GLOBAL MARKETS-Oil crumbles, bond prices up on economy fears

* U.S. shares give up most gains, global index falls* Dollar index rebounds, euro hit by German data* Oil falls, S&P 500 energy sector enters bear market(Updates prices)By Rodrigo CamposNEW YORK, Oct 14 (Reuters) - Brent crude prices marked their biggest decline in more than three years on Tuesday and U.S. and German debt attracted buyers on lingering anxiety over world economic growth.Wall Street