Abreak that separates older metamorphic rocks from younger sedimentary rocks immediately above them is a type of unconformity called a(n)

Non conformity is the type of unconformity separating the 2 rock strata.

Explanation:

There are 4 types of unconformities, among which the non conformity is the type of unconformity that separates two rock types while one among them is younger and the other is older. It usually exists between young sedimentary rocks and old igneous or metamorphic rocks. The non conformity refers to the old erosional  feature on the underlying rock. It also suggests that, a long term uplift has taken place.