Breakdown of hydrous minerals during lithospheric removal as a magmatic trigger beneath the Bolivian Altiplano

The Altiplano-Puna plateau of South America is recognized as a likely setting of Neogene to Recent convective removal of continental mantle lithosphere and lower crust. The precise timing, style, scale, and the relationship of removal with both subduction-related and intraplate volcanism, however, remain loosely constrained. Major and trace elemental variations and spatiotemporal patterns of volcanism are largely controlled by the lithospheric dynamics that trigger melting and are therefore potentially powerful indicators of regional processes involved in plateau growth. In this study, new 40Ar/39Ar age determinations and whole-rock geochemical data from lavas along a 120 km, arc-normal transect (~19.7 deg. S) in the central Altiplano region of Western Bolivia are presented. These data are combined with regional data from the literature that show further systematic variations in spatiotemporal patterns and trace element geochemistry. Rear-arc enrichment in select elements (e.g. K, Ti, Nb, Ta, Rb) is explained as the result of the breakdown of hydrous minerals within the mantle lithosphere and lower crust as the material is carried to greater depths and pressures during density-induced foundering. This breakdown is invoked as a magmatic trigger for lavas erupted across the entire Altiplano-Puna plateau during discrete foundering events that accompany shortening and plateau growth. Subduction-related magmatism occurring beneath the arc front enhances this process and results in a similar, but distinct, geochemical signature.