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Source character, mixing, fractionation and alkali metasomatismin in Palaeoproterozoic greenstone dykes, Dannemora area, NE Bergslagen region, Sweden
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
2013 (English)In: Geological Magazine, ISSN 0016-7568, E-ISSN 1469-5081, Vol. 151, no 04, 573-590 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Cambridge Academic, 2013. Vol. 151, no 04, 573-590 p.
Keyword [en]
Svecofennian, Bergslagen region, Dannemora, greenstone dykes, alkali metasomatism
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
URN: urn:nbn:se:uu:diva-214689DOI: 10.1017/S0016756813000551ISI: 000337751000001OAI: oai:DiVA.org:uu-214689DiVA: diva2:685336
Available from: 2014-01-09 Created: 2014-01-09 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Stratigraphy and Geochemistry of the Palaeoproterozoic Dannemora inlier, north-eastern Bergslagen region, central Sweden.
Open this publication in new window or tab >>Stratigraphy and Geochemistry of the Palaeoproterozoic Dannemora inlier, north-eastern Bergslagen region, central Sweden.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The Palaeoproterozoic Dannemora inlier is situated in the north-eastern Bergslagen region. The inlier consists of primary and reworked volcanic deposits, stromatolitic limestone and skarn that have been subjected to upper greenschist facies metamorphism. Thicknesses of the different volcanic deposits indicate deposition within a caldera, where syn-volcanic alkali alteration was strong. The deposition was submarine and below wave base in the eastern part of the inlier, but above wave base in the central part where erosion channels together with cross-bedding occurs frequently.

The Dannemora Formation is the volcanosedimentary succession of the inlier. Two borehole profiles, a northern and a southern, cover the whole Formation and show different alteration patterns. A strong depletion of Na2O and enrichment of K2O dominate in the southern profile, whereas this pattern is not as evident in the northern profile. The uppermost section of the totally eight constituting the Formation, is intercalated with ore-bearing dolomitic limestone and skarn, and has experienced at least two episodes of alteration. An anticline has been established lithogeochemically from immobile element ratios and the reoccurrence of an accretionary lapilli bed.

Numerous altered sub-alkaline, calc-alkaline and basaltic dykes have been recorded in the Dannemora inlier. They are the result of mixing and fractionation of at least three magmatic sources and carry a mixed signature of subduction zone and within-plate volcanic tectonic setting.

A seismic profile across the Dannemora inlier images a strong reflector package that dips c. 50° E to the east of the inlier. This package coincides with the polyphase, E-up reverse, brittle-ductile Österbybruk deformation zone (ÖDZ). Yet another steep reflector in the Dannemora ore-field extends to a depth of more than two kilometres. This reflector might represent either a deep-seated iron deposit or a fluid-bearing fault zone.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 50 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1209
Keyword
Dannemora, Bergslagen region, volcaniclastic rocks, sedimentology, metasomatic alteration, lithogeochemistry, reflection seismic
National Category
Geology
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-236872 (URN)978-91-554-9115-4 (ISBN)
Public defence
2015-01-16, Axel Hambergsalen, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2014-12-22 Created: 2014-11-24 Last updated: 2015-02-03

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