Deformation mechanisms and characteristics of the meta-BIFs from an early Proterozoic shear system of the Southern Granulite Terrane (SGT), India

We studied three samples of Banded Iron Formation (BIF) rocks from Southern Granulite Terrane (SGT) of India, which was metamorphosed at high-pressure granulite facies and exhumed from 40 to 50 km depth. We examined the microstructure, texture, and deformation mechanisms of the primary minerals (quartz, magnetite, and hematite). Microstructures, variations in size and shape of quartz grains suggest that the three samples deformed to different intensities and temperatures. The EBSD derived CPOs for quartz and magnetite grains are strong, implying dislocation creep as the dominant mechanism. The presence of distinct subgrains in magnetite, rare in natural samples, attests to the same. Furthermore, twinning on {111}, as evident from the misorientation angle and axis distributions, indicates ductile deformation of magnetite below 400 degrees C. Subgrain boundary trace analysis in selected magnetite grains reveals that multiple-slip planes {111}, {110}, and {100} were operational. Topotaxial replacement, via redox reaction, of magnetite grains along their octahedral planes by hematite is evident. Well-developed CPOs but low (< 1 degrees) GOS values of both magnetite and hematite grains suggest that recovery processes in these were rapid. Post-replacement deformation was weak and did not affect the hematite CPOs that were inherited from the relatively more deformed host magnetite grains.