Complexity of the early Archean Uivak Gneiss: Insights from Tigigakyuk Inlet, Saglek Block, Labrador, Canada and possible correlations with south West Greenland

Abstract

16The Saglek Block of Labrador comprises Eoarchean to Neoarchean lithologies, 17 metamorphosed at high temperature at ca. 2.7 Ga. Here, we investigate the gneisses 18 of Tigigakyuk Inlet, previously identified as the locality exposing the most ancient 19 rocks in the Saglek Block. New geochronological and geochemical results reveal a 20 multistage history. Precise magmatic emplacement ages of 3.75 to 3.71 Ga refine the 21 age of the Uivak Gneiss. Zircon rims and neoblastic grains with low Th/U record 22 metamorphism at ca. 3.6 and 2.8-2.7 Ga. Magmatism between these tectono-23 metamorphic events is recorded by the presence of meta-mafic dykes in the 24 gneisses, gabbroic enclaves in ca. 2.7 Ga syn-tectonic granitoids, as well as by a ca. 25 3.56 Ga age for monzonitic gneiss in which metamorphic zircon is present as 26 xenocrysts. Felsic (TTG) magmatism between ca. 3.75 Ga and 3.71 Ga, as well as 27 metamorphism at both ca. 3.6 Ga and 2.8-2.7 Ga, is also recognised in the Itsaq 28 Gneiss Complex of south West Greenland, and is restricted to the Faeringehavn 29 Terrane. Our new data enable a more rigorous correlation between these formerly 30 conjugate parts of the North Atlantic Craton. 2 31 32 3 64Uivak I and II Gneisses are tectonically juxtaposed and interleaved with supracrustal 65 assemblages, along with several generations of pegmatite (Bridgwater et al., 1975). 66

Gneiss-forming events in the Saglek Block, Labrador; a reappraisal of the Uivak gneiss

“…High-U rims scatter around ca. 3.6 Ga in age and correspond to previous estimates for the timing of high-T metamorphism in the area (Schiøtte et al 1989a;Sałacińska et al 2018).…”

“…Macro-scale tectonic lenses of the Nulliak supracrustal assemblage are intercalated with the Uivak gneisses, and both are cut by metamorphosed plagioclase-phyric mafic Saglek dykes (Bridgwater et al 1975;Collerson 1976, 1977). These units have been correlated with the Itsaq Gneiss Complex in southern West Greenland (Sałacińska et al 2018 and references therein), which is intruded by the plagioclase-phyric Ameralik dyke swarm (Nutman et al 2004b), a likely Saglek dyke correlative. The oldest recognized post-Saglek dyke orthogneiss from the Saglek Block is the felsic Paleoarchean Lister gneiss (Schiøtte et al 1989a).…”

“…The first precise age of the formation of Uivak I gneiss (3732 ± 6 Ma) was determined on zircon by Secondary Ion Mass Spectrometry (SIMS) by Schiøtte et al (1989a). Further high-precision SIMS zircon ages were provided by Kusiak et al (2018), Sałacińska et al (2018) and Vezinet et al (2018). Vezinet et al (2018) have identified a felsic gneiss with a protolith age of 3.86 ± 0.01 Ga. Isotopic U-Pb dating of zircon grains also identified a significant amount of older zircon, up to ca.…”

“…Early mapping (Bridgwater et al 1975;Hurst et al 1975;Collerson 1976, 1977) followed by geochronological studies (Collerson and Bridgwater 1979;Shiotte et al 1989a, b;Bridgwater and Schiøtte 1991) defined and dated major lithologies. More recently, a resurgence of interest in the Saglek Block has led to several detailed studies (Krogh and Kamo 2006;Shimojo et al 2016;Komiya et al 2017;Kusiak et al 2018;Sałacińska et al 2018) that revised earlier interpretations and produced new controversies (e.g. Tashiro et al 2017;Whitehouse et al 2019), especially regarding the oldest components of the block.…”

“…Tashiro et al 2017;Whitehouse et al 2019), especially regarding the oldest components of the block. Following the study of Sałacińska et al (2018) that advanced understanding of the age and composition of the Uivak I gneiss, this study focuses on orthogneisses previously identified as the younger, but poorly understood, Uivak II gneiss. We present geochemical and geochronological results that test various hypotheses concerning the formation of early-tomid-Archean gneissic protoliths.…”