cometadata/datacite-titles-descriptions-related-identifiers huggingface dataset:
DOI: 10.6084/m9.figshare.3454316.v1
title: Linking the Palaeoproterozoic Nagssugtoqidian and Rinkian orogens through the Disko Bugt region of West Greenland

description: The parallel Trans-Hudson-aged Nagssugtoqidian Orogen and Rinkian Fold Belt of West Greenland have long been considered distinct orogenic belts separated by a zone of low or no Palaeoproterozoic strain in the intervening Disko Bugt region. New U–Pb geochronology confirms the continuity of Palaeoproterozoic deformation and metamorphism through this region, thereby directly linking the Nagssugtoqidian Orogen and Rinkian Fold Belt. Furthermore, two newly identified Archaean blocks in this region are interpreted to represent the southern and northern extensions of the Rae and North Atlantic cratons, respectively. Their boundary comprises a 15 km wide belt of distributed, NW-vergent ductile thrusts that deform ultramafic pods and is interpreted to represent the main collisional suture of a single, variably exhumed, &gt;1100 km wide Nagssugtoqidian–Rinkian orogenic system. We conclude that Palaeoproterozoic convergence of a northern Archaean craton carried a south-facing passive margin sedimentary sequence (Karrat Group) towards a south-dipping subduction zone along the North Atlantic Craton. Terminal collision created a high-grade core centred on the magmatic arc of the central Nagssugtoqidian Orogen and a NW-vergent fold and thrust belt in the Rinkian Fold Belt.
DOI: 10.6084/m9.figshare.3454313
title: Laser ablation <sup>40</sup>Ar/<sup>39</sup>Ar dating of metamorphic fabrics in the Caledonides of north Ireland

description: <em>In situ</em> <sup>40</sup>Ar/<sup>39</sup>Ar dating of two texturally distinct biotite generations within a phyllite from the Caledonian orogen of Ireland was performed using a high spatial resolution laser microprobe. In this sample, an older generation of biotite (Bt<sub>1</sub>) is aligned subparallel to a steeply dipping crenulation foliation, which itself is overprinted by a regionally developed shallow-dipping foliation. The second biotite generation (Bt<sub>2</sub>) occurs as randomly oriented porphyroblasts that overprint and therefore postdate both foliations. A dataset of 89 ages from a single thick section was acquired for six Bt<sub>1</sub> and eight Bt<sub>2</sub> crystals. The two biotite generations yielded two statistically distinct age distributions with Bt<sub>1</sub> maxima at <em>c</em>. 433 Ma and Bt<sub>2</sub> maxima at <em>c</em>. 417 Ma. A small number of analyses (11% Bt<sub>1</sub> and 26% Bt<sub>2</sub>) from within crystals yielded ‘main population’ ages that conflict with the dominant age populations and relative chronology established by field relationships. Discordant analyses from the earlier Bt<sub>1</sub> crystals are younger (than the Bt<sub>2</sub> population) and probably reflect intracrystalline partial resetting during a later thermal perturbation. The discordant Bt<sub>2</sub> analyses are predominantly older (than the Bt<sub>2</sub> population) and are interpreted as indicating localized excess <sup>40</sup>Ar. Such dis cordant analyses indicate intracrystalline variation reflecting both excess <sup>40</sup>Ar (Bt<sub>2</sub> population) and <sup>40</sup>Ar loss (Bt<sub>1</sub> population), and were therefore discarded when determining geologically meaningful ages.
DOI: 10.6084/m9.figshare.3454313.v1
title: Laser ablation <sup>40</sup>Ar/<sup>39</sup>Ar dating of metamorphic fabrics in the Caledonides of north Ireland

