Greenstone belts in the central Godthåbsfjord region, southern West Greenland

In 2004 the Geological Survey of Denmark and Greenland (GEUS) initiated a study of the origin and tectono-metamorphic evolution of greenstone belts and important regional structures in the central Godthåbsfjord region, southern West Greenland (Fig. 1; Hollis et al. 2004). Like other Archaean belts worldwide, these greenstone belts are locally host to gold mineralisation. Their complexity requires a combination of detailed geological mapping, geochemistry, petrographic work and geochronological studies to develop models of their geological setting, evolution and gold mineralisation

Knowledge-driven mineral potential modelling: applying the Mineral Systems Approach to the west Kimberley, Australia

Regional prospectivity analysis of the west Kimberley has been undertaken using the results of geophysical structural interpretation and the mineral systems approach (MSA) to mineral potential modelling. Ore deposits are small expressions of much larger Earth processes and systems focusing mass and energy flux at multiple scales. The MSA provides a framework within which metal sources, fluid transport mechanisms and traps can be recognised and represented by predictor maps. Predictor maps act as proxies for mineral system processes and include distance to a particular geological feature, locations of rheological and chemical contrast, structural complexity and location of mantle-tapping structures. The approach to mineral potential modelling taken here combines different predictor maps in a knowledge-driven inference framework order to identify likely zones of mineralisation. Nickel-sulphide analysis indicates that the Inglis Fault and Yampi Fold Belt localises areas of increased mineral potential. Carbonate-hosted base metal prospectivity is restricted to regions overlying basement highs in the Lennard Shelf. A buried NW extension of the Oscar Range interpreted from geophysics is identified as a region of high mineral potential. Gold potential is mostly associated with the Inglis Fault, the Yampi Fold Belt and the central portion of the King Leopold Orogen. The centre, west and east of the King Leopold Orogen and parts of the Yampi Fold Belt show small, but high, regions of mineral potential for stratiform-hosted base metal deposits. The northwest and east of the King Leopold Orogen show small regions of tin-tungsten mineral potential. Intrusion-related base metal mineral potential is restricted to the Wotjulum Porphyry in the Yampi Fold Belt and the northern central part of the King Leopold Orogen

Apatite: a U-Pb thermochronometer or geochronometer?

Apatite is an accessory mineral that is frequently found in both igneous and clastic sedimentary rocks. It is conventionally considered to be characterized by a closure temperature range between 375 and 600 °C and hence has been employed to address mid-temperature thermochronology questions relevant to the reconstruction of thermal events in the middle to lower crust. However, questions remain as to whether apatite faithfully records thermally-activated volume diffusion profiles, or rather is influenced by recrystallization and new growth processes. We present a case study of two apatite samples from the Akia Terrane in Greenland that help chart some of the post magmatic history of this region. Apatite in a tonalitic gneiss has distinct U-enriched rims and its U-Pb apparent ages correlate with Mn chemistry, with a high Mn group yielding an age of c. 2813 Ma. The U-Pb and trace element chemistry and morphology support an interpretation in which these apatite crystals are originally igneous and record cooling after metamorphism, with subsequent generation of discrete new rims. Epidote observed in the sample implies a <600 °C fluid infiltration event associated with apatite rims. The second sample, from a granitic leucosome, contains apparently homogeneous apatite, however U-Pb analyses define two distinct discordia arrays with different common Pb components. An older, c. 2490 Ma, component is associated with elevated Sr, whereas a younger, c. 1800 Ma, component has lower Sr concentration. A depth profile reveals an older core with progressively younger ages towards a compositionally discrete late Paleoproterozoic rim. The chemical and age profiles do not directly correspond, implying different diffusion rates between trace elements and U and Pb. The variation in core ages is interpreted to reflect radiogenic-Pb loss from a metamorphic population during new rim growth. The younger, c. 1800 Ma U-Pb age is interpreted to date new apatite growth from a compositionally distinct reservoir driven by tectonothermal and fluid activity, consistent with regional mica Ar-Ar ages. Results from these two samples show that recrystallization, dissolution and regrowth processes likely formed the younger rim overgrowths, and at temperatures below those often considered to be closure temperatures for Pb diffusion in apatite. The results from these samples imply many apatite grains may not record simple thermally activated Pb diffusion profiles and cautions against inversion of apatite U-Pb data to thermal histories without detailed knowledge of the grain growth/alteration processes

Low-pressure metamorphism during Archaean crustal growth: a low-strain zone in the northern Nagssugtoqidian orogen, West Greenland

