Detrital garnet petrology challenges Paleoproterozoic UHP metamorphism in Western Greenland

The global onset and evolution of subduction-driven plate tectonics is one of the most debated topics in Earth sciences. Although very diverging views are hold, most observations indicate a transition from a stagnant- to a mobile-lid regime in the late Archean (e.g. Palin et al. 2020). Since then, geothermal gradients (T/P) of metamorphic rocks decreased, interpreted as an evolution from warm and shallow to cold and steep subduction (e.g. Brown et al. 2006), which may explain the oldest unequivocal evidence of UHP metamorphism at ~620 Ma (Jahn et al. 2001). By contrast, extreme UHP conditions of ~7 GPa at ~975 °C have been supposed for ~1.8 Ga crustal rocks in Western Greenland, mainly based on carbonaceous inclusions in garnet interpreted as diamond partially replaced by graphite as well as oriented inclusions of a hydrous phase interpreted as orthopyroxene exsolution from a majoritic precursor (Glassley et al. 2014).

In order to find mineralogical evidence for UHP metamorphism, like coesite, we used a detrital approach that has been demonstrated to be very powerful (Schönig et al. 2018, 2019, 2020; Baldwin et al. 2021). Modern sands from eight catchments draining the proposed UHP terrane in Western Greenland were extensively screened by semi-automated Raman heavy-mineral analysis (n = 52,130 grains) and electron microprobe analysis of garnet major-element chemistry as well as hyperspectral Raman imaging (>680 million spectra) of mineral-inclusion assemblages (n = 2,674 garnets). In all samples, amphibole, garnet, orthopyroxene, augitic clinopyroxene, and sillimanite represent the majority of heavy minerals, reflecting erosional material sourced from amphibolite- to granulite-facies rocks. Garnet chemistry and mineral inclusion assemblages, particularly the common co-existence of sillimanite and rutile inclusions, indicate a major garnet growth stage at MP to HP granulite-facies conditions. Though, lower garnet X_Mg and/or higher X_Mn and/or X_Ca as well as a more frequent occurrence of hydrous mineral inclusions (amphibole and phlogopite-biotite) of a smaller proportion (~20 %) imply garnet growth at lower temperature conditions, interpreted as relicts of prograde metamorphism. Garnets predicted to be grown at the highest P conditions (~6 %) commonly host inclusions of augitic clinopyroxene, amphibole, plagioclase, and quartz, mainly indicating HP amphibolite facies conditions that in maximum may have reached the transition zone between amphibolite- and eclogite-facies conditions. Furthermore, we show that neither the reported existence of diamond nor the interpretation of a majoritic precursor hold against a critical re-assessment. Overall, the total absence of minerals indicating UHP conditions (like coesite and diamond) and even HP conditions (like omphacite or glaucophane) in our large detrital dataset as well as alternative interpretations for reported UHP indicators strongly challenge the existence of a Paleoproterozoic UHP terrane in Western Greenland.