Clinopyroxene phenocrysts from the Księginki nephelinite (SW Poland)

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Abstract

The Eocene nephelinite from Księginki quarry (SW Poland) contains five types of clinopyroxene phenocrysts varying by texture and chemical composition. Type I phenocrysts are formed of Mg-rich (mg# = 0.93–0.88) homogenous cores, patchy mantle and zoned rims. Abundant type II is less magnesian (mg# = 0.65–0.88) and consists of spongy or spongy-patchy core surrounded by zoned rims, whilst in type III (mg# = 0.69–0.84), the cores are massive but patchy. The mg# of cores of type IV phenocrysts is slightly lower than that of type I (0.79–0.89), but its cores are either massive or patchy. Type V is very scarce and consist of relatively Mg-poor (mg# = 0.75–0.77) core enveloped by nonpatchy, sometimes zoned mantle and zoned outer rim. Chemical composition of type I and type IV cores suggests that they are xenocrysts introduced into the nephelinite from disintegrated peridotite and clinopyroxenitic xenoliths, respectively. Type V is also of xenocrystic nature, but its source rock was significantly more evolved than mantlederived ones. Types II and III are possibly cognates from the host nephelinite or a melt related to the nephelinite. All the types of phenocrysts suffered from disequilibrium with the nephelinitic (or proto-nephelinitic) melt or dissolution during adiabatic uplift. Linear variation in chemical composition of phenocrysts of Księginki nephelinite suggests its evolution because of fractional crystallisation, without significant influence of other differentiation processes.

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