IGEBA   23946
INSTITUTO DE GEOCIENCIAS BASICAS, APLICADAS Y AMBIENTALES DE BUENOS AIRES
Unidad Ejecutora - UE
artículos
Título:
Petrology and geochemistry of Carboniferous siliciclastics from the Argentine Frontal Cordillera: A test of methods for interpreting provenance and tectonic setting
Autor/es:
SPALLETTI, L; LIMARINO, C.; COLOMBO, F.
Revista:
JOURNAL OF SOUTH AMERICAN EARTH SCIENCES
Editorial:
PERGAMON-ELSEVIER SCIENCE LTD
Referencias:
Lugar: Amsterdam; Año: 2012 vol. 36 p. 32 - 54
ISSN:
0895-9811
Resumen:
Petrological and geochemical characteristics of
sandstones and shales from the Carboniferous Cerro Agua Negra Formation
(Río Blanco basin, western Argentina) are discussed. The sandstones are
mostly feldspathic 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
Protoprecordillera, a NS trending mountainous area that separated the
Paganzo and Río 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 Río
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 SiO2 in the arenites and of Al2O3 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.