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

Abstract

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