Hans Schöberg

Aeolian dust in East Antarctica (EPICA‐Dome C and Vostok): Provenance during glacial ages over the last 800 kyr

Aeolian mineral dust archived in Antarctic ice cores represents a key proxy for Quaternary climate evolution. The longest and most detailed dust and climate sequences from polar ice are provided today by the Vostok and by the EPICA-Dome C (EDC) ice cores. Here we investigate the geographic provenance of dust windborne to East Antarctica during Early and Middle Pleistocene glacial ages using strontium and neodymium isotopes as tracers. The isotopic signature of Antarctic dust points towards a dominant South American origin during Marine Isotopic Stage (MIS) 8, 10, 12, and back to MIS 16 and 20 as deduced from EDC core. Data provide evidence for a persistent overall westerly circulation pattern allowing efficient transfer of dust from South America to the interior of Antarctica over the last 800 kyr. Some small but significant dissimilarity between old and recent glacial ages suggests a slightly reduced Patagonian contribution during ancient glaciations.

U-Pb dating of the post-kinematic Sveconorwegian (Grenvillian) Bohus granite, SW Sweden: evidence of restitic zircon

Abstract U-Pb zircon isotopic studies of a representative granite type and a pegmatite-aplite belonging to the post-kinematic Sveconorwegian monzogranitic Bohus granite show that a part of the zircon material has preserved isotopic characteristics of the protolith and consequently yields erroneous ages. However, monazite and xenotime from the aplite-pegmatite yield almost concordant 206 Pb 238 U ages, at 919±5 Ma and 922±5 Ma, respectively, which is interpreted to be the crystallisation age of the major portion of the Bohus granite. The present investigation advocates, owing to the preservation of the protolith signature in the zircon U-Pb data, that restite unmixing is one mechanism which contributes to the chemical variation of the weakly peraluminous Bohus granite. The geochemical and petrographic characteristics of the Bohus granite indicate that the different magma surges constituted anatectic, crust-derived, water-undersaturated liquids with varying amounts of suspended crystals. Crystal fractionation and restite unmixing from this crystal mush, together with differential partial melting, were the principal magmamodifying mechanisms during the generation of the Bohus granite massif.

Change of Sm-Nd isotope composition during weathering of till

Weathering of till in northern Sweden results in the formation of well-developed spodosols. The till is dominated by 1.9–1.8 Ga granitic material. The REE are among the elements most strongly depleted during weathering, and the loss of REE from the E-horizon decreases as the atomic number increases. To study if weathering leads to a change of the Nd isotope composition, we have analysed the Nd isotopic composition of the various horizons including living plants and humus of two profiles of weathered till (typic haplocryods) in northern Sweden. As much as between 65.6 and 75.3% of the Sm and Nd in the <0.2 mm fraction has been lost from the E-horizon, and between 32.5 and 54.7% from the B-horizon. Nd has been lost to a slightly greater extent than Sm. The two C-horizon samples have eNd(0) values of −22.1 and −23.2. Corresponding E-horizon values are −18.1 and −20.2. The B-horizon values are intermediate between the values of the E and C horizons. It is concluded that the weathering leads to a change in the Sm/Nd ratio resulting in a change of the Sm-Nd isotope composition. The plant and humus samples deviate even more from the unweathered till. For one station the results could be interpreted as if the Sm and Nd taken up by the plants had similar isotope characteristics as the amounts of these elements released by weathering in the E-horizon. For the other station it is probable that the Nd isotope composition of the organic samples is dominated by Nd released by till weathering which, however, is mixed with another Nd-source, possibly an airborne component. The explanation to the change of isotope compostion in the till is that a larger proportion of the Nd released by weathering is released from minerals with a lower Sm/Nd ratio than the bulk soil, compared with the amount released from minerals with a higher Sm/Nd ratio. Although the various REE-carrying minerals had the same initial Nd isotopic composition, 1.8–1.9 Ga of decay of 147Sm to 143Nd has resulted in a higher present 143Nd/144Nd ratio in the minerals with a higher Sm/Nd ratio.

