Evolution of Caledonian deformation fabrics under eclogite and amphibolite facies at Vårdalsneset, Western Gneiss Region, Norway

The Vårdalsneset eclogite situated in the Western Gneiss Region, SW Norway, is a well preserved tectonite giving information about the deformation regimes active in the lower crust during crustal thickening and subsequent exhumation. The eclogite constitutes layers and lenses variably retrograded to amphibolite and is composed of garnet and omphacite with varying amounts of barroisite, actinolite, clinozoisite, kyanite, quartz, paragonite, phengite and rutile. The rocks record a five‐stage evolution connected to Caledonian burial and subsequent exhumation. (1) A prograde evolution through amphibolite facies (T =490±63 °C) is inferred from garnet cores with amphibole inclusions and bell‐shaped Mn profile. (2) Formation of L>S‐tectonite eclogite (T =680±20 °C, P=16±2 kbar) related to the subduction of continental crust during the Caledonian orogeny. Lack of asymmetrical fabrics and orientation of eclogite facies extensional veins indicate that the deformation regime during formation of the L>S fabric was coaxial. (3) Formation of sub‐horizontal eclogite facies foliation in which the finite stretching direction had changed by approximately 90°. Disruption of eclogite lenses and layers between symmetric shear zones characterizes the dominantly coaxial deformation regime of stage 3. Locally occurring mylonitic eclogites (T =690±20 °C, P=15±1.5 kbar) with top‐W kinematics may indicate, however, that non‐coaxial deformation was also active at eclogite facies conditions. (4) Development of a widespread regional amphibolite facies foliation (T =564±44 °C, P<10.3–8.1 kbar), quartz veins and development of conjugate shear zones indicate that coaxial vertical shortening and sub‐horizontal stretching were active during exhumation from eclogite to amphibolite facies conditions. (5) Amphibolite facies mylonites mainly formed under non‐coaxial top‐W movement are related to large‐scale movement on the extensional detachments active during the late‐orogenic extension of the Caledonides. The structural and metamorphic evolution of the Vårdalsneset eclogite and related areas support the exhumation model, including an extensional detachment in the upper crust and overall coaxial deformation in the lower crust.     

Discovery of diamond in the Tromsø Nappe,Scandinavian Caledonides (N. Norway)

We report the first finding of diamond in crustal rocks from the Tromsø Nappe of the North Norwegian Caledonides. Diamond occurs in situ as inclusions in garnet from gneiss at Tønsvika near Tromsø. The rock is composed essentially of garnet, biotite, white mica, quartz and plagioclase, minor constituents include kyanite, zoisite, rutile, tourmaline, amphibole, zircon, apatite and carbonates (magnesite, dolomite, calcite). The microdiamond, identified by micro‐Raman spectroscopy, is cuboidal to octahedral in shape and ranges from 5 to 50 μm in diameter. The diamond occurs as single grains and as composite diamond + carbonate inclusions. Diamond vibration bands show a downshift from 1 332 to 1 325 cm^?1, the majority of Raman peaks are centred between 1 332 and 1 330 cm^?1 and all peaks exhibit a full width at half maximum between 3 and 5 cm^?1. Several spectra show Raman bands typical for disordered and ordered graphite (sp²‐bonded carbon) indicating partial transformation of diamond to graphite. The calculated peak P–T conditions for the diamond‐bearing sample are 3.5 ± 0.5 GPa and 770 ± 50 °C. Metamorphic diamond found in situ in crustal rocks of the Tromsø Nappe thus provides unequivocal evidence for ultrahigh pressure metamorphism in this allochthonous unit of the Scandinavian Caledonides. Deep continental subduction, most probably in the Late Ordovician and shortly before or during the initial collision between Baltica and Laurentia, was required to stabilize the diamond at UHP conditions.     

