SHRIMP U-Pb dating and subsidence analysis of pre-Andean Paganzo Basin, northwestern Argentina: Implications for late Paleozoic tectonic evolution of western Gondwana

EZPELETA, M., ASTINI, R.A., DÁVILA, F.M. AND CAWOOD, P.

Alemania

Simposio; LAK; 2009

The Upper Paleozoic basins of western Argentina have been extensively studied since the earliest twentieth century from a sedimentological-stratigraphic and biostratigraphic point of view. Nevertheless, the tectonic setting is still debated. These basins have been considered from pure extensional systems (e.g. Salfity and Gorustovich, 1983) and pull-apart basin (Fernández Seveso and Tankard, 1995), to flexural forelands generated by tectonic loading (e.g. López Gamundí et al., 1994; Ramos, 2000). In order to contribute to understand the evolution of these Carboniferous-Permian basins, we carry out a provenance study using U-Pb SHRIMP detrital zircon dating and complementary subsidence analyses. These data together to previous sedimentological and stratigraphic information assist us to unravel the tectonic scenario of the Gondwanan basins along west-central Argentina. The stratigraphy can be separated into two different regions, the western depocenter between the Cordillera and the western Sierras Pampeanas and the central-east depocenter located within the eastern Sierras Pampeanas. The western depocenters are formed, from bottom to top, by the Punta del Agua, Río del Peñón and Mulas Muertas Fms, and the central-eastern depocenters by the Malanzán, Las Pircas and De la Cuesta Fms. From the U-Pb detrital zircon ages (<10% discordant), we differentiate major age peaks as follows: In the western region of the basin, 337 ±23 (n=3), 1060 ±25 (n=7), 1196 ±74 (n=3), and 1345 ±62 (n=4) Ma for Punta del Agua Fm and 487 ±11(n=5) and 1126 ±93 (n=4) Ma for Río del Peñón Fm. In the eastern basins: 365 ±89 Ma, (n=2), 471 ±22 Ma, (n=3), 522 ±94 Ma, (n=2) and 1138 ±16 (n=3) Ma for the Malanzán Fm; 457,2 ± 9 (n=9) for the Las Pircas; and 466 ±54, (n=3), 1018 ±56 (n=4) Ma for the de la Cuesta Fms. In the western depocenter, the youngest zircon ages in the volcaniclastic Punta del Agua Fm (ca. 337 Ma, late Mississipian) are interpreted as autochthonous related to recycling of andesitic and acidic flows in the same unit. These ages are similar to those of volcanic rocks in the southern Puna (ca. 334 Ma., Martina et al., 2007) immediately to the northeast. The Mesoproterozoic detrital input is more consistent with a provenance from the basement highs (presently western Sierras Pampeanas assumed to be part of the large Cuyania terrane), immediately to the east-southeast. Paleocurrent and composition of granitic conglomerates interfingering with volcanic deposits indicate a provenance from Sierras Pampeanas at the east. In the eastern depocenter, the zircon ages of early Pennsylvanian glacial deposits (Malanzán Fm), indicate that these deposits are a local cover for the eastern Sierras Pampeanas, including a Cambrian basement (recycling Mesoproterozoic ages) intruded by Ordovician and late Devonian granites. Paleocurrents and composition of conglomerates support a provenance mainly from the east. During late Pennsylvanian- early Permian, an important postglacial transgression took place in the basins of western Argentina. Conglomeratic wedges of the Las Pircas Fm in the eastern domain and fluvial-deltaic sequence of the Río del Peñón Fm in the western domain, show Ordovician and Mesoproterozoic detrital inputs that indicate a provenance from the eastern Sierras Pampeanas. Paleocurrent and composition are consistent with such provenance suggesting a depositional continuity between these two units. Early Permian recurrence of alluvial-fluvial-ephemeral lakes indicates a definite continental setting for Paganzo Basin. In the eastern-central domain, Ordovician detrital ages in the De la Cuesta Fm are consistent with the onlapping relationship with the Ordovician granites that have been the local source. The lack of Cambrian and Neoproterozoic ages together with a major Mesoproterozoic population suggest a western provenance (western Sierras Pampeanas) where the host of Ordovician granites are Grenville rocks. By contrast to the east, the host of Ordovician granitic plutons yields important Cambrian and Neoproterozoic ages (Collo et al., 2009). The paleogeographic reconstruction allows to interpret that during the late Mississipian-early Permian, the regional slope of the Paganzo Basin is from the Pampean Ranges at the east toward the current Cordillera. The continentalization of the basin during the Permian, is associated with the inversion of the regional slope toward the east. From the analysis of the cumulative subsidence curve two different stages arise. The first stage is better explained by lithospheric stretching as indicated by the large and rapid amount of subsidence. The second stage is more common of flexural basins, driven by increasing tectonic loads indicated from the exponential arrangement. This later foreland stage is coeval with the occurrence of arc volcanism and plutonism of the Choiyoi Complex, exposed in the Cordillera to the west. The flattening of the subsidence curve between both stages would represent a sag interval and local unconformities developed during the Early Permian. The provenance analysis together with our new subsidence curve suggest the late Paleozoic Paganzo basin was formed in two stages: (a) a backarc extensional basin system with a major detrital supply (extrabasinal) from the east and intense intrabasinal volcanism, and b) a flexural foreland system with provenance from the Cordillera and progradation of clastic systems toward the east, similar to the modern Andean foreland.