Caledonian Metamorphism Research Articles

Remagnetisation of the Caerfai Group (lower Cambrian, SW Wales) in hot geothermal fluids during Caledonian (pre-Acadian) metamorphism

Palaeomagnetic data from the Lower Cambrian red beds of the Caerfai Group in Pembrokeshire, South Wales have been revisited. Original studies all produced closely similar directional data with the combined results yielding a palaeomagnetic pole at 21.8oS; 355.5°E corresponding to a time of ~430 Ma when compared with the APWP of Stable Europe and Baltica. The results of this study indicate that remagnetisation occurred during Caledonian metamorphism, prior to Acadian deformation in the area. The magnetic remanence is carried by fine grained hematite distributed throughout the sequence and which appears to have been acquired during the duration of a single chron. This time zone is traditional early Silurian (late Llandovery) and corresponds to the docking of Baltica and east Avalonia with the Laurentian continent. The proposed age of the remagnetisation event is consistent with radiometric ages from epizonal authigenic illites in overlying Middle Cambrian bentonites at Porth-y-Rhaw and elsewhere in the Welsh Basin and other parts of Avalonia including Charnwood block and Brabantia. The Caerfai Group comprises texturally immature litharenites with a mineral assemblage including chlorite, biotite, muscovite, graphite, and epidote consistent with a greenschist facies source area which remains unidentified. A key component of the Caerfai mineral assemblage are magnetite crystals (typically 30–60 μm) deposited during contemporaneous volcanic activity. Deep burial during the drift of Avalonia, indicated by clay transformations and overpressure in the Caerfai Bay Mudstone, was associated with the alteration of iron-bearing minerals and the dissolution of magnetite. During collision of Avalonia/Baltica with Laurentia a geothermal cell with epizonal metamorphic conditions (>300 °C) and fluids enriched in iron, probably in the form of iron chloride complexes was established. The remagnetisation event was triggered when the geothermal cell was cooled, most likely by the infiltration of meteoric water, and the reaction of iron chloride with water produced the widespread precipitation of fine-grained hematite and the formation of a chemical remanent magnetisation (CRM).

Early Neoproterozoic magmatism and Caledonian metamorphism recorded by the Mårma terrane, Seve Nappe Complex, northern Swedish Caledonides

Petrology, geochronology and bulk-rock chemistry are combined to investigate the early Neoproterozoic magmatism and Cambrian–Ordovician metamorphism in the northern Swedish Caledonides. This work includes several lithologies of the Mårma terrane in the Seve Nappe Complex exposed in the Kebnekaise region. U–Pb zircon geochronology yielded crystallization ages of 835 ± 8 Ma for a mylonitic orthogneiss, 864 ± 3 Ma for the Vistas Granite and 840 ± 7 Ma for an intruded granitic dyke, whereas a gabbro and a granodiorite intrusion gave U–Pb zircon crystallization ages of 856 ± 3 Ma and 850 ± 1 Ma, respectively. U–Pb monazite dating of the mylonitic orthogneiss gave an upper intercept age of 841 ± 7 Ma and a lower intercept age of 443 ± 20 Ma. Pressure–temperature estimates derived for the mylonitic orthogneiss reveal metamorphic peak-pressure and peak-temperature of 10.5–11.75 kbar and 550–610°C and 7.4–8.1 kbar at 615–675°C, respectively. Metamorphic pressure–temperature estimates for the Vistas Granite yield 6.5–7.5 kbar at 600–625°C. Whole-rock chemistry coupled with U–Pb geochronology indicates that bimodal magmatism was related to attempted rifting of Rodinia between 870 and 840 Ma. Supplementary material: Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6675075 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Xenotime reveals Caledonian hydrothermal overprint on Neoproterozoic Cu mineralization, East Greenland

