Ba-Nb-Th-La DIAGRAMS USED TO IDENTIFY TECTONIC ENVIRONMENTS OF OPHIOLITE
蛇绿岩生成构造环境的Ba-Th-Nb-La判别图

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.