The relative roles of crystal settling and in situ crystallization in the formation of igneous bodies are still a subject of debate in igneous petrology. Both processes predict that minerals must become more evolved in composition inward. Our study of three dolerite sills from Siberia indicates, however, that this is not always the case. In particular, the ∼100-m-thick Vavukansky dolerite sill shows a systematic inward increase in An content of plagioclase across lower (from ∼An63 to ∼An82) and upper (from ∼ An63 to ∼An80) marginal zones. This is followed by inward decreases in the An content of plagioclase in the interior part of the sill that ends in a sandwich horizon (An55–58). These trends in plagioclase composition are not consistent with either approach, but can be explained by in situ crystallization involving two distinct stages. During the initial stage the sill evolved as an open system that was continuously filled by magmas, becoming more primitive with time. This gave rise to marginal zones with minerals showing reverse compositional trends (e.g., an inward increase in the An content of plagioclase). During the subsequent stage the sill evolved as a closed system by fractional crystallization. This resulted in minerals showing normal compositional trends (e.g., an inward decrease in the An content of plagioclase). Several other mafic sills and layered intrusions show similar variations in plagioclase compositions, indicating that open-system, in situ crystallization may be a common process in the origin of igneous bodies.