Over the past several years, an expanding dataset of U-Pb isotopic ages from zircon and monazite in the gneisses of the Saglek Block, Labrador have confirmed a predominance of Eoarchean granitoid (namely tonalite-trondhjemite-granodiorite or TTG) protoliths that broadly correspond to those in the more extensively studied Itsaq Gneiss Complex of southwest Greenland [1]. Whereas some studies have pointed to continuous crustal growth from 3.9 to 3.6 Ga, beginning with relics of supracrustals that predate the oldest granitoid crust, investigation of a large number of samples along a 100 km coastal section, involving careful statistical handling of complex ion microprobe data and interpretation of growth modification features in zircon grains, instead indicate an episodic evolution. A few felsic orthogneisses with protolith ages scattered between 3860 and 3770 Ma have been identified in several places, but with no evidence of continuity beyond the outcrop scale. These are subsumed by a widespread generation of 3750-3700 Ma Uivak TTG gneisses. Both generations of TTG crust have been recycled into ca 3.6 Ga granite sensu stricto and trondhjemite during a period that culminated in high-grade metamorphism at the end of the Eoarchean. Local discontinuous tectonic enclaves of mafic to felsic supracrustal rocks have been interpreted as predating the Uivak gneiss as far back as >3.9 Ma, but instead represent much younger metasediments [2] and felsic metavolcanics with ages either similar to or younger than protoliths of the Uivak gneiss. The majority are tectonic enclaves without clear contacts with TTG gneiss, and in places where contacts are observed the intrusive phase is not proven to be Eoarchean. Occurrences of ca 3.55 Ga dioritic and felsic metavolcanics indicate a change in magmatism following a major tectono-metamorphic event at 3.6 Ga that is also recognised in West Greenland (Nutman, 2018). Following this was a half-billion year hiatus in magmatic activity, with the exception of an unknown number of dolerite dykes so commonly seen after the cratonisation of Archean crust, and which are likely belong to the ca. 3.4 Ga Ameralik swarm in Greenland. The dominant generation of Uivak Gneiss is sufficiently well defined to distinguish it in age from TTG protoliths in the Isukasia and Færinghavn Terranes of the Itsaq Gneiss, at 3.6 Ga. Although the tectonic nature of the 3.6 Ga event is obscured by subsequent tectonometamorphism in the Saglek Block [3], the wide distribution of of metamorphic zircon raises the possibility of a mobile-lid tectonic regime that brought togther distinct pieces of continental crust and which culminated in orogenesis and cratonisation at the end of the Eoarchean.

Reference(s)

[1] Nutman A. (2018) in Kranendonk et al. (eds) Earth’s Oldest Rocks, 2nd edition; 

[2] Whitehouse et al. (2019) Precambrian Res. 70-81.; 

[3] Dunkley et al. (2020) J. Geol. Soc., jgs2018-153.