The Archaean Thrym Complex, exposed in southeastern Greenland, is part of the North Atlantic Craton. Its southernmost part is in a transitional zone containing Archaean orthogneiss to the north, and Palaeoproterozoic paragneiss and granitic rocks to the south that are part of the Ketilidian Orogen. This 50 km wide transitional zone includes ca. 2830 Ma orthogneiss that is similar in age to orthogneiss in the central and northern parts of the complex, and ca. 2740 Ma orthogneiss that is synchronous with the ca. 2790–2700 Ma Skjoldungen Orogeny. A significant difference between this zone and the complex to the north is the presence of greenschist-facies sedimentary rocks unconformably overlying orthogneiss and intruded by ca. 1820–1800 Ma granitic and mafic to intermediate intrusions. These intrusions are within the ca. 1854–1799 Ma age-range of the Julianehåb Igneous Complex, with suites, both older and younger than ca. 1830 Ma, that are not restricted to specific zones within the Ketilidian Orogen. The geochemistry of the Palaeoproterozoic mafic to intermediate rocks in the transitional zone shows contamination by orthogneiss from the Thrym Complex, and emplacement within an oceanic-continental transition in a within-plate setting unrelated to subduction. In contrast, the Palaeoproterozoic granites were sourced entirely from Archaean orthogneiss of the Thrym Complex, making them technically I-type granites that have inherited their petrogenetic signature from the orthogneiss. As a result, the Palaeoproterozoic granites and intermediate to mafic igneous rocks have similar chondrite-normalised patterns to the orthogneiss in the Archaean complex. This demonstrates that discrimination diagrams cannot be assumed to be unequivocal for the rocks studied unless the geochemistry of the source rocks is also known.
- Programområde 4: Mineralske råstoffer