Microscopic analysis of garnet morphology in Santander Massif sillimanite-cordierite hornfels: Constraints on crystal nucleation and growth

Abstract

Understanding crystal growth in high-grade metamorphic rocks provides unique insights into metamorphic processes and geological evolution. This study investigates the petrographic characteristics, crystal growth mechanisms, and chemical composition of garnets from the Santander Massif of Colombia, a region known for its high-grade metamorphic rocks. Petrographic analysis reveals a discrepancy between anhedral to subhedral garnet forms on an outcrop scale and euhedral forms in hand samples, which may be due to limited or heterogeneous pressure and temperature conditions or spatial constraints at the outcrop scale, and more homogeneous conditions at the hand sample scale. Advanced techniques, including scanning electron microscopy (SEM) and atomic force microscopy (AFM), show that garnet crystals predominantly exhibit trapezohedral {211} and dodecahedral {110} faces, indicating different growth mechanisms. Electron probe microanalysis (EPMA) reveals a homogeneous chemical composition in the garnets, predominantly almandine with minor pyrope, grossular, and spessartine components. The application of cathodoluminescence (CL) and differential interference contrast (DIC) microscopy provides additional insights into mineral characteristics and growth features, contributing to a comprehensive understanding of garnet formation. These findings offer new valuable perspectives on the metamorphic conditions and processes of the rocks that form the Santander Massif.