SEDIMENTARY AND VOLCANIC INTERBEDDING IN LAS MÁQUINAS CREEK, PALEOGENE BACK ARC IN THE HIGH ANDES OF SAN JUAN PROVINCE, NORTHWESTERN ARGENTINA

In the Valle del Cura region in the vicinity of the Cerro de Las Tortolas area, near the international border with Chile, a thick sequence of volcanic rocks of Tertiary age is exposed. Their most common lithology corresponds to andesites, dacites and acidic pyroclastic rocks. Local stratigraphy and specially the lower section of the series can be established at the Las Máquinas Creek. There is exposed a volcano/sedimentary sequence made up of lithic- and feldspar-rich sandstones, tuffs and tuffaceous sandstones with rhyolitic and rhyodacitic lavic interbedding. These flows are brownish red in color with homogeneous to strongly banded structure, the latter with eutaxitic texture. They have a low phenocrysts/groundmass ratio. Common phenocrysts are argilitized alkali feldspar and oxidized botitic flakes. Felsitic groundmass is made up of spherulites and axiolitic textures. Some varieties have sodium-rich plagioclase.

Lavas are silica rich, with SiO₂ percentage varying between 70% and 75%, and alumina between 12% and 13%. Their trace elements ratios (Rb vs. (Y+Nb), Th-Hf/3-Ta) point to an evolved volcanic arc with a withinplate component. La/Yb ratio is greater than 12 and their relation to MORB confirms this trend.

This unit is covered by the Miocene volcanic rocks of Cerro de las Tortolas Formation, is intruded by Las Máquinas Basalts, and is also covered by the Vacas Heladas Ignimbrite, the last volcanic episode registered in the area. The rocks described in this essay correlate with outcrops of similar characteristic located very close to the north of this area and are considered to be part of the Valle del Cura Tuff Formation of Eocene-lower Oligocene age and represent the local expression of the volcano-sedimentary activity in the backarc during the considered time.