Initial Hf isotope compositions in magmatic zircon from early Proterozoic rocks from the Gawler Craton, Australia: A test for zircon model ages

Abstract

We evaluate the validity of age information obtained from two-stage Hf model ages in zircon. Multiple isotope investigations of minerals from the Kiana Granite of the Archean Sleafordian Complex from the southern Gawler Craton, Australia, reveal a complex geologic evolution of the suite. Rubidium–Sr mineral ages define a tectono-thermal event at 1673.4 ± 4.2 Ma, associated with the Kimban orogeny. ID-TIMS U–Pb ages from single zircons yield an upper concordia intercept at 2348 ± 21 Ma, which marks the time of magmatic emplacement. Whole-rock Nd model ages yield crustal residence ages of ∼ 2.8 Ga. For each single zircon grain, the Lu–Hf isotope systematic was determined by solution MC-ICPMS. Two-stage Hf model ages for the magmatic zircons vary by as much as 500 Myrs, depending on the choice of Lu/Hf for the source of the host rock, putting further emphasis on the need to constrain the source Lu/Hf of a rock used for model age calculations. An average Lu/Hf for mafic crust was assumed for the source reservoir of the rocks to calculate its Hf isotope evolution through time, which yields zircon model ages of ∼ 3.5 Ga that coincide with the initial growth episode postulated for the Gondwana supercontinent. It is demonstrated that two-stage zircon model ages deduced from initial Hf isotopes can provide important age information for the source of igneous rocks, and consequently on important crustal formation events, even when conventional geochronologic investigations fail, e.g., due to thermo-tectonic overprints or mixing.