Tiling Fire-place Hearth / underlayment?

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I have a Majestic pre-fab fireplace (log+gas burning). The hearth (area in front of fireplace) consisted of ceramic tile affixed directly to the floor. I'm replacing the tile on the hearth, but first installing hardwood flooring, so I need to put down 3/4" plywood as underlayment. Can I run the plywood all the way up to the fireplace, then just tile over it for the hearth, or should I use a non-combustible material as underlayment where the hearth will be? Currently the subfloor is a 1" layer of lightweight acoustical concrete over wood. They had tiled & carpeted directly over this concrete.