Keywords

1 Introduction

Rock samples when subjected to external magnetic field, certain minerals within the sample show same strength of induced magnetization in all directions and are considered as “magnetically isotropic”. On the contrary, there are few minerals in the rocks, which have different strength of induced magnetization in preferred directions are called as “magnetically anisotropic”. Such minerals have an Anisotropy of Magnetic Susceptibility (AMS). Variation in susceptibility in preferred directions creates susceptibility ellipsoid (Tarling and Hrouda 1993), having three principal axes—long, intermediate and short. These axes are the principal susceptibility axes represented in SI units as K1, K2 and K3, where K1 ≥ K2 ≥ K3. The axes of susceptibility ellipsoid generally show a preferred orientation of principal axes of the strain ellipsoid in tectonically deformed rocks (Hrouda and Janak 1976; Rathore 1979, Borradaile and Tarling 1981, 1984; Hrouda 1982; Borradaile and Mothersill 1984; Borradaile 1987, 1991; Borradaile and Alford 1987; Borradaile and Henry 1997).

In the present paper, we provide AMS data generated on quartzites of Champaner Group belonging to the Aravalli Supergroup with an aim of demonstrating the last deformation imprint recorded within this rock. With the help of the AMS data, strong correlation has been established between the deformation signatures pertaining to the Champaner Group along with its basement and the neighboring Pre-Cambrian stratigraphic units. The present attempt is to explore the AMS fabrics from Champaner Group of rocks, particularly quartzites, as emphases regarding the emplacement record of granite, to understand the SAMB tectonics operated at its southern margin.

2 Geological and Structural Setting

The Champaner Group, a part of upper Aravallis is exposed at the eastern most fringe of Gujarat, India (Gupta et al. 1997) (Fig. 1). It bears an important link between the main Aravalli domains with the Central Indian Tectonic Zone (CITZ). The Champaner Group consists variety of rock types such as quartzites of manganiferous and non-manganiferous variety, polymict and oligomict metaconglomerates, carbonaceous phyllites and schists, spotted slates, hornfelses, manganiferous phyllites, biotite schists, dolomitic limestones, calc-silicate rock and meta-greywackes, which overlies the Paleo-Proterozoic Pre-Champaner Gneissic Complex (PCGC) located at Chhota Udepur region–i.e. “considered basement”, comprising granite-gneisses, quartzites and pelitic gneisses (Gupta et al. 1992, 1997; Merh 1995; Srikarni and Das 1996; Karanth and Das 2000; Das 2003) (Fig. 2). The Champaner rocks show greenschist facies metamoprhism and hornfelses are developed at the contact aureole with granite intrusive. (Das et al. 2009; Joshi and Limaye 2018; Joshi 2019a, b). Granites located in and around Champaner Group indicate syn-to-post emplacement record. Timing of granites suggest a wide span of dates from 1.1 to 0.93 Ga (Joshi and Limaye 2018).

Fig. 1
figure 1

lithostratigraphic map of SAMB, NW, India, specifying location of the Champaner Group. Modified after Mamtani and Greiling (2005). Square indicates the study area magnetic foliation data presented within Godhra granite is after Mamtani (2014)

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Fig. 2
figure 2

Geological map of the Champaner Group, after Joshi 2019b

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Structurally, the rocks including quartzites, show poorly deformed signatures having three phases of deformation, viz. D1, D2 and D3. The three phases of deformation have resulted into F1, F2 and F3 folds, respectively. D1-D2 resulted into F1-F2 are coaxial having ESE-WNW and E-W trend, respectively, while the D3 shows folds having NNW-SSE to NNE-SSW trend. Combination of F1-3 folds have resulted into various interference fold patterns on regional as well as on the outcrop scale. The Type-III interference fold pattern has formed on account of regional deformation resulted due to combinations of F1-2 folds, whereas the Type-I interference pattern superimposed orthogonally over Type-III, which led to the development of domes. This last axial trace generated on account of superimposition is produced by later granitic pulse, represents F3 folds (Joshi 2019a, b). In addition, there has been observed evidences of out-of-sequence deformation (i.e. trends, which not concomitant with F1, F2 and F3) demonstrate varied trends and meso-scale interference patterns inside the detached and isolated patches of calc-silicates, caught up within the granites (Joshi and Limaye 2018).

