What is the term for the original rock from which a metamorphic rock is formed?

The term "parent rock" refers to the original rock from which a metamorphic rock is formed through the process of metamorphism. This process typically involves the alteration of the mineral composition and structure of the parent rock due to changes in temperature, pressure, or the presence of chemically active fluids.

For instance, limestone, which is a sedimentary rock, can be transformed into marble, a type of metamorphic rock, under specific conditions of heat and pressure. The original limestone is considered the parent rock because it is the source material that undergoes metamorphic changes.

In contrast, the other terms do not carry the same specific geological meaning. "Source rock" is often used in exploration geology to refer to the original rock that generates hydrocarbons, not specifically in the context of metamorphism. "Ground rock" and "foundation rock" are not standard geological terms related to metamorphic processes and might lead to confusion when discussing rock transformation. Thus, "parent rock" is the most accurate and widely recognized term within the context of metamorphic geology.