Age and chemical constraints of Volcán Tunupa: Implications for behind arc volcanism in the Bolivian central Andes

New ⁴⁰Ar/³⁹Ar age determinations of groundmass separates and whole-rock geochemical data constrain the Pleistocene eruptive history of Volcán Tunupa, a glacially-dissected composite volcano (~50 km³) situated between the Salar de Uyuni and Salar de Coipasa. Tunupa erupted ~110 km east of the arc front of the Western Cordillera of the central Andes near the eastern edge of the Intersalar Volcanic Field, an arc-perpendicular expression of volcanism that extends to the central Altiplano basin of Bolivia. ⁴⁰Ar/³⁹Ar age determinations indicate that the edifice was constructed between ~1.40 and 1.55 Ma, whereas nearby Cerro Huayrana lavas erupted ~ 11 Ma. Published ages from the Western Cordillera that are concordant with both Tunupa and Huayrana lavas demonstrate that the central Altiplano lavas are a long-lived expression of behind arc volcanism. The Tunupa lavas define a calc-alkaline trend from trachyandesite to trachydacite (wt.% SiO₂ = 60.6 - 63.6; wt.% K₂O + Na₂O = 7.5 - 8.3) and are overlain by younger, more silicic (wt.% SiO₂ = 66) trachydacitic domes. Major element compositions of Tunupa and Huayrana are enriched in FeO and TiO₂ compared to the arc front. These lavas are also enriched in high field strength elements, notably Nb and Ta, and are characterized by considerably lower Ba/Nb and La/Ta ratios than arc front lavas in northern Chile. The geochemical and spatiotemporal patterns of the behind arc Tunupa and Huayrana lavas suggest different petrogenetic histories from typical central Andean arc lavas.