oa Probing the ‘Zircon Zip’: Geochemistry

Zircon megacrysts feature in basalt fields along the Australian-Asian continental margins and form an Indo-Pacific petrological puzzle. These xenocrysts show magmatic growth features under cathodoluminescence imaging. Representative zircons were studied from several sites along this zone (3 Australian, 2 South East Asian, 1 far eastern Russian).

Zircon geochemistry was characterised by electron microprobe and laser ablation inductively coupled plasma mass spectrometry analysis. Hf/Zr ratios (0.013 –0.022) suggest undersaturated to intermediate parental melts, while U/Th (0.2 – 1.7) suggests both fluids and silicate melts participated. Variations in rare earth element contents (av 74 – 2800ppm) and in chondrite normalised parameters Ce^*, Eu^* and Yb/Sm suggest different chemical fractionations took place both within and between zircon sites.

Geochemical signatures for these zircons, compared with those from known lithologies (statistical trees) suggest lamproitic, granitoid and volatile-rich mixed parents provided the zircons.