摘要
本文根据冲绳海槽的地下结构、地温场特征、以及浮岩中斑晶矿物的结晶温度和榴辉岩的熔融相关系等，综合分析了冲绳海槽酸性浮岩岩浆的活动模式。提出：在冲绳海槽中部和东部地下１５ｋｍ以下的地幔物质部分熔融，产生橄榄拉斑玄武质岩浆，而在海槽中部地壳下结构层中（７—１５ｋｍ）物质部分熔融产生少量安山质岩浆。因此，在冲绳海槽地下存在有双层岩浆房结构，并且二者之间在中部有岩浆通道。来自地幔的拉斑玄武质岩浆在下部地壳中一方面受到地壳物质的混染，另一方面早期结晶的基性矿物会发生部分熔蚀。与此同时，还会有新的结晶作用。在岩浆中将同时存在有三种结晶相：一是自地幔中带入的地幔物质结晶相，其８７Ｓｒ／８６Ｓｒ比值低（０．７０４左右）；二是地壳物质熔融残余物或由地壳岩浆中结晶的矿物，其物源为地壳，８７Ｓｒ／８６Ｓｒ比值高（０．７０９左右）；还有一种矿物是地幔岩浆受地壳物质混染后的产物，其８７Ｓｒ／８６Ｓｒ比值中等。岩浆在一个较长时间稳定的环境中（应该对应于冲绳海槽的扩张间隙）发生充分的结晶分异作用而变成富含ＳｉＯ２的酸性岩浆（浮岩岩浆）。
Based on the subbottom structure,geotemperature field characters,crystalline temperature of phenocryst in pumice and phase diagram for eclogite melting,the primary analysis on acidic pumice magmatism suggested that the partial melting of the mantle at the depth of about 15km results in olivine tholeiitic magma at the east and middle of the trough,and the partial melting of the lower crust about 7～15km deep generates andesitic magma at the middle trough.Therefore,there is a two layer magma chamber structure under the middle Okinawa Trough and the magma pathway between the two chambers exists at the middle trough.This model can fairly interpret the fact that the phenocrysts of the pumice are composed of three generational minerals:basic minerals with lower 87 Sr / 86 Sr ratio of about 0 704 crystallized in the mantle,acidic minerals with higher 87 Sr / 86 Sr ratio of about 0 709 generated from crust derived magma and the minerals with medium 87 Sr / 86 Sr value crystallized from the mantle derived magma contaminated by the crust materials.
出处
《海洋地质与第四纪地质》
CAS
CSCD
北大核心
1997年第1期59-66,共8页
Marine Geology & Quaternary Geology
基金
国家自然科学基金
