Alta Sierra de Somún Curá: the youngest volcanic complex of Somún Curá Magmatic Province

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

Cochabamba

Simposio; 11Th South American Symposium on Isotope Geology; 2018

Colegio de Geologos de Bolivia

The Somún Curá Magmatic Province, in north central Argentine Patagonia, is characterized by a series of events with a temporal distribution that most authors group into plateau and post-plateau volcanism. The post-plateau events built up the most important volcanic edifices which correspond to sierras showing lithological and eruptive complexity according to which different Volcanic Complexes were defined. The latter are, in turn, integrated into the Quiñelaf Superunit. In the northeastern of the basaltic plateau, the Alta Sierra de Somún Curá Volcanic Complex (ASSVC) was emplaced, representing the smaller volcanic complex.The Cerro Corona volcano is part of the (ASSVC). It is a shield volcano of polygenic character, constituted by basaltic and trachytic-trachyandesitic lava flows and lava domes. The presence of varied disequilibrium textures in phenocrysts indicates that the trachytes are the result of changes in the physical conditions of the magmatic reservoir, suggesting the influence of a hotter basaltic magma located at the base of the reservoir with or without some mafic recharge.Due to its stratigraphic position, the ASSVC has always been considered as young volcanism, either Quaternary or mainly Late Oligocene to Miocene based on K/Ar radiometric dates.Since there is a special interest in determining the chronology of trachytes, a specimen of Cerro Corona trachyte dome was selected to perform U-Th-Pb Sensitive High Mass Resolution Ion MicroProbe (SHRIMP) analysis. The SHRIMP determinations in zircons of this unit were carried out at Curtin University, Perth. Because the low amount of radiogenic lead (206Pb content ranges from 0.1 to 0.3 ppm) the timing of counts of 206Pb was increased from the usual 10 seconds to 30 seconds. The presented age was calculated using TuffZirc algorithm (Isoplot software) corresponding to the 206Pb/238U age of 10.47 +0.31 -0.47 Ma. This Late Miocene ? Tortonian age confirms that the Alta Sierra de Somún Curá complex is the youngest in comparison with the other post plateau complexes scattered over the Oligocene basaltic plateau. An analysis of the published data of different complexes points to a range of ages from the older southern complexes like Sierra de Los Chacays (24.87 +0.57 -0.21) to the younger northern centers like the Alta Sierra de Somún Curá. The Pire Mahuida Complex, located in the central part of the plateau, shows an age of 17.62 +0.56 -0.15, similar to the Barril Niyeu complex dated preliminary at 20.2 (± 0.9).Therefor a northwards shift of magmatic activity may be confidently inferred for the post-plateau assemblages within the Somún Curá setting.