  We study the dynamics of quantum and classical correlations in the presence
of nondissipative decoherence. We discover a class of initial states for which
the quantum correlations, quantified by the quantum discord, are not destroyed
by decoherence for times t < \bar{t}. In this initial time interval classical
correlations decay. For t > \bar{t}, on the other hand, classical correlations
do not change in time and only quantum correlations are lost due to the
interaction with the environment. Therefore, at the transition time \bar{t} the
open system dynamics exhibits a sudden transition from classical to quantum
decoherence regime.
