Role of viscous folding in magma mixing

Abstract

Magma mixing is a complex phenomenon and occurs through the interplay of a number of processes. Recent developments in microfluidics and chaotic mixing experiments have enabled us to better understand the complex processes associated with magma mixing from the hybrid rocks of the Ghansura Rhyolite Dome (GRD) of Proterozoic Chotanagpur Granite Gneiss Complex (CGGC), Eastern India. The hybrid rocks of GRD have preserved amphibole-rich microzones (ARM) and microfolded mineral veins resulting from the mixing of felsic (rhyolitic) and mafic (basaltic) melts. Based on textural features and mineral chemical data interpretations, we infer that viscous folding can be an important mechanism to obtain chemical equilibrium in a magma mixing scenario. Magma mixing is a chaotic process brought about by the interplay of advection (i.e. stretching and folding) and diffusion. In our scenario, stretching and folding of the mineral veins or filaments were achieved through viscous folding, followed by chemical diffusion between the contrasting magmas to enhance magma mixing. Our findings reveal that when mafic magma carrying phenocrysts of augite came in contact with the felsic magma, there was diffusion of heat from the hotter mafic phase to the colder felsic phase followed by diffusion of elemental components. Diffusion of elements like H+, Al3+ and other cations from the felsic phase to the mafic phase converted clinopyroxene phenocrysts in the mafic system into aggregates of small amphibole crystals. The formation of abundant amphibole microcrysts greatly increased the viscosity of the mafic system, allowing the amphibole-rich magma to venture into the surrounding felsic melt as veins and undergo viscous folding to facilitate mixing.