Palaeoproterozoic oblique plate convergence in South Greenland: A reappraisal of the Ketilidian Orogen

New field and petrographic data are presented for a revised four-fold division of the Ketilidian Orogen into a Border Zone in the north, followed to the south by the Julianehåb batholith, the Psammite Zone and the Pelite Zone, as the basis for a new plate tectonic model. The Border Zone within the southern margin of the Archaean foreland contains shallow marine, volcano-sedimentary basins, granites and appinite dykes which are regarded as part of an incipient backarc. The wedge-shaped Julianehåb batholith comprises polyphase calc-alkaline granites, granodiorites, tonalites and hornblende diorites with swarms of appinite dykes. The batholith was emplaced during sinistral transpression as a result of oblique convergence between the Archaean continental plate to the north and an oceanic plate (no longer preserved) to the south. New Rb-Sr isotopic data from the southern part of the batholith confirm previous views that it consists of juvenile Palaeoproterozoic crust. The batholith is interpreted as the root of a volcanic arc that was the provenance of, and in part the basement to, polymict conglomerates, arkosic arenites, semi-pelites and pelites in the Psammite and Pelite Zones, deposited in intra-arc basins and the inner part of a forearc between the batholith and an ocean to the south. The sedimentary rocks and their subordinate intercalations of basalt and andesite were intensely migmatized and deformed in high temperature-low pressure conditions prior to the emplacement of broadly concordant sheets of granites and related rocks of the rapakivi suite. Appinite dykes were emplaced at various stages of the migmatization and deformation. We attribute the thermal effects which characterize the Ketilidian oblique convergent system to voluminous basaltic underplating in accord with recent work on the rapakivi suite and associated metamorphism.