科科斯脊玄武岩斜长石矿物化学及地质意义

综合大洋钻探计划(IODP) 334和344航次在U1381站位处的两个钻孔(A孔和C孔)获得了中美洲西海岸外科科斯脊基底拉斑玄武岩，对其岩浆过程开展研究可为理解其岩石成因提供重要依据。本文对科科斯脊玄武岩中斜长石斑晶和微晶进行了详细的原位主微量元素分析，结果表明，斜长石种属为培长石、拉长石及少量中长石。部分斜长石斑晶具有正环带结构；但多数斜长石斑晶不具有明显环带，仅从核部到边部存在微弱的成分变化。斜长石斑晶与微晶的微量元素差别较大:斜长石斑晶富集轻稀土和大离子亲石元素、亏损高场强元素，且具有明显的Eu正异常；斜长石微晶不相容元素含量通常高于斜长石斑晶。根据斜长石温度计计算获得斜长石斑晶结晶温度为1 050～1 253℃，斜长石微晶结晶温度为866～1 033℃。基于以上特征，推测斜长石斑晶核部是相对原始岩浆的产物，而斑晶边部以及微晶是演化岩浆的结晶产物。斜长石斑晶的成分变化及熔蚀麻点结构是由于岩浆补给及岩浆减压上升造成的。最后，本研究推测科科斯脊基底玄武岩来自于开放的岩浆房，且岩浆房内可能存在原始岩浆的不断注入及岩浆对流。

Mineral chemistry and geological significance of plagioclases hosted by basalts from the Cocos Ridge

During the Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344, tholeiitic basalt basement samples of the Cocos Ridge offshore central America were drilled in holes U1381A and U1381C at Site U1381. Studying on these samples will provide some important clues for understanding their petrogenesis. In this study we carried out in-situ major and trace elements analysis for plagioclase phenocrysts and microlites hosted by these basalt samples. The results show that plagioclases are bytownite, labradorite, with a small number of andesine. Some of the plagioclase phenocrysts show positive compositional zoning, while others only have weak compositional changes from the core to the rim of the phenocrysts. The trace element compositions of plagioclase phenocrysts and microlites are quite different. The plagioclase phenocrysts are enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements, and have obvious positive Eu anomalies. The contents of most incompatible elements of microlites are higher than those of phenocrysts. We used the classical igneous plagioclase thermometer to calculate the crystallization temperature of plagioclase phenocrysts and microlites as follows, i.e., 1 050?1 253℃ for plagioclase phenocrysts, and 866?1 033℃ for plagioclase microlites. Finally, we suggest that the core of plagioclase phenocrysts is crystallized from earlier relatively primitive magma, while the rim of the phenocrysts and microlites are crystallization products of continuous evolved magma compositions. The corrosion structure and positive compositional zoning of plagioclase phenocrysts may be mainly caused by some processes such as upwelling decompression and magma recharging. We suggest that the Cocos Ridge basaltic magma was originated from a magma chamber with continuous injection and convection of primitive magma.