3 Anisotropy of Magnetic Susceptibility Studies on Quartzites, Principle and Methodology

Quartzites of the Champaner Group are hard, massive, compact; at times flaggy and whitish to dark grey and/or black in colour. They are highly jointed with variable grain size from fine to coarse and display varieties of primary sedimentary structures at different locations of the study area. By and large quartzites of the study area exhibit E-W to NW-SE strikes with variable dip directions. Petrographic study of quartzites reveal granoblastic polygonal, mosaic with two granular boundary varieties (i.e.) in-equigranular interlobate and seriate interlobate. The former type is observed in the quartz grains showing sutured boundaries with large scale variability in grain size, whereas the latter shows less size variation with sutured grain boundaries. At places the grains show inclusions of mica flakes especially Mn-rich-muscovite having colourless to faint green pleochroism (Patel et al. 2016).

The present section deals with the Anisotropy of Magnetic Susceptibility (AMS) studies carried out on quartzites of the Champaner Group. Oriented samples were collected at 12 localities across the Champaner Group and more than 60 cores were analyzed for the AMS study. These measurements were done using KLY-4 Kappabridge at Department of Geology and Geo-physics, Indian Institute of Technology, Kharagpur. The instrument gave the results for the following parameters: (1) the magnitude and orientations of three principal axes of the susceptibility ellipsoid viz. K1, K2 and K3, where K1 ≥ K2 ≥ K3; (2) mean susceptibility (km); (3) degree of anisotropy (Pj); (4) shape parameter (T); (5) magnitude of foliation (F) and (6) lineation (L) (Table 1). The quartzites of the study area reflect low Km values, having the mean susceptibility as 47.37 × 10−6 SI units. Some quartzites reveal exceptionally high mean susceptibility values more than 250 × 10−6 SI units, due to the presence of biotite content.

Table 1 AMS data of the quartzites of the Champaner Group
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4 Result and AMS Data Interpretation

The orientations of all three principal axes K1, K2 and K3 have been plotted using lower hemisphere stereographic projection. The plane passes through K1-K2 and essentially matching with K3 pole defines the magnetic foliation (F). The F plane in the given figure is plotted with the dotted line. The stereographic net is also aided with the planar structure S0 (bedding plane) acquired through field and has been depicted by means of continuous line (Fig. 3). There are total 12 samples, viz. SN1-4, SN6, CPR1, CPR3-5 and CPR7-9, out of which 61 cores have been analyzed.

Fig. 3
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AMS orientation data plotted as lower hemisphere projections for the Champaner Group. Dotted great circles represent the magnetic foliation, whereas continuous great circles represent the S0 reading recorded from the field. Filled square, triangle and circle indicate orientations of K1, K2 and K3 respectively

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As mentioned in the geological and structural setting section, the Champaner Group shows three phases of deformation, viz. D1, D2 and D3, of which first two phases were co-axial (F1 ~ ESE-WNW; F2 ~ E-W) and the last phase of deformation has N-S trend. The response of all three phases of deformation has been recorded within the quartzites of the Champaner Group. The samples collected from the southern part of the study area, viz. CPR1, 3, 4 and 5 show dominance of F1 and F2 folds. The CPR1 sample has a dominance of F1 fabric on an outcrop scale due to the presence of mesoscale folds, but shows dominance of magnetic foliation in nearly E-W direction. The superimposition of F2 fabric over F1 is clearly seen in the sample collected from NW of Masabar (CPR1). CPR3 sample from the F2 limb portion show oblique relationship of the magnetic foliation with the bedding. CPR4 and 5 collected from F2 limb, depicts similar trend of bedding and magnetic foliation. These samples show the effect of F2 folds present in the southern part of the study area. The signatures of F3 fold or preferred orientation along N-S direction are not seen in the present samples. This suggests that the last episode of deformation did not show any mineral alignment in the southern part of the study area.

