Zircon inheritance in mafic inclusions from Bega batholith granites, southeastern Australia: An ion microprobe study

Abstract

Ion microprobe U‐Th‐Pb dating zircons from four mafic inclusions from the Palaeozoic granites of the Glenbog and Blue Gum suites, Bega Batholith, shows that while most of the zircons are probably magmatic, the cores of some are premagmatic. The mean 206Pb/238U ages of the magmatic zircons are 412±3 Ma and 414±2 Ma for two inclusions from the Glenbog Granodiorite, 406±6 Ma for an inclusion from the Anembo Granodiorite, and 377±3 Ma for an inclusion from the Blue Gum Tonalite. The mafic inclusions and inherited zircons come from the granites' source rocks. The abundance of inherited zircons in the inclusions is less than in the granites. Most of the inherited zircons, as in the granites, are 430–630 Ma old, which probably represents the time interval of underplating of the crust prior to granite genesis, or a period of prolonged metamorphism of earlier underplated material. Rare Proterozoic inherited zircons are probably from a sedimentary component in the source. The inclusions do not appear to contain inherited zircons ∼1000 Ma old, which are common in the granites.