There isn't a single "original" rock that transforms into metamorphic rock. Instead, any igneous or sedimentary rock can be subjected to heat, pressure, and/or chemical reactions to become metamorphic.

Here are some examples:

* Igneous rocks: Granite can transform into gneiss, basalt can become amphibolite.

* Sedimentary rocks: Limestone turns into marble, sandstone can become quartzite, and shale can morph into slate.

So, the name of the original rock is dependent on the specific metamorphic rock you're interested in.