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- Dahlin, Peter, 1974-, et al.
(författare)
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Source character, mixing, fractionation and alkali metasomatismin in Palaeoproterozoic greenstone dykes, Dannemora area, NE Bergslagen region, Sweden
- 2013
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Ingår i: Geological Magazine. - Cambridge : Cambridge Academic. - 0016-7568 .- 1469-5081. ; 151:04, s. 573-590
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Johansson, Åke, et al.
(författare)
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A geochronological review of magmatism along the external margin of Columbia and in the Grenville-age orogens forming the core of Rodinia
- 2022
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 371, s. 1-43
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Tidskriftsartikel (refereegranskat)abstract
- A total of 4344 magmatic U-Pb ages in the range 2300 to 800 Ma have been compiled from the Great Proterozoic Accretionary Orogen along the margin of the Columbia / Nuna supercontinent and from the subsequent Grenvillian collisional orogens forming the core of Rodinia. The age data are derived from Laurentia (North America and Greenland, n = 1212), Baltica (NE Europe, n = 1922), Amazonia (central South America, n = 625), Kalahari (southern Africa and Dronning Maud Land in East Antarctica, n = 386), and western Australia (n = 199). Laurentia, Baltica, and Amazonia (and possibly other cratons) most likely formed a ca. 10 000-km-long external active continental margin of Columbia from its assembly at ca. 1800 Ma until its dispersal at ca. 1260 Ma, after which all cratons studied were involved in the Rodinia-forming Grenvillian orogeny. However, the magmatic record is not smooth and even but highly irregular, with marked peaks and troughs, both for individual cratons and the combined data set. Magmatic peaks typically range in duration from a few tens of million years up to around hundred million years, with intervening troughs of comparable length. Some magmatic peaks are observed on multiple cratons, either by coincidence or because of paleogeographic proximity and common tectonic setting, while others are not. The best overall correlation, 0.617, is observed between Baltica and Amazonia, consistent with (but not definitive proof of) their being close neighbours in a SAMBA-like configuration at least in Columbia, and perhaps having shared the same peri-Columbian subduction system for a considerable time. Correlation factors between Laurentia and Baltica, or Laurentia and Amazonia, are below 0.14. Comparison between the Grenville Province in northeastern Laurentia and the Sveconorwegian Province in southwestern Fennoscandia (Baltica) shows some striking similarities, especially in the Mesoproterozoic, but also exhibits differences in the timing of events, especially during the final Grenville-Sveconorwegian collision, when the Sveconorwegian evolution seems to lag behind by some tens of million years. Between the other cratons, the evolution before and during the final Grenvillian collision is also largely diachronous. After 900 Ma, magmatic activity had ceased in all areas investigated, attesting to the position of most of them within the stable interior of Rodinia.
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- Johansson, Åke
(författare)
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A refined U-Pb age for the Stockholm granite at Frescati, east-central Sweden
- 2019
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 141:1, s. 40-47
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Tidskriftsartikel (refereegranskat)abstract
- A sample of fine-grained grey Stockholm granite from the Frescati area just north of central Stockholm, east-central Sweden, earlier dated to 1803 +23/−19 Ma by the U-Pb zircon method using TIMS on multigrain fractions, has been reanalyzed using the Nordsim ion microprobe. The new age obtained, 1792±4 Ma, is more precise, and replaces the earlier highly discordant date. It agrees well with other ages for the formation of the Stockholm-type granites and related pegmatites, indicating an age of around 1.79 Ga for this late-orogenic Svecofennian granite magmatism. The Stockholm granite thus formed toward the end of the 1.83–1.79 Ga late Svecofennian metamorphic phase, and crosscuts earlier formed migmatitic gneiss structures in a brittle manner at the present-day level of exposure.
