You can narrow it down by where it shows on the plant, if its new growth its an immobile nutrient def, if its on old growth its a mobile nutrient def. The mobile nutrients are N P K Mg S and the immobile nutrients are Ca + all the micro nutrients, since yours is on old growth and its heavy necrosis during flower its probably K or P. Its most likely not P because cannabis does not need large amounts of P during any growth stage and it could turs the leaves a purple color along with yellow/brown edges.
If its at the base of your plants then its definitely a mobile nutrient def, which could be from a lack of the nutrient, too much of 1 nutrient can lockout another, or your pH is too far out of range.
IMO I think its a lack of 1 or more nutrients, but just to be safe let me know what nutes your using, the amount per gallon of water, the frequency of use, if you use plain water in between feedings, and the pH and EC of your nutrient solution before and after its in your pots. That info will help me help you.
P.S. There is no prob answering your questions bro, I enjoy helping others if I can. Also, Google 'deficiency and abundance chart for cannabis' or something like that and one of the results should show a chart of leaves with multiple deficiencies and how they look.
If you can't find it I will post it for you when I'm able to get back on my desktop computer.