The Metroid Prime trilogy is easily an adventure game series. Here's the thing, the Prime games play exactly like all the other 2D Metroid titles before it, except from a first person view. It just so happens that Samus has a beam weapon attached to her right arm in which to shoot enemies. Just because we switch the perspective, and Samus shoots enemies, it doesn't the game has transcended its genre. The core gameplay of a 2D Metroid is still there, intact, unchanged.
How about this, think of Super Metroid, only this time imagine playing it in 1st person view. It would still be Super Metroid, but you're viewing the gaming world through Samus' visor as opposed to watching the action unfold from a sideview. The only change is perspective, not genre.