Abstract
Diamond - formed during the collision of continents - has been found in continent-continent collision zones (orogens) of Phanerozoic age only, which may indicate a change in plate tectonic conditions within the last 600 Ma.
We use mineral Al isopleths, isotope systems and (micro-)structural relations in mantle fragments enclosed in the Scandinavian Caledonides to demonstrate that the tectonic transport of these `orogenic peridotites' towards 200 km depth, the deep diamond stability field, occurred at the Early Silurian and destroyed old (Proterozoic) pyroxene exsolution microstructures in garnet. Post-peak metamorphic conditions in peridotite overlap with those in surrounding Early Devonian crustal eclogite and imply a long-lived continental plate exhumation through the entire Silurian without major crustal melting. Cold collision regimes are capable to preserve ultra-high pressure metamorphosed continental margins in a mature planet.
