| 1. |
- Kalsbeek, Feiko, et al.
(författare)
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Enigmatic 1146 ± 4 Ma old granite in the southeastern rim of the West African craton, now part of the Dahomeyan orogenic belt in Ghana
- 2020
-
Ingår i: Journal of African Earth Sciences. - : Elsevier BV. - 1464-343X. ; 167
-
Tidskriftsartikel (refereegranskat)abstract
- A unique exposure of a strongly deformed late Mesoproterozoic augen-granite (U–Pb zircon age 1146 ± 4 Ma) was detected within rocks on the eastern rim of the West African Craton. These rocks, together with their sedimentary cover, were reworked during the Neoproterozoic Pan-African Dahomeyan orogeny, and are now incorporated in the Dahomeyan orogenic belt of southeast Ghana. To the authors’ knowledge no similar ages have been reported from elsewhere in the West African Craton or in the Pan-African orogenic belts farther north or their continuation in northeastern Brazil. Most other granitoid rocks in the Dahomeyan belt are of Palaeoproterozoic age. Two of them were dated for this study and yielded Concordia ages of 2067 ± 15 Ma and 2119 ± 13 Ma. Lower intercepts around 600 Ma in Wetherill discordia diagrams demonstrate disturbance of the U–Pb systems during the Pan-African orogenic cycle. Chemical data indicate that the Palaeoproterozoic granitoid rocks are arc-type granites (sensu lato), while the Mesoproterozoic augen-granite has the chemical composition of an alkaline within-plate granite. Chemical, whole rock Sm–Nd and zircon Lu–Hf data suggest that the magma that yielded the augen-granite was formed by melting within a mafic Palaeoproterozoic source at moderate crustal depth. The occurrence of a Mesoproterozoic granite in the (now) eastern margin of the West African Craton is evidence of a previously unknown igneous event within this terrain.
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| 2. |
- Be'eri-Shlevin, Yaron, et al.
(författare)
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Provenance of Neoproterozoic sediments in the Sarv nappes (Middle Allochthon) of the Scandinavian Caledonides : LA-ICP-MS and SIMS U-Pb dating of detrital zircons
- 2011
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 187:1-2, s. 181-200
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Tidskriftsartikel (refereegranskat)abstract
- We present U-Pb age data for detrital zircons from dike-intruded Neoproterozoic sedimentary rocks of the Caledonian Middle Allochthon in central Sweden and Norway. Detrital zircons from 11 samples from the Sarv, Saetra and upper Leksdal nappes (informally referred to as the Sarv nappes) are clustered within ca. 0.9-1.75 Ga, but display a bimodal distribution with major ca. 1.45-1.75 Ga and ca. 0.9-1.2 Ga components. An apparent increase of younger (0.9-1.2 Ga) components to the northwest reflects varying source terranes. Detrital zircons from an additional sample from the lower part of the Leksdal Nappe, of uncertain affiliation to the Sarv has a prominent 1.75-1.85 Ga component supporting previous suggestions that this part of the nappe belonged to a more proximal basin. Comparison of the Sarv age probability patterns with data from basement windows and basement slices within the Middle Allochthon in central Sweden and Norway supports the derivation of the sediments from the attenuated Baltican continental crust on which they were presumably deposited. Similar comparisons suggest that derivation from the southern segment of the Fennoscandian Shield or from eastern segments of Laurentia is less likely, mostly because they include also older components. We infer that the ca. 200 km wide belt of attenuated Baltican continental crust included northern extensions of Mesoproterozoic to early Neoproterozoic terranes exposed in the southern part of the Fennoscandian Shield and the easternmost part of Laurentia, which at ca. 900 Ma were still adjacent. Pre-1.75 Ga terranes of the Fennoscandian Shield were probably isolated from the Sarv distal basin(s) by intracratonic basins and uplifted margins associated with early development of this extended continental crust. The significantly older ages in the lower part of the Leksdal Nappe and its inferred more proximal position support this model. The proposed northern extension of Mesoproterozoic-early Neoproterozoic terranes can explain in a simpler way the occurrence of such detritus in many Caledonide-Appalachian allochthons exposed at the margins of the North Atlantic, with no need to infer large displacement along the axis of the Caledonide Orogen or to postulate selective transport of Grenville-age material from the south over large distances.One of our Sarv samples located at the Norwegian coast revealed Caledonian reworking at ca. 395 Ma. This age agrees with ages of late-tectonic amphibolite-facies metamorphism and pegmatite intrusion recorded in this part of the Caledonides.
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| 3. |
- Björkman, Anne, 1981, et al.
(författare)
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Plant functional trait change across a warming tundra biome
- 2018
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 562:7725, s. 57-62
-
Tidskriftsartikel (refereegranskat)abstract
- The tundra is warming more rapidly than any other biome on Earth, and the potential ramifications are far-reaching because of global feedback effects between vegetation and climate. A better understanding of how environmental factors shape plant structure and function is crucial for predicting the consequences of environmental change for ecosystem functioning. Here we explore the biome-wide relationships between temperature, moisture and seven key plant functional traits both across space and over three decades of warming at 117 tundra locations. Spatial temperature–trait relationships were generally strong but soil moisture had a marked influence on the strength and direction of these relationships, highlighting the potentially important influence of changes in water availability on future trait shifts in tundra plant communities. Community height increased with warming across all sites over the past three decades, but other traits lagged far behind predicted rates of change. Our findings highlight the challenge of using space-for-time substitution to predict the functional consequences of future warming and suggest that functions that are tied closely to plant height will experience the most rapid change. They also reveal the strength with which environmental factors shape biotic communities at the coldest extremes of the planet and will help to improve projections of functional changes in tundra ecosystems with climate warming.
