LATE ORDOVICIAN HIGH-PRESSURE REGIONAL METAMORPHISM IN SCOTLAND: CALEDONIAN OROGENESIS PREDATED THE CLOSURE OF IAPETUS

We document newly recognised Late Ordovician high-pressure (HP) metamorphism in the Scottish Caledonides. Garnet growth at ca. 450-445 Ma within metabasic rocks was associated with formation of kyanite-bearing assemblages in meta-pelites and peak conditions of 1.0–1.2 GPa and 700–780°C. This requires the burial of rock to ~40 km depth driven by crustal shortening and thickening that followed Late Cambrian-Early Ordovician Grampian arc-continent collision but predated the final Silurian closure of the Iapetus Ocean, south of the Midland Valley. There are several potential drivers for this regional HP metamorphic event including: (1) continued collision of the Midland Valley terrane with Laurentia, (2) subduction flip followed by collision of outboard terranes, or (3) subduction flip followed by flat slab subduction. This latter scenario could be comparable to the Sevier and Laramide Orogenies in the North American Cordillera. Irrespective of the preferred tectonic model, the HP metamorphism at ca. 450-445 Ma was followed by Scandian nappe stacking and lower pressure metamorphism at ca. 444-415 Ma forming a single protracted orogenic phase prior to the final closure of Iapetus. Therefore, terminal continental collision need not be the only significant driver of orogenesis.