Norwegian Caledonides Research Articles

Micro-scale element migration during eclogitisation in the Bergen arcs (Norway): A case study on the role of fluids and deformation

Understanding chemical element mobility during high-pressure metamorphism is paramount to the knowledge of the transformations occurring during the course of eclogite-facies metamorphism. In particular, the role of deformation and fluid circulation appear essential. In order to better decipher this role, we investigated two eclogite-facies samples from the Lindås Nappe in the Bergen arcs (Norwegian Caledonides), which are related to a fluid/deformation metamorphic event. This study, based on detailed microtextural investigations, coupled with major and trace element analyses, either in situ EMPA and LA-ICP-MS or by conventional ICP-MS nebulisation on mineral fractions, illustrates the complexity of chemical reactions accompanying the eclogite- and amphibolite-facies metamorphism. A single eclogite-facies mineral species two eclogite-facies (i.e. phengite and epidote) depicts various chemical features depending on its textural location at thin-section scale. The correlation with the mineralogical reaction occurring in each textural site demonstrates that the compositional variations are inherited from the precursor minerals. Thus, the composition of eclogite-facies minerals is locally controlled by the composition of few hundred-micron domains. On the contrary, alteration phases of the amphibolite facies (symplectite and calcic amphibole) display clear enrichment in LILE, Pb, Sr and LREE compared to their precursor minerals (omphacite and garnet). This supports that the transport of elements at hand-sample scale was enhanced during the retrogression and elements were efficiently delocalised from one textural site to another. The contrasting behaviour of element mobility between peak eclogite-facies metamorphism and retrogression can be explained in terms of rate processes and fluid/deformation activity. During eclogitisation, deformation allows the formation of textural and mineralogical microdomains. Contemporaneously the fluid circulation favours the development of heterogeneities by transport of elements in excess towards areas protected from deformation where phengite-bearing quartz lenses crystallise. This, together with the very fast recrystallisation of eclogite-facies minerals, results in a heterogeneous redistribution of elements at sample scale and in a chemical disequilibrium between different microdomains of hundred-micron size. During retrogression in the amphibolite facies, coeval with long-term ductile deformation, the fluids act as an efficient vector to redistribute the elements from a given textural site to another and, thus, allow to reach, at hand-sample scale, a new chemical equilibrium between the different crystallising phases.

Short-lived mafic magmatism at 560–570 Ma in the northern Norwegian Caledonides: U–Pb zircon ages from the Seiland Igneous Province

The Seiland Igneous Province (SIP) of northern Norway comprises a suite of mainly gabbroic plutons, with subordinate ultramafic, syenitic and felsic intrusions. Several intrusions from the Seiland Igneous Province have been dated by ID-TIMS U–Pb zircon and monazite analyses. The Hasvik Gabbro on the island of Sørøy, previously assigned an age of 700±33 Ma by Sm–Nd, yields a U–Pb zircon age of 562±6 Ma, within error of the Storelv Gabbro (569±5 Ma) and a diorite associated with the Breivikbotn Gabbro (571±4 Ma). Various intrusions on the Øksfjord peninsula give nearly identical ages of 565±9 Ma (gabbro), 566±4 Ma (monzonite), 565±5 Ma (monzodiorite), 570±9 Ma (norite), and 566±1 Ma (orthopyroxenite). These ages overlap with those from Sørøy, and define a single and short-lived period of gabbroic (to felsic) magmatism for the region between 570 and 560 Ma, pre-dating a subordinate episode of alkalic magmatism at 530–520 Ma. The U–Pb ages contradict the previous geochronological interpretation for the Finnmark area, which implied a period of 250 m.y. for the emplacement of the SIP intrusions. The new age data also clearly distinguish the Seiland intrusions, emplaced into the Sørøy Group metasediments of the Kalak Nappe Complex, from several older granitic intrusions (c. 850 to 600 Ma) that cut the Sørøy Group farther east and south. The coincident ages of the different Seiland intrusive bodies also contradict the previous structural model for the area, which posits that the different gabbro bodies were emplaced at intervals, with compressional deformation affecting the gabbros between periods of intrusion. The short time span between the main plutonic phases strongly suggests that the mechanism for the emplacement of mafic magma operated in a single, probably extensional, tectonic regime. The mafic intrusions were later deformed and metamorphosed to at least amphibolite facies, most likely by the Scandian (420 Ma) phase of the Caledonian Orogeny.

