蛇绿岩记录的大洋地幔内熔体迁移过程

Ophiolitic Records of Melt Migration Processes in Oceanic Mantle

大洋岩石圈形成演化的关键过程是熔体如何从软流圈地幔中抽取、迁移汇集并最终形成洋壳。蛇绿岩可能为揭示上述过程提供关键对象。前人研究发现,熔体在地幔中主要通过网脉状纯橄岩通道发生迁移汇聚,这种地幔通道与壳幔过渡带和洋壳内的岩浆体系共同构成大洋扩张中心的巨型熔体抽取系统。地幔深部来源的熔体在通过纯橄岩通道时,会发生复杂的熔岩相互作用,使得熔体和地幔的组成均发生变化,形成大洋岩石圈壳-幔组成解耦现象。在不同的大洋扩张中心,地幔内熔体迁移方式和过程是多样的,导致大洋岩石圈的结构和组成复杂多变。我国西藏雅江带蛇绿岩形成于多种构造体制(洋中脊、俯冲带等),其地幔内发育较为新鲜的纯橄岩通道样品,如纯橄岩、铬铁矿岩、辉石岩等。这些样品能够为深入研究地幔纯橄岩通道的成因、熔体在通道内的反应迁移过程,以及不同构造体制下大洋岩石圈壳-幔组成解耦与否的原因,提供新的窗口,同时有助于进一步认知大洋扩张中心巨型岩浆-动力系统、壳幔圈层分异和地球物质循环过程。

The key processes of the formation and evolution of oceanic lithosphere are how melts were extracted from asthenospheric mantle,then migrated and converged,and eventually formed the oceanic crust.Ophiolite may be targets for revealing these processes.Previous studies have suggested that the melts migrated and accumulated mainly along the vein-like dunite channels in the mantle.This type of channels,combined with magmatic systems within the crust-mantle transition zone and the oceanic crust,formed the giant melt plumbing system within the oceanic spreading center.When the melts derived from deep asthenospheric mantle flow through the dunite channels,complex interactions between melts and mantle would occur to have generated observable compositional variations between the migrating melts and the interacted lithospheric mantle,resulting in the compositional decoupling between oceanic lithospheric crust and mantle.Within different oceanic spreading centers,the melt migration manners and processes within the mantle are various,leading to the complexity and variation of the structure and composition of oceanic lithosphere.The Yarlung Zangbo(YZ)ophiolites in Tibet of China formed in various tectonic settings(e.g.,mid-ocean ridges,subduction zones).The relatively fresh dunite channel samples,such as dunite,chromitite and pyroxenite,have been observed within the YZ ophiolitic mantle.The studies of these samples can provide new windows for deeply investigating the genesis of mantle dunite channels,the melt reaction and migration processes within the channels and the reasons for the compositional decoupling or not between oceanic crust and mantle formed within different tectonic regimes.Studies of these samples are also helpful to better understand the giant magmatic-dynamic system under the oceanic spreading center,the differentiation of crust and mantle,and the mass cycling processes within the Earth.