Long-lasting formation and metamorphic alteration of Paleoarchean (~3.3 Ga) peridotite bodies within the Eoarchean Itsaq Gneiss Complex, southwest Greenland

Ancient terrestrial rocks are pivotal for deciphering the evolution of early Earth. The Ujaragssuit Nunât ultramafic complex (UNUC), found as enclaves within the best-preserved and largest Eoarchean tract, the Itsaq Gneiss Complex, is known for preserving the Earth’s oldest chromitites. This has attracted extensive scientific attention, for instance providing unique Hadean mantle records. Yet, the associated peridotites have received comparatively less attention. Here, we provide detailed petrographic observations and integrated mineral and whole-rock geochemical analyses to clarify the petrogenesis and post-magmatic processes of these peridotites. An in-depth examination of mineral and whole-rock geochemical compositions reveals the critical role of olivine and minor orthopyroxene as cumulus minerals in controlling their chemical compositions. The pronounced enrichment of light rare earth element (LREE) relative to heavy REE (HREE) is associated with the intercumulus tholeiitic parental melt after fractional crystallization. The Re-depletion Os model ages (TRD), primarily derived from chromitites, dunites and amphibole-free harzburgites, are clustered around 3.3 Ga, suggesting this as the formation age of the UNUC. In contrast, the much younger TRD ages (<2.3 Ga) observed in amphibole harzburgites are likely a consequence of pervasive regional metamorphism, coinciding with the formation of low-pressure metamorphic amphiboles. Furthermore, the UNUC cumulates are estimated to derive from a HSE-poor mantle source that has received 41±5% of full late accretion recorded in modern mantle. Further work on mass-independent isotopes is expected to elucidate the origin of the accretionary materials.