Northern Scandinavian Caledonides Research Articles

A hyperextension assemblage, imbricated in Archean–Paleoproterozoic crust, at the base of the Kalak Nappe Complex in the northern Scandinavian Caledonides

The break-up of continents generates dyke swarms, basins and hyperextension assemblages. Once incorporated into new orogens, the latter can provide crucial information about origins and tectonic processes. The Nålfjell Complex in the Caledonian Skillefjord Nappe of northern Norway has many of the characteristics of a hyperextension assemblage, notably the presence of solitary serpentinite bodies exposed by the exhumation of serpentinized mantle and now embedded in schists, amphibolites, mylonites, marbles and felsic gneisses. The Skillefjord Nappe is a lithologically diverse, imbricated and discontinuous allochthon. It consists of felsic gneisses and dykes dated by zircon U–Pb to c. 3100, 2940, 2830, 2510 and 1800–1750 Ma. These rocks yield titanite ages of 2810–2700, 1750, 1660–1590 and 430–420 Ma. Metagabbro intruded at 1995 Ma. The ages and evolution of the Skillefjord Nappe are distinct from those of the structurally higher Svaertholt Terrane (>1030 Ma sediments deformed and intruded by granite at 980–950 Ma) and the Sørøy Terrane (characterized by a multi-stage 900–500 Ma evolution). The assembly of these disparate elements was completed at 420 Ma. The timing of hyperextension is uncertain. The serpentinites have a primitive Pb isotopic composition indicating old depleted mantle lithosphere. Extensional processes from the Paleoproterozoic to Ediacaran are considered. Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/cc/caledonian-wilson-cycle

Late Neoproterozoic extended continental margin development recorded by the Seve Nappe Complex of the northern Scandinavian Caledonides

The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record the opening and closure of the Iapetus Ocean, culminating with the collision of Baltica and Laurentia. The Seve Nappe Complex (SNC) of the Scandinavian Caledonides comprises relics of the outermost Baltoscandian passive margin that were subducted to mantle depths during ocean closure. Subduction of these rocks overprinted much of the Neoproterozoic record for Iapetus Ocean formation, and as a result, much of the work conducted in the SNC has focused solely on the Caledonian orogenic history.In this study, we combined petrological and geochronological work to expand the knowledge about the Neoproterozoic metamorphic history of the Baltoscandian margin in the understudied Váivančohkka-Salmmečohkat region of the northern Scandinavian Caledonides. The work focused on rocks that belong to the upper gneiss unit, which constitutes part of SNC in the region. The unit comprises migmatitic paragneisses and garnet-mica schist containing metamafic bodies. U-Pb zircon and Th-U-total Pb monazite dating of the migmatitic paragneiss yielded consistent age of metamorphism in 602 ± 5 Ma and 599 ± 3 Ma, respectively. A similar U-Pb age of 604 ± 8 Ma was obtained for the zircon from the leucocratic vein transecting the amphibolite within the studied gneiss. Interestingly, no Caledonian ages were identified. Likewise, no evidence for high or ultra-high pressure conditions was found, neither in the gneisses/schists nor in the metamafic rocks. Petrographic observations and calculated metamorphic P-T conditions indicate that rocks belonging to upper gneiss unit underwent upper-amphibolite facies metamorphism in a melt stability field; 8.0–10.5 kbar at 750–790 °C.Since the studied rocks underwent high-grade metamorphism in the Ediacaran and lack obvious evidence for Caledonian high-pressure metamorphism, the studied part of the SNC offers an extraordinary insight into the Late Neoproterozoic history of the hot, extended Baltoscandian margin.

