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- Skiöld, Torbjörn, et al.
(författare)
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Chronology of Proterozoic orogenic processes at the Archaean continental margin in northern Sweden
- 1993
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 64:1-4, s. 225-238
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Tidskriftsartikel (refereegranskat)abstract
- The Proterozoic Svecofennian orogeny was characterized by the rapid formation of great amounts of juvenile continental crust and extensive remobilization of older crust. This paper considers U---Pb zircon ages and some geochemical and lithostratigraphic features of the Svecofennian in the continental-margin area between the Skellefte ore district and the boundary of the Archaean craton farther north in Sweden. New U---Pb zircon ages constrain the igneous activity of Svecofennian crust formation in northern Sweden to between 1930 and 1870 Ma ago. The same time constraints apply also to crust formation farther east in Finland. Orogeny along the entire Archaean-Proterozoic boundary zone was thus simultaneous and does not represent an east-to-west event succession. It is argued that rocks with similar major element compositions but distinctly different trace element characteristics were formed simultaneously but in different plate-tectonic environments. By ≈ 1875 Ma ago, the Svecofennian volcanic arc had matured and a variety of syn- to late-orogenic igneous rocks appeared in both tensional and compressional settings. Shortly thereafter, the Svecofennian magmatic activity ceased altogether, probably as a result of collision between the arc and the Archaean continent in the north. It is also suggested that pre-Svecofennian rifting of the Archaean craton had created a passive continental margin and that the transition to an active margin with subsequent island-arc magmatism and subduction beneath the Archaean craton commenced prior to 1930 Ma ago. It may well have been initiated by a relative drift of the Archaean craton towards the present southwest as a consequence of the formation of the collisional Proterozoic Lapland Granulite Belt in the northernmost Baltic Shield.
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- Stegmann, Bo
(författare)
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Försurad skogsmark i Gävleborgs län 1949-1989 : En undersökning av markförsurningens omfattning under tiden 1949-1989
- 1990
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- I slutet av 1940-talet anlades en kraftledning längs med södra Norrlandskusten. 1949 påbörjades ett avsnitt av kraftledningen genom Gävleborgs län. I början av 1960-talet anlades en parallell kraftledning och 1962 påbörjades avsnittet genom Gävleborgs län.Jordprover togs i samband med anläggandet av kraftledningarna. Resistivitet och PH uppmättes för att utröna korrosionsrisken på kraftledningsstolparna. 1989 upprepades provtagningar vid 25 stolpar längs med kraftledningen. Mark-PH uppmättes för att se förändringar i pH under perioderna 1949–1962, 1962–1989 och 1949–1989. Kalkhalten bestämdes för att se markens eventuella buffringsförmåga mot pH- sänkning.Provgropar grävdes i kraftledningsgatan 15–2 m in skogen. Provtagning gjordes från samma djup som 1949 och 1962. pH mättes på samma sätt som 1949 och 1962 dvs. I vattensuspension. Mätvärdena grupperades i 4 nivåer, 0,0m, 0,2–0,3m, 0,4–0,5m och 0,6–0,7m. Minskningen i pH- medelvärden är signifikant på alla nivåer utom två. Vid uträkningen av pH-medelvärden har pH behandlats som icke- logaritmiska värden. pH- medelvärden från andra markundersökningar som resultaten från Gävleborgsundersökningen jämförs med är framräknade på likartat sätt. I bilaga har pH- medelvärden framräknats som logaritmiska värden. Kalkhalten bestämdes för samtliga mineraljordprover och inget av dessa innehöll en kalkhalt >0,1%. Deposition av luftburna försurande ämnen har för tiden 1949–1989 varit den dominerande försurningskällan.
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- Stegmann, Bo
(författare)
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Näringsbalansen i skogsmark i Gävleborg : en pilotstudie
- 1994
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- En preliminär nettobalansberäkning gjordes för det utbytbara Ca-, K- och Mg- förrådet i morän i skogsmark. Beräkningen avser en tidsperiod på 80-100år, motsvarande en skogsgeneration. I redovisningen av beräkningarna uttryckes dessa värden per år.
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- Öhlander, Björn, et al.
(författare)
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Delineation and character of the Archaean-Proterozoic boundary in northern Sweden
- 1993
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 64:1-4, s. 67-84
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Tidskriftsartikel (refereegranskat)abstract
- Before the deposition of a Proterozoic cover and the repeated Proterozoic reworking of the older rocks, the presently exposed Archaean areas in northern Sweden formed part of a coherent craton. In the present study, we have used Sm---Nd isotopic analyses of Proterozoic granitoids and metavolcanics to delineate the Archaean palaeoboundary. In a regional context, the transition from strongly negative εNd(t) values in the northeast to positive values in the southwest is distinct, and approximately defines the border of the old craton. The Archaean palaeoboundary extends in a WNW direction, and is subparallel to the longitudinal axis of the Skellefte sulphide ore district but it is situated ≈ 100 km farther to the north. The ≈ 1.9 Ga old granitoids on the two sides of the palaeoboundary were all formed in compressional environments, but those situated to the north have higher contents of LILE and LREE at similar contents of Si. This indicates that they were generated in an area with thicker crust and supports the location of the Archaean-Proterozoic palaeoboundary. There is no simple correlation between the Archaean palaeoboundary, as defined by the isotopic results, and any of the major fracture systems as interpreted from regional geophysical measurements. Reflection seismic work indicates that juvenile volcanic-arc terrains to the south have been thrust onto the Archaean craton. Possible thrust faults have been identified from aeromagnetic measurements. Rifting of the Archaean craton created a passive margin ≈ 2.0 Ga ago. Spreading shifted to convergence with subduction beneath the Archaean continent ≈ 1.9 Ga ago. Subsequently, the resulting juvenile volcanic arc collided with the old continent, and the Archaean palaeoboundary as existing today was formed by a collision characterized by overthrusting. The boundary then was disturbed by later deformation predominantly along NNE-trending fracture systems.
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- Amakawa, Hiroshi, et al.
(författare)
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Isotopic compositions of Ce, Nd and Sr in ferromanganese nodules from the Pacific and Atlantic Oceans, the Baltic and Barents Seas, and the Gulf of Bothnia
- 1991
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Ingår i: Earth and Planetary Science Letters. - 0012-821X .- 1385-013X. ; 105:4, s. 554-565
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Tidskriftsartikel (refereegranskat)abstract
- Ferromanganese nodules from the Pacific and Atlantic Oceans, the Barents and Baltic Seas, and the Gulf of Bothnia were analyzed for the isotopic compositions of Ce, Nd and Sr and the abundances of REE, Ba and Sr. REE patterns of Barents, Baltic and Bothnian samples show no Ce anomaly, or even a negative one, in contrast to the positive anomaly observed for the Pacific and Atlantic samples. Moreover, the Baltic and Bothnian samples have distinctly low εNd values; average εNd values of the four regions are as follows: Pacific -5, Barents -10, Atlantic -11 and Baltic inclusive of Gulf of Bothnia -19. The characteristic low εNd values of the Baltic samples are indicative of the influence of Precambrian rocks from the Baltic shield. Of particular interest is the feature of the Ce isotopic composition that εCe values of the samples from the Pacific are negative and those from the other three regions positive. This novel finding might suggest a difference in sources of Ce between the Pacific and other regions. These results demonstrate that Ce isotopic ratios can be a useful tracer in marine geochemistry, in combination with isotopic compositions of Nd and Sr.
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