中太平洋CA海山玄武岩中斜长石化学成分特征及地质意义

摘要

Chemical composition of plagioclase phenocryst zone,plagioclase crystallite and silicate of basalts in CA Seamount (sample CAD21),was examined using EPMA method and X-Ray fluorescence spectrometry (XRF) on basis of petrography.The basalts of CA Seamount are intraplate basalts formed by the activity of mantle plume.The core and the edge of zones of plagioclase phenocrysts show nonsequence optical extinction phenomenon,indicating a discontinuous zoning structure.The sequence of zones is labradorite to bytownite from edge to core,showing a normal zoning in the process of magmatic evolution.Such a phenomenon is constrained by melt composition and temperature-pressure condition.Average temperatures of plagioclase phenocrysts are 1 281 ℃ (temperature of magmatic source),1 198℃ (temperature of magmatic chamber) and 1 071 ℃ (temperature of magmatic exhalation) respectively.The small temperature difference indicates a rapidly eruption process,in accord with the formation and eruption characteristics of oceanic island basalt magma.%在对CA海山玄武岩CAD21样品岩相学研究基础上,运用电子探针和X-荧光光谱法(XRF)对中太平洋CA海山斜长石斑晶中的环带、斜长石微晶和玄武岩中的硅酸盐进行了化学成分研究.CA海山玄武岩为地幔柱成因的板内玄武岩;斜长石斑晶具有环带结构,环带核部与边部为不连续消光,是不连续环带;环带核部为培长石,边部为拉长石,是岩浆演化过程中形成的正环带,其成因受岩浆演化过程中熔体组分及温、压条件的共同制约.斜长石斑晶核部、边部及斜长石微晶估算温度平均值分别为1 281,1 198和1 071℃,分别代表了岩浆源区、岩浆房及岩浆喷发温度,三者温度差值较小,这和洋岛玄武质岩浆的形成及喷发特点相吻合.