TS7.11

EDI
The Caledonian orogen of the North Atlantic region: a natural laboratory for studying tectonic processes

The Caledonian mountain belt represents a world-class example of a deeply denudated Himalayan-style orogen. The exposed crustal sections allow the study of all stages of the Wilson cycle and may contribute to our understanding of many fundamental processes in Earth Sciences, including (1) continental-rifting, break-up and ocean formation, (2) subduction, (3) marginal basin formation, (4) arc-continent and continental collisions, (5) (U)HP metamorphism, (6) orogenic wedge formation and dynamics, (7) the formation and evolution of crustal-scale shear zones, (8) fluid-rock interactions, (9) ductile and brittle deformation mechanisms, and (10) the dynamics of late- to post-orogenic extension and deep crustal exhumation.

This session aims to bring together scientists studying rocks and geological processes from all stages of the Caledonian Wilson cycle, i.e. from rifting to collision and post-orogenic extension, and welcomes sedimentological, petrological, geochemical, geochronological, geophysical, structural, and modelling contributions that help to improve our understanding of the Caledonides and mountain belts in general.

Co-organized by GMPV11
Convener: Jaroslaw Majka | Co-conveners: Deta Gasser, Johannes Jakob, Holger Stunitz
vPICO presentations
| Thu, 29 Apr, 15:30–17:00 (CEST)

vPICO presentations: Thu, 29 Apr

Chairpersons: Jaroslaw Majka, Deta Gasser, Johannes Jakob
15:30–15:35
The Appalachians, British Caledonides and their links to the Scandinavian Caledonides
15:35–15:40
|
EGU21-10080
|
ECS
|
solicited
|
Highlight
Joseph P. Gonzalez, Suzanne L. Baldwin, Jay B. Thomas, William O. Nachlas, Paul G. Fitzgerald, and Pierre Lanari

The Caledonian orogen formed following Paleozoic subduction of the Iapetus Ocean and preserves evidence of ultrahigh-pressure (UHP) metamorphism and exhumation of crustal rocks from mantle depths. The Appalachian orogen similarly formed in the Paleozoic following subduction of Iapetus Ocean crust, but evidence for (U)HP metamorphism in exhumed Appalachian rocks has been challenging to identify. We present results from a metapelite from high-pressure rocks of the Tillotson Peak Complex in the northern Appalachians, which formed during the middle-Ordovician Taconic orogeny. This sample contained mm-cm scale garnet porphyroblasts that host abundant mineral inclusions. Confocal Raman microspectroscopy of inclusions in the rims of a garnet porphyroblast identified relic coesite, preserved as a bi-mineralic inclusion composed of coesite in α-quartz. Raman depth profiling and 2-dimensional mapping indicate the relic coesite is ~10 μm3, suggesting that mineralogical evidence of UHP metamorphism in the Appalachians may be preserved only as μm-scale inclusions contained in polymetamorphosed rocks. We applied quantitative WDS X-ray maps acquired with electron microprobe, quartz-in-garnet elastic thermobarometry, and Zr-in-rutile trace element thermometry to further constrain the metamorphic history of the coesite-bearing metapelite. Garnet zoning patterns in conjunction with elastic and trace element thermobarometry applied to co-entrapped mineral inclusions suggest that garnet nucleated at 14-15.5 kbar and 420-520 °C, and continuously crystallized to 15-19.5 kbar and 470-560 °C during subduction zone metamorphism. Peak metamorphic conditions based on the stability field of coesite and on Zr-in-rutile thermometry from inclusions in the garnet rims suggest UHP metamorphism at >28 kbar and 530 °C. UHP metamorphism of pelitic sediments within the Taconic paleo-subduction zone invite comparisons with similar UHP rocks in the Caledonian orogeny. Future studies of UHP metamorphism in the Appalachian orogen will focus on constraining: 1) the spatial and temporal scales of UHP metamorphism, 2) the retrograde/exhumation P–T path of the coesite-bearing metapelite, and 3) the P–T history of other nearby metamorphic units, such as the Tillotson peak metabasites, to evaluate if these units shared a similar metamorphic history.

How to cite: Gonzalez, J. P., Baldwin, S. L., Thomas, J. B., Nachlas, W. O., Fitzgerald, P. G., and Lanari, P.: Petrologic constraints on subduction zone metamorphism from a coesite-bearing metapelite in the Northern Appalachian Orogen, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10080, https://doi.org/10.5194/egusphere-egu21-10080, 2021.

15:40–15:42
|
EGU21-6526
|
ECS
Lewis Evason, Anna Bird, Eddie Dempsey, Kit Hardman, Martin Smith, and Rob Strachan

The Grampian Shear Zone (GSZ) represents a highly deformed tectonostratigraphic contact between the Proterozoic metamorphic rocks of the Dalradian Group from the underlying high grade metamorphic Neoproterozoic rocks of the Badenoch Group within the Grampian Highlands. The nature (tectonic suture or palaeo-unconformity), age and structure of the GSZ and indeed the underling Badenoch Group are poorly constrained. Previous studies of the GSZ and synkinematic (intruded during shearing) pegmatites found therein, yielded metamorphic/deformation (and magmatic) ages ranging from c.a. 808 to 440 M. This study reinvestigates this shearzone using in-situ (within section) petrochonological analysis on a range of U-Pb and Rb-Sr chronometers – Monazite, zircon, titanite, rutile and mica. Carrying out this analysis in-situ and using a variety of minerals allows us to directly date deformation fabrics over a wide range of deformation temperatures, giving us a far more detailed picture of the events recorded within these rocks. Large monazite grains (≥100μm) were mapped using in-situ LA-ICP-MS to show within grain variation of major elements and REEs. Monazite U-Pb spot analysis from the GSZ has yielded ages ranging from 784.11 ± 1.2Ma to 442.58 ± 0.58Ma. The same analysis was performed on a sample from the Grampian group which yielded an age of 441.34 ± 037Ma. In addition to this monazite data, in-situ U-Pb Titanite analysis from the Badenoch Group gave ages of 526.96 ± 1.33 Ma from a metabasite sample, with a metasedimentary sample giving a range of titanite U Pb ages from 540 to 460Ma. These age ranges show that the Badnoch Group and the GSZ have recorded a complex polyorogenic history relative to the “simple” overlying Dalradian metasediments. We propose that the Grampian Shear Zone represents a deep-seated Knoydartian (808 to 784Ma) age shear zone within the meso-Neoproterozoic Badenoch Group. This shear zone was then reactivated during the Grampian phase of the Caledonian Orogeny resulting in the tectonic emplacement of the Dalradian metasediments above the Badenoch group.

How to cite: Evason, L., Bird, A., Dempsey, E., Hardman, K., Smith, M., and Strachan, R.: Unravelling the enigmatic Grampian Shear Zone: In-situ monazite and titanite U-Pb analysis of the juxtaposed Badenoch and Grampian Groups, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6526, https://doi.org/10.5194/egusphere-egu21-6526, 2021.

15:42–15:44
|
EGU21-722
|
ECS
Timothy Armitage, Robert Holdsworth, Robin Strachan, Thomas Zach, Diana Alvarez-Ruiz, and Eddie Dempsey

Ductile shear zones are heterogeneous areas of strain localisation which often display variation in strain geometry and combinations of coaxial and non-coaxial deformation. One such heterogeneous shear zone is the c. 2 km thick Uyea Shear Zone (USZ) in northwest Mainland Shetland (UK), which separates variably deformed Neoarchaean orthogneisses in its footwall from Neoproterozoic metasediments in its hanging wall (Fig. a). The USZ is characterised by decimetre-scale layers of dip-slip thrusting and extension, strike-slip sinistral and dextral shear senses and interleaved ultramylonitic coaxially deformed horizons. Within the zones of transition between shear sense layers, mineral lineations swing from foliation down-dip to foliation-parallel in kinematically compatible, anticlockwise/clockwise-rotations on a local and regional scale (Fig. b). Rb-Sr dating of white mica grains via laser ablation indicates a c. 440-425 Ma Caledonian age for dip-slip and strike-slip layers and an 800 Ma Neoproterozoic age for coaxial layers. Quartz opening angles and microstructures suggest an upper-greenschist to lower-amphibolite facies temperature for deformation. We propose that a Neoproterozoic, coaxial event is overprinted by Caledonian sinistral transpression under upper greenschist/lower amphibolite facies conditions. Interleaved kinematics and mineral lineation swings are attributed to result from differential flow rates resulting in vertical and lateral extrusion and indicate regional-scale sinistral transpression during the Caledonian orogeny in NW Shetland. This study highlights the importance of linking geochronology to microstructures in a poly-deformed terrane and is a rare example of a highly heterogeneous shear zone in which both vertical and lateral extrusion occurred during transpression.

