New U-Th-Pb SHRIMP ages of the Complejo Volcánico Agua de la Piedra, Provincia Magmática Somún Curá, Patagonia, Argentina
During the Cenozoic, the northern extra-Andean Patagonia was characterized by an extensive magmatism, mainly Oligocene (25–28Ma) and basaltic, i.e. the Somún Curá plateau. Younger, post-plateau magmatism produces several polygenetic volcanic complexes (Alta Sierra de Somún Curá, Sierra de Apas, Sierra de Telsen, Sierra de Talagapa, Sierra Pire Mahuida, and Sierra de Chacays) and minor monogenetic centres (Trayén Niyeu Volcano, Dos Hermanos Hills), developed from Late Oligocene through Miocene times. Up to the present, the Agua de la Piedra Volcanic Complex has been considered a post-shield eruptive assemblage. This volcanic building is composed of basal pyroclastic flows of trachytic composition and lava flows and domes which span from trachyte and lesser amounts of rhyolite to late basaltic flows. The series is characterized by its alkaline composition and displays a typical compositional gap in the mesosilicic realm. The evolution from the basic rocks to the trachyte-rhyolite differentiates was controlled by early olivine-clinopyroxene fractionation and later by plagioclase + K feldspar crystallization. There are scarse geocronological data of this volcanic series. In order to precisely date the bimodal sequence, a trachyte lava flow (Puesto Navarrete; 32.32 +0.48 -0.98 Ma), which constitutes the core of the Complex, and another rhyolite flow (Puesto Suárez; 32.01 +0.51 -0.34 Ma) were selected to perform U–Pb Sensitive High Resolution Ion MicroProbe (SHRIMP) analyses. The basaltic facies was dated by K-Ar methodology at 24.4 ± 0.9 Ma. All in all, the ages pose new time constraints in the Agua de la Piedra Volcanic Complex within the evolutive context of the Somún Curá Magmatic Province. The new geochronological information obtained indicate that the trachytic and rhyolitic rocks would have formed synchronously or shortly before the main event that formed the plateau, while the ages of the basaltic rocks are bound to be coeval with other post plateau volcanic complexes.