Apatite is one of the accessory minerals in the UHP metamorphosed eclogites at Northern Dulan belt, North Qaidam. It appears in three kinds of occurrences: (1) included in garnet which often shows cracks along the apatite grains, (2) coexisting with omphacite, rutile and/or clinozoisite in matrix and often surrounded by garnet, and (3) coexisting with retrograded minerals. The three eclogite samples examined in the present study are enriched in garnet. Two of them contain up to 80 vol.% garnet and the other is a porphyry of medium-grained garnet. They commonly show cracks and features of retrograde metamorphism, such as fissure-filling of secondary minerals including calcite or greenschist facies minerals. There are two size-ranges of well-oriented sulfide minerals included in apatite. One is nanometer-sized sulfide needles (50 ¡Ñ 20 ~ 870 ¡Ñ 120 nm) and particles (55 ~ 370 nm). The other is micrometer-sized sulfide needles (~20 ¡Ñ 0.5 £gm) and rods (~2.5 ¡Ñ 0.5£gm). Fluid inclusions and the micrometer-sized sulfide minerals commonly occur in the apatite grains that are near the cracks. Both nanometer- and micrometer-sized sulfide minerals are elongated with their long axes being normal or parallel to the c axis of the apatite. We used SEM-EDS and TEM-EDS to analyze and found that the sulfide minerals are troilite, pyrrhotite, Cu-bearing pyrrhotite and chalcopyrite. There are two sets of preferred crystallographic orientations for the dominated troilite and host apatite. The rod troilite is elongated along its a axis and <001>troilite // <001>apatite, <48-3>troilite ∡ <13-3>apatite = ~ 0.6º, (2-10)troilite // (3-10)apatite, <100>troilite ∡ <100>apatite = ~ 10º. The needle troilite is also elongated along its a axis and <001>troilite ¡æ <001>apatite, <-110>troilite ∡ <-12-2>apatite = ~ 1.3º, (11-2)troilite // (0-1-1)apatite. The preferred crystallographic orientation relationships, in terms of the c axis of troilite being parallel or normal to the c axis of host apatite, are similar to those for oriented quartz precipitates and omphacite hosts in the previous studies. According to the observations that only few sulfide minerals included in other minerals, the occurrences of apatites, and the microtextures of sulfide minerals, we suggest that the origin of sulfide minerals may relate to metasomatism during plate subduction. Metal ions such as iron, copper, cobalt and nickel were carried by chlorine- and sulfur-enriched fluids, which might be trapped as primary fluid inclusions in the apatite. The sulfide minerals then formed at the sites of fluid inclusions with the aid of fluids and available ions.

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.

Oriented needle-shaped or rod-shaped quartz precipitates occur in clinopyroxenes have been commonly observed in eclogites or garnet peridotites from the high pressure or ultra-high pressure (HP/UHP) metamorphic belts, and their occurrence has been used as an indicator of UHP metamorphism. However, the origin of those quartz precipitates and their crystallographic orientation relationships with clinopyroxene hosts are still not clear. In order to understand the formation mechanisms and environments of the quartz precipitates, the present study has used electron backscattered diffraction (EBSD) analysis, petrographic and scanning electron microscopy, and electron microprobe analysis to study textural features, mineral assemblages, mineral compositions, and crystallographic orientation relationships of mineral precipitates in the omphacite from Xitieshan, North Qaidam UHP metamorphic belt. The results show that the oriented rod-like precipitates in the omphacite hosts are mainly composed of quartz + edenite, and the rods are 5~20

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.