How to cite: Armitage, T., Holdsworth, R., Strachan, R., Zach, T., Alvarez-Ruiz, D., and Dempsey, E.: Complex kinematics in a major ductile shear zone, NW Shetland: Evidence of ductile extrusion during Caledonian transpression, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-722, https://doi.org/10.5194/egusphere-egu21-722, 2021.

15:44–15:46
|
EGU21-10091
Bernard Bingen, Espen Torgersen, and Morgan Ganerød

Geological mapping, zircon U–Pb dating of 28 samples, and mica 40Ar–39Ar dating of 7 samples in the Stavanger–Ryfylke region (Stavanger, Suldal, Nedstrand, Randøy) characterizes the tectonostratigraphy of the southernmost nappes in the Scandinavian Caledonides. Four main tectonostratigraphic levels are described. (1) The lowest phyllite/mica schist nappes –Buadalen, Holmasjø, Lower Finse, Synnfjell– represent the Cambro–Ordovician sediment cover of the Baltic margin. (2) The overlying nappes –Madla, Storheia, Dyrskard, Hallingskarvet– consist of felsic metaigneous rocks with a consistent age between c.1525 and 1493 Ma. They host c.1040 Ma intrusives and c.1025 Ma Sveconorwegian metamorphism. They likely represent transported Baltican (Sveconorwegian) basement, widely exposed in S Norway. (3) The overlying nappes –Sola, Boknafjord, Kvitenut, Revseggi– are more diverse and lack counterparts in the exposed Baltican crust. The Sola nappe, near Stavanger, comprises a marine succession –Kolnes succession– of mica schist, metasandstone, marble, amphibolite and felsic metavolcanic rocks. The metavolcanic rocks –Snøda metadacite–rhyolite– are fine-grained mica gneisses, with calc-alkaline composition. Their extrusion age of c.941–934 Ma date deposition of the sequence. Detrital zircons in a metasandstone sample (n=138) yield main age modes at c.1040, 1150 and 1395 Ma, as well as significant Paleoproterozoic and Archaean modes. The Kolnes succession was affected by Taconian/Grampian metamorphism peaking in eclogite-facies conditions between c.471 and 458 Ma (Smit et al., 2010), followed by regional cooling around 445–435 Ma. Leucogranite bodies (c.429 Ma) cut the Grampian fabric. Several 40Ar–39Ar white mica and biotite plateau ages constrain the timing of Scandian top-to-the SE nappe stacking at c.420 Ma. The Boknafjord nappe in Nedstrand comprises a c.932 Ma augen gneiss, overlain successively by amphibolite and mica schist units. Preliminary detrital zircon data (n=11) imply an Ordovician (<459 Ma) deposition for the mica schist. (4) The highest nappes –Karmsund and Hardangerfjord– host the Karmøy and Bømlo ophiolite complexes. These complexes comprise a c.493 Ma supra subduction zone ophiolite, intruded by c.485–466 Ma volcanic arc plutonic rocks, and unconformably overlain by fossiliferous upper Ordovician (<c.445 Ma) clastic sediments (Pedersen and Dunning, 1997).

We propose that the Iapetan Karmøy–Bømlo ophiolite complexes were accreted onto the Kolnes succession on the Laurentian side of the Iapetus realm, during the Grampian orogeny, before integration of both in the Scandian nappe pile. The age of HP metamorphism in the Kolnes succession (471–458 Ma) matches the inferred timing for obduction of the Karmøy–Bømlo complexes (485–448 Ma). The evidence for a Laurentian margin obduction stems from a conspicuous similarity with Shetland. On Shetland, the c.492 Ma Unst–Fetlar ophiolite complex was obducted during the Grampian orogeny onto Neoproterozoic Laurentian marine sequences (psammite-marble-mica gneiss) of the Westing, Yell Sound and East Mainland successions. The Westing and Yell Sound successions are characterized by a c. 944–925 Ma, Renlandian, high-grade metamorphism, a dominant detrital zircon mode at 1030 Ma, and common Archean detrital zircons. They correlate well with the Kolnes succession and suggest an ancestry along the Neoproterozoic Renlandian active margin of Laurentia and Rodinia, before opening of Iapetus.

How to cite: Bingen, B., Torgersen, E., and Ganerød, M.: Tectonostratigraphy of the Southernmost Scandinavian Caledonides: testing the Shetland correlation and the Laurentian/Renlandian link, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10091, https://doi.org/10.5194/egusphere-egu21-10091, 2021.

The Baltica outer margin prior to Iapetus opening
15:46–15:48
|
EGU21-16408
David G. Gee and Jarosław Majka

In the Scandes, the lower thrust sheets of the Caledonian allochthons provide unambiguous stratigraphic evidence of correlation with the successions of the Baltoscandian platform. Cambrian successions, including the Alum Shale Formation, providing the footwall for the main Caledonian decollement in Scandinavia, can be followed at least 200 km westwards from the thrust front into the hinterland of the orogen. The overlying early Palaeozoic strata provide evidence of facies changes into foreland basin deposits in the mid Ordovician and early Silurian; also of Ediacaran and Cryogenian successions, including Marinoan tillites. The amount of internal shortening in the Lower Allochthon is not uncontroversial, but certainly amounts to more than 100 km, implying that all the overlying alllochthons in the Scandes were derived from west of the Norwegian coast.

The metamorphic grade of the units in the Lower Allochthon increases from low to high greenschist facies, from the thrust front westwards into the deep hinterland. Overlying thrust sheets of the Middle Allochthon are of higher metamorphic grade and more ductilely deformed. The basal parts are usually dominated by basement-derived units and Neoproterozoic sedimentary rocks. They are overthrust by dolerite dyke-intruded thrust sheets, the Särv Nappes, with host-rocks dominated by Cryogenian and Ediacaran sandstones, the former including subordinate limestones and Marinoan tillites. The Baltoscandian margin dolerite dyke swarms amount to up to c. 35% of these thrust sheets.

The overlying, highest tectonic units in the Middle Allochthon (the Seve Nappe Complex, SNC) are of amphibolite and higher metamorphic grade. They include a greater variety of lithologies, including some that are very similar to those in the underlying Särv Nappes (e,g. quartzites and eclogitized dolerites). The metasedimentay host rocks include a wide range of paragneisses and marbles. Abundant mafic rocks include metamorphosed gabbros, basalts and peridotites and, together with the dyke swarms, can totally dominate the composition of some thrust sheets. The similar geochemistry and early Ediacaran age (c. 600 Ma) of the mafic rocks in the Särv and Seve nappes define the Baltoscandian outermost margin and continent-ocean transition zone (COT). Iapetus Ocean terranes comprise the overlying thrust sheets of the Upper Allochthon (e.g. the Köli Nappe Complex).

The metamorphism of the different thrust sheets in the SNC provide clear evidence that some parts were subducted; others not. A wide range of isotope age data constrain the timing of subduction, with the earliest ages in the mid Cambrian (c. 505 Ma) to early Ordovician (c. 483 Ma). It has been suggested that the deposition of the Alum Shale Formation on the Baltscandian platform, was related to this early Caledonian subduction. A more probable interpretation is that subduction along the outermost edge of this highly extended COT did not influence the edge of the platform till the early Tremadoc.

Some authors have introduced cryptic sutures into the Baltoscandian outer margin, described above. They should reassess their data and better define the evidence for their conviction.

How to cite: Gee, D. G. and Majka, J.: Reconstructing the Ediacaran to Ordovician history of the Baltoscandian Margin of continent Baltica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16408, https://doi.org/10.5194/egusphere-egu21-16408, 2021.

15:48–15:50
|
EGU21-6121
|
ECS
Rodolphe Lescoutre, Bjarne Almqvist, Hemin Koyi, Olivier Galland, Peter Hedin, Sonia Brahimi, Henning Lorenz, and Christopher Juhlin

The role of inheritance in localizing basement deformation in the foreland has been demonstrated in orogens in different parts of the world. In the external domain of the central Scandinavian Caledonides, questions remain about the amount and the distribution of deformation accommodated by the Baltica basement during Caledonian orogeny. However, to answer these questions, it is necessary to understand the architecture of the Baltica crust underneath the Caledonian nappes and to determine the occurrence of potential detachment horizons or inherited structures that accommodated the shortening.

