Deformation partitioning during folding of banded iron formation

Domainal electron and optical microscopy and c-axis fabric analysis are uti lized to document microstructures and the associated deformation partitioni ng between crystal plastic, brittle and fluid-assisted deformation mechanis ms during folding of Proterozoic banded iron formation (a hematite-quartz-c alcite multilayer) from the Quadrilatero Ferrifero granite-greenstone terra in (southeastern Brazil). The operation of different mechanisms was partial ly determined by the contrasting theologic response of these three minerals at greenschist facies metamorphic conditions. Fracturing was the main defo rmation process in hematite, while solution-transfer accounted for part of the deformation in both calcite and quartz. Notably, there is strong intera ction among the different deformation mechanisms, which have influenced eac h other in various ways. Fracturing of hematite in hinge zones of folds cau sed opening of gaps, which were sealed by direct precipitation of silica fr om 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 o f hematite partially controlled solution-precipitation creep in quartz. Het erogeneous access of fluid into the deforming medium also affected the defo rmation processes. Inhomogeneous deformation in the quartz-calcite aggregat es generated intergranular porosity and increased fluid access, with soluti on-transfer becoming dominant in these domains. In contrast, the relatively more homogeneous deformation in the pure quartz aggregates served to maint ain well-adjusted grain boundaries and reduced fluid access into the interg ranular space, such that these domains deformed uniformly by crystal plasti c processes, at relatively dry conditions. (C) 2001 Elsevier Science Ltd. A ll rights reserved.