Central Scandinavian Caledonides Research Articles

Mid-Ordovician stratigraphy and volcanism in the Hølonda area, Scandinavian Caledonides: complex tectonomagmatic development following arc–continent collision near the Laurentian margin of Iapetus

The Hølonda area of the central Scandinavian Caledonides is a key for models of the Caledonian orogen owing to its Ordovician fauna of Laurentian affinity, now stranded on the Baltic side during opening of the North Atlantic. Here, we present a revised stratigraphic and tectonomagmatic model based on remapping, sedimentology, igneous geochemistry, Nd and Sr isotopes and geochronology. The Hølonda Group ( c. 470–461 Ma) reflects a transition from subaerial and shallow-marine deposition on a continental shelf to deeper-water sedimentation along a subsiding slope. Adakitic and mid-ocean ridge basalt type magmatism at c. 468 Ma was succeeded by benmoreitic–rhyolitic, shoshonitic, calc-alkaline and ultra-alkaline volcanism at c. 467–465 Ma. The complex magmatism followed arc–continent collision along a microcontinent outboard of Laurentia, associated with subduction polarity flip and slab rollback. This led to rifting and opening of a wide basin and its adjoining shelf on thickened orogenic lithosphere. Associated mantle upwelling and partial melting of depleted and variably metasomatized mantle occurred in a tectonomagmatic setting comparable with that of the central Mediterranean. The Hølonda–Ilfjellet setting is unique along the Caledonian–Appalachian orogen, possibly reflecting interaction with Laurentia-derived continental terranes at the northeastern end of the Taconian–Grampian orogenic tract. Supplementary material: Analytical data and supplementary figures are available at https://doi.org/10.6084/m9.figshare.c.6368922 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

Exhumation of continental margin rocks from mantle depths to orogenic foreland: example from the Seve Nappe Complex of the central Scandinavian Caledonides

Exhumation of continental margin rocks from mantle depths to orogenic foreland: example from the Seve Nappe Complex of the central Scandinavian Caledonides

Two-stage regional rare-element pegmatite formation at Tysfjord, Norway: implications for the timing of late Svecofennian and late Caledonian high-temperature events

Pegmatite fields within granite plutons are commonly considered to have formed from residual melts of their host. This is not always true as demonstrated by the Tysfjord granite gneiss and its two groups of pegmatites. The Tysfjord granite gneiss, exposed in a tectonic window of the Caledonides of northern Norway, is part of the transscandinavian igneous belt (TIB) that includes several phases of granitic magmatism. In the northern Hamarøy area (Drag-Finnøy), where most rare-element pegmatites occur, Paleoproterozoic and metamorphosed Group 1 allanite–(Ce)–fluorite metapegmatites have similar bulk rock chemical composition as the TIB granite gneiss rocks, indicating that these pegmatites are residual melts. Group 1 metapegmatites, which are up to 400 m in size, are among the largest known intra-plutonic pegmatites with Nb–Y–F (NYF) signature. The formation of these unusually large granite-hosted NYF pegmatites may have been facilitated by the overall high F content of TIB granite gneisses. Undeformed Group 2 amazonite–tourmaline pegmatites yield columbite and zircon U–Pb ages in the range 400–379 Ma. These pegmatites are interpreted to be anatectic melts that formed from the partial melting of Tysfjord granite gneiss. Group 2 pegmatites, including those from Træna Island and the Sjona tectonic window (400 and 414 Ma), formed during late Caledonian ductile shearing and incipient unroofing of the central Scandinavian Caledonides and record progressively younger ages of this event from SW to NE.