Constraining the age of many types of ore deposits remains challenging because of the lack of radiogenic isotopes incorporated into common ore-forming minerals. The timing of pre-Caledonian-hosted Cu mineralization along the entire c. 1200 km long East Greenland Caledonides remains virtually unknown, hampering our knowledge of ore deposit timing and genesis in a frontier exploration region. Here, automated mineral analysis of a series of nodular, disseminated and vein-hosted Cu- ± Pb-mineralized metasedimentary rocks in central East Greenland reveals detrital zircon and hydrothermal xenotime, both amenable to U–Pb geochronology. Detrital zircon geochronology of a co-deposited quartzite reveals an age distribution highly similar to the Cryogenian ( c. 700 Ma) upper Eleanore Bay Supergroup. Hydrothermal xenotime U–Pb analyses adjacent to nodular and disseminated chalcocite across three proximal samples have variable amounts of common Pb that together yield a well-defined single discordia with a lower concordia intercept of 438 ± 13 Ma (2 σ ). This age is within uncertainty of the onset of Caledonian regional metamorphism and granitoid production and clearly post-dates deposition of the upper Eleanore Bay Supergroup by several hundred million years. Considering a published chalcocite Pb–Pb isochron age of 680 ± 65 Ma, the hydrothermal xenotime U–Pb ages imply that Caledonian-driven fluid activity, sourced from metamorphic reactions or from granitoids, remobilized diagenetic Cu and Pb mineralization. Chalcocite Pb–Pb isotopes show that dissolved and reprecipitated portions are volumetrically minor, radiogenic and Pb-poor, implying that fluids stripped most of the Pb from the system. Thus, it is likely that remobilization was localized on the grain scale, although some Cu and Pb was transported away from diagenetic sites, perhaps into veins. Although Caledonian metamorphism and granitoid emplacement is widespread in central East Greenland, the full extent of their roles in upgrading Cu mineralization remains to be ascertained. Supplementary material: Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6675384 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Establishing Provenance from Highly Impoverished Heavy Mineral Suites: Detrital Apatite and Zircon Geochronology of Central North Sea Triassic Sandstones

A study of Triassic sandstones in the central North Sea, UK, has shown that combined detrital zircon and apatite geochronology and apatite trace element analysis is a powerful tool for reconstructing provenance for sandstones with diagenetically impoverished heavy mineral suites. Sandstones in the earlier part of the succession (Bunter Sandstone Member and Judy Sandstone Member) have characteristics that indicate derivation from Moinian–Dalradian metasediments affected by Caledonian tectonothermal events, in conjunction with a Palaeoproterozoic-Archaean source unaffected by Caledonian metamorphism. Palaeogeographic reconstructions indicate that the sediment cannot have been input directly from either of these cratonic areas. This, in conjunction with the presence of common rounded apatite, indicates that recycling is the most likely possibility. The zircon-apatite association in the younger Joanne Sandstone Member sandstones indicates derivation from lithologies with mid-Proterozoic zircons (either crystalline basement or metasediments in the Caledonian Nappes), subjected to Caledonian metamorphism to generate early Palaeozoic apatites. This combination is compatible with a source region in southern and western Norway. The low degree of textural maturity associated with the detrital apatite, together with the unimodal Caledonian age grouping, indicates the Joanne sandstones have a strong first-cycle component.

Palaeomagnetic investigations across Hinlopenstretet border zone: from Caledonian metamorphosed rocks of Ny Friesland to foreland facies of Nordaustlandet (NE Svalbard)

New demagnetization results of 286 palaeomagnetic specimens from 15 sites in Neoproterozoic–Lower Paleozoic sequences of Ny Friesland–Nordaustlandet (the Caledonian Eastern Svalbard Terrane) are presented. New in situ 40 Ar/ 39 Ar ultraviolet mass spectrometry (Uv-MS) age determinations on amphibolites and mylonites from the Ny Friesland cluster in the 340–460 Ma interval correspond broadly to the Caledonian thermal event. Older ages are observed but are more scattered and probably reflect partial resetting or argon gain of Proterozoic micas. Palaeomagnetic, petrological and structural data confirm a west to east decrease in the degree of the Caledonian metamorphism and remagnetization across Hinlopenstretet, which is interpreted here to coincide with the limit of Caledonian east-directed overthrusting. No pre-Caledonian palaeomagnetic record was identified in investigated amphibolites and metacarbonates of Ny Friesland. Pre-folding pre-Caledonian, potentially Neoproterozoic (primary?) magnetization was recognized only in non-metamorphosed Murchisonfjorden (Nordaustlandet) carbonates. However, selective palaeomagnetic palaeopoles from Murchisonfjorden also fall into Caledonian sensu lato and Mesozoic sectors of the Laurussia reference path. This also indicates that regions east of Hinlopenstretet and east of the Caledonian front could be partially subjected to Caledonian remagnetization. Mesozoic palaeomagnetic overprints are potentially related to the injection of Mesozoic dolerites in eastern Svalbard. Here, we discuss two concepts of post-Caledonian rotations that could affect the palaeomagnetic record of the Eastern Svalbard Terrane: rotations around vertical axes related to strike-slip faults and rotations around horizontal axes related to normal listric faults that originated during extension in the Devonian basin and the opening of the Atlantic and Arctic oceans. Supplementary material: (1) A detailed description of applied rock-magnetic and palaeomagnetic procedures, (2) detailed description of investigated geochronological samples, (3) in situ Uv-MS 40 Ar/ 39 Ar isotopic age determination results, (4) mineralogy of ferromagnetic phases, (5) hysteresis parameters, (6) distribution of the NRM components and great circles in palaeomagnetic sites and (7) anisotropy of magnetic susceptibility are available at https://doi.org/10.6084/m9.figshare.c.6132018

