Deformation partitioning during folding of banded iron formation

Abstract

Domainal electron and optical microscopy and c-axis fabric analysis are utilized to document microstructures and the associated deformation partitioning between crystal plastic, brittle and fluid-assisted deformation mechanisms during folding of Proterozoic banded iron formation (a hematite–quartz–calcite multilayer) from the Quadrilátero Ferrı́fero granite–greenstone terrain (southeastern Brazil). The operation of different mechanisms was partially determined by the contrasting rheologic response of these three minerals at greenschist facies metamorphic conditions. Fracturing was the main deformation process in hematite, while solution-transfer accounted for part of the deformation in both calcite and quartz. Notably, there is strong interaction among the different deformation mechanisms, which have influenced each other in various ways. Fracturing of hematite in hinge zones of folds caused opening of gaps, which were sealed by direct precipitation of silica from the fluid phase. As a consequence, a chemical potential gradient between crystals and fluid phase was produced, and quartz was dissolved to restore the thermodynamical crystal–fluid equilibrium. Thus, brittle deformation of hematite partially controlled solution-precipitation creep in quartz. Heterogeneous access of fluid into the deforming medium also affected the deformation processes. Inhomogeneous deformation in the quartz–calcite aggregates generated intergranular porosity and increased fluid access, with solution-transfer becoming dominant in these domains. In contrast, the relatively more homogeneous deformation in the pure quartz aggregates served to maintain well-adjusted grain boundaries and reduced fluid access into the intergranular space, such that these domains deformed uniformly by crystal plastic processes, at relatively dry conditions.