The structure and timing of lateral escape during the Scandian Orogeny: A combined strain and geochronological investigation in Finnmark, Arctic Norwegian Caledonides

Determining the timing, duration and mechanism of tectonic events within an orogenic cycle, such as ocean subduction, continent–continent collision or gravitational collapse, is challenging, especially in ancient orogenic belts. Variations in the tectonic transport direction, however, can be used as a guide to these stages of orogeny. While thrust sheets within the Caledonian allochthon in north Norway were emplaced broadly eastwards perpendicular to the trend of the orogen, many features indicate material transport in other orientations. One dominant feature of the Magerøy Nappe, sitting above and infolded with the Kalak Nappe Complex, is the development of a strong N–S lineation orthogonal to the main transport direction. Strain measurements, in part determined by a new method, are used, in the context of the regional structural data to identify the critical stage in orogeny when compressional forces are balanced by orogen-parallel lateral escape. Quantitative 3-D strain estimation in the Magerøy Nappe indicates prolate deformation with c. 50% horizontal shortening parallel to the thrusting direction (E–W) and c. 200% extension along the orogenic strike (N–S) with c. 30% vertical shortening. Temporal constraint on this fabric is provided by Ar–Ar isotopic analysis of undeformed white mica in cross-cutting granitic pegmatites. These data show that prolate deformation occurred before the white mica cooling age of 416 ± 4 Ma, while the previously determined depositional age of the Hellefjord Schist indicates that it occurred after 438 ± 4 Ma. A granitic pegmatite that intruded the Hellefjord Schist after an initial deformation phase but during or prior to a later deformation, has been dated at 431 ± 2 Ma by U–Pb zircon. A previous lower age constraint on this deformation of 428 ± 5 Ma is given by metamorphic zircon overgrowths on fractured grains. These results constrain the continental collision between Baltica and Laurentia in Finnmark to the interval c. 431–428 Ma. Placed in a regional context, these results indicate that lateral escape was orthogonal to the thrusting direction and occurred during the continent–continent collision stage in the Scandian Orogeny before gravitationally driven collapse.

Evolution of lavas in the Late Ordovician/Early Silurian Solund‐Stavfjord Ophiolite Complex, West Norway

The geochemistry of the volcanic sequence of the Late Ordovician/Early Silurian Solund‐Stavfjord Ophiolite Complex (SSOC) of the Western Norwegian Caledonides has been investigated along 12 stratigraphic sections spaced over a lateral distance of ∼60 km. The metabasalts commonly show a cyclic organization, comprising sheet flows followed by pillow lavas and with volcanic breccias at the top of some volcanic units. The proportions of the volcanic rock types vary considerably along‐axis, even within short distances. The robust nature of the sheeted dike complex, the proportion and regional distribution of the various volcanic rocks, the aphyric nature of the metabasalts, as well as the predominance of Fe‐Ti basalts suggest that the SSOC formed at an intermediate to fast spreading ridge, probably within a back‐arc basin. The variations in the concentrations of the incompatible (e.g., Zr) and compatible (e.g., Cr) elements in the lavas are substantial, i.e., from 49–384 ppm and 66–443 ppm, respectively. Nd isotopic ratios show only minor variations, suggesting that the lavas were generated from an isotopically uniform source. The variations in the incompatible elements (represented by Zr) define stratigraphic units ∼25 to 150 m thick, with either gradual decreases or increases in Zr concentrations, while Cr may show different trends. The different Zr‐Cr covariations are interpreted in the light of a numerical model in which fractional crystallization and mixing of various magmas are the principal processes. Through upwardly Zr‐increasing units, Cr generally decreases and estimated magma densities increase, compatible with progressive fractional crystallization. However, through upwardly Zr‐decreasing units, Cr and estimated magma densities either increase or decrease, trends that are attributed to hybridization in frequently replenished magma chambers. Within short lateral distances (<1 km), the geochemical stratigraphy changes dramatically, excluding eruption from a homogeneous, axially continuous magma chamber. Instead, we propose that the volcanic sequence was fed from small, separate magma lenses that evolved independently of each other.

Enigmatic nature of thick sedimentary carbonates depleted in 13C beyond the canonical mantle value: The challenges to our understanding of the terrestrial carbon cycle

