Granite emplacement mechanisms and tectonic controls: inferences from deformation studies

ABSTRACT This paper is a structural and tectonic approach to the emplacement and deformation of granitoids. The main methods available in structural geology are briefly reviewed and this emphasises that (a) a wealth of data, particularly strain and shear sense, which pertain to these problems, can be determined in and around plutons; (b) given the nature, unlike many other crustal rock types, of granites to crystallise from isotropic through weakly anisotropic crystal suspension fluids, that deformation which has occurred in these states may not be well preserved; and (c) it is entirely possible, using this methodology, to separate deformation resulting from externally originating tectonic stresses from that which is associated with internal magma-related stresses. It is also recommended that the genetically-based Cloosian classification of granite fabrics and structures into “primary” (magmatic flow/magmatic flow current) and “secondary”, be abandoned and that a more observationally-based approach which classifies granite deformation fabrics and structures according to their time of occurrence relative to the crystallisation state of the congealing magma, be adopted (i.e. pre-full crystallisation deformation and crystal plastic strain deformation). Examples of recent, structurally based, studies of emplacement mechanisms of plutons within tectonic settings are described and these show that, in general, space for magma can be created by the combination of tectonically-created cavities and internal magma-related buoyancy. This occurs in both transcurrent and extensional tectonic settings and there is no reason to doubt that it can happen in compressive-contractional regimes. It is concluded that transient and permanent space creation, such as may be exploited by available magmas, is a typical feature of the tectonically stressed and deforming lithosphere and this, in combination with the natural buoyancy and ascending tendency of magmas, can generate the varied emplacement mechanisms of granites.