One of the first detailed accounts of Precambrian supracrustal rocks in central West Greenland came from a small group of islands and skerries a few kilometres north-east of Aasiaat (Fig. 1). In 1948, K. Ellitsgaard-Rasmussen spent a few days on the islands and published a metamorphic study of their low-grade greenstones and aluminous clastic rocks (Ellitsgaard-Rasmussen 1954). He observed a striking dissimilarity between these supracrustal rocks and the grey gneisses found in most of the Aasiaat region, although the latter were at that time also assumed to be of supracrustal origin. He furthermore noted that the regional significance of the islands should be pursued, and that the island of Maniitsoq 4 km west of the small islands might hold a key to their interpretation. More than 50 years were to elapse before the islands were surveyed again in July 2003, during field work for the Ikamiut map sheet in the northern Nagssugtoqidian orogen (van Gool et al. 2002). The collision of two Archaean continents during the c. 1850 Ma Nagssugtoqidian orogeny caused intensive structural and thermal reworking at up to granulite facies grade in most of central West Greenland; see Connelly et al. (2000) and van Gool et al. (2002). The small islands north-east of Aasiaat are indeed regionally important, because they document a previously unrecognised low-grade, low-strain domain of presumed Archaean age that has largely escaped the Nagssugtoqidian orogeny, and as predicted by Ellitsgaard-Rasmussen (1954) a clue to their significance was found on Maniitsoq

North Atlantic Craton architecture revealed by kimberlite-hosted crustal zircons

The Maniitsoq project is supported by the Ministry of Mineral Resources, Government of Greenland. NJG and PAC thank Australian Research Council grant FL160100168 for financial support. ON is supported by Australian Research Council grant FT140101062 and the Melbourne TIE team.Archean cratons are composites of terranes formed at different times, juxtaposed during craton assembly. Cratons are underpinned by a deep lithospheric root, and models for the development of this cratonic lithosphere include both vertical and horizontal accretion. How different Archean terranes at the surface are reflected vertically within the lithosphere, which might inform on modes of formation, is poorly constrained. Kimberlites, which originate from significant depths within the upper mantle, sample cratonic interiors. The North Atlantic Craton, West Greenland, comprises Eoarchean and Mesoarchean gneiss terranes – the latter including the Akia Terrane – assembled during the late Archean. We report U–Pb and Hf isotopic, and trace element, data measured in zircon xenocrysts from a Neoproterozoic (557 Ma) kimberlite which intruded the Mesoarchean Akia Terrane. The zircon trace element profiles suggest they crystallized from evolved magmas, and their Eo- to Neoarchean U–Pb ages match the surrounding gneiss terranes, and highlight that magmatism was episodic. Zircon Hf isotope values lie within two crustal evolution trends: a Mesoarchean trend and an Eoarchean trend. The Eoarchean trend is anchored on 3.8 Ga orthogneiss, and includes 3.6–3.5 Ga, 2.7 and 2.5–2.4 Ga aged zircons. The Mesoarchean Akia Terrane may have been built upon mafic crust, in which case all zircons whose Hf isotopes lie within the Eoarchean trend were derived from the surrounding Eoarchean gneiss terranes, emplaced under the Akia Terrane after ca. 2.97 or 2.7 Ga, perhaps during late Archean terrane assembly. Kimberlite-hosted peridotite rhenium depletion model ages suggest a late Archean stabilization for the lithospheric mantle. The zircon data support a model of lithospheric growth via tectonic stacking for the North Atlantic Craton.Publisher PDFPeer reviewe

Zircon fingerprint of the Neoproterozoic North Atlantic: Perspectives from East Greenland

Correlations across the once-contiguous North Atlantic region remain challenging, particularly for pre-Caledonian packages. Here, we present new zircon U-Pb and Hf isotope data from five metasedimentary samples and three granites from East Greenland to assess the age and provenance of Meso- to Neoproterozoic successions in this area, and to evaluate their correlatives across the North Atlantic region. Detrital zircon U-Pb data yield maximum depositional ages of 936 ± 15 Ma (2σ) that indicate the metasedimentary rocks of this region of East Greenland are probably a component of the Neoproterozoic Nathorst Land Group (lower Eleonore Bay Supergroup). Intruding granites are exclusively of Caledonian age at 426 ± 1 Ma (2σ) and contain a significant xenocrystic cargo with comparable ages to detrital zircon in the host metasedimentary sequences. Detrital zircon age components are concentrated between ca. 1850 and 920 Ma, with prominent peaks at ca. 1620 Ma, 1450 Ma and 1080 Ma, and additional subcomponents at 2900–2600 Ma (~4%) and 2030–1940 Ma (~2%). Provenance of this detritus is likely from the East Laurentian margin via axial drainage to East Greenland. Statistical analyses of a newly-compiled circum-Atlantic detrital zircon dataset corroborates a model in which sedimentation occurred in three distinct megasequences: (1) 1020–950 Ma, as a response to the opening of the Asgard Sea that followed Laurentia–Baltica collision, (2) 920–840 Ma, recording the onset of further rifting of the North Atlantic and attempted breakup of Rodinia, and (3) 740–635 Ma, concomitant with the breakup of Rodinia and ending with the Marinoan glaciation. The first two megasequences show strong U-Pb and εHf similarities in both space and time across the North Atlantic. However, there is a significant increase in detrital zircon disparity between the second and third megasequences, which we propose is associated with increased compartmentalization of depocentres with localized supply and distinct sediment routing pathways during continental breakup

