Formation of garnet clusters during polyphase metamorphism

摘要: Pre-Alpine garnets of Variscan age from metapelitic basement units in Northern Italy were strongly retrogressed at near-surface conditions prior to Alpine contact metamorphism. The replacement by sheet silicates caused a significant volume increase during retrogression, producing pervasive fracturing. Up several hundreds angular fragments formed each crystal. Electron backscatter diffraction analysis documents maximum misorientation ~22° some as result local rotation New garnet growth is observed on the metamorphic overprinting, resulting clusters. Fragments can be identified due calcium-rich domains. Fragment orientations inherited, and only minor new nucleation occurred. These develop features typically associated with multiple models, but here they reflect events. We propose that clusters indicative events, which separated period intense retrograde alteration.