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

• Applicant: Dr. Leo Jakob Millonig
• Subject Area: Palaeontology
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 510971915

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.

• DFG Programme: Research Grants