Here's why:

* Metamorphic facies are defined by the specific mineral assemblages that form under a particular range of pressure and temperature conditions.

* The same minerals will form under similar conditions, regardless of the original rock type.

* For example, the greenschist facies is characterized by the minerals chlorite, epidote, albite, and actinolite. This assemblage forms at relatively low pressures and temperatures.

* Rocks from different starting materials (e.g., shale, basalt) can transform into greenschist facies rocks if they experience the same metamorphic conditions.

So, while the original rock type (protolith) can influence the specific details of the metamorphic rock, the mineral assemblage is the key factor in determining the metamorphic facies.