Origin and significance of volcanic garnet: A detailed petro-mineralogical study of the almandine-bearing andesite of Breziny (Central Slovakia Volcanic Field, Western Carpathians, central Europe)

Abstract

Almandine-rich garnets from a Neogene andesite of Slovakia can be divided into two main types. Garnet megacrysts are magmatic and form a chemically homogeneous group that contains, on average, about 5 wt% CaO and 4.5 wt% MgO as petrogenetically significant components. Garnets occurring in lithic fragments and garnets aggregated in garnetite lenses are characterised by Ca-poor cores (CaO < 2 wt%) that testify for a two-step history and correspond respectively to inherited pre-anatectic and peritectic garnets. Available experimental data show that the composition of magmatic garnet megacrysts is compatible with a peritectic origin, through the fluid-absent melting of an immature metasedimentary protolith or a tonalitic gneiss. However, thermal evolution evidenced by zircons shielded in garnet rather suggests that garnet nucleated and grew by cooling of a hybrid magma pool, resulting from the complete mixing of crust- and mantle-derived melts.