Benchmark BSE Sensex tanked 824 points to settle at a fresh seven-month low on Monday following heavy selling in IT and oil & gas shares amid weak global trends.The 30-share BSE barometer plunged by 824.29 points or 1.08 per cent to close at 75,366.17 with 23 of its constituents ending lower and seven with gains.During the day the index moved between a high of 75,925.72 and a low of 75,267.59.