Using Metamorphic Textures to Infer Deformation and Thermodynamic Histories

Garnet porphyroblasts are commonly used to determine P-T conditions in metamorphic rock assemblages. However, the actual process of porphyroblast nucleation is still poorly understood and garnet growth may occur at different conditions than those predicted by equilibrium thermodynamics. It is typically assumed that the Gibbs free energy of a system can be used to predict the growth of garnet at a given P and T, but here a new idea is proposed that in nature, growth does not occur until the chemical potential of each garnet-forming component departs from its equilibrium (pre-nucleation) state. Similar thermodynamic modeling results from a variety of metamorphic settings, rock types and apparent degrees of overstep indicate these deviations likely control the extent of overstep in the garnet-forming reaction in many natural samples. The process of metamorphic differentiation is then used to explain microstructures observed in the Haimanta Group, NW India that contain quartz-rich and mica-rich layers, with numerous garnet porphyroblasts appearing solely in the quartz-rich layers. This process is deformation-controlled, allowing the microstructures to be used as kinematic indicators to determine the deformation history of the Haimanta Group. The Elle Microstructural Modeling Program, the Basil deformation code and observed chemical zoning of garnets are used to prove metamorphic differentiation as the process responsible for garnet growth, meaning a singular, prograde event resulted in garnet growth in the Haimanta Group, in agreement with published monazite data for theses samples.