Granite magmatism and extensional tectonics in Southern Greenland

Abstract

AbstractThe southernmost part of the early Proterozoic Ketilidian mobile belt (1·9–1·7 Ga) of southern Greenland comprises a thick series of largely flat‐lying migmatitic metasediments and metavolcanics of upper amphibolite to lower granulite facies. Regional tectonics are extensional and there is little structural evidence for earlier crustal thickening. Several episodes of granite magmatism are recorded, culminating in the late‐ to post‐orogenic rapakivi granites, which are associated with with a coeval suite of norites. These were emplaced at ca. 1740 Ma, roughly synchronously with the peak of regional metamorphism (2–3·5 kbar, 650–800°C). The rapakivi granites have been emplaced into major crustal‐scale extensional shear zones and have ramp‐flat geometries. ‘Black’ and ‘white’ rapakivi granites are found. The mineralogy of the black granites is dominated by orthopyroxene–amphibole assemblages with rare fayalite. In the white granites, biotite and amphibole are the only Fe–Mg silicates. All Fe–Mg silicates have high Fe/(Fe+Mg) ratios. In terms of bulk chemistry, the suite is bimodal. Both norites and granites are strongly fractionated and have Fe/(Fe+Mg) ratios of 0·50–0·65 (norites) and 0·65–0·90 (granites). REE modelling shows that the granites can be derived from 40 per cent partial melting of a metasedimentary protolith. The sequence of magmatic events in this region reflects the progressive metamorphism and dehydration melting of juvenile sediments in an extending basin. The heat source for melting may have come from the underplating of olivine tholeiite melts generated during lithospheric extension.