In this work, we study the lithological and structural architecture of the Baltica basement in central Sweden, east and west of the present-day Caledonian front. The aim is twofold: 1) identifying the main geological features of the Fennoscandian Shield and their regional extent underneath the Caledonian nappes to the west, and 2) to address their role in accommodating deformation during Caledonian orogeny.

The study area is characterized by mainly ~1.8 Ga granitic bodies intruded by various generations of mafic intrusions and locally bounded by major crustal shear zones. On the one hand, based on seismic interpretations, magnetic and gravimetry forward modeling and mapping, and results from the recently drilled COSC-2 borehole (as part of the Collisional Orogeny in the Scandinavian Caledonides (COSC) drilling project), we show that the basement underlying the Caledonian nappes is characterized by inclined to sub-horizontal mafic intrusions with large extent, emplaced at mid-crustal level. We propose that these intrusions are similar in size, geometry, and potentially age, to the 1.25 Ga Central Scandinavian Dolerite Group (CSDG) that are mapped as 100’s km long elliptic bodies or described as saucer-shaped intrusions further east. On the other hand, based on observations from COSC-2 drill cores and previous studies, analogue modelling and 2D seismic restoration, we propose that favorably oriented intrusions influenced, at least partly, crustal shortening in this area by localizing deformation along their margins. At a regional scale, we discuss the distribution of thick-skinned and thin-skinned deformation at the present-day orogenic front. On a broader scale, this study raises the question regarding the influence of pre-existing mafic intrusions in controlling the structural evolution and the segmentation of orogenic or rift systems in general.

How to cite: Lescoutre, R., Almqvist, B., Koyi, H., Galland, O., Hedin, P., Brahimi, S., Lorenz, H., and Juhlin, C.: Large-scale flat-lying mafic intrusions in the granitic Baltica crust of central Sweden and implications for basement deformation during Caledonian orogeny, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6121, https://doi.org/10.5194/egusphere-egu21-6121, 2021.

15:50–15:52
|
EGU21-11471
|
ECS
Sabine Rousku, Erika Nääs, Christopher Barnes, Abigail Barker, and Jarosław Majka

The Seve Nappe Complex (SNC) of the Scandinavian Caledonides comprises Neoproterozoic sedimentary and igneous rocks that experienced high-pressure metamorphism and deformation during subduction and exhumation. Fieldwork was conducted in the Kebnekaise region in northern Sweden, focusing on the Aurek metagabbro and the Vistas metaigneous rocks within the Vássačorru Igneous Complex (VIC), hosted within SNC metasediments. Field observations show that the Aurek metagabbro is locally sheared with well-defined foliation and lineation. In contrast, the Vistas metaigneous rocks, consisting of both granite and gabbro bodies, are only locally foliated. Furthermore, the granite is intruded by ENE-WSW striking dolerite and rhyolite dykes that parallel the local foliation, and are weakly deformed, whereas a NNE-SSW striking syenite dyke is observed in a portion of undeformed gabbro.

The Aurek metagabbro mineral assemblages consist of garnet, amphibole, plagioclase, biotite, chlorite, and pyroxene. The Vistas gabbro and dolerite dyke both consist of plagioclase, pyroxene, and amphibole. The Vistas granites and rhyolite dyke include quartz, feldspar, biotite, muscovite, ± garnet, and the syenite dyke contains feldspar, plagioclase, pyroxene, amphibole, quartz, and biotite. The Vistas metaigneous rocks generally show primary igneous assemblages.

Bulk rock chemistry shows that the Aurek and Vistas gabbros, and the Vistas dolerite dyke, are classified as tholeiites. For the Aurek gabbros, Th/Yb of 0.06-1.86 and Nb/Yb of 0.11-5.14 indicate that they have N-MORB to E-MORB compositions, with possible crustal input. The Vistas gabbro (Th/Yb of 0.09 and Nb/Yb of 1.15) and the dolerite dyke (Th/Yb of 0.12 and Nb/Yb of 0.66) also suggest such trend. The Vistas granites, rhyolite, and syenite dyke all have calc-alkaline composition. Trace elements confirm volcanic arc affinity for the granites and the syenite dyke (Nb: 3.1-5.9 ppm, Rb: 116.5-177.5 ppm, Y: 12.9-18.0 ppm, Ta: 0.3-0.4 ppm, Yb: 2.04-3.19 ppm), whereas the rhyolite dyke (Nb: 38.2 ppm, Rb: 247.8 ppm, Y: 72.6 ppm, Ta: 2.8 ppm and Yb: 12.62 ppm) reflects a within plate setting.

Combining the field relationship with geochemistry of the studied metaigneous rocks, we tentatively propose that the VIC is composed of three pulses of magmatism: (1) mafic MORB magmatism represented by the gabbros, emplaced in an extensional regime; (2) felsic calc-alkaline magmatism represented by granites and syenite, emplaced in an active continental margin environment; and (3) bimodal within-plate magmatism or crustal assimilation in a volcanic arc represented by dolerite and rhyolite dykes. However, the only existing age is from U-Pb zircon dating of the Vistas granite, which yielded 845±14 Ma (Paulsson & Andreasson, 2002). Further zircon U-Pb geochronology will be conducted to obtain ages of the various lithologies of the VIC to better understand temporal relationships and to link the VIC with tectonic events in the Scandinavian Caledonides.

This study was supported by the National Science Centre (Poland) grant no. 2019/33/B/ST10/01728 to J. Majka.

References

Paulsson, O., Andreasson, P.-G., 2002. Attempted break-up of Rodinia at 850 Ma: Geochronological evidence from the Seve-Kalak Superterrane, Scandinavian Caledonides. J. Geol. Soc. 159, 751–761. https://doi.org/10.1144/0016-764901-156

How to cite: Rousku, S., Nääs, E., Barnes, C., Barker, A., and Majka, J.: Deciphering the Vássačorru Igneous Complex within the Seve Nappe Complex, Scandinavian Caledonides , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11471, https://doi.org/10.5194/egusphere-egu21-11471, 2021.

15:52–15:54
|
EGU21-4119
|
Highlight
Christian Tegner, Torgeir B. Andersen, Hans Jørgen Kjøll, Eric L. Brown, Graham Hagen-Peter, Fernando Corfu, Sverre Planke, and Trond H. Torsvik

The origin of Large Igneous Provinces (LIPs) associated with continental breakup and the reconstruction of continents older than c. 320 million years (pre-Pangea) are contentious research problems. Here we study the petrology of a 615 - 590 Myr dolerite dyke complex that intruded rift-basins of the magma-rich margin of Baltica and now is exposed in the Scandinavian Caledonides. These dykes are part of the Central Iapetus Magmatic Province (CIMP), a LIP emplaced in Baltica and Laurentia during opening of the Iapetus Ocean within the Caledonian Wilson Cycle. The >1000 km long dyke complex displays lateral geochemical zonation from enriched to depleted basaltic compositions from south to north. Geochemical modelling of major and trace elements shows these compositions are best explained by melting hot mantle 75-250°C above ambient mantle. Although the trace element modelling solutions are non-unique, the best explanation involves melting a laterally zoned mantle plume with enriched and depleted peridotite lithologies, similar to present-day Iceland and to the North Atlantic Igneous Province. The origin of CIMP appears to have involved several mantle plumes. This is best explained if rifting and breakup magmatism coincided with plume generation zones at the margins of a Large Low Shear-wave Velocity Province (LLSVP) at the core mantle boundary. If the LLSVPs are quasi-stationary back in time as suggested in recent geodynamic models, the CIMP provides a guide for reconstructing the paleogeography of Baltica and Laurentia 615 million years ago to the LLSVP now positioned under the Pacific Ocean. Our results provide a stimulus for using LIPs as piercing points for plate reconstructions.

How to cite: Tegner, C., Andersen, T. B., Kjøll, H. J., Brown, E. L., Hagen-Peter, G., Corfu, F., Planke, S., and Torsvik, T. H.: A mantle plume origin for the Scandinavian Dyke Complex: a “piercing point” for 615 Ma plate reconstruction of Baltica?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4119, https://doi.org/10.5194/egusphere-egu21-4119, 2021.

