ZIRCON INCLUSIONS IN CORUNDUM MEGACRYSTS .1. TRACE-ELEMENT GEOCHEMISTRY AND CLUES TO THE ORIGIN OF CORUNDUM MEGACRYSTS IN ALKALI BASALTS

Zircons enclosed in corundum megacrysts from basalts have a shea prism atic habit with the prism slightly more developed than the pyramidal f aces. The {110} prism is dominant compared with {100}, indicating crys tallisation from alkaline-peralkaline environments. Rare zircon decomp osition reactions suggest the possible presence of a primary SiO2 phas e in the source paragenesis for corundum megacrysts. The zircon inclus ions 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 (Sigma REE up to 5000 ppm). They form a cogenetic group that is charac terised by extreme REE fractionation with preference for the HREE vary ing from 30-100x chondritic Sm to 5000-20000X chondritic Lu. REE parti tioning 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 prese nt paper, show concave-down REE patterns with a maximum Sm concentrati on 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 characteris tics of zircon inclusions provide evidence that corundum megacrysts mu st have crystallised from alkaline and highly evolved melts under very reducing conditions, implying that the corundum is unlikely to have f ormed as high pressure phenocrysts in the host mafic magmas.