Biotite generates significant radiogenic Sr and is thus suitable for Rb–Sr dating, which can be achieved directly in thin section via laser ablation and provides a means to directly date growth of this important fabric-forming mineral or its cooling. In contrast, apatite has an extremely low Rb/Sr ratio and hence offers a useful tool to estimate source ⁸⁷Sr/⁸⁶Sr_i. Plagioclase also contains significant Sr and may further assist in understanding the rocks Sr isotopic system. However, the fluid source of growing or recrystallizing apatite, plagioclase, and biotite need not be the same. Compounding the complexity, apatite may gain radiogenic Sr from biotite during recrystallization or dissolution–reprecipitation that accompanies metamorphism. In this contribution, we present a case study from the Kalak Nappe Complex in the Scandinavian Caledonides, Arctic Norway, where a combined geochronology program, critically supported by biotite Rb–Sr dating, helps to resolve the conspicuous inverted metamorphic field gradient found upwards through the nappe stack in this orogen. Apatite U–Pb ages of 417–413 Ma in the upper nappes are interpreted as cooling through the Pb retention zone. Such ages contrast with the suggestion of older, partially reset apatite ages in the lower nappes. Biotite Rb–Sr ages of c. 413 Ma in the upper nappes are similar to U–Pb ages of coexisting apatite but conspicuously different from c. 433 Ma biotite Rb–Sr ages in the lower nappes. Ages of c. 433 Ma are comparable to muscovite ⁴⁰Ar/³⁹Ar and titanite and monazite U–Pb ages throughout the complex and may directly date mineral growth at peak thermal conditions during the Scandian orogenic event. Apatite ⁸⁷Sr/⁸⁶Sr ratios also track the influence of metamorphism and imply more radiogenic signatures relative to relic plagioclase in samples which have greater overprinting in the upper nappes of the complex. Phase equilibrium modelling establishes Palaeozoic peak P–T conditions of 610–750 °C at <12 kbar in the upper nappes, comparable to peak conditions previously established for the lower nappes. Combined thermal modelling of the post-peak Scandian orogenic history based on published ⁴⁰Ar/³⁹Ar, paired with new Rb–Sr, and U–Pb data, implies a slower cooling pathway in the upper nappes relative to the lower nappes. We reconcile this thermal history with a geodynamic model of obduction of a hot back-arc, as represented by the Silurian Magerøy Nappe, onto the upper nappes of the Kalak Nappe Complex at c. 430 Ma. These results demonstrate that in-situ laser ablation Rb–Sr dating of biotite provides a powerful tool for resolving orogenic tectonothermal histories in orogens flushed with excess argon.

黑云母产生显着的放射成因 Sr，因此适用于 Rb-Sr 测年，这可以通过激光烧蚀直接在薄片中实现，并提供一种直接测年这种重要的织物形成矿物的生长或其冷却的方法。相比之下，磷灰石具有极低的 Rb/Sr 比率，因此提供了一个有用的工具来估计源⁸⁷ Sr/ ⁸⁶ Sr _i. 斜长石还含有大量的 Sr，可以进一步帮助了解岩石的 Sr 同位素系统。然而，生长或重结晶磷灰石、斜长石和黑云母的流体源不必相同。使复杂性更加复杂的是，磷灰石可能会在伴随变质作用的重结晶或溶解-再沉淀过程中从黑云母中获得放射性 Sr。在这篇文章中，我们介绍了挪威北极斯堪的纳维亚 Caledonides 的 Kalak Nappe Complex 的案例研究，其中一个组合地质年代学计划，得到黑云母 Rb-Sr 测年的大力支持，有助于解决向上发现的显着倒变质场梯度在这个造山带中的推覆层。上推覆层中 417-413 Ma 的磷灰石 U-Pb 年龄被解释为通过 Pb 保留区冷却。这样的年龄与较低推覆层中较旧的、部分重置的磷灰石年龄形成对比。c. 黑云母 Rb-Sr 年龄。上推覆中的 413 Ma 与共存磷灰石的 U-Pb 年龄相似，但与 c 明显不同。433 Ma 黑云母 Rb-Sr 年龄在下推覆层中。时代 c. 433 Ma堪比白云母⁴⁰ Ar/ ³⁹ Ar 以及钛石和独居石 U-Pb 在整个复合体中老化，并且可以直接测出斯堪地亚造山事件期间峰值热条件下矿物生长的年代。磷灰石⁸⁷ Sr/ ⁸⁶ Sr 比率也跟踪变质作用的影响，并暗示相对于在复合体上部推拿中具有更大叠印的样品中的残余斜长石具有更多的放射特征。相平衡模型建立了 610–750 °C 的古生代峰值 P-T 条件，在上推带 <12 kbar，与先前为下推带建立的峰值条件相当。基于已发表的⁴⁰ Ar/ ³⁹峰后斯堪的亚造山历史的组合热模拟Ar 与新的 Rb-Sr 和 U-Pb 数据相结合，意味着相对于下部推带，上部推带中的冷却路径较慢。我们将这一热历史与热后弧的地球动力学模型相协调，以志留纪 Magerøy Nappe 为代表，在 c. 的 Kalak Nappe 复合体的上部推覆上。430 马。这些结果表明，黑云母的原位激光烧蚀 Rb-Sr 测年为解决被过量氩冲刷的造山带中的造山构造热历史提供了强有力的工具。