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| 4. |
- Cornell, David H., 1948, et al.
(författare)
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Dating detrital zircon from the gold-bearing Ventersdorp Contact Reef in the Ventersdorp Supergroup of South Africa
- 2021
-
Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 357
-
Tidskriftsartikel (refereegranskat)abstract
- The Ventersdorp Contact Reef (VCR) at the base of the >4.5 km-thick volcanosedimentary Ventersdorp Supergroup unconformably overlies the Witwatersrand Supergroup and is the second youngest mineable reef in the Witwatersrand Goldfields. The volcanic rocks of the Ventersdorp Supergroup are predominantly mafic, affected by low-grade thermal metamorphism and difficult to date. Only the Makwassie Formation in the upper Platberg Group of the main Ventersdorp repository has been reliably dated on four felsic volcanic samples at 2720 ± 2 Ma. The actual timing of Ventersdorp volcanism and the duration of the three recognised lithostratigraphic groups remains enigmatic, despite much research and heroic attempts to synthesize the available data. In this work detrital zircon grains from VCR conglomerates were U-Pb dated in order to improve the time constraints on the Klipriviersberg Group at the base of the Ventersdorp Supergroup. The six youngest grains in VCR samples were reliably dated at 2799 ± 9 Ma. The Klipriviersberg Group and the Ventersdorp Supergroup is thus younger than 2808 Ma and the supergroup is older than the 2642 Ma Vryburg Formation at the base of the Transvaal Supergroup. Comparisons of detrital grain dates confirm that the VCR was largely derived from erosion products of the underlying Witwatersrand Supergroup, however the youngest VCR grains are ~20 Ma younger and may have been derived directly from magmatic rocks in the provenance or a felsic facet of the synchronous komatiitic Klipriviersberg volcanism. Multi-grain analyses of discordant grains show that recent lead loss is predominant. However about 5% of the data show the effect of complex Mesoproterozoic lead loss, which can yield ages as much as 150 Ma too young in 10% discordant data. This was found in grains with high Th-induced radiation damage, providing a criterion for data rejection. The proposed large igneous provinces dated between 2791 and 2683 Ma, based mainly on dated mafic dykes, which are not in contact with supracrustal Ventersdorp rocks, do fit the established time constraints and might provide a key to Ventersdorp chronostratigraphy. However only the proposed 2754–2709 Ma Platberg volcanic province is based on reliably dated Platberg Group volcanic rocks.
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| 5. |
- Cornell, David H., 1948, et al.
(författare)
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Geochronology of Mesoproterozoic hybrid intrusions in the Konkiep Terrane, Namibia, from passive to active continental margin in the Namaqua-Natal Wilson Cycle
- 2015
-
Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 265, s. 166-188
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Tidskriftsartikel (refereegranskat)abstract
- tThis work investigates a number of gabbro – granite hybrid rocks exposed in the Great Escarpmentbetween the coastal Namib desert and the inland plateau of Namibia. They form part of the tectonostratigraphic Konkiep Terrane which lies between the Mesoproterozoic Namaqua-Natal Province and largelyPalaeoproterozoic Rehoboth Province in southern Africa. Microbeam U–Pb and Lu–Hf analyses of zirconwere used to investigate the geochronology of these rocks which have previously been assigned to theMooirivier Metamorphic Complex or the Kairab Formation. The oldest hybrid rocks at Neuhof Valley areU–Pb dated at 1359 ± 6 Ma and cut by 1347 ± 8 Ma unfoliated granite dykes, but have the youngest Lu–Hfcrustal residence ages between 1690 and 1430 Ma. Together with nearby volcanic rocks of the 1333 ± 6 Ma Neuhof and 1327 ± 10 Ma Welverdiend Formations which we dated, they are thought to reflect rifting of the Rehoboth Province followed by the ocean development (by seafloor spreading) phase of the NamaquaWilson Cycle.The hybrid rocks at Goede Hoop Foothills are U–Pb dated at 1230 ± 6 Ma with a number of xenocrysts upto 1532 Ma. Similar hybrid rocks at Hauchabfontein have 20–30 Ma younger U–Pb ages of 1211 ± 19 Maand 1202 ± 7 Ma (two samples), they formed by gabbro intruding an 1195 ± 5 Ma granite (within errorbut older from field relations) and contain many xenocrysts with U–Pb ages 1755 ± 29 Ma (n = 7), 1367and 1260 ± 17 Ma (n = 24). All the Goede Hoop Foothills and Hauchabfontein hybrid rocks have old Lu–Hfcrustal residence ages from 1935 to 2000 Ma and one sample up to 2320 Ma, reflecting involvement of Palaeoproterozoic Rehoboth Province crust. This 1230 to 1200 Ma group of hybrid rocks correspondsin age to the arc magmatism recognised in the Namaqua-Natal Province to the west and south of theKonkiep Terrane. The hybrid rocks probably formed during the closure of the Namaqua Ocean by eastwardsubduction under the Rehoboth Province, corresponding to the previously proposed Rehoboth MagmaticArc and forming the Konkiep Terrane by massive intrusions into the margin of the Rehoboth Province.The hybrid rocks we investigated in the Konkiep Terrane are thus related to the rifting phase andthe pre-collisional subduction phase of the Namaqua Wilson Cycle. The Arc-related magmatism endedabruptly at 1200 Ma as the Namaqua-Natal terranes were assembled by collisions and subduction ceased.We did not find evidence for the 1200 to 1150 Ma post-collisional metamorphic cycle and magmatism which prevailed in the adjacent Grünau Terrane of the Namaqua Province, suggesting that the KonkiepTerrane did not experience crustal thickening during the Namaqua collision and terrane assembly. Anundeformed felsic lava at Sukses which we dated at 1105 ± 10 Ma corresponds in age to the Langberg Formation in the Rehoboth Terrane. It is coeval with the Keimoes Suite of post-tectonic intrusions andyounger Koras lavas in the Namaqua-Natal Province and these ∼1100 Ma rocks all extend the area of theplume-related Umkomdo Large Igneous Province documented on the Kaapvaal Craton. This part of theKonkiep Terrane originated in the Rehoboth Province and was then modified by magmatic intrusionscoupled to the evolution of the Namaqua Natal Province.
