In-situ laser ablation Lu–Hf geochronology of garnet across the Western Gneiss Region: Campaign-style dating of metamorphism

The development of in-situ laser ablation Lu–Hf geochronology of apatite, xenotime and garnet has opened avenues to quickly and directly date geological processes. We demonstrate the first use of campaign-style in-situ Lu–Hf geochronology of garnet across the high- to ultrahigh-pressure Western Gneiss Region in Norway. Mafic eclogites from this region have been the focus of much work, and were clearly formed during continental subduction during the Caledonian Orogen. However, abundant quartzofeldspathic and pelitic lithologies record a more complex history, with some preserving polymetamorphic age data, and most containing no indication of high-pressure mineral assemblages formed during subduction. Twenty metapelitic and felsic samples spanning 160 lateral kilometers across the Western Gneiss Region have been analysed using garnet Lu–Hf geochronology. The results reveal Caledonian ages for the majority of the garnets, suggesting some quartzofeldspathic and metapelitic lithologies were reactive and grew garnet during high- to ultrahigh-pressure metamorphism. However, two ultrahigh-pressure eclogite locations, Verpeneset and Fjørtoft, preserve both Caledonian and Neoproterozoic-aged garnets. Despite significant uncertainties on some of the Lu–Hf geochronologic ages, laser ablation Lu–Hf efficiently identifies the polymetamorphic history of parts of the Western Gneiss Region, illustrating the effectiveness of this novel analytical method for rapid mapping of metamorphic ages.Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycleSupplementary material:https://doi.org/10.6084/m9.figshare.c.5715453

Halogens in Eclogite Facies Minerals from the Western Gneiss Region, Norway

Ultra-high-pressure (UHP) eclogites and ultramafites and associated fluid inclusions from the Western Gneiss Region, Norwegian Caledonides, have been analysed for F, Cl, Br and I using electron-probe micro-analysis, time-of-flight secondary ion mass spectrometry and neutron-irradiated noble gas mass spectrometry. Textures of multi-phase and fluid inclusions in the cores of silicate grains indicate formation during growth of the host crystal at UHP. Halogens are predominantly hosted by fluid inclusions with a minor component from mineral inclusions such as biotite, phengite, amphibole and apatite. The reconstructed fluid composition contains between 11.3 and 12.1 wt% Cl, 870 and 8900 ppm Br and 6 and 169 ppm I. F/Cl ratios indicate efficient fractionation of F from Cl by hydrous mineral crystallisation. Heavy halogen ratios are higher than modern seawater by up to two orders of magnitude for Br/Cl and up to three orders of magnitude for I/Cl. No correlation exists between Cl and Br or I, while Br and I show good correlation, suggesting that Cl behaved differently to Br and I during subduction. Evolution to higher Br/Cl ratios is similar to trends defined by eclogitic hydration reactions and seawater evaporation, indicating preferential removal of Cl from the fluid during UHP metamorphism. This study, by analogy, offers a field model for an alternative source (continental crust) and mechanism (metasomatism by partial melts or supercritical fluids) by which halogens may be transferred to and stored in the sub-continental lithospheric mantle during transient subduction of a continental margin.

In-situ U-(Th-)Pb dating and REE analysis of zircon and monazite in the Grt-bearing gneisses from Gossa: Tracing early subduction into the highest-grade domains of the Western Gneiss Region, Norway

