First off, I found this site by accident today researching geothermal pond loops. Fantastic resource! There is a great forum for almost every subject if you can find it.

I would not feel comfortable placing an earthen dam on unfractured granite w/o a cuttoff trench to key in the core of the dam. W/O the cutoff trench, it would be easy for the water to move along the bedrock and destabilize the whole embankment. However, with a 4&1/2' deep pond you may not have enough hydrostatic pressure to matter that much. There are lots of variables that make giving/following advise on these matters hit or miss. FWIW, if the exposed rock is under where the dam will be, I'd "key" it in.

Good Luck!