Joaquim Daniel de Liz

The evolution of Neoproterozoic magmatism in Southernmost Brazil: shoshonitic, high-K tholeiitic and silica-saturated, sodic alkaline volcanism in post-collisional basins

The Neoproterozoic shoshonitic and mildly alkaline bimodal volcanism of Southernmost Brazil is represented by rock assemblages associated to sedimentary successions, deposited in strike-slip basins formed at the post-collisional stages of the Brasilian/Pan-African orogenic cycle. The best-preserved volcano sedimentary associations occur in the Camaquã and Campo Alegre Basins, respectively in the Sul-riograndense and Catarinense Shields and are outside the main shear belts or overlying the unaffected basement areas. These basins are characterized by alternation of volcanic cycles and siliciclastic sedimentation developed dominantly on a continental setting under subaerial conditions. This volcanism and the coeval plutonism evolved from high-K tholeiitic and calc-alkaline to shoshonitic and ended with a silica-saturated sodic alkaline magmatism, and its evolution were developed during at least 60 Ma. The compositional variation and evolution of post-collisional magmatism in southern Brazil are interpreted as the result mainly of melting of a heterogeneous mantle source, which includes garnet-phlogopite-bearing peridotites, veined-peridotites with abundant hydrated phases, such as amphibole, apatite and phlogopite, and eventually with the addition of an asthenospheric component. The subduction-related metasomatic character of post-collisional magmatism mantle sources in southern Brazil is put in evidence by Nb-negative anomalies and isotope features typical of EM1 sources.

Neoproterozoic, Mildly Alkaline, Bimodal Volcanism in Southern Brazil: Geological and Geochemical Aspects

Neoproterozoic magmatism in southern Brazil is associated with translithospheric shear belts and strike-slip basins in a post-collisional setting related to the last stages of the Brasiliano—Pan African orogenic cycle. It evolved from an association of high-K calc-alkaline, leucocratic peraluminous and continental tholeiitic magmas, to shoshonitic, and eventually to sodic mildly alkaline series. Bimodal volcanism in the last group was coeval with subaerial siliciclastic sedimentation in all post-collisional basins preserved in the region. Three magmatic associations were identified in the bimodal volcanism: (1) low-Ti basalts and rhyolites; (2) high-Ti basalts and rhyolites; and (3) high-Nb rhyolites and basalts. Basic parental magmas were produced from sources related to EM1-type mantle previously modified by Brasiliano subduction, whereas silicic high- and low-Ti rocks were probably produced through fractional crystallization. Significant crustal contamination was recognized only in some slightly peraluminous rhyolites with low contents of HFS elements. High-Nb rhyolites, the youngest volcanic rocks, probably reflect the participation of asthenospheric components in the source, and mark the last magmatic event related to the Brasiliano-Pan African orogenic cycle in southern Brazil.

Rochas dioríticas do Platô da Ramada, Rio Grande do Sul, e sua relação com o vulcanismo alcalino da Formação Acampamento Velho, Neoproterozoico do Escudo Sul-Rio-Grandense

Acampamento Velho. Esta unidade e constituida por rochas vulcânicas de composicao predominantemente acida, com termos basicos associados, e corresponde a porcao extrusiva do magmatismo alcalino sodico de idade Neoproterozoica (570-550 Ma), vinculado aos estagios pos-colisionais do Ciclo Brasiliano-Panafricano no Escudo Sul-Rio-Grandense. O corpo intrusivo varia desde termos dioriticos ate quartzo-monzoniticos e sua afinidade alcalina sodica e atestada pelos padroes de elementos litofilos de grande raio ionico (LILE), elementos de elevado potencial ionico (HFSE) e elementos terras raras (ETR), cujas caracteristicas indicam sua correlacao com o magmatismo alto Ti da Formacao Acampamento Velho, descrito no Plato da Ramada. A evolucao do magmatismo alcalino da Formacao Acampamento Velho nesta regiao pode ser explicada, principalmente, por processos de cristalizacao fracionada envolvendo tres estagios principais, conforme resultados obtidos por meio de modelamentos petrogeneticos apresentados neste artigo. No entanto, face ao grande volume de rochas rioliticas de alta silica e a complexidade que envolve a geracao e a evolucao destes liquidos, a possibilidade de que tenham ocorrido processos de cristalizacao fracionada associados possivelmente a mecanismos de assimilacao crustal e grande. A identificacao e a caracterizacao de rochas intermediarias relacionadas a Formacao Acampamento Velho no Plato da Ramada indicam, portanto, que o magmatismo provavelmente evoluiu desde composicoes basicas ate acidas. Porem, a assertiva relacionada a bimodalidade do vulcanismo ainda permanece, devido a ausencia de lavas andesiticas. Tal ausencia pode ser atribuida a uma ‘barreira de densidade’ gerada pela diferenciacao dos liquidos basicos para intermediarios enriquecidos em FeO, os quais, por esta razao, estacionariam nos niveis crustais rasos.