Thermal granulite‐facies metamorphism with diffuse retrogression in Archaean orthogneisses, Fiskefjord, southern West Greenland

Abstract

Around Fiskefjord, southern West Greenland, Archaean amphibolite‐facies, granulite‐facies and retrograde orthogneisses occur in lithological and structural continuity with each other. The granulite‐facies rocks here—and elsewhere in West Greenland—are surrounded by extensive areas of retrograde gneisses. Both the prograde and retrograde metamorphism took place in a major event of continental crust formation c. 3000 Ma ago, which gave rise to granulite‐facies conditions in part of the rock complex exposed today. In the Fiskefjord area distributions of major and trace elements, as well as strontium and lead isotopes, show that the fades transformations were accompanied by pronounced metasomatism, and mineral chemistry indicates that the hydrous retrograde metamorphism took place under amphibolite‐facies conditions and was gradual and incomplete. The metamorphic and metasomatic processes in the Fiskefjord area are believed to have been controlled by heat from continuous intracrustal injection of large masses of tonalitic magma, which caused gradual dehydration and partial melting, followed by liberation of aqueous fluids during crystallization of anatectic melts. These fluids partially retrograded previously dehydrated gneisses. In contrast, South Indian high‐grade gneisses have mainly prograde amphibolite–granulite‐facies transitions which are distinct and well preserved, later than penetrative deformation, and are likely to have been controlled by CO2 streaming. These amphibolite–granulite‐facies transitions are reported to be near‐isochemical. It is suggested that there are (at least) two different kinds of granulite‐facies metamorphism: a near‐isochemical prograde type in stabilized tectonic environments, perhaps controlled by influx of CO2 (e.g. in South India) and significantly post‐dating original crust formation; and a fluid‐deficient type with widespread anatexis, hydrous retrogression and metasomatism, which takes place during accretion of continental crust, and in which heat is the governing factor (e.g. in southern West Greenland).