| 1. |
- Bergman, Stefan, et al.
(author)
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Detrital zircons in late Svecofennian metasandstones in central Sweden and southern Finland
- 2006
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In: Bulletin of The Geological Society of Finland. ; , s. 184-
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Conference paper (peer-reviewed)abstract
- Deformed and metamorphosed quartz-rich sandstones in southern Finland and east-central Sweden were presumed to be deposited in late Svecofennian time (< c. 1.88 Ga). On the basis of this assumption detrital zircons were selected from four localities (Luukkola, Pyhäntaka and Tiirismaa in Finland and Hamrånge in Sweden) for U-Pb SIMS determination.The obtained ages define three age groups, 2.90 2.40 Ga, 2.15 1.96 Ga and 1.92 1.84 Ga, with no correlation between age and Th/U-ratio. The groups are similar in all four samples, which supports correlation, and they are comparable to previously reported detrital ages in this part of the Fennoscandian Shield. The oldest zircon analysed gave an age of 3.32 Ga (Tiirismaa).The maximum ages of sedimentation (and of subsequent deformation and metamorphism), indicated by the youngest zircon, from the four localities are 1842±10 Ma (Luukkola), 1865±11 Ma (Pyhäntaka), 1848±13 Ma (Tiirismaa), and 1855±10 Ma (Hamrånge), respectively. Possible source rocks for these zircons are found in and in the vicinity of the vast Ljusdal Batholith in Sweden. Importantly, rocks with such ages are rare in Finland.It is concluded that one or several sedimentary basins covered an area of at least 400x500 km at c. 1.85 1.82 Ga ago in the Fennoscandian Shield. Basin formation can be constrained between 1.86 and 1.83 Ga, the latter age by onset of regional metamorphism in southern Finland.Stratigraphically below the metasandstone at Hamrånge is a metadacite with an U-Pb zircon TIMS age of 1888±6 Ma. This indicates affinity to the metavolcanic rocks in Bergslagen to the south, rather than to the younger equivalents at Loos to the northwest. It also suggests a significant time gap of >30 Ma between the volcanism and sand deposition in the Hamrånge area.
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| 2. |
- Matthews, Bethany E., et al.
(author)
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Micro- and Nanoscale Surface Analysis of Late Iron Age Glass from Broborg, a Vitrified Swedish Hillfort
- 2023
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In: Microscopy and Microanalysis. - : Oxford University Press. - 1431-9276 .- 1435-8115. ; 29:1, s. 50-68
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Journal article (peer-reviewed)abstract
- Archaeological glasses with prolonged exposure to biogeochemical processes in the environment can be used to understand glass alteration, which is important for the safe disposal of vitrified nuclear waste. Samples of mafic and felsic glasses with different chemistries, formed from melting amphibolitic and granitoid rocks, were obtained from Broborg, a Swedish Iron Age hillfort. Glasses were excavated from the top of the hillfort wall and from the wall interior. A detailed microscopic, spectroscopic, and diffraction study of surficial textures and chemistries were conducted on these glasses. Felsic glass chemistry was uniform, with a smooth surface showing limited chemical alteration (<150 nm), irrespective of the position in the wall. Mafic glass was heterogeneous, with pyroxene, spinel, feldspar, and quartz crystals in the glassy matrix. Mafic glass surfaces in contact with topsoil were rougher than those within the wall and had carbon-rich material consistent with microbial colonization. Limited evidence for chemical or physical alteration of mafic glass was found; the thin melt film that coated all exposed surfaces remained intact, despite exposure to hydraulically unsaturated conditions, topsoil, and associated microbiome for over 1,500 years. This supports the assumption that aluminosilicate nuclear waste glasses will have a high chemical durability in near-surface disposal facilities.
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| 3. |
- McCloy, John S., et al.
