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Lithogeochemistry and alteration of a Palaeoproterozoic metavolcanic succession in the Dannemora area, Bergslagen region, central Sweden
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The rocks in the Dannemora area constitute the Dannemora Formation, which can be divided into eight different sections, A-H, on the basis of stratigraphy and geochemistry. The great thickness and alteration pattern of the deposit indicate deposition in a caldera environment, related to both intensive and waning volcanic stages and hydrothermal processes. The partly strong alteration is shown by the sericite dominated matrix and the completely sericite replaced plagioclase and volcaniclastic clasts. The northern part of the Dannemora area in general shows a lower degree of alteration than the central part in comparable stratigraphic levels, which implies that these parts were located in different positions to hydrothermal cells.

A lapilli-bearing ash layer is present between the massive pyroclastic beds in section A and C that have distinctly different alteration patterns suggesting that the strong alteration in section A occurred before deposition of section C. During the waning volcanic stage, the calmer environment allowed stromatolitic limestone interlayered with thin horizons of ash-siltstone to deposit. During subsequent dolomitisation of the limestone the interlayered ash-siltstone was affected by great enrichment of CaO and depletion of MgO. Despite this strong alteration this section is classified as least altered by alteration index, which most likely reflect a two stage alteration with initial alkali enrichment, followed by CaO enrichment that “reset” the alteration index. The great enrichment of Ba in the upper member is supported by the occurrence of hyalophane. On the basis of lithogeochemical signature of immobile element ratios together with the reoccurrence of an accretionary lapilli-bed the position of a previously inferred anticline has been established.

National Category
URN: urn:nbn:se:uu:diva-236878OAI: oai:DiVA.org:uu-236878DiVA: diva2:765754
Available from: 2014-11-25 Created: 2014-11-25 Last updated: 2015-02-03Bibliographically 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.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1209
Dannemora, Bergslagen region, volcaniclastic rocks, sedimentology, metasomatic alteration, lithogeochemistry, reflection seismic
National Category
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
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)
Available from: 2014-12-22 Created: 2014-11-24 Last updated: 2015-02-03

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