Late Ordovician regional high-pressure metamorphism in Scotland: Caledonian metamorphic climax predated the closure of Iapetus

We document newly recognised Late Ordovician high-pressure (HP) metamorphism in the Scottish Caledonides. Garnet growth at ca. 455–445 Ma within metabasic rocks from the Ross of Mull is constrained to pressures >0.9 GPa and was associated with formation of kyanite-bearing assemblages in meta-pelites that equilibrated at 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 post-dated Late Cambrian-Early Ordovician Grampian arc-continent collision but predated the final Silurian closure of the Iapetus Ocean, south of the Midland Valley. Similar garnet bearing amphibolites dated at ca. 455–445 Ma on the Scottish north coast and Shetland, suggest that this period of HP metamorphism was a regional feature across the Northern Highland Terrane and Shetland. This was followed by Scandian nappe stacking and lower pressure metamorphism at ca. 444–415 Ma, potentially forming a single protracted orogenic phase prior to the final closure of Iapetus. There are several potential drivers for this Late Ordovician event including: (1) subduction flip south of the Midland Valley Terrane to NW-directed subduction followed by collision of cryptic outboard terranes and/or Baltica, (2) continued SE directed subduction and collision of the Midland Valley terrane with Laurentia, or (3) subduction-flip followed by NW-directed flat slab subduction causing protracted accretionary orogenesis. Irrespective of the preferred tectonic model, the climax of Caledonian orogenesis in Scotland predated terminal continental collision.