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The role of mafic dykes in the petrogenesis of the Archean Siilinjärvi carbonatite complex, east-Central Finland
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics.ORCID iD: 0000-0002-5433-0908
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Mineralogy Petrology and Tectonics. Geology and Mineralogy, Åbo Akademi University, Akatemiankatu 1, 20500 Turku, Finland.ORCID iD: 0000-0001-5581-3613
Åbo Akademi University.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.ORCID iD: 0000-0003-1241-2988
2019 (English)In: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 342-343, p. 468-479Article in journal (Refereed) Published
Abstract [en]

The Archean (~2.6 Ga) Siilinjärvi carbonatite complex in east-central Finland is crosscut by a few ultramafic lamprophyre dykes, together with a broad array of more evolved mafic dykes that range in composition from foidites to various types of alkali basalts. A possible genetic link between the primitive lamprophyres and the carbonatite complex has previously been hypothesised, but their exact relations have been unclear due to the regional metamorphic overprint (i.e., greenschist facies). Here we focus on the petrology and petrography of the mafic dykes, and integrate the data to present a coherent model that can explain the genesis of the Siilinjärvi carbonatite complex. Field-relations, in combination with petrography and geochemistry, indicate that there are at least three generations of mafic dykes present. The oldest dykes (Generation I) are strongly deformed, and inferred to have been emplaced shortly after the formation of the complex itself. These dykes can be divided into two groups (i.e., ultramafic lamprophyres and Group A), where Group A comprises foidites characterised by low SiO2 (41.4–51.5 wt%) and high alkali (>10 wt% K2O) content. We interpret the foiditic magmas to have evolved from primitive ultramafic lamprophyres by fractionating a clinopyroxene-olivine dominated mineral assemblage that was devoid of feldspar. This fractionation path forced alkali-enrichment in the magmas belonging to Group A, which pushed them into the miscibility gap, and resulted in liquid immiscibility that produced moderately alkaline conjugate carbonatite(s). Subsequent fractionation of the conjugate carbonatite by predominantly calcite and apatite produced the mineralogically homogeneous carbonatite cumulate that is exposed at Siilinjärvi. Younger, less deformed, mafic dykes (belonging to Generations II and III) exhibit trace element characteristics, broadly similar to basaltic dyke swarms in the region. The younger dykes are characterised by the presence of large plagioclase crystals in thin sections. Crystallisation of a feldspar-bearing mineral assemblage resulted in only moderate enrichment of alkalis with increased fractionation, which caused the younger dykes to evolve along the more common basalt-to-trachyte series. Thus, the magmas belonging to Generations II and III at Siilinjärvi never fulfilled the conditions required to produce carbonatites by liquid immiscibility.

Place, publisher, year, edition, pages
2019. Vol. 342-343, p. 468-479
Keywords [en]
Siilinjarvi, Carbonatite, Ultramafic lamprohyre, Archean, Fractional crystallisation, Liquid immiscibility
National Category
Geochemistry
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
URN: urn:nbn:se:uu:diva-385377DOI: 10.1016/j.lithos.2019.06.011ISI: 000477091500030OAI: oai:DiVA.org:uu-385377DiVA, id: diva2:1323967
Funder
Vinnova, 2014–06238Swedish Research Council, 2018-03702The Geological Survey of Sweden (SGU), 311-1485/2017Available from: 2019-06-13 Created: 2019-06-13 Last updated: 2019-09-20Bibliographically approved

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Mattsson, Hannes B.Högdahl, KarinMalehmir, Alireza

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