Discussion of Torridonian tourmaline-quartz pebbles and the Precambrian crust northwest of Britain

Abstract

M r A. D. G ibbs writes: Allen et al. suggest that tourmaline-quartz pebbles in the Torridonian of Scotland may be used to reconstruct contemporary erosion levels in the Precambrian rocks to the northwest. The authors state that the pebbles are indicators of granite intrusions in the pre-Ketilidian block at a level much higher than the present erosion surface. In the course of my mapping in the Buksefjorden region to the south of Godthab, West Greenland, (Ghadwick & Coe 1972) radiating tourmaline aggregates up to 0.7 m in diameter in quartz segregations have been found along the margins of pegmatites in Malene supracrustal sillimanite gneisses and amphibolites. The pegmatites are associated with the roof zone of the Qprqut granite, which is a high level sheeted complex. The Qprqut granite intrudes amphibolite facies gneisses of Amitsoq and Nuk types (McGregor 1973) with ages around 3.7 b.y. and 3.2 b.y. respectively (Black et al. 1971). The granite complex gives an age around 2.6 b.y. (unpublished age quoted in Black et al. 1973), i.e. within the range quoted by Allen et al. for the Torridonian pebbles. The erosion of the roof zones of granites such as the Qorqut at or near the present level of erosion could easily provide a source of quartz-tourmaline pebbles such as those described by the authors. I acknowledge the assistance of K. Ellitsgaard-Rasmussen of the Greenland Geological Survey. P rofessor J. S utton replied on behalf of the authors: Mr Gibbs suggestion is most welcome. It will be interesting to see if