CPR 7 and SN2 sample collected from the eastern part of the study area from Narukot dome clearly signifies similar relationship of S0 and S1 in these quartzites. The SN2 sample collected from F1 fold limb and CPR 7 sample from F2 limb gives nearer results of F1 and F2 fabric. It suggests that although, there is a dominance of N-S deformation in the eastern part of the study area, the preferred magnetic fabric parallel to F3 is absent. Such condition has occurred due to the presence of F2 fold dominance in the Narukot area. Further to the west of the Narukot dome, the samples collected along the axial trace of the F2 fold (SN3, SN4 and SN6), show prominence of E-W magnetic foliation. The D2 imprint recorded within these rocks clearly suggests that the central part of the Champaner Group has a dominance of N-S shortening. The western and northern part of the study area shows the magnetic fabric parallel to the D3 deformation of the Champaner Group in the samples viz. CPR 8, CPR 9 and SN1. This magnetic fabric ranges from N-S to NE-SW; however there is no mesoscopic scale imprint of the late deformation in the present localities.

Jelinek plot (Pj vs. T) (Fig. 4), suggest that majority of the cores fall in the oblate field (flattening field) and few cores from the quartzites show their affinity towards prolate field (constrictional field). In order to examine the type of strain, the available AMS data has been plotted as Flinn diagram (F vs. L), which holds good correlation with the Jelinek plot. Most of the AMS data of cores from quartzites of the Champaner Group represented in the Flinn diagram concentrated in the oblate field and few around K = 1 line, which separates the prolate and the oblate field. This is an indication of the plane strain condition (Fig. 4).

Fig. 4
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Jelinek plots (at left) and Flinn diagram (at right) for the quartzites of the Champaner Group

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5 Discussions

5.1 Significance of AMS Study for the Study Area

The AMS data results from quartzites of Champaner Group indicate that there exist two prominent striking planes of magnetic foliation, viz (i) ENE-WSW to E-W and (ii) N-S to NE-SW. The former range of strikes represent the first two phases (i.e. D1 = F1 ~ ESE-WNW and D2 = F2 ~ E-W), while the later orientation matches with the third deformational phase (i.e. D3 = F3 ~ NNW-SSE to NNE-SSW) recorded within the Champaner Group along with its basement.

Advancement in the present technique indicate that the Anisotropy of Magnetic Susceptibility (AMS) studies have been used to deduce the time relationship between emplacement/fabric development and regional deformation (Mamtani and Greiling 2005); in kinematic analysis and vorticity quantification of syn-tectonic granitoids (Tarling and Hrouda 1993; Benn et al. 1998, 1999, 2001; Majumder and Mamtani 2009; Mamtani et al. 2013); to capture imprints of superposed deformation in granitoids (Mondal 2018; Mamtani et al. 2019). In the light of this extensive work on Godhra and other granitoids, authors posit their AMS data set generated on quartzites by correlating it with the existing literature on structural and magnetic records related to the SAMB.

