Self-induced incipient `eclogitization' of metagranitoids at closed-system conditions

The incipient development of diagnostic high-pressure assemblages –the `eclogitization'– of granitoids, such as plagioclase-breakdown and small-scale formation of garnet and phengite does not require exogenous hydration because unlike dry protoliths like basalt/gabbro or granulite, granitoids s.l. contain crystallographically-bound H₂O in biotite. During high-pressure overprint, partial biotite dehydration-breakdown causes a localized increase in the chemical potential of H₂O (µH₂O). Diffusion of H₂O into nearby plagioclase induces the formation of diagnostic eclogite-facies assemblages of jadeite–zoisite–K-feldspar–quartz ± kyanite ± phengite that pervasively replace former cm-sized plagioclase without requiring the participation of free H₂O. Depending on P–T evolution, similar textures may involve albite instead of jadeite. Plagioclase-breakdown may also occur due to simple burial because compression leads to an increase of µH₂O, without requiring additional influx of H₂O at the texture scale. However, diffusion of biotite-derived H₂O into plagioclase sites likely favors reaction due to its catalytic effect. In parallel, ~100 µm-thick complementary coronae involving garnet phengite–quartz develop at former biotite–plagioclase/K-feldspar interfaces due to the coupled diffusion of FeO–MgO–H₂O from biotite towards feldspars, and minor CaO in the opposite direction. The reaction textures likely create structural weaknesses and preferential fluid pathways, thereby promoting further hydration, deformation and equilibration along the prograde path. If exogenous H₂O is introduced, it is accommodated in phengite growing at the expense of igneous K-feldspar and possibly in epidote-group minerals. Upon decompression, such hydrated rocks would dehydrate, thereby favoring fluid-assisted retrogression and loss of diagnostic eclogite-facies assemblages at lower pressure. Whereas the prograde reaction textures are only preserved at closed-system conditions and in the absence of deformation, they are suggested to commonly form during orogenic metamorphism of granitoids and quartzofeldspathic gneisses that dominate the continental crust in high-pressure terranes such as the Western Italian Alps or the Western Gneiss Region (Norway).