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Journal of Structural Geology 82 (2016) 48e59

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Fabric analysis of quartzites with negative magnetic susceptibility e

Does AMS provide information of SPO or CPO of quartz?

A.R. Renjith a, *, Manish A. Mamtani a, Janos L. Urai b

a Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, West Bengal, Indiab Structural Geology, Tectonics and Geomechanics, RWTH Aachen University, Lochnerstrasse 4-20, D-52056 Aachen, Germany

a r t i c l e i n f o

Article history:Received 18 September 2015Received in revised form4 November 2015Accepted 4 November 2015Available online 14 November 2015

Keywords:Anisotropy of magnetic susceptibilityShape preferred orientationCrystallographic preferred orientationQuartzite

* Corresponding author.E-mail addresses: renjithar.gg@iitkgp.ac.in (A.R.

ernet.in (M.A. Mamtani), j.urai@ged.rwth-aachen.de (

http://dx.doi.org/10.1016/j.jsg.2015.11.0050191-8141/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

We ask the question whether petrofabric data from anisotropy of magnetic susceptibility (AMS) analysisof deformed quartzites gives information about shape preferred orientation (SPO) or crystallographicpreferred orientation (CPO) of quartz. Since quartz is diamagnetic and has a negative magnetic sus-ceptibility, 11 samples of nearly pure quartzites with a negative magnetic susceptibility were chosen forthis study. After performing AMS analysis, electron backscatter diffraction (EBSD) analysis was done inthin sections prepared parallel to the K1K3 plane of the AMS ellipsoid. Results show that in all thesamples quartz SPO is sub-parallel to the orientation of the magnetic foliation. However, in most samplesno clear correspondance is observed between quartz CPO and K1 (magnetic lineation) direction. This iscontrary to the parallelism observed between K1 direction and orientation of quartz c-axis in the case ofundeformed single quartz crystal. Pole figures of quartz indicate that quartz c-axis tends to be parallel toK1 direction only in the case where intracrystalline deformation of quartz is accommodated by prism <c>slip. It is therefore established that AMS investigation of quartz from deformed rocks gives information ofSPO. Thus, it is concluded that petrofabric information of quartzite obtained from AMS is a manifestationof its shape anisotropy and not crystallographic preferred orientation.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

The measurement of low-field anisotropy of magnetic suscep-tibility (AMS) is a useful tool for petrofabric analysis of deformedrocks (Tarling and Hrouda, 1993). This method involves inducingmagnetism in an oriented sample in different directions andmeasurement of the induced magnetization in each direction. AMSof an individual mineral depends on its crystallography (crystallineanisotropy) and/or shape (shape anisotropy). In case of minerals ofthe cubic crystal system (e.g., magnetite), shape anisotropy is thedominant controlling factor because these minerals lack a crystal-lographic anisotropy. In minerals of all the other crystal structures,induction of magnetization is believed to take place along the longcrystallographic axis (the “easy” axis) (Tarling and Hrouda, 1993;Borradaile and Jackson, 2010). In addition, if the crystalline “easy”axis and long (shape) axis share the same orientation, then themagnetic anisotropy of the mineral is maximized. Thus, if a rock

Renjith), mamtani@gg.iitkgp.J.L. Urai).

contains minerals with shape and/or crystallographic preferredorientation (CPO), it possesses a stronger magnetic susceptibility inthe direction of the preferred orientation as compared to otherdirections. This anisotropy is visualized as AMS ellipsoid, which is asecond rank tensor. In most rocks the orientations of principal axesof AMS ellipsoid (K1 > K2 > K3) correlate well with orientations ofstructural features (finite strain axes), flow axes or some axis ofkinematic importance (Borradaile and Jackson, 2010; Mamtani,2014). This has made AMS a useful tool in Structural Geologyresearch because in rocks where deformation related fabric ele-ments are not visible to the human-eye, they can be deduced fromorientations of the AMS ellipsoid.

Although studies integrating AMS and CPO data are relativelyscarce, it has been theoretically understood that orientations ofprincipal axes of AMS ellipsoid and crystal axes are parallel inorthorhombic and tetragonal crystals; this relationship becomescomplex in monoclinic and triclinic crystal systems, more so inrocks that are commonly polycrystalline and polymineralic(Borradaile and Jackson, 2010). AMS of a rock has contribution fromall the different mineral phases present in it viz. paramagnetic,diamagnetic and ferromagnetic sensu lato (s.l). If a rock has positive

A.R. Renjith et al. / Journal of Structural Geology 82 (2016) 48e59 49

magnetic susceptibility, it implies that the AMS is dominated by theparamagnetic minerals (e.g., biotite, hornblende etc.) and/orferromagnetic (s.l) minerals (e.g., magnetite, hematite etc.) presentwithin it (Rochette et al., 1992; Tarling and Hrouda, 1993; Raposoand Berqu�o, 2008). There are several studies on deformed rockswith positive magnetic susceptibility that document (a) similaritybetween shape of AMS and strain ellipsoid (b) parallelism betweenshape preferred orientation (SPO) of elongated para/ferromagneticminerals and magnetic fabric, and (c) positive correlation betweenstrain intensity gradient and variation in degree of magneticanisotropy (e.g., Rathore, 1979; Hrouda, 1982; Borradaile andAlford, 1987; Archanjo et al., 1995; Mukherji et al., 2004; Senet al., 2005; Sen and Mamtani, 2006; Borradaile and Jackson,2010; Raposo et al., 2014; Mamtani, 2014).

