Competing roles of evolving P-T conditions, equilibration volume and chemical potential landscape in the formation of corona texture: a case study from the Southern Granulite Terrane, India

Corona texture is defined by the development of partial or complete rim of one or more minerals around a central reactant mineral depicting limited mass transport (local equilibration) in the length scale of the coronitic layers. The mineral assemblages forming corona texture in a metamorphic rock are commonly used to trace the P-T-X conditions through which the rock evolved during various tectonic processes. However, without a proper assessment of the changes in the equilibration volume (EV) and its effect on the mineralogy, any petrological interpretation deduced from the coronitic texture may be incorrect. In this study, we demonstrate that the double corona texture, observed in a suite of Mg-Al rich ortho-amphibole cordierite-bearing rock from the Cauvery Shear Systemin (Southern Granulite Terrane, India), developed in response to the continuously evolving EV. The studied rock contains aluminosilicate porphyroblasts that are set in a matrix of ortho-amphibole ± quartz. The aluminosilicate porphyroblasts are rimmed successively by an inner symplectic corona of sapphirine + cordierite, and an outer mono-mineralic corona of cordierite (near ortho-amphibole). Locally, patches of corundum with a rind of cordierite grow preferentially along the interface of aluminosilicate and the inner symplectic corona. Based on detailed petrography and mineral composition analyses, the corona textures are interpreted to have formed through a sequence of different chemical reactions that occurred in local micro-domains. We calculated quantitative P-T pseudosection in a NCFMASHT (Na2O-CaO-FeO-MgO-Al2O3-SiO2- H2O-TiO2) system and activity-adjusted P-T petrogenetic grid in a MASH (MgO-Al2O3-SiO2- H2O) system which, together, suggest that the coronitic assemblages were formed in response to a steeply decompressive retrograde P-T path from >8.8 kbar to <6 kbar, at a nearly constant temperature of ~700°C. Changes in EV in response to the limited transport of chemical components during the formation of corona texture were investigated through isothermal P-μMgO, P-μSiO2, and P-μMgO-μSiO2 MASH diagrams. Our results quantitatively model the continuously changing chemical potential landscape (P-μMgO- μSiO2 evolution path) around the central aluminosilicate porphyroblast within the corona-bearing micro-domain. The path demonstrates that a gradually shrinking EV around the central aluminosilicate during retrogression led to the sequential change of mineral reactions and equilibrium mineral assemblages, and resulted in the formation of multiple coronae. Unavailability of fluids and/or rapid exhumation is considered as the most dominant factors responsible for the decreasing elemental mobility and the consequent shrinking in EV in the studied rock.