Carbon and oxygen isotope measurements of 66 samples from the 60 m-thick variegated marble in the Upper Allochthon of the Norwegian Caledonides have a mean δ 13C carb of −8.4 ± 0.9‰ (V-PDB), and a mean δ 18O of 20.2 ± 2.2‰ (V-SMOW). The variegated marble is overlain by 150 m-thick pale grey marble characterised by mean δ 13C carb of −6.5 ± 0.8‰ ( n = 25) and underlain by dark grey marbles with a mean δ 13C carb of +4.8 ± 1.1‰ ( n = 61). This tripartite unit of an poorly constrained age—but between Neoproterozoic and Early Silurian—discontinuously developed over a distance of 500 km, is likely to represent one of the largest isotopically anomalous sedimentary carbonate formations yet reported. The marbles depleted in 13C beyond the canonical mantle value of −6‰ show no obvious evidence of post-sedimentary repartitioning of carbon isotopes. Several other carbonate formations deposited within approximately 680–540 Ma time interval (Chenchinskaya, Nikolskaya and Torginskay in Siberia, Ingta in NW Canada, Shuram in Central Oman, Trezona and Wonoka in South Australia) are several hundred meters thick, developed over a distance of hundreds of kilometres, and all show a similar depletion in 13C beyond the mantle value, for reasons that are not well understood. The existence of these carbonates represents a challenging problem for our current understanding of global carbon geodynamics. Changes in the ratio of reduced/oxidised carbon sequestered in sediments, a methane hydrate release or ‘zero’ biological productivity, if applied separately, cannot explain carbon isotope characteristics and formation of these carbonates. We tentatively propose that several factors associated with unusual geodynamic and palaeoclimatic scenarios developed between 600 and 540 Ma might have been involved in the extreme lowering of the isotopic composition of carbon entering the global Earth surface environment. This period was marked by the retreat of Neoproterozoic glaciers and break-up of the supercontinent Rodinia. The late-postglacial warming might have induced a massive release of methane hydrates extremely enriched in 12C. The ‘death’ of Rodinia was marked by unusually rapid (approximately 20 cm/year) motion of newly formed continental plates suggesting vigorous mantle convection and an enhanced restructuring of the lowest compartments of the Earth. This could provide a flux of 12C-rich material from the isotopically light asthenosphere–mantle source ( δ 13C = −25 to −15‰) to the surface. Considerable reconfiguration of the continental and sea areas of the Earth might have triggered enhanced weathering of previously deposited 12C-rich organic matter. In presenting this working hypothesis we also question the completeness of our understanding of the terrestrial carbon cycle, and the popular assumption that the isotopic composition of carbon entering the global Earth surface environment was always around −6‰.

THE DEVELOPMENT OF A MARGARITE CORUNDUM BLACKWALL BY METASOMATIC ALTERATION OF A SLICE OF MICA SCHIST IN ULTRAMAFIC ROCK, KVESJOEN, NORWEGIAN CALEDONIDES

An unusual occurrence of blackwall rock occurs on a promontory in Lake Kvesjoen, in the Norwegian Caledonides, at the contact between a lens of ultramafic rock (Fo‐En-rock) 200 m in diameter and amphibolite-facies metapelitic country-rocks. The blackwall contains corundum, staurolite, tourmaline and four micas: muscovite, paragonite, margarite and biotite. Accessories include allanite‐epidote, apatite, ilmenite, rutile, monazite, and zircon. The blackwall evolved from a normal quartz-rich staurolite‐garnet‐biotite metapelite by progressive loss of SiO2 to the silica-deficient Fo-rich ultramafic rock at about 570°C and 6.5 kbar. Thermodynamic modeling of the metasomatic process predicts first the disappearance of garnet and excess quartz, and then the production of corundum from excess Al2O3. Subsequent desilication of plagioclase then results in the formation of paragonite and margarite in addition to the “primary” muscovite and biotite. Abundant development of biotite at the expense of staurolite and muscovite at a late stage of the process leads to the observed dark biotite-rich corundum‐margarite rock. The model process predicts the observed blackwall mineralogy and also suggests that the corundum‐margarite rock represents a mature endproduct of the metasomatic reaction. The SiO2 lost by the metapelite reacted with forsterite and enstatite of the ultramafic rock to form talc. The chemical potential of SiO2 imposed by the talc‐forsterite assemblage was much lower than that given by quartz saturation in the metapelite under the prevailing conditions during the active process. The resulting gradient in chemical potential was the prime driving force in the formation of the blackwall.

Geochemical preservation potential of high-grade calcite marble versus dolomite marble: implication for isotope chemostratigraphy

