The Paraná-Etendeka province

Abstract

Stratigraphic data and40Ar-39Ar ages for the Early Cretaceous Paraná- Etendeka flood basalts indicate that the main magmatic episode lasted for several m.y. (129-134 Ma) and was linked to the northward opening of the South Atlantic Ocean, but with some earlier magmatism (135-138 Ma) found inland far from the eventual oceanic rift. The regional distribution of distinct high-Ti/Y (Urubici, Pitanga, Paranapanema, Ribeira) and low-Ti/Y (Gramado, Esmeralda) magma types in the lavas and associated dyke swarms implies that magma generation occurred over a wide area and involved different mantle sources. Low MgO contents (3-7 wt%) indicate extensive fractional crystallisation, and upper crustal assimilation was important in the evolution of the Gramado magmas. However, Paraná basalts that are considered to be uncontaminated by crust have trace element and isotope characteristics (e.g. Nb/La < 0.8; εNdi < 0) and major element features that appear to require mantle sources distinct from those of oceanic basalts. The minor, late-stage, Esmeralda magma type is an exception, requiring a component from incompatible-element- depleted asthenosphere. The role of the Tristan mantle plume appears to have been largely passive, with conductive heating facilitating mobilisation of old lithospheric material. Significant rhyolitic eruptions (>1000 km3) that can be correlated across the Atlantic Ocean accompanied the final magmatic phase in the southeast Paraná and the Etendeka. The flood basalts post-date most estimates for the Jurassic-Cretaceous boundary, ruling out any link to a faunal extinction.