The Ketilidian orogen of South Greenland: geochronology tectonics, magmatism, and fore-arc accretion during Palaeoproterozoic oblique convergence

摘要

Recent fieldwork, geochemistry, and U-Pb geochronology in the Palaeoproterozoic Ketilidian orogen provide substantial insights into the timing and mechanisms of its magmatic, sedimentary, and tectonic accretion. From north to sooth 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 continent-continent 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 1895-1870 Ma dextral transpression in the Makkovik orogen, Labrador Sinistral transpression and I-type granite emplacement followed at 1848-1805 Ma. In the northeast, limited geochronology indicates deformation and metamorphism at ca. 1800 Ma. The calc-alkaline Julianehab batholith was largely emplaced between 1854-1795 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 (HT-LP) metamorphism and anatexis occurred at 1795-1785 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 1755-1732 Ma and folded into broad arches and narrow synclinal cusps compatible with late-stage sinistral transpression.