Compared to AMS studies on rocks with positive susceptibility(high para/ferro-magnetic mineral content), there are only a fewinvestigations on rocks with negative magnetic susceptibility (suchas pure/nearly pure quartzite and marble). AMS in such rocks iscontrolled by diamagnetic mineral phases such as quartz (inquartzite) and calcite (in marble). Although the crystallography ofthese minerals is well established, and there exists considerableknowledge about the relation between deformation and active slipsystems in them (e.g., Hobbs et al., 1976; Nicolas and Poirier, 1976),there are only a few studies aimed at correlating their CPO withAMS data. In the case of calcite bearing rocks, an integration of AMSwith Electron Backscatter Diffraction (EBSD) data has shown a goodcorrelation between CPO and AMS (e.g., de Wall et al., 2000;Almqvist et al., 2010). However, similar studies dealing withquartz and quartzite are lacking. Moreover, of the limited studiesdone on quartzites, almost all of them dealt with samples havingpositive magnetic susceptibility (e.g. Mamtani et al., 1999; Tripathyet al., 2009; Mamtani and Sengupta, 2010; Vishnu et al., 2010;Mamtani and Vishnu, 2012). Quartz is diamagnetic, and has anegative magnetic susceptibility of �13.4� 10�6 SI units (Tarlingand Hrouda, 1993). This implies that in quartzites with positivemagnetic susceptibility, there is a dominant contribution frompara/ferro-magnetic minor mineral phases as compared to themajor diamagnetic mineral phase (quartz). To the best of ourknowledge there are no significant AMS studies of (nearly) purequartzites that have a negative magnetic susceptibility. Experi-ments done on single crystal of quartz indicate that the ratio ofmagnetic susceptibility along c-axis to basal plane is very low (1.01;Nye, 1957; cf Hrouda, 1986). This was experimentally reconfirmedby Hrouda (1986). The latter also reported that in some casesmagnetic susceptibility of single quartz crystal is higher along the c-axis than in the ab plane, while in some cases it is the reverse (alsosee Rochette et al., 1992). Borradaile and Jackson (2010) have statedthat in a single undeformed crystal of quartz, the longest axis of theAMS ellipsoid (i.e., the direction with the most negative suscepti-bility) corresponds to the c-axis. Magnetic susceptibility along c-axis is �13.7 mSI while in the basal plane it is �12.5 mSI (see Fig. 7cof Borradaile and Jackson, 2010). However, the above authors statethat because quartz undergoes basal glide under most tectono-metamorphic deformation conditions, c-axis of deformed quartzoccurs at high-angle to the foliation, thus producing inverse fabric.However, there is no detailed investigation and documentation onhow the CPO, SPO and AMS in pure/nearly-pure quartzites (withnegative magnetic susceptibility) are related to one-another. Someof the important questions that remain to be fully answered inquartzites with negative susceptibility are:

(a) Is K1 direction parallel to the c-axis of quartz (the easy axis)?(b) What is the relation between SPO and CPO of quartz?(c) How are CPO and SPO related to the principal directions of

the AMS ellipsoid?

(d) How does the dominant slip system (and hence temperatureof deformation) influence the relationship between CPO andAMS?

The present study involves an integration of AMS, SEM basedelectron backscatter diffraction (SEM-EBSD) and SPO analysis ofquartzite samples that have negative magnetic susceptibility toaddress the above listed questions.

2. Sample description and modus operandi

To fulfill the objectives of the present investigation, quartziteswith negative magnetic susceptibility from two different regionshave been studied.

2.1. Quartzites from Rengali region (India)

Quartzite samples from the Rengali Province located in easternpart of India were collected for this study. These rocks lie in thenorthern part of Eastern Ghats Mobile Belt in eastern India (inset,Fig. 1a). The samples were taken from a quartzite ridge that strikesin NWeSE direction, and extends for a distance of ~70 km situatedto the north of the Kerajang Shear Zone (KSZ). The rocks of theregion are known to have undergone three deformation events (D1to D3). According to Misra and Gupta (2014), D1 and D2 took placeduring the late Archaean time under amphibolite to granulite faciesconditions. D3 took place under greenschist facies conditions at490e470 Ma, and was accompanied with shearing and myloniti-zation along the KSZ (see Misra and Gupta, 2014 and referencestherein). Nine oriented blocks (each having dimension, ca.10 cm � 10 cm � 10 cm) were collected for AMS as well asmicrostructural investigations. Since the rocks did not possesswell-defined visible foliation (Fig. 1b), they were ideal for AMSinvestigation in order to recognize the orientation of fabricelements.

Cylindrical cores of 25.4 mm diameter and 22 mm height weredrilled from each oriented block. AMS analysis was done using theKLY-4S Kappabridge (AGICO, Czech Republic) housed in theDepartment of Geology & Geophysics, Indian Institute of Technol-ogy (IIT) Kharagpur (India). Multiple cores were analyzed from eachblock; the programme SUFAR (AGICO, Czech Republic) was used forthe measurement of AMS parameters viz. Km (mean susceptibility),Pj (degree of magnetic anisotropy), T (shape parameter), and ori-entations of three principal axes of the AMS ellipsoid (K1 > K2 > K3).Subsequently, the programme Anisoft 4.2 (AGICO, Czech Republic)was used to calculate Jelínek statistics, and determine mean valuesof AMS parameters for each block (using AMS data from individualcores of every block). The calculated values of all parameters aretabulated in Table 1. The reader is referred to Appendix A.1 for amore elaborate description of the AMS methodology and formulaeused for calculation of various parameters. As listed in Table 1, allthe investigated quartzite samples from Rengali have a negativemagnetic susceptibility ranging between �13.6 � 10�6 SI unitsto �3.06 � 10�6 SI units; this implies negligible contribution fromother mineral phases. Microstructural studies also reveal that thesamples are almost pure quartzites and contain <3% of other (mi-nor) mineral phases. In most of the samples, muscovite is the minorphase, which is known to be paramagnetic with a susceptibility of165 � 10�6 SI units (Borradaile et al., 1987). Since the susceptibil-ities of all the samples are negative, it can be assumed that thepresence of minor amounts of muscovite does not affect the AMSsignificantly. In the quartzite sample Rn258B, sillimanite is theminor phase. This sample has a mean susceptibility of�13.3� 10�6

