Lightest paddle for turing

In my experience, under 450g you are going to make compromises, as it may break, depending on the blade surface, your weight, strength, and paddling technique.
So with the years, most brands settled to 450g for their production lines to avoid too much breakage in the hands of the general public.

For instance, in the Gong line, for a 645cm blade (8" wide) with the same shape, the first ones were at 350g, but I broke them quite easily with my 100kg and hard surf paddling on takeoffs. Then they weighted 400g, and I rarely broke them, and now at 450g, I have never broken one. But I am sure my 60kg daughter could use a 350g without problems. So Gong decided to be on the safe side, and stay at 450g minimum. But note that the 7" wide blades are a bit lighter, at 430g.

Note that I keep an old 350g 8' paddle religiously in my van (350g in hands is a magical feeling), but I only use it in relaxed conditions, as I know I would break it if I attempted my usual full force take off.

So, you may want to look for paddles with a small blade, or custom paddles, or specific ones (for girls or lightweights). And don't expect the manufacturer to consider breaking as covered by the warranty.

For touring and not surfing, the lightest paddle I have used is the Blackfish Viento 520. 380gr cut at 74". I would however never take it surfing. Very strong catch and power so even too big for long touring sessions but perfect for giving it all on a 10~15km course.
When it gets messy I switch to the Salish 500, about the same weight.
Surfing calls for stronger constructions, as noted.

Starboard Enduro Prepreg Carbon S35 M (85) is 432g