Plagioclase crystals characterized by compositional zoning are typical of the Zigana Granitoid (ZG), NE Turkey. The zoned crystals, which show textural equilibrium with the assemblage quartz + plagioclase + K-feldspar + hornblende + biotite + pyroxene,+ magnetite + ilmenite, exhibit oscillatory zoning. The zoned plagioclase crystals, ranging in size from 3 to 6 mm, are oval and larger than normal lath-shaped crystals. The zoned crystals are divided petrographically into two main types: (i) crystals zones only at their rims, with spongy cores; and (ii) completely zoned ones. Composition of the zoned crystals ranges from An(23) (oligoclase) to An(70) (labradorite). Plagioclase crystals with oscillatory zoning are governed by strong substitution mechanisms of Ca2+ double left right arrow Na+, Al3+ double left right arrow Si4+ and weak substitution of Fe3+ for Si4+. The Fe2O3(t) (wt %) contents of the subject plagioclases increase with increasing An content. The petrographic and compositional changes from rim to rim (oscillatory zoning), and positive correlation between Fe2O3(t) (wt %) and An (mol %) contents of the zoned plagioclases, suggest recurrent mafic magma injection into a felsic magma chamber, and thus magma interaction as a dominant magmatic process during the evolution of the Zigana Granitoid (ZG).