(Mis)Identification of magmatic and exhumation ages by detrital zircon U-Pb and He double dating: A case study from the Bergell-Gonfolite system (European Alps)

Reliable interpretation of detrital thermochronometric datasets requires correct attribution of these ages as either the record of exhumational cooling or the record of post-magmatic cooling independent of tectonic or erosional exhumation. A classic approach for identifying magmatic cooling ages is through double dating leveraging paired high- and low-temperature geo/thermochronologic systems, which should yield, within error, indistinguishable ages from the same grain. On the contrary, low-temperature thermochronometric ages that are younger than their corresponding crystallization ages are mostly invariably interpreted to record exhumation. Here, we test this last assumption by applying a detrital zircon U-Pb and (U-Th)/He double-dating approach to a well-constrained source-to-sink system in the southern European Alps, archiving the progressive unroofing of the Bergell-Novate volcanic-plutonic complex and associated country rocks. We depth-profile U-Pb dated unpolished detrital zircon grains and performed (U-Th)/He analysis on non-volcanic/plutonic grains. Of the double-dated grains, 40% yielded (U-Th)/He ages overlapped in age with Bergell-Novate magmatism (32-30 and 27-24 Ma). Despite yielding (U-Th)/He ages younger than their corresponding U-Pb ages, these ages record post-magmatic cooling within the contact aureole and not exhumation. Our findings indicate that only a fraction of the grains yielding magmatic He ages can be identified by double dating, and that the assumption that all the remaining grains constrain exhumation can be potentially misleading. Finally, we present improved criteria for the interpretation of detrital zircon thermochronometric double-dating results and conclude that many previous interpretations based on a classic double-dating approach should be reconsidered in syn-magmatic orogenic systems.