High‐temperature, decompressional equilibration of the eclogite facies orogenic root (Western Gneiss Region, Norway)

Abstract

AbstractGarnet–clinopyroxene mafic rocks have been investigated in the outer coastal area of the northwestern‐most part of the Western Gneiss Region (WGR), South Norway. The garnet–clinopyroxene rocks occur as lenses with amphibolitized and deformed margins ranging in size from 1 m2 up to 2–3 km2. They are regionally widespread and included in migmatitic gneisses, mica schists and amphibolites. The mafic lenses vary from fine‐ to coarse‐grained with a strain variation from massive to coaxial S > L tectonite fabric. Garnet is Alm42‐53Prp17‐35Grs20‐33Sps0‐3. Clinopyroxene is a Na–Al diopside (En34‐43Fs8‐17Wo48‐52) with Al up to 0.50 a.p.f.u. and Na content up to Jd24. Garnet and clinopyroxene occur in an assemblage with edenitic‐pargasitic amphibole (Ti < 0.32 a.p.f.u.), plagioclase (An16‐43Ab57‐71), quartz, locally biotite (Mg# = 0.0.46–0.56; Ti = 0.51–0.59 a.p.f.u.), calcite, epidote and accessory rutile, ilmenite, zircon and apatite. Garnet porphyroblasts occur commonly as euhedral crystals, and locally with corroded rims surrounded by a corona of plagioclase or amphibole–plagioclase. Growth of secondary garnet is locally observed in S > L tectonite rock. Clinopyroxene occurs as elongated subhedral crystals forming a strong fabric, or as a coarse symplectite with plagioclase. Amphibole is present as matrix grains in the garnet–clinopyroxene assemblage, but occurs also in coronas on garnet as symplectite with plagioclase, or as replacement textures on clinopyroxene. Secondary titanite is produced on rutile, and spinel+plagioclase on ilmenite. The P–T evolution is modelled by P–T pseudosections (TheriakDomino software), thermobarometry and by mapping of garnet chemistry. Garnet porphyroblasts show a decrease in CaO and MnO, an increase in MgO and variable FeO with resulting increasing Mg# from core to rim, indicating growth under increasing temperatures and decompression. Calculation of garnet+clinopyroxene+plagioclase+quartz+rutile stability combined with garnet and clinopyroxene isopleths of grossular and Mg# composition yields a maximum temperature metamorphism of 1.4–1.8 GPa and >900°C. The P–T modelling supports high‐P granulite facies conditions for the equilibration of the garnet–clinopyroxene‐dominated mafic lenses. The maximum temperature metamorphism is associated with partial melting. In addition, an outermost small Mn increase, and local reversal of the Mg# ratio and CaO‐flattening in garnet of the mafic lenses are interpreted as retrogression into amphibolite facies. This is in accordance of mineral replacement of clinopyroxene to amphibole and titanite growth on rutile. The data support an evolution where the eclogite facies crust in the northwestern‐most coastal part of WGR underwent decompression during heating into high‐P granulite facies conditions, followed by cooling and amphibolitization. Our investigation gives a regional petrological documentation and illustrates an extensive high‐T equilibration in the Caledonian root zone subsequent to the deep crustal burial.