U-Pb Zircon Dating of Migmatitic Paragneisses and Garnet Amphibolite from the High Pressure Seve Nappe Complex in Kittelfjäll, Swedish Caledonides

The Seve Nappe Complex exposed in the Kittelfjäll area of the northern Scandinavian Caledonides comprises a volcano-sedimentary succession representing the Baltica passive margin, which was metamorphosed during the Iapetus Ocean closure. Garnet amphibolites, together with their host migmatitic paragneisses, record a potential (U)HP event followed by decompression-driven migmatization. The garnet amphibolites were originally thought to represent retrogressively altered granulites. The petrological and geochemical features of a studied garnet amphibolite allow for speculation about a peridotitic origin. Zirconium (Zr) content in rutile inclusions hosted in garnet in paragneisses points to near-peak temperatures between 738 °C and 780 °C, which is in agreement with the c. 774 °C obtained from the matrix rutile in the garnet amphibolite. The matrix rutile in multiple paragneiss samples records temperatures below 655 °C and 726 °C. Whereas the LA-ICP-MS U-Pb dating of zircon cores revealed the age spectrum from Paleoproterozoic to early Paleozoic, suggesting a detrital origin of zircon cores in paragneisses, the metamorphic zircon rims show an Early Ordovician cluster c. 475–469 Ma. Additionally, zircon cores and rims from the garnet amphibolite yielded an age of c. 473 Ma. The REE patterns of the Caledonian zircon rims from the paragneisses show overall low LREE concentrations, different from declining to rising trends in HREE (LuN/GdN = 0.49–38.76). Despite the textural differences, the cores and rims in zircon from the garnet amphibolite show similar REE patterns of low LREE and flat to rising HREE (LuN/GdN = 3.96–65.13). All zircon rims in both lithologies display a negative Eu anomaly. Hence, we interpret the reported ages as the growth of metamorphic zircon during migmatization, under granulite facies conditions related to exhumation from (U)HP conditions.

Deep-seated Carbonatite Intrusion and Metasomatism in the UHP Tromsø Nappe, Northern Scandinavian Caledonides—a Natural Example of Generation of Carbonatite from Carbonated Eclogite

Deep-seated Carbonatite Intrusion and Metasomatism in the UHP Tromsø Nappe, Northern Scandinavian Caledonides—a Natural Example of Generation of Carbonatite from Carbonated Eclogite

Chromium-rich kyanite, magnesiostaurolite and corundum in ultrahigh-pressure eclogites (examples from Pohorje Mountains, Slovenia and Tromsø Nappe, Norway)

Chromium-rich kyanite, magnesiostaurolite and corundum (ruby) were found in ultrahigh-pressure (UHP) eclogites associated with garnet peridotite and serpentinite from Pohorje Mountains (Eastern Alps) and the Tromso Nappe (Northern Scandinavian Caledonides). The concentrations of Cr 2 O 3 in kyanite and magnesiostaurolite from the Pohorje Mts. (17.0 and 15.0 wt%, respectively) are the highest Cr values ever reported for these minerals, and the Cr content of corundum from this area (≤12.2 wt% Cr 2 O 3 ) also belong to the highest reported from natural samples. The Cr-rich kyanite of turquoise-blue colour contains inclusions of chromite (Cr* = 0.74–0.78) and Cr-rich magnesiostaurolite (6 to 15 wt% Cr 2 O 3 ; X Mg = 0.7–0.8). Kyanite is commonly surrounded by a corona composed of the assemblage Cr-bearing sapphirine (up to 5.5 wt% Cr 2 O 3 ) + corundum + Al-rich spinel + plagioclase. Blue-green, Cr-rich magnesiostaurolite (5.4–8.5 wt% Cr 2 O 3 ; X Mg = 0.76–0.82) forming ≤500 μm large crystals is associated with Cr-rich kyanite, Cr-rich spinel and Cr-rich corundum which are surrounded by pargasitic amphibole and zoisite. Chromium enrichment in kyanite, magnesiostaurolite and corundum was controlled by substitution for Al in the octahedral sites (Cr 3+ Al −1 ). Magnesiostaurolite compositional variations indicate at least two additional substitutions: Fe 2+ Mg −1 and M 2+ Si(Al,Cr) −2 , where M 2+ are the divalent cations Mg, Fe, Mn, Ni and Zn. The highest Si content and near absence of tetrahedrally coordinated Al was observed in the most Cr-rich magnesiostaurolite. Chromian kyanite, magnesiostaurolite and ruby formed by reactions involving primary chromite derived from a magmatic protolith characterized by high Cr, Mg, Al and low Si, during prograde metamorphism and formation of eclogite at HP-UHP conditions.

