1 Introduction

The EGMB of India consists of outcrops of multiply deformed, high-grade metamorphic rocks belonging to granulite facies of Grenvillian age (1.1–0.95 Ga) (Bhattacharya et al. 1998; Krause et al. 2001; Dobmeier and Raith 2003). The granulites are intruded by several anorthosite massifs (Chatterjee 1960; De 1969), Balugaon and Rambha anorthosite massifs being most characteristic (Ray 1952; Perraju 1960, 1973; De 1969). Stable magnetization is common in the massif anorthosites (McEnroe et al. 2002; Robinson et al. 2002, 2004; Brown and McEnroe 2004, 2008, 2012). Rock magnetic and AMS studies are very important on these rocks for correlation to regional tectonics. AMS is a useful tool to study about deformation and tectonics, which can be obtained from the various statistical parameters and orientation attributes related to AMS (Borradaile and Alford 1987; Borradaile 1991; Parés and van der Pluijm 2002; Till et al. 2010; Ferré et al. 2014; Fodor et al. 2020; Agarwal et al. 2021). One of such high-grade metamorphic terrain is EGMB, which is studied by many workers over last few decades mostly using petrology, geochronology, geochemical studies, etc. (Sengupta et al. 1999; Dobmeier and Simmat 2002; Krause et al. 2001 and references there in). However, no detailed magnetic studies have been done in this terrain. Magnetic fabrics are reported from few metamorphic terrains, such as Oddanchatram anorthosite, Tamil Nadu (Satyanarayana et al. 2003), Southern Granulite terrain (Mondal et al. 2009), Chotonagpur Granite Gneissic layered complex (Chatterjee et al. 2018b) and Kondapalle–Pangidi layered complex (Gain et al. 2022). The present attempt is related to both rock magnetic and AMS studies of the granulite rocks of the EGMB and the anorthosite intrusives at Balugaon and Rambha. The aim of the study is to identify the different magnetic remanence carrier residing in the rocks and tectonic implications by different generations of magnetic minerals. Another important identification is the magnetic fabric analysis by anisotropy of magnetic susceptibility (AMS) in relation to the deformational history of the study area.

2 Geology, sampling and methods

The Proterozoic EGMB is a high-grade metamorphic belt along the eastern coast of the Indian peninsula which extends for about 1600 km from Cuttack in Odisha to Nellore in Andhra Pradesh. The area under study is in and around Chilka Lake, Odisha and is located along the NE boundary of the Eastern Ghats Belt (figure 1). The EGB has a trend of NE–SW covering an area of about 50,000 km2. The massif-type anorthosites have widespread occurrences with the adjacent high-grade metamorphic rocks of the Proterozoic time (Anderson 1969; Isachsen 1969; Sighinolfi and Gorgoni 1975; Berrang 1996). The tectonic evolution around the Chilka Lake area is known (Dobmeier and Simmat 2002). The anorthosite massifs are surrounded by high grade metamorphic rocks such as khondalite (Walker 1902), charnockite, leptynite (Sen 1987), mafic granulite, calc-silicate granulites. All these rocks underwent high-temperature to ultra-high temperature (HT-UHT) metamorphism (Dobmeier and Simmat 2002).

Figure 1
figure 1

General geological map of Balugaon–Rambha anorthosite massif and its surrounding granulites, Eastern Ghats Mobile Belt, India (after Dobmeier and Simmat 2002).

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The present study is conducted within a latitude range of 19º27′–19º58′N and longitude 85º00′–85º25′E. In and around Chilka Lake area (CHL), a total of 38 sites were visited, amongst which 25 sites were selected for the detailed study. One oriented block sample, along with a few chip samples, were collected from each site. A total of 25 block samples were collected from individual sites in the studied area. Among these, 12 sites are from granulites, 9 from anorthosite and 4 from leptynite. Mesoscopic structural data were also collected from the exposed structural features in the field. Each of the samples was drilled out at the Rock Cutting Laboratory, Jadavpur University, Kolkata, India, using a non-magnetic drill bit. From each block sample at least 6 cores of 2.2 cm height and 2.54 cm diameter were drilled out. A total of 180 cores were drilled for magnetic measurements. AMS studies of these cores were carried out using Bartington Susceptibility Meter (MS-2). MS-2 Susceptibility Meter (Bartington, UK) was used for the low-field AMS analysis at 0.46 kHz. It works in synchronous operation with the AMS-Bar software, which measures susceptibility in 18 different orientations. On the basis of these 18 different orientations, the principal susceptibility axes' direction, along with corresponding susceptibility values and different parameters, are obtained as outcomes in the Geophysical Laboratory, Department of Geological Sciences, Jadavpur University, Kolkata, India. Based on this analysis, magnetic data and respective parameters are obtained.

