Chemical stratigraphy of the Parana basalt succession in western Uruguay: further evidence for the diachronous nature of the Parana magma types

An important issue in the study of continental flood basalt provinces concerns whether chemically defined magma types have chronological significance. Relevant to this debate, we present important findings based on new major and trace element geochemical data on lavas from three drill holes in western Uruguay which intersect up to 718 m of the Parana basalt succession. The lava pile here consists of Paranapanema lavas overlain by widespread Gramado lavas and one rare Esmeralda lava. The occurrence of Paranapanema lavas beneath Gramado lavas is the reverse of the magma type stratigraphy of the Parana flood basalt pile further north in Brazil. This provides evidence, independent of isotopic dating, that in the Parana, magma types are not chronostratigraphic. The simultaneous eruption of different magma types suggests that the spatial and temporal distribution of magma types reflects the sub-crustal distribution of distinct source regions inferred to lie within the lithospheric mantle.