Origin of megacrysts and pyroxenite xenoliths from the Pliocene alkali basalts of the Pannonian Basin (Hungary)

Clinopyroxene and amphibole megacrysts and type 11 pyroxenite xenoliths occur in young (mainly Pliocene) alkali basalts of the Pannonian Basin. Major and trace element contents (the latter were determined by LAM-ICP-MS) of the clinopyroxenes and amphiboles suggest that megacrysts and type 11 xenoliths share a common origin. Both suites are fragments of igneous cumulates and pegmatitic veins that crystallised from alkali basaltic melts in the upper mantle at 11-14kbar pressures. Trace element contents of the calculated hypothetical melts in equilibrium with the megacrysts and type 11 xenoliths are very similar to those of the host alkali basalts, but (87)Sr/(86)Sr ratios are significantly lower and (143)Nd/(144)Nd ratios are higher in the megacrysts and xenoliths than in the Pliocene alkali basalts that host them. The source rocks of the megacrysts and type 11 xenoliths had probably crystallised from earlier intrusions of alkali basaltic melts in the lithospheric mantle and were disrupted later by more voluminous intrusions of compositionally similar but isotopically more enriched alkali basaltic magmas.