Does continental crust transform during eclogite facies metamorphism?

Eclogites within exhumed continental collision zones indicate regional burial to depths of at least 60km, and often more than 100km in the coesite-stable, ultra-high pressure (UHP) eclogite facies. Garnet, omphacitic pyroxene, high-Si mica, kyanite +/- coesite should grow at the expense of low-P minerals in most felsic compositions, if equilibrium obtained at these conditions. The quartzofeldspathic rocks that comprise the bulk of eclogite facies terranes, however, contain mainly amphibolite facies, plagioclase-bearing assemblages. To what extent these lower-P minerals persisted metastably during (U)HP metamorphism, or whether they grew afterwards, reflects closely upon crustal parameters such as density, strength and seismic character. The Nordfjord area in western Norway offers a detailed view into a large crustal section that was subducted into the eclogite facies. The degree of transformation in typical pelite, paragneiss, granitic and granodioritic gneiss was assessed by modelling the equilibrium assemblage, comparing it with existing parageneses in these rocks and using U/Th-Pb zircon geochronology from laser ablation ICPMS to establish the history of mineral growth. U-Pb dates define a period of zircon recrystallization and new growth accompanying burial and metamorphism lasting from 430 to 400Ma. Eclogite facies mafic rock (similar to 2vol.% of crust) is the most transformed composition and records the ambient peak conditions. Rare garnet-bearing pelitic rocks (<10vol.% of crust) preserve a mostly prograde mineral evolution to near-peak conditions; REE concentrations in zircon indicate that garnet was present after 425Ma and feldspar broke down after 410Ma. Felsic gneiss - by far the most abundant rock type - is dominated by quartz + biotite + feldspar, but minor zoisite/epidote, phengitic white mica, garnet and rutile point to a prograde HP overprint. Relict textures indicate that much of the microstructural framework of plagioclase, K-feldspar, and perhaps biotite, persisted through at least 25Ma of burial, and ultimately UHP metamorphism. The signature reaction of the eclogite facies in felsic rocks - jadeite/omphacite growth from plagioclase - cannot be deduced from the presence of pyroxene or its breakdown products. We conclude that prograde dehydration in orthogneiss leads to fluid absent conditions, impeding equilibration beyond similar to high-P amphibolite facies.