Going to disagree with my grommie here... magnesium deficiency is first spotted on lower leaves, not on top... BUT I agree that dimming your lights is going to help ... my thoughts are that this is happening due to heat stress - and you're certainly much too humid for the flowering stage! So try to drop both your temps (70 would be great) and your humidity (it should be about 40% not 60%!). Making sure your getting lots of fresh air to your grow will help as well... You're going to want to fix this so you don't end up with foxtailing, too!
Good luck!