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Microdiamond on Åreskutan confirms regional UHP metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.ORCID iD: 0000-0001-7346-1770
Slovak Academy of Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. AGH - University of Science and Technology, Department of Mineralogy, Petrography and Geochemistry.ORCID iD: 0000-0002-6792-6866
Slovak Academy of Sciences.
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2017 (English)In: Journal of Metamorphic Geology, ISSN 0263-4929, E-ISSN 1525-1314, Vol. 35, 541-564 p.Article in journal (Refereed) Published
Abstract [en]

Metamorphic diamond in crustal rocks provides important information on the deep subduction of continental crust. Here, we present a new occurrence of diamond within the Seve Nappe Complex (SNC) of the Scandinavian Caledonides, on angstrom reskutan in Jamtland County, Sweden. Microdiamond is found insitu as single and composite (diamond+carbonate) inclusions within garnet, in kyanite-bearing paragneisses. The rocks preserve the primary peak pressure assemblage of Ca,Mg-rich garnet+phengite+kyanite+rutile, with polycrystalline quartz surrounded by radial cracks indicating breakdown of coesite. Calculated P-T conditions for this stage are 830-840 degrees C and 4.1-4.2GPa, in the diamond stability field. The ultrahigh-pressure (UHP) assemblage has been variably overprinted under granulite facies conditions of 850-860 degrees C and 1.0-1.1GPa, leading to formation of Ca,Mg-poor garnet+biotite+plagioclase+K-feldspar+sillimanite+ilmenite+quartz. This overprint was the result of nearly isothermal decompression, which is corroborated by Ti-in-quartz thermometry. Chemical Th-U-Pb dating of monazite yields ages between 445 and 435Ma, which are interpreted to record post-UHP exhumation of the diamond-bearing rocks. The new discovery of microdiamond on angstrom reskutan, together with other evidence of ultrahigh-pressure metamorphism (UHPM) within gneisses, eclogites and peridotites elsewhere in the SNC, provide compelling arguments for regional (at least 200km along strike of the unit) UHPM of substantial parts of this far-travelled allochthon. The occurrence of UHPM in both rheologically weak (gneisses) and strong lithologies (eclogites, peridotites) speaks against the presence of large tectonic overpressure during metamorphism.

Place, publisher, year, edition, pages
2017. Vol. 35, 541-564 p.
National Category
Geology
Identifiers
URN: urn:nbn:se:uu:diva-332532DOI: 10.1111/jmg.12244ISI: 000402647600004OAI: oai:DiVA.org:uu-332532DiVA: diva2:1153297
Funder
The Royal Swedish Academy of Sciences, GS2015-0006
Available from: 2017-10-30 Created: 2017-10-30 Last updated: 2017-11-07Bibliographically 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. 63 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1595
Keyword
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: 2017-11-17

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Klonowska, IwonaMajka, JaroslawGee, David

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