Graham sorry for the great delay in answering your question.
The quick answer is that you can use granite dust in your compost as long as it is well mixed through - it has a tendency to pan or form crusts if present in quantity..
Will it do the same as the volcanic rock dust -no it has less beneficial minerals in it.
Quarry workers in our area used to spread granite dust on the gardens to great effect.
Thanks for the reply Ian and no problem about the delay.
I can see that it would pan or crust if not mixed completely, and as it isn't as beneficial as volcanic dust I probably won't consider it.
Thanks
Graham