Age of the Glen Dessary Syenite, Inverness-shire: diachronous Palaeozoic metamorphism Across the Great Glen

Apparently igneous zircons from the Glen Dessary syenite complex give concordant U-Pb ages averaging 456 ± 5 Ma which is taken to date crystallization. The complex was deformed and metamorphosed to amphibolite facies subsequent to emplacement so that this age is a maximum for these orogenic events in the western Moine Series. Other age data indicate that amphibolite facies conditions associated with pegmatite emplacement continued until ca. 430 Ma contrasting with the north-east Highlands where tectonism and metamorphism was completed by 460 Ma ago. The disparity between hornblende and mica K-Ar ages suggests that cooling from 500 to 300°C took about 20 Ma. Higher blocking temperatures are indicated for U-Pb in monazite and sphene than for U-Pb in apatite.