摘要
花岗岩与大地构造环境之间的关系是花岗岩研究的热门话题,许多人认为,利用地球化学标志可以判别花岗岩形成的大地构造环境。勿庸置疑,花岗岩构造环境判别方法是仿效玄武岩提出来的。因此,本文从回顾玄武岩构造环境判别开始,详细剖析了Pearce et al(1984b)和Barbalin(1999)关于花岗岩构造环境判别的研究成果,指出了花岗岩构造环境判别中存在的问题。我们认为,花岗岩地球化学性质主要反映的是花岗岩源区的性质和构造环境,而非花岗岩形成时的构造环境。本文按照全球花岗岩的分布将花岗岩分为产于大洋及其边缘(海岸)的、产于板块边缘和陆内与碰撞有关的和产于陆块内部的三类花岗岩。(1)产于大洋及其边缘(海岸)的花岗岩源于洋壳类型的玄武岩(MORB、IAT、OIB等),花岗岩具明显的地幔印记ε_(Nd)(t)同位素比值高,Sr同位素比值低),大体可以用现有的判别图判别其形成的构造环境。(2)与碰撞作用有关的花岗岩大多分布在陆块边缘,同碰撞和后碰撞指的是构造(变形)事件,与板块构造环境(洋脊、岛弧、洋岛、裂谷等)在概念上是不同的。区分同碰撞和碰撞后花岗岩不能单靠花岗岩的地球化学标志,也不能单靠花岗岩构造判别图,而应当从岩石组合和岩石性质两方面入手:碰撞有利于形成埃达克岩和(具低Sr低Yb特征的)淡色花岗岩;碰撞后的伸展背景有利于形成非常低Sr高Yb的A型花岗岩。(3)产于陆块内部的花岗岩其形成主要与地幔来源的热有关,花岗岩的地球化学性质主要决定于源岩及形成时的深度,与地表浅层构造作用和事件无关。研究表明,地球上只有大约10%的花岗岩可以探讨其形成的构造环境,20%左右的花岗岩需要研究它们与构造事件的关系(同碰撞或后碰撞),而约70%的产于陆壳上的花岗岩,既无从考虑其形成的构造环境,也无需研究其与构造事件的关系。因此,花岗岩构造环境判别不具有普遍的意义。文中指出,花岗岩构造环境判别方法既存在理论上的不足和概念上的混淆,也存在操作上的困难,需要重新思考。
It is commonly accepted that the geochemistry of granitic rocks can be used to discriminate the tectonic settings when they formed.However,more and more evidence shows that the tectonic settings of some granitic intrusions cannot be well constrained if only based on their geochemical characteristics.Basically the discrimination diagrams for the tectonic settings of granites were created on the same theory as those for basalts.This study reviews the origin of the discrimination diagrams of basalts and the work of Pearce et al. (1984b)and barbalin(1999)on the discrimination diagrams of granites,and suggests that the geochemistry of granites is actually related to the nature and tectonic setting of their parental magmas rather than the granites themselves.On the basis of distribution of granites around the world,there are three types of granites:(1)granites in oceanic crust and oceanic margin which are derived from basaltic magmas(MORB,IAT and OIB)with apparent mantle contribution(high positiveε_(Nd)(t)values and low initial Sr ratios); (2)collision-related granites in the continental margin including syn-collisional and post-collisional granites.They both are related to tectonic(deformation)events in the shallower depth,rather than tectonic setting.The difference between them is not only the geochemistry of rocks but also the assemblages of rocks.For example,adakites and low-Sr and-Yb leucogranites formed during collision whereas low-Sr and high-Yb granites and typical A-type granites with very low Sr and High Yb formed during the extension period after the collision and associated with within-plate basalts;(3)granites within the continent crust are usually generated by crustal anaxesis induced by the heat from the mantle such that the geochemistry of the granites is closely dependant on the composition of source rocks and the depth where the source rocks melted,again not related to the tectonic setting when granites formed.It is shown in this study that the proportion of above three types of granites in the present world is approximately～10%,～20% and～70%, respectively.Therefore,about 70% granites that occurred within the continental crust are not necessary to discriminate the tectonic setting when they formed.The discrimination diagrams for the tectonic settings of granites are only suitable to the granites originally formed in the oceanic crust.It is probably a misleading idea to discriminate the tectonic setting for the granites in the continental crust.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2007年第11期2683-2698,共16页
Acta Petrologica Sinica
基金
国家重点基础研究发展规划项目(2002CB412601)资助的项目。
关键词
花岗岩
玄武岩
构造环境
同碰撞
碰撞后
Granite
Basalt
Tectonic setting
Syn-collision
Post-collision