Geochemical characteristics of the Feragen ultramafic body, central Scandinavian Caledonides: Implications for petrogenesis and tectonic significance

Geochemical characteristics of the Feragen ultramafic body, central Scandinavian Caledonides: Implications for petrogenesis and tectonic significance

Pressure, temperature and lithological dependence of seismic and magnetic susceptibility anisotropy in amphibolites and gneisses from the central Scandinavian Caledonides

As a petrofabric indicator, anisotropy of magnetic susceptibility (AMS) can potentially be used to infer seismic properties of rocks, and in particular seismic anisotropy. To evaluate the link between AMS and seismic anisotropy we present laboratory measurements of elastic wave velocities and anisotropy of magnetic susceptibility (AMS) for eight samples from the deep drilling investigation forming a part of the Collisional Orogeny in the Scandinavian Caledonides (COSC) project. The samples consist of a representative suite of mid crustal, deformed rock types, namely felsic and biotite-rich gneisses, and amphibolites (mafic gneisses). Compressional (P) and shear (S) waves were measured at confining pressures from room pressure to 600 MPa and temperature from room condition to 600 °C. Seismic anisotropy changes with increasing temperature and pressure, where the effect of pressure is more significant than temperature. Increasing pressure, considering the range of samples, results in an increase in mean wave speed values from 4.52 to 7.86 km/s for P waves and from 2.75 to 4.09 km/s for S waves. Biotite gneiss and amphibolite exhibit the highest anisotropy with P wave anisotropy (AVp) in the ranges of ~9% to ~20%, and maximum S- wave anisotropy exceeds 10%. In contrast, Felsic gneisses are significantly less anisotropic, with AVp of <7% and AVs of <6%. Up to 20% anisotropy may be generated by microcracks at 600 MPa and 600 °C, which is likely originating from thermal expansion of anisotropic minerals. An agreement is found between AMS and seismic anisotropy, although this is only a case if mean magnetic susceptibility (kmean) ranges between ~1 × 10−5 to ~1 × 10−3 [SI]. Such kmean values are common in rocks dominated by paramagnetic matrix minerals. Based on our results we propose that such samples are the most likely to be useful for the prediction of seismic anisotropy based on their AMS.

Seismic anisotropy of mid crustal orogenic nappes and their bounding structures: An example from the Middle Allochthon (Seve Nappe) of the Central Scandinavian Caledonides

We report compositional, microstructural and seismic properties from 24 samples collected from the Middle Allochthon (Seve Nappe) of the central Scandinavian Caledonides, and its bounding shear zones. The samples stem both from field outcrops and the continental drilling project COSC-1 and include quartzofeldspathic gneisses, hornblende gneisses, amphibolites, marbles, calc-silicates, quartzites and mica schists, of medium to high-strain. Seismic velocities and anisotropy of P (AVp) and S (AVs) waves of these samples were calculated using microstructural and crystal preferred orientation data obtained from Electron Backscatter Diffraction analysis (EBSD). Mica-schist exhibits the highest anisotropy (AVP ~ 31%; max AVs ~34%), followed by hornblende-dominated rocks (AVp ~5–13%; max AVs 5–10%) and quartzites (AVp ~6.5–10.5%; max AVs ~7.5–12%). Lowest anisotropy is found in calc-silicate rocks (AVp ~4%; max AVs 3–4%), where the symmetry of anisotropy is more complex due to the contribution to anisotropy from several phases. Anisotropy is attributed to: 1) modal mineral composition, in particular mica and amphibole content, 2) CPO intensity, 3) crystallization of anisotropic minerals from fluids circulating in the shear zone (calc-silicates and amphibolites), and to a lesser extent 4) compositional banding of minerals with contrasting elastic properties and density. Our results link observed anisotropy to the rock composition and strain in a representative section across the Central Scandinavian Caledonides and indicate that the entire Seve Nappe is seismically anisotropic. Strain has partitioned on the nappe scale, and likely on the microstructural scale. High- strain shear zones that develop at boundaries of the allochthon and internally within the allochthon show higher anisotropy than a more moderately strained interior of the nappe. The Seve Nappe may be considered as a template for deforming, ductile and flowing middle crust, which is in line with general observations of seismic anisotropy in mid-crustal settings.

