geochemistry and petrogenesis of tholeiitic dykes from the

Geochemistry and petrogenesis of tholeiitic dykes fromthe Chotanagpur Gneissic Complex, eastern India

RAHUL PATEL1,2, RAVI SHANKAR

2, D SRINIVASA SARMA1,2,*

and AUROVINDA PANDA1,2

1Academy of ScientiBc and Innovative Research (AcSIR), Ghaziabad 201 002, India.

2CSIR–National Geophysical Research Institute (CSIR–NGRI), Hyderabad 500 007, India.*Corresponding author. e-mail: [email protected]

MS received 18 September 2020; revised 6 March 2021; accepted 21 March 2021

In this study, we present the geochemical analysis on 14 samples from seven distinct E–W trendingmaBc dykes from the Chotanagpur Gneissic Complex. These dykes have not been studied previouslyand highlight the importance of igneous and tectonic processes in the Chotanagpur Gneissic Complex.The dykes, in terms of modal mineralogy, do not show notable variations, but textural variations arewell noticed. These dykes are characterised as basalt and basaltic andesite (SiO2 = 45.48–54.03 wt.%;Mg# = 21–68.5), and comparable with E-MORB type tholeiitic magma series. The dykes are classiBedinto two groups, low Mg# and high Mg# dykes, based on their Mg# and silica content. The majorfractionating mineral phases are olivine, plagioclase, and pyroxene. The dykes are variably contami-nated with crustal input, as shown by Nb/U vs. (Th/Nb)PM, Th/Nb vs. La/Nb, and Th/Yb vs. Nb/Yb.The dykes also underwent post-magmatic hydrothermal alteration after their emplacement. Semi-quantitative trace element modelling suggests that these dykes are derived by partial melting withinspinel peridotite-rich mantle and spinel–garnet peridotite-rich mantle source in the transition zone. Thelow Mg# dykes are consistent with 3–15% partial melting curves, whereas high Mg# dykes arecomparable with 15% melting curve. Finally, we present a conceptual and simpliBed model for the E–Wtrending maBc dykes of the CGC, based on the geochemical data of the present study and the availableinformation.

Keywords. Chotanagpur Gneissic Complex (CGC); maBc dykes; geochemistry; trace element modelling;petrogenesis.

1. Introduction

Large Igneous Provinces (LIPs) are referred as abroad spectrum of igneous bodies (e.g., giantdolerite dyke swarms, sill provinces, continentaland oceanic Cood basalts, layered maBc–ultra-maBc intrusions, and volcanic rifted margin), withareal and volume extent of[1 km2 and[0.1 km3,respectively, but temporally conBned to a maxi-mum time limit of 50 Ma, and represented by

short duration (1–5 Ma) magmatic pulses (CoDnand Eldholm 2005; Bryan and Ernst 2008 andreferences therein). Over the past decades, maBcdyke swarms of the continental shield regionshave emerged as useful tools for understandingLIPs, as they provide valuable insights into thespatial and temporal picture of short-lived magmapulses (Halls 1982; Fahrig 1987; Halls and Fahrig1987; Parker et al. 1990; Baer 1995; Ernst andBuchan 2001; Hanski et al. 2006; Bleeker and

J. Earth Syst. Sci. (2021) 130:160 � Indian Academy of Scienceshttps://doi.org/10.1007/s12040-021-01646-7 (0123456789().,-volV)(0123456789().,-volV)

Ernst 2006; Srivastava et al. 2014). LIPs occur inboth oceanic and continental environments, butcompared to oceanic, the variability of magmaticrocks is more diversiBed in continental regions(Kumar and Ratna 2008). Continental rifting is atypical feature associated with LIPs that initiatedeither in response to lithospheric stretching orplume impingements and consequently resulted ina broad spectrum of magmatic rocks (Turcotteand Emerman 1983).Dykes are classiBed as the salient extensional,

magmatic tabular bodies that constraint theknowledge about various domains of geodynamics,such as mantle chemistry, magma transportation,and the interplay of crust–mantle interaction(Halls 1982; Bleeker and Ernst 2006 and refer-ences therein). MaBc dykes commonly occur inArchean terrains, Proterozoic rift valleys, sedi-mentary basins, and large igneous provinces(Halls and Fahrig 1987; Parker et al. 1990; Hanskiet al. 2006; Srivastava et al. 2009, 2014). Theformation of continental crust in the mobile beltsettings and its geodynamics is challenging todecipher because of the scarcity of fresh outcrops,which are subsequently altered and acquiredcomplex metamorphic nature through geologicalevolution of the Earth. However, maBc dykescould be a potential candidate to examine theabove-mentioned queries due to their relativelyresistive nature in comparison to other igneousrocks.The ENE–WSW to E–W trending maBc

