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

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