Turkish Journal of Earth Sciences




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 Ca^{2+} Na^+, Al^{3+} Si^{4+} and weak substitution of Fe^{3+} for Si^{4+}. The Fe_2O_3^{(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 Fe_2O_3^{(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).

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