| 1. |
- Harris, M., et al.
(författare)
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Chronostratigraphic correlation of the volcanic 1.21 Ga Barby and Haiber Flats Formations in the Sinclair Supergroup of Namibia
- 2021
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Ingår i: Journal of African Earth Sciences. - : Elsevier BV. - 1464-343X. ; 178
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Tidskriftsartikel (refereegranskat)abstract
- This work investigates Mesoproterozoic volcanic rocks of the Haiber Flats, Barby, Welverdiend and Kairab Formations, and associated intrusive rocks in the Konkiep Terrane. Various correlations have been made in the past and recent dating work yields contrasting ages. U–Pb dating of zircons is applied in combination with Sm–Nd and Lu–Hf isotopes to determine the age, assess correlation and ascertain the much-debated tectonic setting. Ion probe U–Pb zircon dating yields ages of 1218 ± 5 Ma, 1212 ± 4 Ma and 1212 ± 5 Ma (all errors 2σ) for three samples of the Haiber Flats Formation and 1219 ± 13 Ma, 1215 ± 19 Ma for the Barby Formation. The ages obtained for the Barby Formation agree with the 1217 ± 2 Ma and 1214 ± 5 Ma ages reported from previous investigations. Rocks mapped as the supposedly older Kairab rhyolite and felsic tuff gave younger ages of 1220 ± 4 Ma and 1222 ± 10 Ma, respectively, which are coeval with the Barby and Haiber Flats Formations. Two Gorab Gabbro samples, previously mapped as Kairab Formation, yield ages of 1341 ± 8 Ma and 1336 ± 8 Ma. These older ages are within error of two Welverdiend Formation samples which gave ages of 1344 ± 6 Ma and 1337 ± 9 Ma, and the formation was previously dated in the type area at 1327 ± 10 Ma. These ages confirm the reassignment of rocks originally mapped as tholeiitic Barby Formation into the Welverdiend Formation, north of a newly identified stratigraphic hiatus corresponding to the geophysically defined Wêreldend Lineament. Sm–Nd and Lu–Hf isotopic signatures for the Barby, Haiber Flats and coeval Kairab samples indicate that they are co-magmatic and derived from partly depleted mantle contaminated by older crust. TDM model ages of these samples range from 2.5 to 1.5 Ga with a major 2.2 to 1.8 Ga group. This suggests that the older crustal component was Paleoproterozoic but included some Archean material. The Welverdiend Formation and Gorab Gabbros show similarity in age, Sm–Nd and Lu–Hf characteristics and were probably derived from partially depleted mantle, or from a well-homogenised mixture of depleted mantle and older crust. REE diagrams indicate enrichment in LREE (La–Gd) and a flat HREE pattern (Tb to Lu). On spidergrams, most samples are characterised by negative Nb–Ta anomalies and positive K and Pb anomalies which indicate a subduction-related setting (with positive Sr anomaly) or a reworked crustal origin. The similarity in ages, isotopic signatures and geochemistry confirms the correlation of the Barby and Haiber Flats Formation. © 2021 Elsevier Ltd
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| 2. |
- Cornell, David H., 1948, et al.
(författare)
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Dating of Guperas Formation rhyolites changes the stratigraphy of the Mesoproterozoic Sinclair Supergroup of Namibia
- 2020
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Ingår i: South African Journal of Geology. - : Geological Society of South Africa. - 1012-0750 .- 1996-8590. ; 123:4, s. 633-648
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Tidskriftsartikel (refereegranskat)abstract
- The volcanosedimentary Guperas Formation contains the youngest volcanic rocks of the Sinclair Supergroup in the Konkiep Terrane of southern Namibia. Precise U-Pb zircon microbeam dating shows that the Guperas Formation as mapped includes felsic volcanic rocks which belong to both the first (1.37 to 1.33 Ga) and the third (1.11 to 1.07 Ga) magmatic cycle of the Sinclair Supergroup. Volcanic rocks of the 'true' Guperas Formation are dated by three samples, with a combined age of 1108 +/- 10 Ma. The sedimentary rocks mapped as Guperas Formation are also distinguished by two different detrital age spectra into the similar to 1 100 Ma true Guperas Formation and the Aruab Member of the similar to 1 217 Ma Barby Formation. Geochronology now resolves the previous stratigraphic separation of the very similar Nubib and Rooiberg (Sonntag) Granites. The two small outcrops of 1 334 +/- 5 Ma Rooiberg Granite are now shown to be part of the regional 1 334 +/- 8 Ma Nubib Granite batholith. The Konkiep Terrane was affected by faulting and shear zones, but was only gently folded and not involved in regional metamorphism, despite its proximity to the Namaqua-Natal Province to the southwest. This is due to the Konkiep Terrane having a thick and strong continental basement which may have formed as part of the mainly Palaeoproterozoic Rehoboth Province. IIowever no Palaeoproterozoic rocks are exposed in the Konkiep Terrane, which is now interpreted as an unaffiliated terrane. The three cycles of extrusive and plutonic magmatism in the Sinclair Supergroup formed in chronologically distinct periods and different tectonic settings, which requires revision of the stratigraphic nomenclature. The Konkiep Group is replaced by three new groups which are separated by >100 million-year unconformities. The Betta Group, represented by the mainly volcanic Kumbis, Nagatis and Welverdiend formations in the first magmatic cycle, probably formed in a passive continental rift setting due to breakup of the Rehoboth Province between 1 374 and 1 334 Ma. The Vergenoeg Group, represented by the sedimentary Kunjas and volcanic Barby and Haiber Flats formations, formed in a subduction setting at the margin of the Konkiep Terrane. This similar to 1 217 to 1204 Ma magmatic cycle ended with the accretion of Namaqua-Natal terranes to the Kalahari Craton. The similar to 1 100 Ma Ganaams Group, represented by the volcanic Guperas Formation and sedimentary Aubures Formation, was the result of interplay between the continentalscale Umkondo mantle heating event and movements between crustal blocks following the Namaqua-Natal collisional orogeny.
