Experimental studies on the formation of inclusions in plagioclases from metatonalites, Hohe Tauern, Austria (lower temperature stability limit of the paragenesis anorthite plus potash feldspar)

Ackermand, D.; Franz, Karl

doi:10.1007/bf00397373

Cited by 17 publications

(1 citation statement)

References 8 publications

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“…A late Proterozoic–early Palaeozoic sedimentary origin precludes inheritance of earlier phases or textures, and therefore in GDS and other calcic metasedimentary rocks, the assemblage of clinozoisite‐epidote, sodic plagioclase, rutile, phengitic muscovite ± garnet reflects Scandian metamorphism. Studies on similar compositions (Ackermand & Karl, ; Hörmann & Raith, ; Le Goff & Ballévre, ; Rotzler et al ., ; Carswell et al ., ) have shown that such an assemblage indicates at least high‐ P amphibolite facies (400–600 °C, >1.6 GPa), produced from consumption of prograde greenschist facies minerals such as biotite, plagioclase, K‐feldspar and titanite.…”

Section: Discussionmentioning

confidence: 99%

Does continental crust transform during eclogite facies metamorphism?

Young

Kylander‐Clark

2015

Journal Metamorphic Geology

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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.

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