Göran Åberg

Precambrian geochronology of south-eastern Sweden

Abstract Geochronological investigations in south-eastern Sweden, based on U–Pb, Rb-Sr and K–Ar determinations, yield ages between 2465 Ma and 900 Ma. The oldest age, 2465 Ma, is obtained on zircons from the Vastervik quartzite. Intrusion of synkinematic Svecokarelian granites occurred between 1970 Ma and 1840 Ma ago. Intrusion of late-kinematic Svecokarelian granites then followed, accompanied by wide-spread migmatization, deformation and folding affecting the older rocks. Between 1730 Ma and 1655 Ma, different generations of the Smaland porphyries and Smaland granites were emplaced. The last dated activity in postorogenic time was the intrusion of the Smaland dyke porphyries 1655 Ma ago. The effect of the intrusion of the Smaland granites was intense in the Vastervik area and caused a resetting of the Rb-Sr ages of the older granites to 1665–1655 Ma. The lowering of the K–Ar ages to approximately 1400 Ma is probably due to anorogenic intrusions, like the Gotemar granite at 1380 Ma, and metamorphic event...

87Sr/86Sr, Na, F, Sr, and La in skeletal fish debris as a measure of the paleosalinity of fossil-fish habitats

Forty-six samples of apatite from lower vertebrates (ostracoderms, placoderms, elasmobranchs, actinopterygians, and crossopterygians), ranging in age from Silurian to Recent, were analyzed to determine each specimen9s 87 Sr/ 86 Sr isotopic ratio and content of the elements Na, F, Sr, and La. Our aim was to determine whether these chemical parameters can yield reliable information about the paleosalinity conditions of the ancient fish habitats. In recent times, the uniform 87 Sr/ 86 Sr isotopic ratio and Na, F, and Sr abundances of sea water in general differ characteristically from the corresponding ratios and abundances of fresh waters and low-saline brackish waters. For evaluation of the data on fossils, analyses were made on recent skeletal-fish apatite derived from different waters with known Sr-isotopic ratio and salinity. Salinity interpretations based on our chemical analyses are generally congruent with the salinity conditions indicated by other paleoenvironmental parameters inherent in the fossil-bearing strata. Some fossil specimens, however, gave signals seemingly incompatible with prevailing opinions regarding the environment in which the embedding strata formed. In some cases, the divergences could be accounted for by erratic occurrences of reworked material. Other cases, such as ostracoderm and placoderm remains from the Old Red Sandstone, are less easy to unravel. The method tested in this study may be a useful tool, clarifying, for example, salinity conditions during formation of various Old Red Sandstone deposits.

Zircon ages of metavolcanic and synorogenic granitic rocks from the Svärdsjö and Yxsjöberg areas, south central Sweden

Abstract U-Pb dating of zircons from the Yxsjoberg and Svardsjo leptites yields a discordia age of 1900 ± 19 Ma and for zircons from the synorogenic granite near Svardsjo, 1873+11 -10 Ma. Calculation with the systematics of Wickman (1983: Uranium-lead isotope systematics: The case of crystals with discrete cores. Lithos 16, in press) gives an age of 1925 Ma for the granite zircons.

Radiometric dating of Precambrian rocks in Småland, southeastern Sweden

Abstract Dating of zircons with the U-Pb method in southeastern Sweden gives ages of 1837 Ma on volcanics, considered as postorogenic to the Svecokarelian, and 1825 Ma on an arkose (possibly a sheared synorogenic granite). These figures coincide with previous U-Pb ages in southeastern Sweden. Zircons from an older preorogenic gneiss and a metasediment do not fit a U-Pb discordia line but imply a previous history back to 2500 Ma ago. Zircons from a dike porphyry intruding the ‘younger’ volcanics do not yield a meaningful age, probably due to contamination. When compiling the presented U-Pb data with previous Rb-Sr and K-Ar ages the orogenic activity from synorogenic through postorogenic times seems to be continuous in southeastern Sweden.

Testing 87Sr/86Sr as a paleosalinity indicator on mixed marine, brackish-water and terrestrial vertebrate skeletal apatite in late Paleocene-early Eocene near-coastal sediments, Mississippi

Abstract Mixed marine, brackish-water and terrestrial vertebrate skeletal apatite is found in the late Paleocene-early Eocene Tuscahoma and Bashi Formations of Mississippi. The co-Occurrence in these near-coastal marine deposits of skeletal apatite with different provenances offers a unique opportunity for unraveling the effects of diagenesis on apatite 87Sr/86Sr composition, with bearings on the use of this isotope pair as paleosalinity indicator. The results show that the Sr isotopes of all originally poorly ordered phases of skeletal apatite have extensively re-equilibrated with pore water with significantly lower 87Sr/86Sr ratios than late Paleocene-early Eocene seawater. For example, marine shark and ray teeth from the Bashi Formation all have clearly nonmarine 87Sr/86Sr ratios, 0.7073-0.7075, compared with a coeval seawater ratio in the range 0.7077-0.7078. However, separated shark tooth enamel, with a high original crystallinity, appears to have retained all or most of its in vivo Sr isotopic signature until the present. Here we also show that 87Sr/86Sr results on recent vertebrate skeletal material from different environments are in good agreement with expected values for respective habitat. Earlier analyses of calcite material from the Bashi Formation indicated the existence of a shift in seawater 87Sr/86Sr to a lower value at the Paleocene-Eocene boundary. The strong influence of pore water with low, nonmarine 87Sr/86Sr on the apatite in the Bashi Formation suggests that the proposed isotopic shift may be an artifact related to diagenetic processes.

