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

Abstract

The Early Cretaceous volcanic rocks of southern Uruguay comprise mafic and felsic volcanics. The position of these outcrops at the southern edge of the 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 Y 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 asthenospheric affinities (high Nb/La, low 87Sr/86Sr). The felsic volcanics are divided into two series, the Lavalleja Series and the Aigua Series. The Lavalleja Series are chemically and isotopically similar to the Parana-Etendeka low-Ti rhyolites, and are considered to be related to the Treinte Y Tres lavas by extensive fractionation and crustal assimilation. The Aigua Series have low 143Nd/144Nd(i) and low 87Sr/86Sr(i) and unlike the rhyolites of the Parana, are interpreted as melts of pre-existing mafic lower crust that subsequently 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.