Systematics of clinopyroxene phenocrysts, megacrysts, and cumulates in Tertiary basalts of southern Slovakia with implications in the structure of lithospheric mantle

Abstract

: Tertiary alkali basalts in the South-Slovakian Basin contain homogeneous clinopyroxene megacrysts, composite phenocrysts with chemically and optically homogeneous olive-green cores overgrown by oscillatory zoned rims, and fragments of clinopyroxene-rich igneous cumulates. Discrimination based on Ca, Na, Ti and Cr concentrations defined clinopyroxenes with alkalic, tholeiitic and transitional affinities. Single-pyroxene thermobarometry revealed that the incipient clinopyroxene crystallization occurred at 1300–1350 °C and 2.0–2.2 GPa, thus identifying the zone of mantle melting in a depth of >70 km. The highest frequency of pressure data from clinopyroxene-rich mafic cumulates, betwen 0.7 and 1.2 GPa, indicate stagnant basaltic reservoirs located beneath the Moho discontinuity, 30–43 km deep. Late fractionation products of alkali basalt derivatives may have occurred as laccoliths and dykes in the lower crust. Thermodynamic modelling of mineral assemblages of cumulate xenoliths revealed that parental melts for both alkalic and tholeiitic affinities were silica-undersaturated (46–46.5 wt. % SiO 2 ) and moderately alkalic (index of alkalinity 2.0–3.8, Mg# ~50) basalt to basanite, with pre-eruptive water contents between 1 and 2 wt. %. Amphibole–biotite– titanite–ilmenite–plagioclase cumulates recorded the advanced stage of fractionation of more calcic basalt at 900–920 °C and 0.65 GPa. Deep-seated basalt reservoirs have been less alkalic than erupted lavas, the latter showing a Mg depletion and an increasing aluminium saturation index diagnostic of the extensive crystal separation on the way to the surface. Olive-green megacrysts and cores of zoned phenocrysts originated at lower temperatures and higher pressures than their autocrystic rims. The megacrystic assemblage of Fe-diopside–Al-augite, Mg-calcite, apatite, ulvöspinel, and disintegrated amphibole crystallized from evolved, relatively cold (~950–1100 °C), Fe-rich carbonatite–alkalic silicate melt within the depth interval of 26–53 km, corresponding to a lithostatic pressure of 0.7–1.5 GPa. The silicate–carbonate–phosphate melt fraction probably originated in the subducting slab of oceanic crust.