Metamorphic Barrow Zones in Scottish Highlands

The Barrow zones of the Scottish Highlands are located in the southern part of the Grampian Highlands terrane, which is bound to the south by the Highland Boundary Fault. The zones are a series of mineral assemblages in metamorphic mudstones, which start at chlorite grade and are then defined by the sequential first appearance of biotite, garnet, staurolite, kyanite and sillimanite. The zones were first recognised and mapped by British survey geologist George Barrow (Barrow, 1912), and represent increasing metamorphic grade to the northwest away from the Highland Boundary Fault. The metamorphism occurred at c. 470 Ma during the Grampian orogeny, a short-lived tectonometamorphic event involving the collision of the Laurentian margin with the Midland Valley Arc within the longer Caledonian orogeny (Viete et al., 2012). In the type sections of Glen Clova and Glen Esk in the central portion of the Barrow zones, reported metamorphic conditions vary from 500-650 °C and 5-7 kbar in the garnet, staurolite, kyanite and sillimanite zones (Vorhies and Ague, 2011). Higher-pressure conditions are present to the west, and lower-pressure/high-temperature conditions to the east, which transitions into a different sequence of metamorphic mineral assemblages known as the ‘Buchan’ zones.