Garnet-omphacite gneisses from the island of Otrøy situated in Western Gneiss Region (WGR), Norway, Scandinavian Caledonides, were examined within this study. The WGR is one of the planets most studied ultrahigh-pressure (UHP) terranes. The studied gneisses are hosts for UHP garnet peridotites and eclogites. The presence of the high pressure mineral assemblage including e.g. omphacite and phengite together with assumed remnants of pseudomorphic transition of formerly stable coesite present in omphacite and garnet, suggest formation of the Otrøy gneisses during ultrahigh pressure metamorphism (UHPM). However, geothermobarometry based on the mineral assemblage composed of garnet + clinopyroxene + phengite yielded pressure-temperature conditions of c. 880˚C and 2.2 GPa, characteristic for just high pressure metamorphism.  Nevertheless, it can be concluded that the Otrøy gneisses were formed due to the deep burial of continental crust during the continent-continent collision. This study provides insights into the understanding of the deep subduction of continental crust and expands our knowledge about the complex metamorphic and tectonic evolution of the WGR and the Scandinavian Caledonides. / Granat-omphacitförande gnejser från ön Otrøy i Western Gneiss Region (WGR), Norge, Skandinaviska Kaledoniderna, har undersöks i den här studien. WGR är ett av världens mest studerade område för ultrahögtryckmetamorfa bergarter. I den här typen av gnejser förekommer linser av ultrahögtrycksbergarter så som granat-peridotiter och eklogiter. I den studerade Otrøygnejsen förekommer bland annat phengit och omphacit med inneslutningar av sannolika pseudomorfer efter coesit. Detta visar på att Otrøygnejsen troligen har bildats under metamorfos vid ultrahöga tryck. Geotermobarometriundersökningen, baserat på granat + clinopyroxen + phengitsystemet, visar att tryck- och temperaturförhållandena var ca.880 ˚C och 2.2 GPa. Det innebär att gnejsen metamorfoserats inom intervallet karaktäristiskt för högtrycksmetamorfos. Likväl, kan det fastställas att Otrøygnejsen bildats till följd av en djup subduktion under kontinent-kontinentkollision. Den här studien bidrar till en ökad förståelse av processerna som påverkar den kontinentala skorpan vid djup subduktion och vidgar våra kunskaper om den komplexa metamorfa och tektoniska utvecklingen i WGR och de Skandinaviska Kaledoniderna.

The tectonometamorphic histories of allochthonous nappe units that were deeply buried and subsequently exhumed during the Scandian orogeny in the Western Gneiss Region (WGR) of Norway are poorly constrained and understood. One such unit is the Blåhø Nappe which underlies northern Harøya and Finnøya, two islands in the Nordøyane ultrahigh-pressure (UHP) metamorphic domain. The Blåhø Nappe contains a lower unit of garnet amphibolite gneiss with interlayered garnet-bearing quartzofeldspathic gneiss and marble, and an upper unit of migmatitic aluminous gneiss. Southern Harøya comprises orthogneisses attributed to the Baltican continental crust, and is separated from the Blåhø Nappe by the Finnøya migmatitic shear zone (FMSZ). Field and petrographic observations from these islands suggest that the Blåhø Nappe experienced peak metamorphism at high pressure amphibolite to granulite facies conditions before being overprinted by relatively lower pressure amphibolite facies conditions. In contrast, the adjacent Baltican basement gneiss contains coesite-eclogite pods, which attests to UHP conditions. However, the basement rocks are also overprinted by an amphibolite facies assemblage. To understand the implications of these observations, the metamorphic history of the Blåhø Nappe was investigated, and compared to that of the basement. This thesis presents thermobarometric and geochronologic analyses used to define a metamorphic pressure-temperature-time (PTt) path for the Blåhø Nappe on Harøya and Finnøya. The results suggest prograde metamorphism between ca. 440 Ma and 415 Ma, peak temperature metamorphism at 860?C and 15 kbar at around 410 Ma, equilibration at amphibolite-facies conditions of 680?C and 9 kbar by 395 Ma, and cooling below Ar-closure T in muscovite by ca. 360 Ma. The PTt data from the Blåhø Nappe demonstrate that these rocks did not share the UHP history of the adjacent basement rocks, but that both experienced similar amphibolite facies metamorphism and deformation. These units were likely juxtaposed along the FMSZ during isothermal decompression from their respective maximum burial depths to ~30 km depth.

