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

Introduction

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.

Samples and Methodology

Data from three quartzite samples are discussed here. The samples were taken from the Rengali region of eastern India (Fig. 1A). The samples lie in the vicinity of a shear zone (Kerajang Shear Zone), and are dynamically recrystallized. For a detailed description of regional geology and tectonics, the reader is referred to Misra and Gupta (2014). Fig. 1B highlights that the quartzites lack a clearly visible foliation, due to which the X, Y and Z direction of the strain ellipsoid cannot be identified from field structures. To establish the tectonic reference frame, anisotropy of magnetic susceptibility (AMS) study of cylindrical cores extracted from oriented quartzite samples was performed using the KLY-4S Kappabridge (AGICO, Czech Republic) housed in the Department of Geology & Geophysics, IIT Kharagpur (India). AMS is a well-accepted method for petrofabric analysis, and it helps establish the orientation of the three principal axes of the AMS ellipsoid (K₁ > K₂ > K₃), which are equated with the three principal axes of the strain ellipsoid (X > Y > Z). The K₁K₂ plane helps identify the XY (foliation) plane, while the K₁K₃ is equivalent to XZ section of the strain ellipsoid (e.g. Mamtani and Vishnu, 2012; Mamtani et al., 2011; 2013; Mamtani 2014; Renjith and Mamtani, 2014). In this study, EBSD analysis have been done on thin section prepared parallel to the K₁K₃ plane. As mentioned in Fig. 2 (flowchart), the orientation of long shape axis of individual quartz grains obtained from EBSD analysis were used to calculate the intensity of SPO, while the grain boundary map was used to determine strain.

Fig. 1A. Map of India showing the location of Rengali (study area).

Fig. 1B. Representative quartzite sample from Rengali. It maybe noted that in this study EBSD analysis of quartz was done in the K1 K3 section of the AMS ellipsoid.

Fig. 2. Flow chart highlighting the methodology adopted in this study

Results and Discussion

Fig. 3A shows a representative photomicrograph of one of the quartzite thin sections studied. Inverse pole figure (IPF) map of the same obtained from EBSD data is presented in Fig. 3B. Post-acquisition processing of EBSD data with HKL Channel5 software helped in preparing grain boundary map (Fig. 3C), and in automated tabulation of the grain size and shape data. For the present study, orientation of long shape axis of individual quartz grains was used to calculate the concentration parameter (k; Piazolo and Passchier, 2002), which quantifies the intensity of SPO. Moreover, the grain boundary map was saved as a TIF image, which was processed in MATLAB based programme AMOCADO (Gerik and Kruhl, 2009) in order to calculate the anisotropy of the shape fabric defined by quartz grains, which is also a measure of strain.

Table 1. Quartz fabric anisotropy obtained from three quartzite samples investigated in the present study.

Table 1 shows that the samples with a strong SPO intensity (k), also have a higher E (strain). In addition, it is also noted that shape anisotropy and strain have a one-to-one correlation with the degree of magnetic anisotropy (P_j), which is a measure of the eccentricity of the AMS ellipsoid. All these initial results on 3 quartzite samples indicate the robustness of EBSD data in performing SPO and strain analyses.

Fig. 3: (A) Photomicrograph of a quartzite showing dynamically recrystallized quartz grains.

(B) and (C) are IPF and grain boundary maps, respectively, of the area shown in (A) that were generated by postacquisition processing of EBSD data using HKL Channel5 software.

(D) Analysis of anisotropy of the grain boundary pattern in (C) using the programme AMOCADO (Gerik and Kruhl, 2009). Here the aspect ratio of the obtained ellipse is 1.464, which can be equated to 2D strain. Note that the reference frame is K1 K3 plane of the AMS ellipsoid, which is equivalent to XZ of the strain ellipsoid.

It is important to note that it would have been impossible to have similar data from routine petrographic images (photomicrographs), digital image analysis, or by manual digitization of grain boundaries. This would not only be time consuming, but also inaccurate because the dynamically recrystallized nature of quartz does not allow clear distinction of grain boundaries under a petrographic microscope. Hence, the authors believe that using grain boundary and shape data from EBSD analysis provides all the basic information for detailed SPO and strain analyses in deformed quartzites. Similar studies on more samples will further help in establishing EBSD based statistical information in quantification of microstructures.

Acknowledgments

Data presented in this note are part of an on-going doctoral research being carried out by Renjith, A.R. at the Indian Institute of Technology (IIT) Kharagpur (India). Financial assistance by the institute is gratefully acknowledged. Niloy Bhowmik is thanked for assisting in carrying out EBSD analysis at CRF (IIT Kharagpur). Last, but not the least, the authors are grateful to Oxford Instruments for supporting EBSD research activities of the Fabric Analysis Lab (FAL), Department of Geology & Geophysics, IIT Kharagpur.

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