Darren, at least around these parts (Kansas City), the frost line is 30 inches. Which means that we have to dig footings for deck posts at least that deep so that water doesn't freeze underneath the posts and concrete footings and lift and drop the deck. Also, patios around here typically are only 4-6 inches thick, so what we "HAVE" to do is reinforce the thickness of the patio anywhere that a post will reside on it... i.e., making that area 30 inches below the surface filled with concrete. Considering the large size of the deck you plan on building I would strongly suggest more than a mere 4-6 inches of concrete patio for supporting that much weight!
Anyway, my two cents on your situation, and hopefully this helps!