Uivak II augen gneiss from the Saglek Block, Labrador: the current state of play

Abstract

Interpreting Archean geology is often challenging due to the rocks having obscure field relationships and polymetamorphic histories (Kusiak et al. 2019; Dunkley et al. 2020). In such circumstances, U-Pb isotopic analysis of zircon is crucial for revealing the geological history. This study investigates Archean gneisses from the Saglek Block in Canada, which record magmatic and metamorphic history between ca 3.9 Ga and 2.5 Ga. The predominant lithology is the Uivak gneiss which is primarily composed of tonalite-trondhjemite-granodiorite (TTG) with subordinate intermediate to mafic components. Uivak gneiss is traditionally divided into Uivak I and Uivak II, where Uivak I is grey gneiss and Uivak II is characterized by augen texture and Fe-rich geochemistry (Collerson and Bridgwater, 1979). Ages for the magmatic protoliths of Uivak I are >3.6 Ga, whereas Uivak II ages vary between ca 3.6-3.3 Ga (Sałacińska et al. 2019; Wasilewski et al. 2021 and references therein). This study presents geochemical and U-Pb zircon geochronology from Mentzel and Maidmonts Islands. Augen gneiss on Mentzel Island fits the definition of Uivak II augen gneiss and yield a U-Pb zircon age of ca 3.3 Ga. A similar age was reported for Maidmonts gneiss (Sałacińska et al. 2019) and Illuilik gneiss (Wasilewski et al. 2021). On Mentzel Island, granitic bodies intruded the augen gneiss at ca 2.7 Ga and 2.5 Ga during high-T metamorphism. New dating confirms that augen gneiss on Mentzel Island and elsewhere in the Saglek Block belongs to Uivak II gneisses of ca 3.3 Ga. Variations in rare earth element concentration between different ca 3.3 Ga rocks can be attributed to the involvement of different crustal components in the magmatic protolith. On Maidmonts Island, the augen gneiss intrudes grey gneiss with a protolith age of ca 3.7 Ga, which confirms deformation and metamorphism of Uivak I gneiss before ca 3.3 Ga. This research was funded by NCN grants UMO2019/34/H/ST10/00619 to MAK.