Early Cretaceous (Aptian) uplift of Patagonia recorded on detrital zircons population from the Patagonian Andes

GHIGLIONE, MATÍAS; NAIPAUER, MAXIMILIANO; BARBERON, VANESA; RONDA, GONZALO; PEREYRA-FERNÁNDEZ, ELIANA; RAMOS, VICTOR; AGUIRRE-URRETA, MARÍA BEATRIZ; VALENCIA, VICTOR

Córdoba

Congreso; XIX Congreso Geológico Argentino; 2014

The southern Patagonian Andes exposes sediments of the Austral-Magallanes foreland basin system along 800 km, characterized by a southward axial sediment dispersal pattern, and a younging of basal orogenic sequences on the same direction. Detrital and volcanic zircon U-Pb geochronology was undertaken on the first coarse-grained regressive units represented by the Río Belgrano and Río Tarde Formations at the northern end of the basin in Lago Pueyrredón, revealing the maximum age for the ending of sag conditions. The maximum depositional age of the basal green sandstones of Río Belgrano Formation is 115.1 ± 2.1 Ma based on the youngest single zircon age, although a more conservative estimate would be the represented by a ~122 Ma age-peak (Early Aptian), including 36% of the 98 dated zircons. The youngest single zircon of the overlaying red coarse sandstones of Río Tarde Formation yields an age of 118.4 ± 1.5 Ma, however the ~ 122 Ma peak (31% of dated zircons) is also present. The top of the sequence was dated by U/Pb from a volcanic tuff from the uppers levels of Río Tarde Formation, indicating an eruption age of 111.9 ± 1.5 Ma. Recent dated Baqueró Group by CA-TIMS U-Pb zircon age of 118 ± 0.09 Ma for a tuff at its base, i.e. indicating that compresional deformation of the Deseado Massif took place and ended before 118 Ma, in concordance with our data. Our data is also in agreement with the presence of an angular unconformity separating the Divisadero Group from the underlying folded Lower Cretaceous volcanic and sedimentary rocks indicating that sometime between 121 and 118 Ma (Aptian), the sector located immediately to the north of our study zone was uplifted. Also in the North Patagonian massif, an unconformity exists between Neocomian rocks and overlying Albian beds of the Chubut Group. This can be associated with a sedimentary zircons Triassic to Middle Jurassic age peak, and abundant volcanic litic recorded on the Neuquen basin, during deposition of the Aptian-Albian Rayoso Formation and an angular unconformity between the Rayoso and Neuquén Group, that can be interpreted under the light of our regional correlation as coming from the V1 Jurassic volcanic stage indicating the Aptian uplift of the North Patagonian Massif. While the 179-188 Ma age peak from the V1 volcanic stage is not present on detrital zircon populations from Punta Barrosa (~101-91 Ma), and younger foreland sequences, the 122 Ma peak is a pervasive characteristic in foreland sequences from Última Esperanza and Peninsula Brunswick, and could correspond to the southern continuation of the Aysen volcanic arc, although its roots are not registered on the dating of the Patagonian Batholith south of ~48°SL, and could have been derived from erosion of northern foreland sequences or the Deseado Massif.