The Skuggliberga unit of the Oppdal area, central Scandinavian Caledonides: calc-alkaline pyroclastic volcanism in a fluvial to shallow-marine basin following a mid-Ordovician orogenic event

The Skuggliberga unit of the Oppdal area, central Scandinavian Caledonides: calc-alkaline pyroclastic volcanism in a fluvial to shallow-marine basin following a mid-Ordovician orogenic event

Core‐Log‐Seismic Integration in Metamorphic Rocks and Its Implication for the Regional Geology: A Case Study for the ICDP Drilling Project COSC‐1, Sweden

AbstractContinental collision causes deformation in the crust along shear zones. However, the physical and chemical conditions at which these zones operate and the deformation processes that enable up to hundreds of km of tectonic transport are still unclear because of the depth at which they occur and the challenges in imaging them. Ancient exhumed collision zones allow us to investigate these processes much better, for example at the COSC‐1 borehole in the central Scandinavian Caledonides. This study combines data from the COSC‐1 borehole with different seismic measurements to provide constraints on the spatial lithological and textural configuration of the Seve Nappe Complex. This is one of the few studies that shows that core‐log‐seismic integration in metamorphic rocks allows to identify the spatial distribution of major lithological units. Especially gamma ray logs in combination with density data are powerful tools to distinguish between mafic and felsic lithologies in log‐core correlation. Our results indicate that reflections along the borehole are primarily caused by compositional rather than textural changes. Reflections in the Seve Nappe Complex are not as distinct as in greater depths but continuous and several of them can be linked to magmatic intrusions, which have been metamorphically overprinted. Their setting indicates that the Seve Nappe Complex consists of the remnants of a volcanic continental margin. Our results suggest that ductile‐deformed middle crustal reflectivity is primarily a function of pre‐orogenic lithological variations which has to be considered when deciphering mountain building processes.

U-Pb baddeleyite age for the Ottfjället Dyke Swarm, central Scandinavian Caledonides: new constraints on Ediacaran opening of the Iapetus Ocean and glaciations on Baltica

ABSTRACT The Ottfjället Dyke Swarm (ODS) is a prominent component of the Ediacaran mafic magmatism associated with opening of the Iapetus ocean, and hosted in the Särv Nappe, Middle Allochthon of the Scandinavian Caledonides. A U-Pb baddeleyite age of 596.3 ± 1.5 Ma for a thick, well preserved, plagioclase-phyric dolerite dyke in Härjedalen, Sweden, dates emplacement of the swarm. The age represents a robust, inheritance-free reference age for variably deformed and metamorphosed tholeiitic dykes in sandstone-dominated sequences of the lower part of the Middle Allochthon, representing the proximal, rifted Baltoscandian margin preceding the opening of Iapetus. The new age is within the narrow time span between 610 and 595 Ma defined by the most reliable age estimates for mafic dykes in structurally higher nappes (upper part of Middle Allochthon), representing the distal margin during the opening of Iapetus. The Ottfjället Dyke Swarm cuts the Tossåsfjället Group succession, which includes sabkha-related carbonate platform and diamictite couples, one of several correlated Neoproterozoic glaciogenic successions in Scandinavia. The intrusion age of ca. 596 Ma therefore sets a minimum age for the glaciogenic successions. It implies that Neoproterozoic glaciations in Scandinavia predate the ca. 580 Ma Gaskiers glaciation event and are probably part of the ca. 635 Ma Marinoan “Snowball-Earth”-type glaciation.

Absolute timing of Caledonian orogenic wedge assembly, Central Sweden, constrained by Rb–Sr multi-mineral isochron data

Multi-mineral Rb–Sr isochron ages for mylonites of the Caledonian nappe pile (Jämtland, Sweden) constrain the orogenic evolution of the central Scandinavian Caledonides. An age of 437.8 ± 3.9 Ma from a metamorphic leucosome in the central nappe stack dates crystallization of migmatized gneiss. Thirteen isotopic ages derived from mylonites across all structural levels are between 434.6 ± 6.9 Ma and 426.3 ± 3.1 Ma. These results, complemented by a dense network of kinematic field data, are interpreted to reflect the timing of protracted top-to-the-ESE general shear under decreasing amphibolite- to upper-greenschist-facies conditions across all nappes. Three samples, derived from the top, middle and bottom of the nappe pile, yield additional deformation ages between 416.0 ± 3.6 Ma and 410.1 ± 3.0 Ma. These ages are interpreted to represent post-assembly imbrication of the nappe stack under decreasing metamorphic conditions. The new tectonochronologic data are consistent with a tectonic model that explains nappe stacking in Jämtland by three stages of ESE-directed in- and out-of-sequence thrusting. In the first stage, postdating earlier subduction–exhumation of the high-grade metamorphic Seve Nappe Complex, the latter was emplaced in-sequence in the lower/mid-crust above nappes in its footwall (foreland) at c. 430 Ma. This event is related to the collision of Baltica with an island arc. Concurrently at c. 430 Ma, determined by paleogeography and structurally unrelated to the first stage, second-stage out-of-sequence thrusting emplaced the Köli Nappe Complex atop the developing nappe stack during final Baltica–Laurentia collision. Eventually, imbrication and exhumation of the composite Jämtland nappe stack in the uppermost middle crust during the third stage at c. 414 Ma was caused by continuous underthrusting of Baltica beneath Laurentia. This three-stage thrust model reflects the complex interactions of Baltica–arc–Laurentia collisions that were accommodated by two merging subduction zones.

