后弧（reararc）岩浆作用的特征及其意义

The rea arc magmatism characteristics and significance

后弧岩浆作用（rear arc magmatism）是一个新的术语，国内文献大多认为与弧后（back arc）相当，也译为弧后。实际上rear arc 不同于back arc，前者仍然属于弧的范围，而后者已不属于弧结构。目前，对后弧岩浆作用的研究还十分有限， 原因一是 rear arc 出露较少，二是 rear arc 的鉴别标志不清楚。本文尝试对后弧玄武岩（rear arc basalt， RAB）作一个简单的介绍， 并采用对大量数据进行分析比较的方法与典型的岛弧玄武岩（IAB）和弧后盆地玄武岩（BAB）作一个对比。研究表明，后弧玄武岩主要由中-高 K 钙碱性和钾玄岩系列组成， 与典型的 IAB 和 BAB 相比， RAB 富集 Na2O、K2O、P2O5 ，贫CaO。后弧岩浆作用的微量元素具有典型的弧岩浆岩的特点，但LILE 及HFSE 比典型的岛弧岩浆的含量更富集，LREE 明显高于岛弧岩浆岩。 与岛弧岩浆相似，后弧岩浆同样具有明显的Nb-Ta 负异常。研究表明，上述3 类玄武岩很难区分开。但是，BAB 和RAB之间还是有一些不同的，如Sc/Nb-Ba/Y、Cu/P2O5-Y/Zr、Sc/Nb-Sr/Y 以及F2O3 /Zr-Y/Zr 等判别图。本文作者指出，后弧岩浆作用的提出完善了弧结构：一个完整的弧，从海沟向弧的方向，随着板块的俯冲作用，岩浆源区深度增加，地壳混染程度增加，依次出现前弧、弧和后弧岩浆作用， 至弧的后部，洋壳拉张，出现弧后盆地。前弧以玻安岩为代表，弧主要是IAB，后弧为碱性玄武岩，弧后则为MORB（＋IAB 的印记）。显然，后弧岩浆作用的提出，对古造山带岛弧结构的恢复、古俯冲方向的确定是有积极意义的。

Rear arc magmatism is a new term in our country and many of our documents discuss and translate it as “back arc magmatism”. Actually, rear arc is different from back arc which is no longer a part of arc structure, rear arc is still belong to arc environment. At present, the worldwide study of rear arc is very limited because it’s outcrops are less and the sign signature to identify it is still ambiguous. In this paper we try to introduce rear arc magmatism briefly and contrast the rear arc basalts（RAB）with typical island basalts（IAB）and back arc basalts. The study shows that, rear arc basalt is mainly composed of medium-high K alkaline and shoshonite series rich in Na2O, K2O, P2O5 and poor in CaO compared with typical IAB and BAB. The rear arc trace elements have the same signatures as typical island arc magma, but more enriched in LILE and HFSE, and it’s LREE is also obviously higher than the typical island arc magma. Compared with the typical island arc magma, rear arc magma has the same clear Nb-Ta negative abnormal. It’s hard to set this three basalts totally apart, but there is some differences between BAB and RAB, like Sc/Nb-Ba/Y, Cu/P2O5-Y/ZrlSc/Nb-Sr/Y, F2O3 /Zr-Y/Zr figures shows. The identify of rear arc magmatism complete the arc structure： from trench to arc direction, with the strengthen of slab subduction, increase of the depth of magma source and crust contamination, a complete arc structure should contain front arc, arc and back arc magmatism. While the back arc basin appears in the back of an arc as a result of oceanic crust extension. Meawhile, front arc mainly composed of Boninite, arc is IAB, rear arc mainly contains alkaline series and back arc usually made of MORB（＋IAB）. Clearly, the come up of rear arc magmatism has a positive significance for rebuilt the ancient orogenic belt arc structure and determine the ancient subduction direction.