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 towards igneous events and metamorphic anataxis; additionally, zircon is highly refractory and frequently polycyclic. Garnet, a rock-forming and thus commonly-occuring 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 towards the most recent garnet-crystallising 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/²⁰⁶Pbc values at the time of crystallization are near unity for the garnet analysed in this study, suggesting that garnet does not significantly partition U from Pb during crystallization, at least for the generally almandine-rich garnets analysed 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.