Of the 3 basic rock types, the type that was once one form but has changed to another due to heat & pressure
[DOC] [TLE] Metamorphic Rocks - Tulane University [PAR] Sep 17, 2015 ... Protolith can be any type of rock and sometimes the changes in texture ... 
Metamorphism therefore occurs at temperatures and pressures ... burial, tectonic 
stress, heating by magma or interactions with fluids.