Source character, mixing, fractionation and alkali metasomatismin in Palaeoproterozoic greenstone dykes, Dannemora area, NE Bergslagen region, Sweden

The geochemical and isotopic characteristics of metamorphosed Svecofennian maficdykes from the Dannemora area in the NE part of the Bergslagen region in central Sweden wereinvestigated and compared to mafic intrusive rocks in their vicinity. The dykes, with an inferred ageof c. 1860–1870 Ma, are cal...

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Bibliographic Details
Published in:Geological magazine 2013, Vol.151 (4), p.573
Main Authors: Dahlin, Peter, Johansson, Åke, Andersson, Ulf B.
Format: Article
Language:English
Subjects:
alkali metasomatism
Bergslagen region
Dannemora
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Geovetenskap med inriktning mot mineralogi, petrologi och tektonik
greenstone dykes
Svecofennian
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Summary:The geochemical and isotopic characteristics of metamorphosed Svecofennian maficdykes from the Dannemora area in the NE part of the Bergslagen region in central Sweden wereinvestigated and compared to mafic intrusive rocks in their vicinity. The dykes, with an inferred ageof c. 1860–1870 Ma, are calc-alkaline, sub-alkaline and basaltic in composition and have a mixedsubduction and within-plate geochemical affinity. They are the result of mixing of at least three mantlesource components with similar basaltic major element composition, but different concentrations ofincompatible trace elements. Magma M1 is strongly enriched both in Rare Earth Elements (REE)and High-Field-Strength Elements (HFSE); magma M2 is highly enriched in Large-Ion LithophileElements (LILE, except Sr) with only moderate enrichment in HFSE and REE (particularly low inHeavy Rare Earth Elements); and magma M3 is enriched in Sr and has a flat REE profile. MagmaM3 also has a somewhat more positive (depleted) initial εNd value of +1.8, compared to +0.4 to +0.5 for magmas M1 and M2. The magma evolution was controlled by a mixture of fractionation (mainlyaffecting the compatible elements) and mixing, best seen in the incompatible element concentrationsand the Nd isotope data. The basaltic overall composition indicates little or no wholesale contaminationby upper continental crust, but the dykes have undergone later metasomatic changes mainly affectingthe alkali elements.
ISSN:0016-7568
1469-5081
DOI:10.1017/S0016756813000551