A comparison of the geochronology and geochemistry of plagioclase-dominated granitoids across a major terrane boundary in the SW Balitic Shield

Abstract To make reconstructions of the Proterozoic palaeogeography meaningful, the terranes making up the individual shields must first be identified and the timing of terrane accretion determined. New chemical and isotopic data confirm the terrane-separating character of a major shear zone, the Mylonite Zone, in the Southwestern Scandinavian Gneiss Complex. These data relate to plagioclase-dominated granitoids to the east of the Mylonite Zone. Two zircon upper-intercept age determinations result in 1674+24−19 and 1688+10−10 Ma. Two separate crystals from the 1688-Ma rock give two single-zircon evaporation ages: one crystal gives 1646 ± 172 Ma (2σ) and the other 1654 ± 46 Ma (2s). Earlier investigations have pointed to vast amounts of similar looking granitoids to the west of the Mylonite Zone. However, these rocks are younger, approximately 1.60 Ga. The granitoids to the west of the Mylonite Zone are typical calc-alkaline rocks with a pronounced increase in the content of mafic minerals concurrent with a decrease in quartz. The trend of the eastern rocks is different, mainly involving changes in the quartz/total feldspar ratio. At the same SiO2 content, the eastern rocks are lower in MgO and CaO and higher in (Na2O + K2O) and especially in Ba than their western counterparts. Also other elements like Ni show differences between the two suites. We conclude that these similar looking rocks belong to two different suites that are different in age and have different chemical signatures, probably caused by different source-rock chemistries. ϵNd1.68-values in the eastern suite range from + 2.0 to 2.5, suggesting that assimilation of large amounts of old continental rocks did not take place. Results from Sr-isotope determinations support this suggestion. The present results suggest, together with older data, that no granitoid rocks older than approximately 0.9 Ga are common to the two terranes separated by the Mylonite Zone. There is thus no compelling evidence suggesting that the two terranes formed one entity before the Sveconorwegian (Grenvillian) orogeny.

Age of porphyry-type deposits in the Skellefte District, northern sweden

Abstract The Skellefte District in northern Sweden consists of metamorphosed Lower Proterozoic submarine volcanic, sedimentary, and intrusive rocks. Several massive sulphide deposits occur in the volcanics, and several small porphyry-type deposits exist in the oldest granitoids, which are coeval with the volcanics. The volcanic rocks have been dated previously at 1.88 Ga while the oldest granitoids have an age of 1.89 Ga. The aim of this study was to establish the timing of the porphyry systems. For this purpose, an intrusive quartz-feldspar porphyry, associated with the Tallberg porphyry-type deposit, has been dated. The U-Pb zircon age is 1886+15 -95 Ma which is within the error limits of both the host tonalites (the oldest Jorn granitoids) and the lower Skellefte volcanic rocks, hosting the massive sulphides. While the massive sulphide deposits were formed on the 1.88–1.89 Ga old, early Proterozoic seafloor, the porphyry-type deposits formed farther north inside the marginal arc, both types of deposit ...

Age and geochemistry of granites associated with Momineralizations in western Bergslagen, Sweden

Abstract The Bergslagen area, south-central Sweden, where large amounts of Fe, Cu, Zn, Pb, Au, Ag, Mo and W have been produced for several centuries, is one of the oldest mining districts in Europe. Two episodes of magmatism and ore formation can be distinguished: (1) an “older”, Early Svecofennian phase that generated 1890-1850 Ma old calc-alkaline, continental-margin volcanic and plutonic units related to the Svecofennian orogeny; and (2) a “younger” phase comprising potassiumrich granitoids, which intruded over a period extending from 1825 to 1680 Ma during the final stage of, or subsequent to, the regional metamorphism and deformation related to the Svecofennian orogeny. In recent years, considerable confusion has dominated the discussion concerning the timing and setting of the MoW-forming granites in western Bergslagen. In this study, one of the most important intrusions of undeformed granite in western Bergslagen has been investigated in regard to geochemistry and UPb zircon age. The granite (Skalhojden) has a peraluminous, silica- and potassium-rich chemistry as well as a uniformly flat REE-pattern with a marked negative Eu anomaly. The UPb data yielded an age of 1758±8 Ma. These data show convincingly that the granite is comparable to other 1800-1680 Ma MoW-bearing granitoids elsewhere in the Fennoscandian Shield but are in strong conflict with hypotheses on Early Svecofennian rift-related crust- and ore-forming processes recently proposed for western Bergslagen. A revision of the models of crustal growth and ore formation in that area is suggested.

