全球岛弧玄武岩数据挖掘——在玄武岩判别图上的表现及初步解释

Global IAB data excavation: The performance in basalt discrimination diagrams and preliminary interpretation

MORB(洋中脊玄武岩)、OIB(洋岛玄武岩)和IAB(岛弧玄武岩)是学术界最关心的3种玄武岩类型,其中尤以与板块消减作用有关的岛弧岩浆活动备受关注。岛弧可分为洋内岛弧和大陆边缘岛弧(活动陆缘弧)2类。对IAB进行讨论,重点探讨IAB的识别。IAT(岛弧拉斑玄武岩)和IAB是前弧、岛弧和后弧岩浆作用的产物,其中,后弧组分更具多样性,它不同于弧后玄武岩,前者属于弧的范围,而后者形成的动力学过程与俯冲系统有关,但其是独立的构造单元,尽管其岩浆作用可能仍受到俯冲流体的影响。前人对IAB进行了大量研究,提出了多种构造环境判别图解,并得到广泛应用。尝试应用全球玄武岩数据来验证上述判别图的可信度,研究发现,可信度高的判别图不多,且大多与Th、Ta(Nb)和Ti元素有关的,如Hf-Th-Ta(Nb)、TiZr-Sr和Th/Yb-Ta/Yb图,其余判别图的判别效果可信度低且具多解性,建议谨慎使用。IAB与MORB和OIB的区别主要体现在Nb-Ta亏损的特征上,是否受到俯冲流体的影响是区分IAB与MORB和OIB最重要的标志。

MORB, OIB and IAB (arc calc- alkaline basalt) have aroused much interest among geologists, with particular attention paid to igneous activities in island arcs related to plate subduction. Such island arcs can be divided into island arc and continental margin arc (active epicontinental arc). This paper discusses the IAB, mainly focusing on the identification of IAB. The IAT (island arc tholeiite) and the IAB are products of the fore-arc, the island arc and the rear - arc magmatism. Among them, the rear-arc is more diversified in composition and is different from back-arc (back arc): the former belongs to the scope of the arc, while the latter is related to the subduction system in the kinetics of formation; nevertheless, the back-arc is an independent tectonic unit, although its magmatism might still be affected by the subduction metasomatic fluids. Previous researchers made detailed studies of the IAB and put forward a variety of tectonic environment discrimination diagrams which have been widely used. In this paper, the authors tried to apply the global basalt data to verify the credibility of the discriminant figures. However, there only exist very few highly credible discrimination diagrams, and these figures are mostly related to Th, Ta (Nb), and Ti elements, such as the figures of Hf -Th-Ta (Nb), Ti-Zr-Sr and Th/Yb-Ta/Yb, whereas the rest of the discriminant figures are of low credibility and characterized by multiple solutions,and hence should be used prudently. Researches show that the difference between the IAB and MORB, OIB mainly finds expression in the depletion of Nb- Ta , and this suggests that the most important criterion to distinguish the IAB from MORB and OIB is whether they are affected by subduction fluids or not.