The Mesoarchaean Tasiusarsuaq terrane of southern West Greenland consists of Tonalite-trondhjemite-granodiorite gneisses and, locally, polymetamorphic mafic and ultramafic rocks. The terrane experienced medium-pressure granulite facies conditions during M1A in the Neoarchean, resulting in the development of two-pyroxene melanosome assemblages in mafic granulites containing garnet-bearing leucosome. Reworking of these rocks during retrogression introduced garnet to the melanosome in the form of overgrowths, coronas and grain necklaces that separate the mafic minerals from plagioclase. NCFMASHTO pseudosection modelling constrains the peak metamorphism during M1A to ~850°C and 7.5kbar at fluid-saturated conditions. Following M 1A, the rocks retained their M1A H2O content and became fluid-undersaturated as they underwent near-isobaric cooling to ~700°C and 6.5-7kbar, prior to reworking during M1B. These low H2O contents allowed for the formation of garnet overgrowths and coronas during M1B. The stability of garnet is greatly increased to lower pressure and temperature in fluid-absent, fluid-undersaturated mafic rocks, indicating that fluid and melt loss during initial granulite facies metamorphism is essential for the introduction of garnet, and the formation of garnet coronas, during retrogression. The occurrence of garnet coronas is consistent with, but not unique to, near-isobaric cooling paths.
- Garnet coronas
- Mafic granulites
- Pseudosection modelling
- Southern West Greenland
- Programme Area 4: Mineral Resources