The Trump administration on Tuesday sought to allay growing fears among immigrant communities over wide-ranging new directives to ramp up enforcement against illegal immigrants, insisting that the measures are not intended to produce “mass deportations.” Federal officials cautioned that many of the changes detailed in a pair of memos from Homeland Security Secretary John F. Kelly will take time to implement because of costs and logistical challenges, and that Border Patrol agents and immigration officers will use their expanded powers with care and discretion.