Are discrimination diagrams always indicative of correct tectonic settings of granites? Some crucial questions on granite study

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: ( I) granites in oceanic crust and oceanic margin which are derived from basaltic magmas (MORB, IAT and OIB) with apparent mantle contribution ( high positive epsilon(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.