Modelagem geoquímica da gênese e evolução do magmatismo da formação Acampamento Velho, Caçapava do Sul e Dom Pedrito, RS

Abstract

The Acampamento Velho Formation represents the second volcanic event in the Camaquã Basin. The volcanic rocks of this formation are partially represented mainly in Caçapava do Sul, Dom Pedrito and Vila Nova do Sul cities. These rocks are comprised dominantly by acidic with subordinate basic terms, beyond the absence of intermediate extrusive terms, characterizing a bimodal volcanism. In order to identify and quantify the processes responsible for the genesis and evolution of the magma from which these rocks were formed, the realization of a geochemistry modeling is propose. In this sense, deterministic models are used, which describe through mathematical equations the behavior of some of the major and trace chemical elements during the various geological processes, to characterize the formation and evolution of the magma of this unit. The processes modeled in this work include partial melting, fractional crystallization, crustal assimilation and magma mixing. The quantification process was realized in two consecutive stages of modeling. The first consisted of mass balance, through Petrograph™ software, and involves the major elements. The second was the modeling with the trace elements, performed through the Petromodeler™ software. In order to better understand the techniques used and the process acting in a magmatic system, an extensive bibliography study of the work already produced of the theme and the realized in the study area was realized. As a result, values were obtained indicating the partial melting rate of the mantle and the crust, as well as the analysis of the behavior of the trace elements in relation to the mixture of these two magmas of different origin and compositions. The results suggest a mantle melting at different rates, which varied from 19% to 28% for the genesis of the basic rocks of the Acampamento Velho Formation. The acid rocks were formed by the action of the crystal fractionation mechanisms of the basic magma and the mixture between the magmas generated by the melting of the mantle and the crust. The fractionation of crystals would have occurred at varying rates from the basic magma until the formation of the most evolved magmas, when this fractionation reached 90% of the initial magma. During magmatic evolution, the magma of intermediate composition might have not been formed, or have not been involved in volcanic events, being instead, solidified as plutonic or subvolcanic terms. This model is compared with other models described in the literature and analyzed with respect to the mechanisms present in the environment of formation of these rocks, namely, the post-collision environment.