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| 6. |
- Johansson, Åke, 1955-
(författare)
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A Tentative Model for the Origin of A-Type Granitoids
- 2023
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Ingår i: Minerals. - : MDPI AG. - 2075-163X. ; 13:2, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- A-type granites are typically formed in stable intra-plate, back-arc or postcollisional settingsand are characterized by highly ferroan and potassic major element compositions, and by strong enrichment in incompatible trace elements. Unlike I-, S- and M-type granites, where the letters denote the dominant source material (igneous, sedimentary or mantle derived), there is no consensus on the source and processes giving rise to A-type magmas. In this contribution, a conceptual model for the origin of A-type granitoids, using the Bornholm A-type granitoid complex in southern Fennoscandia as an example, is presented. In this model, underplated mantle-derived basaltic magma may develop into intermediate and siliceous A-type magma, which is ferroan, potassic and highly enriched in incompatible trace elements, through a combination of fractional crystallization leading to cumulate formation, and partial melting and crustal assimilation, in a process akin to zone refining in metallurgy. The key factor is a relatively stable tectonic environment (postcollisional, anorogenic, or extensional), where there is little or no replenishment of more primitive basaltic magma to the system, allowing it to attain more evolved, enriched and extreme compositions. The A-type granitoids may then be viewed as a more evolved counterpart of subduction-related I-type granitoids.
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| 7. |
- Johansson, Åke, 1955-
(författare)
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Berättelsen om ett jordeliv
- 2014
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Ingår i: Geologiskt forum. - : Geologiska Föreningen. - 1104-4721. ; 21:84, s. 31-31
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 8. |
- Johansson, Åke
(författare)
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Cleaning up the record – revised U-Pb zircon ages and new Hf isotope data from southern Sweden
- 2021
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 143:4, s. 328-359
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Tidskriftsartikel (refereegranskat)abstract
- Ten samples of orthogneisses and granitoids from the Eastern Segment of the Sveconorwegian Orogen in southern Sweden, previously dated by ID-TIMS on zircon, have been dated anew using SIMS spot analysis of individual zircon grains, leading to more reliable and in most cases also more precise revised magmatic crystallization ages. A gneissic monzonite within the Protogine Zone south of Alvesta in Småland yields a revised U-Pb age of ca. 1725 Ma, four samples of orthogneiss from Skåne all yield revised ages between 1690 and 1700 Ma, while two samples of coarse-grained granitoid gneiss in the same region yield ages between 1680 and 1690 Ma. These revised ages are between 15 and 250 m.y. older than previously obtained TIMS ages. Two samples of the Gumlösa-Glimåkra granite along the Protogine Zone in northern Skåne and one sample of related syenite yield ages around 1220 Ma, similar but more precise compared to the previous ages. The U-Pb zircon data have been complemented by Hf isotope analysis by LA-ICP-MS on the same zircon grains, and previously obtained initial Sr and Nd whole-rock isotope data have been recalculated to the revised crystallization ages. The Sr isotope data scatter, while the revised initial εNd values fall between +1 and +2 for the older granitoids and orthogneisses, and close to 0 for the 1220 Ma intrusives along the Protogine Zone. Initial εHf in magmatic undisturbed zircons shows relatively little spread within each sample, between 2 and 4 Epsilon units, disregarding occasional outliers, with average values for the 1725 - 1680 Ma rocks falling between +3and +5.5, and at ca. +1.5 in the 1220 Ma rocks. In spite of the limited variation, there is a covariation between initial εNd and initial εHf in the older rocks, suggesting either mixing between two isotopically distinct magma sources, or one magma source which is isotopically heterogeneous. The isotopic signatures of the 1220 Ma intrusives along the Protogine Zone is indicative of juvenile mantle input totheir magmas, rather than pure crustal melting.
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| 9. |
- Johansson, Åke
(författare)
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Comment on Li et al. (2023): A dynamic 2000—540 Ma Earth history: From cratonic amalgamation to the age of supercontinent cycle
- 2023
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Ingår i: Earth-Science Reviews. - 0012-8252 .- 1872-6828. ; 241, s. 1-3
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Tidskriftsartikel (populärvet., debatt m.m.)abstract
- The paper “A dynamic 2000—540 Ma Earth history: From cratonic amalgamation to the age of supercontinentcycle” that was recently published in Earth-Science Reviews by Li et al. (2023) is an impressive piece of work, putting together data for the Proterozoic supercontinent cycle into a coherent model including both maps and animations. Nevertheless, as they write themselves, there will no doubt be room for improvements of their model, some of which I hope to contribute with in this comment from my Baltica perspective, in particular when it comes to the relations between Baltica, Amazonia and West Africa.
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