Análise de microestruturas e texturas cristalográficas em gnaisses milonitizados do Sudoeste Norueguês

Abstract

Resumo: No sudoeste da Noruega observa-se um embasamento que foi intensamente deformado durante a formacao da Orogenia Caledoniana. Pouco se sabe sobre as texturas cristalograficas das rochas da regiao, portanto, em busca da caracterizacao das microestruturas e texturas cristalograficas geradas a partir dessa deformacao, foram selecionadas dez amostras de gnaisses milonitizados. Essas amostras foram separadas em dois grupos distintos. O Grupo 1 e constituido por amostras do Oygarden Complex, localizado proximo a Zona de Cisalhamento do Arco de Bergen, enquanto o Grupo 2, engloba as rochas da regiao denominada de Cinturao Baltico, na Zona de Cisalhamento do Fiorde de Hardanger. Foram preparadas laminas delgadas para a analise microscopica e subsequente investigacao por EBSD (Electron Backscatter Diffraction). Em geral, as amostras exibem foliacao anastomosada/milonitica, estrutura S-C, extincao ondulante e porfiroclastos de feldspato e quartzo frequentemente cercados por bandas de quartzo. No Grupo 2, observa-se tambem microestruturas do tipo mica fish e porfiroclastos de quartzo com formas em "(Delta)" e "(Fi)". A recristalizacao observada e dinamica, expressa principalmente pelo bulging (BLG), com ocorrencia de rotacao de subgraos (SGR). Essas feicoes indicam deformacao por meio de um fluxo plastico, imposta por mecanismos de fluencia de deslocacoes (dislocation creep). As texturas cristalograficas foram analisadas no quartzo. As figuras de polo (Pole Figures - PF) e as figuras de polo inverso (Inverse Pole Figure - IPF) do quartzo indicam um sistema de deslizamento principal prismatico {m} <a>, com ativacao dos sistemas romboedricos positivo {r} <a> e negativo {z} <a>. Esses sistemas romboedricos sao expressos principalmente atraves dos angulos de desorientacao (misorientation), que apresentam picos em torno de 60º e indicam geminacao Dauphine no quartzo. A SGR tambem foi observada atraves dos angulos de desorientacao abaixo de 15º. Com base nessas analises, sugere-se que as rochas passaram por dois processos de deformacao distintos. O primeiro ocorreu em temperaturas no intervalo de 400 . 500 ºC, esta relacionado as zonas de cisalhamento extensionais da regiao e apresenta evidencias como o sistema de deslizamento prismatico {m} <a>, a foliacao anastomosada/milonitica e microestruturas, como par S-C, mica fish e porfiroclastos com caudas de recristalizacao. O segundo processo, caracterizado por temperaturas mais baixas, esta relacionado a reativacao das zonas de cisalhamento e e evidenciado pela cloritizacao das micas, pela recristalizacao do tipo bulging e pelos sistemas de deslizamento romboedricos e basais identificados no quartzo

Abstract: In southwestern Norway a basement that was intensely deformed during the formation of the Caledonian Orogeny can be seen. Little is known about the crystallographic textures of the rocks in the region, therefore, in the quest to characterize the microstructures and crystallographic textures generated from this deformation, ten samples of mylonitized gneisses were selected. These samples were separated into two distinct groups. Group 1 consists of samples from the Oygarden Complex, located near the Bergen Arc Shear Zone, while Group 2 encompasses rocks from the region known as the Baltic Shield, in the Hardangerfjord Shear Zone. Thin sections were prepared for microscopic analysis and subsequent investigation using Electron Backscatter Diffraction (EBSD). In general, the samples exhibit anastomosed/milonitic foliation, S-C structures, undulatory extinction, and feldspar and quartz porphyroclasts often surrounded by quartz bands. In Group 2, microstructures such as mica fish and quartz porphyroclasts with "(Delta)" and "(Fi)" shapes are also observed. The observed recrystallization is dynamic, primarily expressed through bulging (BLG), with the occurrence of subgrain rotation (SGR). These features indicate deformation through plastic flow imposed by dislocation creep mechanisms. The crystallographic textures were analyzed in quartz. The quartz pole figures (PF) and inverse pole figures (IPF) indicate a primary prism slip system {m} <a>, with activation of positive {r} <a> and negative {z} <a> rhombohedral systems. These rhombohedral systems are mainly expressed through misorientation angles, which peak around 60º and indicate Dauphine twinning in quartz. SGR was also observed through misorientation angles below 15º. Based on these analyses, it is suggested that the rocks underwent two distinct deformation processes. The first occurred at temperatures in the range of 400 . 500 ºC, is related to the extensional shear zones of the region, and is evidenced by the prismatic slip system {m} <a>, anastomosed/milonitic foliation and microstructures, such as S-C structures, mica fish and porphyroclasts with recrystallization tails. The second process, characterized by lower temperatures, is related to the reactivation of shear zones and is evidenced by mica chloritization, bulging recrystallization and the identification of rhombohedral and basal slip systems in quartz