Petrographic and geochemical comparison between Early to Middle Cambrian and Uppermost Cambrian-Lower Ordovician quartz-rich sandstones in NW Argentina

Category Geochemistry
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Zimmermann, Udo۱; Augustsson, Carita۲; Bahlburg, Heinrich۲; Buatois, Luis۳; Büld, Mareike۲; Mلngano, Gabriela۳
Holding Date 23 September 2008

In northwest Argentina quartz-arenitic rocks of Early to Middle Cambrian age (Mesón Group) are exposed in several outcrops with macroscopically similar rocks of Late Cambrian to Early Ordovician age (Cobres and Santa Victoria Groups, and the Tolar Chico, Santa Rosita and San Bernardo formations) and are difficult to differentiate. Petrographically, the rocks of the Mesón Group are dominated by sub-angular to rounded quartz, mostly bi-modally distributed. The smaller the grain size, the more angular are the quartz grains. Lithoclasts are represented mainly by well-rounded metasedimentary grains. Small detrital mica and well-rounded zircon also occur. In one outcrop (Angosto de Perchel) subrounded zircons of c. 100µm length were found, most probably of magmatic origin. Matrix content is rarely higher than 5%. The samples from the Upper Cambrian-Lower Ordovician successions are generally poorly sorted and the grains are angular to sub-rounded. Lithoclasts derived from sedimentary rocks (mostly siltstones) and quartz-rich metamorphic protoliths are common. Pyroclastic layers are intercalated with massive quartz-arenites in the San Bernardo Formation of Early Ordovician age. Geochemical analyses show that Th/U ratios are similar between the two successions and point to a relatively strong recycling, but the Uppermost Cambrian-Lower Ordovician samples are generally enriched in Th. The Lower to Middle Cambrian samples reveal higher Nb/Y, but similar Zr/Ti ratios to the Upper Cambrian-Lower Ordovician units. The Uppermost Cambrian-Lower Ordovician sedimentary rocks have a dacitic to rhyodacitic composition with only the sandstones of the Santa Rosita Formation scattering from a silica-rich (high Zr/Ti ratios) composition to partly alkaline (high Nb/Y ratios) in other samples. The volcaniclastic rocks of the San Bernardo Formation are of dacitic to andesitic composition. Provenance indicating ratios like Ti/Zr, Th/Sc, Zr/Sc and La/Sc show for the rocks of the Meson Group a strongly recycled characteristic, especially for its oldest formation (Lizoite Formation). Based on Zr/Sc ratios, rocks of the Santa Rosita and the San Bernardo formations seem to be much more reworked than those of the Méson Group. Some sedimentary rocks from the Cobres Goup are less recycled with a typical upper continental crust composition. Based on macroscopic appearance and petrographic studies, rocks of the Méson Group are expected to be strongly recycled. Despite this, sorting most probably reduced the amount of Zr in the Mesón Group arenites in contrast to the quartz-rich rocks of the Lower Ordovician rocks. Nevertheless, only slight differences are detected between both sedimentary cycles based on geochemistry. The Early-Middle Cambrian quartz-arenites are chemically more homogeneous. The Lower Ordovician succession might have been deposited under different sedimentary conditions to cause a larger grain-size variety and a higher concentration of incompatible elements.