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

Abstract

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 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).