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

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/206Pbc 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.