(author)
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Reproduction of melting behavior for vitrified hillforts based on amphibolite, granite, and basalt lithologies
- 2021
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In: Scientific Reports. - : Springer Nature. - 2045-2322. ; 11
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Journal article (peer-reviewed)abstract
- European Bronze and Iron Age vitrified hillforts have been known since the 1700s, but archaeological interpretations regarding their function and use are still debated. We carried out a series of experiments to constrain conditions that led to the vitrification of the inner wall rocks in the hillfort at Broborg, Sweden. Potential source rocks were collected locally and heat treated in the laboratory, varying maximum temperature, cooling rate, and starting particle size. Crystalline and amorphous phases were quantified using X-ray diffraction both in situ, during heating and cooling, and ex situ, after heating and quenching. Textures, phases, and glass compositions obtained were compared with those for rock samples from the vitrified part of the wall, as well as with equilibrium crystallization calculations. ‘Dark glass’ and its associated minerals formed from amphibolite or dolerite rocks melted at 1000–1200 °C under reducing atmosphere then slow cooled. ‘Clear glass’ formed from non-equilibrium partial melting of feldspar in granitoid rocks. This study aids archaeological forensic investigation of vitrified hillforts and interpretation of source rock material by mapping mineralogical changes and glass production under various heating conditions.
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| 4. |
- Ogenhall, Erik, 1967-, et al.
(author)
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Deformation history of the Palaeoproterozoic Svecofennian supracrustal rocks at Hamrånge, south-central Sweden
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Other publication (other academic/artistic)abstract
- The west-central part of the Palaeoproterzoic Fennoscandian Shield consists of different lithological units: the marine Bothnian basin, granitoids formed in an active continental margin (the Ljusdal Domain, LjD), and a continental back-arc magmatic region (the Bergslagen province). Geochemical signatures indicate that the supracrustal Hamrånge Group (HG) represents an additional unit - a fragment of an oceanic island arc that has been accreted to the continental margin granitoids. Structural data show that the 1.9 Ga supracrustal rocks in the lower part of the stratigraphy of the HG have been affected by three episodes of deformation (D1-D3). On the other hand, the ≤1.86 Ga quartzite in the uppermost part probably has been affected only by the latter two deformational phases. The nature of D1 is poorly understood, but may represent the early accretion of the island arc to the continental margin. A NW-SE compressional phase during D2 resulted in F2-folds and thrusts in the supracrustal rocks, and thrusts are also observed in the footwall granitoids. Microstructures demonstrate that metamorphism outlasted and possibly peaked after D2, and structural data imply a period of flattening subsequent to the D2-thrusting, but prior to D3. During D3 the area was affected by a regional N-S compression causing upright E-plunging F3-folding. This F3-folding matured to steep ductile shear zones, which belong to a system of 1.82-1.80 Ga NW-SE shear zones transecting the central part of the Fennoscandian Shield. The structural evolution recorded in the Hamrånge area, suggesting D2-thrusting towards NW and subsequent N-S (D3) shortening, is in accordance with the accepted tectonic models of the Fennoscandian Shield. The structural configuration of the area is partly comparable, at a smaller scale, to a major tectonic unit in the 0.5 Ga Scandinavian Caledonides, the Lower Seve Nappe (LSN), suggesting similarities in structural evolution of orogenies through time. An important difference is the relative timing of deformation and metamorphism. However, significant differences in tectonostratigraphy suggest that the HG is not far-transported and that the HG and tectonically underlying granitoids of the LjD originally had a close spatial relationship.
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| 5. |
- Ogenhall, Erik, 1967-
(author)
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Geological Evolution of the Supracrustal Palaeoproterozoic Hamrånge Group: A Svecofennian Case Study
- 2010
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Doctoral thesis (other academic/artistic)abstract
- The work presented in this thesis utilizes several geological methods to investigate the origin and evolution of the supracrustal rocks in the Palaeoproterozoic Hamrånge Group (HG) in the south-central Swedish Svecofennian. The first paper is based on whole-rock geochemistry to show the plate tectonic setting of volcanic rocks within the HG. This indicates that the environment was probably an oceanic volcanic arc. Geochronology, used in paper two, shows that the volcanism was active at 1888±6 Ma and that the sediments forming the stratigraphically overlying quartzite were deposited after 1855±10 Ma, with provenance ages overlapping both the volcanic rocks and the 1.86-1.84 Ga continental margin Ljusdal granitoids. In the third paper, thermobarometry was applied to samples from the HG, the migmatitic Ockelbo sub-domain to the south, and the 1.81 Ga Hagsta Gneiss Zone (HGZ) that separate these two units. The results show distinct differences in the metamorphic conditions that have affected the HG and the Ockelbo sub-domain, supporting previous interpretations that the HGZ is an important crustal structure, possibly a terrane or domain boundary. Paper four deals with the structural geology of the Hamrånge area. The study shows that the volcanic rocks and the underlying mica schist have been subjected to three deformation episodes (D1-D3), while the uppermost quartzite was most likely only affected by D2 and D3. While structures related to D1 are rarely seen, D2 resulted in a penetrative foliation, strong lineations and NW-vergent folding and thrusting. D3 is a result of a N-S compression that formed regional E-W folds and steep, ca. NW-SE shear zones, e.g. the HGZ. The results presented in this thesis, integrated with previously published data, outline a model for the geological evolution of the Hamrånge area: At 1.89 Ga a volcanic arc formed that subsequently collided with a continental margin resulting in the first deformation episode, D1, and probably a metamorphic event. This was possibly followed by an extensional period, after 1855±10 Ma, forming a basin that accumulated sediments later to form the quartzite stratigraphically on top of the volcanic rocks. The second deformation episode, D2, formed a fold-thrust belt when the supracrustal HG was thrusted to the NW, on top of the 1.86-1-84 Ga Ljusdal Domain. Flattening and a second metamorphic period followed this thickening of the crust. The last ductile deformation, D3, caused by regional tectonic forces, resulted in F3-folds that matured into ca. 1.8 Ga large-scale, steep shear zones transecting the Fennoscandian Shield.
