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 $$ . contain crystallographically bound H₂O in biotite. During high-pressure overprint, partial biotite breakdown causes a localized increase in the chemical potential of H₂O (μH₂O). Transport 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 pressure-temperature evolution, similar textures may involve albite instead of jadeite, consistent with the general absence of Na-clinopyroxene in high-pressure metagranitoids and kindred gneisses. 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, the addition of biotite-derived H₂O into plagioclase sites likely increases reaction rates. 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 that facilitate further hydration and/or deformation along the prograde path, thereby obliterating the textures. 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 favouring 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 and the Western Gneiss Region (Norway).