it looks like nutrient burn to me, but it could be alot of things. it does look like a magnesium deficieny too.
this is what i would do....as jcd says, flush first. so if it is nute burn, this will be taken care of. also if it is a deficiency it can be caused from a lockout of a nutrient caused by an excess of another nutrient. i am not saying that this is what has happened, but flushing kind of kill two birds with one stone in trying to fix a plant problem like this. then either lessen your nutes for a little bit and see if that works or take your route and supplement magnesium or calcium to see if that helps. calcium is a rare deficiency so i would try that last. good luck.
also, good job on your grow. those buds look delicious. you should look at jigfresh's grow journals, he has some purple kush that looked just like that and turned out great. you will have some top notch bud in your hands soon.