Metamorphic record of accretionary processes in the Neoarchaean

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In collaboration with the Geological Survey of Denmark and Greenland, this project aims to investigate the metamorphic record of the processes of Neoarchaean terrane accretion in the Nuuk region of southern West Greenland. The region is one of a few windows into Archaean mid- to lower continental crust, and therefore provides an ideal site to study the processes that formed the early continental crust. Terrane amalgamation in the Nuuk region was associated with extensive high-grade metamorphism and thrusting of the Tre Brødre and Tasiusarsuaq terranes on top of the Færingehavn. Preliminary work to this project indicates that these terranes constitute a Neoarchaean paired metamorphic belt, including (1) a lower plate (Færingehavn terrane) containing relatively high pressure low temperature metamorphic assemblages, and (2) an upper plate (Tre Brødre and Tasiusarsuaq terranes), dominated by granulite facies conditions along a distinctly hotter apparent geothermal gradient. Thus, the data support the view that the formation of Neoarchaean continental crust occurred along convergence-related magmatic arcs. Moreover, the available geochronology and geochemical data indicate that the upper plate preserves a prolonged and episodic record of subduction-related igneous activity that lasted for ca. 250 million years, suggesting that Archaean subduction was associated with repeated high temperature metamorphic events. Despite this, crustal convergence apparently did not result in the deep subduction of the lower plate, implying that Archaean convergence rates were much slower than today, or that subduction was intermittent and associated with a more frequent slab breakoff or rollback. This project aims to further investigate the metamorphic record and thermal structure of the upper plate, focusing on the orogen hinterland and back-arc region of the orogen. This will involve the reconstruction of the P-T-t-d record of different areas by means of pseudosection modelling and U-Pb dating. The overall goal is to better assess the style of convergence in the Nuuk region, and to clarify whether the thermal structure of the upper plate is consistent with a model of intermittent subduction.