from what I've read, nute burn, but also get a thermometer in there. How high are your lights off the canopy, and when you put your hand under it, how hot is it on your hand. If you can stand it pretty easy, then most likely nute burn.
i've just recently adjusted the height of the lights from the plants and when i place my hand between the light and the plant, it does not burn. I've been flushing the plants (the last couple of watering days) and yet there are still new brown and yellow areas appearing on other leaves.
definately nute burn, BUT, dont worry grasshopper, Flush it with a gal or so of water, and dont feed for a week. The new growth will tell ya all is well. The burnt leaves will still photosynthisize energy for the plant.