Although I haven't come across this problem myself, I have done some research and would say, judging from the first (smaller) photo, that this is probably a magnesium deficiency, to which citrus trees are fairly prone:
Mg -- Always on the oldest leaves first. Bottom center of the leaf is green, and the end and sides are yellow or orange, making an inverted "v" or "Christmas tree" shape to the green part, as it zigzags with the veins.
Magnesium deficiency produces a very distinct yellowing towards the apex of the older leaves, with a triangular area remaining green at the leaf base. Affected leaves fall prematurely in late summer and autumn. The defoliation can be heavy when a large crop is being carried. If the deficiency is not corrected it weakens trees.
Magnesium deficiency in citrus
Apparently, it can be treated with a solution of Epsom Salt (magnesium sulphate, 2 teaspoons dissolved in a litre of water) applied round the base of the tree or, for a quicker result, as a foliar spray:
Magnesium Deficient Plants
There is some useful information about magnesium and Epsom Salt here