Look up rare earth minerals in alkali granites (nd/or) pegmatite rocks.
Alkali feldpar granites (red granites) and flourite pegmatites sometimes contain minerals like Samarskite and Monazite. The conditions of origin of the hot melt rocks is a story with many possible scripts.
You should really look at the pecise granite block from which your specimens come from and see whats written about it. Theres usually a good series of genesis reports from the prospctors