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

The Saglek Block comprises the northern part of the Nain Province and underwent widespread metamorphism at c. 2.7 Ga, producing the dominant gneissosity and intercalation of supracrustal sequences. Zircon dating of gneiss samples collected along 80 km of the Labrador coast from Ramah Bay in the north to Hebron Fjord in the south confirm the wide spread extent of high-grade metamorphism between 2750 and 2700 Ma. In addition, a distinct event between 2550 and 2510 Ma produced felsic melt with peritectic garnet in metavolcanic gneiss and granoblastic recrystallisation in mafic granulite. Ductile deformation of granite emplaced at c. 2550 Ma indicates that this later event involved a degree of tectonism during high-T metamorphism. Such tectonism may be related to a hypothesised post-2.7 Ga juxtaposition of the predominantly Eoarchean Saglek Block against the Mesoarchean Hopedale Block, along a N–S boundary that extends from the coast near Nain to offshore of Saglek Bay. Evidence of reworking of c. 2.7 Ga gneisses by c. 2.5 Ga tectonothermal activity has been found elsewhere on the margins of the North Atlantic Craton (NAC), of which the Nain Province represents the western margin. In particular, a recent suggestion that c. 2.5 Ga metamorphic ages on the northern margin of the NAC in southwest Greenland indicate the time of the final assembly of the craton, may also apply to the western margin.