dykes, probably Mesoproterozoic to Cretaceous/Tertiary age (Ghose and Chatterjee 2008), areemplaced in the Chotanagpur Gneissic Complex(CGC). The petrogenesis and paleotectonicconditions of emplacement of these dykes arepoorly constrained by few preliminary studies(e.g., Kumar and Ahmad 2007). Also, none ofthe earlier studies have proposed any geochem-ical model to decipher the petrogenesis of thesedykes. No tectonic emplacement model for thesedykes has been proposed yet. In this paper, weaddress these issues and present the geochemicalstudy of maBc dykes, from the proximal regionof Ranchi, Jharkhand, India. We have attemp-ted to propose the petrogenesis as well as paleo-tectonic emplacement conditions based on theresults of this study. Additionally, we haveattempted to quantify the degree of partialmelting and to evaluate the nature of themagma source of the studied dykes throughgeochemical modelling.

2. Geological background

2.1 Nature of the CGC

The Indian subcontinent is mainly subdivided intotwo broad and distinct crustal domains, i.e.,northern and sothern crustal domains, separatedby the ENE–WSW trending central Indian tec-tonic zone (Radhakrishna and Naqvi 1986). Thecratons and the mobile belts are the main consti-tuting features of the Indian subcontinent, withlong geo-tectonic history of their evolution. Thecratons are exposed in most parts of peninsularIndia and the cratons are basement in extra-peninsular India as well, though concealed by thealluvium, whereas the mobile belts covereast–northeast India (Bgure 1a). The cratons wereprimarily stabilised during the Precambrian Era,whereas mobile belts were accreted during Meso toNeoproterozoic Era (Saha and Mazumder 2012;Meert and Pandit 2015; Valdiya 2015; Shellnuttet al. 2019). The Singhbhum Craton, CGC(Bgure 1b), and Shillong plateau are the three maincomponents of the eastern India. The origin of theCGC, in association with the Singhbhum Craton, isnot fully understood as there is no consensuswhether these two should be classiBed as a singlegeological entity or separate ones. Further, theexact geological nature of the CGC, as it shouldbe classiBed as a mobile belt or craton, is still to beconBrmed. However, different opinions are pro-posed to delineate the exact nature of the CGC.According to one hypothesis, the CGC is a distinctand isolated orogenic belt (Ghose 1983;Mukhopadhyay 1988; Ghose and Chatterjee 2008;Mohanty 2012 and references therein). Whereas, asper the others, it is a tectonically stable part of thelithosphere (Naqvi and Rogers 1987; Kumar andAhmad 2007; Sharma 2009).Based on the presence of basic rocks and low-

grade meta-enclaves, the CGC has been classiBedas a cratonic block, which subducted below theSinghbhum Craton, and consequently led to theformation of the Singhbhum mobile belt to thesouth of the CGC (Sharma 2009). In addition tothat, emplacement of many potassic igneousintrusions such as orangeites, lamproites (Srivas-tava et al. 2009, 2014), presence of the komatiitesfrom Palamau district of Jharkhand (Bhattacharyaet al. 2010), metabasite and meta-dykes that aremetamorphosed up to amphibolite facies (Ghoseet al. 2005; Kumar and Ahmad 2007; Srivastavaet al. 2012), and Bve large scale continental rift

160 of 24 J. Earth Syst. Sci. (2021) 130:160

zones, trending E–W to ENE–WSW (Ghose andChatterjee 2008), are the best-cited examples thatsignify CGC as a cratonic block. The Bve largescale continental rift zone essentially controls themagmatic emplacement in the CGC (Ghose andChatterjee 2008). In contrast, frequent occurrencesof high-grade metamorphic units, e.g., granu-lites and gneissic rocks and spatial and temporallink of the CGC with Satpura mobile belt pointsout the CGC as a mobile belt (Mohanty2010, 2012).

2.2 Regional geology

The Proterozoic CGC covers about 100,000 km2.Majority of the terrain is located in the states ofChhattisgarh, Jharkhand, Bihar, and West Bengal(Mahadevan 2002; Acharyya 2003; Sharma 2009;Sanyal and Sengupta 2012). A large part of the

eastern margin is covered by Gangetic sediments,derived due to denudation of Sivaliks (Hossainet al. 2007). The CGC has a complex geologicalhistory due to its multiphase deformation, meta-morphism, and migmatisation. Therefore, it is notentirely possible to establish the complete strati-graphic framework as the original characteristicsof the exposed rock units are imprinted with latergeological processes. However, using the principleof relative dating and available radiometric agesof some selected magmatic rocks, a near satisfac-tory stratigraphy table is prepared (table 1). Thelithological ensemble of the CGC can be classiBedinto three lithostratigraphic units, i.e., crystallinebasement, older meta-sediments and late intrusive(Ghose 1983; Banerji 1991). The evolution of theCGC can be described into three orogenic cycles,viz., the Chotanagpur Orogeny (1.6–1.5 Ga), theSatpura Orogeny (0.9–0.85 Ga) and the Munger