Pressure–temperature–time evolution of the Central East Greenland Caledonides: quantitative constraints on crustal thickening and synorogenic extension

Whereas geologists have known for three‐quarters of a century that there was significant crustal thickening in the central East Greenland Caledonides, the crucial role of extensional faulting during Caledonian orogenesis has only been recognized during the past decade. In this paper, new petrographic and thermobarometric observations are presented from migmatitic metasedimentary gneisses of the Forsblad Fjord region (c. 72.5°N). Samples of the Krummedal Sequence, collected from the footwall of the upper of two significant splays of the main extensional fault system in the region—the Fjord Region Detachment (FRD)—enable us to establish a relative sequence of metamorphism. Our pressure (P)–temperature (T) results imply a clockwise loop in P–T space. As recorded by mineral assemblages in the Krummedal gneisses, prograde metamorphism involved a net increase of c. 4 kbar and 250 °C, with peak conditions of c. 10.5 kbar at 785 °C. Early burial and heating was followed by near‐isothermal decompression of 4.5 kbar, a process which is attributed to roughly 18 km of tectonostratigraphic throw on the upper splay of the FRD. Combining data reported here with the published data, it is estimated that the approximate tectonostratigraphic throw along the lower splay of the FRD was c. 16 km. In situ U–Th–Pb‐monazite electron microprobe dating suggests that the earliest phase of metamorphism recorded in the Krummedal Sequence gneisses of Forsblad Fjord occurred during the Caledonian orogeny. Furthermore, the combination of our new data with existing conventional TIMS U‐Pb and ⁴⁰Ar/³⁹Ar data imply that: (1) movement along the uppermost splay of the FRD (c. 425–423 Ma) occurred at maximum time‐averaged slip‐rates equivalent to c. 9 mm of vertical displacement per year; and (2) that the final stages of metamorphism occurred prior to c. 411 Ma, although part of this denudation was likely accommodated on overlying extensional structures that may have been active more recently. There is close agreement between our data and results from the Krummedal Sequence north of the field area (72.5°?74°N), and rocks of the Smallefjord Sequence (75°?76°N) that are suggested to correlate with the Krummedal Sequence. This leads us to infer that the events recorded in the Forsblad Fjord region are of orogen‐scale significance.     

Reworking of deep-seated gabbros and associated contact metamorphosed paragneisses in the southeastern part of the Seiland Igneous Province, northern Norway

The Seiland Igneous Province of the North Norwegian Caledonides consists of a suite of deep-seated rift-related magmatic rocks emplaced into paragneisses during late Precambrian to Ordovician time. In the south-eastern part of the province, contact metamorphism of the paragneisses and later reworking of intrusives and associated contact aureoles have resulted in the development of three successive metamorphic stages. The contact metamorphic assemblage (M1) Opx + Grt + Qtz + Pl + Kfs + Hc + Ilm ± Crd is preserved in xenolithic rafts of paragneiss within metagabbro. Geothermobarometric calculations yield 930-960d? C and 5-6.5 kbar for the contact metamorphism. M1 was followed by cooling, accompanied by strong shearing, formation of the gneiss foliation and recrystallization at intermediate-P granulite facies conditions (M2). Stable M2 phases are Cpx + Opx + Pl +Ilm ± Hbl in metagabbro and Grt ± Sil ± Opx + Kfs + Qtz + Pl ± Bt + Ilm in host paragneiss. The M2 conditions are estimated to 700-750d? C and 5-7 kbar. A subsequent pressure increase is recorded in the M3 episode, which is associated with recrystallization in narrow ductile shear zones and secondary growth on M2 minerals. M3 is defined by the assemblages Grt + Cpx ± Opx + Pl + Ru + Qtz in metagabbro, and Grt ± Ky + Qtz + Pl ± Kfs + Bt + Ru in host paragneiss. M3 conditions are estimated to 650-700d? C and 8-10 kbar. The substantial pressure increase related to the M2 → M3 transition is interpreted to be a result of (early?) Caledonian overthrusting. Chemical zoning in cordierite and biotite suggest rapid cooling following the M3 event. The proposed P-T-t evolution implies that the tectonic evolution of the Seiland Igneous Province was long (at least 330 Ma) and complex and involved initial rifting and extension followed by crustal thickening and compression.     

P–T paths in the Handöl area, central Scandinavia: record of Caledonian accretion of outboard rocks to the Baltoscandian margin

