Magmatic garnet in the Cordilleran-type Galiléia granitoids of the Araçuaí belt (Brazil): Evidence for crystallization in the lower crust

Abstract

Magmatic garnet, together with epidote, is a rare mineral association in cordilleran-I-type granitoids and of special petrogenetic significance. The metaluminous to slightly peraluminous (ASI = 0.97-1.07) Galiléia batholith (Brazil) is a large (ca. 30,000 km 2 ), Neoproterozoic (ca. 632-570 Ma) weakly foliated calc-alkaline granitoid body, characterized by the widespread occurrence of garnet (grossular 25-43 mol%) and epidote (pistacite 9.3-22.7 mol%). Field, petrographic and mineral chemical evidence indicates that garnet, epidote, biotite as well as white mica crystals (low-Si phengite), are magmatic. There is no difference in bulk rock major and trace element composition between the Galiléia granitoids and other garnet-free cordilleran-type granitoids worldwide. This evidence strongly suggests that the origin of the uncommon garnet + epidote parageneses is related to the conditions of magma crystallization, such as pressure, temperature and water content. Comparison between the mineral assemblages and mineral compositions from this study and those recorded in crystallization experiments on metaluminous calc-alkaline magmas, as well as within garnet-bearing metaluminous volcanic rocks and granitoids, indicates that the supersolidus coexistence of grossular-rich garnet, epidote and white mica is consistent with magma crystallization at pressures greater than 0.8 GPa (above 25 km depth) and at temperatures below 700°C, i.e. near the water saturated solidus. Furthermore, resorption textures around garnet (plagioclase ± quartz coronas) and epidote suggest that these minerals have been partially consumed prior to complete crystallization. These findings demonstrate that at 630 Ma the crust underneath the Araçuaí Orogen was already at least 25-30 km thick and relatively cool. However, this contrasts with the marked high heat flow registered from the neighbour Carlos Chagas Batholith located 50 km to the east. In fact such granitoids record granulite-facies metamorphism at the same pressure and time (ca. 570 Ma) of Galiléia granitoids crystallization. Thus, a more suitable geodynamic scenario is required in order to explain these two contrasting thermal regimes within the same orogen. Eventually, field, petrographic and mineral chemical analogies with similar garnet-bearing granitoids located in the fore-arc settings of the British Columbia subduction zone, possibly imply that the Galiléia granitoids represent "rare" garnet-and epidote-bearing metaluminous Cordilleran-Itype granites which can only form in a fore-arc setting.

Condições de colocação do magmatismo máfico do Domínio Macururé, Sistema Orogênico Sergipano: Maciço Capela

“…A presença de epídoto primário em plútons cálcio-alcalinos é igualmente indicativo de médias a altas pressões (Zen e Hammarstrom, 1984a;Schmidt e Thompson, 1996). Almandina cálcica e epídoto magmáticos é uma associação mineralógica incomum, que tem sido descrita em plútons metaluminosos cristalizados a altas pressões em configurações de arco (Zen e Hammarstrom, 1984b;Narduzzi et al, 2017). Ishihara (1977) propôs que os granitos podem ser divididos nas séries Ilmenita (baixa fO 2 ) e Magnetita (alta fO 2 ).…”

Petrologia do Batóito Muniz Freire, Estado do Espírito Santo

“…Foram selecionadas para análises geoquímicas 24 amostras representativas do BMF, as quais estão relacionadas na tabela 2 e 3. Nos diagramas utilizados, são feitas comparações com amostras da Suíte Galiléia (Nalini, 1997, Nalini et al 2005, Narduzzi et al 2017 .…”

“…(2017)realizaram análises em granada em rochas do Batólito Galiléia e compararam seus resultados com experimentos de cristalização e ocorrências naturais de corpos graníticos de outras regiões do mundo, concluindo que os modelos atuais explicam apenas parcialmente sua composição. Comparando os resultados deNarduzzi et al (2017) com os resultados obtidos para amostras do BMF, notase que ambos possuem composições virtualmente idênticas (figura 20B), bem como as composições de granadas dos granitoides de Bushy Point, Alaska, EUA. Análises de granada de metagranitos do Orógeno Dabie-Sul(Xia et al 2012) também forneceram resultados semelhantes aos dos corpos acima.…”

“…Portanto, afim de discutir a composição relativamente incomum das granadas do BMF, especialmente seu alto teor de Sps% e Grs%, essas serão comparadas as granadas analisadas pelos autores acima mencionados, considerando os corpos e suas características principais. Grs% pode aumentar com o resfriamento do magma em pressões de aproximadamente 12 kbar, embora essa variação não tenha efeito relevante em Sps% Narduzzi et al (2017). recorrem às inclusões para explicar os altos teores concomitantes de espessartita e grossulária.…”

Deformation regime variations across the Neoproterozoic Araçuaí hot orogen (SE Brazil): insights from structural and magnetic fabric analyses

“…consists predominantly of medium-grained tonalites, granodiorites, and subordinate granites, that are either granular or porphyritic. Amphibole and garnet are more abundant in the São Vitor than in the Galileia tonalite (Mondou et al, 2012;Narduzzi et al, 2017). Foliation is defined by the orientation of biotite and amphibole crystals and marked by stretched mafic enclaves.…”