Reconstruction of the effusive and explosive deposits of the Aruri and Salustiano formations in the Tapajós Domain, Southern Amazonian Craton, from field relationship, petrography and geochemistry

Abstract

Proterozoic Silicic Large Igneous Provinces (SLIP) are part of the geological inventory since they record the planet's thermal evolution. The Amazonian Craton hosted the poorly understood Uatumã SLIP, which formed ca. 1.88 Ga years ago. The so-called Uatumã magmatism represents the volcanic products of SLIP. This magmatism also refers to specific stratigraphic units, such as the Uatumã, Iriri, and Iricoumé groups. The Aruri and Salustiano formations within the Iriri Group represent an important SLIP stratigraphy segment transitioning from effusive to explosive volcanism. However, this interval is still undisclosed and lacks a thorough comparison of the Iriri Group stratigraphy. From field observations, detailed petrographic description, new geochemistry data on the Aruri and Salustiano formations, and a compilation of geochemistry data on the Uatumã SLIP, we present the litho-stratigraphy for the two formations and a comparison to the other levels of the SLIP stratigraphy. Our data show a predominance of effusive over explosive volcanism and a set of intermediate rocks that intrude the sequence. The Aruri Formation contains six lithofacies organized as interbedded facies representing coherent rhyolitic lava flows and volcaniclastic rocks (tuffs), which are intruded by massive dacite dykes. The Salustiano Formation contains three lithofacies that comprehend coherent rhyolite flows and ignimbrites. Whole-rock geochemistry data indicate that the felsic facies rocks represent rhyolites to alkali-rhyolites (>70% SiO2) of the high-K calc-alkaline to shoshonite series with metaluminous to a peraluminous character. The rocks are considered as formed in transitional settings from a mature arc to an extensional regime. The comparison to the remainder of the Iriri Group corroborates an interpretation of their origin as subaerial volcanism with coeval effusive and explosive phases in a caldera-type volcanic system and epiclastic rocks deposited in a distal position concerning the volcanic center. The whole-rock geochemistry is similar to the other levels of the Uatumã SLIP and confirms the large extent of the ca. 1.88 Ga years ago in the Amazonian Craton.