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| 6. |
- Cornell, David H., 1948, et al.
(författare)
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Microbeam U-Pb zircon dating of the Makwassie and Goedgenoeg Formations in the Ventersdorp Supergroup of South Africa.
- 2017
-
Ingår i: South African Journal of Geology. - : Geological Society of South Africa. - 1012-0750 .- 1996-8590. ; 120:4, s. 525-540
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Tidskriftsartikel (refereegranskat)abstract
- The Archaean Ventersdorp Supergroup is the most widespread volcanosedimentary cover sequence on the Kaapvaal Craton. It unconformably overlies the Witwatersrand Supergroup and is unconformably overlain by the Transvaal Supergroup. Due to exploration drilling for Witwatersrand gold, the Ventersdorp Supergroup is the globally most extensively drilled sequence. Early efforts to date its volcanic units have commonly been thwarted by pervasive 2.3-2.0 Ga alteration. In-situ U-Pb zircon dating promises a panacea, but the effects of lead mobility and high common lead remain potential pitfalls. The ages of the ‘type’ formations of the Ventersdorp Supergroup sensu stricto are still not well established and much of the published data is on units only considered to be correlated with the ‘type’ formations. Quartz porphyry rhyolites of the Makwassie Formation in the Platberg Group are reliably dated by the laser ablation ICPMS U-Pb zircon method on four samples with a combined age of 2720 ± 2 Ma. This supercedes the first ion probe date of 2709 ± 4 Ma which is now regarded as a minimum age due to probable ancient lead loss and associated high common lead. The published 2714 ± 16 Ma age for the stratigraphically lower Klipriviersberg Group was affected by similar problems. The Makwassie Formation is the only formation of the Ventersdorp Group sensu stricto for which an accurate and reliable date has now been established by more than one precise age determination. Two samples considered to be from the Goedgenoeg Formation, which underlies the Makwassie Formation, were investigated. The formation is best dated by a 2746 ± 9 Ma age on a feldspar porphyry borehole sample from near Wolmaransstad. A sample from a similar rock type also assigned to the Goedgenoeg Formation, which underlies the Makwassie Formation in Wesselton Mine at Kimberley gave an age of 2781 ± 5 Ma. A 30 Ma period of Goedgenoeg volcanism is considered to be unlikely. The older sample may rather be a time-correlate of a stratigraphically lower unit such as the 2785 ± 2 Ma Kanye Formation or the 2769 ± 2 Ma Derdepoort Outlier.
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| 7. |
- Cornell, David H., 1948, et al.
(författare)
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Th-U-Pb zircon geochronology of the Palaeoproterozoic Hartley Formation porphyry by six methods, with age uncertainty approaching 1 Ma
- 2016
-
Ingår i: South African Journal of Geology. - : Geological Society of South Africa. - 0371-7208 .- 1012-0750. ; 119:3, s. 473-494
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Tidskriftsartikel (refereegranskat)abstract
- The Palaeoproterozoic Hartley Formation in the Olifantshoek Group was deposited in one of the rift-related Waterberg (sensu lato) red bed basins which formed on the Kaapvaal Craton after the 2.05 Ga Bushveld intrusions and coeval thermal event. The age of these basins is not well constrained due to the shortage of directly dateable rock types. The Hartley Formation contains rare quartz-porphyry lavas interbedded with the dominant basalts and these provide the means to date the formation by analyses of zircon. In this work zircon from one sample has been dated by six Th-U-Pb methods, namely Laser Ablation ICP Quadrupole Mass Spectrometry, Laser Ablation ICP High-resolution Mass Spectrometry, Laser Ablation ICP Multicollector Mass Spectrometry U-Pb (also Lu-Hf), Nordsim Ion probe U-Pb and Th-Pb; and Krogh method ID-TIMS. Our precise ages give a combined age of 1915.2 ± 1.1 Ma. Including one published ion probe date from the only other known occurrence of quartz porphyry, the results only agree if the quoted analytical errors are increased by 20%, which gives a combined result of 1915.6 ± 1.4 Ma. This is considered a reliable, precise and accurate age for the Hartley Formation and supersedes the published Kober method 207Pb/206Pb age of 1928 ± 4 Ma. The new Lu-Hf zircon data, supported by published whole rock Sm-Nd and Rb-Sr data, suggests that both the dominant basalts and the rare quartz porphyries of the Hartley Formation were derived from mafic source rocks which had been in the crustal domain from Archaean times. By contrast with the intracratonic rifts of the other Waterberg Basins, the Olifantshoek Supergroup reflects the development of a western passive margin as the Archaean Kaapvaal Craton rifted and drifted. This was followed by accretion of the Rehoboth Province along the Kalahari Line, accompanied by the development of the east-vergent Kheis Province thrust complex. This created a larger cratonic block against which the 1.2 Ga collisions of Namaqua-Natal terranes impacted. The Kheis Province now yields ~1.17 Ma cooling ages, reflecting the Namaqua collisions, but the true age of the Kheis event is still enigmatic.
