A Scandian pressure-temperature path for the northern Moine Nappe, Scotland

The Northern Highlands Terrane in northwestern Scotland preserves a polyphase metamorphic history dominated by Scandian (435-415 Ma) tectonism. Phase equilibria modeling, major and trace element compositions in metamorphic phases, and textural observations are used to construct a detailed pressure-temperature-time path for a garnet-staurolite-kyanite-mica schist from the Moine nappe. We employ a statistical method to evaluate the best-fit pressure-temperature conditions based on measured and modeled garnet compositions. This method yields a straightforward multistage P-T path and eliminates some of the ambiguity inherent in comparing thermodynamic models with measured phase compositions. Trace elements in garnet, ilmenite, and rutile, and the observed phase relationships in the sample are in excellent agreement with thermodynamically modeled assemblages and phase compositions. Based on these constraints, relatively rapid exhumation (2.0 km/Myr) and cooling (56°C/Myr) are demonstrated for this area.