Peri-Gondwanan elements in the Caledonian Nappes of Finnmark, Northern Norway: Implications for the paleogeographic framework of the Scandinavian Caledonides

The Kalak Nappe Complex in the northern Scandinavian Caledonides has historically been interpreted as representing the pre-Caledonian margin of Baltica consisting of a Precambrian basement and a late Precambrian to Cambrian cover, which were deformed and intruded by a Late Precambrian alkalic mafic complex during the Cambrian Finnmarkian orogeny. New evidence, however, does not fit the above interpretations very well, but instead shows that the sedimentary rocks were deposited at least in part prior to about 1000 Ma and that the complex underwent repeated tectonism and granitic magmatism prior to the emplacement of the gabbroic / alkalic complex at 570 to 560 Ma. This paper presents new U-Pb data documenting distinct episodes of orogenic activity marked by the emplacement of syn-tectonic anatectic melts and in part by anatexis at about 850 Ma and 700 to 680 Ma. Another event at 600 Ma is seen in the basal unit of the overlying Vaddas Nappe. The timing of this activity is entirely atypical for the autochthonous northern segments of the Baltic Shield, which were formed in the Archean and modified in the Palaeoproterozoic. It appears much more likely that the Kalak Nappe Complex is an exotic terrane that developed outside of Baltica, probably in the pre-Gondwanan realm or the southern Iapetus, and was then translated towards, and accreted, to Baltica during the Scandian collisional phase. In view of our present understanding the various terms previously used to designate orogenic phases (Finnmarkian, Soroyan, Porsanger) are no longer relevant or sufficient for describing the complex geological evolution of the region and we propose to terminate their use.

Prograde garnet-bearing ultramafic rocks from the Tromsø Nappe, northern Scandinavian Caledonides

Garnet-bearing peridotitic rocks closely associated with eclogite within the Tromsø Nappe of the northern Scandinavian Caledonides show good evidence for prograde metamorphism. Early stages are recognized as inclusions of hornblende and chlorite in the cores of large garnet poikiloblasts. Closer to the garnet rim, clinopyroxene and Cr-poor spinel appear as additional inclusion phases. Four suites of spinel inclusions can be distinguished based on optical properties and chemical composition. The innermost suite (suite 1) has the lowest Cr# and highest Mg#. Further rimward, the spinel inclusions gradually change in composition, with increasing Cr# and decreasing Mg#. Spinel is rare in the matrix, but locally chromitic spinel occurs as larger grains. Garnet poikiloblasts are rimmed by a kelyphite zone consisting of Hbl + Cr-poor Spl or Opx ± Cpx + Cr-poor Spl, and locally an inner zone of Na-rich Hbl + Chl. Matrix assemblage in the garnet-bearing peridotitic rocks is Hbl + Chl + Cpx + Ol ± Cr-rich spinel, defining a strong foliation wrapping around garnets and associated kelyphites. Thin layers of garnet-orthopyroxenite and garnet–hornblende–zoisite–chlorite rocks are presumably coeval with the matrix foliation of the peridotitic rocks. In dunitic to harzburgitic compositions large undulatory grains of Ol + Opx ± Chl + Spl apparently define the maximum- P conditions. This assemblage is succeeded by a recrystallized assemblage of Ol ± Tlc ± Mgs, which in turn is overgrown by strain-free poikiloblasts of orthopyroxene, indicating a temperature increase. This is postdated by Tlc + Ath ± Mgs, and finally serpentine. P– T estimates for the inclusion suites of clinopyroxene and spinel in garnet clearly indicate garnet growth and spinel consumption in a regime of increasing P. The inner suite (suite 1) apparently was in equilibrium with garnet, clinopyroxene and olivine at 1.40 GPa, 675 °C, whereas included spinel with maximum Cr# (suite 4) indicate 2.40 GPa at 740 °C. Grt + Opx from garnet-orthopyroxenite give 1.5–1.9 GPa at 740–770 °C, and Grt + Hbl + Zo + Chl from a zoisite-rich rock give 1.75 ± 0.25 GPa at 740 ± 30 °C, interpreted to represent recrystallization during uplift. In dunitic to harzburgitic compositions, early Ol + Opx ± Chl + Spl is succeeded by Ol ± Tlc ± Mgs, which in turn is overgrown by neoblasts of strain-free orthopyroxene, indicating temperature increase. This is postdated by Tlc + Ath ± Mgs, and finally serpentine. The ultramafic rocks in the Tromsø Nappe were locally strongly hydrated before subduction along with associated eclogites and metasedimentary rocks during the early (Ordovician) stages of the Caledonian orogeny.

