The area between two successive isograds in a metamorphic terrain. The zone is named after the lower-grade isograd (see metamorphic grade). For example, the area lying between the garnet isograd and the kyanite isograd for pelitic rocks in a regional metamorphic terrain is termed the garnet zone. The zone defines a set of pressure, temperature, P(H2O), and P(CO2), over which there were no mineralogical reactions in the particular rock composition concerned. Normally, the chemical composition of pelites is used as a marker.