U-Pb geochronology of regional migmatites in East Sutherland, Scotland: evidence for crustal melting during the Caledonian orogeny

Moine semi-pelitic metasediments near the base of the Kirtomy and Naver nap pes, northern Scotland, were partially melted syn-tectonically under low T, high a(H2O) conditions. Migmatites from the Kirtomy and Naver nappes have been dated using SHRIMP U-Pb geochronology. Zircon rims which grew during m igmatization have mean Pb-206/U-238 ages of 461 +/- 13 Ma (2 sigma) (Kirtom y) and 467 +/- 10 Ma (2 sigma) (Naver). These ages demonstrate the reality of a Middle Ordovician (Taconic) tectonothermal event in the Caledonides of northern Scotland. Monazites from the Kirtomy migmatite have a younger mea n (206)pb/U-238 age of 431 +/- 10 Ma (2 sigma). This may date either a late , low amphibolite facies overprint associated with steepening of part of th e nappe pile, cooling through the closure temperature of monazite for radio genic Pb, or cessation of fluid flow along the Kirtomy Thrust. Original detrital zircons from the sedimentary precursors of both migmatite s fall mostly into the age range: 1850-1000 Ma, demonstrating that depositi on of the Moine sediments in Sutherland occurred after c. 1000 Ma, followin g the culmination of Grenvillian orogenic activity in the North Atlantic re gion. Only two Archaean grains were found, demonstrating that Lewisian gnei sses of the foreland to the Caledonian were not the source.