Deformation mechanism of magnetite and hematite in granulite facies
Magnetite and hematite are common accessory minerals in a wide range of crustal rocks and are an important component of some iron ore bodies. Most of the cases, these iron bodies form at lower depth and are free from any ductile deformation. Relative abundance of the magnetic minerals in naturally occurring crustal rock and generation of ore bodies at shallow depth could not attract geoscientists to reveal the deformation mechanism of these particular minerals. Therefore, mechanism of ductile deformation of these minerals is still unrevealed in geology. Here, at Earth Sciences Department, IITK, I am working on microstructural study of rock samples from granulite facies which comprise abundantly of iron oxides (Magnetite and hematite) collected from Southern Granulite Terrain. Further these observations will be utilized to decipher deformation mechanism of earlier foormed magnetite. Ultimate goal of this study is to characterize the P-T conditions and assign an appropriate evolutionary path to the shear zone. Using Scanning Electron Microscope and Electron Backscatter Diffraction techniques has evolved as a major branch of structural geology in tracing deformational histories of rocks. The primary aim of doing such analysis is to extract information about deformation mechanism experienced by the rock and mineral phase associated with the rock.
Magnetite and hematite are present as elongated thin microband along with quartz grains that are showing ribbon structure and undulose extinction. Apart from this, microstructures associated with simple shear deformation is absent in these particular thin sections, which is an indirect evidence of pure-shear deformation of these rocks. However, the mineral assemblage of this rock resembles “granulite facies metamorphism” which in a way contradicts with the microstructural observation of these rocks, e.g. undulose extinction in quartz, indicating low P-T deformation condition. Therefore, thermal recovery and static recrystallization can be a possible explanation for the anonymous microstructural appearance of this high grade metamorphic rock. EBSD technique will aid in studying deformation features that will help in evaluating the rheological behavior of hematite and magnetite. This study will also give detailed information about intra-crystalline deformation of the constituent minerals e.g. activation of different slip system of constituent minerals .However, my study is only concentrating on the intra-crystalline deformation of iron oxides mineral present during ductile deformation of this rock.
Keywords: hematite, magnetite, deformation, mechanism, microstructure, EBSD