The morphology and chemistry of K-feldspar megacrysts from Ikizdere Pluton: evidence for acid and basic magma interactions in granitoid rocks, NE–Turkey

Abstract

Ikizdere Pluton consists of granite, granodiorite, tonalite, monzonite, quartz monzonite containing pinkish colored K-feldspar megacrysts (KFMs). The crystal sizes of the KFMs range from 1 to 4 cm. The lath-shaped megacrysts are uniformly (i.e., randomly) distributed in the host plutonic rocks and have mafic and felsic inclusions whose crystal sizes are smaller than 1 mm. The crystal inclusions are biotite, slightly annitic in composition with X Mg[=Fe tot/(Fe tot+Mg)]=0.50–0.58, amphibole (magnesio-hornblende, X Mg[=Mg/(Mg+Fe tot)]=0.70–0.79), iron-titanium oxide (low titanium magnetit and ilmenite), plagioclase (Ab 75−25An 65−35) and as minor quartz. The compositions of the KFMs range from Or 95Ab 5An 0 to Or 82Ab 17An 1. BaO contents of the megacrysts increase from core to rim. The mafic and felsic inclusions are compositionally similar those of the host rocks. The chemical and textural features of K-feldspar are typical for megacrysts that grew as phenocrysts in dynamic granitoidic magma systems. The overgrowth of KFMs and mafic magma injections (magma mixing) may be related to temperature, pressure and compositional fluctuations in the magma chamber. Remnant of earlier formed K-feldspar crystals remain in the felsic magma system, while the mafic injection can decompose some earlier precipitated KFMs. The remnant of K-feldspar remaining after mafic injection are overgrown by rapid diffusion of Ba, K and Na elements in liquid phase, during the later stages of crystallization of the host magma.