Abstract

Relict high-pressure granulite-facies assemblages have been discovered in mafic sheets within Neoproterozoic Moine Supergroup psammites of northern Scotland. Minerals and fabrics associated with the main 440–430 Ma Caledonian deformation and metamorphism (D2) overprint these assemblages, which preserve peak pressure-temperature (P-T) conditions of 11–12 kbar at 650–700 °C, indicating burial to ∼35 km. The maximum age of the sheets is constrained by the post-Grenvillian ca. 1000 Ma age of the youngest detrital 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 events, two partial clockwise P-T-t-D (time-deformation) paths emerge, demonstrating that the high-P event is not part of the evolution of the <440 Ma Caledonian thrust nappes. The high-P granulite assemblages are interpreted to be a relict of an earlier, Taconic 470–460 Ma history. This new evidence suggests that the tectonic evolution of the Caledonian nappes east of the Moine thrust is more complex than previously thought.

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