Sedimentary petrology and geochemistry of siliciclastic rocks from the Upper Jurassic Tordillo Formation (Neuquén Basin, western Argentina): implications for provenance and tectonic setting.

SPALLETTI, L.; QUERALT, I.; MATHEOS, S.; COLOMBO, F.; MAGGI, J.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

Elsevier

Lugar: Amsterdam; Año: 2006

0895-9811

ABSTRACT The Upper Jurassic Tordillo Formation is exposed along the western edge of the Neuquén Basin (west-central Argentina) and consists of fluvial strata deposited under arid/semiarid conditions. The pebble composition of conglomerates, the mineralogical composition of sandstones and pelitic rocks, and the major- and trace element geochemistry of sandstones, mudstones and primary pyroclastic deposits are evaluated to determine the provenance and tectonic setting of the sedimentary basin. Conglomerates and sandstones were derived almost exclusively from volcanic sources. The stratigraphic sections located to the south show a clast population of conglomerates dominated by silicic volcanic fragments and a predominance of feldspathic litharenites. This framework composition records erosion of Triassic-Jurassic synrift volcaniclastic rocks and basement rocks from the Huincul arch that was exhumed as a result of Late Jurassic inversion. In the northwestern part of the study area, conglomerates show a large proportion of mafic and acidic volcanic rock fragments, while sandstones are characterised by a high content of mafic volcanic rock fragments and plagioclase. These data suggest that the source of these sandstones and conglomerates was primarily the Andean magmatic arc located to the west of the Neuquén basin. The clay mineral assemblage is interpreted as the result of combination of a complex set of factors, such as source rock, climate, transport and diagenesis. Post-depositional processes produced significant variations in the original compositions, especially in the fine-grained deposits. The Tordillo sediments are characterised by moderate SiO2 contents, variable abundances of K2O and Na2O, and  a relatively high proportion of ferromagnesian elements. The degree of chemical weathering in the source area, expressed as the chemical index of alteration (CIA), was low to moderate. The major element geochemistry and the Th/Sc, K, Rb, Co/Th, La/Sc and Cr/Th values point to a significant input of detrital volcanic material of calcalkaline felsic and intermediate composition. However, major element geochemistry was not useful to interpret tectonic setting. Discrimination plots based on immobile trace elements, such as Ti, Zr, La, Sc and Th, showed that most data lie in the field for active continental margin. Yet, geochemical information was not  sufficiently sensitive to differentiate between the two different source areas recognized by petrographic and modal analyses of conglomerates and sandstones.