CONICET | Buscador de Institutos y Recursos Humanos

U?Pb geochronology of large detrital zircons populations is a powerful tool for interpreting sandstone provenance. It has been applied to three Middle Cambrian sandstones from the Precordillera of Argentina with the purpose of using the provenance interpretations to test paleogeographic and paleotectonic models proposed for the Cuyania or Precordillera terrane. Two samples are from thin beds of quartz arenite in the Soldano Member of the La Laja Formation at the northern end of Sierra Chica de Zonda in San Juan Province. The sandstone beds are thin and occur within a stratigraphic succession composed primarily of shallow marine limestone. The sandstone is texturally and mineralogically mature and likely represents a multi-cycle sediment, extensively reworked in the shallow interior sea of the Precordillera platform. Thus, the strongly unimodal age distributions of the detrital zircons populations in the two sandstone samples are surprising and distinctive. All zircon grains fall within a unimodal population of 1688?1200 Ma in one sample (N=95) and within a unimodal population of 1559?1316 Ma in the other (N=86). Of these grains, 23% and 65%, respectively, are within the age range of the North American magmatic gap (1610?1490 Ma), indicating a non-Laurentian provenance. Given the widely held assumption that the basement rocks of Cuyania are of Grenvillian age and the multi-cycle character of the La Laja sandstone samples, the absence of Grenvillian age grains is noteworthy. Possible sources for the zircons and thus the sandstones, in particular those grains with ages within the North American magmatic gap, are present in the southwestern Amazonian craton and include the Cachoeirinha orogen, the Serra da Providência Suite, Santa Helena orogen, the Rio Branco and the the Santo Antonio Intrusive Suites (Geraldes et al., 2004). A very different sample was taken from a sandstone interval in the San Isidro olistolith, which is within the Estancia San Isidro Formation at the section in Quebrada de San Isidro in Mendoza Province (Heredia and Beresi, 2004). The sandstone in the San Isidro olistolith is a poorly sorted arkose likely derived from plutonic basement rocks of tonalitic composition. It is a first cycle sandstone that includes detrital zircons with ages nearly contemporaneous with the time of deposition. Its zircon population (N=94) is dominated by a single, prominent 615?511 Ma age cluster which is indicative of provenance in a Brasiliano orogenic belt; it also includes a few grains with ages (1688?1318 Ma) that are found in the unimodal populations in the La Laja samples. Grains of Grenvillian age are absent. The complete absence of zircons with Grenvillian ages (1200 to 950 Ma) in the La Laja and San Isidro olistolith samples is difficult to reconcile with paleogeographic and geotectonic models in which Cuyania rifted from Laurentia in Cambrian or Ordovician time. In these models, the basement rocks of Cuyania are considered to be of Grenvillian age and Laurentian character. The zircon data indicate that, if the provenance of the Middle Cambrian sandstones is basement rocks of Cuyania, then those basement rocks are not of Grenvillian age nor of Laurentian affinity. If the source area of the sandstones was external to Cuyania, then the source rocks could not have been located in Laurentia. In light of the zircon age distributions, the provenance must have been Gondwana. The evidence for sandstone provenance provided by detrital zircon geochronology is most consistent with models in which Cuyania rifted from the southern margin of West Gondwana. Given a Cambrian association with Gondwana and a post-Ordovician arrival at its present position outboard of the Famatinian magmatic belt (Finney et al., 2003), the Cuyania terrane must have migrated along the southern and western margins of Gondwana during the Ordovician Period.

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