| 1. |
- Kalsbeek, Feiko, et al.
(författare)
-
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.
|
|
| 2. |
- Be'eri-Shlevin, Yaron, et al.
(författare)
-
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
-
Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 187:1-2, s. 181-200
-
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.
|
|
| 3. |
- Björkman, Anne, 1981, et al.
(författare)
-
Plant functional trait change across a warming tundra biome
- 2018
-
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.
|
|
| 4. |
- Cornell, David H., 1948, et al.
(författare)
-
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.
|
|
| 5. |
- Cornell, David H., 1948, et al.
(författare)
-
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
-
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.
|
|
| 6. |
- Cornell, David H., 1948, et al.
(författare)
-
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
-
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.
|
|
| 7. |
- Cornell, David H., 1948, et al.
(författare)
-
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
-
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.
|
|
| 8. |
- Flowerdew, Michael J., et al.
(författare)
-
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.
|
|
| 9. |
|
|
| 10. |
|
|