Timing of deformation, metamorphism and leucogranite intrusion in the lower part of the Seve Nappe Complex in central Jämtland, Swedish Caledonides

ABSTRACT Recent studies in the context of the International Continental Scientific Drilling Project “Collisional Orogeny in the Scandinavian Caledonides” have focused on the importance of the Seve Nappe Complex (SNC) for understanding the subduction history of the Baltoscandian margin during closure of the Iapetus Ocean. In the classical Åre area of western central Jämtland, granulite facies migmatites and leucogranites of the Åreskutan Nappe provide evidence of Early Silurian (c. 440 Ma) high temperature metamorphism and a previous prograde, ultra-high pressure history, with microdiamonds. New LA-ICPMS zircon isotope age investigations of the underlying amphibolite facies Lower Seve Nappes, reported here, have also identified an Early Silurian tectonothermal history with pegmatitic leucogranite (c. 443 Ma) and, at lower structural levels, another felsic intrusion of earliest Middle Ordovician age (c. 469 Ma). The latter intrudes isoclinally folded host rock amphibolites and calcareous psammitic paragneisses and is itself tightly folded. Zircons in an amphibolite proved to be highly discordant but indicate Early Silurian metamorphism during isoclinal folding. Detrital zircons in a paragneiss are dominated by Sveconorwegian populations, but also include a range of younger Neoproterozoic grains down to the Early Ediacaran (c. 600 Ma). This new evidence of early Caledonian deformation and metamorphism indicates that, as farther north in the orogen, the Seve tectonothermal history in central Jämtland probably started early in the Ordovician, or before. Subduction and accretion along the Baltoscandian outer margin occurred prior to Scandian continent-continent collision, with Siluro-Devonian emplacement of the SNC across the foreland basins onto the Baltoscandian platform.

Structural and metamorphic inheritance controls strain partitioning during orogenic shortening (Kalak Nappe Complex, Norwegian Caledonides)

The occurrence of pre-collisional structural and metamorphic fabrics may control the development of new structures during subsequent orogenic deformation. Structural, petrological and geochronological analyses have been performed on selected samples collected along a NW-SE cross section of the Kalak Nappe Complex (KNC) exposed on Kvaløya Island (Finnmark, Norway), in order to define pre-Caledonian or Caledonian affinity of deformation fabrics. Nappes within the KNC experienced different pre-collisional tectonometamorphic histories, resulting in contrasting pre-Caledonian fabrics, which in turn controlled orogen-scale strain partitioning and metamorphic re-equilibration during Caledonian shortening. Caledonian deformation during top-to-SE-directed thrusting occurred at 550–675 °C and 0.8–1.0 GPa in the presence of fluids. Suitably-oriented pre-collisional fabrics were firstly exploited as zones of localized shearing internal to the KNC and subjected to metamorphic re-equilibration during shortening. Fold geometry during Caledonian thrusting was also controlled by the orientation of pre-Caledonian fabrics. SE-verging asymmetric folds were developed after minor tilting of pre-Caledonian upright folds with orogen-parallel hinge in the hinterland consistently with top-to-SE shearing. Shear-parallel folds displaying orogen-perpendicular hinge lines resulted from top-to-SE general shearing of pre-collisional upright folds showing pre-collisional orogen-perpendicular hinge lines. Caledonian metamorphism appears to have been accompanied by infiltration of radiogenic 40Ar-rich fluids, which affected the Ar isotopic system in synkinematic micas.

