Evolution of mafic alkaline melts crystallized in the uppermost lithospheric mantle: A melt inclusion study of olivine-clinopyroxenite xenoliths, Northern Hungary

Olivine-clinopyroxenite xenoliths exhumed in alkali basalts (sensu lato) in the Nógrád-Gömör Volcanic Field (NGVF), northern Hungary, contain abundant silicate melt inclusions. Geothermobarometric calculations indicate that these xenoliths crystallized as cumulates in the upper mantle near the Moho. These cumulate xenoliths are considered to represent a period of Moho underplating by mafic alkaline magmas prior to the onset of Late Tertiary alkaline volcanism in the Carpathian-Pannonian region. The major and trace element compositions of silicate melt inclusions in olivine display an evolutionary trend characterized by a strong decrease in CaO/Al₂O₃. The parental melt of the cumulates was a basanite formed by low-degree (∼2%) partial melting of a garnet peridotite source. The compositional trend of the silicate melt inclusions, textural features, and modelling with pMELTS show that the parental melt evolved by major clinopyroxene and minor olivine crystallization followed by the appearance of amphibole simultaneously with significant resorption of the earlier clinopyroxene and olivine. The resulting residual melt was highly enriched in Al₂ O₃, alkalis and most incompatible trace elements. This type of melt is likely to infiltrate and react with surrounding mantle peridotite as a metasomatic agent. It might also form high-pressure pegmatite-like bodies in the mantle that might be the source of the amphibole and sanidine megacrysts also found in the alkali basalts of the NGVF. Preferential remelting of the later-formed (i.e. lower temperature) mineral assemblage (amphibole, sanidine, residual glass) might have significantly contaminated the host alkaline mafic lavas, increasing their Al₂O₃ and total alkali contents and, therefore, reducing their MgO, FeO and CaO content.