15:54–15:56
|
EGU21-9213
|
ECS
Riccardo Callegari, Katarzyna Walczak, Grzegorz Ziemniak, Christopher Barnes, and Jaroslaw Majka

Here, we present preliminary petrochronological results of paragneisses and schists containing bodies of metamafic rocks belonging the Upper Gneiss unit that occurs within the Seve Nappe Complex (SNC) in the Váivančohkka-Salmmečohkat area, north of the lake Torneträsk in northern Sweden and Norway.

At the outcrop scale, the paragneiss is pervasively foliated and bears features of migmatization. It hosts garnet amphibolite bodies that are locally transected by leucocratic veins. Thin section observations of the paragneiss reveal a mineral assemblage composed of Q+Grt+Amp+Bi±Pl±Ms±Sil±Ru. The leucocratic vein contains Q+Pl+Ms+Bi+Grt+Kfs±Sil. Importantly, some of the studied gneisses contain quartz, exhibiting lobate boundaries, as well as garnet surrounded by melt rim. The presence of quartz forming pseudomorphs after melt was also identified and observed to host both monophase and fluid inclusions. All of these microtextures are indicative of partial melting.

Preliminary pressure-temperature estimates derived using conventional geothermobarometry and phase equilibrium modelling corroborated petrographic observations. The peak metamorphic conditions were estimated to 8–10kbar and 800–850°C, i.e., in the stability field of melt.

Uranium-Pb zircon and Th-U-total Pb monazite dating of the migmatitic paragneiss yielded consistent age estimates of 602±5Ma and 599±3Ma, respectively. Nearly the same U-Pb age of 604±7Ma was obtained for the zircon from the leucocratic vein transecting the amphibolite within the studied gneiss. Interestingly, no Caledonian zircon nor monazite were identified. Considering the textural position of the dated zircon and monazite, as well as their chemical character, we suggest that these minerals date the partial melting event recorded by the rocks.

Regionally, we interpret that the Upper Gneiss unit of SNC in the Váivančohkka-Salmmečohkat area could be a northern continuation of the Leavasvággi gneiss associated with the Vassačoru Igneous Complex of SNC in the Kebnekaise region. Notably, the latter reveals evidence of high temperature metamorphism at c. 600Ma (Paulsson and Andréasson 2002) and its mafic component (see also Rousku et al. in this session) could be an equivalent to the metamafic rocks enclosed within the Upper Gneiss unit. The Leavasvággi gneiss and the Upper Gneiss unit together with similar rocks farther north in Indre Troms and in Corrovare which also yield a c. 610-600Ma age of high grade overprint (Gee et al. 2016; Kjøll et al. 2019). Altogether, these areas with only localized Caledonian influence diverge from traditional models developed for the SNC farther south and offer an additional insight into the development of the late Neoproterozoic margin of Baltica at the early stages of Iapetus opening.

This study was supported by the National Science Centre (Poland) grant no. 2019/33/B/ST10/01728 to J. Majka.

References

Gee et al. 2016. Baltoscandian margin, Sveconorwegian crust lost by subduction during Caledonian collisional orogeny. GFF 139, 36–51.

Kjøll et al. 2019. Timing of break-up and thermal evolution of a pre-Caledonian  Neoproterozoic exhumed magma-rich rifted margin. Tectonics 38, 1843-1862.

Paulsson & Andréasson 2002. Attempted break-up of Rodinia at 850 Ma: geochronological evidence from the Seve–Kalak Superterrane, Scandinavian Caledonides. JGS, 159, 751-761.

How to cite: Callegari, R., Walczak, K., Ziemniak, G., Barnes, C., and Majka, J.: Late Neoproterozoic granulite facies metamorphism of the Upper Gneiss unit (Seve Nappe Complex) in the Váivančohkka-Salmmečohkat area, northern Scandinavian Caledonides , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9213, https://doi.org/10.5194/egusphere-egu21-9213, 2021.

Pre-Scandian subduction-collision events recorded in the Middle and Upper/Uppermost Allochthons
15:56–15:58
|
EGU21-13097
|
ECS
Iwona Klonowska, Anna Ladenberger, David G. Gee, Pauline Jeanneret, and Yuan Li

The new LA-ICP-MS zircon isotope age data from paragneiss, amphibolite and two leucogranite intrusions in the Lower Seve Nappe of the Åre synform in the Caledonides of central Jämtland provide evidence of both Silurian and Ordovician tectonothermal histories. Well established concordant c. 468 and c. 470 Ma magmatic ages for the Så quarry leucogranite, which cut earlier foliations and folds in the host-rock amphibolites and paragneisses, imply a tectonothermal history prior to the Middle Ordovician (c. 469 Ma), perhaps synchronous with what has been previously recognized in the Seve Nappe Complex of Norrbotten (e.g. Root & Corfu, 2012), 400 km farther north in the Swedish Caledonides, and very recently also in the Middle Seve Nappe in central Jämtland (Walczak et al. 2020).

The field relationships and data presented here show that magmatic activity occurred during the early Silurian (c. 443 Ma) and earlier during the Early to Middle Ordovician (c. 469 Ma), and that deformation and metamorphism took place both prior to and after c. 469 Ma. The Lower Seve rocks from the nearby COSC-1 drill core have been metamorphosed in the upper amphibolite facies, however, the remnants of the high-pressure metamorphic history are preserved in the relic minerals, including high-silica white mica, in the garnet-bearing mica schists. The exact age of the high-pressure metamorphism is not known so far; however, it predates the 460-430 Ma amphibolite facies deformation recorded by titanites in the amphibolites (Giuntoli et al. 2020).    

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 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 the Scandian continent-continent collision, with Siluro-Devonian emplacement of the Seve Nappe Complex across the foreland basins onto the Baltoscandian platform.

References:

Giuntoli, F., Menegon, L., Warren, C.J., Darling, J., Anderson, M.W. 2020. Tectonics, 39, e2020TC006267, https://doi.org/10.1029/2020TC006267.

Root, D., Corfu, F. 2012. Contributions to Mineralogy and Petrology, 163, 769-788, https://doi.org/10.1007/s00410-011-0698-0.

Walczak, K., Barnes, C.J., Majka, J., Gee, D.G. Klonowska, I., 2020. Geoscience Frontiers (in press), https://doi.org/10.1016/j.gsf.2020.11.009.

This work is financially supported by the National Science Centre (Poland) research project no. 2018/29/B/ST10/02315 and is part of the ICDP project “Collisional Orogeny of the Scandinavian Caledonides.”

How to cite: Klonowska, I., Ladenberger, A., Gee, D. G., Jeanneret, P., and Li, Y.: Timing of deformation, metamorphism and leucogranite intrusion in the lower part of the Seve Nappe Complex in central Jämtland, Swedish Caledonides, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13097, https://doi.org/10.5194/egusphere-egu21-13097, 2021.

15:58–16:00
|
EGU21-8943
|
ECS
Christopher Barnes, Jarosław Majka, David Schneider, Mattia Gilio, Matteo Alvaro, Michał Bukała, and Matthijs Smit

