Integrating EBSD and magnetic susceptibility data to decipher intensity of SPO and strain in deformed quartzite

Measurement of intensity of shape preferred orientation (SPO) is an important objective of many structural geological investigations. This not only helps in fabric quantification, but also enables in better correlation of the rock fabric with strain and strain-intensity variations that may have implications in regional tectonics. There are several methods available to quantify SPO – e.g. Launeau and Robin (1996), Piazolo and Passchier (2002), Gerik and Kruhl (2009). These methods involve recognition of grain shapes and boundaries using automated digital image analysis, or they require images where grain boundaries are already mapped. However, in rocks such as deformed quartzite, these methods are difficult to implement because quartz grains are often dynamically recrystallized. The accurate mapping of grain boundaries of dynamically recrystallized quartz using images taken under a petrographic microscope is almost impossible – manually, as well as using automated digital image analysis programmes. To trace quartz grain boundaries with reasonable accuracy, it is essential to have crystallographic information, because it is well-accepted that crystallographic misorientation of ~10° between adjacent regions demarcates a grain boundary (e.g., Prior et al., 1999). Moreover, in deformed rocks, the shape of minerals (and hence the SPO) is influenced by strain. Hence, for a meaningful interpretation of grain shapes, it is important to evaluate SPO in one of the principal planes of the strain ellipsoid – generally, this is the XY (foliation) plane or the XZ plane (parallel to stretching lineation and perpendicular to foliation) of the strain ellipsoid. In this note, the authors demonstrate the robustness of using grain shape data obtained on the basis of crystallographic information from EBSD analysis to quantify SPO and strain in XZ section of deformed quartzite. EBSD data were generated using NordlysMax2 EBSD detector fitted in Carl Zeiss Auriga Compact FEG-SEM housed in the Central Research Facility (CRF), Indian Institute of Technology (IIT), Kharagpur (India). Automated data acquisition and indexing were done using AZtec® software, while post-acquisition processing was done using HKL Channel5 software.