Strontium in rivers of the Baltic Basin

The rivers in the Baltic Basin drain a mixture of bedrocks ranging from Mesozoic-Paleozoic sediments in the south to Proterozoic-Archean intrusives in the north. The rivers in the sedimentary basin in the south have high concentrations of Sr, in the interval 100–500 µg l−1 while the87Sr/86Sr ratio is close to that of seawater, i.e. 0.71. The northern rivers in the Precambrian shield area on the other hand have low Sr concentrations of 15–50 µg l−1 with high87Sr/86Sr ratios of about to 0.73 (0.721–0.745). The riverine input of dissolved Sr to the brackish Baltic Sea approaches 60 tons year−1, with a weighted mean concentration approaching 130 µg l−1 and a weighted mean87Sr/86Sr ratio close to 0.712. Although the sedimentary area in the south supplies only about 43% of the total river discharge, it gives about 88% of the total Sr input. Because of this and the strong regional riverine variation in87Sr/86Sr ratio, Sr and its isotopes seem to be a convenient tool to unveil mixing relations of water masses in the northern Baltic Sea, provided high resolution analyses are applied. For an overall characterization of water mixing in the Baltic Sea, the Nd system will be superior to that of Sr.

Radiometric dating of the Horrsjö granite, south central Sweden

Abstract U-Pb analyses of zircons from the Horrsjo granite, south central Sweden, give an upper intercept with concordia of 1850+8 −7 Ma (1 sigma). The Horrsjo granite accordingly belongs to the synkinematic Svecokarelian intrusions. The zircons have no visible cores. Their positions on discordia are reversed in relation to the normal case, the smallest crystals having less than half the uranium content of the largest. This may be due to uranium loss caused by superimposed events of ser- and postorogenic Svecokarelian magmatism in the area. However, a recalculation with the new U-Pb systematics of Wickman (Wickman, F. E., 1983: Uranium-lead isotope systematics: The case of crystals with discrete cores. Lithos 16) yields an age of 1946 Ma. The age of 1850 Ma is accordingly a minimum age.

Radiometric dating of Svecokarelian metarhyolites and prekinematic granitoids from Bergslagen, south central Sweden

Abstract Rb-Sr whole rock isochron dating of Svecokarelian prekinematic granitoids from a single outcrop ca. 20 km NNE of Ludvika, south central Sweden, gives an age of 1799 ± 10 Ma. Zircons separated from one of the granitoid samples yield a U-Pb discordia age of 1869+9 -8 Ma. Regional sampling of similar granitoids in the Ludvika area results merely in a Rb-Sr whole rock errorchron of 1770 ± 10 Ma, thus emphasizing the importance of restricting sampling in older metamorphic terrains to a small area with good geological control. Rb-Sr whole rock dating of Svecokarelian metarhyolites (leptites) from the Yxsjoberg area, WSW of Ludvika, fails to give a meaningful age.

The Götemar granite — isotopic and geochemical evidence for a complex history of an anorogenic granite

Abstract The Gotemar massif, located in south-eastern Sweden, consists mainly of a coarse-grained homogeneous alkali feldspar granite with subordinate fine-grained varieties. A multi-radiometric dating approach gives a variety of discordant data depending on method and material. The coarse-grained Gotemar granite is believed to have intruded and crystallized between 1400 and 1350 Ma ago. The isotopic data, together with Ti, Zr, Y and Nb ratios, strongly resemble those of the Jungfrun granite which represents another anorogenic intrusion in the area. This suggests a common origin for these two granites. Rb-Sr dating of the fine-grained aplitic varieties of the Gotemar granite gives an age of 1255 Ma. It is not clear whether these represent late differentiates, or if their age is lowered by late-stage hydrothermal activity.

Radiometrie dating of the serorogenic Svecokarelian Enkullen and Fjällberg granites, south central Sweden

Abstract Radiometric dating of the serorogenic Svecokarelian Enkullen and Fjallberg granites, south central Sweden, yield two parallel Rb-Sr whole-rock isochron ages of 1755 ± 17 Ma and 1758 ± 14 Ma, respectively. The initial 87Sr/86Sr ratios arc also similar; 0.7063 + 0.0056 and 0.7065 ± 0.0034, respectively. A U-Pb dating of zircons from the Fjallberg granite also gives a corrsponding age, i.e., an upper intercept with concordia of 1748+74 -45 Ma. A recalculation of the zircons with the U-Pb model of Wickman (1983) yields an age of 1765 Ma. The agreement between the Rb-Sr and U-Pb datings strongly indicates that they represent true primary crystallizing ages.