Surprise surprise -

This is my Tango this morning. Tango is an irradiated seed version of Murcott. This was a FW tree bought last summer that is coming back from severe twig tie-back over the winter. When I got it last year, there was at least one branch that appeared stressed with yellow-splotched leaves, and that branch eventually died back. The new shoots that came from the bottom (above the graft line) in late winter, matured to be the dark green curled leaves in the above pic. It has been doing a new growth spurt with blooms the past month and the new leaves appear "normal". As a note, this is planted in 5:1:1.

I recall asking about this before and I think Brian had chimed in about the tendency for this to happen with some mandarins but it usually corrects with new growth. I am thinking (and this had been mentioned in some threads last summer) that for my purchase of FW trees last summer - they may have suffered from the excessive rains that California experienced and some trees they shipped last year were severely stressed - notably with respect to whatever rootstock this was on and how the graft had to handle that.

Yours may not be FW but may have had a similar issue of a stressor that is finally expressing via the older leaves.