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Plate tectonic settings of the Svecofennian Palaeoproterozoic volcanic rocks at Hamrånge and Loos, south central Sweden, based on geochemical data
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
2007 (English)In: GFF, ISSN 1103-5897, Vol. 129, no 3, 211-226 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2007. Vol. 129, no 3, 211-226 p.
Keyword [en]
Hamrånge, Loos, Ljusdal batholith, Bergslagen, basalt, geochemistry, discrimination diagram, tectonic setting
National Category
Earth and Related Environmental Sciences
URN: urn:nbn:se:uu:diva-14380DOI: 10.1080/11035890701293211ISI: 000250713200003OAI: oai:DiVA.org:uu-14380DiVA: diva2:42150
Available from: 2008-01-30 Created: 2008-01-30 Last updated: 2011-01-19Bibliographically approved
In thesis
1. Geological Evolution of the Supracrustal Palaeoproterozoic Hamrånge Group: A Svecofennian Case Study
Open this publication in new window or tab >>Geological Evolution of the Supracrustal Palaeoproterozoic Hamrånge Group: A Svecofennian Case Study
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. 53 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 738
Hamrånge, Ljusdal, Hagsta Gneiss Zone, geochemistry, geochronology, thermobarometry, metamorphism, fold-thrust belt.
National Category
Earth and Related Environmental Sciences
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
urn:nbn:se:uu:diva-119357 (URN)978-91-554-7795-0 (ISBN)
Public defence
2010-05-12, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Available from: 2010-04-20 Created: 2010-02-25 Last updated: 2010-04-20Bibliographically approved

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