Geological processes for zircon + sandstone?

Zircon crystals formed when a granite magma crystallised. Since then, geological processes have caused some of these crystals to end up with grains in a sandstone. What sequence of events would account for this?

and what implications would this make for the age of the sandstone assuming that the 206Pb/238U ratio is 0.90 with the radiometric decay worked out as:

t = 1/k x ln (1+ D/P)
    where
    t = the age of the sample
    D = number of atoms of the daughter isotope in the sample
    P = number of atoms of the parent isotope in the sample
    k = decay constant of the parent isotope
    ln = natural logarithm

The half-life of 238U is given to us as 4.47 billion years, which equals 0.693/k. So our formula becomes:
t = 4.47 x10^9 /.693 x ln (1+ .9)
t = 6.45 x10^9 x 0.642 =4.14 x10^9 years