The evolution of orogenic belts – LA-MC-ICPMS U‒Pb dating of metamorphic garnet

造山带是碰撞构造最壮观的地形表现。它们见证了形成和塑造大陆的古代和现代构造过程。区域变质岩对于解释造山带的复杂演化具有特别重要的意义，因为它们保留了关于潜在构造过程的独特的压力-温度（P-T）和时间（t）信息。在这种背景下，石榴石是最终的岩石年代计，它使我们能够比任何其他矿物更详细地表征区域变质岩的P-T-t演化。目前，Lu-Hf和Sm-Nd同位素系统是最常用的区域变质石榴石定年方法，但这两种方法都需要时间和劳动密集型的同位素稀释（ID）技术，这阻碍了它们更广泛的应用。最近的研究（Millonig等人，2020; Schannor等人，2021），但是，表明低U（~1-100 ng/g）石榴石的U-Pb测年LA-ICPMS代表了一个可行的除了我们的地质年代学工具箱具有更高的样品吞吐量和空间分辨率（~100 µm激光光斑直径）相比，传统的解决方案为基础的方法。该项目的目标是全面建立这种最先进的方法的准确性，精度，适用性和局限性，并评估其适用于多变质石榴石中记录的多个造山旋回的定年。为了实现这一目标，我们将应用LA-ICPMS U-Pb测年石榴石样品，以前已经使用Sm-Nd或Lu-Hf同位素系统，并从阿杜拉推覆体，阿尔卑斯山中部的多变质石榴石。含石榴石的样品将通过光学和扫描电子显微镜、主量和微量元素分析以及LA-ICPMS U-Pb测年进行严格的调查。本项目的研究结果将全面展示变质石榴子石的LA-ICPMS U-Pb定年是否有潜力成为岩石年代学研究的常规工具，这将大大提高我们重建和解释区域变质岩的P-T-Pb历史及其形成的构造环境的能力。因此，我们认为，这种方法有可能彻底改变我们对不断塑造地球表面的主要构造过程的理解，包括（超级）大陆的组装以及大陆地壳的生长和循环。

Orogenic belts are the most spectacular topographic manifestations of collisional tectonics. They bear witness to ancient and current tectonic processes that form and shape the continents. Regional metamorphic rocks are of particular importance for deciphering the complex evolution of orogenic belts as they preserve unique pressure-temperature (P–T) and temporal (t) information about the underlying tectonic processes. In this context, garnet is the ultimate petrochronometer that allows us to characterize the P–T–t evolution of regional metamorphic rocks in much greater detail than would bepossible with any other mineral. Currently, the Lu–Hf and Sm–Nd isotope systems are most commonly employed to date regional metamorphic garnet, but both procedures require time- and laborintensive isotope dilution (ID) techniques, which hinders their morewidespread application. Recent studies (Millonig et al., 2020; Schannor et al., 2021), however, demonstrate that U–Pb dating of low-U (~1–100 ng/g) garnet by LA-ICPMS represents a feasible addition to our geochronology toolbox with a much higher samplethroughput and spatial resolution (~100 µm laser spot diameter) compared to conventional solution-based methods. The objectives of this project are to comprehensively establish the accuracy, precision, applicability and limitations of this state-of‒the-art method, and to assess its suitability for dating multiple orogenic cycles recorded in polymetamorphic garnet. To achieve this, we will apply LA-ICPMS U-Pb dating to garnet samples that have previously been dated using the Sm‒Nd or Lu‒Hf isotope systems, and to polymetamorphic garnet from the Adula nappe, Central Alps. The garnet-bearing samples will be subjected to rigorous investigations by optical and scanningelectron microscopy, major and trace element analysis, and U-Pb dating by LA-ICPMS. The results of this project will comprehensively demonstrate, if LA-ICPMS U‒Pb dating of metamorphic garnet has the potential to become a routine tool for petrochronological studies, which will substantially improve our ability to reconstruct and interpretthe P‒T‒t histories of regional metamorphic rocks and the tectonic settings in which they formed. As such, this method has in our view the potential to revolutionize our understanding of the major tectonic processes that constantly shape the face of the Earth, including the assembly of (super)continents and the growth and recycling ofcontinental crust.