Caledonian metamorphism of metasediments from Franz Josef Land

ABSTRACTThe petrography, detrital zircon age distribution and 40Ar–39Ar ages of three samples from the Nagurskaya drill core of westernmost Franz Josef Land have been analyzed and are compared to similar rocks from the Lomonosov Ridge. The analyzed rocks, from near the base of the drill hole, consist of fine-grained psammitic to semipelitic schists, metamorphosed under greenschist-facies conditions. They are lithologically very similar to the recently analyzed metasediments from the Lomonosov Ridge. The detrital zircon age spectra of the samples from both the Lomonosov Ridge and beneath Franz Josef Land span the Meso- to Palaeoproterozoic with a main peak around 1.6 Ga, similar to lower Neoproterozoic metasedimentary rocks in East Greenland, on Svalbard and northern Norway, as well as from Cambrian sediments in Estonia and Silurian–Devonian sediments on Novaya Zemlya. Biotite in metasedimentary rocks from the Nagurskaya drill core indicate an Early Devonian 40Ar–39Ar metamorphic age of c. 400 Ma, possibly superimposed on an earlier, Ordovician history similar to that of the metasediments from the Lomonosov Ridge at 470 Ma. Previously published 40Ar–39Ar analyses of Nagurskaya muscovite yielded c. 600 Ma ages, characteristic of the Timanian Orogen. Together with the new biotite data, these isotope ages favor the interpretation that the Caledonian suture is located between Svalbard and Franz Josef Land, and the Caledonian deformation front between the latter and Novaya Zemlya.

Rare earth elements and Sm-Nd isotope redistribution in apatite and accessory minerals in retrogressed lower crust material (Bergen Arcs, Norway)

In the Bergen Arcs (Norway), Grenvillian granulite retrogressed under eclogite- and amphibolite-facies conditions during Caledonian subduction/collision offer a unique opportunity to investigate rare earth elements (REE) and Sm-Nd isotope redistribution in accessory minerals during fluid-assisted metamorphism. Our sampling targeted apatite-bearing REE-rich protoliths (mangerite and jotunite) that preserve distinct mineral assemblages, depending on the external fluid availability and metamorphic conditions. REE concentrations in apatite are the highest in the granulite. Two populations are present: magmatic apatite (Ap1) relics that occur as inclusions in ilmenite-hematite, and intergranular apatite (Ap2) formed under granulite-facies conditions. The presence of abundant needle-like monazite and sulphide inclusions in Ap2 indicate that granulite reactions were fluid assisted. A thin (typically <10μm) rim of REE-rich epidote (Ep1) commonly surrounds Ap2. In these accessory minerals, U and Th contents are too low, or grains are too small, for in situ U-Th-Pb dating. Sm-Nd isotope data of Ap2, monazite and Ep1 give an isochron age of 601±69Ma, which is interpreted to represent a partially reset Grenvillian age, affected by Caledonian fluid-assisted mineral growth. In amphibolitized samples, granulite Ap2 is replaced by apatite (Ap3) with lower REE contents and no monazite inclusions. The REE released by this replacement are redistributed in a corona of epidote group minerals (Ep2) surrounding Ap3. The in situ Sm-Nd isotope data for Ep2 and titanite, found in replacement of ilmenite-hematite, return an isochron age of 395±65Ma, recording the timing of amphibolite-facies mineral growth when fluids were introduced into the rock. In eclogitized samples, eclogitic apatite (Ap4) occurs as polycrystalline aggregates, suggesting for a complex replacement process during deformation. REE contents of Ap4 are low, as REE originally contained in the precursor apatite were redistributed mainly into zoisite. Apatite shielded as inclusions in ilmenite and garnet preserve the REE-rich signature of the initial magmatic (Ap1) and granulite (Ap2) apatite, indicating these grains did not undergo further re-equilibration during Caledonian metamorphism. The resistance of apatite to compositional re-equilibration in this case confirms the petrological potential of apatite inclusions shielded in chemically inert minerals to track early magmatic, or metamorphic, crystallisation stages.