5.2 AMS of Adjoining Areas

The structural records from Pre-Cambrian stratigraphic units of SAMB, viz. (1) the Banded Gneisses (BG); (2) the Lunavada Group (LG); (3) the Champaner Group (CG) along with its basement (Pre-Champaner Gneissic Complex-PCGC); (4) the Godhra Granites (GG); signifies that each of the unit that belongs to the SAMB, except GG have underwent at least three phases of deformation (Table 2). Moreover, the magnetic records derived through BG, LG and GG, by the earlier workers state that these magnetic trends are coeval with the last phase of deformation (having strikes ~ WNW to WSW and plunge ~ NW to W), recorded within the SAMB (Mamtani and Greiling, 2005). Unlike in the case of CG, where the latest magnetic foliation (having strikes ~ N-S to NE-SW), demonstrating the last phase of deformation within CG, show a distinct heterogeneity in terms of trends recorded during the waning phase of SAMB. In addition, the strain estimation derived with the help of degree of magnetic anisotropy value (P′ or Pj) from Godhra granite samples, suggest that southern part of GG, in proximity to CITZ tends to show inflated P′ or Pj values, implying the rocks have accommodated higher strain condition than their northern counter parts (Mamtani et al. 2019). However, the quartzite samples collected from the eastern part of the Champaner Group, having predominantly N-S to NE-SW magnetic foliation reflects higher P′ or Pj value, suggests that the eastern part of the CG have accommodated high strain condition than any of the other direction (Table 1).

Table 2 Summary of deformation events recorded in neighbouring Precambrian Supracrustals and its relationship with the Champaner Group along with its basement
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In order to understand the heterogeneity in terms of magnetic records, the study of granite located in and around the CG becomes vital. On the basis of structural data proposed by Joshi and Limaye (2018), suggests that the granites located in and around the Champaner region shows evidences of prolong emplacement records. One such evidence has been proposed by a field photograph published in Joshi and Limaye (2014), where, the coarse grained later pulse having intrusive relationship with the comparative fine grained granite variety. Also, the evidences suggested by Joshi (2019a) on genesis of interference fold patterns within CG, explained forceful emplacement of granite deforming the country rocks along N-S trend, on the basis of the model suggested by He et al. (2009) for Fangshan pluton, SW Beijing, China. In addition to that, the existing geo-chemical data of granites located in and around CG is in favor of two pulses revealing prior ‘S’ type granite (Goyal et al. 1997) and later the ‘A’ type variety representing transitional or post-orogenic uplift (suggested by Maithani et al. 1998 and Goyal et al. 2001). Finally, the Geophysical studies carried out within this terrain corroborate to the fact that there lies a sub-surface pluton beneath the CG, which has eventually up rooted the country rocks along with the pre-existing granitic pulse during the later stages (Joshi et al. 2018).

Hence based on the above facts it can be stated that the possibility of prolong emplacement record, which governed the tectonics of SAMB from its southern margin cannot be ruled out. At the end of regional deformation the granite pulse was still active at the southern margin deforming the Champaner metasediments along with its basement, to give rise different trends. These trends being restricted to the Champaner Group and its basement, have not been recorded in the neighboring supracrustals and thus how show heterogeneity in terms of last deformation imprint within the SAMB (Table 2).

6 Conclusion

  1. 1.

    The AMS studies on quartzites from Champaner Group indicate two prominent striking planes of magnetic foliation, viz (i) ENE-WSW to E-W and (ii) N-S to NE-SW. These trends matches with the three deformational phases occurred within the present group, along with its basement (i.e. D1 = F1 ~ ESE-WNW and D2 = F2 ~ E-W; D3 = F3 ~ NNW-SSE to NNE-SSW).

  2. 2.

    Correlating these magnetic fabrics of CG with the available structural and magnetic records from SAMB, suggest that there lies a distinct heterogeneity in case of latest trends of CG and SAMB.

  3. 3.

    Degree of magnetic anisotropy value (P′ or Pj) from quartzite samples indicate that the eastern part of the Champaner Group has accommodated high strain condition than any of the other directions.

  4. 4.

    The above facts are in favor of indicating the prolong emplacement record of GG, which governed the tectonics of SAMB from its southern margin.

  5. 5.

    During the waning phase of regional deformation of CG, the emplacement was still continued to deformed the Champaner metasediments along N-S trend, along with its basement.

  6. 6.

    As this latest trend was confined to the CG and its basement, have not been recorded with the neighboring supracrustals of SAMB.