Early rifting during marginal basin development: Petrography, microstructure, and detrital zircon U-Pb geochronology of the Lapataia Formation, Argentine Fuegian Andes

Although being one of the most efficient tools to constrain the stratigraphy of sedimentary basins, detrital zircon geochronology may present limitations under certain conditions, and isotopic ages should be treated with caution before assigning a depositional age. This is the case of depositional environments that lack coeval volcanism, where there may exist a significant time gap between the age of the youngest detrital zircon available and the actual time of deposition of a sedimentary rock. In this paper, we put on a comprehensive analysis in which detrital geochronology is aided with other reliable geological constraints. As a case study, we evaluate the stratigraphic age and provenance of the Lapataia Formation from the Fuegian Andes of southernmost South America, a low-grade metamorphic unit with a problematic stratigraphic definition in the context of the Rocas Verdes basin. Detrital ages show very wide age spectra, with a younger peak at ca. 267 Ma. However, based on detailed petrography and microstructural observations, as well as stratigraphic correlations at a regional scale, we conclude that the Lapataia Formation represents the earliest stages of Jurassic rifting during the opening of the Rocas Verdes basin. Erosion from older basement blocks in an environment lacking coeval volcanic activity explains the occurrence of Paleozoic and older grains, and the lack of younger zircons. This work offers a multifaceted approach, which may be essential to assist radiometric detrital geochronology at certain stages during basin evolution.