ABUJA, June 7 (Reuters) - Nigeria naira slid pass 500 per dollar on the black market on Monday to fall to a more than 3-1/2-year low, two weeks after the country devalued its official rate to the dollar to adopt a weaker spot rate. Meanwhile, trading volumes fell 14.49% last week to $1.07 billion across the currency markets, data from the exchange regulator, FMDQ Securities Exchange showed on Monday. (Reporting by Chijioke Ohuocha; Editing by Toby Chopra) Our Standards: The Thomson Reuters Trust Principles.