Chemical composition and K–Ar age of Phengite from Barrovian metapelites, Loch Leven, Scotland

Abstract

New K–Ar ages of phengite concentrates from the Barrovian metapelites of the Loch Leven area, Scotland yield 398 ± 10, 406 ± 11, 405 ± 11, 400 ± 11 and 445 ± 12 Ma. All the phengites have homogeneous mineral compositions, and the metamorphic temperatures are below the closure temperature of the K–Ar phengite system; this suggests that the time of metamorphic recrystallization was between the significant age-gap of ca. 445 and ca. 400 Ma. Integration of our K–Ar ages with previous geological and geochronological data from the Loch Leven area and from the Moine Thrust, which is the major lower boundary of the Barrovian metamorphic belt, suggests that the lower boundary is present as the Fort William Slide in the Loch Leven area.