Rheology of Granitic Magmas during ascent and emplacement

Considerable progress has been made over the last decade in understanding the static rheological properties of granitic magmas in the continental crust. The effect of volatiles (H₂O and CO₂) on the interstitial melt phase as consolidation proceeds are identified as important compositional factors governing the rheodynamic behavior of the mixture, along with the oxidation state. Although the strengths of granitic magmas over the crystallisation interval are still poorly constrained, theoretical investigations suggest that during magma ascent, yield strengths of the order of 8 kPa are required to retard completely the upwards flow in metre-wide conduits. In low Bagnold number magma suspensions with moderate crystal contents (solidosities 0.1 ≤φ ≤ 0.3), viscous fluctuations may lead to flow differentiation by shear-enhanced diffusion. AMS and microstructural studies support the idea that granite plutons are intruded as crystal-poor liquids (≤φ),with fabric and foliation development restricted to the final stages of emplacement. If so, then these fabrics contain no information on the ascent (vertical transport) history of the magma. Deformation of a magmatic mush during pluton emplacement can enhance significantly the pressure gradient in the melt, resulting in a range of local macroscopic flow structures including layering, crystal alignment and other mechanical instabilities such as shear zones. As the suspension viscosity will vary with stress rate, it is not clear how the timing of proposed rheological transitions formulated from simple equations for static magma suspensions, apply to mixtures undergoing shear. New theories of magmas as multiphase flows are required if the full complexity of granitic magma rheology is to be resolved.