P-T conditions in the Moines of the Central Highlands, Scotland

Abstract

P-T conditions of regional Caledonian metamorphism recorded by calc-silicate assemblages in Moine rocks from the Central Highlands vary from 550°C, 7 ± 2 kbar, near Loch Laggan, to 670°C, 10 ± 2 kbar in the central Spey valley. The pressure conditions correspond to 30–35 km of tectonic cover in the Central Highlands during the metamorphic temperature maxima. These estimates place much of the Moines of the Central Highlands in the kyanite zone. Spatial variations in estimated pressure conditions at comparable temperatures suggest differential initial tectonic cover thickness and/or differential erosion over the Caledonide orogen. Numerical models of these alternative thermal regimes predict different P–T histories for the metamorphic rocks of the Central Highlands. Mineral assemblages exhumed by differential erosion, related to different cover thickness, record maximum temperature conditions at progressively earlier times with increasing recorded pressure. These time relations are reversed for differential erosion of initially uniform cover thickness. These results have general application to metamorphic rocks developed within zones of tectonic thickening.