Detrital garnet petrology challenges Paleoproterozoic ultrahigh-pressure metamorphism in western Greenland

• Published in: European Journal of Mineralogy
• Main Authors: Schönig, Jan, Benner, Carsten, Meinhold, Guido, Eynatten, Hilmar, Lünsdorf, N. Keno
• Format: Text
• Language: English
• Published: 2023
• Subjects: Greenland; Nordre Strømfjord; Nordre strømfjord
• Online Access: https://doi.org/10.5194/ejm-35-479-2023; https://ejm.copernicus.org/articles/35/479/2023/

Modern-style plate tectonics is characterised by the global operation of cold and deep subduction involving blueschist facies and ultrahigh-pressure metamorphism. This has been a common process since the Neoproterozoic, but a couple of studies indicate similar processes were active in the Paleoproterozoic, at least on the local scale. Particularly conspicuous are extreme ultrahigh-pressure conditions of ∼ 7 GPa at thermal gradients < 150 ∘ C GPa −1 proposed for metamorphic rocks of the Nordre Strømfjord shear zone in the western part of the Paleoproterozoic Nagssugtoqidian Orogen of Greenland. By acquiring a large dataset of heavy minerals ( n = 52 130) and garnet major-element composition integrated with mineral inclusion analysis ( n =2669 ) from modern sands representing fresh and naturally mixed erosional material from the metamorphic rocks, we here intensely screened the area for potential occurrences of ultrahigh-pressure rocks and put constraints on the metamorphic evolution. Apart from the absence of any indications pointing to ultrahigh-pressure and low-temperature–high-pressure metamorphism, the results are well in accordance with a common Paleoproterozoic subduction–collision metamorphic evolution along a Barrovian-type intermediate temperature and pressure gradient with a pressure peak at the amphibolite–granulite–eclogite-facies transition and a temperature peak at medium- to high-pressure granulite-facies conditions. In addition, we discuss that all “evidence” for ultrahigh-pressure metamorphism proposed in the literature for rocks of this area is equivocal. Accordingly, the Nordre Strømfjord shear zone is not an example of modern-style plate tectonics in the Paleoproterozoic or of very low thermal gradients and extreme pressure conditions in general.