description: <em>In situ</em> <sup>40</sup>Ar/<sup>39</sup>Ar dating of two texturally distinct biotite generations within a phyllite from the Caledonian orogen of Ireland was performed using a high spatial resolution laser microprobe. In this sample, an older generation of biotite (Bt<sub>1</sub>) is aligned subparallel to a steeply dipping crenulation foliation, which itself is overprinted by a regionally developed shallow-dipping foliation. The second biotite generation (Bt<sub>2</sub>) occurs as randomly oriented porphyroblasts that overprint and therefore postdate both foliations. A dataset of 89 ages from a single thick section was acquired for six Bt<sub>1</sub> and eight Bt<sub>2</sub> crystals. The two biotite generations yielded two statistically distinct age distributions with Bt<sub>1</sub> maxima at <em>c</em>. 433 Ma and Bt<sub>2</sub> maxima at <em>c</em>. 417 Ma. A small number of analyses (11% Bt<sub>1</sub> and 26% Bt<sub>2</sub>) from within crystals yielded ‘main population’ ages that conflict with the dominant age populations and relative chronology established by field relationships. Discordant analyses from the earlier Bt<sub>1</sub> crystals are younger (than the Bt<sub>2</sub> population) and probably reflect intracrystalline partial resetting during a later thermal perturbation. The discordant Bt<sub>2</sub> analyses are predominantly older (than the Bt<sub>2</sub> population) and are interpreted as indicating localized excess <sup>40</sup>Ar. Such dis cordant analyses indicate intracrystalline variation reflecting both excess <sup>40</sup>Ar (Bt<sub>2</sub> population) and <sup>40</sup>Ar loss (Bt<sub>1</sub> population), and were therefore discarded when determining geologically meaningful ages.
DOI: 10.6084/m9.figshare.3454310
title: Precise <sup>40</sup>Ar/<sup>39</sup>Ar geochronology for the upper Koobi Fora Formation, Turkana Basin, northern Kenya

description: The Plio-Pleistocene Koobi Fora Formation, about 560 m thick, crops out east of Lake Turkana and is part of the much larger depositional system of the Omo–Turkana Basin. The upper half of the Koobi Fora Formation from just below the KBS Tuff to above the Chari Tuff is particularly notable for its wealth of hominid fossils and archaeological sites. Silicic tuffaceous horizons have provided the basis for stratigraphic subdivision and correlation. Pumice clasts within the tuffs contain anorthoclase phenocrysts, ideal for <sup>40</sup>Ar/<sup>39</sup>Ar single-crystal dating. Feldspars from pumice clasts in about 15 tephra within the stratigraphic interval from the KBS Tuff to the Silbo Tuff have yielded precise ages that allow much finer definition of the numerical time framework for the sedimentary sequence between the KBS Tuff (1.869 ± 0.021 Ma) and the Chari Tuff (1.383 ± 0.028 Ma) and to yet higher in the sequence to the SilboTuff (0.751 ± 0.022 Ma). These results provide a precise and accurate time scale for the upper part of the sequence in the whole of the Omo–Turkana Basin. A number of these tuffs are recognized elsewhere in East Africa; thus ages determined at Koobi Fora also apply to the wider region.
DOI: 10.6084/m9.figshare.3454310.v1
title: Precise <sup>40</sup>Ar/<sup>39</sup>Ar geochronology for the upper Koobi Fora Formation, Turkana Basin, northern Kenya

description: The Plio-Pleistocene Koobi Fora Formation, about 560 m thick, crops out east of Lake Turkana and is part of the much larger depositional system of the Omo–Turkana Basin. The upper half of the Koobi Fora Formation from just below the KBS Tuff to above the Chari Tuff is particularly notable for its wealth of hominid fossils and archaeological sites. Silicic tuffaceous horizons have provided the basis for stratigraphic subdivision and correlation. Pumice clasts within the tuffs contain anorthoclase phenocrysts, ideal for <sup>40</sup>Ar/<sup>39</sup>Ar single-crystal dating. Feldspars from pumice clasts in about 15 tephra within the stratigraphic interval from the KBS Tuff to the Silbo Tuff have yielded precise ages that allow much finer definition of the numerical time framework for the sedimentary sequence between the KBS Tuff (1.869 ± 0.021 Ma) and the Chari Tuff (1.383 ± 0.028 Ma) and to yet higher in the sequence to the SilboTuff (0.751 ± 0.022 Ma). These results provide a precise and accurate time scale for the upper part of the sequence in the whole of the Omo–Turkana Basin. A number of these tuffs are recognized elsewhere in East Africa; thus ages determined at Koobi Fora also apply to the wider region.
DOI: 10.6084/m9.figshare.3454307
title: Sequence of tuffs between the KBS Tuff and the Chari Tuff in the Turkana Basin, Kenya and Ethiopia