Metamorphic Zonation by Out‐of‐Sequence Thrusting at Back‐Stepping Subduction Zones: Sequential Accretion of the Caledonian Internides, Central Sweden

AbstractExhumation of the high‐grade metamorphic Seve Nappe Complex and its emplacement between lower‐grade nappes has been related to wedge extrusion in the central Scandinavian Caledonides. To test this hypothesis, the kinematic evolution of the Caledonian nappe pile is studied by systematic structural mapping in central and northern Jämtland, Sweden. Structural data, combined with petrological and quartz microstructure observations, document pervasive top‐to‐the‐ESE, foreland‐directed shearing under progressively decreasing metamorphic grade across the entire nappe pile. Mylonitic foliation, foliation‐parallel boudinage, and abundant top‐to‐the‐ESE and rare, scattered top‐to‐the‐WNW shear‐sense indicators imply foreland‐directed general shear. This deformation regime caused exhumation by concurrent thrusting and vertical ductile thinning. We propose a specific succession of in‐ and out‐of‐sequence thrusts that generated the metamorphic zonation. Our model envisions in‐sequence propagation of thrusts during exhumation of the Seve Nappe Complex, related to subduction of Baltica beneath a volcanic arc within Iapetus. Concurrently, Iapetus subducted beneath Laurentia farther to the west. When Iapetus was closed, Baltica subduction stepped westward and continued beneath Laurentia. The back stepping of subduction at the onset of continental collision caused out‐of‐sequence propagation of the orogenic wedge. Thrusting cut downsection across the existing tectonostratigraphy, emplacing units of lower metamorphic grade above the high‐grade Seve Nappe Complex. This imbrication generated the present metamorphic zonation of the Caledonian nappe pile during sustained convergence between Laurentia and Baltica.

Image log analysis of in situ stress orientation, breakout growth, and natural geologic structures to 2.5 km depth in central Scandinavian Caledonides: Results from the COSC‐1 borehole

AbstractStress‐induced borehole deformation analysis in the Collisional Orogeny in the Scandinavian Caledonide deep scientific borehole establishes in situ stress orientation in a poorly characterized region in central Sweden. Two acoustic televiewer logging campaigns, with more than 1 year between campaigns, provide detailed images along the full length of the 2.5 km deep borehole for breakout, drilling‐induced tensile fracture (DITF), and natural occurring structural analysis. Borehole breakouts occur in 13 distinct zones along total length of 22 m, indicating an average maximum horizontal stress, SHmax, orientation of 127° ± 12°. Infrequent DITFs are constrained within one zone from 786 to 787 m depth (SHmax orientation: 121° ± 07°). These SHmax orientations are in agreement with the general trend in Scandinavia and are in accordance with many mechanisms that generate crustal stress (e.g., ridge push, topographic loading, and mantel driven stresses). The unique acquisition of image logs in two successions allows for analysis of time‐dependent borehole deformation, indicating that six breakout zones have crept, both along the borehole axis and radially around the borehole. Strong dynamic moduli measured on core samples and an inferred weak in situ stress anisotropy inhibit the formation of breakouts and DITFs. Natural fracture orientation below 800 m is congruent to extensional or hybrid brittle shear failure along the same trend as the current SHmax. Analysis of foliation in the image logs reinforces the interpretation that the discontinuous seismic reflectors with fluctuating dip observed in seismic profiles are due to recumbent folding and boudinage.

