Petrogenesis of Miocene subvolcanic rocks in the Western Outer Carpathians (southeastern Moravia, Czech Republic)

Abstract

Neogene subvolcanic rocks in southeastern Moravia form numerous dykes and laccoliths, ranging from clinopyroxene–amphibole and amphibole trachybasalt, through trachyandesite, to biotite–amphibole trachydacite. Leucocratic and melanocratic cumulate gabbro and basalt enclaves up to 70 cm in diameter are rarely present, respectively, within the trachydacite and trachyandesite. The parental magmas rose along tensional fissures spatially related to the Nezdenice Fault but probably never reached the surface. The range of major (e.g., SiO2 44–62 wt. %, mg# 20–65) and trace-element compositions can be explained through magma mixing and mingling and subsequent fractional crystallization. Mineral chemistry shows limited compositional variation of mafic minerals. Diopside phenocrysts indicate narrow ranges of XMg (0.65–0.84) and usually display normal zoning with small Mg-rich cores and Fe-rich rims. Phlogopites from the trachydacite and gabbro enclaves show a mutually similar composition (XFe 0.36–0.43 and IVAl 2.44–2.59). Amphiboles from individual samples of basalt, trachybasalt and trachyandesite are likewise chemically relatively homogeneous (XMg 0.51–0.86, Si 5.78–6.55). Chemical compositions of amphibole phenocrysts from the trachybasalts and trachyandesites indicate multi-stage crystallization at depth of 32 to 21 km for this magmatic system. Systematic changes in Si, Ti, VIAl, XMg contents in amphiboles from trachydacites and gabbro enclaves can be explained by fractional crystallization in a shallower magma reservoir (~20–10 km).