            The Seve Nappe Complex (SNC) of the Scandinavian Caledonides represents portions of the Baltican margin that were subducted to mantle depths. Eclogite-bearing sub-units of the SNC provide a record of this important step in orogen development. One such sub-unit is the Vaimok Lens of the SNC in southern Norrbotten. The Vaimok Lens constitutes eclogites hosted within metasedimentary rocks that reached ultra-high pressure (UHP) conditions in the Cambrian/Early Ordovician period. The metasedimentary rocks are typically composed of quartz, white mica, garnet, plagioclase, biotite, clinozoisite, apatite and titanite, and show a pervasive ‘S2’ foliation that developed during exhumation. Garnet is recognized as a relic of prograde metamorphism during subduction, whereas the other minerals represent retrogressive metamorphism during exhumation. To resolve the timing of prograde metamorphism, Lu-Hf geochronology was conducted on metasediment-hosted garnet that preserves prograde, bell-shaped Mn-zoning with a chemical formula of Alm69-59Grs32-24Sps13-2Prp5-2. The results indicate garnet growth at 495.3 ± 2.6 Ma. Quartz-in-garnet (QuiG) elastic geobarometry was also conducted on garnet from the same sample, providing pressures of 0.9-1.3 GPa, calculated at 500-700°C. Six samples were obtained for in-situ 40Ar/39Ar geochronology, targeting white mica defining the S2 foliation. Samples can be classified as: 1) low-strain (n: 3), with large (>400 µm width), undeformed micas that are chemically homogeneous (XCel: 0.24-0.35), which yielded a weighted average 40Ar/39Ar population of 470.5 ± 5.9 Ma; 2) high-strain (n: 3), with small (<300 µm width) mica fish with heterogeneous chemistry (XCel: 0.03-0.27), which provided weighted average 40Ar/39Ar populations of 447.6 ± 2.6 Ma and 431.1 ± 4.1 Ma. An additional sample from the basal thrust of the lens that contains large (>300 µm width), homogeneous (XCel: 0.24-0.34) mica was also dated, yielding a population of 414.1 ± 5.8 Ma. Altogether, the data indicates that the Vaimok Lens was subducting by c. 495 Ma. The lens underwent post-decompression cooling at c. 470 Ma, possibly decompressing to 0.9-1.3 GPa by this time. This would equate to an exhumation rate of 3-9 mm/yr. Imbrication of the SNC in southern Norrbotten is taken to be c. 447 Ma. Scandian deformation was active by c. 431 Ma and led to overthrusting of the SNC onto subjacent nappes by latest c. 414 Ma. Both the timescale for subduction and the rates of exhumation for the Vaimok Lens reflect subduction-exhumation dynamics of large UHP terranes. Furthermore, the timing of imbrication and Scandian deformation in southern Norrbotten is similar to estimates along strike of the SNC. These results indicate that the SNC acted as a large UHP terrane that underwent a ~25 Myr cycle of subduction and exhumation during the late Cambrian/Early Ordovician, before being deformed and partially dismembered in subsequent accretionary and collisional events.

 

Research funded by National Science Centre (Poland) project no. 2014/14/E/ST10/00321 to J. Majka.

How to cite: Barnes, C., Majka, J., Schneider, D., Gilio, M., Alvaro, M., Bukała, M., and Smit, M.: The subduction, exhumation, and deformation history of the Vaimok Lens, Seve Nappe Complex, Scandinavian Caledonides, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8943, https://doi.org/10.5194/egusphere-egu21-8943, 2021.

16:00–16:02
|
EGU21-12823
|
ECS
Daniel Buczko, Magdalena Matusiak-Małek, Jarosław Majka, Iwona Klonowska, and Grzegorz Ziemniak

The Scandinavian Caledonides comprise numerous ultramafic bodies emplaced within metamorphic nappe complexes. A hypothetical suture between the most distal crustal units representing Baltican margin (Seve Nappe Complex, SNC) with the oceanic Iapetian terranes (Köli Nappe Complex) is abundant in such occurrences. Here we present preliminary data on garnet/spinel peridotites/pyroxenites from SNC in central and northern parts of Swedish Jämtland county. The presented results are a part of a project involving regional study focused on orogenic peridotites (mostly spinel-bearing) of Seve and Köli nappe complexes. 

The ultramafic bodies in the study area range from a meters to kilometer scale and comprise: 1) garnet peridotites, 2) spinel peridotites, 3) spinel pyroxenites and 4) garnet pyroxenites. Individual outcrops often record different levels of serpentinisation. 

The Grt-peridotites are usually harzburgites (sparsely dunites/lherzolites) with an assemblage of Ol+Opx+Cpx+Amph+Grt+Spl.  Minerals within the Grt-peridotites are characterised by Ol Fo=~90-91 and Mg# in pyroxenes 90-92 and 92-96 (enstatite and diopside/Cr-diopside, respectively). Garnet is pyrope with end-members Prp=60-69%, Usp=0-4% and Cr#=0.5-4. Amphibole (pargasite; Mg#=88-92) typically occurs as patches or rims around Grt and often host significant amounts of Spl. The spinel has an intermediate composition between hercynite-spinel and magnesiochromite-chromite (Cr#=41-55, Mg#=40-57). 

The spinel peridotites are formed of Ol+Opx+Amph+Chl+Spl and classify mostly as harzburgites/dunites. Olivine and Opx (enstatite, rarely Cr-enstatite; often as porphyrocrysts) show a high range of Fo/Mg# values (90-95 and 90-94, respectively). Amphibole (tremolite; Mg#=91-96) is usually evenly distributed within the rock, while Chl is often associated with grain boundaries. Spinel has a chromite composition (Cr#=82-100, Mg#=5-10). Within single large (~0.5mm) spinel grains, cores with higher Mg# (~23) and lower Cr# (~82) can be observed.

The garnet pyroxenites are websterites characterised by lower Mg# (88-90) in enstatite, presence of Al-diopside and lower Cr# (<0.5) in pyrope than in peridotites. The Spl-pyroxenites are orthopyroxenites with Mg# in enstatite (86-88) lower than in peridotitic orthopyroxene.

The presented preliminary data suggest that lithologies formed under different pressures (i.e. Grt and Spl facies) and must have recorded different evolution paths. Garnet ultramafics mineralogy resembles typical “mantle” assemblage with Prg suggesting possible metamorphic input also for other consisting phases (similarly to M2 paragenesis described in [1]). While the Grt ultramafic rocks and their evolution has been a subject of several studies before, the Spl ultramafics are relatively understudied and can shed new light on the evolution of SNC. The composition of Spl peridotites represents a mixture of typical “magmatic” mantle phases with metamorphic minerals (Amph+Chl). Very high Mg# values and occurrence of 120° triple point junctions in Ol (also described in [2]) suggest complex genesis, which probably includes serpentinisation (+exhumation?) followed by deserpentinisation. This indicates that the Spl ultramafics of SNC might have been subducted after their primary serpentinisation, which can be related either to emplacement and exhumation of ultramafics during Rodinia breakup or derivation from shallow, serpentinised “wet” mantle wedge in the subduction zone. 

Research founded by Polish National Science Centre grant no. 2019/35/N/ST10/00519.

[1] Gilio et al. (2015). Lithos 230, 1-16.
[2] Clos et al. (2014). Lithos 192-195, 8-20.

How to cite: Buczko, D., Matusiak-Małek, M., Majka, J., Klonowska, I., and Ziemniak, G.: Regional study of orogenic ultramafics of the Seve Nappe Complex, Scandinavian Caledonides - preliminary results from northern and central Jämtland, Sweden, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12823, https://doi.org/10.5194/egusphere-egu21-12823, 2021.

16:02–16:04
|
EGU21-3394
|
Highlight
Deta Gasser, Tor Grenne, Bjørgunn Dalslåen, Trond Slagstad, David Roberts, Torkil S. Røhr, and Øyvind Skår

U-Pb age spectra of detrital zircons are widely used to estimate maximum depositional ages (MDA) for sedimentary successions of various age. Different methods have been proposed for calculating an MDA. The most common are based on calculated ages of either the youngest single grain (YSG), the youngest grain cluster composed of three or more grains that overlap at 2σ (YGC 2σ), or the youngest graphical peak (YPP). Many of these methods produce MDAs consistent with biostratigraphic age or the radiometric age of volcanic horizons within the same unit; however, several studies have shown that MDA estimates based on detrital zircon can be younger than the true depositional age, particularly in active tectonic settings, indicating that the methods should be applied with care for successions where independent depositional age control is lacking.

In this contribution we present a compilation of 27 detrital zircon samples from Ordovician to Silurian strata from a part of the Trondheim Nappe Complex of the central Scandinavian Caledonides. The samples belong to six stratigraphically distinct units with independent age control from fossils, dated volcanic horizons or bracketing units of known age. These successions represent various marginal basins filled during the closing stages of the Iapetus Ocean in an overall active tectonic setting with detritus from both continental landmasses and Cambro-Ordovician island arcs. Shortly after deposition, the successions were folded and metamorphosed at up to greenschist facies during Taconian accretionary events and/or the Scandian continent-continent collision.