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| 3. |
- Cornell, David H., 1948, et al.
(författare)
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Zircon xenocrysts obscured the zircon date for the lower Koras Group, southern Africa
- 2023
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Ingår i: South African Journal of Geology. - 1012-0750. ; 126:2, s. 177-194
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Tidskriftsartikel (refereegranskat)abstract
- The Koras Group is a bimodal volcanosedimentary group located in post-tectonic grabens in a foreland thrust complex in the Kaaien Terrane of the Mesoproterozoic Namaqua-Natal Province of southern Africa. It contains two sequences of mafic and felsic volcanic rocks with an unconformity between them, only the lower sequence being slightly folded. The Koras Group was long regarded as having formed at the end of the 1 210 to 1 000 Ma Namaqua Orogeny, because it lacks the severe deformation and metamorphism of the underlying rocks, with igneous minerals preserved in many samples. Following years of unsuccessful attempts to precisely date the volcanic rocks, the first two ion probe U-Pb zircon studies both reported ages of-1 172 Ma for the Swartkopsleegte Formation felsic lava in the slightly folded lower sequence (based on relatively few dated zircons) and-1 100 Ma for the Leeuwdraai Formation rhyolite in the undeformed upper sequence. Thus a major 70 m.y. hiatus seemed apparent between the lower and upper sequences despite their similar geochemistry and rift-related setting. This gave rise to models which envisaged the Kaaien Terrane being unaffected by the syn-to late-tectonic deformation, migmatisation and granite intrusions, documented between 1 200 and 1 150 Ma in the adjoining Namaqua-Natal terranes to the west.A high-pressure (10 kbar) metamorphic event, recognised in the Kaaien Terrane basement just south of hardly deformed Koras Group exposures and dated at 1 150 Ma, is inconsistent with such models. A re-investigation and microbeam dating campaign on the Koras Group confirms the 1 101 & PLUSMN; 2 Ma (n = 6) age for felsic volcanic rocks of the upper sequence, but establishes a new reliable age of 1 114 & PLUSMN; 4 Ma for the lower one (n = 2). The 1 170 ages obtained in the earlier two studies were revisited and are now considered to reflect the age of zircon xenocrysts from the source rocks, which dominate the zircon population of some Swartkopsleegte Formation samples.Several criteria to distinguish autocrystic (magmatic) from antecrystic (age-overlappping xenocrystic) data points were investigated. One sample had high Th levels in only the younger zircons, but histograms of sufficiently precise 207Pb/206Pb ages provided the main criterion. Calculations of zircon crystallisation temperature intervals were not useful in predicting the abundance or proportions of magmatic and antecryst zircons. A multi-episode model of magmatic generation and crystallisation events is probably appropriate. In cases when felsic volcanic samples yield few zircons, care must be taken to avoid the problem exposed in this study.The Koras Group sediments have similar detrital zircon U/Pb age distributions to those of the Rehoboth Basement Inlier. This supports the concept that the Kaaien Terrane originated as the southern part of the Rehoboth Province.
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| 4. |
- Harris, M., et al.
(författare)
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The Kumbis and Nagatis Formations and the Helmeringhausen Gabbro: Oldest undeformed rocks of the Sinclair Supergroup in Namibia
- 2020
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Ingår i: Journal of African Earth Sciences. - : Elsevier BV. - 1464-343X. ; 165
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Tidskriftsartikel (refereegranskat)abstract
- The Mesoproterozoic rocks of the Kumbis and Nagatis Formations and Helmeringhausen Gabbro in Southern Namibia are now recognised as the oldest rocks in the sedimentary and volcanic Sinclair Supergroup. The stratigraphic subdivision and plate tectonic setting are still debated and reliable ages are sparse. This work investigates the relatively undeformed felsic and mafic lava of the Kumbis Formation, rhyolitic lava of the Nagatis Formation and the intrusive Helmeringhausen Gabbro. Microscopic and field investigation reveals no evidence of regional metamorphism in these units. Ion probe U-Pb zircon dating yields ages of 1374 +/- 7 Ma to 1358 +/- 5 Ma (2 sigma) for the Kumbis Formation, 1363 +/- 11 Ma for the Nagatis Formation and 1372 +/- 12 Ma for the Helmeringhausen Gabbro. These dates are in contrast to the published stratigraphy in which the Helmeringhausen Gabbro was regarded as much older than the Kumbis Formation and both were excluded from the Sinclair Supergroup, whereas the Nagatis Formation was considered to be younger and was included as the oldest unit of the Sinclair Supergroup. Lu-Hf data on zircon indicates different magma sources for the Kumbis and Nagatis Formations, implying that although they are time equivalent, they are not co-magmatic. The geochemical data suggests that the most felsic rocks contain large amounts of older crustal material, from which their apparent subduction signatures were inherited. The more mafic rocks also contain minor crustal components mixed with material derived from depleted mantle. The Helmeringhausen Gabbro is similar to coeval hybrid gabbros at Neuhof Valley 150 Km to the north and they are grouped in the intrusive Helmering Suite. Taking the regional context into account, these oldest rocks of the Sinclair Supergroup probably formed in a rift environment which split the Rehoboth Province apart at the beginning of the Mesoproterozoic Namaqua-Natal Wilson Cycle.
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