The lower temperature limit, at metamorphism takes place, marks the boundary between metamorphism and diagenesis.

• Lower T = 100-150°C - excludes any diagenetic changes. Some workers consider any changes not occurring under atmospheric conditions to be the result of metamorphism.
• High T - upper boundary at 700-900°C, over a wide pressure range, with excess water, depending on bulk composition of the rock.

Generally considered to end when large scale melts of igneous character are produced.