description: This paper describes a sequence of tuffs between the KBS and the Chari Tuff of Omo Group formations in Kenya and Ethiopia. These tuffs have recently been shown to be 1.87 ± 0.02 Ma and 1.38 ± 0.03 Ma in age, respectively. The sequence of tuffs that is derived is consistent with <sup>40</sup>Ar/<sup>39</sup>Ar ages reported separately, and provides the stratigraphic framework for interpreting those ages. Further, new correlations are established to the Konso Formation in southern Ethiopia. As drainage from the Ethiopian Rift to the Omo–Turkana Basin developed after deposition of the Konso Formation, pumice clasts in tuffs of the Omo–Turkana Basin probably were transported there by the Omo River. The tuffs are divided into five groups on the basis of their stratigraphic position in relation to extensive ash layers. The sequence of tuffs has import for the placement and age of archaeological sites in the Koobi Fora Formation, and for ages of mammalian faunas (including hominids). Many tuffs were deposited during a 90 ka interval during which Mediterranean sapropels are lacking, suggesting that Nile flow was reduced, and that the level of a lake that occupied the Omo–Turkana Basin at the time was low. Thus the record of climatic influence on deposition in the Omo–Turkana Basin, previously shown for the Kibish Formation (≤200 ka), extends at least to early Pleistocene time.
DOI: 10.6084/m9.figshare.3454307.v1
title: Sequence of tuffs between the KBS Tuff and the Chari Tuff in the Turkana Basin, Kenya and Ethiopia

description: This paper describes a sequence of tuffs between the KBS and the Chari Tuff of Omo Group formations in Kenya and Ethiopia. These tuffs have recently been shown to be 1.87 ± 0.02 Ma and 1.38 ± 0.03 Ma in age, respectively. The sequence of tuffs that is derived is consistent with <sup>40</sup>Ar/<sup>39</sup>Ar ages reported separately, and provides the stratigraphic framework for interpreting those ages. Further, new correlations are established to the Konso Formation in southern Ethiopia. As drainage from the Ethiopian Rift to the Omo–Turkana Basin developed after deposition of the Konso Formation, pumice clasts in tuffs of the Omo–Turkana Basin probably were transported there by the Omo River. The tuffs are divided into five groups on the basis of their stratigraphic position in relation to extensive ash layers. The sequence of tuffs has import for the placement and age of archaeological sites in the Koobi Fora Formation, and for ages of mammalian faunas (including hominids). Many tuffs were deposited during a 90 ka interval during which Mediterranean sapropels are lacking, suggesting that Nile flow was reduced, and that the level of a lake that occupied the Omo–Turkana Basin at the time was low. Thus the record of climatic influence on deposition in the Omo–Turkana Basin, previously shown for the Kibish Formation (≤200 ka), extends at least to early Pleistocene time.
DOI: 10.6084/m9.figshare.3454304
title: Basic and ultrabasic volcanic rocks from the Argyll Group (Dalradian) of NE Scotland

description: The Dalradian Supergroup of the Scottish Highlands is a largely metasedimentary succession of Neoproterozoic to Early Cambrian age, metamorphosed during the Caledonian Orogeny. The rocks were deposited on the Laurentian margin during and following the break-up of Rodinia. This rift setting is evidenced, in the upper half of the succession, by the presence of several volcanic sequences. A significant development of these volcanic rocks occurs in the NE Grampian Highlands. There, the Blackwater Formation, within the Argyll Group of the Huntly area, is dominated by basic and ultrabasic metavolcanic rocks, with intercalated metasedimentary rocks. The rocks were metamorphosed in the amphibolite facies, with the Mg contents of their dominant amphiboles apparently reflecting those of the whole-rock protoliths. The protoliths ranged from ultrabasic picritic types (MgO up to 35 wt%) through high-magnesia basalts to basaltic andesites and andesites. The magmas feeding the volcanism were of tholeiitic affinity, broadly similar to metavolcanic rocks elsewhere in the Dalradian. Higher-Nb and lower-Nb groups can be distinguished. The inferred parental magmas were basalts with MgO <em>c.</em> 10 wt%. The ultrabasic rocks formed by accumulation of olivine and minor Cr-spinel within the parental basalts. Some more evolved rocks show evidence of minor (5–10 wt%) accumulation of Fe–Ti oxides. Primary magmas of the Blackwater metavolcanic rocks were generated from a mantle source, or sources, relatively enriched compared to the MORB source. It is speculated that all the Dalradian metavolcanic rocks represent varying degrees of mixing of magmas from this source and a depleted, MORB-like, mantle source.
DOI: 10.6084/m9.figshare.3454304.v1
title: Basic and ultrabasic volcanic rocks from the Argyll Group (Dalradian) of NE Scotland

