U-Pb AGE OF DETRITAL ZIRCON FROM CAÑADON WILLIAMS, NORTHEAST CHUBUT PROVINCE, PATAGONIA: IMPLICATIONS FOR THE PROVENANCE OF THE CHUBUT GROUP SEDIMENTARY SEQUENCES

NAVARRO, E.L.; BELOUSOVA, E.; NAVARRO, E.; ASTINI R.A.,

Córdoba

Congreso; XIX Congreso Geológico Argentino; 2014

Asociación Geológica Argentina

The Chubut Group constitutes the most widespread sedimentary unit in NE of Patagonia, characterized by variable-energy fluvial deposits. Hitherto the deposition of the whole Chubut Group is considered of an early Valanginian - Maastrichtian interval age. Deposits of the NE of the Chubut province were assigned to the Gorro Frigio Fm (Nakayama et. al. 1978) and particularly, sedimentary sequences near the Telsen locality were associated with the Cerro Barcino Upper Member (Aptian) of this Fm (Chebli et al. 1975). In the area, a robust geochronological age indicated from the analysis of detrital zircons corresponds to the maximum depositional age of ca. 106 Ma (Albian) (Navarro and Astini 2012). In Cañadón Williams (this study), 20 km NW of Telsen (Fig. 1), the Chubut Group underlie the Somún Curá Fm (26 Ma) (Oligocene) and a low angle discordance separates the two units throughout the area. A Laser Ablation Mass Spectrometry (LA-ICP-MS) analysis of 56 detrital zircons from a medium to coarse sandstone was carried out at the Centre for Geochemical Evolution and Metallogeny of Continents (GEMOC) at Macquarie University, Australia, in order to determine the age of the Chubut Group. These sandstones were interpreted to represent shallow channels associated with lacustrine deposits, not described until now for the Chubut Group in the Telsen area, which constitutes the oldest sedimentary outcrops in the study area. The distribution curve for the U-Pb age (Probability plots, Fig. 2) obtained using Isoplot program (Ludwig 2003), shows two populations, the old one at 181 ± 1 Ma (n=52) and the young at 109 ± 2 Ma (n=4), the latter interpreted as the maximum deposition age. The young ages are consistent with events of Albian vocaniclastic episodes in the central Patagonian Andes between the Somuncurá massif to the north and the Deseado massif to the south (Ramos 1978). The old population matches the age of the volcanic Marifil Fm; Ar40/Ar39 ages of ca. 182 Ma (Rapela and Pankhurst 1993, Pankhurst and Rapela 1995, Féraud et al. 1999) that were obtained for this formation at the East and Southeast of the study area. The Marifil Fm as a provenance source is supported by east ? southeasterly paleocurrents measured at the Cañadón Williams section. Furthermore, the single main population at 182 Ma indicate a restricted supplying provenance at 109 Ma for the Chubut Group in NE Patagonia. The abundance of these Jurassic ages (52 zircons) suggests a close proximity of the source area for the Chubut Group. Cathodoluminescence images show mostly euhedral crystals, some with pyramidal terminations, although acicular and equant habits are present. Zircon external morphology also supports the suggestion that these grains were not transported from a distant source. Their internal structure is characterized by well-developed growth zoning and xenocrystic cores are rare. The grains have generally a ratio length/width close to 2 but sometimes reach to 4, indicating a relatively slow cooling at the time of their formation (Corfu et al. 2003). All these features suggest igneous origin, which is further supported by Th/U values above 0.5 in most of the grains (Vavra et al. 1999).