Ediacaran Na-alkaline Acampamento Velho volcanism in the Ramada Plateau, southernmost Brazil: Sr–Nd–Pb isotopic data and petrogenetic evolution

Abstract In the Ramada Plateau, there is an expressive succession of volcanic rocks from the Acampamento Velho Formation related to the post-collisional period of the Pan-African/Brasiliano orogeny. This unit is predominantly composed of felsic pyroclastic rocks (ignimbrites) with subordinate rhyolitic and basaltic flows, as well as subvolcanic basalts, trachytes, and rhyolites generated between approximately 580 and 550 Ma. These rocks have a sodic alkaline affinity, which is typical for a post-collisional environment. Sr-Nd-Pb isotopic data suggest the rocks of the Acampamento Velho Formation in the Ramada Plateau were produced by the fusion of an asthenospheric mantle, modified by previous subduction with an EMI-type mantle signature. Petrogenetic modelling using isotopes, major and trace elements allowed us to conclude that fractional crystallization processes were more effective in differentiating the basic-intermediated terms involving the fractionation of approximately 60% of melt. For the intermediate and acid compositions, a greater discrepancy between the modelling values for the major/trace elements and isotopes was observed despite the occurrence of fractional crystallization associated with crustal assimilation, especially in trace elements and isotopes diagrams. Field and petrographic evidence, as well as parabolic geochemical trends allowed us to conclude that the intermediate and acid terms of the Acampamento Velho Formation magmatism evolved largely by magma mixing processes