Recent fieldwork, geochemistry, and UPb geochronology in the Palaeoproterozoic Ketilidian orogen provide substantial insights into the timing and mechanisms of its magmatic, sedimentary, and tectonic accretion. From north to south the orogen comprises an Archaean foreland and Border Zone, a calc-alkaline arc, and a migmatized fore arc. Contrasting the marginally older Nagssugtoqidian orogen of central West Greenland, the Ketilidian orogen is juvenile, lacks evidence of continentcontinent collision, and probably evolved during northward subduction of an oceanic plate under the Archaean craton, with a suture south of the present orogen. Palaeoproterozoic dolerite dyke emplacement into the cratonic margin was followed by deposition of Ketilidian cover rocks. Thrusting and dextral transpression before 1848 Ma in the northwest may correlate with 18951870 Ma dextral transpression in the Makkovik orogen, Labrador. Sinistral transpression and I-type granite emplacement followed at 18481805 Ma. In the northeast, limited geochronology indicates deformation and metamorphism at ca. 1800 Ma. The calc-alkaline Julianehåb batholith was largely emplaced between 18541795 Ma during sinistral transpression, giving rise to steep magmatic fabrics and northeast-trending shear zones. Until 1790 Ma, the proximal fore-arc basin (Psammite Zone) received coarse detritus from the batholith, and turbidity currents swept sands and muds into distal parts. Fore-arc sedimentation, pervasive deformation, high temperature low pressure (HTLP) metamorphism and anatexis occurred at 17951785 Ma: flat-lying planar fabrics with top-to-northeast transport were due to tectonic decoupling at the outboard batholith margin during continued transpression. Rapakivi granite sheets were emplaced at 17551732 Ma and folded into broad arches and narrow synclinal cusps compatible with late-stage sinistral transpression.
An unconformable sedimentary succession deposited between
c.
2130–1848 Ma on Archaean gneisses of the foreland of the Palaeoproterozoic Ketilidian orogen includes a layer with coarse sand-sized silicate spherules. The layer is
c.
1 m thick and consists mainly of coarse diagenetic dolomite. In addition to
c.
18% spherules, the layer also contains 3% well-sorted, very fine quartzose sand and 6% larger intraclasts of chert and carbonate. The spherules were previously interpreted as microfossils (
Vallenia
sp.) because of their spheroidal shapes and inclusions of carbonaceous matter. The spherules are reinterpreted as replaced impact ejecta because they have shapes typical of splash-form microtektites, some contain possible examples of replaced skeletal spinel crystals, perlitic cracks and devitrification spherulites, and non-spherical particles with shapes and textures of typical glassy and scoriaceous volcanic ash are absent. The carbonaceous matter is attributed to hydrocarbons that migrated into the spherule layer from elsewhere in the sedimentary succession. The spherules were reworked after deposition, probably as a result of turbidity currents or storm- or impact-induced waves. Analysis of one spherule-bearing sample revealed only 0.02 ppb iridium, a value comparable with low iridium abundances in distal layers of other terrestrial impact ejecta. The spherules in South Greenland are the first distal impact ejecta recognized in mid-Precambrian strata. They represent a major impact because their aggregate thickness exceeds the thickest spherule accumulations reported from the Cretaceous-Tertiary boundary layer. Given their loosely constrained age and the implied scale of the impact, the Ketilidian spherules could be distal ejecta from either the Vredefort, South Africa, (
c.
2025 Ma) or Sudbury, Canada, (
c.
1850 Ma) impacts.
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