Early Cretaceous basaltic and rhyolitic magmatism in southern Uruguay associated with the opening of the South Atlantic

The Early Cretaceous volcanic rocks of southern Uruguay comprise mafic and felsic volcanics. The position of these outcrops at the southern edge of th e Parana-Etendeka continental flood basalt province provides an opportunity to investigate possible lateral variations in both mafic and more evolved rock types towards the margins of such an area of plume-related magmatism. The mafic lavas are divided into two compositionally distinct magma types. The more voluminous Treinte gamma Tres magma type is similar to the low-Ti basalts of the Parana flood basalt province. The Santa Lucia magma type is a distinct and rare basalt type with ocean-island basalt type asthenospheri c affinities (high Nb/La, low Sr-87/Sr-86(i)). The felsic volcanics are div ided into two series, the Lavalleja Series and the Aigua Series. The Lavall eja Series are chemically and isotopically similar to the Parana-Etendeka l ow-Ti rhyolites, and are considered to be related to the Treinte gamma Tres lavas by extensive fractionation and crustal assimilation. The Aigua Serie s extensive fractionation and crustal assimilation. The Aigua Series have l ow Nd-143/Nd-144(i) and low Sr-87/Sr-86(i) and unlike the rhyolites of the Parana, are interpreted as melts of pre-existing mafic lower crust that sub sequently underwent extreme fractionation. The differences observed in the felsic suites may be linked to differences in the volumes of the associated basalts and the amounts of extension.