Some REE-bearing (mostly monazite) mineral accumulations can be observed in meta-sedimentary rocks.
Also, the hydrothermal fluids sourced from dynamo-thermal metamorphism percolating in porous-permeable rocks can remobilize the REE from prior minerals. This process can be observed in REE-bearing skarns. You can access further information in the below article;
Article Mary Kathleen metamorphic-hydrothermal uranium - rare-earth ...