Pb-isotope ratios and the petrogenesis of the Tunkillia Suite, Gawler Craton

<i>In situ</i> Pb-isotope ratios of alkali feldspars from laser ablation ICP-MS (LA-ICP-MS) were used to constrain melt-sources of the <i>ca</i> 1690–1670 Ma Tunkillia Suite, an important suite of intrusive rocks located in the Archean–Mesoproterozoic Gawler Craton, South Australia. Tunkillia Suite magmatism has been variously attributed to either a supra-subduction or post-collisional setting. A robust interpretation of the Tunkillia Suite’s tectonic setting is important for models of the Proterozoic evolution of the Gawler Craton and Australian continent. The Pb-isotopic results indicate a regional variation involving at least two crustal Pb components corresponding to an ancient, U-depleted reservoir in the western Tunkillia Suite and another that reflects mantle–crustal hybridisation. Inheritance of unradiogenic Pb-isotope ratios seen in the western Tunkillia Suite (²⁰⁶Pb/²⁰⁴Pb = 15.060–15.785, ²⁰⁷Pb/²⁰⁴Pb = 15.154–15.426, ²⁰⁸Pb/²⁰⁴Pb = 34.963–35.766) are a globally rare phenomenon that are linked to inheritance from U/Pb-depleted granulites. Pb-isotopic reservoirs containing unradiogenic Pb are among the least melt-fertile sources and are inconsistent with granites generated during continent–continent collision. In contrast, the eastern Tunkillia Suite displays Pb-isotope ratios indicative of crust–mantle mixing, with a volumetrically dominant mantle component (²⁰⁶Pb/²⁰⁴Pb = 15.945–16.143, ²⁰⁷Pb/²⁰⁴Pb = 15.325–15.414, ²⁰⁸Pb/²⁰⁴Pb = 35.330–35.757). These values are consistent with published εNd_{(1680 Ma)} (+2.6 to −2.1) and εHf_{(1680 Ma)} (+5.9 to +0.7) signatures that confirm the role of mantle-melts in the petrogenesis of the Tunkillia Suite. The radiogenic isotopic signatures of the Tunkillia Suite contrast with the syn-collisional granitoids generated from crustal anatexis during the 1730–1700 Ma Kimban Orogeny (εNd_(t) −11.7 to −0.7) and are consistent with the arc-related <i>ca</i> 1630 Ma St Peters Suite granitoids (εNd_(t) +2.31 to −3.46). In addition, isotopic arrays for the Tunkillia Suite demonstrate a similar mixing phenomenon involved in continental arc magmatism of the Andean Cordillera. We contend that the Tunkillia Suite magmatic event is characterised by widespread crustal–mantle hybridisation, favouring a continental arc-like setting.KEY POINTSPb- and Nd-isotopic similarities of Tunkillia Suite to modern continental arc magmas.The Tunkillia Suite has geochemical similarities to modern continental arc magmas.Inheritance of unradiogenic Pb-isotopic signatures in the Tunkillia Suite require mafic melt hybridisation.The Tunkillia Suite should be reconsidered to reflect the onset of late Paleoproterozoic subduction-related tectonism in the Gawler Craton. Pb- and Nd-isotopic similarities of Tunkillia Suite to modern continental arc magmas. The Tunkillia Suite has geochemical similarities to modern continental arc magmas. Inheritance of unradiogenic Pb-isotopic signatures in the Tunkillia Suite require mafic melt hybridisation. The Tunkillia Suite should be reconsidered to reflect the onset of late Paleoproterozoic subduction-related tectonism in the Gawler Craton.