The evolution of Eastern Tornquist-Paleoasian Ocean and subsequent continental collisions: A case study from the Western Tatra Mountains, Central Western Carpathians (Poland)

The crystalline basement of the Tatra Mountains in the Central Western Carpathians, forms part of the European Variscides and contains fragments of Gondwanan provenance. Metabasite rocks of MORB affinity in the Tatra Mountains are represented by two suites of amphibolites present in two metamorphic units (the Ornak and Goryczkowa Units) intercalated with metapelitic rocks. They are interpreted as relics of ocean crust, with zircon δ18OVSMOW values of 4.97–6.96‰. Zircon REE patterns suggest oxidizing to strongly oxidizing conditions in the parent mantle-derived basaltic magma. LA-MC-ICP-MS U-Pb dating of magmatic zircon cores yields a crystallization age of c. 560Ma, with inherited components at c. 600Ma, corresponding to the Pannotia break-up event and to the formation of the Eastern Tornquist–Paleoasian Ocean.However, the zircon rims of both suites yield evidence for two different geological histories. Zircon rims from the Ornak amphibolites record two overgrowth phases. The older rims, dated at 387±8Ma are interpreted as the result of an early stage of Variscan uplift while the younger rims dated at 342±9Ma are attributed to late Variscan collisional processes. They are characterized by high δ18OVSMOW values of 7.34–9.54‰ and are associated with migmatization related to the closure of the Rheic Ocean.Zircon rims from the Goryczkowa amphibolites yield evidence of metamorphism at 512±5Ma, subsequent Caledonian metamorphism at 447±14Ma, followed by two stages of Variscan metamorphism at 372±12Ma and 339±7Ma, the latter marking the final closure of the Rheic Ocean during late-Variscan collision.The presented data are the first direct dating of ocean crust formation in the eastern prolongation of the Tornquist Ocean, which formed a probable link to the Paleoasian Ocean.

The ore potential of saprolites and variably ferruginised associated lithologies on Fosen Peninsula, Trondheimsfjord area, Central Norway

Results of a regional-scale lithogeochemical study of 57 saprolites and variably altered, spatially associated lithologies of a metavolcano-sedimentary succession in the Fosen-Trondheimsfjord area, Central Norway, are presented. Analyses of 53 elements after aqua regia digestion revealed Pb, Cu, Zn, Mo, As, Au, Ag, Bi, Cd, Mn, Sb, Se and V median concentrations in saprolite samples to be above those of spatially associated greenschist- to amphibolite-facies metabasite, gneiss and schist. Single analyses of saprolites are as high as 1055ppb Ag, 87ppm Pb and 8.8ppm Se. These metal concentrations are subeconomic and below concentrations of geochemically fresh samples of known ore deposits in the Trondheimsfjord and adjacent areas. Based on the absence of clay minerals in the saprolites and other signs of weathering other than sulphide dissolution, supergene metal enrichment beneath the ferruginised zones of the Fosen-Trondheimsfjord area is unlikely to have occurred. Instead, saprolites and strongly ferruginised rocks of the study area are interpreted to be the product of superficial alteration and weathering of locally abundant primary pyrite in metabasite and pyrrhotite in metasiliciclastic horizons of the supracrustal lithologies. Distinct element associations in the various rock types are identified by cluster and robust principal component analyses, which may indicate that primary compositional features have been retained during the pervasive Caledonian metamorphism.

The Baklia Fault Zone – a regional strike-slip zone splitting Prins Karls Foreland (Svalbard). [abstract

The Baklia Fault Zone – a regional strike-slip zone splitting Prins Karls Foreland (Svalbard). [abstract

