We present a nonlinear dynamical model for oscillatory zoning in plagioclase based on a simple isothermal constitutive undercooling mechanism. A phenomenological partitioning is introduced to relate the concentration of An in the melt at the interface with the concentration in the solid. The non-linearities in the model result from the coupling of the growth velocity with the local An concentration and from the boundary condition at the interface. The consideration of a nonlinear boundary condition is new and generalizes previous nonlinear growth models. It is shown that parameter values exist for which oscillatory solutions are possible via a Hopf bifurcation. As the system is driven further out of equilibrium, the model shows the development of chaotic solutions via a period-doubling sequence. ¿ American Geophysical Union 1996