description: The Dalradian Supergroup of the Scottish Highlands is a largely metasedimentary succession of Neoproterozoic to Early Cambrian age, metamorphosed during the Caledonian Orogeny. The rocks were deposited on the Laurentian margin during and following the break-up of Rodinia. This rift setting is evidenced, in the upper half of the succession, by the presence of several volcanic sequences. A significant development of these volcanic rocks occurs in the NE Grampian Highlands. There, the Blackwater Formation, within the Argyll Group of the Huntly area, is dominated by basic and ultrabasic metavolcanic rocks, with intercalated metasedimentary rocks. The rocks were metamorphosed in the amphibolite facies, with the Mg contents of their dominant amphiboles apparently reflecting those of the whole-rock protoliths. The protoliths ranged from ultrabasic picritic types (MgO up to 35 wt%) through high-magnesia basalts to basaltic andesites and andesites. The magmas feeding the volcanism were of tholeiitic affinity, broadly similar to metavolcanic rocks elsewhere in the Dalradian. Higher-Nb and lower-Nb groups can be distinguished. The inferred parental magmas were basalts with MgO <em>c.</em> 10 wt%. The ultrabasic rocks formed by accumulation of olivine and minor Cr-spinel within the parental basalts. Some more evolved rocks show evidence of minor (5–10 wt%) accumulation of Fe–Ti oxides. Primary magmas of the Blackwater metavolcanic rocks were generated from a mantle source, or sources, relatively enriched compared to the MORB source. It is speculated that all the Dalradian metavolcanic rocks represent varying degrees of mixing of magmas from this source and a depleted, MORB-like, mantle source.
DOI: 10.6084/m9.figshare.3454301
title: K–Ar and <sup>40</sup>Ar/<sup>39</sup>Ar geochronology of supergene processes in the Atacama Desert, Northern Chile: tectonic and climatic relations

description: New K–Ar and <sup>40</sup>Ar/<sup>39</sup>Ar data obtained for 29 samples of supergene alunite-, jarosite- and hollandite-group minerals from the Central Depression of Northern Chile document almost continuous supergene oxidation from 33 to 9 Ma. The downward migration of supergene oxidation zones is used to calculate maximum denudation rates of 16–35 m Ma<sup>−1</sup>. These calculated rates are sensitive to pre-existing topography and show that the present relief observed in the Central Depression represents a modified Miocene landscape. The results presented here are in good agreement with previously published geochronological data for the Atacama Desert. The combination of all published data, including the results presented here, defines a continuous skewed distribution, with a maximum between 21 and 14 Ma. The relative paucity of ages greater than 30 Ma reflects the removal of supergene profiles by erosion during and immediately after the Incaic compressive deformation. From 29 to 9 Ma, supergene oxidation profiles were generated under semi-arid conditions throughout the Atacama region. Ages between 9 and 5 Ma on the northern and southern margins of the desert mark the end of supergene oxidation processes, consistent with a Pliocene onset of present-day hyperaridity.
DOI: 10.6084/m9.figshare.3454301.v1
title: K–Ar and <sup>40</sup>Ar/<sup>39</sup>Ar geochronology of supergene processes in the Atacama Desert, Northern Chile: tectonic and climatic relations

description: New K–Ar and <sup>40</sup>Ar/<sup>39</sup>Ar data obtained for 29 samples of supergene alunite-, jarosite- and hollandite-group minerals from the Central Depression of Northern Chile document almost continuous supergene oxidation from 33 to 9 Ma. The downward migration of supergene oxidation zones is used to calculate maximum denudation rates of 16–35 m Ma<sup>−1</sup>. These calculated rates are sensitive to pre-existing topography and show that the present relief observed in the Central Depression represents a modified Miocene landscape. The results presented here are in good agreement with previously published geochronological data for the Atacama Desert. The combination of all published data, including the results presented here, defines a continuous skewed distribution, with a maximum between 21 and 14 Ma. The relative paucity of ages greater than 30 Ma reflects the removal of supergene profiles by erosion during and immediately after the Incaic compressive deformation. From 29 to 9 Ma, supergene oxidation profiles were generated under semi-arid conditions throughout the Atacama region. Ages between 9 and 5 Ma on the northern and southern margins of the desert mark the end of supergene oxidation processes, consistent with a Pliocene onset of present-day hyperaridity.
DOI: 10.6084/m9.figshare.3454298
title: Mid-Jurassic age for the Botany Bay Group: implications for Weddell Sea Basin creation and southern hemisphere biostratigraphy