The Seve–Köli Nappe Complex is widespread in the Scandinavian Caledonides and is composed of units representing parts of the Baltoscandian margin (Seve Nappes) now overlain by magmatic–sedimentary rocks (Köli Nappes) derived from west of this margin. The metamorphic evolution of Köli and Seve units has been studied in the Handöl area, central Scandinavian Caledonides, where a fragmented ophiolite with cover sequence in the lower Köli units is thrust over the higher grade Seve units. Thermobarometry constrains metamorphic conditions to 490–570° C/950–600 MPa, with a slight downwards increase in grade, for the lower Köli (Bunnerviken lens), 520–620° C/1000–600 MPa for the upper Seve (Täljstensvalen Complex), 630–740° C/750–650 MPa for the middle Seve (Snasahögarna Nappe) and 480–600° C/1150–1000 MPa for the lower Seve (Blåhammarfjället Nappe).
P–T paths during garnet growth have been constructed for all units, except the highest grade middle Seve. These paths record heating at the base of the Köli and cooling in the underlying Seve units. Pressure increase during garnet growth is indicated for all units leading to anticlockwise P–T paths in the Seve. The results imply thermal convergence with time for all units and spatial convergence in metamorphic grade in the Köli. It is suggested that the contrasting metamorphic histories on either side of the Seve–Köli boundary resulted from the emplacement of relatively colder Köli rocks on top of relatively hotter Seve rocks and that emplacement of structurally higher units contributed to the increase in pressure.     

Pressure-Temperature-Time Constraints for the Seve Nappe of the Singis-Tjuoltajaure area Central Norrbotten Caledonides,Sweden : Implications for Early Caledonian Marginal Baltica

Abstract

This study provides some of the first integrated P-T-t constraints for the evolution of the Finnmarkian tectonothermal event within the northern Scandinavian Caledonides. Samples from units within the Seve Nappe of the Singis-Tjuoltajaure region contain assemblages which allow the application of well calibrateted thermoharometers. New results include : I ) Eclogite grade rocks of the Aurek Assemblage yield temperatures and pressures in excess of 12 kb and 730° С. 2) Temperatures and pressures obtained for seven samples from the Savotjåkka Assemblage range from 571766” С and 8.9-13.6 kb. These pressures correspond to burial depths of approximately 30-45 km during the Finn-markian (190 Ma) for the outer margin of Baltica. when combined with ⁴⁰Ar/³⁹Ar data, uplift rates of .2-.4 mm/yr during the early Finnmarkian arc obtained, and 3) Within the Vidja Assemblage a pressure and temperature of 7.3 ± 1.7 kb and 616 ± 60° is obtained. These conditions are consistent with the interpretation of a late Finnmarkian (450 Ma) Intra-Seve juxtaposition of the Vidja and Aurek Assemblages after approximately 20-30 km of slow uplift from peak pressures recorded during the early Finnmarkian.

This study provides new constraints which need to be considered in future tectonic models. These constraints include : I ) The Savopakte Assemblage records high pressures and high temperatures during the Finnmarkian and 2) Finnmarkian uplift rates of .2-.4 mm/yr were likely for marginal Baltica after peak metamorphism. Rates of this magnitude may be accounted for solely by slow erosion and do not require (but do not preclude) more complex tectonic interpretations. A tectonic model provided by the Late Cenozoic thrust belts of the Apennine system of the mediterranean region, may lead to new insights into the Early Paleozoic evolution of the Scandinavian Caledonides.     

Eclogites and polyphase P–T cycling in the Caledonian Uppermost Allochthon in Troms, northern Norway

Eclogites in the Tromsø area, northern Norway, are intimately associated with meta-supracrustals within the Uppermost Allochthon of the Scandinavian Caledonides (the Tromsø Nappe Complex). The whole sequence, which includes pelitic to semipelitic schists and gneisses, marbles and calc-silicate rocks, quartzofeldspathic gneisses, metabasites and ultramafites, has undergone three main deformational/metamorphic events (D₁/M₁, D₂/M₂ and D₃/M₃). Detailed structural, microtextural and mineral chemical studies have made it possible to construct separate P–T paths for these three events. Chemically zoned late syn- to post-D₁ garnets with inclusions of Bt, Pl and Qtz in Ky-bearing metapelites indicate a prograde evolution from 636°C, 12.48 kbar to c. 720°C, 14–15 kbar. This latter result is in agreement with Grt–Cpx geothermometry and Grt–Cpx–Pl–Qtz geobarometry on eclogites and trondhjemitic to dioritic gneisses. Maximum pressures at c. 675°C probably reached 17–18 kbar based on Cpx–Pl–Qtz inclusions in eclogitic garnets, and Grt–Ky–Pl–Qtz and Jd–Ab–Qtz in trondhjemitic gneisses. Post-D₁/pre-D₂ decompressional breakdown of the high-P assemblages indicates a substantial drop in pressure at this stage. Inclusions and chemical zoning in syn- to post-D₂ garnets from metapelites record a second episode of prograde metamorphism, from 552°C, 7.95 kbar, passing through a maximum pressure of 10.64 kbar at 644°C, with final equilibration at c. 665°C, 9–10 kbar. The corresponding apparently co-facial paragenesis Grt + Cpx + Pl + Qtz in metabasites yields c. 635°C, 8–10 kbar. In the metapelites post-D₃, Grt in apparent equilibrium with Bt, Phe and Pl yield c. 630°C, 9 kbar. The D₁/M₁ and D₂/M₂ episodes are exclusively recorded in the Tromsø Nappe Complex and must thus pre-date the emplacement of this allochthonous unit on top of the underlying Lyngen Nappe, while the D₃/M₃ episode is common for the two units. A previously published Sm–Nd mineral isochron (Grt–Cpx–Am) on a partly retrograded and recrystallized ecologite of 598 ± 107 Ma represents either the timing of formation of the eclogites or the post-eclogite/pre-D₂ decompression stage, while a Rb–Sr whole rock isochron of an apparently post-D₁/pre-D₂ granite of 433 ± 11 Ma is consistent with a K–Ar age of post-D₁/pre-D₂ amphiboles from a retrograded eclogite of 437 ± 16 Ma which most likely record cooling below the 475–500°C isotherm after the M₃ metamorphism.     

