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| 2. |
- Billström, Kjell, et al.
(författare)
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U-Pb zircon age of the Rostberget W-occurrence, northern Sweden
- 1989
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Ingår i: Geologiska föreningens i Stockholm förhandlingar. - : Informa UK Limited. - 0016-786X. ; 111:2, s. 121-126
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Tidskriftsartikel (refereegranskat)abstract
- Anomalously U-rich granites of the so-called Revsund type, associated with W- and Sn-enrichments, occur in the Gunnarn area close to Storuman in northern Sweden. At Rostberget, a swarm of thin undeformed wolframite-bearing greisen veins are situated in the interior of the Joran granite intrusion. We have dated zircons from a sample of the greisen veins with the U-Pb method. The veins are less than 1 cm wide and it is likely that the sample also contains zircons from the immediately surrounding altered granite. The mineralizing event and connected metasomatic processes probably released Zr from the granite which became available for zircon growth processes in the greisen veins. One notable feature, probably due to the greisen-forming processes, is the very high common Pb concentrations characterizing the zircons. By varying the assumed common Pb composition in the calculations, the effect on resulting intercept ages has been evaluated. The obtained age interval (1775-1782 Ma) is typical of formation ages for Revsund granites, which indicates that the mineralization event at Rostberget followed closely upon the granite emplacement. We conclude that the Revsund-type granites, which cover large areas, were generated in a narrow time interval 1790-1775 Ma ago.
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| 3. |
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| 4. |
- Skiöld, Torbjörn, et al.
(författare)
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Chronology and geochemistry of late Svecofennian processes in northern Sweden
- 1989
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Ingår i: Geologiska föreningens i Stockholm förhandlingar. - : Informa UK Limited. - 0016-786X. ; 111:4, s. 347-354
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Tidskriftsartikel (refereegranskat)abstract
- Radiometric and geochemical data have been used to characterize the concluding stages of the Svecofennian orogeny in northern Sweden. U-Pb zircon ages of 1879 +- 7 and 1858 +- 9 Ma have been obtained from two unfoliated granitoids of the North Svecofennian Sub-province. The ages are distinctly different from each other and indicate late- to post-orogenic Svecofennian magmatic activity 1860 Ma ago. The older rock type forms a diorite-quartz monzonite-adamellite trend, while samples of the younger type are adamellitic. Trace element data are not in accordance with a model age where the younger type is formed by continuous magmatic differentiation from rocks of the older type. We propose a new 1910-1860 Ma delimitation for the emplacement of all igneous Svecofennian rocks in northern Sweden and conclude that the widespread potassic 1800-1770 Ma granites of that area should not be considered part of the Svecofennian orogeny, but of some later and separate event.
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| 5. |
- Skiöld, Torbjörn, et al.
(författare)
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Early Proterozoic crust-mantle interaction at a continental margin in northern Sweden
- 1989
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 45:1-3, s. 19-26
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Tidskriftsartikel (refereegranskat)abstract
- On the basis of recent geochemical and Sm-Nd isotopic data, alternative models for the early Proterozoic evolution of the Archaean craton of the Baltic Shield and its marginal areas in northern Sweden are discussed. Before the deposition of a craton cover and the subsequent Svecofennian orogenic activity, the Archaean areas probably formed a continuous domain. The 1.89-1.86 Ga old early Svecofennian granitoids found close to exposed Archaean rocks were created largely by the remobilization of Archaean crust. To the south of the Archaean Domain, however, chronologically equivalent granitoids are made up of matter with short crustal residence times. That area, the Skellefte district, represents a new addition to the continental crust of the Baltic Shield ≈ 1.90 Ga ago. The particulars of the crustal accretion process are still controversial. However, recent data indicate different tectonic developments in the Skellefte district and in the vicinity of the Archaean-Proterozoic boundary farther north. Apparently, the one-time edge of the Archaean craton was situated somewhere between these two areas, but appears to have had a rather more northerly position than previously assumed.
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| 6. |
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| 7. |
- Öhlander, Björn, et al.
(författare)
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Geochemistry of the Proterozoic wolframite-bearing greisen veins and the associated granite at Rostberget, northern Sweden
- 1989
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 78:2, s. 135-150
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Tidskriftsartikel (refereegranskat)abstract
- At Rostberget in northern Sweden, a swarm of thin, undeformed wolframite-bearing greisen veins occurs within the 1.78-Ga-old Joran granite intrusion. This is a microcline-rich porphyritic granite probably generated with sediments as important source materials. When the Joran granite crystallized, an aqueous fluid, rich in volatiles and W, was active in the central and apical parts of the intrusion. This fluid escaped through fractures caused by tectonic rupturing, the immediate surroundings of the fractures became altered, and the greisen veins were formed. The contents of SiO2, Na2O, Sr, V, Zr and LREE decreased during this greisenization, while the contents of Fe2O3, MnO, MgO, CaO, H2O, W, Sn, Mo, Cu, Rb, Sc, Pb, Be, Nb, HREE and probably Ag increased. K2O, TiO2, P2O5, Ba, Zn and Y were not immobile, but no consistent trends of gain or loss were found. Neutralization of the acid vapour by alteration reactions was probably important for controlling the precipitation of wolframite. Accessory minerals contain most of the REE in all sample types. The gain of HREE during the greisenization was caused mainly by deposition of fluorite (and xenotime), and the loss of LREE was the result of dissolution of monazite (and allanite). No change in Eu content was observed. The enrichment of HREE and the presence of fluorite and minor topaz, indicate alteration from a F-rich fluid, but do not necessarily imply that W was transported as F− complexes.
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| 9. |
- Öhlander, Björn, et al.
(författare)
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Regional implications of U-Pb zircon dating of two proterozoic granites associated with molybdenite mineralized aplites in northern Sweden
- 1989
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Ingår i: Geologiska föreningens i Stockholm förhandlingar. - : Informa UK Limited. - 0016-786X. ; 111:3, s. 229-238
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Tidskriftsartikel (refereegranskat)abstract
- To contribute to the understanding of the timing of the molybdenite mineralizing events in the Proterozoic of Sweden, two granites associated with molybdenite enriched aplites at Allebuoda and Kåtaberget in northern Sweden have been dated by the U-Pb zircon method. The result is 1892+-62 Ma for the Kåtaberget granite, and 1767+-9 Ma for the Allebuoda granite. As the granitoids in the Svecofennian region generally fall into two major age groups, 1890-1860 Ma and 1800-1770 Ma respectively, this suggests that the Kåtaberget granite belongs to the older group and the Allebuoda granite to the younger group. Sulphur isotopic analyses suggest a larger admixture of sediments in the source material of the Allebuoda granite and aplite than in corresponding rocks at Kåtaberget. The results of this and previous studies suggest that the major Mo- and W-occurrences in the Precambrian of Sweden are associated with granites belonging to the 1800-1770 Ma group.
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