冲绳海槽黑色与白色浮岩特征及其对岩浆演化的指示

Petrological characteristics of black and white pumice from the Okinawa Trough: Implications for magmatic evolution

为了认识黑色与白色浮岩的岩石特征及成因,使用人工重砂、元素地球化学等分析技术,研究冲绳海槽两类浮岩中元素及矿物组成特征。研究显示,冲绳海槽黑色和白色浮岩具有相似全岩化学组成,都落入流纹岩区,稀土配分曲线明显呈右倾,具负Eu异常,富集不相容元素,岩石物理性质差别是导致其颜色差异的主要因素。两类浮岩岩石学特征的不同主要体现在:(1)白色浮岩锆石和辉石都具有两种类型,黑色浮岩仅具有其中一种;(2)黑色浮岩中发育钛铁矿、钛磁铁矿与基质构成的珠状构造,且基质玻璃中密集分布磁铁矿雏晶,而白色浮岩不具备上述岩石学特征。综合分析海槽两类浮岩岩石学特征所蕴含的岩浆演化信息,推测两类浮岩具有相同的玄武质岩浆源区,富挥发组分的玄武质母岩浆上升进入地壳,形成初级岩浆房,房内结晶分异后残留的酸性岩浆,沿海槽构造薄弱带向上运移,岩浆运移过程中有少部分进入分支断裂系统形成次级岩浆房,期间初级岩浆房接受多次基性岩浆注入;初级和次级岩浆房中的酸性岩浆分别独立喷出海底,前者形成白色浮岩,后者形成黑色浮岩。此外,演化过程中岩浆性质逐渐由氧化性向还原性转变。

Pumice is the most widely distributed volcanic rock in the Okinawa Trough and includes black and white pumice. To better understand the black and white rock characteristics and genesis of this pumice, we employed arti- ficial sand and element geochemistry analysis techniques to investigate the two types of pumice elements and min- eral composition characteristics. Specifically, we used artificial panning and element geochemical data to determine the characteristics and genesis of the black and white pumice. The element compositions of these two pumice species are nearly uniform and belong to the rhyolite category. On the basis of the chondrite-normalized rare earth element （REE） abundances, we observed light REE enrichment in all samples with negative abnormal Eu. All samples were rich in incompatible elements. The physical features of the pumice led to their color differences. The petrology of the pumices displayed only minor variances, such as （1） white pumice contained two species of zircon and pyroxene and black pumice only zircon and pyroxene; （2） bead-shaped structures were only distributed in the black pumice wherein titanium magnetite and ilmenite surrounded by glass were distributed. Based on the above petrological characteristics of the pumice, we could depict the evolution process of the plumbing system. The magma plumbing systems producing the white and black pumice share the same mantle source that generated parental basaltic magma. Primary and deeper magma chambers that formed the white pumice developed while the parental magma was plumbed into the crust. The shallow magma chamber that produced the black pumice was connected to a branch of the magma conduit of a deeper chamber. During the evolution of these two plumbing systems, magma was repeatedly added to the chambers. In addition, the redox feature of the magma gradually transformed from oxidative to reductive during the evolutionary process.