2015/01/06 by D. J. Young, Andrew Kylander‐Clark, A. R. C. Kylander‐Clark · 76 citations
Computer Science · Earth and Planetary Sciences · #Coesite #Continental crust #Crust #Eclogite #Facies #Geochemistry #Geochemistry and Geologic Mapping #Geological and Geochemical Analysis #Geology #Geomorphology #Gneiss #Granulite #High-pressure geophysics and materials #Kyanite #Metamorphic facies #Metamorphic rock #Metamorphism #Paleontology #Petrology #Subduction #Tectonics #Terrane #Zircon
paper · doi:10.1111/jmg.12123
published in Journal of Metamorphic Geology 33(4), 331-357 (Wiley)
openalex publication_date 2015/01/06 · crossref created 2015/01/06 · crossref issued 2015/02/01 · crossref published 2015/02/01 · crossref published-online 2015/02/01 · crossref published-print 2015/05/01 · crossref deposited 2023/10/02 · openalex created_date 2025/10/10 · crossref indexed 2026/08/04 · openalex updated_date 2026/08/05
Abstract Eclogites within exhumed continental collision zones indicate regional burial to depths of at least 60 km, and often more than 100 km 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 400 Ma. Eclogite facies mafic rock (~2 vol.% of crust) is the most transformed composition and records the ambient peak conditions. Rare garnet‐bearing pelitic rocks (<10 vol.% of crust) preserve a mostly prograde mineral evolution to near‐peak conditions; REE concentrations in zircon indicate that garnet was present after 425 Ma and feldspar broke down after 410 Ma. 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 25 Ma 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 ~high‐ P amphibolite facies.