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The role of mafic dykes in the petrogenesis of the Archean Siilinjärvi carbonatite complex, east-Central Finland
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.ORCID-id: 0000-0002-5433-0908
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik. Geology and Mineralogy, Åbo Akademi University, Akatemiankatu 1, 20500 Turku, Finland.ORCID-id: 0000-0001-5581-3613
Åbo Akademi University.
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.ORCID-id: 0000-0003-1241-2988
2019 (Engelska)Ingår i: Lithos, ISSN 0024-4937, E-ISSN 1872-6143, Vol. 342-343, s. 468-479Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
2019. Vol. 342-343, s. 468-479
Nyckelord [en]
Siilinjarvi, Carbonatite, Ultramafic lamprohyre, Archean, Fractional crystallisation, Liquid immiscibility
Nationell ämneskategori
Geokemi
Forskningsämne
Geovetenskap med inriktning mot mineralogi, petrologi och tektonik
Identifikatorer
URN: urn:nbn:se:uu:diva-385377DOI: 10.1016/j.lithos.2019.06.011ISI: 000477091500030OAI: oai:DiVA.org:uu-385377DiVA, id: diva2:1323967
Forskningsfinansiär
Vinnova, 2014–06238Vetenskapsrådet, 2018-03702Sveriges Geologiska Undersökning (SGU), 311-1485/2017Tillgänglig från: 2019-06-13 Skapad: 2019-06-13 Senast uppdaterad: 2019-09-20Bibliografiskt granskad

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Mattsson, Hannes B.Högdahl, KarinMalehmir, Alireza

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