If you are dead set on this, and lets be honest a fire pit on the deck is pretty nice, I'd set it up on gravel and start a fire, then check the temperature directly under the heat shield of the fire pit with an infrared temp gauge. We use the Heatcon Heat Forming Kit to bend composite boards for curves, and as an example, the Trex Transcend line only needs 245° for 45 minutes to make the deck boards mailable enough to bend into a pretzle. Especially with long term, moderate heat like a fire pit. In some places, they usually require a more. Available Sizes: 12' x 12' Combo - 109.95. Composite decks can have a gas fire pit, depending on the model and base. This provides a gap for airflow and keeps moisture from gathering under the pad. While most composite decking is non-combustable, they all have a very low bending/melting points. The Deck Protect fire pit mat is available in (5) sizes and includes the mat and a sturdy rack that raises the mat off the deck. If you made legs longer, or used actual firebrick as your base, you have a better chance. I don't think you will have that covered with a couple of pavers and some sand. 'Place fire pit on Dirt, Stone, gravel, or Sand.
Take a look at the use and care instructions on Home Depot's website. I would say no, even with a couple of pavers, but I also don't have a Phd in fire technologies.