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| 8. |
- Flowerdew, Michael J., et al.
(författare)
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The Importance of Eurekan Mountains on Cenozoic Sediment Routing on the Western Barents Shelf
- 2023
-
Ingår i: Geosciences. - : MDPI AG. - 2076-3263. ; 13:3, s. 91-91
-
Tidskriftsartikel (refereegranskat)abstract
- The importance of topography generated by Eocene Eurekan deformation as a sediment source for sandstones deposited on the western Barents Shelf margin is evaluated through a sediment provenance study conducted on wellbore materials retrieved from Spitsbergen and from the Vestbakken Volcanic Province and the Sørvestsnaget Basin in the southwest Barents Sea. A variety of complementary techniques record a provenance change across the Paleocene-Eocene boundary in wellbore BH 10-2008, which samples Paleogene strata of the Central Tertiary Basin in Spitsbergen. Sandstones containing K-feldspar with radiogenic Pb isotopic compositions, chrome spinel in the heavy mineral assemblage, and detrital zircons and rutiles with prominent Palaeoproterozoic and Late Palaeozoic—Early Mesozoic U-Pb age populations are up-section replaced by sandstone containing albitic plagioclase feldspar, metasedimentary schist rock fragments, a heavy mineral assemblage with abundant chloritoid, metamorphic apatite with low REE contents, metapelitic rutile with Silurian U-Pb ages and zircons with predominantly Archaean and Palaeoproterozoic U-Pb age populations. Our results clearly demonstrate the well-known regional change in source area from an exposed Barents Shelf terrain east of the Central Tertiary Basin during the Paleocene to the emerging Eurekan mountains west and north of the Central Tertiary Basin during the Eocene. Eocene sandstones deposited in the marginal basins of the southwestern Barents Shelf, which were sampled in wellbores 7316/5-1 and 7216/11-1S, contain elements of both the Eurekan and the eastern Barents Shelf provenance signatures. The mixing of the two sand types and delivery to the southwest margin of the Barents Shelf is consistent with a fill and spill model for the Central Teritary Basin, with transport of Eurekan-derived sediment east then south hundreds of kilometres across the Shelf.
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| 9. |
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| 10. |
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| 11. |
- Gee, David Godfrey, et al.
(författare)
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Detrital zircon signatures of the Baltoscandian margin along the Arctic Circle Caledonides in Sweden: The Sveconorwegian connection
- 2015
-
Ingår i: Precambrian Research. - : Elsevier. - 0301-9268 .- 1872-7433. ; 265, s. 40-56
-
Tidskriftsartikel (refereegranskat)abstract
- New evidence is presented here that the Sveconorwegian Orogen continued northwards from type areas in southwestern Scandinavia along the Baltoscandian outer margin into the high Arctic. The Silver Road (Silvervägen) profile through the Scandinavian Caledonides, located in Sweden along the Arctic Circle at 66–67◦ N, provides a full section through the tectonostratigraphy of the Baltoscandian margin from the Autochthon, via the Lower Allochthon to the upperment parts of the Middle Allochthon. Metamorphic grade increases upwards through the nappes, being low greenschist facies at lowest levels and increas- ing to eclogite grade in the highest parts of the Seve Nappe Complex, the latter being related to early Ordovician subduction of the Baltoscandian outermost margin. The sedimentary rocks range in age from Neoproterozoic to Ordovician and provide evidence of the changes of environment from the Baltoscan- dian platform, westwards out over the Cryogenian rifted margin to the continent-ocean transition zone; also the Ordovician foreland basin. Twelve samples of psammites from the different tectonostratigraphic levels have yielded U/Pb detrital zircon age-signatures that reflect the changing character of their pro- venance. Autochthonous sandstones are derived from late Paleoproterozoic (1800–1950 Ma) crystalline rocks in the vicinity to the east of the thrust front. Ediacaran-early Cambrian quartzites of the Lower Allochthon also yield mainly late Paleoproterozoic zircon signatures, but with subordinate Mesopro- terozoic and late Archaean populations, whilst mid Ordovician, W-derived foreland basin turbidites are dominated by Sveconorwegian (950–1100 Ma) signatures, with subordinate older Mesoproterozoic to latest Paleoproterozoic populations. All samples from the lower parts of the Middle Allochthon (lacking dolerite dykes) have signatures that are dominated by latest Paleoproterozoic and early Mesoproterozoic ages, with subordinate populations down to Sveconorwegian ages; the latter dominate the overlying Särv nappes and also the Seve Nappe Complex, where c. 945 Ma rhyodacites have been previously reported. This evidence of Sveconorwegian source rocks in the hinterland, taken together with previously pub- lished detrital zircon data farther south and north of the Arctic Circle, clearly favours the interpretation that the Sveconorwegian Orogen, during the Neoproterozoic, extended along the entire Baltoscandian outer margin into the high Arctic.
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| 12. |
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| 13. |
- Gee, David G., et al.
