Zircon inclusions in corundum megacrysts: I. Trace element geochemistry and clues to the origin of corundum megacrysts in alkali basalts

Zircons enclosed in corundum megacrysts from basalts have a short prismatic habit with the prism slightly more developed than the pyramidal faces. The {110} prism is dominant compared with {100}, indicating crystallisation from alkaline-peralkaline environments. Rare zircon decomposition reactions suggest the possible presence of a primary SiO₂ phase in the source paragenesis for corundum megacrysts. The zircon inclusions are geochemically distinctive: high Y (up to 1.2%), U (UO₂ up to 1.7 wt%), Th (ThO₂ up to more than 2 wt%), Hf (up to 3.4 wt%), and REE (∑REE up to 5000 ppm). They form a cogenetic group that is characterised by extreme REE fractionation with preference for the HREE varying from 30-100X chondritic Sm to 5000-20000X chondritic Lu. REE partitioning between zircon and silicate liquid is evaluated. The parental melts calculated for the zircon inclusions and other corundum-related zircons, using zircon/melt partition coefficients derived in the present paper, show concave-down REE patterns with a maximum Sm concentration close to 100X chondritic Sm abundance. Such REE patterns represent highly evolved melts after extensive fractionation of low-Ca feldspar prior to and throughout the crystallisation of zircon. The characteristics of zircon inclusions provide evidence that corundum megacrysts must have crystallised from alkaline and highly evolved melts under very reducing conditions, implying that the corundum is unlikely to have formed as high pressure phenocrysts in the host mafic magmas.