Late Jurassic event of ignimbrite flare-up linked to extensional tectonics: The beginnings of Andean volcanism in southern Patagonia (~46°S, Chile).

Abstract

Understanding the origin of Late Jurassic volcanism in southern Patagonia is crucial for unraveling the early Andean orogenic evolution. However, radiometric dating is not connected to stratigraphic analysis along the South Patagonian Andes, which obscures the real duration of the Late Jurassic magmatic activity. In this contribution, we present the results of a volcanic stratigraphy analysis, complemented by structural and petrographic data, on a thick succession of acidic volcanogenic rocks in the Laguna Verde district of southern Chile located along the south shore of General Carrera-Buenos Aires Lake. Through the recognition of igneous stratigraphy, we strategically sampled representative volcanogenic rocks that cover the entire duration of eruptive activity. By doing so, the new U–Pb zircon magmatic ages, combined with a compilation of U–Pb crystallization ages from the South Patagonian Andes, allows us to constrain the volcanic activity in the study area to a period of 8 My (~ 155–146 Ma, V3 stage) and 11 My considering age inherent errors. The field recognition of normal faults and the syn-kinematic emplacement of sub-volcanic bodies, which are inferred to conform to a ring-fault system, along with the presence of a thick succession of ignimbrites, suggest that the syn-extensional volcanic emplacement occurred in a caldera volcanic environment. This setting was responsible for the short-lived, voluminous eruptions. Furthermore, the high Th/U zircon ratios identified for the ~ 155–150 Ma period indicate the climax of extensional tectonics. The integration of these data supports the hypothesis that retreating-mode subduction played a major role in producing ignimbrite flare-ups.