Complex sentence: In late 1960, Dion decided to leave the group (the decision partly due to a heroin abuse problem), but the Belmonts carried on with Mastrangelo singing the lead vocal parts.
Split into simpler sentences: In late 1960, Dion decided to leave the group (the decision partly due to a heroin abuse problem). They carried on as "The Belmonts" with Mastrangelo singing the lead vocal parts.