Formation of multistage garnet grains by fragmentation and overgrowth constrained by microstructural and microchemical mapping

Garnet is an exceptionally useful mineral for reconstructing the evolution of metamorphic rocks that have experienced multiple tectonic or thermal events. Understanding how garnet crystallizes and its mechanical behaviour, is important for establishing a petrological and temporal record of metamorphism and deformation, and to recognize multiple geologic stages within the growth history of an individual crystal. In this study, we integrate fine-scale microstructural (EBSD) and microchemical (LA-ICP-MS mapping) data obtained on a polycyclic garnet-bearing micaschist from the Alpine belt. Results suggest that fragmentation of pre-Alpine garnet porphyroblasts occurred during the late pre-Alpine exhumation and/or the onset of the Alpine burial, such that the older pre-Alpine garnet fragments were transported/redistributed during Alpine deformation and acted as new nucleation sites for Alpine garnet growth. These processes produced a bimodal garnet size distribution (macro mm-sized and micro sub-mm-sized grains). Thermodynamic modelling indicate that Alpine garnet grew during the final stage of burial (from 1.9 GPa 480 °C to 2.0 GPa 520 °C) and early exhumation (down to 1.6 GPa 540 °C) forming continuous idioblastic rims on macro- and micro-grains, and sealing fractures preserved in pre-Alpine garnet porphyroblasts. We propose that fragmentation-overgrowth processes coupled with ductile deformation in polycyclic rocks may produce a bimodal garnet size distribution and form multistage crystals resembling neoblasts. This study highlights the importance of linking microstructural (EBSD) and microchemical (LA-ICP-MS mapping) data by providing valuable information about the dominant deformation mechanisms at a given site by identifying potential links between major/trace element mobility and crystal deformation.