Metamorphic rocks are formed when existing rocks (igneous, sedimentary, or even other metamorphic rocks) are transformed by heat, pressure, or chemical reactions. So, the "parent rocks" of a metamorphic rock can be:

* Igneous rocks: These are formed from the cooling and solidification of magma or lava. Examples include granite, basalt, and gabbro.

* Sedimentary rocks: These are formed from the accumulation and cementation of sediments. Examples include sandstone, limestone, and shale.

* Other metamorphic rocks: Metamorphism can occur in multiple stages, meaning a metamorphic rock can be transformed again to form a new metamorphic rock.

The specific parent rock will influence the type of metamorphic rock that forms. For example:

* Marble is formed from the metamorphism of limestone.

* Slate is formed from the metamorphism of shale.

* Gneiss can be formed from the metamorphism of granite, sandstone, or even other metamorphic rocks.

It's important to note that the parent rock's composition and the conditions of metamorphism (heat, pressure, and chemical reactions) all play a role in determining the final metamorphic rock.