a b

Figure 1. (a) SimpliBed regional geological map of India showing the distribution of major geological components; the cratons,sedimentary basins, and mobile belts (Mukherjee et al. 2017). (b) SimpliBed regional geological map of the Chotanagpur GneissicComplex; In the northern, the CGC is bounded by Son Narmada normal fault/rift and Mahakoshal fold belt that is an extensionof the Satpura mobile belt, Eastern side is covered with alluvial dominated Indo-Gangetic Plane, southern side is bounded bySinghbhum mobile belt intruded with Dalma volcanic, and in the northern side, Gondwana basins are exposed (Acharyya 2003;Srivastava et al. 2012). Note: Sample locations are marked with solid red colour ellipses.

J. Earth Syst. Sci. (2021) 130:160 of 24 160

Orogeny (0.42–0.35 Ga) (Ghosh 1983; Banerji1991). However, according to Singh (1998), thelithostratigraphic units have classiBed as litho-demic units as they do not follow the ‘law ofsuperposition’ providing the multiphase deforma-tion and reworking nature of the CGC. Hence, therocks are divided into three lithodemic groups,

i.e., CCGC group, Koderma group, and Rajgirgroup. As mentioned earlier, the magmatic eventsemplaced in the CGC are controlled by Bve majorrift zones (Chatterjee et al. 2008). These includeNorth Purulia Shear Zone, South Purulia ShearZone, Damodar Graben, South Narmada SouthFault, and N–S striking rift zone that coincides

Table 1. Precambrian and Phanerozoic events stratigraphy of Chotanagpur Gneissic Complex, Eastern India.

Periods/eraDeformation

eventsLithostratigraphic

units and igneous intrusionsLithostratigraphic units description and

available ages of igneous intrusions

Early-Cretaceous Post D2/D3

Lower-Cretaceous Post D2/D3

Permo-Carboniferous Post D2

Meso-Neoproterozoic Pre to Syn D2

Pre and Syn D2

Mesoproterozoic

Post D1- Pre D2

Syn D1

Paleoproterozoic

Pre D 1

Pre-paleoproterozoic and paleoproterozoic

References for lithostratigraphic units:1. Mohanty (2012)2. Sanyal and Sengupta (2012)3. Chatterjee and Ghosh (2008)

References for age data:C-1: Ar–Ar whole rock ages Kent et al. (2002); C-2: Ar–Ar whole rock ages Kent et al. (2002); C-3: Ar–Ar whole rock ages Kent et al. (1997); C-4: Ar–Ar whole rock ages Kent et al. (1997);C-5: SHRIMP U–Pb zircon age Mazumder et al. (2010);C-6: ID-TIMS U–Pb dating of zircon Chatterjee et al. (2008)


with Rajmahal basin. Chronologically, the riftzones have been placed into three time periods,i.e., (1) Late Paleoproterozoic to Early Mesopro-terozoic, (2) Early Neoproterozoic, and (3) EarlyCretaceous; which respectively coincide with theSatpura Orogeny (1.7–1.5 Ga), Grenville Orogeny(*1.0 Ga) and the break-up of the easternGondwanaland (115–118 Ma).The basement of CGC is thought to have con-

sisted of Paleoproterozoic granite and gneissicrock units, into which numerous magmatic unitsare intruded (table 1). The ultramaBc and maBcrocks of unknown geochemical aDnity thatemplaced at 1925 ± 110 are suspected to be theoldest magmatic units (Rekha et al. 2011). Suitesof peridotite komatiites from Palamau district ofJharkhand (Bhattacharya et al. 2010), indicatethat the mantle activities have occurred in theterrain. Further, many other ultramaBc suitesthough mostly metamorphosed in close associationwith low Ti–V magnetite deposits, are alsoexposed in the vicinity of Aurangabad–Koel valley(Ghosh and Chatterjee 2008). Chatterjee et al.(2008) studied the 1.55 Ga (U–Pb zircon) gab-broic anorthosites from the Saltora region ofBengal and suggested the geodynamic significanceof the eastern margin of the CGC (table 1). Thegabbroic anorthosites are divided into two groups,namely, low Ti tholeiite and high Ti tholeiite,with an aDnity of T-MORB (resemble charac-teristics of transitional basalt) and E-MORB,respectively (Ghose 2005). Several episodes andvarieties of granites in the CGC are identiBed andform one of the most widely distributed rocks(Bgure 1b and table 1). Barabazar granites are theoldest (*1771 Ma) reported granites, exposed inSouth Purulia Shear Zones (Dwivedi et al. 2011).Saikia et al. (2017) reported the preliminary geo-chemistry and geochronology of I-type granitesfrom Bathani volcano-sedimentary sequences andproposed their emplacement in volcanic arc set-tings during 1.7–1.6 Ga. Other notable granitesinclude Hazaribagh granite, more hypersthenebearing granites, Mica belt granite, Nagam gran-ites, Gumla granites, and Raigarh granites.

