Reconstruction of the Grampian episode in Scotland: its place in the Caledonian Orogeny

A study of the composition of detrital garnets from Ordovician siliciclastics from the Midland Valley and Southern Uplands terranes of Scotland reveals characteristics of the: metamorphic sources very similar to the Grampian terrane plus a 'lost' blueschist-eclogite terrane. The radiometric ages of detrital muscovite from the Southern Uplands overlaps the ages of the metamorphic muscovite from the Grampian terrane. The depositional age of the earliest Midland Valley detritus is Llanvim; in the Southern Uplands it is Caradoc, These observations support the hypothesis that flysch was formed when the Grampian terrane was suddenly uplifted into mountains which were immediately eroded into neighboring basins and trenches. Thus the Grampian, Midland Valley and Southern Uplands terranes were not exotic to each other in the Upper Ordovician. Therefore the main compressional orogeny (i.e. Peri-Laurentian island are versus Laurentian continental margin collision) in the Grampian terrane in Scotland was post-Cambrian and began in the late Arenig. New radiometric age dates for Grampian terrane synmetamorphic granites and gabbros and post-metamorphic granites in Scotland and Ireland plus mineral cooling ages support the case for a relatively short <15 Ma catastrophic are-continent collisional orogeny between <similar to>480 and similar to 465 Ma. Thus the Caledonian Orogeny is an example of one which was episodic with a short catastrophic collisional tectonometamorphic and magmatic (Grampian) episode followed by a long period of isostatic adjustment, decompression melting, erosion, subduction and batholith formation that lasted 70 Ma. Orogeny stopped when Avalonia docked (i.e. the Scottish Scandian episode). (C) 2001 Elsevier Science B.V. All rights reserved.