Abstract

In situ Pb-isotope ratios of alkali feldspars from laser ablation ICP-MS (LA-ICP-MS) were used to constrain melt-sources of the ca 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 ca 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.

1. KEY POINTS

2. Pb- and Nd-isotopic similarities of Tunkillia Suite to modern continental arc magmas.

3. The Tunkillia Suite has geochemical similarities to modern continental arc magmas.

4. Inheritance of unradiogenic Pb-isotopic signatures in the Tunkillia Suite require mafic melt hybridisation.

5. The Tunkillia Suite should be reconsidered to reflect the onset of late Paleoproterozoic subduction-related tectonism in the Gawler Craton.

摘要

来自激光烧蚀 ICP-MS (LA-ICP-MS) 的碱性长石的原位Pb-同位素比被用于限制钙的熔体来源1690-1670 Ma Tunkillia Suite，位于南澳大利亚太古宙-中元古代 Gawler Craton 的一组重要侵入岩。Tunkillia Suite 岩浆作用被不同地归因于超俯冲或碰撞后环境。Tunkillia Suite 的构造环境的有力解释对于高勒克拉通和澳大利亚大陆的元古代演化模型很重要。Pb 同位素结果表明区域变化涉及至少两个地壳 Pb 组分，对应于 Tunkillia 套件西部的一个古老的 U 型贫化储层，另一个反映地幔-地壳杂交。在 Tunkillia Suite 西部看到的非放射性铅同位素比率的继承 ( ²⁰⁶ Pb/ ²⁰⁴ Pb = 15.060–15.785, ²⁰⁷Pb/ ²⁰⁴ Pb = 15.154–15.426, ²⁰⁸ Pb/ ²⁰⁴ Pb = 34.963–35.766) 是一种全球罕见的现象，与 U/Pb 贫化麻粒岩的遗传有关。含有非放射性铅的铅同位素储层是最不适合熔体的来源之一，并且与大陆-大陆碰撞过程中产生的花岗岩不一致。与此相反，东部Tunkillia套件显示器的Pb-同位素指示壳幔混合的，具有体积主导地幔成分（²⁰⁶ PB / ²⁰⁴ PB = 15.945-16.143，²⁰⁷的Pb / ²⁰⁴ PB = 15.325-15.414，²⁰⁸的Pb / ²⁰⁴铅 = 35.330–35.757）。这些值与公布的 εNd 一致_{(1680 Ma)} (+2.6 to -2.1) 和 εHf _{(1680 Ma)} (+5.9 to +0.7) 特征证实了地幔熔体在 Tunkillia Suite 岩石形成中的作用。Tunkillia Suite 的放射性同位素特征与 1730-1700 Ma Kimban 造山运动期间地壳深熔产生的同碰撞花岗岩形成对比（εNd _(t) -11.7 到 -0.7），并且与弧相关的约1630 Ma St 一致Peters Suite 花岗岩 (εNd _(t)+2.31 至 -3.46）。此外，Tunkillia Suite 的同位素阵列显示了与安第斯山脉大陆弧岩浆作用有关的类似混合现象。我们认为 Tunkillia Suite 岩浆事件的特征是广泛的地壳-地幔杂交，有利于大陆弧形环境。

1. 关键点

2. Tunkillia Suite 与现代大陆弧岩浆的 Pb 和 Nd 同位素相似性。

3. Tunkillia Suite 与现代大陆弧岩浆具有地球化学相似性。

4. Tunkillia Suite 中非放射性铅同位素特征的继承需要镁铁质熔体杂交。

5. 应该重新考虑 Tunkillia Suite 以反映 Gawler Craton 中晚古元古代俯冲相关构造活动的开始。