A generalized inversion statistic is introduced
on k-tuples of semistandard tableaux. It is shown that the
cospin of a semistandard k-ribbon tableau is equal to the
generalized inversion number of its k-quotient. This leads to a
branching formula for the q-analogue of Littlewood-Richardson
coefficients defined by Lascoux, Leclerc, and Thibon. This
branching formula generalizes a recurrence of Garsia and Procesi
involving Kostka-Foulkes polynomials.