Unravelling dates through the ages: geochronology of the Scottish metamorphic complexes

Abstract

AbstractThe paper by Giletti et al. (1961) is seen as a major landmark in the evolution of dating techniques in polymetamorphic terrains. We consider certain critical issues from each of the main complexes of the Scottish Highlands studied by Giletti et al. to illustrate how subsequent developments in geochronological methodology have influenced our understanding of metamorphic belts. Lewisian examples focus on the formation of Archaean crust, and the age of the main high-grade metamorphism and the Scourie dyke swarm. The antiquity of Moinian sedimentation, its relationship to the Torridonian sandstones, and the timing of Precambrian metamorphism have been controversial issues. The timing and nature of Caledonian orogenesis, most clearly expressed in the Dalradian complex, have been the focal points for the refinement of radiometric investigation. These complexes have been subject to successive developments in methodology, with ever-tighter constraints from Rb-Sr and K-Ar mineral dating, through Rb-Sr and Pb-Pb whole-rock studies, U-Pb dating of bulk zircon fractions, and Sm-Nd whole-rock and mineral investigation, up to the latest technologies of single-grain zircon and ion microprobe analysis. The rocks have released their secrets reluctantly, and many of the questions posed in 1961 have still not been definitively answered. However, the hope of unambiguous solution leads towards greater efforts, ever more reliable data, and a clearer evolutionary picture.