Formation and evolution of the Høgtuva beryllium deposit, Norway

Metagranite in the Hogtuva tectonic window, Nordland, Norway, hosts several spatially restricted Be mineralisations with phenakite as the main ore mineral. Geochronology of magmatic zircon (zircon-I) indicates crystallisation of phenakite-bearing and beryl-free metaluminous granite at 1787 ± 57 Ma and of phenakite- and beryl (beryl-I)-bearing peraluminous aplites and pegmatites at 1710 ± 59 Ma. Crystallisation of texturally distinct zircon (zircon-II) cogenetic with fluorite in peraluminous aplites and pegmatites occurred at 434 ± 14 Ma during Caledonian metamorphism in Be-mineralised and barren metagranite. Breakdown of phenakite post-dates growth of zircon-II and resulted in the crystallisation of metamorphic hogtuvaite (Ca2 $${\text{Fe}}_{3}^{2 + } {\text{Fe}}_{3}^{3 + }$$ Si4BeAlO20) in metaluminous metagranite and metamorphic beryl (beryl-II) in peraluminous metamorphosed aplites and pegmatites. Qualitative and quantitative assessment of mineral reactions implies that Be has not been mobilised to significant degrees during metamorphism, in spite of the presence of a fluorine-rich fluid, and that phenakite underwent in situ metamorphic reactions. Hogtuvaite crystallised as a metamorphic phase at the expense of phenakite, the anorthite component in albitic plagioclase, magnetite and possibly clinopyroxene. In peraluminous environments, the breakdown of phenakite to metamorphic beryl-II is accompanied by the formation of albite. A decreasing activity ratio of a(K+/H+) in the metamorphic fluid results in the reaction of K-feldspar and phenakite to beryl-II. Further decrease in a(K+/H+) finally results in complete transformation of the remaining K-feldspar into albite. The metamorphic breakdown of phenakite in the distinct Be-mineralisation types at Hogtuva is probably controlled by changes in the activities of Na+, K+ and H+ in the coexisting metamorphic fluid.

Behaviour of geochronometers and timing of metamorphic reactions during deformation at lower crustal conditions: phase equilibrium modelling and U–Pb dating of zircon, monazite, rutile and titanite from the Kalak Nappe Complex, northern Norway

AbstractThis study investigates the behaviour of the geochronometers zircon, monazite, rutile and titanite in polyphase lower crustal rocks of the Kalak Nappe Complex, northern Norway. A pressure–temperature–time–deformation path is constructed by combining microstructural observations with P–T conditions derived from phase equilibrium modelling and U–Pb dating. The following tectonometamorphic evolution is deduced: A subvertical S1 fabric formed at ~730–775 °C and ~6.3–9.8 kbar, above the wet solidus in the sillimanite and kyanite stability fields. The event is dated at 702 ± 5 Ma by high‐U zircon in a leucosome. Monazite grains that grew in the S1 fabric show surprisingly little variation in chemical composition compared to a large spread in (concordant) U–Pb dates from c. 800 to 600 Ma. This age spread could either represent protracted growth of monazite during high‐grade metamorphism, or represent partially reset ages due to high‐T diffusion. Both cases imply that elevated temperatures of &gt;600 °C persisted for over c. 200 Ma, indicating relatively static conditions at lower crustal levels for most of the Neoproterozoic. The S1 fabric was overprinted by a subhorizontal S2 fabric, which formed at ~600–660 °C and ~10–12 kbar. Rutile that originally grew during the S1‐forming event lost its Zr‐in‐rutile and U–Pb signatures during the S2‐forming event. It records Zr‐in‐rutile temperatures of 550–660 °C and Caledonian ages of 440–420 Ma. Titanite grew at the expense of rutile at slightly lower temperatures of ~550 °C during ongoing S2 deformation; U–Pb ages of c. 440–430 Ma date its crystallization, giving a minimum estimate for the age of Caledonian metamorphism and the duration of Caledonian shearing. This study shows that (i) monazite can have a large spread in U–Pb dates despite a homogeneous composition; (ii) rutile may lose its Zr‐in‐rutile and U–Pb signature during an amphibolite facies overprint; and (iii) titanite may record crystallization ages during retrograde shearing. Therefore, in order to correctly interpret U–Pb ages from different geochronometers in a polyphase deformation and reaction history, they are ideally combined with microstructural observations and phase equilibrium modelling to derive a complete P–T–t–d path.

New palaeomagnetic data from metamorphosed carbonates of Western Oscar II Land, Western Spitsbergen