SI units (Table 1), which is almost identical to that of pure quartzcrystal. This also implies that the presence of minor amount of

Fig. 1. (a) Generalized geological map of the Rengali region in eastern India (after Misra and Gupta, 2014) showing locations of 9 sites from where oriented quartzite blocks weretaken for anisotropy of magnetic susceptibility (AMS) analysis. One such block (sample Rn248B) is shown in (b). After AMS analysis, orientations of the three principal axes of theAMS ellipsoid viz. K1, K2 and K3 (where, K1 > K2 > K3) were marked on each block. Thin sections were prepared parallel to the three principal planes of the AMS ellipsoid formicrostructural investigations as shown in (c) (sample Rn2). See Section 2 for details.

Table 1AMS data of quartzites from Rengali (India) and Naxos (Greece). Km ¼Mean susceptibility; Pj¼ corrected degree of magnetic anisotropy; T¼ shape parameter. K1, K2 and K3 areDeclination/Inclination (D/I) of the three principal axes of the AMS ellipsoid (where K1 > K2 > K3). Note: K1 is the orientation of the magnetic lineation and K3 is pole to themagnetic foliation (K1K2) plane. It may also be noted that the Naxos samples had well developed stretching lineation and foliation that respectively define X direction, and XYplane of the strain ellipsoid. These are parallel to K1 direction, and K1K2 plane of the AMS ellipsoid, respectively (see Fig. 2).

Samplenumber

Km

(�10�6 SI units)Pj T K1

(D/I)K2

(D/I)K3

(D/I)

Rengali (India)Rn248B �13.6 1.088 �0.182 310.4/5.2 216.8/34.9 47.7/54.6Rn254 �13.5 1.073 �0.432 291.7/27.3 94.1/61.5 197.9/7.4Rn258B �13.3 1.092 0.105 1/68 267/1.6 176.4/22Rn248A �13.0 1.129 �0.092 229.1/29.8 77.3/52.4 196.6/20.6Rn252 �11.3 1.170 0.127 13.2/86.1 119.4/1.1 209.5/3.7Rn257B �9.64 1.602 0.111 354.4/58.4 146.4/28.5 243.4/12.5Rn251 �9.16 1.090 0.126 243/24.9 137.2/30.5 4.9/48.7Rn247 �6.56 1.335 0.004 76.4/36.7 310.5/38.1 192.7/30.8Rn2 �3.06 1.294 �0.171 358.5/58.8 98.4/5.9 191.9/30.5Naxos (Greece)JU1 �0.893 2.273 �0.305 183.7/28.6 80.6/22.6 318.2/52.2JU2 �5.08 1.292 0.204 186.8/6.9 88.2/51 282.3/38.1

A.R. Renjith et al. / Journal of Structural Geology 82 (2016) 48e5950

sillimanite does not have a significant effect on the AMS of thequartzite. It was stated above that previous experimental researchshowed that single undeformed crystal of quartz has low anisot-ropy of 1.01 (Hrouda, 1986). However, the deformed quartzitesstudied here (Table 1) have higher Pj value; the minimum recorded

is 1.073 (sample Rn254) and the maximum is 1.602 (sample Rn257B). Since the minor phases in the quartzites are <3%, these highPj values are a manifestation of the fabric defined by the deformedquartz grains.

After performing AMS analysis, the orientations of K1, K2 and K3

A.R. Renjith et al. / Journal of Structural Geology 82 (2016) 48e59 51

were plotted on the respective blocks, and rock slices were cutalong three principal planes of the AMS ellipsoid (K1K2, K1K3 andK2K3 plane; see Fig. 1c). The relation between AMS and strain el-lipsoids in deformed rocks has been the subject of several paststudies (e.g., Hrouda, 1982, 1993; Borradaile and Alford, 1987;Borradaile and Henry, 1997; Je�zek and Hrouda, 2002; Borradaileand Jackson, 2004, 2010; Burmeister et al., 2009; Mamtani andVishnu, 2012; Ferr�e et al., 2014 amongst others). It is well estab-lished that orientations of K1, K2 and K3 axes of AMS ellipsoidrecorded in deformed rocks correlate well with orientations of X, Yand Z axis of the strain ellipsoid, respectively. In several studiesvariation in the degree of magnetic anisotropy (Pj, which is ameasure of the eccentricity of the AMS ellipsoid), has been suc-cessfully applied to gauge strain intensity variations (e.g.,Borradaile and Alford, 1987; Mukherji et al., 2004; Sen et al., 2005;Sen and Mamtani, 2006; Majumder and Mamtani, 2009; Levi andWeinberger, 2011; Issachar et al., 2015). With special reference toquartzites, Mamtani and Vishnu (2012) established a good corre-lation between shape of AMS and strain ellipsoid by measuring 2Dstrain and SPO intensity in 3 principal sections of the AMS ellipsoid(K1K2, K1K3 and K2K3 plane). In light of the several earlier studies(mentioned above; also see Renjith and Mamtani, 2014; Mamtaniand Renjith, 2015) that highlight the usefulness of AMS in identi-fication of principal directions of the strain ellipsoid, for the pur-pose of the present investigation, oriented thin sections preparedparallel to the K1K3 plane of the AMS ellipsoid were chosen forfurther microstructural and SEM-EBSD analysis; this section isequated with the XZ section of the strain ellipsoid. Detailed pre-sentation of the results obtained is given in Section 3.

