A multi-method approach to constrain the age of eruption and post-depositional processes in a Lower Jurassic ignimbrite from the Marifil Volcanic Complex, eastern North Patagonian Massif

Abstract

In the eastern North Patagonian Massif, the Marifil Volcanic Complex corresponds to an Early Jurassic magmatic event. In this study, we evaluate new geochronological and thermochronological data obtained from an ignimbrite of the complex. U–Pb zircon ages from the pumice fraction reveal a Concordia age of 184.4 ± 1.1 Ma. Conversely, U–Pb zircon ages from the whole-rock sample evidence a Concordia age of 190.4 ± 0.6 Ma. Fission track techniques were applied on the same whole-rock sample. Zircon fission track performed on the same zircon-crystals dated by U–Pb yielded an age of 172.5 ± 8.1 Ma, while apatite fission track evidenced a cooling age of 179.1 ± 13.5 Ma. Apatite and zircon crystals describe uniform morphologies with unaltered euhedral to subhedral grains and well-developed crystalline forms. The results suggest that the age of eruption and deposition of the ignimbrite is better explained by the pumice zircon age rather than the whole-rock zircon age. Inherited grain-ages and petrographic evidence support provenance from older, probably cogenetic, magmatic products. Fission track ages reflect a post-depositional process probably related to diagenetic mineral changes.