The metamorphism of mafic rocks: studies on reaction textures, local equilibrium and thermodynamic modelling

• Autor(a) principal: Santos, Caio Arthur
• Data de Publicação: 2019
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Biblioteca Digital de Teses e Dissertações da USP
• Texto Completo: http://www.teses.usp.br/teses/disponiveis/44/44143/tde-30052019-111018/
• topic: Modelamento termodinâmico; not available; Petrologia metamórfica; Rochas metamáficas; Texturas reacionais

Despite the fact that metamafic rocks form one of the best studied groups of metamorphic rocks, there are still aspects of their metamorphism that are not well understood. Traditionally, studies in metamorphic petrology assume that, at peak conditions, rocks attained chemical equilibrium at a scale comparable with a thin section or hand-sample, but this may not be the case when there is little access of fluid or melt. Another important aspect is the ability of petrologists to model metamorphic processes, which depends critically on information such as thermodynamic data of relevant mineral phases. In this thesis, these different aspects of metamorphism are accessed along four separate contributions. First, the most recent set of activity-composition models available for mafic rocks is evaluated through a series of pseudosections for rocks for which there are independent thermobarometric data. The comparison between the phase diagrams and the independent data shows that, although successful some cases, these activity-composition models are not suitable to rocks predicted to be unsaturated in SiO2, as, in these cases, the stability fields of diopside and hornblende are overestimated; additionally the stability field of garnet is underestimatedin all cases. In the second contribution, a suite of metagabbros from Mantiqueira Complex (SE Brazil), with different degrees of hydration/reequilibration, are analyzed using petrography, mineral chemistry and thermodynamic modelling. Thermodynamic modelling shows that the presence of garnet in partially reequilibrated rocks is linked to low quantities of H2O, showing its importance as a system component, in addition to its kinetic role. On the other hand, features such as mineral zoning and pseudomorphic replacements are linked to the relative mobility of the different cations, specially the low mobility of Al. Even the most hydrated samples still show inherited local equilibrium features. The third and fourth contributions deal with the retroeclogites of São Sebastião do Paraíso (SE Brazil): in the third, through a combination of thermodynamic modelling and Zr-in-rutile and Ti-in-quartz thermobarometers, the P-T path of these rocks is constrained to a maximum T of ?730 °C and maximum P between ?16 and ?19 kbar, along a clockwise path. The conditions are compatible with a collisional tectonic setting. In the fourth contribution the textural and mineralogical evolution of the retroeglogites is investigated through petrography, mineral chemistry and thermodynamic modelling. From the least to the most hydrated/reequilibrated rocks, reaction textures become diffuse and mineral compositions tend to converge towards predicted compositions, although not actually reaching them, what can be ascribed to the persistence of inherited features, such as the presence of garnet. As in the second contribution, the low mobility of Al is one of the controlling factors in texture development, along with the high mobility of Na. The picture of metamorphism of mafic rocks that emerge from this work is one of an irregular process, where local equilibrium features perisist even after full hydration. Moreover, there are issues with the data used to model metamorphic processes. Nonetheless, if thermodynamic modelling is used cautiously and in combination with other methods, meaningful results can be obtained.