1. Cenozoic behind-arc volcanism in the Bolivian Andes, South America: implications for mantle melt generation and lithospheric structure
- Author
-
Leonore Hoke and Simon Lamb
- Subjects
Volcanic rock ,Basalt ,geography ,geography.geographical_feature_category ,Subduction ,Mantle wedge ,Volcanic arc ,Asthenosphere ,Geochemistry ,Geology ,Mafic ,Mantle (geology) - Abstract
We develop a model of arc and behind-arc volcanism, as well as constraining the evolution of the lithospheric mantle structure beneath the Bolivian Andes during the last 25 Ma, using new geochronological and geochemical data on behind-arc mafic volcanic rocks, together with a regional isotopic study of geothermal helium emissions, and a comparison with seismic tomographic images of the same region. Helium isotopes measured in natural gas emissions in geothermal and mineral-water springs suggest recent mantle melting in a wide zone extending at c . 20°S from a few tens of kilometres west of the volcanic arc to over 300 km behind the arc, where the subducted slab is at depths increasing from c . 100 to c . 250 km. During the Neogene, there has been behind-arc mafic–felsic magmatism right across the high Andes between 17° and 22°S, in the Altiplano and western margin of the Eastern Cordillera. Modelling of Plio-Pleistocene basalts and basaltic andesites ( c . 15% depleted mid-ocean ridge basalt (MORB) source at depths between c . 75 km and c . 110 km for mafic volcanic rocks that crop out c . 65 km and c . 90 km for those that crop out >100 km behind the arc. REE inversions for Oligo-Miocene shoshonites and basalts ( c . 25–21 Ma) suggest that they originated at significantly shallower depths ( c . 45 km to c . 100 km) from a slightly enriched MORB source. Eruption of shoshonitic lavas also occurred in the Altiplano between 13 and 11 Ma, and widespread behind-arc intermediate–felsic volcanism has been more or less continuous since the early Miocene. All this suggests that since c . 25 Ma there has been a thin ( c . 25 Ma by the opening up of a mantle wedge and inflow of hot asthenosphere, as a consequence of the steepening of the subducted slab and detachment of hydrated and weakened mantle lithosphere and possibly mafic lower crust.
- Published
- 2007