Thermal causes of the Dalradian metamorphism in the central Highlands of Scotland

Synopsis Analysis of carbonate and pelitic mineral assemblages at the base of the Dalradian Series near Spean Bridge, Inverness-shire, leads to the conclusion that the temperature at the peak of regional metamorphism reached 535°C at a depth of 18.5 Km. In the light of the general parallelism of the garnet isograd (taken as nearly isothermal) and the base of the Dalradian, the thermal event is modelled in one dimension using an average heat production for the Dalradian of 4.0 × 10 −1 3 cal/cm 3 s and a temperature-dependent thermal conductivity with room-temperature average value of 7.8 × 10 −3 cal/cm s K. It is argued that the thermal inertia of the upper crustal rocks during metamorphism is offset by their concurrent erosion and that a steady state model is a reasonable approximation to the complex transient thermal history over the 20–40 m.y. period during which deformation, heating and erosion were taking place. The heat flow through the base of the Dalradian at the metamorphic climax was about 1.3–1.6 × 10 −6 cal/cm 2 s; this is attributed to heat production within an underlying 10–20 Km thick Moinian basement together with a normal, or slightly higher than normal, conductive flux from the mantle beneath. If a Lewisian-like lower continental crust underlies the Moinian it would have reached temperatures high enough to permit partial fusion and could be the source of calc-alkaline largely post-metamorphic-climax igneous rocks found in the area.