Corundum (sapphire) and zircon relationships, Lava Plains gem fields, NE Australia: integrated mineralogy, geochemistry, age determination, genesis and geographical typing

Gem minerals at Lava Plains, northeast Queensland, offer further insights into mantle-crustal gem-formation under young basalt fields. Combined mineralogy, U-Pb age determination, oxygen isotope and petrological data on megacrysts and meta-aluminosilicate xenoliths establish a geochemical evolution in sapphire, zircon formation between 5 to 2 Ma. Sapphire megacrysts with magmatic signatures (Fe/Mg similar to 100-1000, Ga/Mg 3-18) grew with similar to 3 Ma micro-zircons of both mantle (delta O-18 4.5-5.6%) and crustal (delta O-18 9.5-10.1 parts per thousand) affinities. Zircon megacrysts (3 +/- 1 Ma) show mantle and crustal characteristics, but most grew at crustal temperatures (600-800 degrees C). Xenolith studies suggest hydrous silicate melts and fluids initiated from amphibolized mantle infiltrated into kyanite+sapphire granulitic crust (800 degrees C, 0.7 GPa). This metasomatized the sapphire (Fe/Mg similar to 50-120, Ga/Mg similar to 3-11), left relict metastable sillimanite-corundum-quartz and produced minerals enriched in high field strength, large ion lithophile and rare earth elements. The gem suite suggests a syenitic parentage before its basaltic transport. Geographical trace-element typing of the sapphire megacrysts against other eastern Australian sapphires suggests a phonolitic involvement.