Rb-Sr chronology of the Carn Chuinneag Intrusion, Ross-shire, Scotland

The Carn Chuinneag granite complex in the northern Scottish Highlands was regionally metamorphosed. Rb-Sr isochron diagrams reveal the presence of a major system and a subsystem of cogenetic rocks and minerals. The major system, consisting of specimens of total rock, indicates an age of intrusion of 530±10 m.y. The subsystem, consisting of mineral grains in intimate association in a hand specimen, indicates an age of metamorphism (homogenization) of 390±5 m.y. (late Caledonian). The late-Caledonian age is obtained from other rocks associated with the main granites. The question arises whether migration of isotopes within the rock during the intense Caledonian metamorphism may invalidate the significance of the 530-m.y. isochron as indicating the true age of emplacement. A theoretical study shows that the consequences of isotopic transfer in multiphase systems are that the original alignment of isochron points is destroyed, and it is not regained until complete exchange produces a new isochron with zero slope. The whole-rock isochron may therefore be confidently interpreted as indicating a true primary age.