<p>To better understand the subduction–exhumation cycles of the Baltoscandian margin that reached (U)HP depths during the Caledonian orogeny, we have performed in-situ U-(Th-)Pb dating coupled with REE analysis of zircon and ± monazite in four samples from the supracrustal rocks of the Blåhø Nappe on Gossa island in the Western Gneiss Region (WGR) of Norway. We dated two garnet-plagioclase-biotite gneisses and two garnet-plagioclase-amphibole gneisses. Our research focused on deciphering the early metamorphic evolution of these complex rocks that have been overprinted by exhumation-related structures and pervasive retrogressive metamorphism.</p><p>The dated zircon grains are spherical or slightly elongated in shape, some of which display clear multi-stage growth features. Only one grain armored by garnet preserved an older detrital core that yielded early Neoproterozoic dates between 1.1-1.0 Ga. This grain does not provide any Caledonian signal. Younger individual <sup>20</sup><sup>6</sup>Pb/<sup>2</sup><sup>38</sup>U dates show three distinct populations that yield three concordia ages, each obtained from distinctly different compositional domains, the oldest from cores and the two youngest from overgrowths. The cores are characterized by HREE enrichment (high Lu/Gd ratios ca. 14.5), high Th/U ratios (> 0.1), and large Eu anomalies. They yield a concordia age of 474 ± 6.4 Ma. These cores can be rimmed by two different types of zircon overgrowth. The first overgrowth type (1) displays the same REE pattern as the cores and gives a concordia age of 444± 4.3 Ma. The second overgrowth type (2) shows a very weak Eu anomaly, no HREE enrichment (low Lu/Gd ratios ca. 2.37) and a very low Th/U ratios (<0.1). These yield a concordia age of 416± 3.7 Ma. The two older U–Pb zircon age populations are tentatively interpreted as reflecting two distinct metamorphic events or a prolonged episode of metamorphism. The youngest concordant metamorphic zircon dates a high grade, probably (U)HP, metamorphic overprint at ca. 416 Ma, subsequent to the previous events. Analyses performed on monazite provided complementary age records to those obtained on zircon. Monazite grains are weakly zoned, exhibit wormy shapes and are aligned with the youngest foliation. Th–U–total Pb dating of monazite, coupled with major and trace element mapping of monazite, yielded a very homogeneous age of 382 ± 1.6 Ma (n=65) interpreted to date the late shearing, which possibly accommodated a late stage of exhumation.</p><p>Funded by the National Science Centre (Poland) project no. 2014/14/E/ST10/00321.</p><p> </p>

Large meta-eclogite massifs within the Western Gneiss Region, Scandinavian Caledonides: Subducted ocean-continent transition?

<p>The northern part of the Western Gneiss Region (WGR) has distinctive belts of allochthonous metasediments and mafic rocks lying within tight infolds into the Baltica basement. They outcrop from the Grong-Olden Window to the Norwegian coast, possibly as far SW as Sørøyane, predominantly comprising metapelite and amphibolite with psammite, marble, calc-silicate, local large eclogite (>4km) lenses and ultramafites. These supracrustal lithotectonic units are attributed to the Blåhø Nappe, correlated with the Seve Nappe Complex (SNC) in its main outcrop in Sweden, which is considered to represent the pre-Caledonian continent-ocean transition (COT) of Baltica. They closely resemble the Lower Seve Nappe in northern Sweden where large amphibolite massifs with marbles are common, along with local eclogites. At least some have geochemical characteristics of spilitised extrusive MORB basalt in contrast to the better known, Neoproterozoic Baltoscandian Dyke Swarm in the SNC.</p><p>In the WGR near Molde a >10km long massif of such “amphibolite” at Tverrfjella commonly exhibits a relict high-P granulite precursor that has, in turn, overprinted eclogite. It encloses marble, scapolite-bearing calc-silicate, garnet peridotite (harzburgite) and Cu ores. Marble and meta-eclogite are intermixed which, along with its high Na spilitic character, suggests that the protolith was extrusive. Limited geochemical data suggest MORB composition. P-T estimates for eclogites in adjacent belts suggest UHP, possibly diamond-stable, conditions; in Sørøyane the well-known Ulsteinvik eclogite contains coesite. In the Molde area some of the mafic rocks and metasediments have partially melted. Eclogite metamorphism was Scandian in the Tverrfjell massif at 418 ± 11 Ma, with similar ages but tighter errors for adjacent belts and Ulsteinvik. These are significantly younger than ages for (U)HP metamorphism in the main SNC outcrop in Sweden, where early Ordovician subduction with a latest Ordovician granulite overprint is recorded. However, metapelites in other Blåhø-like supracrustal belts in the WGR do seem to record this earlier history as does one eclogite, consistent with the “double-dunk” hypothesis in this hinterland region. The protolith age of the metabasalts is unknown; analogy with the BDS suggests Neoproterozoic, but some zircon data from the WGR may suggest magmatic crystallisation during the Ordovician.<sup></sup>O-isotopes indicate that the marbles were Palaeozoic, rather than Proterozoic, carbonates. Overall, the available literature data show that some large mafic massifs in the WGR, with associated metasediments and peridotites, are allochthonous with respect to Baltica basement; they represent major additions of extrusive basalt to a far-distal COT or fully oceanic basin that have been subducted at least once during the Caledonian Wilson cycle. Isotopic data hint that at least some of their protoliths are unusually young. These supracrustal belts certainly merit closer attention.</p>

Exhumation History of the Western Gneiss Region Revealed Through Symplectite Thermobarometry

