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

摘要

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 11-13 kbar at 650-700 ℃, indicating burial to ～35 km. the maximum age of th 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 Mione thrust is more complex than previously thought.