This thesis seeks to improve our understanding of the processes involved in continental collision zones, with a particular focus on subduction-exhumation. The main objective of this work has been to define the tectonometamorphic evolution of the deeply subducted Seve Nappe Complex (SNC) in the Scandinavian Caledonides. I utilize mineralogy, petrology and geochronology to constrain the P-T-t paths of the SNC rocks in Sweden. The research has focused on the high grade rocks of the SNC and resulted in the discovery of metamorphic diamonds within the gneisses in west-central Jämtland and southern Västerbotten. Microdiamonds provided evidence for the ultra-high pressure metamorphism (UHPM) and subduction of continental rocks to mantle depths. The UHPM in these rocks was confirmed by calculations of the P-T conditions. The UHPM is further recorded by eclogites and garnet pyroxenites from northern Jämtland and eclogites from Norrbotten. All these findings provide compelling evidence for regional UHPM of vast parts of the SNC (at least 400 km along the strike of this allochthonous unit). The SNC rocks followed nearly isothermal decompression paths and paragneisses have locally experienced partial melting during exhumation. Formation of the peculiar Ba- and Ti-enriched dark mica in the Tväråklumparna metasediments is related to the latter stage. In-situ monazite dating of the diamond-bearing gneisses from west-central Jämtland supports previous geochronological data inferring that the peak of metamorphism is probably Middle Ordovician and was followed by Early Silurian partial melting. The exact timing of the UHPM here still remains to be resolved. The Lu-Hf garnet and U-Pb zircon dating of eclogite and gneiss from northern Jämtland confirms the Middle Ordovician age of the UHP-HP metamorphism of the SNC rocks. The chemical dating of monazite from the Marsfjället gneiss suggests an earlier UHP history of the Seve rocks in southern Västerbotten as a post-UHP uplift is dated to ca. 470 Ma. Based on the P-T-t data obtained in this thesis, particularly on the evidence for Middle Ordovician UHPM and subsequent Silurian exhumation, a new tectonic model for the Scandinavian Caledonides has been proposed. The outcomes of this thesis therefore improve our understanding of the tectonometamorphic history of the Caledonides.

The St. Cyr klippe hosts well preserved to variably retrogressed eclogites found as sub-meter to hundreds of meter scale lenses within quartzofeldspathic schists in the Yukon-Tanana terrane, Canadian Cordillera. The St. Cyr area consists of structurally imbricated, polydeformed, and polymetamorphosed units of continental arc and oceanic crust. The eclogite-bearing quartzofeldspathic schists form a 30 by 6 kilometer thick, northwest-striking, coherent package. The schists consist of metasediments and felsic intrusives that are intercalated on the tens of meter scale. The presence of phengite and Permian age zircon crystallized under eclogite facies metamorphic conditions indicates that the eclogite was metamorphosed in situ with its quartzofeldspathic host. I investigated the metamorphic evolution of the eclogite-facies rocks in the St. Cyr klippe using isochemical phase equilibrium thermodynamic (pseudosection) modeling. I constructed P-T pseudosections in the system Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O3-SiO2-TiO2-H2O for the bulk-rock composition of an eclogite and a host metatonalite. In combination with petrology and mineral compositions, St. Cyr eclogites followed a five-stage clockwise P-T path. Peak pressure conditions for the eclogites and metatonalites reached up to 3.2 GPa, well within the coesite stability field, indicating the eclogites reached ultrahigh-pressure conditions. Decompression during exhumation occurred with a corresponding temperature increase. SHRIMP-RG zircon dating shows that the protolith of the eclogites formed within the Yukon-Tanana terrane during early, continental arc activity, between 364 and 380 Ma, while the metatonalite protolith formed at approximately 334 Ma, during the Little Salmon Cycle of the Klinkit phase of Yukon-Tanana arc activity. Both the eclogites and the metatonalites were then subducted to mantle depths and metamorphosed to ultrahigh-pressure conditions during the late Permian, between 266 and 271 Ma. The results of our study suggest portions of the Yukon-Tanana terrane were subducted to high-pressure and ultrahigh-pressure conditions. This is the first report of ultrahigh-pressure metamorphism in the accreted terranes of the North American Cordillera. Petrological, geochemical, geochronological, and structural relationships link the eclogites at St. Cyr to other eclogite localities in Yukon, indicating the high-pressure assemblages form a larger lithotectonic unit within the Yukon-Tanana terrane.