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| 6. |
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| 7. |
- Ogenhall, Erik
(author)
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Plate tectonic settings of the Svecofennian Palaeoproterozoic volcanic rocks at Hamrånge and Loos, south central Sweden, based on geochemical data
- 2007
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In: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 129:3, s. 211-226
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Journal article (peer-reviewed)abstract
- The Palaeoproterozoic Hamrånge and Loos groups in central Sweden contain similar metavolcanic and metasedimentary rocks that have been considered to make up the upper part of the Svecofennian stratigraphy. Both occur in kilometre scale refolded synforms with partly similar deformation history, rock types, stratigraphy and metamorphic grade. Water deposition of the supracrustal sequences of both the Hamrånge and Loos areas is shown by primary features like pillows in the metabasalts and cross-bedding and ripple marks in the metasedimentary rocks. Geochemical analyses of the metabasalts demonstrate that they are altered and spilitic to some extent. The volcanism at Hamrånge was probably a continuous felsic to mafic suite, rather than bimodal like in Loos and western Bergslagen. The mafic rocks from both areas show all signatures of being tholeiitic, but with obvious differences. The metabasalts from Hamrånge have signatures suggesting an oceanic volcanic arc (VAB) setting. Similar rocks from Loos show several features indicating a mid ocean ridge (MORB) setting but also some signatures of a volcanic arc setting. The results from Loos can be explained by a transitional setting with a VAB influenced MORB found in back arc basins (BAB). The geochemical results from the two areas show no signs of a continental extensional or within plate (WP) setting as suggested for the Bergslagen area to the south.
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| 8. |
- Ogenhall, Erik
(author)
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Plate tectonic settings of the Svecofennian volcanic rocks at Hamrånge and Loos, central Sweden
- 2006
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In: Bulletin of The Geological Society of Finland. ; , s. 184-
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Conference paper (peer-reviewed)abstract
- The Palaeoproterozoic Hamrånge and Loos formations in central Sweden contain similar metavolcanic and sedimentary rocks that have been considered to make up the upper part of the Svecofennian stratigraphy. Both formations occur in kilometer scale refolded synforms with similar deformation history, rock types, stratigraphy and metamorphic grade.The surrounding granitoids (of Ljusdal type) are interpreted and partly demonstrated to be somewhat younger than the supracrustal rocks dated at 1.86 Ga in Loos, and 1.89 Ga in Hamrånge (Bergman et al., this volume), and consequently inferred to intrude the latter. The ca. 1.85 Ga Ljusdal granitoids, referred to as calc-alkaline, early orogenic tonalite to granodiorite, is somewhat younger than similar rocks in the Bothnian basin to the north and in the Bergslagen arc to the south that have been dated at 1.89-1.87 Ga.The supracrustal sequences of both Hamrånge and Loos show primary features like pillows and amygdules in the basalts and cross-bedding and ripple marks in the sediments, revealing that they were deposited in water. Geochemical analyses of the basalts show that they are altered and spilitic to some extent.The volcanism at Hamrånge was probably a continuous felsic to mafic extrusive event, rather than bimodal like in Loos and western Bergslagen. The rocks from both areas show all signatures of being tholeiitic, but the geochemical results show no signs of a continental extensional setting like that interpreted for the western part of Bergslagen.The geochemical signatures of Hamrånge basalts suggest a volcanic arc (VAB) setting, probably of oceanic origin. Similar rocks from Loos show several features indicating a mid ocean ridge (MORB) environment but also some signatures of a volcanic arc setting. The results from Loos can be explained by a transitional setting with a VAB influenced MORB, found in back arc basins (BAB) or intra arc basins.
