Sm–Nd dating and cooling history of Scourian granulites, Sutherland

Abstract

The Scourian gneisses of the Lewisian complex of north-west Scotland have been the subject of many geochronological investigations. Sm–Nd whole-rock measurements suggest that the protoliths of the Lewisian complex differentiated 2.92±0.05 Gyr ago from an approximately chrondritic mantle¹. Rb–Sr and Pb–Pb whole-rock ages between ∼2.8 and 2.6 Gyr reflect subsequent depletion in Rb and U (refs 2, 3) which has generally been associated with granulite facies metamorphism. Zircons which are thought to have crystallized during this metamorphism have also given U–Pb ages of 2.66 Gyr (ref. 4). This granulite facies metamorphism, involving pressures in excess of 10 kbar and peak temperatures estimated between 820±50 °C (ref. 5) and 1,250 °C (refs 6,7), affects a terrain including supracrustal rocks⁷ and thus implies a major tectonic event. We report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism. Such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in Sm/Nd ratios; garnet, especially, is strongly enriched in Sm (ref. 8). Data for three garnetiferous basic gneiss assemblages suggest that closure of the Sm–Nd system occurred significantly later than the peak of the metamorphism.