Stirred not shaken; critical evaluation of a proposed Archean meteorite impact in West Greenland

Funding Information: The Ministry of Mineral Resources and Labour, Greenland Government supported field and analytical work.Large meteorite impacts have a profound effect on the Earth's geosphere, atmosphere, hydrosphere and biosphere. It is widely accepted that the early Earth was subject to intense bombardment from 4.5 to 3.8 Ga, yet evidence for subsequent bolide impacts during the Archean Eon (4.0 to 2.5 Ga) is sparse. However, understanding the timing and magnitude of these early events is important, as they may have triggered significant change points to global geochemical cycles. The Maniitsoq region of southern West Greenland has been proposed to record a ∼3.0 Ga meteorite impact, which, if confirmed, would be the oldest and only known impact structure to have survived from the Archean. Such an ancient structure would provide the first insight into the style, setting, and possible environmental effects of impact bombardment continuing into the late Archean. Here, using field mapping, geochronology, isotope geochemistry, and electron backscatter diffraction mapping of 5,587 zircon grains from the Maniitsoq region (rock and fluvial sediment samples), we test the hypothesis that the Maniitsoq structure represents Earth's earliest known impact structure. Our comprehensive survey shows that previously proposed impact-related geological features, ranging from microscopic structures at the mineral scale to macroscopic structures at the terrane scale, as well as the age and geochemistry of the rocks in the Maniitsoq region, can be explained through endogenic (non-impact) processes. Despite the higher impact flux, intact craters from the Archean Eon remain elusive on Earth.Publisher PDFPeer reviewe

The Mesoarchaean Akia terrane, West Greenland, revisited : new insights based on spatial integration of geophysics, field observation, geochemistry and geochronology

NJG thanks Australian Research Council grant FL160100168 for financial support.The northern part of the North Atlantic Craton (NAC) in southern West Greenland comprises a large tract of exposed Meso-Neoarchaean continental crust, divided into the ca 3300–2900 Ma Akia and ca 2900–2500 Ma Tuno terranes. We combine aeromagnetic, stream sediment geochemical, new litho-chemical and zircon geochronological data with previously published data to re-evaluate the crustal architecture and evolution of the Akia terrane and its boundary towards the Tuno terrane. The previously recognised, but overlooked, Alanngua complex, situated between the Akia and Tuno terranes is bounded by aeromagnetic lineaments interpreted as Neoarchaean shear zones and has a distinct spectrum of Neoarchaean magmatic and metamorphic zircon ages that are rare in the Akia terrane. The Alanngua complex comprises components derived from both the Akia and Tuno terranes and is interpreted as a tectonic melange created during the Neoarchaean assembly of the NAC. Within the Akia terrane, the chemistry of orthogneiss samples indicate that a large percentage is too mafic to classify as TTG s.s., implying that not only partial melting of mafic crust, but also some yet unaddressed mantle involvement is necessary in their formation. Previous models for the generation of the ca. 3015–2990 Ma quartz-dioritic Finnefjeld and Taserssuaq complexes conflict with their geochemical variation. The complexes are spatially associated with strong aeromagnetic responses that are interpreted to reflect a large gabbro-diorite intrusion, and we propose that the protoliths of the Finnefjeld and Taserssuaq complexes are genetically linked to such intrusion. Formed at same time are carbonatite, high-Mg gabbro and tonalite-trondhjemite, and we propose that this wide spectrum of rocks could have formed by lithospheric and crustal melting in response to asthenospheric upwelling possibly in an extensional setting. Periods of extensive magmatism in the Akia terrane were previously recognised at ca. 3220-3180 Ma and 3070-2970 Ma. We now subdivide the latter period into three episodes: juvenile basaltic-andesitic volcanism at 3070–3050 Ma; tonalitic and dioritic plutonism at 3050–3020 Ma, and gabbroic-dioritic plus tonalitic-trondhjemitic plutonism at 3020–2985 Ma. This last episode was immediately followed by crustal reworking during collision at 2980–2950 Ma.Publisher PDFPeer reviewe