The Silurian to Permian history of a metamorphic core complex in Lofoten, northern Scandinavian Caledonides

The Lofoten archipelago exposes Precambrian Baltic basement and Caledonian allochthonous sequences within a 1000 km long chain of gravity and magnetic highs and structural culminations along the extended, British and Norwegian continental shelf. Previous regional geophysical studies indicate that post‐Caledonian extension and development of the northern Norwegian shelf occurred during broadly defined Carboniferous‐Permian, Cretaceous‐Jurassic, and early Tertiary events. Structures related to these events are known to young westward. We report field, structural, and 40Ar/39Ar thermochronologic data from rocks exposed in Lofoten that further define the history and style of post‐Caledonian extension. The islands of southwest Lofoten also represent the most outboard exposures of Caledonian basement in northern Norway that presumably formed the middle to deep crustal core of the orogen. Metasedimentary rocks and penetratively deformed basement in Lofoten record high‐grade Silurian‐Devonian metamorphism and top‐to‐the‐east (hereinafter tops‐east) thrusting followed by episodes of Late Devonian to Early Carboniferous, tops‐west, ductile extension which progressed into oblique left‐slip movements. The structural style and timing of Silurian contraction in this area are remarkably similar to that determined for the more forelandward areas on the mainland, ∼120 km to the east, supporting the inference that distal parts of the Baltic continental margin that were once deeply subducted are presently exposed in Lofoten. The timings of post‐Devonian structural events that affected rocks in Lofoten are partially constrained by the ages of unconformities and strata known to be preserved in graben flanking the Lofoten culmination. The radiometric age and structural data presented in this study, in combination with stratigraphic constraints, suggest a westward progression through time of extensional deformation over a protracted interval of Silurian to Permian time. The latest, Permian extension in Lofoten is largely characterized by brittle structures that formed at conditions substantially less than 300°C. Compared to the exhumation history of the southern Western Gneiss Region, the depth of Caledonian, continental (A‐type) subduction and subsequent unroofing of Lofoten are of lesser magnitude, and the present erosional level remained in the middle crust for a much longer interval of time. The Permian 40Ar/39Ar mineral ages documented in this study are the youngest such ages yet identified in Scandinavia. These ages relate to episodes of deformation and cooling in response to extensional tectonic events that occurred roughly 100 m.y. after comparable effects identified on the Caledonian mainland. Our preferred explanation for the Carboniferous‐Permian radiometric ages, structural evolution, and stratigraphic data for Lofoten is that they all developed in the context of a long‐lived Cordilleran‐style metamorphic core complex.

Subduction-flip during Iapetus Ocean closure and Baltica-Laurentia collision, Scandinavian Caledonides

Evidence is presented here from the northern Scandinavian Caledonides for development of an extensional basin of Ashgill to Mid Llandovery age along the Baltoscandian margin immediately prior to Baltica–Laurentia collision. U/Pb multigrain and ion microprobe zircon dating of plagiogranites in the Halti Igneous Complex complement previous baddeleyite and zircon dating of a dolerite dyke, and zircon dating of anatectic granite; they demonstrate that this dunite, troctolite, gabbro, sheeted-dyke complex ranges in age from c. 445 to 435 Ma. The dolerite dykes intruded and melted arkoses of inferred Neoproterozoic age. This evidence, taken together with previous documentation of ophiolites (Solund–Stavfjord), ophiolite-like associations (Sulitjelma Igneous Complex) and several other mafic suites (e.g. Råna, Artfjället) of Ashgill to Llandovery age further south in the northern Scandinavian Caledonides, implies that Scandian collisional orogeny along this nearly 2000-km-long mountain belt was immediately preceeded by development of short-lived marginal basins. The latter developed during the final closure of the Iapetus Ocean and are inferred to be of back-arc origin, some (perhaps all) related to E-dipping subduction. Collision of the continents at c. 435 Ma is inferred to have induced a flip in subduction polarity, leading to underthrusting of Laurentia by Baltica.