Seismic anisotropy in mid to lower orogenic crust: Insights from laboratory measurements of Vp and Vs in drill core from central Scandinavian Caledonides

Recent drilling of the first Collisional Orogeny in the Scandinavian Caledonides scientific borehole (COSC-1) near Åre, Sweden permitted a laboratory investigation of seismic anisotropy on high metamorphic grade and highly deformed core samples. The 2.5km deep borehole crosscuts the amphibolite-grade Lower Seve Nappe and intersects a high-strain shear zone in the lowermost 800m. Measurements of ultrasonic compressional (Vp) and shear (Vs) velocities are conducted at room temperature and pressures ranging from room conditions up to 260MPa on six core sections that represent the most abundant lithologies in the borehole. The core sections consist of two amphibolites, a calc-silicate gneiss, a felsic gneiss, an amphibole-rich gneiss, and a garnet-bearing micaschist from the shear zone. Three mutually perpendicular samples were taken to characterize the anisotropy induced by the clear foliation and lineation. The intrinsic (crack-free velocities) Vp0 and Vs0 in the direction perpendicular to foliation ranges from 5.51 to 6.67km/s and 3.31 to 4.13km/s, respectively. In the direction parallel to foliation the Vp0 and Vs0 ranges from 6.31 to 7.25km/s and 3.53 to 4.35km/s, respectively. Vp anisotropy ranges from 3% in the calc-silicate gneiss to 19% in the micaschist. In the upper crustal seismic reflection survey around the COSC-1 borehole, reflection coefficient analysis suggests that reflectors occur due to impedance contrast between commonly occurring amphibolites and gneisses in the upper 1800m and the micaschists below. When extrapolated to mid-crustal levels the analysis indicates that both rock types can produce reflection coefficients between adjacent lithologies in excess of 0.1. Similarities in lithologies, shear zone thickness, and reflectivity pattern in the Central Scandinavian Caledonides compared to other orogens (e.g., the Himalaya) demonstrate the importance of these measurements as a proxy for in-situ strongly anisotropic shear zones in the middle crust.

Traces of the Transscandinavian Igneous Belt in the central Scandinavian Caledonides: U-Pb zircon dating and geochemistry of crystalline basement rocks in the Middle Allochthon

Basement-slices are frequent components in the lower nappes of the central Scandinavian Caledonides. New geochronological and geochemical data provide evidence that three of these basement-slices in the Middle Allochthon are derived from the Transscandinavian Igneous Belt. Dated samples in this study comprise a quartz monzonite from the Stalon Nappe Complex and a quartz monzodiorite and a monzonite from the Ammarnäs Nappe Complex. A Ce/Yb-Ta/Yb-plot of the mafic rocks from the Ammarnäs Nappe Complex suggests a calc-alkaline to shoshonitic geochemical character. Low Ni- and Cr-contents and low Mg# indicate that the mafic rocks originated from an already evolved magma. Enrichment of Fe-Ti oxides and V>200 ppm indicate presence of cumulus phases to various degrees. These basement-derived rocks in the Middle Allochthon yielded concordant U-Pb zircon SIMS ages of 1799 ± 10 Ma, 1787 ± 6 Ma and 1797 ± 5 Ma. They are therefore interpreted to represent rocks detached from the Palaeoproterozoic Transscandinavian Igneous Belt and incorporated in the Middle Allochthon during Scandian orogeny.

Early- to mid-Silurian extrusion wedge tectonics in the central Scandinavian Caledonides

