Riolitos Neoproterozóicos pós-colisionais na área do santuário, sul do Brasil: litoquímica, química mineral e origem das heterogeneidades texturais

Abstract

Petrographic studies on the hipabissal rhyolites from Acampamento Velho Alloformation (~580Ma) in the Santuario area, Vila Nova do Sul, southern Brazil, led to the identification of distinct textural patterns: (i) hemicrystalline and (ii) microcrystalline, related to local chemical gradients and volatile percolation along fractures, and (iii) spherulitic, probably related with high temperature (~700 0 C) devitrification processes. Mineral chemistry indicates that these rocks are made of sanidine, ferrowinchite and ferrobarroisite, Ti-magnetite and ilmenite, a mineral assemblage common in rhyolites of comendiitic affinity. Geothermometry using the composition of Fe-Ti oxides indicates crystallization temperatures of 850 o C-886 o C. The Santuario rhyolites are compositionally classified as highsilica, metaluminous to peralkaline, and in relation to pantelleritic peralkaline types they show lower amounts of Fe, Mn, Na, and Ti, higher contents of Al, Ca and Mg , and lack sodic ferromagnesian phases. The Santuario rhyolites show high contents of REE, with moderate fractionation of LREE to HREE, like most alkaline rhyolites from post-orogenic settings. Spidergrams for incompatible elements are also consistent with rhyolites of post-orogenic settings, associated with magmas related to subductionmodified mantle sources. This magmatism is probably associated to the late phases of Brasiliano-Pan-African Cycle in southern Brazil .