What is mesozone in geology ?

Mesozone - Metamorphism

Metamorphic facies diagram
The mesozone is the zone of medium intensity of regional metamorphism. Its lower limits (around 500°C) and upper (around 650°C) are defined respectively by the appearance of biotite and by those, together, of sillimanite and potassium feldspar. The mesozone is the domain of mica schists and gneisses with biotite and muscovite, staurolite and/or garnet, and most of the amphibolite facies.