This study investigates the ultrahigh pressure (UHP) metamorphic and metasomatic processes involved in the transient subduction-exhumation of continental crust to over 100km depths during a collisional orogeny and the implications for the evolution of the continental crust and crust-mantle interaction at depth. The study focuses on garnet websterites (orthopyroxene eclogites) and zoned, clinopyroxenite-garnetite veining features present in a range of eclogite-facies, crustal metamafic-ultramafic bodies hosted within the Western Gneiss Region (WGR), western Norway. The structural occurrences and textures of some of these crustal garnet websterites are seemingly unique to the WGR but little research has focused on their origin or from a metasomatic perspective. Based on field and petrographic observations, a metasomatic origin is attributed to vein-associated garnet websterites at Kolmannskog, Myrbærneset and Svartberget. A metamorphic origin is attributed to body domains at Nybø, Kolmannskog, Båtneset and Myrbærneset and a combined metamorphic-metasomatic origin is attributed to garnet websterite body domains at Årsheimneset and Remøysunde and inferred at Grytting and Eiksunddal. UHP P-T conditions are obtained from garnet websterites of ~3.7GPa, 740°C at Nybø, ~3.1GPa, 670°C at Grytting, ~3.5GPa, 700°C at Årsheimneset, ~3.6GPa, 815°C at Remøysunde, ~3.0GPa, 750°C at Kolmannskog and ~3.85GPa, 790°C at Svartberget. On this basis, it is proposed the Nordøyane UHP domain be extended eastwards to incorporate the Kolmannskog locality which lies outside its currently defined boundary. Constructed P-T paths suggest the northern Nordøyane UHP domain experienced ~100°C higher temperatures than the southern Nordfjord-Stadlandet UHP domain but experienced similar pressures implying a lower regional P-T gradient than previously established. P-T paths also suggest UHP, vein-forming metasomatism occurred prior to peak temperatures. U-Pb isotopic dating of zircon and monazites in garnetite vein cores dates UHP metasomatism at 414±5.6Ma at Årsheimneset and 410±2.6Ma at Svartberget. The fluid responsible for UHP metasomatism is considered to be a Si-Al-K-H2O-rich supercritical liquid produced in the surrounding country rock associated with the breakdown of phengite with a Na-LILE-LREE-HFSE-P enrichment signature. The major element composition of the fluid added to the Svartberget body is calculated to be 48-60% SiO¬2, 17-27% Al2O3, 3-11% K2O, <10% MgO, CaO and FeO, 3-6% Na2O, <4% P2O5¬, <1% TiO2 and MnO with an overall, undersaturated-saturated sialic, syenitic character hybridised through interaction with the garnet peridotite body margins. The continental fluid-mafic-ultramafic rock systems studied imply a zoned metasomatic unit forms at the interface between subducted continental crust and above mantle wedge at depths of ≥120-130km and along any fluid pathways penetrating into the mantle transferring abundant alkalis, water and trace elements into the mantle. Fluid-mantle interaction is proposed to form abundant biotite and amphibole and zones of garnet websterite, biotite websterite and biotite clinopyroxenite with lenses of eclogite and/or accessory phase (rutile, zircon, monazite, apatite, xenotime)-rich garnetite ±glimmerite selvages where residual fluids accumulate. Subcontinental mantle metasomatism may be associated with UHP, supercritical liquids derived from subducted, eclogite-facies, continental crust rather than oceanic crust as the continental crust is a greater source of the Si, alkalis, trace elements and water which characterise mantle metasomatism.