The Qârusuk dykes: post-Nûk, pre-Qôrqut granitoid magmatism in the Godthåb region, southern West Greenland

Abstract

The Qarusuk dykes are minor intrusions of granitic and trondhjemitic gneiss emplaced in the Godthåb region between 2700 and 2600 Ma ago. They post-date the Nuk gneisses and their subsequent regional deformation and metamorphism but pre-date the essentiaUy post-tectonic Qôrqut granite complex. They are associated in time and commonly in place with intense ductile deformation concentrated in distinct belts along which there was considerable movement of fluids accompanied by retrogression of earlier mineral assemblages. Outside the Godthåb district dykes of similar character post-date the regional c. 2800 Ma granulite facies metamorphism. The dykes are siliceous with low Fet + Mg0 and high Na20 + K20. Variation in Na2O/K2O is at least in part attributed to exchange between magma and percolating fluids. Non-magmatic controls are thought to affect trace element (including REE) and isotopic compositions. The overall chemical character is consistent with derivation by partial melting of sialic rocks associated with the influx of hydrous fluids associated with shear belt formation.