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- Perdahl, Jan-Anders, et al.
(author)
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The marine-continental transition of the early Proterozoic Skellefte-Arvidsjaur volcanic arc in the Bure area, northern Sweden
- 1994
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In: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 116:3, s. 133-138
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Journal article (peer-reviewed)abstract
- It is generally assumed that c. 1.9 Ga ago the northern part of the Fennoscandian (Baltic) Shield comprised a continental margin along which plate tectonism and subduction took place. The main division of the region is a marine domain towards the south and a continental domain towards the north. At Bure in northern Sweden, the marine-continental transition is exposed and the lithologies and palaeotectonic environment of this area are investigated in the present paper. The results show that the former lithostratigraphic division has to be modified and a reconstruction of the geological development is necessary. The Bure supracrustal sequence shows a successive change from a marine schist-greywacke-basaltic environment in the stratigraphically lowest part (Stalo Formation) to a continental volcanic environment in the upper part (Bure Formation). The continental volcanic rocks are intermediate-felsic and mildly alkaline. Minor intercalations of similar volcanic rocks occur within the marine sequence. The youngest event in the Bure area is the deposition of the Loito Formation which consists of a red conglomerate-sandstone lying on top of the Bure and Stalo volcanic rocks. The chemical character and lithological associations infer that the Bure Formation volcanic rocks were deposited in an extensional environment and may constitute the late stage member of the calc-alkaline volcanism that occurs further east, the Arvidsjaur Porphyries. Continental bimodal and slightly alkaline volcanic rocks occur north and northwest of the Bure area and would, together with the continental Bure volcanic rocks, form a separate group, here collectively referred to as the Arjeplog Porphyries
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- Romer, Rolf L.
(author)
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Deformation-related Paleozoic radiogenic lead and strontium additions in Proterozoic marbles from the Rombak-Sjangeli basement culmination, Scandinavian Caledonides
- 1994
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In: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 116:1, s. 23-29
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Journal article (peer-reviewed)abstract
- The Sjangeli supracrustal belt occurs at the eastern margin of the Rombak-Sjangeli basement culmination that involves Proterozoic basement, which became incorporated into the Caledonian orogen as old basement faults were reactivated. The lead isotopic composition of early Proterozoic marbles from the Sjangeli supracrustal belt fall in the 206Pb/204Pb-207Pb/204Pb diagram on a linear array that is interpreted as a mixing line. Using the age of granite intrusion and Proterozoic metamorphism as source age for the lead (t1≈ 1.8 Ga), the corresponding mixing age is t1≈ 0.4 Ga, which suggests that the radiogenic lead was introduced during the Caledonian deformation of the marbles. The isotopic compositions of lead and strontium, which also is highly variable and radiogenic, are positively correlated, indicating that radiogenic lead and strontium were introduced together. Combining lead and strontium isotope data from the marbles and Paleozoic galena-calcite veins demonstrates that the lead and strontium were derived from local sources with isotopically distinct characteristics. Sample suites from deformed marbles that define linear arrays in the 206pb/204pb-207pb/204pb diagram probably represent mixing lines rather than secondary isochrons.
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- Romer, Rolf L.
(author)
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Rb-Sr data structure a possible cause for differences in Rb-Sr whole-rock and U-Pb zircon ages
- 1994
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In: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 116:2, s. 93-103
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Journal article (peer-reviewed)abstract
- Differences between U‐Pb zircon and Rb‐Sr whole‐rock ages are often interpreted to be due to later geologic disturbances such as low‐grade metamorphism and slow cooling. Yet, apparently contrasting ages can also be due to erroneous ages derived from non‐ideal systems. Their incorporation into geological models results in distorted time scales and may ultimately lead to the suggestion of geologic processes and events that are not real. I illustrate this point discussing the geochronologic database that originally was used (1) to constrain the evolution of silicic crust in the Svecofennian area, (2) to define the geographic extent of the Sveconorwegian‐Grenville orogeny in north Norway and east Greenland, (3) to determine the age of thrusting in the southern Swedish Caledonides, and (4) to confine the Apparent Polar Wander Path (APWP) of the Baltic Shield.
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- Romer, Rolf L., et al.
(author)
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U-Pb age of the Yxsjoberg tungsten-skarn deposit, Sweden
- 1994
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In: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 116:3, s. 161-166
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Journal article (peer-reviewed)abstract
- The Yxsjoberg scheelite-skarn deposit of western Bergslagen (south-central Sweden) formed in Palaeoproterozoic time when aqueous fluids migrating along lithologic discontinuities replaced limestones in acid volcanic rocks. The age of the mineralization and, thus, the source of the fluids are controversial. U-Pb dating of titanite that formed during the skarnification of the limestones yields an age of 1789+ or -2 Ma. This age is not compatible with an exhalative origin of the deposit or a genetic relation to Svecofennian arc magmatism, as has been suggested. Instead, the U-Pb titanite age corresponds to the age of Y, Rb, F, and Nb-rich post-kinematic granitoids from the Bergslagen area and granitoids of the Transscandinavian Igneous Belt (TIB) farther to the west. As the TIB granitoids are not fertile with respect to W-mineralizations, the U-Pb titanite age indicates that the post-kinematic granitoids were the source of heat, metals, and probably fluids for the formation of the Yxsjoberg scheelite-skarn deposit
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- Wohlfarth, Barbara, et al.
(author)
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Ice recession and depositional environment in the Blekinge archipelago of the Baltic Ice Lake.
- 1994
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In: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 116:1, s. 3-12
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Journal article (peer-reviewed)abstract
- The Swedish varved clays, deposited in the former Baltic Ice Lake during the deglaciation, constitute an excellent tool to decipher the ice recession with annual precision. New clay varve diagrams from Blekinge, southern Sweden, enable to place the deglaciation of sites situated in northern Blekinge between the local varve years -39 to +77, which preliminary corresponds to 12,971-12,855 v.y. BP. Detailed and systematic investigations demonstrate, that plant macrofossils can be found in the varved clays, if suitable sites, close to the former shoreline of the Baltic Ice Lake are chosen. In Blekinge the coastline formed an archipelago-like landscape with small, deep, incised fjords and numerous islands similar to today's. In-wash of organic matter in near shore areas, commenced with the beginning of the Bolling warming peak (c. 12,800-12,700 v.y. BP) and is reflected by the relatively higher amount of macroscopic plant and animal remains. In varved clays deposited during the Older Dryas stadial (<12,700 v.y. BP) considerably less macroremains were recorded, probably due to less melt water out-flow and/or change in the vegetation.
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