From each of the block samples, a portion was mortared into powder for the rock magnetic analysis at the Palaeomagnetism Laboratory, CSIR – National Geophysical Research Institute (NGRI), Hyderabad, India. The rock magnetic measurements were conducted using an advanced variable field translation balance (AVFTB). Thermo-magnetic, isothermal remanent magnetization (IRM) and hysteresis loops were determined from the rock magnetic analysis. For the petrological study, polished thin sections were prepared from the collected chip samples. The optical information and the textural relationship were studied under a petrological microscope at Jadavpur University. Oxide phases were studied under reflected light setup.

3 Mineralogy and rock magnetic results

3.1 Petrography

Mineralogy and the textural relationship between silicates and opaque minerals are identified under the transmitted light. The granulite is composed of plagioclase, orthopyroxene, k-feldspar with minor quartz and biotite (figure 2a). The feldspar and pyroxene both are present as phenocrysts and as finer grains throughout the interstitial spaces forming granoblastic texture in the orthopyroxene granulites (figure 2b). Coronal garnet is also observed here (figure 2b). Anorthosite contains ~70% plagioclase (figure 2c). Prominent triple junction of the plagioclase phenocrysts is also seen, depicting granoblastic texture (figure 2d). Anorthosite is composed of coarse-grained feldspar and can even preserve stable magnetisations due to the presence of crystallographically oriented Fe–Ti oxides as primary coarse grains, as inclusions in silicate minerals and as secondary oxide grains, which make it an important element in the palaeomagnetic study. Leptynite consists of plagioclase, garnet, biotite, and quartz in decreasing order. Gneissic banding is observed at some places (figure 2e) in leptynite. Opaque minerals are also present throughout, along grain boundaries and along fractures of silicate grains. Silicates, as well as opaque minerals, are present as inclusions within garnet (figure 2f). Opaque minerals are present in all the rock samples in different proportions.

Figure 2
figure 2

Photomicrographs showing different rock types encountered in the study area. (a) Orthopyroxene granulite displaying granoblastic texture defined by orthopyroxene, plagioclase, k-feldspar with minor biotite and opaque (XPL); (b) Orthopyroxene granulite displaying granoblastic texture defined by orthopyroxene, plagioclase, k-feldspar with minor biotite and coronal garnet (PPL); (c) Anorthosite defined by ample amount of plagioclase feldspar (XPL); (d) Plagioclase porphyroblast shows triple junction (XPL); (e) Alternate band of Qtz+Fs and opq+bt in leptynite (PPL), and (f) Porphyroblastic garnet showing inclusion of quartz and opaque, presence of biotite and opaque along the grain boundary (PPL). PPL: Plane polarised light, XPL: Cross polarised light.

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3.2 Magnetic mineralogy

In the studied rock samples, both high-temperature (Haggerty 1976) and low-temperature (Johnson and Hall 1978) oxidation of Fe–Ti oxide grains are present. In the study area, fresh oxide grains with prominent grain boundaries indicate high-temperature oxidised grain, C-1 stage (figure 3a, b) present in granulite and anorthosite. In anorthosite, ilmenite lath present within the titano-magnetite grain denoting the C-3 stage of oxidation, i.e., a higher oxidation stage (figure 3c) is also present. Thick ilmenite lath sandwiched within Ti-rich layers in granulite depicts the later stage of C-3 (figure 3d). In the case of low-temperature oxidation, fresh grain without any crack indicates ‘Stage-1’ (figure 4a), where the grain has not experienced any low-temperature oxidation. ‘Stage 2’ (figure 4b) was identified by the development of microscopic cracks (Akimoto et al. 1984) all around the periphery of the grain during the initiation of low-temperature oxidation. The stage of oxidation increases to ‘Stage-3’ (figure 4c) when the cracks start to migrate inward towards the centre from the periphery, resulting in the cracks filling up by silicates, Stage-4 (figure 4d). Iron oxides are also seen along the grain boundaries (figure 5a), along cleavage planes and along fractures of silicates (figure 5b). They are the secondary Fe–Ti oxides which look like small broken grains. More than one generation of oxides are seen in the samples.

Figure 3
figure 3

(a) Homogeneous C-1 phase, present within grain interstitial space (irregular shape), (b) homogeneous C-1 phase with euhedral shape as an entity of granoblastic texture; (c) C-3 with exsolved thin ilmenite lamellae developed within Ti–Fe oxide (XPL), and (d) later stage of C-3 with thick ilmenite lath sandwiched with Ti-magnetite. PPL: Plane polarised light, XPL: Cross polarised light.