Early Ordovician U‐Pb metamorphic ages of the eclogite‐bearing Seve Nappes, Northern Scandinavian Caledonides

ABSTRACT Eclogite‐grade metamorphism of the Seve Nappe Complex (SNC) in Norrbotten, Sweden, records the attempted subduction of the Baltic continental margin during the early Palaeozoic evolution of the Iapetus Ocean. Metamorphic titanite sampled from several calcsilicate gneisses of the SNC in Norrbotten occurs as part of a prograde, eclogite facies metamorphic mineral assemblage and yields concordant to nearly concordant U/Pb ages of 500–475 Ma. Later structural disruption of these rocks occurred during the Siluro‐Devonian Scandian phase of the Caledonide orogeny, but the U/Pb systematics show no evidence of a second generation (metamorphic or recrystallized) of titanite, or of post‐Early Ordovician disturbance through Pb loss. Hence the U/Pb ages are believed to record the time of prograde mineral growth during eclogite facies metamorphism of the SNC.These results support earlier Sm/Nd and 40Ar/39Ar studies indicating an Early Ordovician metamorphic age for the eclogitic Norrbotten SNC, and confirm the Early Ordovician destruction of at least this segment of the Palaeozoic passive margin of Baltica. These results indicate that the SNC in the northern Scandinavian Caledonides was subducted and metamorphosed to high grade some 50–70 Myr prior to the high‐grade metamorphism of the SNC in the central Scandinavian Caledonides. This result requires significantly different early Palaeozoic tectonic histories for rocks mapped as SNC in the northern Caledonides and those in the central Caledonides, despite a seemingly similar tectonostratigraphic position and broadly similar high‐grade metamorphism.

Early Ordovician U-Pb metamorphic ages of the eclogite-bearing Seve Nappes, Northern Scandinavian Caledonides

Early Ordovician U-Pb metamorphic ages of the eclogite-bearing Seve Nappes, Northern Scandinavian Caledonides

Structural expressions and tectonic implications of general noncoaxial flow in the midcrust of a collisional orogen: The northern Scandinavian Caledonides

Mapping and strain analysis in the northern Scandinavian Caledonides indicate that one way in which contemporaneous upper crustal extension and deeper thrusting within collisional orogens may be linked in a kinematically compatible manner is through the ductile flow of rocks in the midcrust of these mountain belts. In the Ofoten‐Efjorden region of north Norway, amphibolite facies metamorphism and strong penetrative strain occurred at deep levels of the Caledonian nappe stack as it was thrust eastward onto the western margin of Baltica. Kinematic analysis of deformation in the lower portion of the Caledonian nappe stack indicates that the rock mass underwent heterogeneous noncoaxial flow, with components of foreland directed simple shear and subvertically shortened pure shear. General noncoaxial flow (simultaneous simple shear + pure shear) can accomplish foreland‐directed tectonic transport, subvertical thinning, and transport‐parallel elongation at deep levels of a nappe stack. This pattern of flow is significant technically for two reasons: first, it suggests spreading of the Caledonian allochthon during thrust emplacement, and second, it can form a kinematic link between normal faulting at shallower structural levels and thrusting at the base of the nappes. By analogy with the Caledonides, general noncoaxial flow of midcrustal rocks may represent an important process in other collisional and contractional orogens, particularly in those that have undergone upper crustal extension while thrusting continued in the foreland.

