Eclogites and basement terrane tectonics in the northern arm of the Grenville orogen, NW Scotland

The presence of eclogites within continental crust is a key indicator of collisional orogenesis and they have been used worldwide to assist in the delineation of ancient collisional sutures. Eclogites within the Eastern Glenelg basement inlier of the Northern Highland Terrane (NHT) have been re-dated in order to provide more accurate constraints on the timing of collision within the northern arm of the Grenville Orogen. The eclogites yield dates of c.1200 Ma which are interpreted to record the onset of convergence, and the NHT as a whole is thought to represent the lower plate in successive 1200-1000 Ma collision events. The Eastern Glenelg basement inlier is viewed as a fragment of the leading edge of the NHT basement that was partially subducted along a suture and then obducted back up the subduction channel. Differences in ages of igneous protoliths and intrusive histories (Storey et al., 2010; Strachan et al., 2020b), and metamorphic events (this paper) between the NHT basement and the Laurentian foreland, suggests that they were separate crustal blocks until after c. 1600 Ma. We therefore suggest that: 1) the NHT represents a fragment of Archean-Paleoproterozoic crust that was reworked within the c. 1600-1700 Ga Labradorian-Gothian belt, although whether it was derived from Laurentia or Baltica (Strachan et al., 2020b) is uncertain (Fig. 3), and 2) amalgamation of the NHT with the Laurentian foreland did not occur until the terminal stages of the Grenville collision at c. 1000 Ma.