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Figure 4
figure 4

Low-temperature oxidized phases include (a) homogeneous Stage-1 (PPL); (b) Stage-2 with moderately developed shrinkage cracks (PPL); (c) Stage-2 to Stage-3 with prominent shrinkage cracks (XPL), and (d) Stage-3 with cracks replaced by silicates (XPL). PPL: Plane polarised light, XPL: Cross polarised light.

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Figure 5
figure 5

The Fe–Ti oxides also present as ultrafine grains. (a) Secondary oxide grains along the silicate grain boundary and (b) along the fracture plane. PPL: Plane polarised light, XPL: Cross polarised light.

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3.3 Magnetic hysteresis studies

The hysteresis loops were obtained by applying alternative magnetic fields to the samples until saturation was attained. Hysteresis loops obtained from the samples exhibit a saturated nature with moderately high remanence (figure 6a, b, e, f) and an unsaturated magnetization nature along with low or no remanence (figure 6c, d) defined by separate curves. Characteristic curves show elongation with an undersaturation of magnetisation and very low to low remanence indicating paramagnetic and ferrimagnetic minerals present in the same sample (figure 6c, d). Curves (figure 6a, b, e, f) show saturation of magnetization and remanence indicating ferrimagnetism. Mrs/Ms values fall between 0.03 and 0.41 with an average of 0.17. Hcr/Hc, the ratio of remanent coercivity and coercivity of remanence lies between 0.13 (for leptynite) and 1.03 (for anorthosite) with an average of 0.88 (figure 7). The magnetic hysteresis parameters are plotted in a representative diagram called Day plot (Day et al. 1977), with the variations in magnetic domain states (figure 7).

Figure 6
figure 6

Strong field hysteresis loops for representative samples. Insets represent the hysteresis loops after paramagnetic correction.

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Figure 7
figure 7

Distribution of the domain states of Ti-magnetite in different rocks of Balugaon and Rambha anorthosite massifs along with surrounding granulites (Day et al. 1977).

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3.4 IRM acquisition curves and coercivity spectra

The IRM acquisition curve depicts the saturation obtained by applying a magnetic field to the samples with gradually increasing strength. After a sharp rise, the curve starts to saturate below 300 mT, and the saturation was obtained for some samples CHL 6, CHL 8, CHL 11 and CHL 19 (figure 8a, b, c, d). All the samples have a coercivity <10 mT (figure 8e, f, g, h). The nature of the obtained curves resembles the presence of ferrimagnetic minerals. Both the IRM and coercivity spectra point towards soft magnetic minerals (viz., magnetite or Ti-magnetite).

Figure 8
figure 8

Isothermal remanent magnetization (IRM) acquisition curves and corresponding coercivity spectra (backfield IRM demagnetisation) for representative samples of CHL.

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3.5 Thermo-magnetic studies

The thermo-magnetic study is done to know the Curie point and change in the behaviour of the magnetic minerals within the sample during both the heating and cooling cycles. Identification of the principal magnetic minerals present in the samples is possible by observing the nature of the thermomagnetic curves obtained and by observing the Curie temperature (Tc), where the saturation remanence falls to zero. From the thermomagnetic curves of the CHL samples, it is observed that in most of the curves, the magnetization shows a sharp drop in between ~560° and 580°C during the heating cycle (the red line). The nature of the curves from CHL 5, CHL 11 and CHL 3 (figure 9a, b, e) depict the presence of ferrimagnetic minerals. Both the heating and cooling curves crosscut each other (figure 9d); this cross-over is attributed to the exsolution of the Ti-rich lamella, which is ilmenite (from petrographic studies), making the host Fe-rich. The curve (figure 9c) shows both paramagnetic and ferrimagnetic nature, the paramagnetic nature of the curve tends to stretch the Tc at ~680°C and the ferrimagnetic nature at 580°C. Paramagnetism is indicated in different CHL samples (figure 9c, f).

Figure 9
figure 9

Thermomagnetic (magnetization vs. temperature) curves for representative samples.

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4 Anisotropy of magnetic susceptibility studies

AMS is used to determine the grain alignments within the rock with an ambient stress field during its formation and the prolonged journey till date. All the parameters related to AMS depending upon the rock types are stated in table 1. The mean values of individual AMS parameters are also stated there.

