Marianela Ximena Yasmin Díaz, Sergio Alfredo Marenssi, Carlos Oscar Limarino, Laura Jazmin Schencman
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Revista de la Asociación Geológica Argentina Vol 76, No 2 (2019)
La Formación Vinchina (Mioceno) constituye una sucesión de más de 5000 m de espesor de bancos rojos de ambientes fluviales,lacustres, eólicos y de interacción eólica-fluvial, desarrollados en una cuenca de antepaís fracturado. En este trabajo se presentanlos resultados de análisis geoquímicos (elementos mayoritarios, minoritarios y REE) de 12 muestras de areniscas de diferentes nivelesestratigráficos de la formación a lo largo de la sierra de Los Colorados (noroeste de La Rioja), complementados con estudiospetrográficos. Petrográficamente las muestras son mayormente litoarenitas feldespáticas. Los componentes clásticos indican mezclade aportes de basamento y arco volcánico, con intercalaciones de detritos derivados de una fuente andesítica local, predominantesen la parte media del miembro superior (secuencia depositacional 5). Las modas detríticas caen en los campos de orógeno recicladoy arco disectado en el diagrama Q-F-L. Los datos geoquímicos permitieron utilizar diferentes diagramas discriminantes que indicanprocedencia desde fuentes ígneas félsicas o la mezcla entre fuentes félsicas y máficas. Las relaciones entre óxidos mayoritarios permitendiferenciar a las muestras de la secuencia depositacional 5, con valores de SiO2/Al2O3<5, Na2O/K2O>2 y Fe2O3/K2O>2,5. Porotra parte, el índice CIA y los diagramas ACNK y Th-vs-Th/U sugieren una alteración moderada, consistentemente con investigacionesprevias que indican condiciones climáticas áridas-semiáridas. Por su parte, el diagrama Zr/Sc-vs-Th/Sc indica muy bajo grado dereciclado sedimentario. Finalmente, el patrón de distribución de tierras raras y las relaciones LaN/SmN, GdN/YbN y Eu/Eu* indican unaprocedencia desde un margen continental activo con contribuciones de arco juvenil y corteza continental superior antigua.
The Miocene Vinchina Formation is a thick red-bed succession of more than 5000 m thick deposited in fluvial, lacustrine, eolian, and
fluvial-eolian environments developed in a broken-foreland basin. In this study we present geochemical analyses on 12 sandstone
samples from different stratigraphic levels along the Sierra de Los Colorados (northwestern sector of La Rioja Province). Complementary
petrographic analyses of the same samples were also carried out. Petrographically the samples are mainly feldspathic lithoarenites.
Clast compositions suggest a mix from crystalline basement and volcanic arc sources with intervening pulses of an andesitic
local source especially in the middle part of the upper member (depositional sequence 5). Detrital modes mainly plot in the recycled
orogen and dissected arc fields in Q-F-L diagrams. Geochemical data allowed using different discriminant diagrams that suggest provenance
from felsic igneous sources, or a mixing of felsic and mafic sources. Relationships between major oxides allow differentiation
of sequence 5 samples, which exhibit SiO2/Al2O3<5, Na2O/K2O>2, and Fe2O3/K2O>2.5, that are thought to represent an intrabasinal
andesitic event. The CIA index and ACNK and Th vs. Th/U diagrams suggest a moderate chemical alteration grade, consistent with
previous works indicating an arid to semiarid climate. The Zr/Sc vs. Th/Sc indicates a very low grade of sedimentary recycling related
to a short transport distance. Finally, REE distribution pattern and LaN/SmN, GdN/YbN, and Eu/Eu* relationships are compatible with provenance
from an active continental margin with mixed contributions from a young undifferentiated arc (YUA), an old upper continental
crust (OUC), and young differentiated arc (YDA).
fluvial-eolian environments developed in a broken-foreland basin. In this study we present geochemical analyses on 12 sandstone
samples from different stratigraphic levels along the Sierra de Los Colorados (northwestern sector of La Rioja Province). Complementary
petrographic analyses of the same samples were also carried out. Petrographically the samples are mainly feldspathic lithoarenites.
Clast compositions suggest a mix from crystalline basement and volcanic arc sources with intervening pulses of an andesitic
local source especially in the middle part of the upper member (depositional sequence 5). Detrital modes mainly plot in the recycled
orogen and dissected arc fields in Q-F-L diagrams. Geochemical data allowed using different discriminant diagrams that suggest provenance
from felsic igneous sources, or a mixing of felsic and mafic sources. Relationships between major oxides allow differentiation
of sequence 5 samples, which exhibit SiO2/Al2O3<5, Na2O/K2O>2, and Fe2O3/K2O>2.5, that are thought to represent an intrabasinal
andesitic event. The CIA index and ACNK and Th vs. Th/U diagrams suggest a moderate chemical alteration grade, consistent with
previous works indicating an arid to semiarid climate. The Zr/Sc vs. Th/Sc indicates a very low grade of sedimentary recycling related
to a short transport distance. Finally, REE distribution pattern and LaN/SmN, GdN/YbN, and Eu/Eu* relationships are compatible with provenance
from an active continental margin with mixed contributions from a young undifferentiated arc (YUA), an old upper continental
crust (OUC), and young differentiated arc (YDA).
http://ppct.caicyt.gov.ar/index.php/raga/article/view/15244
