New high-pressure granulite event in the Moine Supergroup, northern Scotland: Implications for Taconic (early Caledonian) crustal evolution

Relict high-pressure granulite-facies assemblages have been discovered in m afic sheets within Neoproterozoic Moine Supergroup psammites of northern Sc otland, Minerals and fabrics associated with the main 440-430 Ma Caledonian deformation and metamorphism (D2) overprint these assemblages, which prese rve peak pressure-temperature (P-T) conditions of 11-12 kbar at 650-700 deg rees C, indicating burial to similar to 35 km. The maximum age of the sheet s is constrained by the post-Grenvillian ca. 1000 Ma age of the youngest de trital zircons in the host psammites. The high-P assemblage is constrained by the age of anatexis in the psammites at 467 +/- 10 Ma. Assembling these data with P-T and chronological data for the D2, D3, and later cooling even ts, two partial clockwise P-T-t-D (time-deformation) paths emerge, demonstr ating that the high-P event is not part of the evolution of the <440 Ma Cal edonian thrust nappes. The high-P granulite assemblages are interpreted to be a relict of an earlier, Taconic 470-460 Ma history. This new evidence su ggests that the tectonic evolution of the Caledonian nappes east of the Moi ne thrust is more complex than previously thought.