Table 1 AMS parameters from samples from different sectors of different rock types.
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The mean magnetic susceptibility (Km) of the CHL samples varies from 2.44 × 10−5 to 704 × 10−5 SI units. The L ranges from 0.6 to 1.9 and F ranges from 1.1 to 2.1. The degree of anisotropy (Pj) is almost uniform in all the samples; values fall between 1.1 and 2.6, and the shape parameter (Tj) shows positive values except in some of the samples. All the Pj values range within a certain limit; the mean value of the individual four sections is >1.05 (table 1), which is the verge value for magnetic fabric which is tectonically controlled (Dvorak and Hrouda 1975; Tarling and Hrouda 1993; Chatterjee et al. 2018a; Mondal et al. 2022). However, the Pj–Tj plots (Jelinek 1981) range from prolate to oblate but mostly oblate in nature (figure 10). The Pj–Km plot (figure 11) is plotted to see the susceptibility change related to the corrected anisotropy.

Figure 10
figure 10

Pj vs. Tj plots for (a) northern sector granulites, (b) southern sector granulites, (c) anorthosite, and (d) leptynite.

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Figure 11
figure 11

Pj vs. Km plots for (a) northern sector granulites, (b) southern sector granulites, (c) anorthosite, and (d) leptynite.

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4.1 Magnetic fabrics

The lower hemisphere equal area plots of all principal susceptibility axes of the different sectors of rock types are more of dispersed all throughout than clustered (figure 12). The α95 value for both K1 and K3 also range widely (table 1). The trend of magnetic lineation and pole-to-magnetic foliation varies lithologically (figures 13 and 14). The magnetic lineation has high plunge and variable trend. The mean of magnetic lineation of different rocks is NW–SE for northern sector granulites, almost E–W for anorthosite, NE–SW in leptynite and NE–SW in southern sector granulites. The magnetic lineation map (figure 15) and magnetic foliation map (figure 16), which represent the magnetic fabric of the study area are provided.

Figure 12
figure 12

Lower hemisphere equal area plots for all the principal susceptibility axes for (a) northern sector granulites, (b) southern sector granulites, (c) anorthosite, and (d) leptynite.

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Figure 13
figure 13

Lower hemisphere equal area plots for the magnetic lineation for (a) northern sector granulites, (b) southern sector granulites, (c) anorthosite, and (d) leptynite.

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Figure 14
figure 14

Lower hemisphere equal area plots showing the magnetic foliation for (a) northern sector granulites, (b) southern sector granulites, (c) anorthosite, and (d) leptynite.

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Figure 15
figure 15

Magnetic lineation distribution of the study area. Different sector plots are shown in individual equal-area diagrams.

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Figure 16
figure 16

Magnetic foliation distribution of the study area. Different sector plots are shown in individual equal-area diagram.

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5 Discussions

5.1 Magnetic mineralogy and remanence carriers

The major rock types of the study region are granulite, anorthosite and leptynite. There are different generations of Fe–Ti oxides present along with their different stages of oxidations. Both low-temperature (<350°C) and high-temperature (>600°C) primary oxides are observed.

A higher temperature oxidation has been reached as evidenced by the homogeneous oxide grain of the C-1 stage (figure 3a, b). The grain with ilmenite lath indicates the C-3 stage (figure 3c), and even later stage of C-3 with thick ilmenite bands sandwiched inside the Fe-rich layer indicates higher oxidation (figure 3d). According to Das et al. (2012), M2–D2 is the peak metamorphic event of the area, i.e., the peak metamorphic assemblage in the studied high-grade metamorphic rocks is attained during this second stage of metamorphism, which is, in turn, coeval with the D2 deformational event. So, the high-temperature Fe-oxides may be syn-tectonic during M2, the highest temperature and highest pressure attained by the rocks. In granulite facies, the high-temperature phases occur during the prograde metamorphism (Wang et al. 2015). During post-peak metamorphism, i.e., the retrograde process or D3 the low temperature oxides may have formed, which suffered low-temperature upliftment related oxidation (Akimoto et al. 1984). Different stages of low-temperature oxides, Stage-1, Stage-2 and Stage-3 (figure 4) are shown. Garnets with oxide inclusions (figure 2f) indicate that those oxides are primary. Some silicates and garnets are highly fractured and have decomposed during D3. The high-temperature oxides also suffered the retrograde path, i.e., the low-temperature oxidation, due to the presence of a corrugated grain boundary (figure 3c, d). There are three generations of Fe–Ti oxides present in the study area. Along the fractures of silicate grains (figure 5b) and along grain boundary (figure 5a), tiny magnetite grains are formed. Magnetite is identified due to the drop of the heating curve at Curie temperature Tc ~580°C and that magnetite provides stable magnetization as the nature of the heating curve and the cooling curve is reversible (figure 9). The hysteresis curve, which shows saturation and remanence, indicates typical characterisation of the presence of ferrimagnetic magnetite (figure 6a, b, e, f; Soumya et al. 2017). The hysteresis curve of the samples CHL 13 and CHL 8 (figure 6c, d) shows little remanence, and the unsaturated nature indicates that in the studied rock samples, both ferrimagnetic and paramagnetic minerals are present. The Day plot indicates the chief magnetic remanence carrier belongs to fine-grained SD Ti-magnetites (figure 7). From the thermomagnetic study of the same sample, CHL 13, the presence of ferrimagnetic magnetite and paramagnetic haematite are observed (figure 9c). By the Tc and nature of the curve, paramagnetic haematite and ferrimagnetic magnetite are also identified (figure 9c). From the rock magnetic study, it is evident that ferrimagnetic magnetite and paramagnetic haematite are present (Satyanarayana et al. 2003).