Research Article| April 01, 2015 Early- to mid-Silurian extrusion wedge tectonics in the central Scandinavian Caledonides Jens C. Grimmer; Jens C. Grimmer 1Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany Search for other works by this author on: GSW Google Scholar Johannes Glodny; Johannes Glodny 2Inorganic and Isotope Geochemistry Section, Chemistry of the Earth Department, GFZ German Research Centre for Geosciences, D-14473 Potsdam, Germany Search for other works by this author on: GSW Google Scholar Kirsten Drüppel; Kirsten Drüppel 1Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany Search for other works by this author on: GSW Google Scholar Reinhard O. Greiling; Reinhard O. Greiling 1Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany Search for other works by this author on: GSW Google Scholar Agnes Kontny Agnes Kontny 1Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information Jens C. Grimmer 1Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany Johannes Glodny 2Inorganic and Isotope Geochemistry Section, Chemistry of the Earth Department, GFZ German Research Centre for Geosciences, D-14473 Potsdam, Germany Kirsten Drüppel 1Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany Reinhard O. Greiling 1Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany Agnes Kontny 1Institute of Applied Geosciences, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany Publisher: Geological Society of America Received: 09 Nov 2014 Revision Received: 23 Jan 2015 Accepted: 27 Jan 2015 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2015 Geological Society of America Geology (2015) 43 (4): 347–350. https://doi.org/10.1130/G36433.1 Article history Received: 09 Nov 2014 Revision Received: 23 Jan 2015 Accepted: 27 Jan 2015 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jens C. Grimmer, Johannes Glodny, Kirsten Drüppel, Reinhard O. Greiling, Agnes Kontny; Early- to mid-Silurian extrusion wedge tectonics in the central Scandinavian Caledonides. Geology 2015;; 43 (4): 347–350. doi: https://doi.org/10.1130/G36433.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We present evidence for an extrusion wedge in the Scandian fold-thrust belt of the central Scandinavian Caledonides (Seve nappe complex). Rb-Sr multimineral geochronology in synkinematic assemblages indicates simultaneous movements at the normal-sense roof shear zone and at the reverse-sense floor shear zone between 434 Ma and 429 Ma. A Sm-Nd age of 462 Ma from a mylonitic garnet mica schist documents prograde garnet growth and possible incipient subduction. Pressure-temperature pseudosection calculations provide evidence for eclogite facies metamorphic conditions and nearly isothermal decompression at ∼670 ± 50 °C from 17.5 to 14.5 kbar in garnet-kyanite mica schists during reverse-sense shearing, and from 15 to 11 kbar in garnet mica schists during normal-sense shearing. These data and the presence of decompression-related pegmatites dated at 434 Ma and 429 Ma indicate that the Seve nappes form a large-scale extrusion wedge. This wedge extends along strike for at least 150 km and marks an early stage of ultrahigh-pressure metamorphism, exhumation, and orogenic wedge formation in this part of the Scandinavian Caledonides predating the major, post–415 Ma ultrahigh-pressure exhumation processes in southwestern Norway. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A comparison of two orogenic margins: central Scandinavian Caledonides and western Outer Carpathians

A comparison of two orogenic margins: central Scandinavian Caledonides and western Outer Carpathians

Evidence for hyperextension along the pre-Caledonian margin of Baltica

We propose that a mantle-peridotite-bearing mélange unit, which has been mapped for more than 400 km in southern Norway, represents vestiges of deep basin(s) formed by hyperextension of the Baltic margin during the inception of the Caledonian Wilson cycle. In the mélange, which occurs below the crystalline nappes of the Middle Allochthon in southern Norway, solitary mantle peridotites, detrital serpentinites, metabasalts, gabbros and deep basin sediments are interlayered and imbricated with allochthonous, coarse-grained siliclastic sediments and slivers of Proterozoic basement rocks. The siliciclastic sediments have detritus derived from Proterozoic rocks, similar to those of the large crystalline nappes and the autochthonous basement of Baltica. The mélange unit shares many similarities with the hyperextended Tethyan margin complexes that have been described from the Alps, the Pyrenees and several segments of the present-day passive ocean margins, and we interpret it to represent an ancient late Proterozoic to Palaeozoic analogue to present-day hyperextended passive margins. Regional maps show that the mélange continues into the central Scandinavian Caledonides and that this basin assemblage may have had a much wider geographical distribution. The new model suggests that a major reinterpretation of the Lower and Middle Allochthons of the Scandinavian Caledonides is required.