The Neoproterozoic Hekkelstrand Marble from the North-Central Norwegian Caledonides contains a calcite marble unit hosting a commercially exploited white dolomite marble deposit, which underwent polyphase amphibolite-facies, Scandian metamorphism. Major and trace element geochemistry, δ 18O, δ 13C and 87Sr/ 86Sr ratios have been used to define isotopic differences between interbedded calcite and dolomite marbles and to assess their post-depositional isotopic resetting. A high strontium concentration in the calcite marble (up to 1200 ppm) suggests an aragonitic lime mudstone precursor, and its least-altered δ 13C (+4‰ V-PDB), δ 18O (27‰ V-SMOW) and 87Sr/ 86Sr (0.70615 to 0.70624) ratios are consistent with 700–600 Ma seawater. The dolomite marbles contain minor relics of the calcite (aragonite?) precursor, despite pervasive dolomitisation. The average Mn (44 ppm) and Fe (1041 ppm) contents of the dolomite marbles are comparable with those measured in the host calcite marbles and much lower than those reported from non-metamorphosed dolostones of a similar Neoproterozoic age. The dolomite marbles have an average Sr content of 115 ppm. An average Mg/Ca ratio of 0.56 is lower than that for stoichiometric dolomite (0.62) and is controlled mainly by remnant inclusions of precursor calcite. 87Sr/ 86Sr ratios range between 0.70725 and 0.71034. The least-altered δ 13C of +6‰ and δ 18O of ∼30‰ suggest a high degree of isotopic preservation of the dolomite. The dolomite marbles are isotopically heavier (both δ 18O and δ 13C) by ∼2‰ on average compared with the calcite marble, thus suggesting a pre-metamorphic, lower-temperature, isotopic equilibrium. The dolomite was apparently originally precipitated from 600 to 700 Ma seawater-dominated fluids. The 87Sr/ 86Sr ratio was modified by post-depositional processes. The high δ 13C and δ 18O values and the low Mn content of the bulk dolomite marbles are not consistent with common meteoric diagenesis or metamorphic alteration. The highest 87Sr/ 86Sr ratio of 0.70868 suggests that the earlier dolomite was apparently modified by ca. 560 Ma seawater or by meteoric water considerably buffered by ca. 560 Ma limestones. The most significant geochemical and isotopic modification of the dolomite took place during metamorphism. This has been observed across the dolomite marble-silicate rock contacts over a limited distance of <0.5 m where C and O isotope systems were reset by up to −4.5‰ and −9.7‰, respectively, the 87Sr/ 86Sr ratio increased by up to 0.0012, whereas the Fe, Mn and Sr concentrations have increased by factors of ∼80, ∼15 and ∼1.5, respectively. The dolomite marbles, despite amphibolite-facies polyphase metamorphic alterations, have preserved high δ 18O and δ 13C values, which can be utilised for chronostratigraphic purposes, whereas 87Sr/ 86Sr ratios provide insights into the composition of the post-depositional fluids.

Evidence of high water content in the deep upper mantle inferred from deformation microstructures

Deep upper-mantle rocks from the Norwegian Caledonides show evidence for large strain deformation in both olivine and garnet under varying water contents. Using microstructural observations, including lattice-preferred orientation of olivine and subgrain boundaries of majoritic garnet, we infer the following deformation history. At depths exceeding ∼150 km, large strain deformation occurred at low stress (∼10 MPa) and modest temperature (∼1300 K), involving high water content (>1000 H/10 6 Si in olivine). This was followed by low strain deformation at lower water content (∼200–1000 H/10 6 Si) and modest stress (∼40 MPa) in the shallower parts. These observations show that the deep upper mantle in this region had a considerably higher water content than the upper mantle near mid-ocean ridges.

Assimilation and crystal accumulation in a mid-crustal magma chamber: the Sausfjellet pluton, north-central Norway

The Sausfjellet pluton is made up of two intrusive units emplaced into high-grade metamorphic rocks of the Helgeland Nappe Complex of the Uppermost Allochthon in the Norwegian Caledonides. The eastern part of the pluton intruded marble and less voluminous calc-silicate and pelitic rocks. The western half is hosted predominantly by semi-pelitic migmatite with intercalated marble. Remelting of the migmatite during pluton emplacement occurred in a thermal aureole as much as 1000 m wide. The early gabbroic unit forms the southeastern part of the body; it consists of hornblende-bearing to hornblende-rich gabbro and diorite which is thought to have crystallized from an H 2O-rich andesitic parental magma. The younger dioritic unit underlies the central and western parts of the pluton, as well as a zone as much as 200 m wide that separates the rest of the pluton from its host rocks (herein the “annular zone”). The interior or central zone of the dioritic unit is pyroxene diorite that is locally interlayered with anorthosite. The western and annular zones are, by comparison, mineralogically heterogeneous. They range from diorite to quartz monzonite and from biotite-bearing two- and three-pyroxene assemblages to biotite–hornblende assemblages. Neither rock type nor mafic assemblage is correlated with position in the pluton or proximity to a contact. Stoped blocks of a distinctive coarse-grained diorite, as well as pyroxene-rich calc-silicates, are present in the gabbroic unit and the central zone of the dioritic unit. The few stoped blocks observed in the western zone of the dioritic unit are predominantly quartz-rich gneiss. Chemical variation in the central zone of the dioritic unit is interpreted to result from accumulation of pyroxenes+plagioclase from an H 2O-poor andesitic parent. These rocks have approximately constant δ 18O of +6.6±0.2‰ and lack evidence of in situ assimilation. Heterogeneities in the western and annular zones of the dioritic unit are reflected in variable, anomalously enriched incompatible element contents and in δ 18O, which ranges from +6.7‰ to +8.6‰. Petrologic models indicate that the magma parental to the central zone could also be parental to the western and annular zones. If so, evolution of the western and annular zone magma was by crystal accumulation and assimilation of metapelitic host rocks. As much as 20% of the mass of the western and annular zones can be ascribed to assimilated material, which apparently entered the magma by stoping. Therefore, the asymmetrical zoning of the pluton is due to differences in host rock compositions and the relative ability of the magma to assimilate its host rocks.