Stratigraphical position and Pb—Pb age of Lower Proterozoic carbonate rocks from the Kalix Greenstone Belt, northern Sweden

Abstract In the Kalix area, a more than 3600 m thick sequence of continental rift-associated, well-preserved metasedimentary and mafic metavolcanic rocks occurs in a Lower Proterozoic greenstone belt which has no connection to other areas of mafic volcanites. These supracrustals are probably resting on an Archaean basement, and overlain by at least 2000 m of schists of marine origin. By analysing for Pb isotopes, an attempt was made to date intertidal dolomites with evenly laminated stromatolites and supratidal, pisolitic dolomite calcretes. The resultant age was 2.1±0.2 Ga. The large uncertainty is partly due to a narrow range of Pb isotopic ratios. Small-scale postdepositional redistribution of lead in the stromatolites is probable. The sample sites are stratigraphically situated just below a 400 m thick sequence of mafic metalavas, which can be correlated with c. 2.1 Ga old mafic metalavas in the Perapohja Schist Belt in adjacent parts of northern Finland. Despite the large uncertainty of the age obtai...

Character and U-Pb zircon age of the Proterozoic Ale granite, northern Sweden

Abstract The Ale granite west of Lulea in northern Sweden forms an irregular, almost elliptical exposure covering an area of roughly 30 by 15 km in size. The central part of the intrusion consists of a homgeneous, quartz- and microcline-rich porphyritic granite, whereas its marginal parts are made up of a heterogeneous granite slightly less rich in quartz. Pegmatites and aplites are common in the surroundings. Zircons from two samples, one from the central part of the Ale granite and one from the border zone, were dated at 1802±3 (2σ) Ma and 1796±2 (2σ) Ma, respectively, by using the U-Pb method. An analysis of Sm-Nd istopes of these samples gave initital eNd values of −5.2 and −3.2, respectively, indicating a significant contribution of Archaean crustal material. Thirteen samples were analysed for major and trace elements. The isotopic and geochemical results suggest that the Ale granite was formed in a compressional environment 1.80 Ga ago, and that mature sediments were not an important source material.

U-Pb age of a granitoid in the banded sequence at Grums, SW Sweden

Abstract Zircons were separated from a granitoid band in a sequence of banded gneisses to the west of the Mylonite Zone in western Varmland, south-western Sweden. The analysis was undertaken to test the hypothesis that the 1650 Ma calc-alkalic Amal granitoids form an important part of the banded sequence. The result is important for the understanding of the early history of south-western Scandinavia. The zircons show weak signs of metamorphism but the upper intercept age - 1643+47 -29 Ma (2σ) — is interpreted to be close to the intrusion age. This result is, to within the errors, identical to the age of the Amal granitoids, and the result is taken to support the test hypothesis.

Some aspects on the subdivision of the Haparanda and Jörn intrusive suites in northern Sweden

Abstract The geographical subdivision between the Haparanda and the Jörn suites of intrusive rocks in northern Sweden has not been very well defined. Early stratigraphical schemes placed these two granitoid suites in two separate orogenic cycles, where the Jörn belonged to the older cycle and Haparanda to the younger. Our present knowledge regarding the isotopic ages of these rocks in northern Sweden has changed this view, but has also made the distinction between the two suites less clear. Based on recent Sm–Nd isotopic work combined with geochemistry and some new U–Pb zircon data, we point out some similarities as well as some differences between the Jörn and Haparanda suites of rocks. Two U–Pb zircon age determinations performed give upper intercept ages of 1891±32 Ma and 1861±19 Ma which are interpreted as maximum ages. The two samples are taken from the Luleå area, on each side of the Archaean–Proterozoic boundary, as defined by Sm–Nd isotopic analyses of c.1.9 Ga old intrusive rocks combined with the southern limit of outcropping Archaean rocks. On the basis of new results together with results from previous studies of areas north and south of the Archaean–Proterozoic boundary, we also suggest how to separate the Haparanda and Jörn suites of rocks due to their geochemical, and isotope geochemical, characteristics. The Haparanda suite generally has negative ϵNd(t) values and was formed within or in marginal parts of the Archaean craton. The Jörn suite was formed in an juvenile, island-arc terrane, that was accreted to the Archaean craton during the later, collisional stages of the Svecokarelian orogeny. In a similar way, we connect the Haparanda suite of rocks with the Archaean craton, and the Jörn suite of rocks with Svecofennian juvenile crust.