The formation of ordered morphologies in thin films of symmetric diblock copolymer melts is considered theoretically. Somewhat surprisingly, under proper boundary conditions the presence of a lamellar chemical pattern on the substrate, being sufficiently pronounced and with the right period, is found to induce the formation of diamond-like morphologies. The phase diagram of the most stable phases on the plane (the substrate period Lx the film width H) is built within the self-consistent field theory numerical procedure. We also discuss the behavior of the order parameter Fourier spectrum at the transitions between the various morphologies.