2.2. Quartzites from Naxos (Greece)

Two quartzite samples from Naxos Island (Greece) were alsoinvestigated. Fig. 2a shows the location of samples on the gener-alized geological map of Naxos (after Krabbendam et al., 2003), thegeology of which is dominated by Alpine metamorphic core com-plex. The oldest rocks are migmatitic gneisses (with Hercynianprotolith), overlain by high-grade Mesozoic metasediments andunmetamorphosed rocks (Miocene age), in that order. The meta-morphic rocks are poly-metamorphosed e M1 metamorphism (ca.45 Ma) is reported to be high-P/low-T event and this is super-imposed by M2 metamorphism (ca. 20e16 Ma), which is a high-T/low-P event. The isograds shown in Fig. 2a are related to the M2metamorphic event, and it has been established that the meta-morphic conditions vary from 400 �C/4e5 kbar in the south to700 �C/6e7 kbar in the magmatic core. It is also known that M2 wasassociated with non-coaxial deformation (top-to-the-north senseof shear) that caused north-south extension and producedmylonites (see Krabbendam et al., 2003; and references therein forfurther details of regional geology/tectonics). For the purpose of thepresent study, two mylonitized quartzites belonging to the meta-morphosed Mesozoic rocks have been investigated. One quartzitesample (marked JU1; Fig. 2a) was collected from the southern partsof Naxos below the Biotite-in isograd, and another sample (JU2)was collected from the northern part (north of Stavros Pass), fromthe same outcrop as the samples studied by Krabbendam et al.(2003). Both the samples have minor amount of graphite (whichis known to be diamagnetic; McClure, 1960) and have a stretchinglineation with gentle plunge towards the south. The foliation in thesamples strikes NEeSW with moderate dip to the southeast (seeFig. 2b,c for lower hemisphere equal area projections of fieldstructural data).

AMS investigation of the two quartzites reveals that both havenegative magnetic susceptibility with high Pj values of 2.273 and1.292, respectively, in sample JU1 and JU2. It is also noted that (a)

orientation of the magnetic foliation (K1K2 plane of the AMSellipsoid) is similar to the mesoscopic foliation present in thesamples, and (b) orientation of magnetic lineation (K1) is similar tothe stretching lineation. This similarity between orientations offield and AMS data is documented in Fig. 2b,c; thus K1K3 section ofthe AMS ellipsoid is parallel to stretching lineation and perpen-dicular to foliation. For further microstructural and SEM-EBSDinvestigation, thin sections parallel to XZ section (¼K1K3 of AMSellipsoid) were studied.

3. Results e Integrating AMS and EBSD data

Petrographic investigation revealed the presence of extensiverecrystallization of quartz in the Rengali quartzites (Fig. 3a-d).Subgrain rotation (SGR) and bulging (BLG) are recognized as thedominant recrystallization mechanisms, implying dynamicrecrystallization under low to medium temperature conditions(Stipp et al., 2002) in some samples such as Rn248B (Fig. 3a). SGRdominates sample Rn252 (Fig. 3b) along with some grain boundarymigration (GBM) recrystallization, which are indicative of mediumto high-T deformation. GBM associated with development of quartzribbons, which takes place at relatively higher temperature (500

�C-

700 �C, Stipp et al., 2002) is dominant in sample Rn2 (Fig. 3c). Thesample also shows subgrains with minor bulging grain boundaries.Sample Rn258B is sillimanite bearing and lies 17 km north of theKSZ (Fig. 1a). It is coarse grained (Fig. 3d) and the quartz grainsshow evidence of GBM and some SGR, indicating medium to high-Tdeformation. It was stated earlier (section 2.1) that rocks of thestudy area have undergone amphibolite to granulite facies meta-morphism during early deformation followed by greenschist faciesmetamorphism during the late deformation. The combination ofGBM, SGR and BLG noted in the quartzite samples discussed above(Fig. 3) is a manifestation of the above metamorphic history. In thecase of the two quartzite samples (JU1 and JU2) from Naxos(Greece), SGR and GBM are observed to be important processes ofdynamic recrystallization. As mentioned in Section 2.2, sample JU2is from a higher isograd domain than JU1. Texturally it is noted thatsample JU2, has coarser quartz grains than the latter (Fig. 3e,f).

Quartz CPO was measured in each thin section (K1K3 plane ofAMS ellipsoid) with the help of SEM based EBSD measurements.Analyses were done using Carl Zeiss Auriga Compact FEG-SEMfitted with NordlysMax2 EBSD detector (Oxford instruments, UK)housed in the Central Research Facility (CRF, IIT Kharagpur, India).EBSD data acquisition was done using AZtec software (Oxford In-struments, UK). Post-acquisition data processing was done usingHKL Channel 5 software (Oxford Instruments, UK), which involvedpreparation of inverse pole figure (IPF) maps, and pole figures(lower hemisphere equal area) of {0001} i.e., c-axis, and {112 0} i.e.,a-axis. Working conditions and other instrumentation and pro-cessing details are described in Appendix A.2. It maybe noted thatthe reference frame in all the maps and figures is K1 (horizontal)and K3 (vertical). Thus the horizontal trace on each diagram rep-resents trace of the magnetic foliation (K1K2 plane of AMS ellip-soid). Representative IPF-K2 map of an area (Fig. 4a) analyzed insample Rn252 is documented in Fig. 4b (also see Appendix A.2 andSupplementary Data-1 and 2). The main objective of the presentstudy is to understand the inter-relation between AMS, SPO andCPO of quartz in deformed quartzites (Section 1). As can be seenfrom Fig. 4, the IPF map enables clearer identification of quartzgrain shapes and grain boundaries as compared to the petrographicphotomicrograph in Fig. 4a. This is also clearly documented in Fig. 5where representative EBSD data of samples Rn248B, Rn2, Rn258B,JU1 and JU2 are presented. HKL Channel 5 software also generatesstatistical information of the orientation of long shape axis of eachmapped quartz grain. Fig. 4c is the rose diagram of orientation of