Archean mantle fragments in Proterozoic crust, Western Gneiss Region, Norway

Small volumes of garnet peridotite occur within large dunite bodies in the Western Gneiss Region of the Norwegian Caledonides. Previous Sm-Nd analyses of garnet peridotite yield Proterozoic ages close to the crystallization ages of the host gneisses. In situ (laser-ablation multicollector–inductively coupled plasma–mass spectrometry) Re-Os analysis of sulfides in the garnet peridotites gives a range of Proterozoic and Archean model ages. An Archean (2.7–3.1 Ga) protolith age is supported by whole-rock Re-Os data for sulfide-free dunites from several bodies. The Archean ages imply a melt-extraction event in the lithospheric mantle that predates the growth of the known Proterozoic upper crust in the region.

Volcanic evolution of oceanic crust in a Late Ordovician back‐arc basin: The Solund‐Stavfjord Ophiolite Complex, West Norway

The stratigraphy of the well‐preserved Solund‐Stavfjord Ophiolite Complex in the West Norwegian Caledonides documents the volcanic evolution of a spreading center in a Late Ordovician back‐arc basin. Basaltic sheet flows, pillow lavas and volcanic breccias are the main components of the ∼470–800 m thick extrusive sequence, and are organized stratigraphically in a cyclic manner. Cyclic units vary in thickness from ∼5 m to 225 m and are typically composed of basal sheet flows or lava flows with large pillows that are succeeded by flows with progressively smaller pillows and volcanic breccias. Thick, independent breccia units also occur in the stratigraphy. In sheet‐flow dominated parts of the sequence the cyclic units are thicker (average 85 m) than in pillow‐dominated parts (average 20 m). Detailed logging of closely spaced profiles (∼1 km or less apart) shows that the proportions of sheet flows, pillow lavas and volcanic breccias varies laterally. Along an axial segment of less than 10 km, the volcanic products change from predominantly sheet flows, reflecting robust volcanic centers, to pillow lavas to volcanic breccias. Sheet‐flow dominated volcanic centers seem to be spaced at intervals of ∼25–30 km, and we tentatively interpret their regularity as an expression of volcanic segmentation at an intermediate‐ to fast spreading center. Observations of modern ocean crust suggest that sheet flows dominate at fast spreading ridges, while pillow lavas dominate at slow‐spreading ridges. Volcanic breccias are apparently rare in both of these environments. These features contrast with the stratigraphy of the Solund‐Stavfjord Ophiolite Complex, where the proportion of different volcanic products varies laterally and volcanic breccias are common. We emphasize the importance of detailed studies of the volcanic stratigraphy of ophiolites, as complements to those of in‐situ oceanic crust, in order to provide a more complete picture of volcanic evolution of oceanic crust in different spreading regimes.

The role of felsic and mafic igneous rocks in deciphering the evolution of thrust-stacked terranes: An example from the north Norwegian Caledonides

Excellent exposure of appropriate structural levels across the trace of the regional Ofoten synform, north Norway, make this an exceptional area in which to investigate the role of felsic and mafic igneous rocks in the amalgamation and final emplacement of a stack of far-traveled terranes. The nappe sequence in Ofoten is composed, from bottom to top, of the Narvik, Ofoten, and Niingen nappe complexes. Each of these nappe complexes contains multiple, internal thrust sheets, or nappes, that were emplaced onto Precambrian-Cambrian rocks of the Baltic shield along east-directed thrusts during the Caledonian orogeny. Distinct contrasts in rock types, structural arguments, and regional correlations indicate that the nappes are suspect or exotic terranes. Whole-rock chemistry, petrography, and field relations of felsic and mafic igneous rocks within each allochthon are used to explore the tectonic setting of each terrane, the crustal levels of their amalgamation, and the relative and absolute timing of their amalgamation. Three different types of felsic intrusions were distinguished in each lithologic package, with the exception of the Evenes Group (Ofoten nappe complex) which has none. Caledonian plutons also are lacking in the underlying Baltic basement complex. The intrusions are separated temporally into early-phase, nappe-specific plutons and late-phase intrusions, some of which cut across nappe boundaries. Most are S- and I-type collisional granitoidal rocks. Early-phase granitoids in the Bogen Group (Ofoten nappe complex), however, have an A-type chemistry and mineralogy, probably indicating either second-cycle melting of a granulite terrane or melting of calc-alkaline rocks. The lack of Caledonian intrusives in both the Evenes Group and the basement requires that allochthons overlying these units rooted to the west. Discrimination of the various tectonic settings based on the geochemistry of the felsic intrusions dovetails closely with that determined from a sampling of mafic rocks. Each nappe originated as a laterally-distinct entity that rode piggyback with the Niingen nappe complex being emplaced first and the composite nappe stack thrust lastly upon the basement complex. Previously reported lithologic correlations with fossiliferous units and U-Pb and 40Ar/39Ar mineral cooling dates combine with new Rb/Sr and U-Pb dates to place the relative nappe-stacking chronology into an absolute timeframe. Metamorphism of the Narvik nappe complex was first at ∼432 Ma, followed by metamorphism of the Niingen around ∼430 Ma, which later metamorphosed the Ofoten by down heating between ∼430 and 425 Ma. The stacking process took only ∼7 my and resulted from the Silurian-Devonian Scandian orogeny. Pre-Scandian relics occur within each of the nappe complexes and were strongly reworked and emplaced during the Scandian event.

