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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
王雄1, 黄朋1, 黄海军1, 胡宁静2
1.中国科学院海洋研究所;2.自然资源部第一海洋研究所
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(An>80,最大为87),而边缘的An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~1300℃);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(~1100℃)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(3-10km)停留较长时间,岩浆温度已经明显下降(960-1020℃),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:
基金项目:国家重点基础研究发展计划(2013CB429702); 中国科学院战略性科技先导专项(XDB42020303, XDA1103030204); 国家自然科学基金(41576055) [Foundation: National Key Basic Research Programme of China, 2013CB429702; Strategic Priority Research Program of the Chinese Academy of Sciences, No.XDB42020303, XDA1103030204; National Natural Science Foundation of China, No.41576055]
Magma Evolution Porocess in the East Rift, Manus Basin: Evidences from Plagioclase Phenocrysts in Lavas
Wang Xiong1, Huang Peng1, Huang Haijun1, Hu Ningjing2
1.Institute of Oceanology, Chinese Academy of Sciences;2.the First Institute of Oceanography, Ministry of Natural Resources
Abstract:
The structure and composition of plagioclase in volcanic rocks record the important information of magmatic evolution. In this study, we systematically analyzed mineralogy of plagioclase phenocrysts in lavas from East Rift, Manus Basin. The results exhibit 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 An values decreased?sharply for rim. And in dacites, the composition of plagioclase phenocrysts changes little from the core to the rim, most of plagioclases have normal zoning with the An value decreasing gradually, and few plagioclases are oscillatory zoning. Combined with calculation results of crystallizing temperature and pressure for phenocrysts, we conclude that the magma evolution process in ER as follows: magma originated by partial melting of mantle at approximately 1300℃, in the early age, due to the large magma supply, the magma has high ascend rate, and after a small degree of fractional crystallization, the magma eventually ejected at a fast velocity and temperature and formed basalt-basaltic andesites. In the late stage of the ER lavas, the magma experienced strong fractional crystallization due to the low supply rate of magma and long stay of magma in the chamber (3-10km, 960-1020℃), and finally formed acidic lavas. And we argue that, magma mixing didn’t play an important role in the magmatic evolution process.
Key words:  Manus Basin  East Rift  Plagioclase  mineralogical characteristics  magmatic evolution process
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