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

Abstract

The Archean gneiss complex of the Saglek Block of Labrador is a part of the North Atlantic Craton, and is correlated with southern West Greenland, both being metamorphosed during a ca. 2.7 Ga event. The main component of the complex is the Eoarchean Uivak orthogneiss, which includes lenses of the Nulliak supracrustal assemblage. Both lithologies are cut by the mafic Saglek metadykes. The Uivak gneisses have been divided into Uivak I grey gneiss and Uivak II augen gneiss. The former underwent ca. 3.6 Ga high-T metamorphism prior to the intrusion of the latter. However, the exact age, nature, and extent of Uivak II gneiss are poorly understood. We present geochemical and geochronological results for both these orthogneisses to help refine the various hypotheses that have been proposed concerning the nature of their protoliths. Magmatic ages of 3746 ± 5 and 3717 ± 6 Ma are consistent with previous estimates for the age of Uivak I gneiss. Uivak II augen gneiss from Maidmonts Island, where there is a clear intrusive relationship between the Uivak II and Uivak I gneissic protoliths, has an age of 3325 ± 3 Ma. This is similar to an homogeneous grey gneiss from St. John's Harbour, with an age of 3318 ± 5 Ma. Grey gneiss from Big Island is distinctively younger (3219 ± 7 Ma), and equivalent to the ca. 3.24 Ga Lister gneiss. Our study shows that granitic gneisses classified as Uivak II were emplaced 200-300 million years after ca. 3.6 Ga metamorphism and deformation of the Uivak I gneiss. The igneous protolith of Uivak II gneiss pre-dates the Lister gneiss by about 100 Ma. The Uivak I and Lister gneisses are geochemically similar, and are both Tonalite-Trondhjemite-Granodiorite (TTG) gneisses, whereas the Uivak II gneiss is a granitoid partially derived from pre-existing crust. We propose abandoning the term 'Uivak II gneiss', and renaming ca. 3.3 Ga granitoids, after the type locality, as Maidmonts gneiss. This restricts the term 'Uivak gneiss' to Eoarchean TTG gneisses and removes the necessity for subdividing them into Uivak I and II.

Two Neoarchean tectonothermal events on the western edge of the North Atlantic Craton, as revealed by SIMS dating of the Saglek Block, Nain Province, Labrador

“…A section of the Saglek Block investigated in recent studies by the same group (Kusiak et al 2018;Sałacinśka et al 2018Sałacinśka et al , 2019Whitehouse et al 2019) extends for 80 km along the Labrador coast, from Ramah Bay in the north to Hebron Fjord in the south (Fig. 1b).…”

“…1b). Gneisses in the southern part of the section formed in several stages throughout the Archean, with the oldest and most abundant TTG-type protoliths (the Uivak gneiss) formed between c. 3850 and 3600 Ma (Schiøtte et al 1989;Komiya et al 2017;Kusiak et al 2018;Sałacinśka et al 2018Sałacinśka et al , 2019, with lesser episodes of felsic plutonism at c. 3.3-3.0, 2.7 and 2.5 Ga (Schiøtte et al 1990(Schiøtte et al , 1992Krogh & Kamo 2006;Komiya et al 2015;Sałacinśka et al 2019). Krogh & Kamo (2006) suggested that TTG gneisses from outcrops around Saglek Bay differ in age across the Handy Fault (Fig.…”

“…1), with c. 3.6 Ga protoliths to the west and c. 3.3 Ga to the east. However, recent dating (Komiya et al 2015;Kusiak et al 2018;Sałacinśka et al 2018Sałacinśka et al , 2019 revealed complicated and tectonized relationships between Eoarchean and younger TTG gneisses on both sides of the fault. Metamorphosed supracrustal assemblages of sedimentary and volcanic rocks, with associated ultramafic gneisses, occur as discontinuous layers within gneissosity typically less than 100 m thick, and have been divided into sparsely distributed pre-or syn-Uivak supracrustal rocks (the Nulliak assemblage) and post-Uivak Meso-to Neoarchean Upernavik supracrustal rocks (Bridgwater & Schiøtte 1991).…”

“…It is unknown whether D 2 significantly postdates D 1 , but because it transposes D 1 fabrics in late Mesoarchean to early Neoarchean Upernavik supracrustal rocks, as well as older TTG gneisses, and no intrusive rocks separate D 1 and D 2 , it is likely that they represent stages of a single tectonothermal event. Dating of gneisses with Eoarchean protoliths has established an additional, much older episode of high-grade metamorphism at c. 3.6 Ga (Sałacinśka et al 2018(Sałacinśka et al , 2019; however, no large-scale structures have been distinguished in the field that relate to this earlier event. In outcrops c. 100 km to the south of Saglek Bay, van Kranendonk & Helmstaedt (1990) described ductile thrusting (F 0 ) in the Upernavik supracrustal rocks and recumbent folding (F n+1 ) in both supracrustal rocks and TTG gneisses during high-T, high-P metamorphism.…”

Occurrence and chemical composition of the Eoarchean carbonate rocks of the Nulliak supracrustal rocks in the Saglek Block of northeastern Labrador, Canada

“…Thus, even if there is no clear evidence that the supracrustal rocks were intruded by the Saglek dyke, the supracrustal rocks intruded by the Uivak Gneiss can be identified as the Nulliak supracrustal rocks. The Uivak Gneiss comprises polygenetic orthogneiss with different magmatic ages of their protoliths from 3.9 to 3.6 Ga, and with composition from tonalite to iron-rich monzodiorite (Collerson, 1983;Komiya et al, 2017;Krogh & Kamo, 2006;Kusiak et al, 2018;Regelous & Collerson, 1996;Sałaci nska et al, 2018;Sałaci nska et al, 2019;Schiøtte et al, 1989aSchiøtte et al, , 1989bShimojo et al, 2016;Vezinet et al, 2018;Wanless, Bridgwater, & Collerson, 1979;Wasteneys, Wardle, & Krogh, 1996). Although the presence of the pre-3.9 Ga gneiss in the Saglek Block is still controversial , it is widely accepted that the protoliths of the Uivak Gneiss were formed in the Eoarchean (> 3.6 Ga).…”