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