Fig. 2. (a) Simplified geological map of Naxos, Greece (after Krabbendam et al., 2003). The map shows the various rock types as well as the metamorphic isograds. For the purpose ofthe present investigation, two mylonitized quartzites were taken. It maybe noted that sample JU2 from the northern part belongs to a relatively higher temperature domain than thesouthern sample (JU1). (b) and (c) are lower hemisphere equal area projections of sample JU2 and JU1, respectively, that document similarity in the orientation of field and magneticdata.

A.R. Renjith et al. / Journal of Structural Geology 82 (2016) 48e5952

long shape axis of the quartz grains mapped in Fig. 4b, and is agraphical representation of the SPO in the analyzed frame. Fig. 4dshows lower hemisphere equal area projections (pole figures) ofquartz {0001} i.e., c-axes, and {1120} i.e., a-axes, respectively. Theseare typical Y-maximum fabrics (Schmid and Casey, 1986), thatindicate intracrystalline deformation of quartz by prism <a> slip,where the quartz c-axes fall at the centre of the pole figure (also seeUnzog and Kurz, 2000). According to Okudaira et al. (1995), thetransition from basal<a> to prism<c> slip occurs between 550 and600 �C, and Y-maximum c-axis (indicative of prism <a> slip) startdeveloping at temperature of 450 �C. Texturally, sample Rn252 isreplete with evidence of quartz having undergone dynamicrecrystallization by SGR and GBM. According to Stipp et al. (2002),dynamic recrystallization of quartz by SGR takes place between 400and 500 �C, while GBMoccurs between 500 and 700 �C. Thus, prism<a> slip in the present samples must have taken place definitelybelow 600 �C, and probably in a temperature range of 450e550 �C,and is indicative of deformation at medium-T (Unzog and Kurz,2000; Passchier and Trouw, 2005). Thus, EBSD data from eachframe provides information about SPO as well as CPO in the K1K3

reference frame, which can be used to evaluate how orientations ofAMS ellipsoid, SPO, CPO and slip systems can be/maybe related.From data of the area mapped in Fig. 4 it is obvious that in thisparticular frame, the mean orientation of the long shape axis ofquartz grains (SPO; Fig. 4c) is parallel to the trace of magneticfoliation (K1K2 plane ¼ horizontal direction). But, the quartz c-axisfabric does not coincide with the K1 direction.

To have a statistically robust database of CPO aswell as SPO fromeach thin section, several different areas (frames) of thin section

were similarly analyzed. Complete data from each thin section arepresented in the form of pole figures and rose diagrams in Fig. 6.The rose diagrams that graphically represent SPO are also plotted inthe same diagram for each sample. Since the reference frame in allthe diagrams is same (K1K3), this enables correlation betweenorientation of AMS ellipsoid, SPO and CPO for every sample. It isnoted that in all the samples, the SPO is either parallel, or sub-parallel to the trace of the K1K2 plane (horizontal). Maximumobliquity (16�) between SPO and K1K2 plane is noted in sampleRn248B. On the other hand, the orientation of quartz c-axis isvariable in the different samples. In some samples it lies close to K2direction (centre of the pole figures¼ Y direction of strain ellipsoid)implying dominance of prism <a> slip (e.g., Fig. 4; also see Fig. 6e).In sample Rn2 (Fig. 6i), the c-axis and a-axis patterns are typical ofprism <c> slip. Here, the c-axis maxima are parallel to K1 direction,and the SPO is also parallel to the K1K2 orientation. In all the othersamples from Rengali (India), where a combination of rhomb <a>and basal <a> is inferred, the c-axis lie at high angle to the K1 di-rection. In the case of Rn258B (Fig. 6c), which is sillimanite bearingquartzite, the overall CPO is indicative of a combination of prism<c>, prism <a>, and rhomb <a> slip.

In the case of the Naxos (Greece) sample JU1 (Fig. 6j), the quartzc-axis form Type-II cross girdle, implying dominance of rhomb <a>slip (Schmid and Casey, 1986; Passchier and Trouw, 2005). SampleJU2 (Fig. 6k) shows a single girdle distribution. EBSD maps of thesesamples are presented in Fig. 5d-e. CPO of both the samples fromNaxos implies simple shear (Schmid and Casey, 1986; Krabbendamet al., 2003). Krabbendam et al. (2003) had documented weakeningof CPO with decreasing grain size in the samples from Naxos. As

Fig. 3. Representative photomicrographs of quartzites from Rengali, India (aed) and Naxos, Greece (eef) documenting textures recorded in thin sections prepared parallel to theK1K3 section of the AMS ellipsoid. Dynamic recrystallization of quartz by bulging (BLG) and subgrain rotation (SGR) are dominant in Fig. 3a (sample Rn248B). SGR is prominent inFig. 3b along with grain boundary migration (GBM) (sample Rn252). SGR and GBM along with development of quartz ribbons are also prominent in Fig. 3c (sample Rn2). SampleRn258B (d) is sillimanite bearing quartzite with very coarse quartz grains that have undergone GBM recrystallization. Fig. 3e (sample JU1) and Fig. 3f (sample JU2) are graphitebearing quartzites, respectively from the lower and higher isograd domains (see Fig. 2a). Note that quartz grains are coarser in (f) as compared to (e).

