Magma fractionation, replenishment, and mixing as inferred from green-core clinopyroxenes in Pliocene basanite, southern Slovakia

Abstract

Electron microprobe analyses were made of 43 green-core clinopyroxene phenocrysts and 5 clinopyroxene megacrysts (233 analyses in total) in two Pliocene basanite samples from the Hungarian-Slovakian border area. There are two varieties of clinopyroxene cores. The most common defines a fractionation trend of Mg#'s [Mg = Mg/(Mg + Fe + Mn)] from 0.84 to 0.42, is Al-rich (9–13 wt.%) and has Al (VI) / Al (IV) ratios of 0.34-0.71 and Ti/Al ratios of 0.12-0.17. They can be anhedral, euhedral and partly resorbed, either homogeneous or oscillatory zoned, and there are sometimes inner and outer cores with compositional gaps between them. Less common are highly resorbed Fe-rich diopside cores with lower Al and Ti but Al (VI)/Al (IV) and Ti/Al ratios similar to those of the first type. Mafic (Mg# 0.85-0.77) clinopyroxene mantles surround all cores and have Al (VI) / Al (IV) and Ti/Al ratios similar to the cores. Finally, all grains have outermost rims with Fe/Mg ratios and Ti compositionally continuous from clinopyroxene mantle compositions. Overall compositions of the rims are identical to those of the basanite groundmass clinopyroxenes. Megacryst compositions resemble those of the first core type and have the same kinds of mantles and rims. We interpret the green-core phenocrysts as originating in a mantle alkalic basalt magma reservoir undergoing fractionation, replenishment and convective mixing. While cores record liquids in various stages of compositional evolution, the clinopyroxene mantles indicate that all cores (and megacrysts) shared a common mafic liquid prior to eruption of the host basalt. Compositional continuity between clinopyroxene mantles and rims suggests that the mantles represent liquid parental to the host basanite. Al and Ti abundances in the Fe-rich diopside cores suggest that they are unrelated to the main core type and megacrysts, and are perhaps wall-rock xenocrysts that crystallized from different highly evolved magma. This study of green-core phenocrysts is basaltic rocks of southern Slovakia identifies processes of magma replenishment and mixing associated with alkalic basalt origins. But because green-core phenocrysts are not common in basalts, special conditions, such as low rates of magma replenishment, may be essential to allow magma evolution to produce these types of pyroxene compositions and zonation.