<p>The Western Gneiss Region (WGR) of Norway, part of the Caledonian Orogenic Belt, is one of the largest and best studied examples of exhumed ultra-high pressure (UHP) continental terrains in the world. This makes it an ideal candidate for studying the poorly understood processes that facilitate and control the exhumation of UHP continental material. Although the WGR is often considered the type example of the eduction model of UHP exhumation (Andersen et al., 1991), validation of exhumation models requires robust estimates of pressure and temperature across the full range of retrograde conditions which follow peak metamorphism. However, such constraints are often difficult to obtain as there is commonly overprinting of early-stage exhumation records during later stages of exhumation.  </p><p>UHP assemblages in the WGR are primarily preserved within numerous mafic eclogite enclaves, making them ideal candidates for studying processes and conditions that occur during exhumation from UHP conditions. In this study, we present detailed Electron Probe Micro-Analyses (EPMA) combined with Scanning Electron and Optical Microscopy characterization from a suite of mafic eclogite samples from the Stadlandet Peninsula of Western Norway. Our analyses focus on diopside–plagioclase (± amphibole) symplectite, which form from breakdown of omphacite during exhumation. Spatial variations in the compositions of minerals within these symplectites reflect a detailed record of P-T conditions during exhumation (Boland & van Roermund, 1983; Joanny et al., 1991; Waters, 2002). We used a novel technique of high resolution, low voltage EPMA, combined with secondary fluorescence corrections, which permits the analysis of individual symplectite lamellae with widths down to 1μm. Retrograde P-T pathways were then constructed from these data using the hornblende-plagioclase thermometer and clinopyroxene-plagioclase-hornblende barometer (Waters, 2002).  </p><p>P-T estimates from the symplectites fall in the range 470-720°C and 3-16 kbar. Combining the P-T arrays with existing peak P-T estimates indicates a two-stage exhumation path, with a steep initial isothermal decompression from depth followed by a more gentle cooling trajectory at lower pressures. The inflection in the exhumation path is estimated to be around 10-15 kbar at 650-700°C. The path shape is usually interpreted to record an initial rapid buoyancy driven exhumation from UHP to the base of the crust or lithosphere, followed by a second stage of slow exhumation to crustal depths. This confirmation of two-stage exhumation paths helps to constrain models of exhumation for the WGR, which in turn provides insights into how UHP terrains exhume globally.</p><p> </p><p>References:</p><p>Andersen, T. B., Jamtveit, B., Dewey, J. F. & Swensson E. (1991). Subduction and Eduction of Continental Crust: Major Mechanisms during Continent-Continent Collision and Orogenic Extensional Collapse, a Model Based on the South Norwegian Caledonides. Terra Nova, 3(3), 303–10</p><p>Boland, J., & van Roermund, H. (1983). Mechanisms of exsolution in omphacites from high temperature, type B, eclogites. Physics and Chemistry of Minerals, 9(1), 30–37.</p><p>Joanny, V., van Roermund, H. & Lardeaux, J. M. (1991). The clinopyroxene/plagioclase symplectite in retrograde eclogites. Geologische Rundschau, 80(2), 303–320</p><p>Waters, D. J. (2002). Clinopyroxene-amphibole-plagioclase symplectites in Norwegian eclogites. Mineralogical Society, Winter Conference, Derby.</p>

Deformation and reaction fabrics in eclogites from the Western Gneiss Region (Norway) - evidence of dehydration reactions attributed to episodic deformation