A.R. Renjith et al. / Journal of Structural Geology 82 (2016) 48e59 53

shown in Fig. 3f, there is a significant grain size variation in sampleJU2. In order to test the relationship between grain size, CPO andSPO in this particular sample, the authors classified the data intothree size classes (>200 mm, 100e200 mm, and 50e100 mm). Theresults are presented as Fig. 7. It is noted that the intensity of CPOdecreases with the decrease in grain size. In an earlier investigation,Krabbendam et al. (2003) inferred that weakening of CPO withdecreasing grain size in the Naxos quartzites is on account ofdiffusion creep and grain boundary sliding. CPO obtained fromEBSD analysis in the present study (Fig. 7) also supports thisinference. However, the orientation of long shape axis of the quartzgrains is parallel to the trace of the magnetic foliation (horizontaldirection in Fig. 7) irrespective of the grain size.

4. Discussion

The results presented above (Section 3) provide a comparison

between AMS, SPO, CPO and active slip systems, thus enabling us toaddress the four objectives that were listed in section-1.

4.1. Orientation of quartz c-axis vis-�a-vis AMS ellipsoid and SPO indeformed quartzites

It is known from experiments that undeformed single crystal ofquartz has a very low anisotropy (1.01; Hrouda, 1986), and themaximum induction (maximum negative susceptibility) directionis expected to be parallel to the quartz c-axis (Borradaile andJackson, 2010). However, as mentioned in section-1, most of thequartzites in nature are known to have higher degree of magneticanisotropy (Pj) (Mamtani et al., 1999; Mamtani and Sengupta, 2010;Vishnu et al., 2010). Even the quartzites investigated here have ahigh Pj value despite the fact that all of them have a negativemagnetic susceptibility, implying that their AMS is dominantlycontrolled by the diamagnetic quartz within them. Of the samples

Fig. 4. Graphical representation of EBSD data from one of the quartzite samples analyzed in this investigation. (a) Photomicrograph of one of the areas (frames) of sample Rn252showing dynamically recrystallized quartz grains. (b) Inverse pole figure (IPF-K2) map of the area enclosed in the white box in (a) obtained by processing EBSD data using HKLChannel 5 software (Oxford Instruments, UK). This software was also used to calculate orientation of long shape axis of the quartz grains, using which a rose diagram was preparedthat is presented in (c). (d) and (e) are, respectively, the pole figures (lower hemisphere equal area; one point per grain) of {0001} i.e., c-axis, and {1120} i.e., a-axis of the quartzgrains in the IPF-K2 map presented in (b). n ¼ number of quartz grains. Contour lines are multiples of uniform density (mud). Note that the thin section is parallel to K1K3 plane ofthe AMS ellipsoid, and the reference frame in all the diagrams is K1 (horizontal) and K3 vertical. Dashed horizontal line in (c) marks the trace of the magnetic foliation (K1K2) plane.

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investigated here, sample Rn254 from Rengali (India) has theminimumPj of 1.073, while sample JU1 fromNaxos (Greece) has thehighest Pj of 2.273 (Table 1). The sample Rn248B has minimum Kmof�13.6� 10�6 SI, which is the value of a pure single quartz crystal.Even this sample has Pj of 1.088. In addition, as noted fromFigs. 4e6, the quartz c-axis maxima are not parallel to the orien-tation of K1 in most cases. This is unlike the result expected for asingle undeformed crystal of quartz, in which the maximum in-duction takes place parallel to the c-axis. In comparison to the c-axis orientation, it is noted that in all the cases the SPO is parallel/sub-parallel to the trace of the K1K2 (magnetic foliation) plane ofthe AMS ellipsoid. The maximum deviation between the K1K2orientation (horizontal) and the SPO (in rose diagrams) is 16� de-grees for sample Rn248B. In samples Rn252, Rn251, Rn247 and Rn2(Fig. 6e, g, h, and i respectively), SPO is parallel to K1K2 orientation.Even in the Naxos samples, which are mylonites having a well-developed stretching lineation and foliation, the SPO is parallel tothe K1K2 orientation. As mentioned in section-3 (above), thisparallelism between SPO and magnetic foliation holds true irre-spective of the grain size of quartz (Fig. 7). All these results implythat AMS analysis in deformed quartzites provides informationabout SPO, and the magnetic foliation is a good approximation ofthe visible foliation. But the quartz c-axis orientation in the samplesis generally not parallel to the K1 orientation, which defines thelong axis of the AMS ellipsoid. The role of slip systems in quartz inthis regard is discussed in the following section.

4.2. Control of slip-systems in deformed quartz on the relationshipbetween CPO and AMS ellipsoid

Data presented in Fig. 6 help in evaluating the relationship be-tween slip-systems in quartz, CPO, SPO and AMS. Since AMS pro-vides information of SPO (Section 4.1 above), the present studyindirectly also helps understand the relation between slip-systemsand AMS. It is well-established that different slip-systems domi-nate at different temperatures in quartz to accommodate intra-crystalline deformation. Basal <a> slip and rhomb <a> slipdominate at low-T, prism <a> at medium-T, and prism <c> slipdominates at high-T (Mainprice et al., 1986; Schmid and Casey,1986; Passchier and Trouw, 2005). A comparison of quartz c- anda-axes presented in Fig. 6, with the CPO patterns obtained by aboveresearchers indicates that most of the samples from Rengali haveundergone intracrystalline deformation by a combination of slipsystems - generally rhomb <a> and prism <a> slip. Only sampleRn2 shows exclusively prism <c> slip (Fig. 6i). The CPO patterns ofNaxos samples also indicate intracrystalline deformation by simpleshear, where rhomb <a> slip dominates, particularly in the coarsergrained parts. And as shown in Fig. 7, the CPO weakens withdecreasing grain size, which indicates increasing diffusion creepand grain boundary sliding as was suggested in the earlier study byKrabbendam et al. (2003). The above data indicate that exceptsample Rn2, all the investigated samples preserve evidence ofmedium to low-T deformation (below 600 �C). And the SPO in all of

