Thermal evolution of metamorphic rocks in the Central Highlands of Scotland

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Abstract
Summary: Conditions of the metamorphic maxima in lower Dalradian kyanite zone pelites from Kinloch Rannoch, Schichallion, Glen Tilt, Glen Ey and Glen Avon are calculated from analyses of coexisting kyanite, garnet, biotite, plagioclase and quartz (staurolite or chlorite also coexist in some samples). Temperatures of 550–620°C were reached at pressures of 9–12 kb corresponding to depths of burial of 35–40 km. Compositional gradients in zoned garnets indicate that pressures were falling at the time maximum temperatures were reached. The high values of pressure are confirmed by observations of the sub-assemblages kyanite-zoisite (not anorthite-margarite) and kyanite-garnet-chlorite-quartz (not staurolite or chloritoid). The pressure of intrusion of a Newer Igneous complex in upper Glen Tilt is 5–6 kb. The present results are contrasted with conditions at a similar structural/stratigraphical level 50 km WNW where comparable temperatures were reached at depths around 20 km. It is concluded that either erosion rates early during metamorphism were lower or the tectonic cover was greater in the Central Highlands than to the west.