* corresponding author Abstract: Palaeomagnetic−petrographic−structural analyses of Proterozoic-Lower Palaeozoic metamorphosed carbonates from 12 locations within Oscar II Land (Western Spitsbergen) have been carried out to determine their usefulness in palaeogeographic reconstructions for Caledonian time. Structural analyses confirm that metacarbonates record several stages of deformation: D1, D2 ductile phases to Caledonian metamorphism and a D3 brittle phase to Late Cretaceous-Paleogene evolution of the West Spitsbergen Fold Belt. The latter is represented by thrust faults, localized folds with strain slip cleavages and late extensional collapse. Petrographic investigations reveal that Caledonian greenschist facies metamorphism was characterized by the high activity of H2O−CO2−rich fluids which pro− moted extensive recrystallization and within−rock spatial reorganization of sampled meta− carbonates. Microscopic, SEM and microprobe analyses exclude the existence of any pri− mary pre−metamorphic ferromagnetic minerals (primary−related to sedimentation and or early diagenesis) and point to metamorphic 4C superstructure (Fe7S8) as the main ferromagnetic carrier in investigated rocks. This is confirmed by the three−component iso− thermal remanent magnetization (IRM) procedures and the results of thermal demagnetiza− tions. In 12 sites a total number of 72 independently oriented palaeomagnetic samples were collected from which 181 specimens were drilled and thermally demagnetized. Sampled metacarbonates are weakly magnetized (NRM <0.2mA/m). The statistically significant palaeomagnetic results were achieved only from 1 of 12 investigated sites. In one site situ− ated in the Western overturned limb of the Holmesletfjellet Syncline intermediate unblock− ing temperatures - pyrrhotite related component WTSJ5M superimposed on the S1 Cal− edonian schistosity was recognized (D = 100.7, I = −21.495% = 5.5, = 58.23). Coinci− dence of WTSJ5M with Silurian-Devonian sector of the Baltica reference path after unfold− ing of the syncline by the angle of 130 suggests synfolding origin of this direction. Further, this suggests that Holmesletfjellet Syncline originated as an open fold and has been trans− formed into an overturned syncline during the Late Caledonian shortening or in the Late Cretaceous-Palaeogene time.

Age of alkaline rocks in the Seiland Igneous Province, Northern Norway

Abstract: Spatially associated carbonatite and syenite complexes at Breivikbotn on the island of Sørøy, in the Caledonian nappes of northern Norway, have been dated by isotope dilution thermal ionization mass spectrometry U–Pb analysis on zircon and titanite. Zircon crystals in malignite and silico-carbonatite contain many inclusions of Th- and U-rich minerals and have been locally affected by dissolution–reprecipitation processes, all features that tend to cause disturbed U–Pb systems. Some alkalic gneisses also exhibit evidence of new zircon growth during the Caledonian metamorphism. The carbonatite was emplaced into the Kalak Nappe Complex at 574 ± 5 Ma, and various alkaline gneisses and nepheline syenite dykes intruded the neighbouring Breivikbotn Gabbro and its metasedimentary host between 580 and 560 Ma. The alkaline rocks were coeval with, and probably genetically related to major mafic and ultramafic magmatism throughout the Seiland Igneous Province, preceding the emplacement of nepheline syenite pegmatites at 530–520 Ma. The ages of the Seiland Igneous Province indicate formation in an extensional setting, the multistage history of the host Kalak Nappe Complex pointing to a potential origin as an Avalonian-type peri-Gondwanan terrane.

Petrology of coronite from the Bergen Arcs Complex, Norway

Petrologic examination of coronites from the Bergen Arcs Complex in Norway revealed that garnet crowns formed due to clinopyroxene interaction with matrix plagioclase and spinel during the Grenville granulite-facies metamorphism (at T ∼ 960°C and P = 1.3 GPa). Along with this, the rocks show evidence of reactions related to superimposed Caledonian eclogite-facies metamorphism. These are microscopic coronas consisting of omphacite, kyanite, corundum, amphibole, and biotite. The rims formed under aqueous conditions with potassium introduction ata T ∼ 710–730°C and P ∼ 1.3–1.5 GPa. Local occurrence of eclogite metamorphism found at a great distance (>100 m) from shear zones of the eclogite metamorphic stage indicates that the whole eclogite succession and not only its local sites (shear zones) were heated to the eclogite-metamorphism temperature.

U–Pb systematics of zircon and titanite from the Gardnos impact structure, Norway: Evidence for impact at 546 Ma?

