I remember reading something by Ashbery, when I was very young, about reading poetry of the past. Poets who read only contemporary poetry do not have the same degree of historical depth, he seemed to be saying. You can't even imitate the poetry of the past, so you can remain more original. That one line struck me: it is a line by the 17th century poet Marvell, but Ashbery uses it for his own purposes. There is defamiliarization. What the hell is a "packet boat" and why would you put a drunk in one? The line means something different in Marvell than in Ashbery. (The Pierre Menard effect?)
Lorca uses a line by Guillén: "Sí, tu niñez, ya fábula de fuentes." It is the last line of a poem by Guillén, and the first line of a poem by Lorca. I would say that the line means something very different in Guillén than in Lorca. These two poets, unlike Marvell and Ashbery, are contemporaries, yet the distance between them is immense.