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

GHIGLIONE MATIAS; NAIPAUER, MAXIMILIANO; BARBERÓN VANESA; RONDA GONZALO; PREYRA-FERNÁNDEZ ELIANA; AGUIRRE-URRETA BEATRIZ; VALENCIA VICTOR; RAMOS VICTOR

Cordoba

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. Although their boundaries are highly diachronic throughout the basin, the stratigraphic and tectonic evolution is sharply divided in lithotectonic sequences: A Jurassic synrift stage composed of a bimodal association of rhyolitic ignimbrites with minor mafic and intermediated lavas (Chon Aike, El Quemado and Tobífera Formations), followed by a thick Early Cretaceous tabular pelitic sag facies (Río Mayer Formation), and Late Cretaceous to Cenozoic heterolitic to coarse, marine and continental sediments from the orogenic-foreland stage sensu lato. The basal foreland stage is represented by regressive sandstones with detrital content derived from uplifted Paleozoic metamorphic basement complexes, juvenile volcanic arc and Jurassic vulcanites from the synrift stage (Fildani et al. 2008, Varela et al. 2012). 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 (Aguirre-Urreta and Ramos, 1981), 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. On the Tres Lagos locality 250 km to the south, deltaic to fluvial basal sequences have an U?Pb age of ~96 Ma from a level of tuffs intercalated at 140 meters above its base (Varela et al. 2012). First marine regressive units located 400 km to the south, on the southern end of this axial depocenter at Última Esperanza region are ca. 17-18 Myr older, and contain zircon grains derived from sources to be found to the west (Paleozoic-Mesozoic metamorphic complexes, the Sarmiento ophiolitic complex, and latterly the Tobífera-El Quemado Formation) and are therefore interpreted has the first pulses of Andean deformation at this latitude. This regressive units are the latest Aptian (~U/Pb zircon age of 101 ± 1.1 Ma) Punta Barrosa and the Coniacian-Santonian (~89-84 Ma) Cerro Toro Formations (review by Bernhardt et al., 2012). This pronounced southward younging trend continues at least another 250 km up to Peninsula Brunswick where foreland basin sedimentation initiated at 88.49 ± 0.61 Ma (McAtamney et al. 2011), around 11 Myr later than Punta Barrosa and 26 Myr after Río Belgrano. The studied detrital zircon population yielded interesting results, instead of having a clear Andean orogenic origin, they rather come from Patagonia. An abundant Mesozoic population characterize both the Rio Tarde and Rio Belgrano Formations, including a 179-188 Ma peak clearly in coincidence with the V1 volcanic stage from Marifil Formation associated to the breakup of Gondwana, developed to the northeast in the North Patagonian Massif (178-188 Ma, Pankhurst et al. 2000). This peak is not represented on detrital zircon populations from Punta Barrosa and younger foreland sequences to the south (Fildani et al. 2008). Subordinate population with zircon ages between ~165 and 173 Ma are also present in both units; these ages are comparable with the V2 volcanic stage (162-172 Ma, Pankhurst et al. 2000). However, zircons of 153-157 Ma (V3) representing the age of El Quemado Complex underlying the sag units, and exposed along the Patagonian thrust front to the west and the Deseado Massif to the east (Pankhurst et al. 2000) are poorly represented. The 179-188 Ma peak seems to indicate the contemporaneous uplift of the North Patagonian Massif during the Aptian, or speculatively from a reworking of detritus from Neocomian fluvial sequences from the Deseado Massif that could have a V1 detrital signature. The next main peak at 122 Ma most probably represents ash fall tuffs from contemporaneous volcanic activity from the arc, and is present in both units. The roots of this volcanism are clearly identified in the North Patagonian Batholith of Aysen to the west, presenting U-Pb ages of 120-130 Ma (Pankhurst et al. 1999; Thomson et al. 2001). This 122 Ma peak is also a pervasive characteristic of detrital zircon populations from Punta Barrosa and younger foreland sequences from Última Esperanza and Peninsula Brunswick. In Rio Tarde Formation appears the inception of a well represented group of ages from the Paleozoic to the Neoproterozoic (27%) with maximum peaks at ~354 Ma, 456-470 Ma, 514 Ma, and 680 Ma. A smaller group with Grenville ages also appear. These old age peaks together with a petrographic analysis showing a large proportion of metamorphic clasts (Barberón et al. 2014 this congress) indicate that there were exposed basement sources, which are comparable with outcrops from the Andean metamorphic complexes located to the west, and in the Deseado Massif to the east. The mixed presence of basement sources, together with V1 Jurassic volcanic stage is an outstanding characteristic of Central Patagonia, suggesting that uplift of the Deseado Massif and probably the western rim of the North Patagonian Massif took place at the time of deposition of Río Belgrano and the basal Río Tarde Formation, in coincidence with in situ data from both provenance sectors. Homovc et al. (2001) describes a deeply eroded unconformity that separates the Baqueró Group from the underlying sequences in the Deseado Massif. This angular unconformity represents a strong compressional event produced during the opening of the Atlantic Ocean affecting the Deseado and San Julian basins (Homovc et al. 2001). Cesari et al. (2011) recently dated the 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 (Suarez et al., 2010). Also in the North Patagonian massif, an unconformity exists between Neocomian rocks and overlying Albian beds (Suarez et al. 2007) of the Chubut Group (Suarez and Marquéz 2007). 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 (Tunik et al. 2010) 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.