(författare)
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The Baltoscandian margin detrital zircon signatures of the central Scandes
- 2014
-
Ingår i: Geological Society, London, Special Publications. ; 390, s. 131-155
-
Tidskriftsartikel (refereegranskat)abstract
- In central parts of the Scandinavian Caledonides, detrital zircon signatures provide evidence of the change in character of the Baltoscandian crystalline basement, from the characteristic Late Palaeoproterozoic granites of the Transscandinavian Igneous Belt (TIB, c. 1650–1850 Ma) in the foreland Autochthon to the typical, mainly Mesoproterozoic-age profile (c. 950–1700 Ma) of the Sveconorwegian Orogen of southwestern Scandinavia in the hinterland. Late Ediacaran to Early Cambrian shallow-marine Vemdal quartzites of the Jämtlandian Nappes (Lower Allochthon) provide strong bimodal signatures with TIB (1700–1800 Ma) and Sveconorwegian, sensu stricto (900–1150 Ma) ages dominant. Mid-Ordovician turbidites (Norråker Formation) of the Lower Allochthon in Sweden, sourced from the west, have unimodal signatures dominated by Sveconorwegian ages with peaks at 1000–1100 Ma, but with subordinate components of older Mesoproterozoic zircons (1200–1650 Ma). Latest Ordovician shallow-marine quartzites also yield bimodal signatures, but are more dispersed than in the Vemdal quartzites. In the greenschist facies lower parts of the Middle Allochthon, the Fuda (Offerdal Nappe) and Särv Nappe signatures are either unimodal or bimodal (950–1100 and/or 1700–1850 Ma), with variable dominance of the younger or older group, and subordinate other Mesoproterozoic components. In the overlying, amphibolite to eclogite facies lower part of the Seve Nappe Complex, where the metasediments are dominated by feldspathic quartzites, calcsilicate-rich psammites and marbles, most units have bimodal signatures similar to the Särv Nappes, but more dispersed; one has a unimodal signature very similar to the Ordovician turbidites of the Jämtlandian Nappes. In the overlying Upper Allochthon, Lower Köli (Baltica-proximal, Virisen Terrane), Late Ordovician quartzites provide unimodal signatures dominated by Sveconorwegian ages (sensu stricto). Further north in the Scandes, previously published zircon signatures in quartzites of the Lower Allochthon are similar to the Vemdal quartzites in Jämtland. Data from the Kalak Nappes at 70°N are in no way exotic to the Sveconorwegian Baltoscandian margin. They do show a Timanian influence (ages of c. 560–610 Ma), as would be expected from the palinspastic reconstructions of the nappes. Thus the detrital zircon signatures reported here and published elsewhere provide supporting evidence for a continuation northwards of the Sveconorwegian Orogen in the Neoproterozoic, from type areas in the south, along the Baltoscandian margin of Baltica into the high Arctic.
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| 14. |
- Kaufmann, Tobias, et al.
(författare)
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Common brain disorders are associated with heritable patterns of apparent aging of the brain
- 2019
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Ingår i: Nature Neuroscience. - : Nature Publishing Group. - 1097-6256 .- 1546-1726. ; 22:10, s. 1617-
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Tidskriftsartikel (refereegranskat)abstract
- Common risk factors for psychiatric and other brain disorders are likely to converge on biological pathways influencing the development and maintenance of brain structure and function across life. Using structural MRI data from 45,615 individuals aged 3-96 years, we demonstrate distinct patterns of apparent brain aging in several brain disorders and reveal genetic pleiotropy between apparent brain aging in healthy individuals and common brain disorders.
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| 15. |
- Lorenz, Henning, 1972-, et al.
(författare)
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Detrital zircon geochronology of Palaeozoic Novaya Zemlya – a key to understanding the basement of the Barents Shelf
- 2013
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Ingår i: Terra Nova. - : Wiley. - 0954-4879 .- 1365-3121. ; 25:6, s. 496-503
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Tidskriftsartikel (refereegranskat)abstract
- The Novaya Zemlya fold-and-thrust-belt is the northern con- tinuation of the late Palaeozoic Uralide Orogen. Little is known about its deeper structure and the basement history of the adjacent Barents and Kara shelves. Based on geological evidence and detrital zircon analysis of 28 samples from the northeastern and stratigraphically deepest part of the archi- pelago, we demonstrate that Cambro-Ordovician turbidite- dominated deposition was almost exclusively sourced from rocks consolidated during the Timanian orogeny (Timanian basement). A profound change in provenance occurred near the end of the Ordovician. Over 90% of the zircons from Silu- rian and about 80% from Devonian strata have ages charac- teristic of the Sveconorwegian Orogen, implying uplift of these rocks in the vicinity of Novaya Zemlya. The presence of Sveconorwegian and Grenvillian rocks in the high Arctic sug- gests revision of recent reconstructions of the Rodinia super- continent, its break-up and subsequent Caledonian orogeny.
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| 16. |
- Majka, Jaroslaw, et al.
