Origin of Zircon Megacrysts from Cenozoic Basalts in Northeastern Cambodia: Evidence from U-Pb Age, Hf-O Isotopes, and Inclusions

AbstractZircon megacrysts found in basalts may provide a window into metasomatic processes and element transport in the upper mantle. However, little is known about the formation conditions of these zircons. Here we report U-Pb ages, Hf-O isotope compositions, trace element data, and mineral inclusion analyses for six large fragments of zircon megacrysts from the Gem village in northeastern Cambodia. The zircons have low contents of U, Th, Y, P, and rare earth elements (REEs; 130–288 ppm). The normalized REE pattern is characterized by a steeply rising slope from light REEs to heavy REEs and positive Ce anomalies. Secondary ion mass spectrometer isotopic analyses on the zircon megacrysts yield a mean U-Pb age of 0.98 ±0.04 Ma (2σ) and homogeneous oxygen isotopic composition with a mean δ18O value of 5.0‰ ± 0.18‰ (2σ), identical to the δ18O mantle value. The 176Hf/177Hf values range from 0.282970 ± 0.000024 (2σ) to 0.283040 ± 0.000016 (2σ), with eHf (0.98 Ma) values of 6.6–9.0, obtained by the laser ablati...