We calculated MDAs by the three methods YSG, YGC 2σ and YPP for all samples based on 206Pb/ 238U ages, applying a rigorous discordance filter of 5% (most studies use 10%), in order to use the most reliable analyses possible. Our analysis shows that the YSG MDA is up to 36 m.y. younger than the known depositional age for 17 of the 27 samples, with up to six individual grains giving too young age estimates in some samples. Hence, YSG MDA obviously does not provide a reliable MDA estimate. Of the YGC 2σ (weighted mean age) estimates, six are still significantly younger than known depositional age; and an additional seven are younger but overlap with the known depositional age when considering the maximum error on the YGC 2σ estimate. The only method which provides an MDA estimate within the age of known deposition or older for all samples is the YPP method.

Our results indicate that statistically robust estimates of MDA from detrital zircon data in such an active orogenic setting are provided only by the YPP method; both the YSG and the YGC 2σ methods provided unreliably young estimates even with a discordance filter of 5% (using a filter of only 10% makes the problem considerably worse). The spuriously young ages of up to six near-concordant grains in some samples is probably due to concealed lead loss, possibly caused by (fluid-assisted?) recrystallisation of zircon domains during regional greenschist-facies metamorphism shortly after deposition.

How to cite: Gasser, D., Grenne, T., Dalslåen, B., Slagstad, T., Roberts, D., S. Røhr, T., and Skår, Ø.: How reliable are maximum depositional age estimates based on detrital zircon? An example from Early Palaeozoic successions of the Trondheim Nappe Complex, Scandinavian Caledonides, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3394, https://doi.org/10.5194/egusphere-egu21-3394, 2021.

16:04–16:06
|
EGU21-7179
|
ECS
Stephan Höpfl, Jiří Konopásek, Holger Stünitz, and Steffen G. Bergh

Deciphering the structural and metamorphic history of the Balsfjord Series in the Upper Allochthon of the Scandinavian Caledonides in northern Norway

Höpfl Stephan1, Konopásek Jiří1, Stünitz Holger1,2 Bergh G., Steffen1

UiT Norges arktiske universitet, Institutt for geovitenskap, stephan.m.hopfl@uit.no

 

1Department of Geosciences, UiT The Arctic University of Norway, Tromsø 9037, Norway

2Institut des Sciences de la Terre (ISTO), Université d’Orleans, Orleans 45100, France

 

The Balsfjord Series is located in the central part of Troms–Finnmark County, northern Norway, and is part of the upper allochthon of the Scandinavian Caledonides. It consists of an Ordovician–Silurian metsedimentary sequence lying on top of the mostly gabbroic Lyngen Magmatic Complex (LMC). The unit exhibits an inverted metamorphic gradient, where the metamorphic conditions increase from the base to the top, from very low grade in the southeast to medium grade in the west and northwest. The Balsfjord Series is sandwiched between two high-grade units, the Nakkedal + Tromsø Nappe Complex in the hanging wall and the Nordmannvik Nappe as the top part of the Reisa Nappe Complex (RNC) in the footwall. The Nakkedal + Tromsø Nappe Complex features metamorphic peak ages of ca. 455–450 Ma and the Nordmannvik Nappe of ca. 430 Ma. The peak metamorphism of the Balsfjord Series has never been dated and the role of the inverted metamorphic gradient is not yet understood. One of the main motivations in this project is to resolve the Caledonian deformation history in the Balsfjord Series, ideally leading to a regional tectonic model explaining the tectonostratigraphic and metamorphic relationships between the abovementioned units.

The Balsfjord Series features two main discernible folding phases. The earlier phase displays tight to isoclinal folds with flat lying axial surfaces parallel to the penetrative foliation. Observed fold axes are parallel with the stretching lineation. These folds are best preserved in the northwestern, upper part of the unit and are syn-metamorphic in certain areas, as they fold original bedding (transposed foliation). A later folding phase is represented by mainly open folds with inclined to steep axial surfaces. Their fold axes are gently plunging with a predominant NE–SW orientation. We interpret these two folding events to be genetically related but slightly diachronous. The earlier folding phase with flat lying axial surfaces was likely generated during nappe thrusting and peak metamorphism of the Balsfjord Series. The subsequent open folding with inclined to steep axial surfaces is explained as a result of continued shearing and shortening of the weaker metapelitic Balsfjord Series against the more rigid gabbroic part of the LMC during the late stages of the Caledonian nappe thrusting.      

Observed thrust kinematics and penetrative retrogression at the bottom of the Nakkedal + Tromsø Nappe Complex suggest that its final exhumation took place during prograde metamorphism of the underlying Balsfjord Series. The ongoing dating of the prograde metamorphism in the Balsfjord series will provide important information about a possible continuity between the timing of peak metamorphism in the Nakkedal + Tromsø Nappe Complex, the Balsfjord series and the underlying RNC.

How to cite: Höpfl, S., Konopásek, J., Stünitz, H., and Bergh, S. G.: Deciphering the structural and metamorphic history of the Balsfjord Series in the Upper Allochthon of the Scandinavian Caledonides in northern Norway , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7179, https://doi.org/10.5194/egusphere-egu21-7179, 2021.

Subduction-exhumation of the Western Gneiss Region
16:06–16:08
|
EGU21-16336
Simon Cuthbert

The northern part of the Western Gneiss Region (WGR) has distinctive belts of allochthonous metasediments and mafic rocks lying within tight infolds into the Baltica basement. They outcrop from the Grong-Olden Window to the Norwegian coast, possibly as far SW as Sørøyane, predominantly comprising metapelite and amphibolite with psammite, marble, calc-silicate, local large eclogite (>4km) lenses and ultramafites. These supracrustal lithotectonic units are attributed to the Blåhø Nappe, correlated with the Seve Nappe Complex (SNC) in its main outcrop in Sweden, which is considered to represent the pre-Caledonian continent-ocean transition (COT) of Baltica. They closely resemble the Lower Seve Nappe in northern Sweden where large amphibolite massifs with marbles are common, along with local eclogites. At least some have geochemical characteristics of spilitised extrusive MORB basalt in contrast to the better known, Neoproterozoic Baltoscandian Dyke Swarm in the SNC.

In the WGR near Molde a >10km long massif of such “amphibolite” at Tverrfjella commonly exhibits a relict high-P granulite precursor that has, in turn, overprinted eclogite. It encloses marble, scapolite-bearing calc-silicate, garnet peridotite (harzburgite) and Cu ores. Marble and meta-eclogite are intermixed which, along with its high Na spilitic character, suggests that the protolith was extrusive. Limited geochemical data suggest MORB composition. P-T estimates for eclogites in adjacent belts suggest UHP, possibly diamond-stable, conditions; in Sørøyane the well-known Ulsteinvik eclogite contains coesite. In the Molde area some of the mafic rocks and metasediments have partially melted. Eclogite metamorphism was Scandian in the Tverrfjell massif at 418 ± 11 Ma, with similar ages but tighter errors for adjacent belts and Ulsteinvik. These are significantly younger than ages for (U)HP metamorphism in the main SNC outcrop in Sweden, where early Ordovician subduction with a latest Ordovician granulite overprint is recorded. However, metapelites in other Blåhø-like supracrustal belts in the WGR do seem to record this earlier history as does one eclogite, consistent with the “double-dunk” hypothesis in this hinterland region. The protolith age of the metabasalts is unknown; analogy with the BDS suggests Neoproterozoic, but some zircon data from the WGR may suggest magmatic crystallisation during the Ordovician.O-isotopes indicate that the marbles were Palaeozoic, rather than Proterozoic, carbonates. Overall, the available literature data show that some large mafic massifs in the WGR, with associated metasediments and peridotites, are allochthonous with respect to Baltica basement; they represent major additions of extrusive basalt to a far-distal COT or fully oceanic basin that have been subducted at least once during the Caledonian Wilson cycle. Isotopic data hint that at least some of their protoliths are unusually young. These supracrustal belts certainly merit closer attention.

How to cite: Cuthbert, S.: Large meta-eclogite massifs within the Western Gneiss Region, Scandinavian Caledonides: Subducted ocean-continent transition?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16336, https://doi.org/10.5194/egusphere-egu21-16336, 2021.

