Evidence for 3650–3600 Ma assembly of the northern end of the Itsaq Gneiss Complex, Greenland: Implication for early Archaean tectonics

Abstract

The world's most extensive, well preserved, early Archaean (>3500 Ma) structures occur in the northern end of the 3850–3560 Ma Itsaq Gneiss Complex, southern west Greenland, an area which includes the 3800–3700 Ma Isua supracrustal belt. These provide a unique opportunity to study early tectonics. The area is dominated by ∼3800–3650 Ma orthogneisses containing migmatized inclusions of ≥3800 Ma metavolcanic, sedimentary, ultramafic, and gabbroic rocks. The thickest and most extensive of these inclusions forms the southern side of the Isua supracrustal belt. Also present are panels of less migmatized 3710–3690 Ma volcanic and sedimentary rocks, in the northern side of the Isua supracrustal belt, and probably also in the Tussaap supracrustal belt in the south of the area. Combined mapping and geochronology has identified mylonites which were active before regional early Archaean folding and 3580–3560 Ma metamorphism. Sensitive high‐resolution ion microprobe dating of premylonitization, synmylonitization, and postmylonitization granite sheets indicates some mylonites were active between 3650 and 3600 Ma. During this period it is proposed that large horizontal motions and interleaving occurred, results of which included juxtaposition of supracrustal sequences in the Isua supracrustal belt with different depositional ages. The tectonic interleaving was most likely due to horizontal motions in the vicinity of plate margins. The new combined field observations and geochronology support hypotheses that some form of plate tectonics at least contributed to early Archaean sialic crustal development.