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 has been previously reported from across the Northern Highland Terrane (NHT) and Shetland, yet the metamorphic conditions are unknown and the tectonic drivers associated with this event are controversial. Here we show that garnets dated at ca. 449 Ma within metabasic rocks on the Ross of Mull (NHT) grew at pressures >0.9 GPa, associated with formation of kyanite-bearing assemblages in meta-pelites that equilibrated at peak conditions of 1.0–1.3 GPa and 740–780°C. This requires the burial of rock to ~40–45 km depth, and the addition of this (now removed) overburden suggests that the crust reached ~70 km in thickness, enough to support a region of high topography ~6km in elevation. The timing of this crustal thickening episode 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. The presence of 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. It 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 the 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 followed by large magnitude sinistral strike slip along the Great Glen Fault, (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 without necessarily requiring either collision with Baltica or large-scale strike slip displacements along the Great Glen Fault.