Southwest Scandinavia, 40–15 kyr BP: palaeogeography and environmental change

Twelve palaeogeographical reconstructions illustrate environmental changes at the southwest rim of the Scandinavian Ice Sheet 40–15 kyr BP. Synchronised land, sea and glacier configurations are based on the lithostratigraphy of tills and intertill sediments. Dating is provided by optically stimulated luminescence and calibrated accelerator mass spectrometry radiocarbon. An interstadial sequence ca. 40–30 kyr BP with boreo‐arctic proglacial fjords and subarctic flora and occasional glaciation in the Baltic was succeeded by a Last Glacial Maximum sequence ca. 30–20 kyr BP, with the closure of fjords and subsequent ice streams in glacial lake basins in Kattegat and the Baltic. Steadily flowing ice from Sweden bordered the Norwegian Channel Ice Stream. A deglaciation sequence ca. 20–15 kyr BP indicates the transgression of arctic waters, retreat of the Swedish ice and advance of Baltic ice streams succeeded by a return to interstadial conditions. When ameliorated ice‐free conditions prevailed in maritime regions, glaciers advanced through the Baltic and when interstadial regimes dominated the Baltic, glaciers expanded off the Norwegian coast. The largest glacier extent was reached in the North Sea around 29 kyr BP, about 22 kyr BP in Denmark and ca. 18 kyr BP in the Baltic. Our model provides new data for future numerical and qualitative landform‐based models. Copyright © 2003 John Wiley & Sons, Ltd.     

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.     

Pull-apart formation and strike-slip partitioning in an obliquely divergent setting, Leka Ophiolite, Norway

The Leka Ophiolite Complex (LOC) is located on the island of Leka, Norway, and belongs to the Uppermost Allochthon of the Scandinavian Caledonides. The rocks of the adjacent mainland and most of the surrounding islands are basement gneisses and supracrustal rocks not related to the ophiolite complex. Paleostress analysis, gravity inversion, and regional geology support a fault-bounded rhombochasm geometry for the LOC. The paleostress inversions revealed two types of tensors, interpreted as small strains: (1) horizontal extension, generally E–W to NE–SW, and (2) horizontal extension in the same direction with an added component of perpendicular horizontal contraction. A strong positive gravity anomaly (25 mGal) is centered on Leka, and gravity inversion indicates that the LOC lies directly below its surface exposures with steep-sided walls and a flat bottom located at 7 km depth. The faults bounding the LOC probably initiated during postorogenic extension in the Scandinavian Caledonides. The faults are regional in scale and are parallel to other NE–SW trending en echelon faults along the Norwegian coastline and on the adjacent mainland.A pull-apart structure explains the down-dropping and subsequent preservation of the LOC, as it is surrounded by rocks from lower structural positions within the nappe stack. The paleostress directions from Leka support a sinistral component of shear along these faults. The gravity inversion is consistent with a fault-bounded geometry. This pull-apart structure, as uniquely recorded by the dense ophiolitic rocks, suggests that strike-slip partitioning was active in an obliquely divergent setting.