This depends on the fluid; If it is water based it can change in mineral composition without having to melt.
If the fluid is a magma high-temperature fluid this can melt the rock and/or bake the rock, which causes a contact aureole to form on the outer rock walls. this metamorphoses the rock wall.
The size of the aureole depends on the heat of the intrusion, its size, and the temperature difference with the wall rocks.