Dating the exhumation of UHP rocks and associated crustal melting in the Norwegian Caledonides

U–Pb and Rb–Sr dating was undertaken in combination with P–T estimates to (1) constrain the time of ultrahigh-pressure (UHP) eclogite formation in the Stadlandet UHP province of Norway, (2) date later crustal melting–migmatization of the eclogite country gneisses, and (3) temporally trace post-migmatite cooling and retrogression under amphibolite facies metamorphic conditions. In contrast to earlier U–Pb studies which used accessory minerals from the gneisses, we focused on the direct dating of minerals defining the HP assemblage. For the eclogite, rutile and omphacite fractions were analyzed for U–Pb, and from an adjacent migmatite leucosome titanites and K-feldspar. For Rb–Sr dating, phengite was measured for the eclogite, and biotite for two leucosome layers of the migmatite–eclogite complex. A U–Pb age of 389±7 (2σ) Ma is obtained if the full set of 12 rutile and five omphacite analyses is regressed (MSWD: 16), and 389±2 Ma for those nine data which strictly satisfy isochron conditions (MSWD: 0.78). The 389-Ma age is interpreted to date equilibration and freezing of the eclogite paragenesis at maximum temperatures of 770 °C, reached during decompression to 1.8 GPa. Decompression from 2.8 to 1.8 GPa occurred in the partial melting domain of granitic crust, with the migmatites being dated at 375±6 Ma by titanite and K-feldspar from an eclogite-adjacent granitic leucosome. This titanite age also shows that the U–Pb chronometer in rutile is very robust to high temperatures—it remained a closed system for at least 14 million years, at temperatures in excess to 650 °C. After decompression and migmatization, exhumation is accompanied by rapid cooling to reach the 300 °C isograde by 357± 9 Ma, determined by a biotite isochron for a leucosome in a slightly shallower structural level. In considering that the time of maximum pressure is bracketed by early zircon crystallization during subduction and later omphacite–rutile equilibration in the eclogites, an exhumation rate of 5 mm/year is deduced for initial exhumation, occurring between 394 and 389 Ma. For subsequent cooling from 770 to 600 °C, we obtain a rate of 2.3±1.3 mm/year. First stages of exhumation most likely occurred under an overall compressional regime, whereas Devonian basin formation is associated to detachment movements during 389–375 Ma exhumation. This period of extension is followed by a much younger, decoupled thermal phase at 327±5 Ma, occurring under static conditions within very restricted zones, most likely in association with the circulation of fluid phases along old discontinuities. Initial isotopic signatures of Sr and Pb substantiate Paleo- to Meso-Proterozoic crust formation times of the Stadlandet UHP province precursor lithologies.

The Devonian Nesna shear zone and adjacent gneiss-cored culminations, North–Central Norwegian Caledonides

The Early to Mid-Devonian strain distribution in the Norwegian Caledonides was characterized by large-magnitude extension, extension-normal shortening and sinistral strike-slip. In SW Norway, previous work has shown that the angle between the orogen and the Devonian maximum elongation trend decreases northward towards a zone of sinistral strike-slip. We provide evidence that: (1) late- to post-orogenic extension was much more important in North–Central Norway than recognized previously; (2) the Early to Mid-Devonian, roughly orogen-parallel, maximum elongation trend persisted over more than 200 km farther north than hitherto recognized; (3) a number of gneiss-cored culminations in North–Central Norway were affected by, and probably owe their present geometry to, extensional shearing and faulting. The Nesna shear zone is identified here as a major, late Early to Mid-Devonian, low-angle extensional shear zone in the North–Central Norwegian Caledonides. In the study area, the Nesna shear zone now constitutes the lower boundary of the Helgeland Nappe Complex, a terrane exotic to Baltica. The present situation of the Helgeland Nappe Complex is thus that of an extensional allochthon. The transport direction along the medium- to low-grade Nesna shear zone was top-to-the-WSW, and thus roughly orogen-parallel. Extensional shearing in the Nesna shear zone may provide an explanation for a westward-younging, 40 Ar/ 39 Ar cooling pattern previously reported from the Ofoten–Lofoten area. Folding of the Nesna shear zone along axes parallel to the extensional transport direction indicates that extension-normal shortening continued at least into late Early and Mid-Devonian times. The WSW–ENE maximum elongation trend probably persisted in North–Central Norway through most of the Devonian period, and accompanied the transition from ductile to brittle deformation in the rocks of the Caledonian nappe-stack. Deformation in low-angle, initially medium-grade, extensional shear zones such as the Nesna shear zone was succeeded by shearing in steeper, low-grade, ductile-to-brittle shear zones that developed along the western margins of gneiss-cored culminations. Temporal overlap between activity in the Nesna shear zone and in the lower-grade shear zones that bound the culminations can be demonstrated. To the east, however, medium-grade extensional shear zones such as the present boundary between the Seve and Köli nappes were probably back-rotated as the culminations developed in the footwall of the low-grade shear zones. From the Mid–Late Devonian to Early Carboniferous, the low-grade, culmination-bounding shear zones constituted the eastern margin of a regional, transtensional system bounded in the south by the sinistral Møre–Trøndelag Fault Complex.