(författare)
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Torellian (c. 640 Ma) metamorphic overprint of Tonian (c. 950 Ma) basement in the Caledonides of southwestern Svalbard
- 2014
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Ingår i: Geological Magazine. - 0016-7568 .- 1469-5081. ; 151:4, s. 732-748
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Tidskriftsartikel (refereegranskat)abstract
- Ion microprobe dating in Wedel Jarlsberg Land, southwestern Spitsbergen, provides new evidence of early Neoproterozoic (c. 950 Ma) meta-igneous rocks, the Berzeliuseggene Igneous Suite, and late Neoproterozoic (c. 640 Ma) amphibolite-facies metamorphism. The older ages are similar to those obtained previously in northwestern Spitsbergen and Nordaustlandet where they are related to the Tonian age Nordaustlandet Orogeny. The younger ages complement those obtained recently from elsewhere in Wedel Jarlsberg Land of Torellian deformation and metamorphism at 640 Ma. The Berzeliuseggene Igneous Suite occurs in gently N-dipping, top-to-the-S-directed thrust sheets on the eastern and western sides of Antoniabreen where it is tectonically intercalated with younger Neoproterozoic sedimentary formations, suggesting that it provided a lower Tonian basement on which upper Tonian to Cryogenian sediments (Deilegga Group) were deposited. They were deformed together during the Torellian Orogeny, prior to deposition of Ediacaran successions (Sofiebogen Group) and overlying Cambro-Ordovician shelf carbonates, and subsequent Caledonian and Cenozoic deformation. The regional importance of the late Neoproterozoic Torellian Orogeny in Svalbard's Southwestern Province and its correlation in time with the Timanian Orogeny in the northern Urals as well as tectonostratigraphic similarities between the Timanides and Pearya (northwestern Ellesmere Island) favour connection of these terranes prior to the opening of the Iapetus Ocean and Caledonian Orogeny.
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| 17. |
- Pettersson, Carl-Henrik, et al.
(författare)
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Detrital zircon U-Pb ages of Late Silurian and Early Devonian sedimentary sequences from Northwestern, Svalbard, implications for regional correlations in the Arctic Caledonides
- 2008
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In the North Atlantic region, Caledonian and Grenvillian terrains are exposed on Svalbard, Greenland, Great Britain, Canada and Scandinavia. Reconstructing the original configuration of these terrains depends partly on determining the age and provenance of the sedimentary rocks overlying the basement. Provenance investigations using detrital zircon U-Pb ages are a powerful tool to explore the palaeogeography and can yield important information how the North Atlantic region evolved through the Grenvillian and Caledonian orogenies. U-Pb laser ablation ICP-MS provenance study of zircons from Late Silurian and Early Devonian coarse siliciclastics of NW Svalbard indicate an east Greenland provenance. The decrease in 410-440 Ma ages and 2600-2800 Ma ages, combined with a large increase in 950-1000 Ma ages up-section suggests that sediment detritus was initially derived from exhumation of the Caledonian Orogen and subsequently from the Grenvillain Orogen.
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| 18. |
- Pettersson, Carl Henrik, et al.
(författare)
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Detrital zircon U-Pb ages of Silurian-Devonian sediments from NW Svalbard: A fragment of Avalonia and Laurentia?
- 2010
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Ingår i: Journal of the Geological Society (London). - : Geological Society of London. - 0016-7649 .- 2041-479X. ; 167:5, s. 1019-1032
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Tidskriftsartikel (refereegranskat)abstract
- Detrital zircon populations from Silurian-Devonian clastic rocks of NW Svalbard were analysed by U-Pb laser ablation inductively coupled plasma mass spectrometry to investigate the pre-Caledonian provenance of Svalbard's Northwestern Terrane. Changes in the resulting age spectra suggest a major shift in sources from the Laurentian-Avalonian suture in the latest Silurian to the local metasedimentary basement of the Northwestern Terrane in the Late Silurian-Early Devonian, and in the Lochkovian to Grenvillian-Sveconorwegian sources. These data, together with structural, additional geochronological and metamorphic data from Svalbard, East Greenland and Avalonia, support the amalgamation of Svalbard as the result of long-distance transport along sinistral strike-slip faults. A unifying model for the final amalgamation of Svalbard, consistent with the stratigraphical and tectonothermal history of Svalbard, involves fragments from the Grampian orogen and Avalonian crust originally accreted to the Laurentian margin being subsequently transported northward along sinistral strike-slip faults during Scandian deformation.
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| 19. |
- Pettersson, Carl-Henrik, et al.
(författare)
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U-Pb Zircon Provenance of Metasedimentary Basement of the Northwestern Terrane, Svalbard : A central East Greenland correlation
- 2007
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Svalbard’s Caledonian and older bedrock consists of three main exotic terranes separated by north-south trending strike-slip faults. Early in the 19th century Kulling (1930, 1934) noted the striking similarity between the Neoproterozoic and Early Paleozoic sedimentary rocks from East Greenland and Svalbard’s Eastern Terrane. After this pioneering work Harland et al. (1969) documented their remarkable similarities through detailed stratigraphic correlation of Neoproterozoic and Early Paleozoic sedimentary rocks. Recent work in the Northwestern and the eastern part of the Eastern terrane shows that this metasedimentary basement has a strong resemblance to the Krummedal and Smallefjord sequences of central East Greenland, with Grenville age (sensu lato) granitoid intrusion followed by Caledonian migmatization and granite genesis. We present the first LA-ICP-MS U-Pb zircon provenance study on quartzites and mica schists from the Kongsfjorden Group of the Northwestern Terrane of Svalbard. These results indicate a strong correlation with the Krummedal of central East Greenland and help to constrain paleogeographic reconstructions of the Northwestern Terrane.
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| 20. |
- Pointon, Michael A., et al.
