Some observations on the structural and metamorphic chronology on Agto and surrounding islands, central West Greenland

Abstract

The report concerns an area situated in granulite facies terrain in the Precambrian Nagssugtoqidian orogenic belt (K/Ar age ca. 1700 m.y.) of central and northern West Greenland. The acid rocks are biotite gneisses, biotite-hypersthene gneisses, granitic gneisses, granites and sillimanite- and garnet-bearing gneisses. The basic rocks are amphibolites, pyroxene amphibolites and their garnetbearing equivalents. The basic rocks can be divided into an old, concordant group and a younger, discordant group. In the area mapped four structural complexes characterised by different orientations of axial planes related to deformations of different times have been mapped. The youngest structures have ENE-striking axial planes - the typical Nagssugtoqidian trend. The discordant metabasites are younger than at least two phases of folding and are preserved in the structural complex characterised by the oldest structures. They cut migmatitic structures and thus they are younger than at least one period of high grade metamorphism. The discordant metabasites are older than the youngest deformation, that which resulted in folds with ENE-striking axial planes, i. e. they are older than the typical Nagssugtoqidian structures. They are older than one period of high grade metamorphism which converted the basic dykes into pyroxene amphibolites. This period of metamorphism is at least partially contemporaneous with the Nagssugtoqidian deformation. The fieId occurrence of the discordant metabasites shows that they intruded into rocks affected by tensional stresses and the author proposes that the intrusion of the basic dykes marks the interval between two orogenic episodes, one Nagssugtoqidian and the other pre-Nagssugtoqidian. If this assumption is correct, all the rocks of the area are pre-Nagssugtoqidian. The minimum temperature during the peak of metamorphism is evaluated using evidence of anatexis and the minimum pressure from the absence of cordierite.