i would use a delimolene  cleaner that the concrete companies sell ..usually has a name like Big Orange, or Citrus Cut, or Citrus Kleen. it should work on the tar..another more toxic answer would be laquer thinner..Prosoco also does sell and Asphalt and Tar Remover....we use the delimolene all the time to remove carpet glue and mastic...very easy and low odor.

From: Roger Konarski <qm144@yahoo.com>
To: Stone and Tile PROS Technical Support <sccpartners@stoneandtilepros.com>
Sent: Sunday, September 23, 2012 9:47 AM
Subject: [sccpartners] Removing tar

I have been asked to remove the rubberized sheeting on some outdoor honed granite. Please see attached pictures. Would appreciate any suggestions on how  to remove what looks like tar and roofing material.
 
Thanks,
RK