蛇绿岩生成构造环境的Ba-Th-Nb-La判别图

Ba-Nb-Th-La DIAGRAMS USED TO IDENTIFY TECTONIC ENVIRONMENTS OF OPHIOLITE

Pearce的Ti-Zr-Y图不能很好区分洋脊和岛弧玄武岩,Ti对岩浆分离结晶作用很敏感,因此用这个图判别蛇绿岩生成构造环境效果不好,本文利用分配系数相近的Ba,Th,Nb,La四个非常不相容元素的(Ba/Nb)-Ba,(Nb/Th)-Nb,(La/Nb)-La和(Ba/Nb)-(Th/Nb)图解很好区分了洋脊、岛弧、洋岛玄武岩。弧后盆玄武岩同时具有洋脊和岛弧玄武岩的特征。这些元素在海水蚀变中较稳定,它们的比值在分离结晶过程中保持不变,因此这些图解有利于判别蛇绿岩(包括熔岩和均质辉长岩)生成的构造环境。

Ti-Zr-Y diagrams proposed by J. A. Pearce have widely been used to identify tectonic settings of ophiolite. However, because of considerable overlap in fields for island arc versus mid-oceanic environments and sensibility of Ti to fractional crystallization in basaltic magma, misidentification of tectonic setting is common using these geochemical diagrams. In this paper, the author proposes a group of new geochemical tecotnic discriminant using 4 very incompatible elements (Ba, Nb, Th, La) with similar partition coefficients. Island arc basalts, Mid-ocean ridge basalts and oceanic island basalts can be separated well by Ba/Nb-Ba, Nb/Th-Nb, La/Nb-La and Ba/Nb-Th/Nb diagrams. Back arc basin basalts distribute in both MORB and IAB fields of these diagrams. These diagrams are also successful to identify tectonic settings of ophiolite which include lava and isotropic babbro, because of inmobility of Nb, La, Th, Ba in sea water alteration and insensibility of their ratios to magma process.