One final thought, now I've practiced it a bit (and to bring the thread full circle......)

The chords in the riff are A, B, C, D, E, F# and G - so it looks like we're in the key of G.

In that second bar, the G# note doesn't belong in that key.

So - in this riff we have a great example of modal interchange!?

I hope that's right - if so, I've definitely learned something new this week.

c.