Deformation partitioning during folding and transposition of quartz layers.

• Autor(a) principal: Lagoeiro, Leonardo Evangelista
• Data de Publicação: 2003
• Outros Autores: Hippertt, João, Lana, Cristiano de Carvalho
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Institucional da UFOP
• Texto Completo: http://www.repositorio.ufop.br/handle/123456789/3953; https://doi.org/10.1016/S0040-1951(02)00585-1
• topic: Deformation partitioning; Microstructures; Dissolution-precipitation creep; Crystal-plasticity; Foliation transposition

Banded iron formation (BIF) from the Quadrilátero Ferrífero (southeastern Brazil) shows a compositional layering with alternating iron-rich and quartz-rich layers. This layering was intensively folded and transposed at a centimeter/millimeter scale through a component of bedding-parallel shear related to flexural slip at middle to high greenschist facies conditions (400–450 jC). The microstructure and c-axis fabrics of normal limbs, inverted limb and hinge zones of a selected isoclinal fold were analyzed combining optical and scanning electron microscopy (SEM) and digital image analysis. In the normal limbs, recrystallized quartz grains show undulose extinction, relatively dry grain boundaries, c-axes at high angle to foliation and a pervasive grain shape fabric (GSF) indicating operation of crystal-plastic processes. In the inverted limb, quartz grains show more serrated and porous (‘‘wet’’) grain boundaries; the GSF is similar to that of the normal limb, but c-axes are oriented at 90j to those of the normal limb. We interpreted these characteristics as reflecting operation of solution-precipitation deformation in inverted limbs, as a consequence of grains having been rotated to an orientation that was hard to basal hai glide, but easy to dissolution-precipitation creep. This deformation partitioning between crystal-plasticity and solution-transfer during folding/ transposition of quartz may explain the common occurrence of layered quartz rocks, where individual layers show alternating caxis fabrics with opposite asymmetries but a consistent GSF orientation. Such characteristics may reflect an earlier event of pervasive folding/transposition of a preexisting layering.