Detrital Garnet Geochronology by In Situ U‐Pb and Lu‐Hf Analysis: A Case Study From the European Alps

Abstract

Detrital geochronology employing the widely‐used zircon U‐Pb proxy is biased toward igneous events and metamorphic anatexis; additionally, zircon is highly refractory and frequently polycyclic. Garnet, a rock‐forming and thus commonly occurring mineral, is predominantly metamorphic and much less refractory. Here, we report in situ U‐Pb and Lu‐Hf ages from detrital garnet hosted in ancient and modern sediments of the European Alps. Both geochronometers are biased toward the most recent garnet‐crystallizing metamorphic event in the source area, with fewer inherited ages. This likely reflects efficient removal of inherited garnet during diagenesis and metamorphism, and is in contrast to detrital zircon, apatite, and rutile U‐Pb data, which largely record pre‐Alpine ages. Neither the U‐Pb nor Lu‐Hf system in garnet exhibits a relationship between age recovery and composition. However, the Lu‐Hf system in garnet yields significantly better age recovery than the U‐Pb system. Estimated initial ²³⁸U/ ²⁰⁶Pb _c values at the time of crystallization are near unity, suggesting that garnet does not significantly partition U from Pb during crystallization, at least for the generally almandine‐rich garnets analyzed in this study. Hence, Lu‐Hf geochronology of detrital garnet offers an effective method to detect and date the most recent phase of mid‐grade metamorphism in sub‐anatectic source areas, in which detrital zircon U‐Pb analysis may be of less utility.

Plain Language Summary

Mountain ranges are characterized by rapid changes in their constituent rocks as these undergo metamorphism to adjust to increasing pressure and temperature during tectonic burial. These metamorphic processes drive mineral crystallization. Once cooled, each mineral acts as a geochemical reservoir isolated from the surrounding environment. Therefore, if a mineral has incorporated a radioactive isotope during crystallization, it can be dated to constrain the timings and rates of metamorphism. As erosion ultimately converts crystalline bedrock to sediment, the geological histories of these processes are preserved in the sediment shed during erosion. Consequently, these histories can be read from sedimentary rocks in adjacent sedimentary basins. Minerals traditionally used to study the sources of these sediments, such as zircons, largely grow from molten rock rather than during metamorphism, and are tough enough to be recycled through multiple tectonic events. The mineral garnet more commonly grows under metamorphic conditions and is thus more effective at directly recording the most recent phases of significant mountain building. Here, we present uranium‐lead and lutetium‐hafnium ages of garnet in modern and ancient sediment from the Alps. We show that garnet preferentially records Alpine events and is thus suitable for provenance studies targeting the most recent mountain building event.

Key Points

• Detrital garnet U‐Pb and Lu‐Hf ages preferentially record the most recent metamorphic event in the source area

• Both systems are less refractory than alternative detrital U‐Pb geochronometers

• Age recovery for Lu‐Hf in garnet is considerably better than for U‐Pb