Fig. 5. Representative EBSD data of samples Rn248B, Rn2, Rn258B, JU1 and JU2 are presented in (a), (b), (c), (d) and (e) respectively. In each row e (i) is the photomicrograph. (ii) isthe inverse pole figure map (IPF-K2) of the area marked in box in (i). (iii) and (iv) are the c- and a-axes (lower hemisphere equal area projections) pole figures, respectively.Contouring of pole figures is based on multiples of uniform density (mud). The maximum density concentration values are 8.90, 8.24, 22.26, 3.51 and 8.68 in (a), (b), (c), (d) and (e),respectively.

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Fig. 6. CPO and SPO data of quartz from quartzites of Rengali, India (aei) and Naxos, Greece (jek). CPO for every sample is presented as pole figures (lower hemisphere equal area)of {0001} i.e., c-axis (first column), and {112 0} i.e., a-axis (second column). Rose diagram (third column) of orientation of long shape axis represents SPO defined by quartz, whichcan be compared with the horizontal (dashed) line; the horizontal line in each diagram is the trace of the magnetic foliation (K1K2 plane), and all diagrams are presented in K1K3

reference frame, where K1, K2 and K3 are the three principal axes of AMS ellipsoid (K1 > K2 > K3). Contouring of pole figures is based on multiples of uniform density (mud). Themaximum density concentration values are (a) 6.22; (b) 7.68; (c) 13.69; (d) 9.35; (e) 27.08; (f) 8.51; (g) 13.69; (h) 6.41; (i) 8.24; (j) 3.51; (k) 5.46. “n” against each pole figurerepresents number of quartz grains. It maybe noted that multiple frames were analyzed to obtain the above data from each sample (except Rn254, Rn2, and JU1, all of which werefine grained, and gave considerable statistical data from a single frame).

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them is parallel/sub-parallel to the K1K2 plane, while the quartz c-axis is not parallel to K1 direction of the AMS ellipsoid.

Sample Rn2 is the only sample that shows intracrystallinedeformation of quartz by prism <c> slip. From previous studies it isknown that quartz undergoes prism <c> slip at high temperature,when quartz c-axes tend to align parallel to the X-direction of strainellipsoid (marked by the stretching lineation; Mainprice et al.,1986; Blumenfeld et al., 1986). As discussed in section-3, the tran-sition from basal <a> to prism <c> slip in quartz occurs at

550e600 �C (Okudaira et al., 1995). It was documented in Figs. 3cand 5b, that quartz grains in sample Rn2 underwent GBM recrys-tallization and also developed quartz ribbons. According to Stippet al. (2002) GBM in quartz is indicative of dynamic recrystalliza-tion at high-T of 500e700 �C. Moreover, prism <c> slip has alsobeen reported from quartz ribbons formed at high-T in earlierstudies (e.g., Festa, 2009). Thus, we infer that Rn2 underwent dy-namic recrystallization by prism <c> slip and developed quartz c-axis maxima proximal to X-direction (¼ K1 direction in AMS

Fig. 7. CPO and SPO data from quartzite sample JU2 (Naxos, Greece) categorized into three classes based on grain size viz. >200 mm (a) 100e200 mm (b) and 50e100 mm (c). In eachrow, c-axis, a-axis and rose diagram of long axis of quartz grains (SPO) are presented in (i), (ii) and (iii), respectively. The minimum and maximum values of density contours(multiples of uniform density ; mud) are shown for each case. It is noted that the maximum mud value decreases with decreasing grain size. This implies weakening of CPO withdecrease in grain size of quartz. However, the orientation of long axis of quartz grains (SPO) that is represented in the rose diagrams is parallel to the trace of the magnetic foliation(K1K2 plane; horizontal direction shown by dashed line) in each case.

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ellipsoid reference frame). Further, the SPO of this sample (Rn2) isalso parallel to the K1K2 plane.

Based on the above discussion, it can be inferred that indeformed quartzites, AMS always gives information of SPO, irre-spective of the temperature of deformation. If deformation takesplace at relatively low temperatures, and intracrystalline defor-mation of quartz is dominantly due to basal <a>, rhomb <a> orprism <a> slip, then the orientation of quartz c-axis does notcoincide with K1 direction of the AMS ellipsoid. Quartz c-axis andK1 coincide only in the case of high-T deformation, when prism<c>slip dominates in quartz.

5. Conclusions

In the past it has been accepted that when an undeformedquartz crystal is placed in an external magnetic field, then induc-tion of magnetization occurs along the easy axis of the crystal,which in the case of quartz is its c-axis. Thus, in the case of anundeformed quartz crystal, its AMS, although weak, is presumed tobe controlled by its crystallography. However, there had been nosystematic study aimed at evaluating the relationship betweenorientation of the AMS ellipsoid of deformed quartz in quartziteand its crystallography (CPO), as well as SPO. Since quartz is

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diamagnetic and has a negative magnetic susceptibility(ca. �13.4 � 10�6 SI units), deformed quartzites having a negativemagnetic susceptibility were chosen in this study from Rengali(India), and Naxos (Greece). Integration of results from AMS anal-ysis, with CPO and SPO data (derived from SEM-EBSD analysis)reveals that in all the samples, the SPO defined by long axis ofquartz grain shapes is sub-parallel to the orientation of the mag-netic foliation. In most cases, the orientation of quartz c-axis (easyaxis of quartz) in deformed quartzites is not parallel to the longestaxis of the AMS ellipsoid (K1 direction). The quartz c-axis tends tobe sub-parallel to K1 only if quartz grains accommodated intra-crystalline deformation by prism <c> slip. Therefore, based on thepresent study it is concluded that AMS analysis of quartzites yieldsinformation about SPO of quartz, and not its CPO. Thus, petrofabricdata obtained from a quartzite using AMS analysis is a manifesta-tion of shape anisotropy of quartz grains in it, rather than crystal-line anisotropy.

