Abstract
Gem minerals at Lava Plains, northeast Queensland, offer further insights into mantle-crustal gemformation 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 ~100-1000, Ga/Mg 3-18) grew with ~3 Ma micro-zircons of both mantle (δ180 4.5-5.6‰) and crustal (δ180 9.5-10.1‰) affinities. Zircon megacrysts (3±1 Ma) show mantle and crustal characteristics, but most grew at crustal temperatures (600-800°C). Xenolith studies suggest hydrous silicate melts and fluids initiated from amphibolized mantle infiltrated into kyanite+sapphire granulitic crust (800°C, 0.7 GPa). This metasomatized the sapphire (Fe/Mg ~50-120, Ga/Mg ~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.
Original language | English |
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Pages (from-to) | 545-581 |
Number of pages | 37 |
Journal | Mineralogical Magazine |
Volume | 79 |
Issue number | 3 |
DOIs | |
Publication status | Published - 2015 |
Keywords
- Australia
- basalt
- corundum
- gemstone genesis
- lithosphere
- sapphires
- trace elements
- zircon