Plagiarism in Fanfic

This is just something I wanted to check other people's opinions on before I get too far into writing this fic.

This fic is a re-write of (part of) Tolkien's The Children of Húrin from the perspective of a particular character, rather than the omniscient perspective of the novel. As such, while I am adding scenes, there are some scenes that are just-- me writing my own version.

Most of the dialogue is original. There's not that much dialogue in the novel, and I can't mantain Tolkien's style. But there are some lines of dialogue where there are recognisable chunks of canon dialogue in their, because either there's just not a good way to rephrase it, or because that phrasing is important to the story. (The chunks are generally no longer than a sentence.)

Now, if these were recognisable chunks of some other novel, than yep, what I'm doing is probably plagiarism. But because these chunks are from the canon source material-- I'm not sure if it is as bad as other types of plagiarism.

What do you think?