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The region studied, located in southern Rio Grande do Sul State, Brazil, 25 km to the northeast of Lavras do Sul, records two volcanic events. The first is the eruption of pyroclastic material and localized flows of alkali-basalt, trachyandesite and andesite composition, corresponding to the early Cambrian Hilario Formation. This event in the region, through tectonic reactivation and explosive expulsion of a large amount of material from the magmatic reservoir chamber, which later faulted and collapsed, has generated an elliptical caldera of 7.2 km × 3.0 km. The rocks in the caldera have derived from partial melting of a spinel lherzolitic or garnet lherzolitic mantle, in a typically orogenic, calc-alkaline environment. They were generated during the final phase of subduction of the Adamastor plate beneath the Rio de la Plata plate in the early Cambrian. The second event occurred in a post-collisional tectonic setting in the Middle Ordovician?, when alkaline magma was emplaced through deep fractures, generating four cones within the collapsed caldera. The rocks in the cones bear the geochemical signature of a more evolved magma when compared to the rocks in the caldera, having been formed through low fusion rates of a garnet-depleted source in the upper mantle. It represents the last phase of the Pan-African-Brasiliano Orogeny, which occurred right after the collision of the Rio de La Plata and Kalahari continental plates, in a retroarc setting. |
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