NEW U-Th-Pb SHRIMP AGE FOR THE SOMÚN CURA POST-PLATEAU SEQUENCES, PATAGONIA, ARGENTINA.

SALANI, F.M.; REMESAL, M. B.; SANTOS S. JOAO ORESTES

Puerto Vallarta

Simposio; X Simposio Sudamericano de Geología Isotópica (SSAGI); 2016

X Simposio Sudamericano de Geología Isotópica

During the Cenozoic, the northern extraandean Patagonia was characterized by an extensive magmatism, mainly Oligocene (25-28Ma) and basaltic, i.e. the Somún Curá plateau. A younger post plateau stage was built by several polygenic volcanic complexes (Alta Sierra de Somún Curá, Sierra de Apas, Sierra de Telsen, Sierra de Talagapa, Sierra Pire Mahuida, and Sierra de Chacays) and monogenetic minor centres (Trayen Niyeu Volcano, Dos Hermanos Hills), developed from Late Oligocene through Miocene times. The current knowledge of the age of this volcanism is based mostly on K-Ar and Rb-Sr determinations; there are scarse U-Pb data referred to as post plateau series. There are U-Pb data available for the Pire Mahuida Volcanic Complex (Salani et al., 2014; Salani et al., 2015) and for the Sierra de los Chacays Volcanic Complex (SCVC) (Cordenons et al. 2014).In this contribution we study the post-plateau bimodal association of the Agua de La Piedra Volcanic Complex (APVC), located at the western border of the Somún Curá plateau (41º 35´ S.L and 41º 45´S.L; 68° 50´ y 69°W.L). The APVC is a volcanic centre composed of basal pyroclastic trachytic flows and younger lava flows and lava domes, which span from early trachyte and lesser amounts of rhyolite to late basaltic flows (Remesal et al., 2002). In order to precisely date the bimodal sequence, a trachyte lava flow (Puesto Navarrete), which constitutes the core of the Complex, was selected.The U-Th-Pb SHRIMP determinations in zircons were performed at Curtin University, Perth. The presented 206Pb/238U age was calculated using the TuffZirc algorithm (Isoplot software), having yielded 32.41 +0.80 -0.25 Ma (Late Oligocene-Rupelian). The post plateau associations share a number of features, e.g. they were developed in an extensional regime, are mainly related to cauldrons, are bimodal and have an alkaline chemical character. However when this volcanic field is compared to the rest of the post plateau associations, it becomes apparent that the Agua de La Piedra Volcanic Complex represents one of the oldest complexes built on the Somún Curá plateau contrary to previous assumptions.