Seamless exchange of models among different modeling tools increasingly becomes a crucial prerequisite for the success of modeldriven engineering. Current best practices use model transformation languages to realize necessary mappings between concepts of the metamodels defining the modeling languages supported by different tools. Existing model transformation languages, however, lack appropriate abstraction mechanisms for resolving recurring kinds of structural heterogeneities one has to primarily cope with when creating such mappings. We propose a framework for building reusable mapping operators which allow the automatic transformation of models. For each mapping operator, the operational semantics is specified on basis of Colored Petri Nets, providing a uniform formalism not only for representing the transformation logic together with the metamodels and the models themselves, but also for executing the transformations, thus facilitating understanding and debugging. To demonstrate the applicability of our approach, we apply the proposed framework for defining a set of mapping operators which are intended to resolve typical structural heterogeneities occurring between the core concepts usually used to define metamodels.