How large a piece of metal, over what length of the line? The dielectric constant of the pcb/substrate is also relevant, as is the actual width of the 50 ohm track and the gap between the ground plane. A 50 ohm track can be made with an infinite range of track to ground plane gaps.
This is probably not something you would want to model on a field simulator, especially since if you had one you wouldn't ask the question.
The practical answer is to try it with and without the metal and see if it makes a difference in your application.
Normally a microstrip line isn't affected if you are two board thicknesses away. Most board thicknesses are 0.060", hence if you are 0.1" away, then there's probably not much affect.