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| 9. |
- Plymale, Andrew E., et al.
(author)
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Niche Partitioning of Microbial Communities at an Ancient Vitrified Hillfort : Implications for Vitrified Radioactive Waste Disposal
- 2021
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In: Geomicrobiology Journal. - : Taylor & Francis. - 0149-0451 .- 1521-0529. ; 38:1, s. 36-56
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Journal article (peer-reviewed)abstract
- Because microbes cannot be eliminated from radioactive waste disposal facilities, the consequences of bio-colonization must be understood. At a pre-Viking era vitrified hillfort, Broborg, Sweden, anthropogenic glass has been subjected to bio-colonization for over 1,500 years. Broborg is used as a habitat analogue for disposed radioactive waste glass to inform how microbial processes might influence long-term glass durability. Electron microscopy and DNA sequencing of surficial material from the Broborg vitrified wall, adjacent soil, and general topsoil show that the ancient glass supports a niche microbial community of bacteria, fungi, and protists potentially involved in glass alteration. Communities associated with the vitrified wall are distinct and less diverse than soil communities. The vitrified niche of the wall and adjacent soil are dominated by lichens, lichen-associated microbes, and other epilithic, endolithic, and epigeic organisms. These organisms exhibit potential bio-corrosive properties, including silicate dissolution, extraction of essential elements, and secretion of geochemically reactive organic acids, that could be detrimental to glass durability. However, long-term biofilms can also possess a homeostatic function that could limit glass alteration. This study documents potential impacts that microbial colonization and niche partitioning can have on glass alteration, and subsequent release of radionuclides from a disposal facility for vitrified radioactive waste.
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| 10. |
- Sjöblom, Rolf, et al.
(author)
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Assessment of the reason for the vitrification of a wall at a hillfort. The example of Broborg in Sweden
- 2022
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In: Journal of Archaeological Science. - : Elsevier. - 2352-409X .- 2352-4103. ; 43
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Journal article (peer-reviewed)abstract
- It was discovered around 250 years ago that some of the rock material in the walls of some hillforts had been subjected to such high temperature that it had vitrified. This prompted a debate as to the reason for it that is still going on today: did the vitrification come about as a result of hostile action, by accident, or for the purpose of constructing the fort? The present paper is based on the recognition that hillforts are different, and therefore should be evaluated individually. All identifiable factors of interest should be included, and especially those that might disprove any alternative. Thus, incentives, competence and petrographic aspects were evaluated for the hillfort named Broborg (dated to the Migration Period, in Sweden A.D. 400–550), and it is concluded that the vitrification here came about for the purpose of constructing the fort.
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| 11. |
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| 12. |
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| 13. |
- Weaver, Jamie L., et al.
(author)
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Pre‐Viking Swedish Hillfort Glass : A Prospective Long‐Term Alteration Analogue for Vitrified Nuclear Waste
- 2018
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In: The International Journal of Applied Glass Science (IJAGS). - : John Wiley & Sons. - 2041-1294 .- 2041-1286. ; 9:4, s. 540-554
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Journal article (peer-reviewed)abstract
- Models for long‐term glass alteration are required to satisfy performance predictions of vitrified nuclear waste in various disposal scenarios. Durability parameters are usually extracted from short‐term laboratory tests, and sometimes checked with long‐term natural experiments on glasses, termed analogues. In this paper, a unique potential ancient glass analogue from Sweden is discussed. The hillfort glass found at Broborg represents a unique case study as a vitrified waste glass analogue to compare to Low Activity Waste glass to be emplaced in near surface conditions at Hanford (Washington State). Glasses at Broborg have similar and dissimilar compositions to LAW glass, allowing the testing of long‐term alteration of different glass chemistries. Additionally, the environmental history of the site is reasonably well documented. Initial investigations on previously collected samples established methodologies for handling and characterizing these artifacts by laboratory methods while preserving their alteration layers and cultural context. Evidence of possible biologically influenced glass alteration, and differential alteration in the two types of glass found at the Broborg site is presented.
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