Discussion of ‘Metamorphic P–T and retrograde path of high-pressure Barrovian metamorphic

First, we would like to commend Aoki et al. (2013) on a careful study and thought-provoking manuscript. Their inter-pretation of the Barrovian metamorphism as a fundamentally retrograde feature offers a refreshing alternative to the more conventional ‘peak-metamorphic ’ models. Aoki et al. (2013) suggested that high-grade rocks from the eastern Grampian Terrane, Scotland, sig-nify high-pressure/high-temperature (1.2–1.4 GPa, 770– 800 °C) metamorphism during the early stages of the Grampian Orogeny. These granulite-facies rocks were over-printed during the Grampian Orogeny by the amphibolite-facies Barrovian metamorphism. Here, with consideration of time scales and heat sources available for metamorphism dur-ing the Grampian Orogeny, we argue that an early Grampian high-temperature metamorphism is highly implausible. The results of structural and geochronological work from the liter-ature provide support for another interpretation of the origins of the Aoki et al. (2013) gneisses: rather than forming during the Grampian Orogeny, they represent Precambrian basement to the Dalradian Supergroup. We present a new latest Meso-proterozoic (1003 ± 6 Ma) U–Pb age for migmatization of the Cowhythe Gneiss, which corroborates the Precambrian basement model for some high-grade metamorphic rocks of the eastern Grampian Terrane. 1. Results and interpretations of Aoki et al. (2013) and background information The study of Aoki et al. (2013) focused on garnetite lenses within garnet amphibolites from the region of Cairn Leuchan, above Glen Muick, in the Grampian Highlands of Scot-land (location shown on Fig. 1). These rocks experienced a high-pressure granulite-facies (HGR) metamorphism and later amphibolite-facies (AM) metamorphism (Aoki et al