16:08–16:10
|
EGU21-223
|
ECS
Samantha March, Renée Tamblyn, Martin Hand, Bruna Carvelho, and Chris Clark

The Western Gneiss Region (WGR), Norway is an archetypal continental ultrahigh-pressure (U)HP terrane with an extensive metamorphic history, recording the subduction and subsequent exhumation of continental crust to depths exceeding 120 km. The vast bulk of past work within the WGR has focused on mafic eclogites. In this study, data from rare garnet-kyanite metapelites in (UHP) domains of the WGR is presented. U–Pb geochronology and trace element compositions in zircon, monazite, apatite, rutile and garnet were acquired, and P–T conditions were calculated by mineral equilibria forward modelling and Zr-in-rutile thermometry. The Ulsteinvik metapelite defines a prograde path that traverses through ~600–710 °C and ~11–14 kbar. Minimum peak conditions are ~750 °C and ~2.9 GPa in an inferred garnet-kyanite-coesite-omphacite-muscovite-rutile-quartz-H2O assemblage. Plagioclase-biotite-quartz intergrowths developed after omphacite-phengite-rutile breakdown on the early retrograde path, followed by cordierite-spinel-plagioclase symplectites after garnet-kyanite-biotite, defining a retrograde P–T point at ~740 °C and ~7 kbar. Late Ordovician-Early Silurian (~470–440 Ma) zircon and rutile age data in Ulsteinvik pre-dates the major Scandian UHP subduction episode in the WGR, interpreted as recording early Caledonian subduction within the Blåhø nappe. Monazite and apatite U-Pb geochronology and trace element data suggest exhumation occurred at ~400 Ma. The Fjørtoft metapelite is a constituent of the Blåhø nappe. Minimum peak P–T conditions are ~1.8 GPa and ~750 °C, with poor peak mineral fidelity attributed to extensive retrograde deformation. Negative Eu anomalies in ~423 Ma monazite suggest retrograde conditions were reached [RJT1] by ~423 Ma. Ulsteinvik and Fjørtoft may have experienced pre-Scandian subduction together within the Blåhø nappe, but record dissimilar histories after this. Two potential scenarios are presented: (1) Ulsteinvik resided within the mantle for 20 million-years longer than Fjørtoft during Scandian subduction, or (2), the samples were exhumed at different times during pre-Scandian subduction of the Blåhø nappe. The preservation of prograde zoning within Ulsteinvik garnets precludes a long-term residence within the mantle and suggests the latter option. In this scenario, the subducting Blåhø nappe experienced a degree of slab tear and partial underplating of the upper plate during the early stages of continental underthrusting. Discrete pieces may have later reattached to the lower plate at different times, partially exhumed, and then subducted to mantle-depths during the Scandian.

How to cite: March, S., Tamblyn, R., Hand, M., Carvelho, B., and Clark, C.: Timescales of continental subduction: Constraints from ultrahigh-pressure metapelites in the Western Gneiss Region, Norway, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-223, https://doi.org/10.5194/egusphere-egu21-223, 2021.

16:10–16:12
|
EGU21-1364
|
ECS
Isabel S. M. Carter, Andrew Parsons, David J. Waters, and Phillip Gopon

The Western Gneiss Region (WGR) of Norway, part of the Caledonian Orogenic Belt, is one of the largest and best studied examples of exhumed ultra-high pressure (UHP) continental terrains in the world. This makes it an ideal candidate for studying the poorly understood processes that facilitate and control the exhumation of UHP continental material. Although the WGR is often considered the type example of the eduction model of UHP exhumation (Andersen et al., 1991), validation of exhumation models requires robust estimates of pressure and temperature across the full range of retrograde conditions which follow peak metamorphism. However, such constraints are often difficult to obtain as there is commonly overprinting of early-stage exhumation records during later stages of exhumation.  

UHP assemblages in the WGR are primarily preserved within numerous mafic eclogite enclaves, making them ideal candidates for studying processes and conditions that occur during exhumation from UHP conditions. In this study, we present detailed Electron Probe Micro-Analyses (EPMA) combined with Scanning Electron and Optical Microscopy characterization from a suite of mafic eclogite samples from the Stadlandet Peninsula of Western Norway. Our analyses focus on diopside–plagioclase (± amphibole) symplectite, which form from breakdown of omphacite during exhumation. Spatial variations in the compositions of minerals within these symplectites reflect a detailed record of P-T conditions during exhumation (Boland & van Roermund, 1983; Joanny et al., 1991; Waters, 2002). We used a novel technique of high resolution, low voltage EPMA, combined with secondary fluorescence corrections, which permits the analysis of individual symplectite lamellae with widths down to 1μm. Retrograde P-T pathways were then constructed from these data using the hornblende-plagioclase thermometer and clinopyroxene-plagioclase-hornblende barometer (Waters, 2002).  

P-T estimates from the symplectites fall in the range 470-720°C and 3-16 kbar. Combining the P-T arrays with existing peak P-T estimates indicates a two-stage exhumation path, with a steep initial isothermal decompression from depth followed by a more gentle cooling trajectory at lower pressures. The inflection in the exhumation path is estimated to be around 10-15 kbar at 650-700°C. The path shape is usually interpreted to record an initial rapid buoyancy driven exhumation from UHP to the base of the crust or lithosphere, followed by a second stage of slow exhumation to crustal depths. This confirmation of two-stage exhumation paths helps to constrain models of exhumation for the WGR, which in turn provides insights into how UHP terrains exhume globally.

 

References:

Andersen, T. B., Jamtveit, B., Dewey, J. F. & Swensson E. (1991). Subduction and Eduction of Continental Crust: Major Mechanisms during Continent-Continent Collision and Orogenic Extensional Collapse, a Model Based on the South Norwegian Caledonides. Terra Nova, 3(3), 303–10

Boland, J., & van Roermund, H. (1983). Mechanisms of exsolution in omphacites from high temperature, type B, eclogites. Physics and Chemistry of Minerals, 9(1), 30–37.

Joanny, V., van Roermund, H. & Lardeaux, J. M. (1991). The clinopyroxene/plagioclase symplectite in retrograde eclogites. Geologische Rundschau, 80(2), 303–320

Waters, D. J. (2002). Clinopyroxene-amphibole-plagioclase symplectites in Norwegian eclogites. Mineralogical Society, Winter Conference, Derby.

How to cite: Carter, I. S. M., Parsons, A., Waters, D. J., and Gopon, P.: Exhumation History of the Western Gneiss Region Revealed Through Symplectite Thermobarometry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1364, https://doi.org/10.5194/egusphere-egu21-1364, 2021.

16:12–16:14
|
EGU21-7275
Pauline Jeanneret, Katarzyna Walczak, Jarosław Majka, Michał Bukała, Simon Cuthbert, and Ellen Kooijman

To better understand the subduction–exhumation cycles of the Baltoscandian margin that reached (U)HP depths during the Caledonian orogeny, we have performed in-situ U-(Th-)Pb dating coupled with REE analysis of zircon and ± monazite in four samples from the supracrustal rocks of the Blåhø Nappe on Gossa island in the Western Gneiss Region (WGR) of Norway. We dated two garnet-plagioclase-biotite gneisses and two garnet-plagioclase-amphibole gneisses. Our research focused on deciphering the early metamorphic evolution of these complex rocks that have been overprinted by exhumation-related structures and pervasive retrogressive metamorphism.

The dated zircon grains are spherical or slightly elongated in shape, some of which display clear multi-stage growth features. Only one grain armored by garnet preserved an older detrital core that yielded early Neoproterozoic dates between 1.1-1.0 Ga. This grain does not provide any Caledonian signal. Younger individual 206Pb/238U dates show three distinct populations that yield three concordia ages, each obtained from distinctly different compositional domains, the oldest from cores and the two youngest from overgrowths. The cores are characterized by HREE enrichment (high Lu/Gd ratios ca. 14.5), high Th/U ratios (> 0.1), and large Eu anomalies. They yield a concordia age of 474 ± 6.4 Ma. These cores can be rimmed by two different types of zircon overgrowth. The first overgrowth type (1) displays the same REE pattern as the cores and gives a concordia age of 444± 4.3 Ma. The second overgrowth type (2) shows a very weak Eu anomaly, no HREE enrichment (low Lu/Gd ratios ca. 2.37) and a very low Th/U ratios (<0.1). These yield a concordia age of 416± 3.7 Ma. The two older U–Pb zircon age populations are tentatively interpreted as reflecting two distinct metamorphic events or a prolonged episode of metamorphism. The youngest concordant metamorphic zircon dates a high grade, probably (U)HP, metamorphic overprint at ca. 416 Ma, subsequent to the previous events. Analyses performed on monazite provided complementary age records to those obtained on zircon. Monazite grains are weakly zoned, exhibit wormy shapes and are aligned with the youngest foliation. Th–U–total Pb dating of monazite, coupled with major and trace element mapping of monazite, yielded a very homogeneous age of 382 ± 1.6 Ma (n=65) interpreted to date the late shearing, which possibly accommodated a late stage of exhumation.

