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

• Ano de defesa: 2019
• Autor(a) principal: Angelo, Tiago Valim
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Biblioteca Digitais de Teses e Dissertações da USP
• Palavras-chave em Português: Anisotropia de susceptibilidade magnética; Anisotropy of magnetic susceptibility; Araçuaí belt; Fábrica magnética; Faixa Araçuaí; Fluxo magmático; Hot orogen; Magmatic flow; Magnetic fabric; Magnetismo de rocha; Orógeno quente; Rock magnetism
• Link de acesso: http://www.teses.usp.br/teses/disponiveis/44/44141/tde-16102019-101332/

The Neoproterozoic Araçuaí belt of East Brazil formed during the amalgamation of Western Gondwana and holds characteristics of a hot collisional belt, involving large amount of magma, partial melting of the middle crust, and slow cooling rates. This work combines structural analysis, magnetic fabric, and geochronological studies in order to access information related to the flow of rocks, deformation history, and structural patterns associated with the behavior of this orogenic setting. Microstructural observations support that deformation of the plutonic bodies occurred in the magmatic state while the host metasedimentary and basement rocks remained in the solid-state. A detailed structural mapping integrating field and anisotropy of magnetic susceptibility (AMS) revealed four domains with contrasting flow patterns. The structural patterns from W to E are characterized by: westward thrusting orthogonal to the belt (region 1), orogen-parallel tranpression induced strain partitioning (regions 2 and 3), and orogen-parallel flow and subsidiary eastward vergence magmatic flow (region 4). Anisotropy of anhysteretic remanent magnetization (AARM) and magnetic mineralogy investigations suggest that the main carriers of the AMS are biotite and/or amphibole in paramagnetic samples (host metassediments and basement unit), and multi-domain (MD) or pseudo-single domain (PSD) magnetite grains in ferromagnetic materials (hosted in plutonic bodies). U-Pb dating of zircons from granitoids in the western arc border and central arc regions (structural regions 1, 2, and 3) reveal that magmatism occurred between 615-567 Ma. Microstructural investigations in the igneous bodies suggest that deformation occurred before complete solidification. This magmatism is associated with the main tectono-metamorphic peak attained by the orogen. To the east (structural region 4), younger bodies were emplaced in a still thermally buffered environment, but after solidification of the anatectic country rock. This late magmatism (540-480 Ma) is associated with a minor tectono-metamorphic peak, and magmatic deformation affected these bodies during that time. In the context of protracted deformation under slow cooling conditions, the composite observed fabric results from the interplay of collision-driven (thrusting and transpression strain partitioning) and gravity-driven (orogen-parallel flow) deformations, induced by the East-West convergence between the São Francisco and Congo Cratons