Paleozoic diamonds within a Precambrian peridotite lens in UHP gneisses of the Norwegian Caledonides

Re–Os data obtained by LAM–ICPMS techniques from sulfides within the Bardane peridotite body in the Western Gneiss Region of the Norwegian Caledonides indicate a Proterozoic, or possibly Archean, origin for the peridotite, probably as depleted residue formed through melt extraction in the mantle. Sm–Nd mineral ages from primary garnet-bearing assemblages in a megacrystic orthopyroxenite within the peridotite indicate the mantle was invaded by melts that crystallized in the garnet-stability field during the mid-Proterozoic. However, microdiamonds within this megacrystic orthopyroxenite formed well after these processes, during the Paleozoic Caledonian Orogeny. Clinopyroxenes show enriched trace element patterns and radiogenic Sr and non-radiogenic Nd isotopic ratios, indicating fertilization of the Bardane peridotite with incompatible elements, presumably including carbon. Metasomatism may have occurred while the peridotite was part of the mantle wedge above an Iapetus subduction zone or it may have occurred after the lenses were introduced into the crust. Textural and field relationships and Sm–Nd mineral results from secondary garnet-bearing assemblages indicate the microdiamonds formed during Caledonian metamorphism, probably during the Scandian Orogeny, when the peridotite body was introduced from the mantle into a subducting crustal gneiss complex and carried to mantle depths within the diamond-stability field. Texturally and mineralogically similar megacrystic orthopyroxenites on the nearby island of Otrøy lack microdiamonds, suggesting they underwent all the above processes except the metasomatic introduction of carbon.

Processes leading to eclogitization (densification) of subducted and tectonically buried crust

The transformation of subducted and tectonically buried crustal rocks to denser eclogite plays a fundamental role in the dynamics of mountain building and crustal recycling. However, a complex of partially eclogitized granulites on the island of Holsnøy in the western part of the Norwegian Caledonides reveals that such densification depends in part on the antecedent history of rock masses and that large bodies of untransformed rock can persist metastably even after long residence times in the lower crust. The rocks on Holsnøy suggest that conversion to eclogite was delayed until lower or subcrustal seismic events allowed aqueous fluids to enter the previously dry and unusually strong granulite complex. Metamorphism then occurred in a spatially heterogeneous manner, with several distinct physical processes involving feedbacks between deformation, fluid infiltration, and chemical reactions operating simultaneously in different parts of the rock mass. Field relations show that the introduction of fluids and conversion to eclogite significantly weakened the rocks. A reservoir‐flux systems model is used to represent the various processes contributing to eclogitization of the granulites, which operated at vastly different rates over disparate intervals of time. The model suggests that the metamorphic process may have been brief (≪1 Myr), and ultimately self‐limiting, arrested by the change in rheology associated with the conversion of brittle granulite to ductile eclogite.

Crustal‐scale boudinage and migmatization of gneiss during their exhumation in the UHP Province of Western Norway

ABSTRACT A field study in the coesite province, the deepest unit of the Norwegian Caledonides, gives new constraints on the rheological behaviour of the continental crust during exhumation. Lithological heterogeneities and differential retrogression led to crustal‐scale boudinage during the late‐orogenic intense E–W stretching event in the footwall of the Nordfjord‐Sogn Detachment. The main gneissic lithologies display a modest but widespread syn‐exhumation migmatization. Textural criteria allow estimation of a 30% fusion rate. Partial melting mostly post‐dates eclogitization and is synchronous with ductile stretching and top‐to‐west shearing. Presented observations suggest that the melt reactions and migmatization resulted in a soft rheology. During subduction to ~ 100 km depth and subsequent exhumation, crustal viscosity can be reduced by up to four orders of magnitude. Models are discussed that consider a transition from a small internal strain of the crust to viscous flow during exhumation.

