Revised stratigraphy and new U-Pb geochronological data are reported for the Nacientes del Teno Formation and lowermost strata of the Río Damas Formation in Maule, Chile (~35.15°S), a succession of sedimentary and volcanic rocks deposited along the western margin of the Neuquén backarc basin during the Middle to Late Jurassic (174–145 Ma). These results provide insights into the location, composition, and timing of coeval arc volcanic systems, and thus represent an important contribution to our understanding of the tectonostratigraphic evolution of the Andean orogen.
New U-Pb geochronological data from detrital and primary volcanic zircon grains provide the first absolute ages (ca. 172–161 Ma) reported for the Nacientes del Teno Formation. Detrital zircon ages include a tightly clustered population ca. 165 Ma that coincides with a Middle Jurassic trough that exists in frequency histogram and relative probability plots of published U-Pb detrital zircon age data sets, challenging the notion of a lull in volcanic arc activity at this time.
Along with the tight distribution of U-Pb detrital zircon ages, sedimentological evidence for proximal volcaniclastic deposition of Nacientes del Teno Formation strata in alternating marine-subaerial environments suggests the nearby presence of andesitic stratovolcanoes located along the western margin of the Neuquén Basin during the Middle Jurassic. These hypothesized volcanoes would have been located farther east than defined in traditional models for the location of the “main magmatic arc” in the Jurassic, suggesting that arc volcanism at this time occurred across a greater width of the continental margin than previously recognized.
A maximum depositional age of ca. 151 Ma calculated from new U-Pb detrital zircon ages agrees with previously published maximum depositional ages for the Río Damas Formation. Sedimentological and geochronological evidence suggest that the Late Jurassic regression represented by the Río Damas Formation resulted from tectonic uplift.