Acknowledgments

This paper is a part of doctoral research being done by ARRfunded by an Institute Research Fellowship of Indian Institute ofTechnology (IIT) Kharagpur, India. Niloy Bhowmik is thanked fortechnical support in carrying out EBSD analysis at the CentralResearch Facility (CRF, IIT Kharagpur, India). Discussions with SaibalGupta and Surajit Misra are gratefully acknowledged. MAM thanksAlexander Minor for giving suggestions on processing of EBSD dataduring the 20th International conference on Deformation Mecha-nisms, Rheology & Tectonics (DRT) in Aachen (Germany). Com-ments and suggestions by two anonymous reviewers, and editorialhandling by Ian Alsop are gratefully acknowledged.

Appendix A. Analytical Techniques

A.1: Measurement of Anisotropy of Magnetic Susceptibility (AMS)

AMS was measured using the KLY-4S Kappabridge (AGICO,Czech Republic) housed in the Department of Geology &Geophysics, Indian Institute of Technology (IIT), Kharagpur (India).This is a fully automatic inductivity bridge with a sensitivity of2 � 10�8 SI units, and all the measurements were made in anexternal magnetic field of 300 A/m. At least 5 cores (each having25.4 mm diameter and 22 mm height) were investigated fromevery oriented sample. All measurements weremade in the spinnermode using the program SUFAR (AGICO, Czech Republic). Dataobtained include orientations and magnitudes of the three prin-cipal axis of the magnetic susceptibility ellipsoid viz. K1, K2 and K3,where K1>K2>K3. The magnitudes are then used to determine thefollowing parameters (after Tarling and Hrouda, 1993):

(a) Mean Susceptibility, Km ¼ (K1 þ K2 þ K3)/3(b) Strength of magnetic foliation, F ¼ (K2�K3)/Km

(c) Strength of magnetic lineation, L ¼ (K1�K2)/Km

(d) Degree of magnetic anisotropy,

Pj ¼ expffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffif2½ðh1 � hmÞ2 þ ðh2 � hmÞ2 þ ðh3 � hmÞ2�g

qHere,

h1 ¼ ln K1, h2 ¼ ln K2, h3 ¼ ln K3 and hm ¼ (h1.h2.h3)1/3, and Pj isa measure of the eccentricity of the AMS ellipsoid.

(e) Shape parameter, T ¼ (2h2�h1�h3)/(h1�h3); this definesthe shape of the AMS ellipsoid and its value variesfrom �1 to þ1; negative and positive T values indicateprolate and oblate shapes, respectively.

AMS data from multiple cores of each sample were processedusing the program Anisoft (version 4.2; AGICO, Czech Republic), in

order to calculate mean values of the various AMS parameters(Jelínek statistics, Jelínek, 1981) for every sample. Data presented inTable 1 were obtained using the above procedure.

A.2: SEM-EBSD Measurements

In this study, EBSD patterns were collected at 25 kV acceleratingvoltage, 1.49 � 10�6 mbar system vacuum, and ~14 mm workingdistance using Carl Zeiss Auriga Compact FEG-SEM fitted withNordlysMax2 EBSD detector (Oxford instruments, UK) housed inthe Central Research Facility (CRF, IIT Kharagpur, India). Dataacquisition and indexing of EBSD patterns were carried out auto-matically using AZtec software (Oxford instruments, UK). Acquireddata were processed using HKL CHANNEL 5 software (Oxford In-struments, UK). On an average 90% of indexing was achieved. For aparticular frame, initially IPF-K2 map and pole figures (single pointper grain) of {0001} i.e., c-axis, and {1120} i.e., a-axis were prepared(using HKL Channel 5) based on the obtained raw (unfiltered) data.Statistical data viz. grain size, and orientation of long shape axis ofeach grain with reference to horizontal direction (trace of K1K2plane of the AMS ellipsoid) were also determined using HKLChannel 5. Rose diagram of orientation of long shape axis of quartzwas prepared, which is a graphical representation of the SPO.Supplementary data-1 shows all this information for one of theframes from sample Rn252. The map prepared from raw data wassubsequently filtered. The filtration process involved wild spikeremoval followed by medium level zero solutions filtering(replacement of non-indexed solutions by the most commonneighbor orientation). IPF-K2 map, and pole figures, as well as rosediagram of orientation of long shape axis of quartz grains based onfiltered data were then plotted. Fig. 4 shows IPF-K2 map and polefigures and rose diagram of the same frame for which unfiltereddata were presented as Supplementary data-1. A comparison of thelatter with Fig. 4 reveals that processing does not lead to any sig-nificant variation in CPO (pole figures), as well as the SPO (rosediagrams). In addition, we also performed misorientation analysisalong profiles (traverses) within quartz grains, in unfiltered as wellas filtered images (Supplementary data-2). No significant differ-ences were noted in the misorientation profiles obtained fromunfiltered and filtered images. Hence, the filtered data are consid-ered to not have generated any artifacts that could affect the overallCPO and SPO information necessary for this study.

Appendix B. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jsg.2015.11.005.

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