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

Modern-style plate tectonics is characterized by the global operation of deep and cold subduction involving ultrahigh-pressure and blueschist-facies metamorphism. This is a common process since the Neoproterozoic, but a couple of studies indicate similar processes have been 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 proposed for metamorphic rocks of the Nordre Strømfjord shear zone in the western part of the Paleoproterozoic Nagssugtoqidian Orogen of Greenland (Glassley et al., 2014). By acquiring a large dataset of heavy minerals (n = 52,130) and garnet major-element composition integrated with mineral inclusion analysis (n = 2,669) 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/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. This is in strong contrast to the proposed ultrahigh-pressure conditions and low geothermal gradients, and challenges the existence of a Paleoproterozoic modern-style plate-tectonic regime in western Greenland.