Metamorphic zone

These zones occur because most metamorphic minerals are only stable in certain intervals of temperature and pressure.

[1] The temperature and pressure at which the mineralogical composition of a rock equilibrated can vary laterally through a metamorphic terrane.

Changes in mineralogical composition in a terrane reflect differences in metamorphic grade of the rocks.

The first or last appearance of an index mineral (the place where a metamorphic reaction is observed) forms an easily recognizable isograd.

When mapping the metamorphic grade of a terrane, a geologist has to take the lithology of the rock in account.

The main lithologies are ultramafic, mafic, felsic (or quartzo-feldspathic), pelitic and calcareous.

In this series of zones, both pressure and temperature increase gradually along the metamorphic gradient.

Schematic map of a terrane with an increasing metamorphic grade . There are two folded lithologies : quartzite (originally sandy sediment) and pelite (originally clayey sediment). The index minerals can only grow in the pelite. The highest indicated isograd is the solidus of hydrated granite , at higher metamorphic grade partial melting occurred in the quartzite.
Metamorphic zones in Scottish Highlands , the region where they were first recognized by geologist George Barrow in 1912. Most of the region has Barrow zones, but an area north of Aberdeen has Buchan zones ( andalusite and biotite ).
Andalusite crystals in Dalradian metamorphic rock at Boyndie Bay in the Buchan metamorphic zone of north-east Scotland