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. 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. 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 rocks in the caldera have derived from partial melting of a spinel lherzolitic or garnet lherzolitic mantle, in a typically orogenic, calc-alkaline environment. 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 first is the eruption of pyroclastic material and localized flows of alkali-basalt, trachyandesite and andesite composition, corresponding to the early Cambrian Hilário Formation. 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.
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