Iron is not mobile in plants, so new leaves, which lack the iron required for chlorophyll production, appear yellow, whereas older leaves, which were created when iron was present and hence chlorophyll could be produced, are green.
Magnolia grandifloras are meant to tolerate dry, alkaline soil, which is what we have at Neats Home Garden, but I think it may have reached the end of its tolerance. Last summer's long dry months may also have stressed and weakened it.
Very few products seem to be available to the amateur gardener for treating mature trees. But I have found one which I am going to try, produced by a company called Solufeed. It's a chelated iron formulation meant to be used for the rapid correction of iron deficiency in most plants. You mix the powder in water, and add it to rainwater, and water in.
After clearing the weeds from under the tree, I’ve treated the tree with Solufeed three times now, and will repeat the application regularly throughout the spring. I will also mulch it heavily with well-rotted horse manure, once I can lay my hands on some. Fingers crossed this combined treatment will help it recover over this year or so.