UNRAVELING DATES THROUGH THE AGES - GEOCHRONOLOGY OF THE SCOTTISH METAMORPHIC COMPLEXES

The paper by Giletti et al. (1961) is seen as a major landmark in the evolution of dating techniques in polymetamorphic terrains. We conside r certain critical issues from each of the main complexes of the Scott ish Highlands studied by Giletti et al. to illustrate how subsequent d evelopments in geochronological methodology have influenced our unders tanding of metamorphic belts. Lewisian examples focus on the formation of Archaean crust, and the age of the main high-grade metamorphism an d the Scourie dyke swarm. The antiquity of Moinian sedimentation, its relationship to the Torridonian sandstones, and the timing of Precambr ian metamorphism have been controversial issues. The timing and nature of Caledonian orogenesis, most clearly expressed in the Dalradian com plex, have been the focal points for the refinement of radiometric inv estigation. These complexes have been subject to successive developmen ts in methodology, with ever-tighter constraints from Rb-Sr and K-Ar m ineral dating, through Rb-Sr and Pb-Pb whole-rock studies, U-Pb dating of bulk zircon fractions, and Sm-Nd whole-rock and mineral investigat ion, up to the latest technologies of single-grain zircon and ion micr oprobe analysis. The rocks have released their secrets reluctantly, an d many of the questions posed in 1961 have still not been definitively answered. However, the hope of unambiguous solution leads towards gre ater efforts, ever more reliable data, and a clearer evolutionary pict ure.