The Heggmovatn supracrustals, North Norway—A late Mesoproterozoic to early Neoproterozoic (1050–930 Ma) terrane of Laurentian origin in the Scandinavian Caledonides

Rocks of the “Heggmovatn dome”, North Central Scandinavian Caledonides have traditionally been interpreted as a fold nappe involving Baltican basement rocks. They are also considered to be comparable in age to the orthogneisses in the Glomfjord dome to the south, the Rishaugfjell dome to the east and the Tysfjord culmination to the north, dated to ca. 1.7–1.8Ga. However, while these basement culminations are generally dominated by orthogneisses, the Heggmo area is composed of alternating metapsammites and metapelites intruded by both pre-and post-tectonic granitoids. U–Pb ID-TIMS ages on magmatic zircons from the syn/post-tectonic granitoids indicate a mid-Silurian emplacement age (ca. 430Ma) whereas zircons from a migmatitic biotite-rich pre-tectonic intrusive indicate a Grenvillian/Sveconorwegian crystallization age (ca. 925Ma). U–Pb LA-ICPMS ages on clastic zircons from the Heggmo metapsammites, coupled with ages from cross-cutting granitoid intrusives constrain the period of deposition of the sedimentary sequence to be between ca. 1050Ma and ca. 930Ma.There are strong similarities in lithologies, depositional age, age of clastic zircon populations and magmatic activity, to Laurentian derived supracrustal sequences, particularly, the Krummedal Sequence in Northeast Greenland. Based on these similarities and on its high tectonic position in the Caledonian nappe pile, the Heggmo rocks are reinterpreted to represent an exotic terrane of Laurentian affinity, probably derived from present day NE Greenland, and thrust onto Baltica as part of the Uppermost Allochthon during the Scandian phase (E. Devonian) of the Caledonian orogeny.

Serpentinites and low-K island arc meta-volcanic rocks in the Lower Köli Nappe of the central Scandinavian Caledonides: Late Cambrian–early Ordovician serpentinite mud volcanoes in a forearc basin?

The late Cambrian to early Ordovician meta-volcanic rocks of the Caledonian Lower Köli Nappe consist mainly of tholeiitic low-K island arc basalts, andesites, and rhyodacites. The dominance of rhyodacites in this meta-volcanic succession raises the question on whether fractional crystallization or partial melting were involved in their origin. Low Mg#, low Cr and Ni contents and compositional trends imply at least two stages of fractional crystallization for the origin of the meta-volcanic rocks. Sedimentary-hosted serpentinites occur stratigraphically below and above the meta-volcanic rocks raising the question on their origin. Geochemical data indicate strongly depleted harzburgitic–dunitic peridotite as precursor rocks of the serpentinites. Unusually high contents of As, Sb, Pb in these serpentinites are not in agreement with a depleted mantle geochemistry, but indicate enrichment by fluids from the subducted slab during serpentinization in the mantle wedge. The massive, detrital, and in places fossiliferous serpentinite bodies within the sedimentary host-rocks point to former serpentinite mud volcanoes within a non-accretionary forearc. Therefore it is suggested that the highly fractionated volcanic rocks were emplaced as lava flows and shallow intrusions in sedimentary forearc successions implying that the Lower Köli Nappe was part of a much larger trench–arc complex involving at least the immediate hanging wall Middle Köli Stikke Nappe.

The Ammarnäs Complex in the central Scandinavian Caledonides: an allochthonous basin fragment in the foreland of the Sveconorwegian orogen?

Terra Nova, 23, 270–279, 2011 Abstract The Ammarnäs Complex is a tectonic unit in the Middle Allochthon of the central Scandinavian Caledonides and consists of several hundred metres of thick uniform meta-greywacke successions of greenschist metamorphic grade containing kilometre-scale lenses of pre-Caledonian crystalline basement rocks. Meta-greywacke samples were analysed for major and trace elements and U-Pb isotopes. Lithoclasts and geochemical data indicate that the greywackes were derived from granitic–granodioritic rocks. Detrital zircon dating yielded 7% Neoproterozoic, 52% Mesoproterozoic and 41% Palaeoproterozoic 207Pb/206Pb ages with two prominent age peaks at 1.64 and 1.5 Ga and a broad distribution of Sveconorwegian (1140–900 Ma) and ‘pre-Sveconorwegian’ (1340–1140 Ma) ages. We interpret the Ammarnäs Complex as a former Neoproterozoic basin in the foreland of the Sveconorwegian orogen, which escaped major pre-Caledonian magmatism–metamorphism and which was most likely deposited between c. 920 and 630 Ma. Terra Nova, 00, 1–10, 2011