New constraints and age determinations for the alkaline volcanism of the Western Sierras Pampeanas, San Juan province, NW Argentina

CASTRO DE MACHUCA, BRÍGIDA; LÓPEZ, MARÍA GIMENA

Cochabamba

Simposio; 11th South American Symposium on Isotope Geology (11th SSAGI); 2018

Colegio de Geólogos de Bolivia

Volcanic rocks (Baldecitos Formation) exposed at Sierra de Valle Fértil, Western Sierras Pampeanas, Argentina, are the result of continental intraplate magmatism linked to an extensional setting after the collapse of the Gondwanide orogen and the early breakup of Gondwana (Late Permian - Early Triassic). Herein we present new radiometric ages of this extension-related alkaline volcanism that clarify the chronology of magmatic processes after the Gondwanide orogeny. Alkaline volcanism, much of it SiO2-undersaturated, comprises lava flows and subvolcanic bodies overlying or intruding the crystalline basement and varying in composition from basanite, alkali basalt, trachybasalt, basaltic trachyandesite to trachyte, phonolite and rhyolite. No absolute age dating has been previously reported for these lithologies; the only available geochronological data are restricted to the Ischigualasto-Villa Union rift basin, farther north of the study area. Magmatism there has been dated on alkali basalts and altered ash/tuff layers interbedded with continental sediments, ranging between 231.4 ± 0.3 My (40Ar/39Ar, Martinez et al. 2011) and 215 ± 5 My (K/Ar, González and Toselli 1975). More recently, a U/Pb zircon age of 252.38 (+0.09/-0.22) My on silicic tuffs from the top of the basal unit of the basin (Talampaya Formation), have been reported (Grulbanson et al. 2015). Samples dated here belong to a basaltic trachyandesite lava flow (Usno Basalt 30°33´54"S - 67°32´29" W) and a shallow-level trachyte intrusion (La Aguadita Trachyte 30°46´24" S - 67°26´16" W). Age determinations were performed at the Laboratory of Geochronology, SERNAGEOMIN, Chile, using the 40Ar/39Ar and K/Ar methods on whole rock, respectively. Analytical specifications on these methods have been given by Arancibia et al. (2006). Errors are quoted at the two sigma (2σ) level and the decay constants are those suggested by Steiger and Jager (1977).Obtained data of 238.1 ± 0.4 My (40Ar/39Ar isochron combined age) for the Usno Basalt and 218 ± 7 My (K/Ar) for La Aguadita Trachyte, are consistent with previous ages established in the Ischigualasto-Villa Union Basin. All available radiometric evidence is compatible with magmatic activity in the area over a period of at least 35 My from the lowest section of the Lower Triassic (Induan) continuing throughout the Triassic to the medium section of the Upper Triassic (Norian). However, if we relate the 252 My-old tuffs to the final stages of Gondwanic silicic volcanism, the age range of alkaline magmatism s. str. would be constrained to a narrower span time since the Middle to Upper Triassic (Ladinian-Norian).