Sm‐Nd isotope data from the Halti‐Ridnitsohkka mafic‐ultramafic complex in the northern Scandinavian Caledonides

Samples from the Halti‐Ridnitsohkka mafic‐ultramafic complex in the northern part of the Scandinavian Caledonides have been studied by the Sm‐Nd isotope dating method. Three samples from the Haiti cumulate sequence (olivine gabbro) gave an age of 564±35 Ma and an initial eNd of 5.7±0.3(theinitial143Nd/144Ndratiois0.51220±6). The magma source is a depleted mantle. Five samples from the Ridnitsohkka gabbro dolente dyke swarm yield data that are too scattered to give any acceptable isochron and age for this unit. There is a significant difference in the amount of Nd in the samples between these two units.The Ridnitsohkka dyke complex is separated from the Haiti cumulates by a shear zone of unclear significance. Previous work on badelleyite and zircon from the Ridnitsohkka dyke complex has yielded Ordovician ages (425–445 Ma). It has not been possible to evaluate the apparent conflict in these results. However, the conclusion here is to place the Halti‐Ridnitsohkka mafic‐ultramafic complex in the Kalak Nappe Complex of the Middle Allochthon and not as an eastern component of the Reisa Nappe Complex of the Upper Allochthon. Matsson, I., 1994; Sm‐Nd isotope data from the Halti‐Ridnitsohkkamafic‐ultramafic complex in the northern Scandinavian Caledonides. GFF, Vol. 116 (Pt. 1, March), pp. 13–16. Stockholm. ISSN 1103–5897.

A tectonic model for Scandian terrane accretion in the northern Scandinavian Caledonides

Muscovite and hornblende 40 Ar/ 39 Ar ages have been obtained from several structural levels within the Caledonian nappe stack of Troms, northern Norway. Meaningful ages range between c. 425 Ma and 370 Ma, suggesting protracted post-metamorphic cooling from Early Silurian to Late Devonian times. Whilst the possibility of earlier ‘Finnmarkian’ (Late Cambrian-Early Ordovician) histories is not precluded by these data, they provide further evidence of intense Scandian tectonothermal activity in this portion of the Scandinavian Caledonides. Three structural units within the nappe pile record hornblende 40 Ar/ 39 Ar ages ranging from 425 to 421 Ma. One of these records a similar muscovite cooling age, taken to indicate rapid nappe exhumation from peak Scandian metamorphism as Laurentia and Baltoscandia collided in the Late Llandovery and Wenlock. All other 40 Ar/ 39 Ar muscovite ages from the nappe pile range between 395 ± 1 and 373 ± 1 Ma. These are thought to track the progression of the 350°C crustal isotherm as it relaxed through the tectonically thickened Caledonian crust. Major internal thrusts appear to have been largely inactive at this time and acted as sites of enhanced fluid flow and retrogression. Further accretion of the orogenic wedge to the Baltoscandian margin was characterized by piggy-back style translation of the nappes on a low-angle basal decollement. This was accompanied by large-scale basement reactivation and out-of-sequence thrusting, resulting in further interleaving of the nappe stack in the Late Devonian (Frasnian).

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

AbstractThis 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 Savotjakka 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 Finnm...

Initial tectonothermal evolution within the Scandinavian Caledonide Accretionary Prism: constraints from 40Ar/39Ar mineral ages within the Seve Nappe Complex, Sarek Mountains, Sweden

In the Caledonide orogen of northern Sweden, the Seve Nappe Complex is dominated by rift facies sedimentary and mafic rocks derived from the Late Proterozoic Baltoscandian miogeocline and offshore‐continent–Iapetus transition. Metamorphic breaks and structural inversions characterize the nappe complex. Within the Sarek Mountains, the Sarektjåkkå Nappe is composed of c. 600‐Ma‐old dolerites with subordinate screens of sedimentary rocks. These lithological elements preserve parageneses which record contact metamorphism at shallow crustal levels. The Sarektjåkkå Nappe is situated between eclogite‐bearing nappes (Mikka and Tsäkkok nappes) which underwent high‐P metamorphism at c. 500 Ma during westward subduction of the Baltoscandian margin. 40Ar/39Ar mineral ages of c. 520–500 Ma are recorded by hornblende within variably foliated amphibolite derived from mafic dyke protoliths within the Sarektjåkkå Nappe. Plateau ages of 500 Ma are displayed by muscovite within the basal thrust of the nappe and are consistent with metamorphic evidence which indicates that the nappe escaped crustal depression as a result of detachment at an early stage of subduction. Cooling ages recorded by hornblende from variably retrogressed eclogites in the entire region are in the range of c. 510–490 Ma and suggest that imbrication of the subducting miogeocline was followed by differential exhumation of the various imbricate sheets. Hornblende cooling ages of 470–460 Ma are recorded from massive dyke protoliths within the Sarektjåkkå Nappe. These are similar to ages reported from the Seve Nappe Complex in the central Scandinavian Caledonides. Probably these date imbrication and uplift related to Early Ordovician arrival of outboard terranes (e.g. island‐arc sequences represented by structurally lower horizons of the Köli Nappes).Metamorphic contrasts and the distinct grouping of mineral cooling ages suggest that the various Seve structural units are themselves internally imbricated, and were individually tectonically uplifted through argon closure temperatures during assembly of the Seve Nappe Complex. The cooling ages of 520–500 Ma recorded within Seve terranes and along terrane boundaries of the Sarek Mountains provide evidence of significant accretionary activity in the northern Scandinavian Caledonides in the Late Cambrian–Early Ordovician.

