海底玄武岩中的中酸性玻璃质熔体研究  

STUDY OF THE INTERMEDIATE-ACID GLASSY MELT IN SEA-FLOOR BASALT

作　　者：陈小明[1] 赵广涛[2] 李巍然[2] 潘燕宁[1] 

机构地区：[1]南京大学金属矿床成矿机制研究国家重点实验室地球科学系,江苏南京210093 [2]青岛海洋大学海洋地球科学学院,山东青岛266003

出　　处：《地学前缘》2001年第3期216-222,共7页Earth Science Frontiers

基　　金：国家自然科学基金资助项目 ( 49972 0 2 5 ;4990 30 0 3)

摘　　要：笔者在冲绳海槽海底玄武岩基质中发现的中酸性玻璃质熔体 ,与以往在基性火山岩中发现的玻璃质熔体存在明显的差异 :( 1)在存在状态上 ,前者呈充填状态存在于细小的基质矿物之间 ,其体积明显受到岩浆冷却速率的制约 ;后者常出现富铁相和富硅相两种熔体共存现象 ,且一种熔体常呈球状分布于另一种熔体中。 ( 2 )在成分演化上 ,前者随着冷却速率的降低 ,成分向酸性方向演化 ;后者的成分与冷却速率间的关系不明显 ,一直表现为富铁相和富硅相两端员成分 ,缺失中间过渡成分。这两种类型的熔体 ,分别反映了幔源岩浆不同的演化过程 :冲绳海槽海底玄武岩中的中酸性玻璃质熔体 ,反映了幔源岩浆结晶分异演化过程 ,并记录了演化过程中各阶段产物的特征 ;而以往在基性火山岩中发现的富铁相和富硅相两种熔体的共存现象 ,反映了幔源岩浆的熔离过程 ,并记录了熔离产物的特征。The silicic rocks formed in spreading axis regions in the ocean crust (e.g. plagiogranites, rhyolites and andesites) are important for understanding the evolution of mantle-derived magmas, and for diagnosing ancient ophiolite suites and their tectonic settings. A number of models have been proposed to explain the origin of these rocks. The intermediate-acid glassy melt found in the groundmass of the sea-floor basalt in Okinawa Through provides an evidence for derivation of silicic magmas by crystal fractionation of mantle-derived magmas. It is different from the one found in the basic volcanic rocks: (1) Concerning the state of existence, the former fills in small matrix minerals, and its volume is restricted by the cooling rate of magma, but for the latter, two melts rich in Si and Fe respectively often coexist and one of them appears to be the spherical shape in the other; (2) Concerning the composition evolution, the composition of the former evolves toward the acid end with decreasing cooling rate. However, there is no obvious relation between the latter's composition and the cooling rate, and they always appear to be two end members rich in Fe and Si respectively, and lack the transition composition. These two kinds of melts reflect two different mantle magmatic evolution: the intermediate-acid glassy melt of the sea-floor basalt in Okinawa Through reflects the crystallization differentiation of the mantle magma and records the features of the products of all evolving stages. The coexistence of the two melts rich in Fe and Si in the previous studies of basic volcanic rocks reflects the magmatic immiscible process and records their product features.

关 键 词：海底玄武岩 玻璃质熔体 幔源岩浆演化 结晶分异 岩浆熔离 

分 类 号：P588.145[天文地球—岩石学]