5.2 Magnetic fabric versus tectonics

Orientation of the K1 in the Chilka Lake area can serve as a proxy for the direction of the stress field during the different orogenic events of the EGMB. This is because the maximum susceptibility axes (K1) align themselves along the direction of maximum elongation or the maximum extensional stress and evidently stay perpendicular to the direction of shortening or the maximum compressive stress (Goldstein 1980; Mondal et al. 2009; Gain et al. 2022). Based on this the magnetic fabric development of the Chilka Lake area can be two staged, each having a relationship with a particular tectonic event in the terrain.


Stage 1 (metamorphism stage): Signature of the first stage in the development of magnetic fabric is recorded in the granulitic rocks of the area. These were the rocks which came into existence at the earliest among all (~2.0 Ga), where the signatures of the retreating phase of orogeny in the Eastern Ghats Belt, followed by subduction during 2–1.7 Ga is preserved (Dasgupta et al. 2013). Based on the orientation of the magnetic fabrics from granulite, the K1 trends NE–SW mostly with some deviations in the southern granulite (figure 13a, b). This proves that during this stage, the distribution of the maximum compressional stress (σ1) was oriented NE–SW, which is also parallel to the regional F1 fold axis due to D1 deformation. However, the magnetic foliation plane developed during this stage is not parallel to the regional schistosity. This reversal in the magnetic fabric away from the mesoscopic one may be due to the high susceptibility magnetite, which is developed during this stage only because of UHT metamorphism in this phase (Sengupta et al. 1999; Dasgupta et al. 2013). High-susceptibility magnetite grains have the capability of rotating the magnetic foliation plane in the direction of mineral growth, deviating it from the mesoscopic schistosity. A certain degree of positive correlation between Pj and Km in both the granulites of southern and northern sectors speaks for the same (figure11a, b).


Stage 2 (magmatism stage): An analogy with the maximum susceptibility axes as a proxy from this σ1 orientation in the anorthosite and leptynite as well, which are the products of post-subduction magmatism, it is noted that the stress field orientation was not constant.

Initially, in the metamorphism stage, the σ1 orientation was NE–SW, which is still somewhat preserved in the magnetic lineation of leptynite, which finally grades to an E–W orientation, and it becomes completely haphazard in the anorthosite. Hence, the leptynite marks the transition between the metamorphism and final magmatism stage. Hence, based on the magnetic fabrics, the leptynites are prior to anorthosite. Mesoscopically also, the D1 and D2 signatures are absent in the anorthosites, and the E–W orientation of the K1 fabric in leptynites is in accordance with the D3 deformation. The felsic magmatism, mainly the anorthosites, have unoriented magnetic fabrics without preserving the regional tectonics because the anorthosites were emplaced after the completion of the D3 phase of deformation, after the emplacement of the leptynites.

6 Conclusions

From the rock magnetic study, it is clear that the ferrimagnetic character and the magnetic remanence carrier of the studied rocks are Ti-magnetite and haematite. The rocks are rich in Ti-magnetite because of their prolonged metamorphism to granulite (Frost and Shive 1986; Shive and Fountain 1988; Shive et al. 1992; Liu et al. 2013). Progressive metamorphism to granulite facies increases the amount of ferrimagnetic Ti-magnetite and magnetite. Conversely, retrogration imprints paramagnetic contents (Wang et al. 2015). In the present case, the scenario is the same here, due to retrograde metamorphism (Sengupta et al. 1999) in the rock's paramagnetic character. The metamorphism took place in the rocks that came into existence (~2.0 Ga), and the following orogeny signatures are captured in the granulites. Based on magnetic fabrics, magnetic lineation trends NE–SW in the granulites, with some exceptions in the southern granulites. This is followed by magmatic events, which are experienced by leptynite, whose K1 trends from NE–SW to E–W. Then comes the anorthosite where there is no such magnetic fabric orientation.