Using geochemistry to establish the igneous provenances of the Neogene continental sedimentary rocks in the Central Depression and Altiplano, Central Andes

Geochemical and mineralogical data from ancient sedimentary strata can be reliable indicators of the provenance of sediments. The heavy mineral assemblages and the major and trace element contents of sedimentary rocks from the Neogene continental successions of the Central Depression in Chile and the Mauri and Corque Basins in the Altiplano of Bolivia reflect volcanic source rocks with different degrees of magmatic differentiation and alkalinity. These results indicate that the source rocks of the Central Depression were less differentiated (andesite to rhyodacite) than those of the Altiplano basins (rhyodacite to rhyolite). The low concentration (cations per formula unit) of total Al (<0.15 pfu) and [Ca+Na] (∼0.82 pfu) in the detrital clinopyroxenes and low [Nb/Y] ratio (<0.7) of whole-rock analyses of sandstones from the Central Depression indicate erosion of a calc-alkaline source rock. By contrast, the high concentration of total Al (>0.15 pfu) and [Ca+Na] (∼0.92 pfu), the variable Ti content (0.01-0.04 pfu) of the clinopyroxenes and the high [Nb/Y] (>0.7) in whole-rock analyses of sandstones from the Mauri Basin indicate erosion of alkaline rock sources. Locally, the low concentration of total Al (<0.15 pfu) in the detrital clinopyroxenes of sandstones from the Corque Basin indicates erosion of a subalkaline source rock. The chemical trends in the sandstones are similar to those in volcanic suites from the Western Cordillera and Eastern Cordillera of the Central Andes.