西南印度洋中脊63.9°E斜长石超斑状玄武岩对超慢速扩张洋脊岩浆过程的指示

Magmatic process beneath the ultra-slow spreading ridge: Petrological studies of a Plagioclase Ultra-phyric Basalt( PUB) from 63. 9° E,Southwest Indian Ridge

人们对超慢速扩张洋中脊深部岩浆过程的了解至今仍十分模糊。我们对西南印度洋洋中脊(Southwest Indian Ridge,SWIR) 63.9°E处采集到的斜长石超斑状玄武岩(Plagioclase Ultra-Phyric Basalt,PUB)进行了岩石学和地球化学研究。样品具有以下几个特征:斜长石斑晶的体积分数高达~25%,而橄榄石斑晶的体积分数约1%;尽管该样品中玻璃的成分与同一洋脊段玄武岩的成分基本一致,但高Fo橄榄石斑晶与玻璃基质的成分不平衡;不同类型的斜长石晶体之间存在成分差异,单个斜长石大斑晶中的An值也呈现出与正常的结晶分异过程不符的环带;斜长石斑晶中发育溶蚀、筛状等不平衡结构。因此,我们认为,斜长石超斑状玄武岩经历了多期次熔体的作用,是由通过密度分选聚集在岩浆房顶部的斜长石斑晶被之后的火山喷发带出海底形成。尽管斜长石超斑状玄武岩与同一洋脊段的非斑状玄武岩之间并不存在母熔体成分上的差别,但超斑状玄武岩的出现进一步反映了超慢速扩张洋壳岩浆活动的多样性。

geochemistry of a Plagioclase Ultra-phyric Basalt( PUB) sample,which is collected at 63. 9° E of the Southwest Indian Ridge( SWIR). This sample contains a large volume of plagioclase phenocrysts( up to ~ 25% modal fraction),but only ~ 1% modal fraction of olivine phenocrysts. Major element composition of glass matrix in the sample is similar to that of other aphyric basalts from the same ridge segment. The olivine phenocrysts are not equilibrated with the glass matrix,and compositions of plagioclase phenocrysts vary largely. One plagioclase phenocryst shows compositional zonation,which is not consistent with simple fractional differentiation of MORB from a single magma chamber. Thus we explain its formation as the mixing and interaction between early formed plagioclase crystals and subsequent multiple-stage melts. At the same time,numerous plagioclase phenocrysts exhibit disequilibrium textures( e. g.resorption texture and sieve texture),which also indicates that they have undergone different magmatic stages. Again,we emphasize that PUBs are the results of accumulation of plagioclase floating at the top of magma chamber and they have been affected by multiple-stage melts before final eruption. The occurrence of PUB here reflects the diversity of magmatic process beneath the ultra-slow spreading SWIR.