Extraction of Pb with artificially too-old ages during stepwise dissolution experiments on Archean zircon

Stepwise HF-dissolution experiments on five Archean zircon populations reveal very systematic patterns of Pb discrimination, releasing at one point Pb with artificially too-old 207Pb/206Pb ages. The experiments involved a first HF dissolution step for 1 h at room temperature and evaporation on a hot-plate that produces Pb with young 207Pb/206Pb ages, followed by a 4 1/2 h HF dissolution step in an oven at 190°C that liberates the excessively old Pb. The final residue yields in most cases U–Pb data that are consistent with the results obtained by the normal selection, abrasion, and total dissolution procedure. In these examples, the too-old ages cannot be easily explained by the presence of an inherited core component but are more likely to indicate segregation of Pb in zircon sub-domains during thermal annealing episodes early in its history, as has been proposed by other workers. Aside from shedding light on these particular aspects of zircon U–Pb systematics, the combined results also provide geologically relevant information concerning the regional evolution of the western Superior Province in Ontario. An age of 2718±3 Ma for a gabbroic unit from the Quetico Subprovince shows that this was coeval to 2722–2718 Ma ultramafic to felsic igneous rocks in the adjacent Shebandowan greenstone belt, including a gabbro body dated in this study at 2725+17/−11 Ma. These age relationships suggest that volcanic units of the Shebandowan greenstone belt were tectonically imbricated in younger sedimentary rocks of the Quetico basin during late Archean convergence. The other three samples represent felsic intrusive units from Geraldton in the Wabigoon subprovince. An age of 2699±1 Ma for an Au-mineralized feldspar porphyry dyke and identical ages of 2690±1 Ma for two phases of the syn-tectonic Croll Lake stock put constraints on the timing of major deformation and hydrothermal activity in the belt.