<p>The eclogites from Vårdalneset, Western Gneiss Region, Norway, show an exceptional large variety of reaction and deformation microfabrics that document the processes and conditions during burial and exhumation. Coarse grained eclogites comprise about 35% omphacite, 25% garnet and 20% amphibole with various amounts of white mica, zoisite, kyanite, rutile, zircon and pyrite. Their fabric is characterized by few mm long and several hundred µm wide amphibole and omphacite grains aligned in the foliation plane with zoned garnet porphyroblasts up to several mm in diameter. In contrast, finer-grained mylonitic eclogites with grain diameters of few hundred µm comprise systematically higher amounts of garnet (45%) and omphacite (35%) and generally less amphibole (< 5%) but similar amounts of zoisite, white mica, rutile and quartz. In the coarse-grained eclogite, amphibole shows evidence of dislocation creep as indicated by undulatory extinction, subgrains and recrystallized grains in necks of boudinaged coarse amphibole layers as well as in contact to garnet. The large garnet porphyroblasts generally show a complex zonation with an inclusion-rich Fe-poor and Mg-rich inner core surrounded by a zone with Fe- and Ca-rich patches and a broad Mg-rich, Ca- and Fe-poor rim. Only at contact to coarse amphibole an additional, a few tens of µm thin serrated rim further enriched in Mg can occur. At the direct contact to such serrated Mg-rich rims, amphibole is partly replaced by a fine-grained quartz-kyanite ± rutile aggregate, indicating dehydration reactions of amphibole. Quartz - kyanite ± rutile aggregates are surrounding garnet also in contact to omphacite, zoisite and to other garnet crystals. The microstructures suggest that deformation and dehydration of amphibole are coupled and played an important role during deformation of the eclogites finally leading to the mylonitic eclogites with higher amounts of garnet and omphacite. Deformation is suggested to have triggered the dehydration reaction by a slight and local increase in temperature. Furthermore, deformation provided additional pathways for the escaping fluids along the increased grain and phase boundary area, as indicated by commonly present quartz within interstitials between recrystallized amphibole grains. In all samples, few µm wide amphibole rims replacing garnets document restricted rehydration-reactions at a later stage. The large variety of the deformation and reaction microfabrics exemplarily show that both deformation and metamorphic reactions did not proceed at long-term continuous conditions, but that both are coupled and occurred episodically.</p>

Timescales of continental subduction: Constraints from ultrahigh-pressure metapelites in the Western Gneiss Region, Norway

<p>The Western Gneiss Region (WGR), Norway is an archetypal continental ultrahigh-pressure (U)HP terrane with an extensive metamorphic history, recording the subduction and subsequent exhumation of continental crust to depths exceeding 120 km. The vast bulk of past work within the WGR has focused on mafic eclogites. In this study, data from rare garnet-kyanite metapelites in (UHP) domains of the WGR is presented. U–Pb geochronology and trace element compositions in zircon, monazite, apatite, rutile and garnet were acquired, and P–T conditions were calculated by mineral equilibria forward modelling and Zr-in-rutile thermometry. The Ulsteinvik metapelite defines a prograde path that traverses through ~600–710 °C and ~11–14 kbar. Minimum peak conditions are ~750 °C and ~2.9 GPa in an inferred garnet-kyanite-coesite-omphacite-muscovite-rutile-quartz-H<sub>2</sub>O assemblage. Plagioclase-biotite-quartz intergrowths developed after omphacite-phengite-rutile breakdown on the early retrograde path, followed by cordierite-spinel-plagioclase symplectites after garnet-kyanite-biotite, defining a retrograde P–T point at ~740 °C and ~7 kbar. Late Ordovician-Early Silurian (~470–440 Ma) zircon and rutile age data in Ulsteinvik pre-dates the major Scandian UHP subduction episode in the WGR, interpreted as recording early Caledonian subduction within the Blåhø nappe. Monazite and apatite U-Pb geochronology and trace element data suggest exhumation occurred at ~400 Ma. The Fjørtoft metapelite is a constituent of the Blåhø nappe. Minimum peak P–T conditions are ~1.8 GPa and ~750 °C, with poor peak mineral fidelity attributed to extensive retrograde deformation. Negative Eu anomalies in ~423 Ma monazite suggest retrograde conditions were reached [RJT1] by ~423 Ma. Ulsteinvik and Fjørtoft may have experienced pre-Scandian subduction together within the Blåhø nappe, but record dissimilar histories after this. Two potential scenarios are presented: (1) Ulsteinvik resided within the mantle for 20 million-years longer than Fjørtoft during Scandian subduction, or (2), the samples were exhumed at different times during pre-Scandian subduction of the Blåhø nappe. The preservation of prograde zoning within Ulsteinvik garnets precludes a long-term residence within the mantle and suggests the latter option. In this scenario, the subducting Blåhø nappe experienced a degree of slab tear and partial underplating of the upper plate during the early stages of continental underthrusting. Discrete pieces may have later reattached to the lower plate at different times, partially exhumed, and then subducted to mantle-depths during the Scandian.</p>

Subcratonic and tectonic evolution of pyroxenite and eclogite with lamellar inclusions in garnet, Western Gneiss Region, Norway

