Crystallization Dynamics of Granite Magma Chambers in the Absence of Regional Stress: Multiphysics Modeling with Natural Examples

Abstract

Numerical models built linking an internally consistent rheological dataset for a cooling granite magma with equations of heat transfer and fluid motion for geometrically different magma chambers cooling at various crustal depths reveal that granite magmas first undergo a short period of chaotic convection, during which wall-rock contamination and magma mixing are possible, followed by a long period of no convective cooling, during which melt segregation occurs. Convection is driven by the negative density gradient generated in the upper cooling zone by melt-to-solid phase transformation. Convection breaks the upper mushy zone and drags the fragments downwards with descending Rayleigh^Taylor fingers. Such fragments can be preserved as microgranular enclaves. The descending Rayleigh^Taylor fingers split low aspect-ratio (sill-like) magma chambers into nearly isolated convection cells. If the magma is initially heterogeneous, this effect divides the chamber into contiguous homogeneous zones with distinct trace element and isotope ratios, and finally results in a pluton with marked lateral compositional variations, easily misinterpreted as different intrusive batches. Convective heat-loss quickly leads most of the magma chamber to critical crystallinity, independently of the vertical coordinate, so that a chamber-wide three-dimensional skeleton of crystals with uniform initial porosity c .0 ·4^0·5 is formed. This configuration is gravitationally unstable; therefore, it spontaneously compacts towards an equilibrium vertical variation of porosity approaching Atty’s Law. In the absence of regional stress, the upwards migration of the inter-crystalline melts, as a result of compaction, is the most effective way of melt^solid segregation and causes vertically zoned plutons with an upper layer of felsic segregates. Granite magma chambers fractionated by these mechanisms will produce short-range differentiation series, from a composition slightly less silicic than the initial magma to high-silica segregates. In the presence of regional stress, tectonic squeezing and shearing during the post-convective stage can expel residual fluid more efficiently and lead to wide-range granite differentiation series, from rocks notably less silicic than the initial magma to high-silica leucogranites.