  We study semigroups generated by general fractional Ornstein-Uhlenbeck
operators acting on $L2(\mathbb R^n)$. We characterize geometrically the
partial Gevrey-type smoothing properties of these semigroups and we sharply
describe the blow-up of the associated seminorms for short times, generalizing
the hypoelliptic and the quadratic cases. As a byproduct of this study, we
establish partial subelliptic estimates enjoyed by fractional
Ornstein-Uhlenbeck operators on the whole space by using interpolation theory.
