300 ℃); 在岩浆活动早期, 由于岩浆供应充足, 岩浆上升过程中速度较快, 仅发生很小程度的分离结晶, 形成少量斑晶矿物, 并以较高的速度、温度(~1 100 ℃)直接喷出海底后形成玄武岩-玄武质安山岩; 至岩浆活动晚期, 岩浆供应量减少, 岩浆在岩浆房(3~10 km)停留较长时间, 岩浆温度已经明显下降(960~1 020 ℃), 发生强烈的分离结晶作用, 岩浆喷出海底形成酸性火山岩; 而在整个岩浆演化过程中, 未有明显的岩浆混合作用发生。 |
| 关键词: 马努斯海盆 东部裂谷 斜长石 矿物学特征 岩浆演化 |
| DOI：10.11759/hykx20200527001 |
| 分类号:P736.3 |
| 基金项目:中国科学院战略性科技先导专项(XDB42020303, XDA1103030204); 国家重点基础研究发展计划(2013CB429702); 国家自然科学基金(41576055) |
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| Magma evolution process in the East Rift, Manus Basin: evidence from plagioclase phenocrysts in lavas |
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WANG Xiong1,2, HUANG Peng1,3,4, HUANG Hai-jun1, HU Ning-jing3,5
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1.Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;2.University of Chinese Academy of Sciences, Beijing 100049, China;3.Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China;4.Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China;5.Key Laboratory of Marine Sedimentary and Environmental Geology, the First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China
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| Abstract: |
| The structure and composition of plagioclase in volcanic rocks provide important information regarding magmatic evolution. In this study, we systematically analyzed the mineralogy of plagioclase phenocrysts in lavas from the East Rift (ER), Manus Basin. Results show that the plagioclase phenocrysts in basaltic andesites have normal zoning, the An values for the core-mantle (An=80-87) of plagioclase belong to high-An plagioclase, and the An values decrease sharply for the rim. In dacites, the composition of plagioclase phenocrysts slightly changes from the core to the rim, most plagioclases have normal zoning with the An value decreasing gradually, and few plagioclases have oscillatory zoning. The calculation results of crystallizing temperature and pressure for phenocrysts show that the magma evolution process in ER involved the following phenomena: 1) Magma originated via partial melting of the mantle at~1300℃; 2) In the early age, magma had a high ascend rate owing to the large magma supply. 3) After a small degree of fractional crystallization, magma eventually ejected at a high velocity and temperature and formed basalt-basaltic andesites. In the late stage of the ER lavas, the magma underwent strong fractional crystallization owing to the low supply rate of magma and long stay of magma in the chamber (3-10 km; 960℃-1020℃) and finally formed acidic lavas. Moreover, we argue that magma mixing did not play an important role in the magmatic evolution process. |
| Key words: Manus Basin East Rift plagioclase mineralogical characteristics magmatic evolution process |