Some of the world's oldest preserved art is the cave art of Europe, most of it in Spain and France.
The above sentence is from IELTS test reading passage, and it is oral English. I want to make sure that the grammatical function of 'most of it in Spain and France' in the sentence. In my opinion, it is a parenthesis, which is to add some new information to the main sentence. This is because there is a comma in front of it. But I am not sure if it's possible that it is the complement of the subject or not.
So I need your kind help.