Funded by the National Science Centre (Poland) project no. 2014/14/E/ST10/00321.

 

How to cite: Jeanneret, P., Walczak, K., Majka, J., Bukała, M., Cuthbert, S., and Kooijman, E.: In-situ U-(Th-)Pb dating and REE analysis of zircon and monazite in the Grt-bearing gneisses from Gossa: Tracing early subduction into the highest-grade domains of the Western Gneiss Region, Norway, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7275, https://doi.org/10.5194/egusphere-egu21-7275, 2021.

The Arctic Caledonides
16:14–16:16
|
EGU21-13315
|
ECS
Margot Patry, Iwona Klonowska, Karolina Kośmińska, and Jarosław Majka

The Isbjørnhamna Group, which crops out in the south-west of Svalbard in the High Arctic, is crucial for understanding Svalbard’s regional geology. It can be traced in southern Wedel Jarlsberg Land and Sørkapp Land, and it consists of a Barrovian-type series of metapelites that were metamorphosed during the Torellian (c. 640Ma; Majka et al. 2008) and overprinted during the Caledonian orogenesis (Majka & Kośmińska, 2017). Although relatively recent petrological study exists, there are certain gaps in it. In order to fill these gaps, we decided to re-investigate these rocks using the most up-to-date petrochronological approach. Hence, we aim to determine the metamorphic history of these rocks in detail, test the hypothesis if there are indeed several orogenic events registered by these rocks and what was a possible exhumation mechanism responsible for uplift of this sequence.

The studied garnet-bearing mica schists preserve four different parageneses, ranging from chloritoid to kyanite metamorphic zones. Here we report on the samples containing chlorite and chloritoid, kyanite, staurolite and both staurolite and kyanite. The studied samples are the same exact rocks that have been previously studied by Majka et al. (2008, 2010) using both geothermobarometry and petrogenetic grids in the KFMASH system. According to those authors the estimated pressure-temperature conditions (P-T) were c. 655°C at 11kbar for the kyanite-bearing shist, c. 624°C at 6.6 to 8.7kbar for the staurolite + kyanite pelite and c. 580°C at 8-9kbar for the staurolite-bearing rock. The chloritoid schist has not been studied previously.

Our preliminary phase equilibrium modelling in the MnNCKFMASHTO system using the Theriak-Domino software indicates P-T conditions of c. 660°C at 7 kbar for the kyanite-schist and c. 575°C at 8 to 9.5kbar for the staurolite-schist, respectively. The chloritoid schist yielded conditions of c. 560°C at 7.5kbar. Further P-T modelling coupled with in-situ Ar-Ar and U-Pb geochronology should allow for much better understanding of the complex geological history of these rocks as well as potential flaws in the previous studies.

 

Research funded by National Science Centre (Poland) project no. 2019/33/B/ST10/01728.

 

References:

Majka & Kośmińska (2017): Arktos, 3:5, 1.17.

Majka et al. (2008): Geological Magazine, 145, 822-830.

Majka et al. (2010): Polar Research, 29, 250-264.        

How to cite: Patry, M., Klonowska, I., Kośmińska, K., and Majka, J.: Re-investigating the Barrovian metamorphic rocks of the Isbjørnhamna Group, Svalbard Caledonides, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13315, https://doi.org/10.5194/egusphere-egu21-13315, 2021.

16:16–16:18
|
EGU21-10492
|
ECS
Wentao Cao, Jane Gilotti, and Hans-Joachim Massonne

Kyanite eclogite from the North-East Greenland Caledonides – the upper plate of the Caledonian orogeny – preserves a mineral assemblage and petrographic texture that are consistent with an initial near-isothermal exhumation path. Two medium-grained kyanite eclogites from the Danmarkshavn area (76°46’N, 18°40’W) located west of the Germania Land shear zone contain the peak assemblage of garnet + omphacite + kyanite + phengite + amphibole + rutile. Subhedral garnet encloses monomineralic omphacite and polymineralic inclusions of clinopyroxene + plagioclase ± quartz ± amphibole ± K-feldspar ± kyanite. X-ray mapping of garnet indicates a homogenous core with a composition of Py51–52Alm28–29Gr19–20Sp0–1, along with a slightly zoned rim of Py54Alm31Gr15Sp1 that is replaced by a corona of symplectitic amphibole + plagioclase. Omphacite (XNa up to 0.41), rarely present in the matrix, is indicated by symplectite of clinopyroxene + amphibole + plagioclase. Symplectites of corundum + plagioclase, spinel + plagioclase and sapphirine + plagioclase replace former kyanite. These symplectites are typically surrounded by a plagioclase corona with decreasing Ca (from XAn = 92–97 to XAn = 47–53) from the symplectite to the matrix. Isochemical phase equilibrium modeling along with homogenous garnet core and peak omphacite compositions yielded a peak metamorphic pressure-temperature (P-T) condition at 1.9 GPa, 840 ˚C. Assuming local equilibrium at the microscopic scale, an attempt to model a symplectite of spinel + sapphirine + plagioclase after kyanite using a pseudosection yielded estimated P-T conditions at 0.8–1.3 GPa and 700–900 ˚C. Integrating the calculated P-T conditions and previous geochronological results, an initial exhumation path from 1.9 GPa to ~1.0 GPa from ~415–390 Ma to ~375 Ma is nearly isothermal at around 800 ˚C.

How to cite: Cao, W., Gilotti, J., and Massonne, H.-J.: Near-isothermal exhumation of lower crust in the Caledonian Orogen: Metamorphic path of kyanite eclogite from the Danmarkshavn area, North-East Greenland Caledonides, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10492, https://doi.org/10.5194/egusphere-egu21-10492, 2021.

16:18–16:20
|
EGU21-15127
Jarosław Majka, Karolina Kośmińska, Jakub Bazarnik, and William C. McClelland

We report on U-Pb zircon dating and bulk rock geochemistry results of intermediate to felsic rocks of the Thores Suite of the Pearya Terrane, northern Ellesmere Island (Arctic Canada).  Our new results together with the previously published data show that the Thores Suite was formed in the Early Ordovician (c. 490-470 Ma) as a part of an island arc. Some of the dated samples revealed common xenocrystic zircon. The latter yielded ages ranging between c. 2690 Ma and c. 520 Ma. The obtained ages of xenocrystic zircon are interpreted to be typical of Laurentia. We propose that the youngest obtained cluster of ages c. 580-570 Ma expresses a component typical for the Timanide Orogen, which is conventionally tied to Baltica. The newdataset sheds light on the history and understanding of the Thores Suite, which used to be explained as an effect of the M’Clintock orogenesis. The latter event was commonly presented as foreign to the major Caledonian orogenesis sensu stricto. In our view, the Thores Suite represents an island arc, which was formed on a fragment of continental crust dismembered during Iapetus opening. Importantly, the age of the Thores island arc is coeval with other island arcs and high pressure metamorphic units of the Scandinavian and Svalbard Caledonides. Thus, it is likely that the Thores volcanic island arc was a part of the larger arc system operating within northern Iapetus. The juxtaposition of the Thores arc with the other successions of the Pearya Terrane is ascribed to a major sinistral strike-slip escape fault-system developed along the northeastern margins of Baltica and Laurentia, broadly concurrent with the main Scandian collision between the two aforementioned continents. This crustal scale fault structure enabled the juxtaposition of numerous crustal blocks of different Precambrian ancestry that can be found in various regions of the current High Arctic, including Svalbard, Greenland and Ellesmere Island.

This research was supported by the National Science Centre (Poland) project no. 2015/17B/ST10/03114 and the internal AGH-UST funding to J. Majka, the internal grant of the Polish Geological Institute - NRI no. 62.9012.2014.00.0 to J. Bazarnik and the National Science Foundation (USA) grant EAR1650022 to J. Gilotti and W. McClelland.

How to cite: Majka, J., Kośmińska, K., Bazarnik, J., and McClelland, W. C.: The Thores volcanic island arc of the Pearya Terrane from Ellesmere Island formed on Precambrian continental crust, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15127, https://doi.org/10.5194/egusphere-egu21-15127, 2021.

16:20–17:00