Cambrian-Ordovician paleogeography of Baltica

Recent paleomagnetic data and existing paleontological evidence show that Baltica occupied temperate southern latitudes during Cambrian and Early Ordovician time, but was inverted with reference to its present orientation. Hence the currently opposed margins of the Baltic and Laurentian shield were not conterminous in the early Paleozoic. Laurentian and Baltic late Precambrian rifting episodes may therefore have occurred along unrelated continental margins. Closure of the Tornquist sea was accompanied by the counterclockwise rotation of Baltica relative to Avalonia in Early to Middle Ordovician time. This rotation, initiated in Late Cambrian-Early Ordovician time, was coeval with the Finnmarkian orogeny in the northern Scandinavian Caledonides.

The Lower-Middle Ordovician palaeofield of Scandinavia: southern Sweden ‘revisited’

Abstract We present a palaeomagnetic pole for the Arenig-Llanvirn Swedish Orthoceras Limestones based on a new analysis of some earlier reported palaeomagnetic data. This new pole, 18° N and 046° E, differs from the original estimate of 30° N and 46° E. The revised pole probably represents the most reliable Ordovician pole from Baltica, although some uncertainties remain concerning the relative importance of diagenetic/depositional controls on remanence acquisition. Based on the new pole from the Swedish Orthoceras Limestone and recent palaeomagnetic results from the northern Scandinavian Caledonides, it is now evident that Baltica lay within southerly latitudes of between 30 and 60° S in Cambro-Ordovician times. Subsequent anticlockwise rotation relative to eastern Avalonia (southern Britain) may indicate dextral shear across the Tornquist suture prior to Mid-Late Ordovician amalgamation of eastern Avalonia and Baltica.

Dyke complexes of the Rohkunborri Nappe, Seve terranes, in the Indre Troms and Pältsa area, northern Scandinavian Caledonides

Dyke complexes of the Rohkunborri Nappe, Seve terranes, in the Indre Troms and Pältsa area, northern Scandinavian Caledonides

Geologic thermobarometry of retrograded metamorphic rocks: An indication of the uplift trajectory of a portion of the northern Scandinavian caledonides

Kyanite grade pelitic schists from the Efjord area (68°15′N, 16°45′E), northern Norway, show abundant textural evidence of retrograde reequilibration during late orogenic uplift and cooling of this part of the Scandinavian Caledonides. Graphical analysis of mineral compositions in seven samples that show a range of retrograde textures indicates that they reached final chemical equilibrium under nonuniform pressure‐temperature conditions. Three element‐partitioning geothermometers and geobarometers yield consistent PT estimates in individual samples, which together define a trend in PT space. The sample showing the fewest signs of retrogression yields the highest pressure and temperature estimates, whereas samples exhibiting abundant reequilibration textures give lower PT estimates. We suggest that the trend indicated by these samples reflects a portion of the PT trajectory assumed by the Efjord area during uplift. Thus the judicious use of element‐partitioning geothermometers and geobarometers on samples showing signs of retrograde metamorphism may yield important new information concerning the late tectonic evolution of metamorphic terrains.