Geochronological constraints on tourmaline formation in the Western Fold Belt of the Mount Isa Inlier, Australia: Evidence for large-scale metamorphism at 1.57 Ga?

Boron metasomatism during two deformational events resulted in tourmaline formation in parts of late Palaeoproterozoic Eastern Creek Volcanics of the Western Fold Belt of the Mount Isa Inlier. Despite low U/Pb ratios (≪1, as measured by laser ablation ICP-MS). When the step leaching process was applied to the tourmaline it process extracted Pb fractions with 206Pb/204Pb up to 76, illustrating the method's ability to efficiently separate radiogenic and common Pb that are hosted in different crystal lattice sites. Tourmaline from two syn-D2 metamorphic/hydrothermal veins at Anderson's Lode and Eldorado yield similar Pb step-leaching ages of 1573 ± 12 and 1577 ± 48 Ma, respectively. Tourmaline from a post-D2, but, pre-D3a, vein at King was dated at 1528 ± 51 Ma. Although there is evidence to suggest some contribution from U-rich inclusions (i.e., zircon and monazite) to some leach steps, the isochrons are thought to provide robust ages for hydrothermal tourmaline formation during two discrete thermal events. This is supported by estimates of Pb closure temperatures in tourmaline derived from ionic porosity models. The ∼1575 Ma ages for D2 also reflect the main metamorphic event in the Western Fold Belt, but are ∼40 myr older than accepted age estimates for this episode. The revised timing for peak metamorphism/tectonism in the Western Fold Belt corresponds with the age of metamorphism in the Eastern Fold Belt, suggesting broadly coeval tectonic histories across the entire Inlier from at least 1600 Ma.