Evolution of Heterogeneous Mantle in the Acampamento Velho and Rodeio Velho Volcanic Events, Camaquã Basin, Southern Brazil.

Abstract: The Camaquã Basin, developed during the last phases of the Brazilian/Pan-African Orogeny and was filled with a thick volcano-sedimentary succesion, in which two volcanic events of alkaline affinity are represented by the Acampamento Velho Alloformation and the Rodeio Velho Member. The Acampamento Velho Alloformation records a bimodal event with a lower association of mafic flows and an upper association of felsic pyroclastic rocks and flows. It was formed during extension, after the subduction of the Adamastor oceanic plate beneath the Rio de La Plata continental plate at the end of the Neoproterozoic III. The second event, the Rodeio Velho Member, represented by mafic flows, intrusions and piroclasts, took place during overall extensional tectonism, probably in the middle Ordovician. Rb, Sr, Sm, and Nd isotopic measurements were carried out on samples from both units. Regardless the event they represent, all the samples display negative values for epsilon Nd, ranging from 2.97 to 10.31 for the Acampamento Velho Alloformation and from 8.39 to 13.92 for the Rodeio Velho Member. The initial ⁸⁷Sr/⁸⁶Sr ratios vary from 0.706 to 0.707 and from 0.704 to 0.707 for the Acampamento Velho Alloformation and Rodeio Velho Member, respectively. Mafic flow deposits in both units show a preferential enrichment in Ba relative to Th. Flow samples from the Rodeio Velho Member also display a distinctive enrichment in the Ba/Th ratio, without a change in the initial Sr, compared to the mafic flow deposits from the Acampamento Velho Alloformation, which show a slight enrichment in those ratios. As for the Acampamento Velho Alloformation, the mafic lavas could be a mixture of depleted mantle-derived basalts plus 20% to 30% of crustal contamination by sediment (probably Neoproterozoic arkosic quartzites). The formation of a magmatic chamber and the separation of the magma into two fractions gave rise initially to the mafic rocks at the base of the Acampamento Velho Alloformation The other magma fraction underwent a significant enrichment in crustal component before the felsic rocks of this Alloformation were formed. The flows from the Rodeio Velho Member originated in a distinct magma chamber, with EM I characteristics that was much more enriched in incompatible elements and depleted in radiogenic Sr. [Copyright &y& Elsevier]