Petrology and geochemistry of Carboniferous siliciclastics from the Argentine Frontal Cordillera: A test of methods for interpreting provenance and tectonic setting

摘要

Petrological and geochemical characteristics of sandstones and shales from the Carboniferous Cerro Agua Negra Formation (Rio Blanco basin, western Argentina) are discussed. The sandstones are mostly feld-spathic litharenites with subordinate litharenites and lithic arkoses, and their modal compositions indicate two major sources. The first represents a recycled orogen and can be identified as the Proto-precordillera, a N-S trending mountainous area that separated the Paganzo and Rio Blanco basins and that was a positive element at least up until the end of the Carboniferous. The second was further east, in the Sierras Pampeanas terrane. It consisted of a variety of metasedimentary and felsic plutonic crystalline rocks. The Protoprecordillera must have been crossed by transverse valleys that facilitated the transfer of Pampean terrigenous material towards the Rio Blanco basin. A lack of volcanogenic sand suggests that the basin was open towards proto-Pacific in the west, rather than being separated from it by a magmatic arc as previously suggested. Bulk geochemical analysis of different lithological types (arenites, wackes and shales) demonstrates a strong relationship between texture and chemical composition of the rocks, even taking into account the immobile elements. The largest differences are between shales and arenites, while the wackes have intermediate compositions. The CIA (chemical index of alteration) indicates partial remobilisation of oxides from source rocks and enrichment of aluminium and potassium in the shales, which reflects their potassic clay mineral composition. Overall, siliciclastics of the Cerro Agua Negra Formation have a similar composition to the upper continental crust (with slight net enrichment of SiO_2 in the arenites and of Al_2O_3 in the shales). Rare earth element profiles reflect terrigenous contributions, since they are enriched in light-REE, have a pronounced negative Eu anomaly and a relative depletion of the heavy-REE. The results show that discrimination plots commonly employed to infer provenance and tectonic setting from siliclastics are compromised where bulk chemical composition is strongly dependent on grain size and sorting.