Barrows zones

views updated

Barrow's zones The original subdivision by George Barrow (1853–1932) of the sequence of mineral changes seen in rocks of pelitic composition in the Glen Esk region of north-eastern Scotland. Each zone is bounded by two isograds, each of which marks the appearance, in the direction of increasing metamorphic grade, of a new index mineral, and is named after the index mineral seen on the lower-grade boundary. (For instance, the kyanite zone has the kyanite isograd as its low-grade boundary and the sillimanite isograd as its high-grade boundary.) Within each zone, no additional minerals appear in the pelites, the constant mineral assemblage representing equilibrium over a range of metamorphic conditions. See BARROVIAN-TYPE METAMORPHISM. Compare BUCHAN METAMORPHIC ZONES.

More From encyclopedia.com