Japan's benchmark Nikkei 225 Stock Average tumbled 2.1 per cent Monday morning. The Nikkei lost 263.88 points, or 2.1 per cent, to trade at 12,297.07 at 11:30 am (0230 GM) at the end of the morning session. The broader-based Topix index was down 17.94 points, or 1.71 per cent, at 1,033.71, according to a report of DPA.