(författare)
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A Multi-proxy Provenance Study of Late Carboniferous to Middle Jurassic Sandstones in the Eastern Sverdrup Basin and Its Bearing on Arctic Palaeogeographic Reconstructions
- 2022
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Ingår i: Geosciences. - : MDPI AG. - 2076-3263. ; 13:1, s. 10-10
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Tidskriftsartikel (refereegranskat)abstract
- A multi-proxy provenance study of Late Carboniferous to Middle Jurassic sandstones from the eastern Sverdrup Basin was undertaken employing optical petrography and heavy mineral analysis, chemical analysis of apatite, garnet and rutile grains, as well as detrital zircon U–Pb geochronology and Hf isotope analysis. Late Carboniferous to Middle Jurassic strata on the southern basin margin are inferred as being predominantly reworked from Silurian to Devonian strata within the adjacent Franklinian Basin succession. Higher-grade metamorphic detritus appeared during Middle to Late Triassic times and indicates exhumation and erosion of lower (Neoproterozoic to Cambrian) levels within the Franklinian Basin succession and/or a direct detrital input from the Canadian-Greenland Shield. The provenance of northern-derived sediments is more enigmatic owing to the subsequent opening of the Arctic Ocean. Northern-derived Middle Permian to Early Triassic sediments were likely derived from proximal areas of the Chukotkan part of the Arctic Alaska-Chukotka microplate. Late Triassic northern-derived sediments have different detrital zircon U–Pb age spectra from Middle Permian to Early Triassic ones and were likely derived from the Uralian orogenic belt and/or the Arctic Uralides. The loss of this sand input during latest Triassic times is interpreted to reflect drainage reorganisation farther upstream on the Barents Shelf. Middle Jurassic sands in the northern and axial parts of the basin were largely reworked from local northern-derived Late Triassic strata. This may have been facilitated by rift flank uplift of the northern basin margin in response to rifting in the adjacent proto-Amerasia Basin.
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| 21. |
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| 22. |
- van Schijndel, Valby, et al.
(författare)
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New microbeam zircon dating insights on migmatites of the former Mooirivier Metamorphic Complex and the Neuhof Formation, Sinclair Group, Namibia
- 2010
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Ingår i: Abstract in ‘Out of Africa: a 140 years with Kevin Burke and Lew Ashwal’. ; 1:1, s. 203-204
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- New microbeam zircon dating insights on migmatites of the former Mooirivier metamorphic complex and the Neuhof formation, Sinclair group, Namibia Valby van Schijndel (1), David H. Cornell (1), Dirk Frei (2) 1) Department of Earth Sciences, The University of Gothenburg, SE40530, Sweden 2) GEUS, Denmark and Greenland Geological Survey, Copenhagen, Denmark In the 1.4-1.0 Ga Sinclair Group and its basement, located in Namibia, the relations and chronostatigraphic succession are not fully understood. This is certainly true for the former Mooirivier Metamorphic Complex (MMC) and the Neuhof Formation. Becker (2008) interpreted the MMC and the Neuhof Formation as part of the Kairab Formation (Hoal, 1990), a heterogeneous metamorphic basement overlain by metavolcanics that are intruded by subvolcanics. This basement is part of the Namaqua Metamorphic Complex and Hoal (1990) separated the metavolcanics from the apparently older basement. The comparison Becker (2008) made was based on the fact that the units lie close to each other and lithological similarities; the MMC seems to be dominated by gneissic intrusive rocks and the Neuhof Formation consists mainly of basic volcanic and sedimentary rocks with closely related intrusives. In this study we used the NordSIM ion probe and a high resolution laser ablation ICPMS to date zircons from two migmatised samples and a red aplite dyke that cuts a migmatite from the MMC on Hauchabfontein farm. This migmatite location is interpreted by Becker (2008) as the gneissic Hauchab Granodiorite. He states that this body intrudes an amphibolite which is indicated by the presence of amphibolite xenoliths in the granodiorite. The second migmatite is located close by to the west. Our field interpretation is that the ‗gneissic granodiorite‘ is the result of a migmatisation event caused by a gabbro intrusion mixing with melted granitoid material. The two migmatite samples are hybrid rocks, consisting of a mixture of mafic and felsic components. Zircons from these hybrid rocks showed large CLbright rims, indicating an overall metamorphic or migmatitic event. However most of the zircons show no age difference between rim and core. These dates are 1205 ±6 and 1220 ±8 Ma and reflect the gabbro intrusion and migmatisation event. Since the cores of the zircons were probably reset, a migmatisation event is favoured above a metamorphic one, since it requires very high temperatures. This also applies to the formation of hybrid mixtures between felsic and mafic components. The migmatisation age is slightly older than the red aplite dyke at 1201±16 Ma. The migmatisation corresponds in age to the Barby Formation which formed during northwestward subduction prior to the ~1200 Ma Namaqua collision. In the Hauchabfontein migmatite we found one concordant zircon core dated at 1367 Ma possibly related to tonalite intrusives at 1375 Ma like the Hammerstein (Becker, 2008) and the Aunis Tonalite (Hoal, 1990), that intrude the Neuhof and Kairab Formations respectively. The second migmatite sample contains four xenocrysts with a combined concordant age of 1748 ±21 Ma. This age indicates the presence of a basement older than the Kairab Formation, like the Elim Complex and Gaub Valley Formation to the north. South of Hachaubfontein on the border of Mooirivier Farm, we sampled a metasediment from the Neuhof Formation. This is a low grade, quartz-rich gneiss, thought to be of volcanic origin. Becker (2008) interpreted fine grained quartz-feldspatic gneisses that occur in the Neuhof Fomation as intrusives because of the young age (~1320 Ma) in comparison to the 1375 Ma Hammerstein Tonalite intrusive. However in our sample we found two age domains 69 at 1330±5 and 1291±9 Ma. This indicates that the sample was not an intrusive but more likely a metasedimentary rock containing detrial zircons derived from two magmatic units. References Becker, T., 2008. in R. McG. Miller (ed) The Geology of Namibia, Vol. 1 Archean to Mesoproterozoic, . Hoal, B.G., 1990. The geology and geochemistry of the Proterozoic Awasib Mountain Terrain, southern Namibia, Mem. Geol.Surv. Namibia, vol. 11, pp 163
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| 23. |
- van Schijndel, Valby, et al.