Isotopic stratigraphy suggests Neoproterozoic ages and Laurentian ancestry for high-grade marbles from the North-Central Norwegian Caledonides

Carbon and strontium isotope stratigraphy has been applied to constrain the depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides. Two marble formations hosted by diverse schists from the Bogen Group, all previously correlated over long distances with a Late Ordovician–Early Silurian, low-grade, fossiliferous succession, have been studied for carbon, oxygen and strontium isotopes. The least altered 87Sr/86Sr ratios ranging between 0.7062 and 0.7068, and the best preserved δ13C values falling between +5.0 and +6.5‰ obtained from two marble formations, are consistent with a seawater composition in the time interval 700–600 Ma. The results obtained do not support the previously proposed correlation of the Bogen Group with an Ordovician–Silurian lithostratigraphic succession further north. The apparent depositional ages suggest that the tectonostratigraphic succession studied is inverted and that the tectonostratigraphy of the region requires revision. The Neoproterozoic depositional ages combined with the palaeogeographic position of Baltica imply that carbonates were initially accumulated in seas on a continental shelf, probably Laurentia, and were tectonically transported onto Baltica during Early Silurian, Scandian collision, at c. 425 Ma. Prospecting for new dolomite marble deposits of the Hekkelstrand type and carbonate-hosted manganese–iron ores should be restricted to 700–600 Ma sequences in the Uppermost Allochthon of the Norwegian Caledonides.

Petrogenesis of the Anorthosite Dyke Swarm of Tromso, North Norway: Experimental Evidence for Hydrous Anatexis of an Alkaline Mafic Complex

The 456 ± 4 Ma Skattøra migmatite complex in the north Norwegian Caledonides consists of migmatitic nepheline-normative metagabbros and amphibolites that are net-veined by numerous nepheline-normative anorthositic and leucodioritic dykes. Plagioclase (An20–50) is the dominant mineral (85–100%) in the dykes and the leucosome, but amphibole is generally present in amounts up to 15%. The following observations strongly suggest formation of the anorthositic magma by anatexis of the surrounding gabbro in the presence of an H2O-bearing fluid phase: (1) the migmatites have plagioclase-rich (anorthositic) leucosomes and amphibole-rich restites; (2) crystallization of amphibole in the anorthositic and leucodioritic dykes suggests high H2O activity; (3) the presence of coarse-grained to pegmatitic dykes and miarolitic cavities indicates a fluid-rich magma; (4) hydration zones that surround many anorthosite dykes suggest that the magma probably expelled H2O-rich fluids during crystallization. Water-saturated melting experiments at 0·5–1·5 GPa and temperatures from 800 to 1000°C have been performed on a nepheline-normative gabbro to test the proposed petrogenesis of the Skattøra anorthosites. The glasses produced close to the solidus are tonalitic in composition, but they become richer in plagioclase at higher temperatures. At and below 1·0 GPa, the residues are composed of amphibole. Experiments above 1·0 GPa produced residual garnet and/or zoisite in addition to amphibole, suggesting that the anorthositic dykes in the Skattøra migmatite complex formed below 1·25 GPa. The experiments show that the high Na2O content of the anorthosite dykes can only be produced if Na is added to the charges. The glass that best fits the composition of the Skattøra dykes was produced at 1·0 GPa and 900°C with 2 wt % Na(OH) added.

Crustal melting in the lower parts of island arcs: an example from the Bremanger Granitoid Complex, west Norwegian Caledonides

The 440±5 Ma Bremanger Granitoid Complex (BGC), west Norwegian Caledonides, comprises tonalites, granodiorites and minor granites. The BGC is coeval with a nearby subduction-related gabbronoritic/dioritic intrusion (the 443±4 Ma Gasoy Intrusion) and an ophiolite complex generated in a back-arc basin (the 443±3 Ma Solund-Stavfjord Ophiolite Complex). The BGC has a restricted isotopic variation (⊂Nd440Ma=+0.04 to –2.21, 87Sr/86Sr440 Ma=0.7045 to 0.7054) indicating a substantial mantle component, but it is enriched in crustal material relative to the primitive parts of the gabbronoritic rocks of the Gasoy Intrusion (⊂Nd440 Ma=+1.67, 87Sr/86Sr440 Ma=0.7048). The chondrite-normalised REE patterns are fractionated [(La/Yb)N ~8 to ~16], smooth or with negative or positive Eu anomalies. Geochemical modelling suggests that the BGC does not represent residual melts produced by fractional crystallisation of basic magma, but is compatible with partial melting of mafic source rocks with various amounts of H2O. Residual garnet is suggested by HREE depletion, indicating a minimum pressure of 9–10 kbar during melting.