Chapter 21: Metamorphism - Faculty web pages

To summarize:• An isograd represents the first appearance of a particularmetamorphic index mineral in the field as one progressesup metamorphic grade• When one crosses an isograd, such as the biotite isograd,one enters the biotite zone• Zones thus have the same name as the isograd that formsthe low-gradeboundary of that zone• Since classic isograds are based on the first appearance ofa mineral, and not its disappearance, an index mineralmay still be stable in higher grade zones