ε_（Nd）－La／Nb、Ba／Nb、Nb／Th图对地幔不均一性研究的意义──岛弧火山岩分类及EMII端元的分解

IMPLICATIONS OF ε_(Nd)-La/Nb, Ba/Nb, Nb/Th DIAGRAMS TO MANTLE HETEROGENEIY──CLASSIFICATION OF ISLAND ARC BASALTS AND DECOMPOSITION OF EMII COMPONENT

本文提出一组εＮｄ－Ｌａ／Ｎｂ、εＮｄ－Ｂａ／Ｎｂ和εＮｄ－Ｎｂ／Ｔｈ图。在这些图中岛弧火山岩呈三角形分布，从而将其源区的三个主要端元（ＭＯＲＢ型亏损地幔、俯冲洋壳析出流体、陆源沉积物）同时显示在一张图上。据此，岛弧火山岩可根据其源区特征划分成两类：二端元型（无陆源沉积物）和三端元型。此外，ＥＭＩＩ型幔源岩石在这些图中也被分成两类：与岛弧火山岩类似的，具有高Ｌａ／Ｎｂ、Ｂａ／Ｎｂ和低Ｎｂ／Ｔｈ的大陆溢流玄武岩和某些橄榄岩包体；具有低Ｌａ／Ｎｂ、Ｂａ／Ｎｂ和高Ｎｂ／Ｔｈ的萨莫德型海岛玄武岩。它们对应的ＥＭＩＩ富集地幔端元也可分为两种：ＥＭＩＩＭ，由俯冲洋壳析出流体交代楔型地幔而成；ＥＭＩＩＳＲ，与陆源沉积物再循环进入对流上地幔有关。

A group of εNa-La/Nb,Ba/Nb, Nb/Th diagrams is used to study mantle heterogeneity.Island arc basalts (IAB) are distributed in a triangle of these diagrams.Three end-member components(the MORB type depleted mantle, the fluid released from subducted oceanic crust and the sediment from the continental crust)of the source of IAB may be displayed in these same diagrams. Two types of IAB are identified. They are the two-component type(with little continental sediments),such as the basalts from Aleutians and New Britain,and the three-component type,such as those from Sunda, Lesser Antilles and Andes.In addition, the EMII type mantlederived rocks may also be divided into two groups.One is exemplified by continental flood basalts and some peridotite xenolithes,similar to IAB, with high La/Nb and Ba/Nb,and low Nb/Th ratios. The other includes Samoa type oceanic island basalts,with low La/Nb and Ba/Nb, and high Nb/Th ratios.The corresponding two sub-components of EMII component are EMIIM, which is related to the metasomatismof lithosphere mantle by fluids released from subducted oceanic crust,and EMIISR, related to the intervention of recycling continental sediments into the convective mantle.