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UHP metamorphism recorded by phengite eclogite from the Caledonides of northern Sweden: P-T path and tectonic implications
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics. AGH Univ Sci & Technol, Fac Geol Geophys & Environm Protect, Krakow, Poland.ORCID iD: 0000-0001-7045-3150
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.ORCID iD: 0000-0001-7346-1770
AGH Univ Sci & Technol, Fac Geol Geophys & Environm Protect, Krakow, Poland.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics. AGH Univ Sci & Technol, Fac Geol Geophys & Environm Protect, Krakow, Poland.ORCID iD: 0000-0002-6792-6866
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2018 (English)In: Journal of Metamorphic Geology, ISSN 0263-4929, E-ISSN 1525-1314, Vol. 36, no 5, p. 547-566Article in journal (Refereed) Published
Abstract [en]

The Seve Nappe Complex (SNC) of the Scandinavian Caledonides records a well‐documented history of high pressure (HP) and ultra‐high pressure (UHP) metamorphism. Eclogites of the SNC occur in two areas in Sweden, namely Jämtland and Norrbotten. The Jämtland eclogites and associated rocks are well‐studied and provide evidence for late Ordovician UHP metamorphism, whereas the Norrbotten eclogites, formed during the late Cambrian (Furongian)/Early Ordovician, have not been studied in such detail, especially in terms of the P–T conditions of their formation. Within the studied eclogite, clinopyroxene contains a high‐Na core and two rims: inner, medium‐Na and outer, low‐Na. Garnet consists of a high‐Ca euhedral core, low‐Ca inner rim and medium‐Ca outer rim. A similar pattern occurs within phengite, where high‐Si cores are enveloped by medium and low‐Si rims. The compositions of the mineral cores, inner rims and outer rims reflect three stages in the metamorphic evolution of the eclogite. Applied Quartz‐in‐Garnet geobarometry, coupled with Zr‐in‐rutile geothermometry reveal that garnet nucleation (E0 stage) took place at 1.5–1.6 GPa and 620–660°C. The eclogite peak‐pressure assemblage developed during the E1 stage, it consists of garnet+omphacite+phengite+rutile+coesite? and yields P–T conditions of 2.8–3.1 GPa and 660–780°C as constrained by conventional geothermobarometry and thermodynamic modelling in the NCKFMMnASHT system. Later, lower‐pressure stages E2 and E3 record conditions of 2.2–2.8 GPa, 680–780°C and 2.1 GPa, 735°C, respectively. The prograde metamorphic evolution of the eclogite is inferred from inclusions of epidote, amphibole and clinopyroxene within garnet. The presence of amphibole–quartz–plagioclase symplectites, secondary epidote/zoisite and titanite replacing rutile record the later retrograde changes taking place at <1.5 GPa (referred as E4 stage). The obtained P–T conditions indicate that the Norrbotten eclogites underwent a metamorphic evolution characterized by a clockwise P–T path with peak metamorphism reaching up to coesite stability field within a relatively cold subduction regime (7.8°C/km). The obtained results provide the first evidence for UHP metamorphism in the SNC above the Arctic Circle and document cold subduction regime and multistage exhumation of the deeply subducted Baltican margin at early stage of the Caledonian Orogeny.

Place, publisher, year, edition, pages
2018. Vol. 36, no 5, p. 547-566
Keywords [en]
QuiG barometry, Seve Nappe Complex, Subduction-exhumation, thermodynamic modelling, Zr in rutile thermometry
National Category
Geology Geochemistry
Identifiers
URN: urn:nbn:se:uu:diva-332529DOI: 10.1111/jmg.12306ISI: 000434276500004OAI: oai:DiVA.org:uu-332529DiVA, id: diva2:1153294
Available from: 2017-10-30 Created: 2017-10-30 Last updated: 2018-10-08Bibliographically approved
In thesis
1. Deep subduction of the Seve Nappe Complex in the Scandinavian Caledonides
Open this publication in new window or tab >>Deep subduction of the Seve Nappe Complex in the Scandinavian Caledonides
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis seeks to improve our understanding of the processes involved in continental collision zones, with a particular focus on subduction-exhumation. The main objective of this work has been to define the tectonometamorphic evolution of the deeply subducted Seve Nappe Complex (SNC) in the Scandinavian Caledonides. I utilize mineralogy, petrology and geochronology to constrain the P-T-t paths of the SNC rocks in Sweden.

The research has focused on the high grade rocks of the SNC and resulted in the discovery of metamorphic diamonds within the gneisses in west-central Jämtland and southern Västerbotten. Microdiamonds provided evidence for the ultra-high pressure metamorphism (UHPM) and subduction of continental rocks to mantle depths. The UHPM in these rocks was confirmed by calculations of the P-T conditions. The UHPM is further recorded by eclogites and garnet pyroxenites from northern Jämtland and eclogites from Norrbotten. All these findings provide compelling evidence for regional UHPM of vast parts of the SNC (at least 400 km along the strike of this allochthonous unit). The SNC rocks followed nearly isothermal decompression paths and paragneisses have locally experienced partial melting during exhumation. Formation of the peculiar Ba- and Ti-enriched dark mica in the Tväråklumparna metasediments is related to the latter stage.

In-situ monazite dating of the diamond-bearing gneisses from west-central Jämtland supports previous geochronological data inferring that the peak of metamorphism is probably Middle Ordovician and was followed by Early Silurian partial melting. The exact timing of the UHPM here still remains to be resolved. The Lu-Hf garnet and U-Pb zircon dating of eclogite and gneiss from northern Jämtland confirms the Middle Ordovician age of the UHP-HP metamorphism of the SNC rocks. The chemical dating of monazite from the Marsfjället gneiss suggests an earlier UHP history of the Seve rocks in southern Västerbotten as a post-UHP uplift is dated to ca. 470 Ma.

Based on the P-T-t data obtained in this thesis, particularly on the evidence for Middle Ordovician UHPM and subsequent Silurian exhumation, a new tectonic model for the Scandinavian Caledonides has been proposed. The outcomes of this thesis therefore improve our understanding of the tectonometamorphic history of the Caledonides.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. p. 63
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1595
Keywords
geothermobarometry, thermodynamic modelling, Raman spectroscopy, diamond, ultrahigh-pressure metamorphism, eclogite, peridotite, gneiss
National Category
Geology
Identifiers
urn:nbn:se:uu:diva-332525 (URN)978-91-513-0139-6 (ISBN)
Public defence
2017-12-08, Hambergsalen, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Research CouncilThe Royal Swedish Academy of SciencesThe Crafoord Foundation
Available from: 2017-11-17 Created: 2017-10-30 Last updated: 2018-03-07

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