Early Proterozoic collision tectonics, and rapakivi granites as intrusions in an extensional thrust-thickened crust: The Ketilidian orogen, South Greenland

Abstract

The Ketilidian orogen, that extends east-west across southern Greenland, formed in the period 1740–1850 Ma. In a north to south profile there are the following interpreted tectonic zones: a shelf-foredeep which has been thrust northwards to the foreland and back-folded and thrust southwards towards the suture, and which overlies Archean basement with Ketilidian granites with an appreciable crustal melt component; a suture with basaltic lavas, gabbros, shear belts, mylonites, copper and gold mineralisation; an Andean-type granitic batholith intruded into an island arc of volcanics, gneissose plutons and layered noritic gabbros; a steep back-arc shear belt of high-strain gneisses; and a flat-lying thrust stack of meta-supracrustals and paragneisses intruded by, largely crustal-melt, rapakivi granites. It is proposed that this profile is analagous to that from the Himalayas to Tibet, and that the rapakivi granites are equivalent to the Baltoro batholith in the Karakoram at the western uplifted end of the Tibetan plateau. Crustal thickening was achieved by thrusting, and ensuing thermal relaxation led to partial dehydration melting of deep continental crust. This provides a new model for the tectonic environment of rapakivi granites as intrusions in extensional zones within a collapsing thrust-thickened crust.