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

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

Regional zircon U-Pb geochronology for the Maniitsoq region, southwest Greenland

The Ministry of Mineral Resources, Government of Greenland, funded this project. Analyses in the JdLC GeoHistory Facility were enabled by instrumentation supported by AuScope (auscope.org.au) and the Australian Government via the National Collaborative Research Infrastructure Strategy. The Tescan Mira3 FEG-SEM was funded through the Australian Research Council LIEF program.Zircon U-Pb geochronology places high-temperature geological events into temporal context. Here, we present a comprehensive zircon U-Pb geochronology dataset for the Meso- to Neoarchean Maniitsoq region in southwest Greenland, which includes the Akia Terrane, Tuno Terrane, and the intervening Alanngua Complex. The magmatic and metamorphic processes recorded in these terranes straddle a key change-point in early Earth geodynamics. This dataset comprises zircon U-Pb ages for 121 samples, including 46 that are newly dated. A principal crystallization peak occurs across all three terranes at ca. 3000 Ma, with subordinate crystallization age peaks at 3200 Ma (Akia Terrane and Alanngua Complex only), 2720 Ma and 2540 Ma. Metamorphic age peaks occur at 2990 Ma, 2820-2700 Ma, 2670-2600 Ma and 2540 Ma. Except for one sample, all dated metamorphic zircon growth after the Neoarchean occurred in the Alanngua Complex or within 20 km of its boundaries. This U-Pb dataset provides an important resource for addressing Earth Science topics as diverse as crustal evolution, fluid-rock interaction and mineral deposit genesis.Publisher PDFPeer reviewe