Oriented lamellar inclusions of pyroxene and rutile in mantle garnet often serve as evidence for majoritic and titaniferous precursor garnets, respectively. We investigated ten new such microstructure-bearing samples from six orogenic peridotite bodies in SW Norway, which originated in the E Greenland mantle lithosphere, petrologically and thermobarometrically. All pyroxenite (nine) and eclogite (one) samples have large (mainly porphyroclastic) garnet containing silicate and oxide inclusions with shape-preferred orientation relationship. These inclusions vary – dependent on their size – systematically in shape (acicular to subprismatic), width (∼50 μm to submicron size), spacing (several 100 to ∼10 μm) and phase (pyroxene to Ti-oxide ± pyroxene). Smaller inclusions can fill the space between larger inclusions, which support the idea of consecutive generations. The larger, early formed lamellae occur least frequent and are most poorly preserved. A younger generation of other inclusions decorates healed cracks cutting across cores but not rims of garnet. These inclusions comprise oxides, silicates, carbonates (aragonite, calcite, magnesite) and fluid components (N2, CO2, H2O). The older, homogeneously distributed inclusions comply texturally and stoichiometrically with an origin by exsolution from excess Si- and Ti-bearing garnet. Their microstructural systematic variation demonstrates a similar early evolution of pyroxenite and eclogite. The younger inclusions in planar structures are ascribed to a metasomatic environment that affected the subcratonic lithosphere. The microstructure-bearing garnets equilibrated at ∼3.7 GPa (840 °C) and ∼3.0 GPa (710 °C), at a cratonic geotherm related to 37–38 mW m−2 surface heat flow. Some associated porphyroclastic grains of Mg-rich pyroxene have exsolution lamellae of Ca-rich pyroxene and vice versa that indicate a preceding cooling event. Projected isobaric cooling paths intersect isopleths for excess Si in garnet at ∼1550 °C, if an internally consistent thermodynamic data set in the system Na2O–CaO–MgO–Al2O3–SiO2 (NCMAS) is applied (or ∼1600 °C if using CMAS). This temperature may confine the crystallisation of the unexsolved garnets at 100–120 km depths of the E Greenland subcratonic lithosphere. Tectonism is indicated in coastal and hinterland samples by porphyroclastic orthopyroxene with Al2O3 concentrations showing W-shaped profiles. Cores of associated large (>200 μm) recrystallised grains have low Al2O3 contents (0.18–0.23 wt.%). Both characteristics typify relatively short intracrystalline Al diffusion lengths and a prograde metamorphism into the diamond stability field. We assign this event to subduction during the Scandian orogeny. Porphyroclastic orthopyroxene in other samples shows U-shaped Al2O3 concentration profiles paired with long Al diffusion lengths (several 100 μm) that exceed the radius of recrystallised grains. Their cores contain high Al2O3 contents (0.65–1.16 wt.%), consistent with a diffusional overprint that obliterated prograde and peak metamorphic records. Unlike Al2O3, the CaO content in porphyroclastic orthopyroxene cores is uniform suggesting that early exhumation was subparallel to Ca isopleths in pressure–temperature space. The depth of sample origin implies that rock bodies of Scandian ultra-high pressure metamorphism occur in nearly the entire area between Nordfjord and Storfjord and from the coast towards ∼100 km in the hinterland, i.e. in a region much larger than anticipated from crustal eclogite.

Segmentation of the Caledonian orogenic infrastructure and exhumation of the Western Gneiss Region during transtensional collapse

The (ultra)high-pressure Western Gneiss Region (WGR) of the Norwegian Caledonides represents an archetypical orogenic infrastructure of a continent-continent collision zone. To test established exhumation models, we synthesize the geochronology and structures of major basement windows and provide new ages from poorly dated areas. Migmatite U-Pb zircon samples date melt crystallization at ∼405 Ma in the Øygarden Complex, expanding the spatial extent of Devonian migmatization. Micas from shear zones in the Øygarden and Gulen domes yield 40Ar/39Ar ages mostly between 405 and 398 Ma, recording exhumation of metamorphic core complexes. On a larger scale, the youngest ages of various geochronometers in different segments of the WGR show abrupt breaks (10 – 30 Myrs) across low-angle detachments and sinistral transfer zones, which also correspond to metamorphic and structural discontinuities. We explain segmentation of the orogenic infrastructure by partitioned post-orogenic transtension due to lateral and vertical rheological contrasts in the orogenic edifice (strong cratonic foreland and orogenic wedge vs. soft infrastructure). Differential crustal stretching dragged out deep levels of the orogenic crust below low-angle detachments and became progressively dominated by sinistral transfer zones. Collapse obliterated the syn-collisional structure of the orogenic root and resulted in the diachronous exhumation of distinct infrastructure segments. Supplementary material:https://doi.org/10.6084/m9.figshare.c.5241710