Zircon and titanite were investigated in impactites of the Gardnos structure, a crater formed in Sveconorwegian (ca. 1 Ga) crust, which was then overridden in the Devonian by Caledonian nappes. Observed deformation features in zircons are granular texture, planar microstructures, and likely the incorporation of organic carbon during impact causing black staining of the zircon grains. The grains were studied by scanning electron microscopy (SEM) and cathode luminescence (CL) and dated by U–Pb isotope dilution - thermo-ionization mass spectrometry (ID-TIMS). Zircon grains without impact related features have U–Pb data showing moderate discordance (5–13%) and indicating formation ages mostly in the range of 1600–1000 Ma, except detrital zircon ages as old as >2481 Ma, reflecting the diversity of target rocks in the area. Titanite with concordant ages of 995–999 Ma dates metamorphism during final juxtaposition of the Telemarkia on the Idefjorden terrane to the east. Zircon grains with demonstrated or presumed shock features yield highly discordant (14–40%) U–Pb data, with a majority of them plotting along an array with a lower intercept of about 340 Ma reflecting the influence of the Caledonian orogeny and recent Pb-loss. One zircon grain was totally reset at 379 Ma during late Caledonian metamorphism, which also caused local growth of new titanite. A specific group of zircon grains yields data with relatively high discordance for moderate U contents, and five of these analyses, including that of a grain with proven granular or aggregate texture, fit a discordia line with an upper intercept of 546 ± 5 Ma. These features are interpreted as indicating zircon break-down to an amorphous state during impact, with subsequent recrystallization into microcrystalline aggregates causing extensive to complete Pb loss. We further suggest that their crystallinity prevented Pb loss during the Caledonian orogeny, while the small subgrain size and increasing metamictisation allowed more recent disturbances. We thus interpret the 546 Ma age as the approximate time of impact.

Fluid evolution in base‐metal sulphide mineral deposits in the metamorphic basement rocks of southwest Scotland and Northern Ireland

AbstractThe Dalradian and Ordovician–Silurian metamorphic basement rocks of southwest Scotland and Northern Ireland host a number of base‐metal sulphide‐bearing vein deposits associated with kilometre‐scale fracture systems. Fluid inclusion microthermometric analysis reveals two distinct fluid types are present at more than half of these deposits. The first is an H2O–CO2–salt fluid, which was probably derived from devolatilization reactions during Caledonian metamorphism. This stage of mineralization in Dalradian rocks was associated with base‐metal deposition and occurred at temperatures between 220 and 360°C and pressures of between 1.6 and 1.9 kbar. Caledonian mineralization in Ordovician–Silurian metamorphic rocks occurred at temperatures between 300 and 360°C and pressures between 0.6 and 1.9 kbar. A later, probably Carboniferous, stage of mineralization was associated with base‐metal sulphide deposition and involved a low to moderate temperature (Th 70 to 240°C), low to moderate salinity (0 to 20 wt% NaCl eq.), H2O–salt fluid. The presence of both fluids at many of the deposits shows that the fractures hosting the deposits acted as long‐term controls for fluid migration and the location of Caledonian metalliferous fluids as well as Carboniferous metalliferous fluids. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Caledonian low-temperature granulite-facies metamorphism in the West Kunlun orogenic belt—SHRIMP Chronological evidence from zircon

The West Kunlun orogenic belt is located at the conjunction of the paleo-Asian tectonic system and the Tethys tectonic system. Petrological and mineralogical studies of the Early Cambrian metamorphic surface crust in this region have shown that in case the metamorphism reached low-temperature granulate facies, the typical mineral assemblage is biotite-garnet-silimanite-K feldspar-plagioclase-quartz. The peak metamorphic temperatures are within the range of 720–740°C and the pressure is 0.6 GPa ±. Three types of metamorphic zircon have been detected in the metamorphic rocks: the complex inclusion-bearing type ; the early relic zircon inclusion-bearing type; and the inclusion-free type. SHRIMP age determination of these three types of metamorphic zircon have revealed that these zircons were formed principally during 400–460 Ma, indicating that pre-Cambrian metamorphic surface crust rocks underwent low-temperature granulite facies metamorphism during the Caledonian. In combination with the geological characteristics of this region, it is considered that when the oceanic basin was closed, there occurred intense intracontinental subduction (type A), bringing part of the Early Cambrian metamorphic basement in this region downwards to the lower crust. Meanwhile, there were accompanied with tectonic deformation at deep levels and medium- to high-grade metamorphism. This study provided important chronological and mineralogical evidence for the exploration of the evolutionary mechanism and process of the West Kunlun Early Paleozoic.