Consensus workshops on the development of an ADHD medication management protocol using QbTest: developing a clinical trial protocol with multidisciplinary stakeholders

BackgroundThe study design and protocol that underpin a randomised controlled trial (RCT) are critical for the ultimate success of the trial. Although RCTs are considered the gold standard for research, there are multiple threats to their validity such as participant recruitment and retention, identifying a meaningful change, and non-adherence to the protocol. For clinical RCTs, involving patients and clinicians in protocol design provides the opportunity to develop research protocols that are meaningful to their target audience and may help overcome some of the inherent threats in conducting RCTs. However, the majority of protocols do not describe the methodology underpinning their development, limiting the amount of learned experience shared between research groups.MethodWith the purpose of reporting a collaborative approach towards developing a protocol, we present the findings from three sequential workshops that were conducted with the aim of developing a protocol to investigate the feasibility of adding a computerised test of attention, impulsivity and activity (QbTest) to medication management of children and young people with Attention deficit hyperactivity disorder (ADHD). Based on previous qualitative interviews with clinicians and families, each workshop prioritised topics for focused discussion. Information from the workshops was fed back to the participants for reflection in advance of the next workshop.ResultsThe workshops involved 21 multi-disciplinary ADHD experts, including clinicians, patient and public involvement (PPI) members, parents of young people with ADHD and researchers. The consensus workshops addressed key research issues such as: the most relevant outcome measures/ resource drivers; methods and time points for data collection; and the clinical protocol for utilising the QbTest, including when best to use this within the medication management process. The resulting protocol details a feasibility RCT design describing these factors.ConclusionProtocols which are co-developed may help overcome some of the risks associated with RCT completion (e.g. recruitment, retention, protocol adherence) and help prioritise outcomes of greater relevance to the populations under study. The methodology has potential value for researchers and organisations developing clinical guidelines, and offers insights into the valuable impact of PPI upon trial design

Geodynamic implications of synchronous Norite and TTG formation in the 3 Ga Maniitsoq Norite Belt, West Greenland

This study was supported by Villum Fonden through grant VKR18978 to K.S. Funding for article fees was supplied by the Ministry of Mineral Resources, Government of Greenland.We present new data for the ∼3.0 Ga Maniitsoq Norite Belt of the Akia Terrane, West Greenland, with the aim of understanding its petrogenesis. The Maniitsoq Norite Belt is hosted in regional tonalite-trondhjemite-granodiorite (TTG) and dioritic orthogneisses, intruded by later sheets of TTG and granite pegmatites, and comprises two main rock types: plagioclase-rich “norites” and pyroxene-rich “melanorites”. Both norites and melanorites have high SiO2 contents (52–60 wt% SiO2), high bulk rock Mg# (0.57–0.83), and low TiO2 contents (0.1–0.7 wt%). Their trace element patterns are defined by depleted heavy Rare-Earth elements, highly enriched light Rare-Earth elements, negative anomalies in Nb, Ta, and Ti, and variable anomalies in Zr, Hf, and Eu. New zircon U-Pb geochronology data and previously published ages establish an emplacement age of 3,013 ± 1 Ma for the majority of the Maniitsoq Norite Belt, with magmatism continuing until 3,001 ± 3 Ma. This ∼12 Myr period of norite magmatism is coeval with an ongoing period of TTG production in the Akia Terrane. Norite Belt emplacement was closely followed by high temperature, low pressure granulite-facies metamorphism at ∼800°C and 900°C/GPa) and that the norite magmas were emplaced into thin crust and lithosphere. Compositions of the norites and melanorites can be explained by derivation from a single mafic parental melt (∼13 wt% MgO), with the norites predominantly accumulating plagioclase and the melanorites predominantly accumulating pyroxene. Evidence from field relationships, the presence of xenocrystic zircon, major element compositions and combined trace element and Hf-isotope modelling suggests the norites were contaminated by assimilation of ∼20–30% continental TTG crust. Geochemical and Hf-Nd isotopic constraints indicate that the norite mantle source was depleted, and that this depletion occurred significantly before the emplacement of the norite magmas. Contemporaneous production of both TTGs and norite, their emplacement in thin crust, and the rapid transition to high temperature, low pressure granulite-facies metamorphism is best explained by their formation in an ultra-hot orogeny. Formation of norites in this setting may be restricted to >2.7 Ga, when geothermal gradients were higher on Earth.Publisher PDFPeer reviewe