(författare)
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Three episodes of crustal development in the Rehoboth Province, Namibia.
- 2011
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Ingår i: The Formation and Evolution of Africa: A Synopsis of 3.8 Ga of Earth History. Geological Society, London, Special Publications. ; 357, s. 27-47
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Tidskriftsartikel (refereegranskat)abstract
- The African continental crust was assembled by a series of orogenies over a period of billions of years mainly in Precambrian times. Tracing the build-up history of this stable crust is not always straightforward due to multiphase deformation and regions with poor outcrop. Episodes of metamorphism and magmatism associated with multiple Wilson cycles are recorded in zircons, which found their way into sediments derived from the hinterland. Dating of zircon populations in detrital rocks can hence provide age spectra which reflect the metamorphic and magmatic events of the region. Microbeam dating of detrital zircon is used to characterize the crustal development history of the Rehoboth Province of southern Africa. We investigated a quartzite of the Late Palaeo-Early Mesoproterozoic Billstein Formation, formed in a continental basin, and a quartz-feldspar arenite layer of the late Mesoproterozoic Langberg Formation conglomerates, immature sediments formed within a felsic volcanic system (both close to Rehoboth Town). The combined data indicate three episodes of crustal evolution in the Rehoboth Province. The oldest phase is only documented in the Billstein quartzite by three 2.98–2.7 Ga Archaean zircons. A Palaeoproterozoic phase between 2.2 and 1.9 Ga is older than any known exposures of the Rehoboth Province. The Billstein quartzite shows a main peak at 1.87 Ga, corresponding to the 1863±10 Ma Elim Formation. The Langberg sample reflects magmatism related to the entire Namaqua–Natal Wilson cycle between c. 1.32 and 1.05 Ga. The absence of zircons of that age range in the Billstein quartzite indicates a pre-Namaqua age for the Billstein Formation. Our data shows that there were at least three episodes of crustal development at 2.98–2.7 Ga, 2.05–1.75 and 1.32–1.1 Ga. We have documented the existence of a previously unrecognized 2.98–2.7 Ga Archaean crustal component, which was probably exposed in the Rehoboth Province during the Palaeoproterozoic and thus indicates a much longer geological history for the Rehoboth Province than previously known.
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| 24. |
- Yousefi, Fazilat, et al.
(författare)
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Geochemistry, petrogenesis and tectonic setting of middle Eocene hypabyssal rocks of the Torud–Ahmad Abad magmatic belt : An implication for evolution of the northern branch of Neo-Tethys Ocean in Iran
- 2017
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Ingår i: Journal of Geochemical Exploration. - : Elsevier. - 0375-6742 .- 1879-1689. ; 178, s. 1-15
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Tidskriftsartikel (refereegranskat)abstract
- The Torud–Ahmad Abad magmatic belt is located in the south-southeast of Shahrood (East of Semnan Province, NE Iran) and lies in the northern part of the Central Iran Structural Zone (CISZ), where a thick sequence of Paleocene to middle Eocene volcanic and volcanosedimentary rocks cropped out. This sequence was intruded by numerous dikes, hypabyssal igneous domes and one small gabbrodioritic intrusion, with compositions ranging from trachybasaltic andesite, trachyandesite, dacite, trachyte, gabbro, diorite and syenite. Various enclaves (cogentic and noncogenetic) with different composition, size and shape have been found in these domes and dikes. These enclaves are evidence of magma mixing and crustal contamination. Geochemically, the studied rocks exhibit a calc-alkaline to high potassium calc-alkaline affinity, and are enriched in LREE and LILE and depleted in HREE and HSFE. Other geochemical characteristics, such as a silica content varying between 59–63 wt% and 51–59 wt%, a Na2O content > 3 wt%, Al2O3 content > 16 wt%, Yb < 1.8 ppm, and Y < 18 ppm, make it possible to classify these rocks as high silica adakite in the Ahmad Abad region and low silica adakite in the Sahl-Razzeh region or at least, adakitic like rocks. Also, depletion of Nb and Ti, and high enrichment in Rb, Ba, K and Th, imply crustal contamination of the mentioned adakitic domes. The petrographical and geochemical evidence show that the magma forming of the high silica adakites has been originated from partial melting of the subducted oceanic slab of Neo-Tethys (Sabzevar–Darouneh branch) in amphibolite to eclogite facies and the low silica adakites formed by partial melting of the metasomatized or modified mantle wedge, above the subduction zone. Gabbroic to syenitic rocks are the products of fractional crystallization of basic magma which originated from a nearly non-modified mantle wedge above the subducted oceanic slab. U-Pb dating of the dacitic and andesitic rocks belong to hypabyssal rocks yielded age of 41.4 ± 0.3 Ma, and 35.5 ± 0.2 Ma respectively and consistent to Middle to Late Eocene.
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