Unearthing the basement of the Central Andes: insights from crustal xenoliths

The continental crust of the Central Andes is the thickest at any subduction on Earth today reaching an estimated 80 km in thickness (Zandt et al., 1994). However, little is known about the nature and geological evolution of the crustal basement upon which the Central Andes sit due to the extensive sedimentary cover sequences which blanket the region today. Crustal xenoliths entrained within Plio-Pleistocene andesitic-dacitic lavas on the Bolivian Altiplano offer a rare insight into the nature of the poorly exposed Central Andean basement. The samples are lithologically diverse ranging from almost pure quartzite to garnet-mica schists, with rarer granulites and several igneous lithologies including diorites and microgranites. This diversity is reflected in their significant geochemical heterogeneity (⁸⁷Sr/⁸⁶Sr: 0.7105-0.7445; ¹⁴³Nd/¹⁴⁴Nd: 0.5118-0.5123; ²⁰⁸Pb/²⁰⁴Pb: 17.25-18.93). Relative trace element abundances and P-T estimates are consistent with sampling of the upper continental crust at ~23 km depth. Additionally these xenoliths provide key crustal end-member compositions for modelling the petrogenesis of Central Andean volcanic rocks. It is hoped that in-situ U-Pb analysis of zircon within this rock suite will provide further insights into the nature and evolution of the Central Andean continental crust. References Zandt, G., Velasco, A. A., and Beck, S. L., (1994). Composition and thickness of southern Altiplano crust, Bolivia. Geology v. 22, pp: 1003-1006.