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

Published online by Cambridge University Press:  13 August 2013

PETER DAHLIN*
Affiliation:
Department of Earth Sciences, Uppsala University, Villavägen 16, SE-75236 Uppsala, Sweden
ÅKE JOHANSSON
Affiliation:
Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50 007, SE-10405 Stockholm, Sweden
ULF B. ANDERSSON
Affiliation:
Department of Earth Sciences, Uppsala University, Villavägen 16, SE-75236 Uppsala, Sweden LKAB, Research and Development, TFG, SE-981 36 Kiruna, Sweden
*
Author for correspondence: [email protected]

Abstract

The geochemical and isotopic characteristics of metamorphosed Svecofennian mafic dykes from the Dannemora area in the NE part of the Bergslagen region in central Sweden were investigated and compared to mafic intrusive rocks in their vicinity. The dykes, with an inferred age of c. 1860–1870 Ma, are calc-alkaline, sub-alkaline and basaltic in composition and have a mixed subduction and within-plate geochemical affinity. They are the result of mixing of at least three mantle source components with similar basaltic major element composition, but different concentrations of incompatible 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 Lithophile Elements (LILE, except Sr) with only moderate enrichment in HFSE and REE (particularly low in Heavy Rare Earth Elements); and magma M3 is enriched in Sr and has a flat REE profile. Magma M3 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 (mainly affecting the compatible elements) and mixing, best seen in the incompatible element concentrations and the Nd isotope data. The basaltic overall composition indicates little or no wholesale contamination by upper continental crust, but the dykes have undergone later metasomatic changes mainly affecting the alkali elements.

Type
Original Articles
Copyright
Copyright © Cambridge University Press 2013 

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