description: New U–Pb zircon ion-microprobe ages from the alluvial conglomerates and flood plain sediments of the Botany Bay Group demonstrate that sedimentation occurred at <strong><em>c.</em></strong> 167 Ma, coeval with rift-related silicic volcanism in the northern Antarctic Peninsula. In contrast, rift-related volcanism and sedimentation in the southern Antarctic Peninsula (Latady Basin) occurred at <strong><em>c.</em></strong> 183 Ma. The new data indicate that syn-rift sedimentation and volcanism was diachronous from south to north, consistent with early opening of the Weddell Sea embayment by anti-clockwise rotation of the Antarctic Peninsula in the Mid-Jurassic. A definitive date for the Botany Bay Group floras has important implications for Southern Hemisphere biostratigraphic correlations.
DOI: 10.6084/m9.figshare.3454298.v1
title: Mid-Jurassic age for the Botany Bay Group: implications for Weddell Sea Basin creation and southern hemisphere biostratigraphy

description: New U–Pb zircon ion-microprobe ages from the alluvial conglomerates and flood plain sediments of the Botany Bay Group demonstrate that sedimentation occurred at <strong><em>c.</em></strong> 167 Ma, coeval with rift-related silicic volcanism in the northern Antarctic Peninsula. In contrast, rift-related volcanism and sedimentation in the southern Antarctic Peninsula (Latady Basin) occurred at <strong><em>c.</em></strong> 183 Ma. The new data indicate that syn-rift sedimentation and volcanism was diachronous from south to north, consistent with early opening of the Weddell Sea embayment by anti-clockwise rotation of the Antarctic Peninsula in the Mid-Jurassic. A definitive date for the Botany Bay Group floras has important implications for Southern Hemisphere biostratigraphic correlations.
DOI: 10.6084/m9.figshare.3454295
title: <sup>40</sup>Ar/<sup>39</sup>Ar laserprobe dating of mylonitic fabrics in a polyorogenic terrane of NW Iberia

description: The tectonothermal evolution of a polyorogenic terrane in the Variscan belt of NW Spain has been constrained by <sup>40</sup>Ar/<sup>39</sup>Ar laserprobe incremental heating experiments on mylonitic fabrics developed in major structures. Transitional levels between HP–HT and IP upper units in the Órdenes Complex where metamorphic and structural records demonstrate two cycles of burial and exhumation were selected for dating. Two groups of ages have been defined: (1) Silurian–Early Devonian, obtained from mylonites of the Fornás extensional detachment, here considered as the minimum age for the start of tectonic exhumation of the HP–HT units and an upper age-limit for the HP–HT event itself; (2) Early to Mid-Devonian, from structures related to the Variscan convergence in the area, which include top-to-the-east thrusts and extensional detachments. A single, younger Carboniferous age obtained from the uppermost allochthonous sequences possibly reflects the final stages of emplacement of the allochthonous complexes. Our data indicate a polyorogenic character for a part of the Iberian allochthonous complexes, including Variscan (<em>sensu stricto</em>) and Early Variscan convergence, as well as an older, Early Palaeozoic cycle.
DOI: 10.6084/m9.figshare.3454295.v1
title: <sup>40</sup>Ar/<sup>39</sup>Ar laserprobe dating of mylonitic fabrics in a polyorogenic terrane of NW Iberia

description: The tectonothermal evolution of a polyorogenic terrane in the Variscan belt of NW Spain has been constrained by <sup>40</sup>Ar/<sup>39</sup>Ar laserprobe incremental heating experiments on mylonitic fabrics developed in major structures. Transitional levels between HP–HT and IP upper units in the Órdenes Complex where metamorphic and structural records demonstrate two cycles of burial and exhumation were selected for dating. Two groups of ages have been defined: (1) Silurian–Early Devonian, obtained from mylonites of the Fornás extensional detachment, here considered as the minimum age for the start of tectonic exhumation of the HP–HT units and an upper age-limit for the HP–HT event itself; (2) Early to Mid-Devonian, from structures related to the Variscan convergence in the area, which include top-to-the-east thrusts and extensional detachments. A single, younger Carboniferous age obtained from the uppermost allochthonous sequences possibly reflects the final stages of emplacement of the allochthonous complexes. Our data indicate a polyorogenic character for a part of the Iberian allochthonous complexes, including Variscan (<em>sensu stricto</em>) and Early Variscan convergence, as well as an older, Early Palaeozoic cycle.