Widespread preservation of cumulative Paleoarchean peridotite bodies within the Eoarchean Itsaq Gneiss Complex, southwest Greenland

The Ujaragssuit Nunât layered intrusion, which is the largest layered intrusion within the Ujaragssuit Nunât complex (UNC) and is known for preserving the Earth’s oldest chromitites, was previously considered to have a minimum age of > 3.8 Ga. However, a recent ∼3.25 Ga age for chromitites from the Ujaragssuit Nunât layered intrusion challenges the previous interpretation that the intrusion predates the adjacent Eoarchean tonalities, raising the possibility that other ultramafic bodies in the UNC may also be younger than previously assumed. To investigate this, we conducted a thorough geochemical study of ultramafic rocks collected throughout the Ujaragssuit region, from both within and beyond the well-studied largest layered intrusion. Mineral and whole-rock geochemical data indicate that their compositions were predominantly controlled by the accumulation of olivine and orthopyroxene, along with trapped intercumulus melts. The Re-depletion Os model ages (T RD ), primarily obtained from dunites, cluster at ∼3.3 Ga. Meanwhile, the amphibole-free harzburgites and the majority of Cr-Al trend amphibole harzburgites display negligible Re addition yield consistent Re-Os model ages (T MA ) of ∼3.3 Ga. Taken together, the best age estimate for ultramafic rocks in the UNC is ∼3.3 Ga. In contrast, the significantly younger T RD ages (<2.3 Ga) and a ∼2.6 Ga Re-Os regression line obtained from amphibole harzburgites likely point to regional Neoarchean metamorphism, responsible for the formation of metamorphic chromites and metamorphic and/or metasomatic amphiboles. These findings demonstrate that the peridotite bodies and chromitites are not Eoarchean relics predating the ∼3.8 Ga gneiss. Instead, they represent pervasive mafic magma intrusions into existing continental crust.