3. Present study

3.1 Field geology

Dykes were observed to be vertical to steeplyinclined and commonly intruding the neighbouring

granitic plutons; however, at times, country rockswere not exposed. The studied dykes trend inE–W direction (Bgure 2a–c). Most of the dykeswere not identiBed in the satellite images due tothick vegetation cover. At many instances,dolerite exposures were seen in contact withquartz veins, indicating that the studied dykesmight have experienced hydrothermal activities.The average length and width of studied dykeswere 500–650 and 20–30 m, respectively. Dykeshave chilled margins or sharp contact with theadjoining country-rocks, indicating a rapidcooling of magma pulses. There was a recog-nisable variation in grain size from margin toaxis of dyke.

3.2 Petrography

Under a microscope, the studied dykes showconsistent variations in textures, with limitedvariations in mineral assemblages (Bgure 3a–e).The studied samples mainly consist of subhedralto anhedral plagioclase and clinopyroxene grainsin variable proportion and subordinate amountof orthopyroxene and amphibole. Lamellartwinning is commonly seen in plagioclase; how-ever, smaller grains lack this feature. Patches ofpyroxene are seen to occur in plagioclase andvice versa bounded by opaques. Arrangement oflath-shaped plagioclase, partially enclosed bysmaller pyroxene crystals forms a sub-ophitictexture (Bgure 3c and d). Anhedral-shapedclinopyroxene grains seem to be more dominant,but noticeably smaller in comparison to plagio-clase crystals (Bgure 3d). The microstructuralarrangements between smaller phenocryst ofclinopyroxene, plagioclase crystals, and glassygroundmass (black colour Fe-rich) give rise toan intergranular texture (Bgure 3e). In the caseof dykes, extreme variations in textures could bedue to: (1) increase in cooling rate from middleto chilled margin of the dyke, and (2) changes incooling rates with depth. Since all the sampleswere collected from the middle part of dyke, itseems that observed variations might havearisen due to magma crystallisation at differentdepths with varied fractionation. Evidence ofhydrothermal activities was observed in the formof quartz veins in the Beld (Bgure 2d), anduralitisation (Bgure 3f) of pyroxene crystals toamphibole.


4. Geochemistry

4.1 Methodology and analytical techniques

In total, 18 samples from seven sites were collected.The samples were collected from the middle ofdykes to avoid the eAect of alteration and con-tamination. Fourteen fresh samples were selected

for geochemical analysis after careful screening ofsamples. Hand specimens were nearly homoge-neous in physical appearance, even though undermicroscope, variations in terms of textures andmineralogy (see section 5.1) were observed. Rocksamples were initially broken to small chips using asledgehammer and were washed adequately beforeprocessing. The chips were crushed in jaw crusherand subsequently pulverized to 200 mesh size usinga ring grinder. After processing each sample, thejaw crusher was cleaned by de-ionised water. Loss-on Ignition (LOI) test was performed on eachsample to determine the presence of moisture.Three grams of powder was taken from each sam-ple into crucibles, then heated for 30 min at 900�C.Samples were cooled in a desiccator at room tem-perature. Then the sample weight was calculatedand deduced from 3 gm to give rise to moisturecontent.Major element chemistry was determined by

Wavelength Dispersive X-Ray FluorescenceSpectrometry (WD-XRF) using Philips MAGIXPRO model 2440 at CSIR- NGRI, Hyderabad.Samples were prepared as pressed pellets formajor element analysis. The measured time limitfor analysis of each sample was 20 min (Krishnaet al. 2007). Approximately, 2 gm of powder fromeach sample was taken into aluminium cups,Blled with boric acid, and pressed for about 20sec at 25 tonnes in a hydraulic pellet pressinginstrument. DNC* was used as reference materialfor major oxides with refrence standard deviation(RSD) value \2%. The trace and rare earth ele-ments (REEs) concentrations were determined byHigh Resolution Inductively Coupled PlasmaMass Spectrometry (HR-ICP-MS: NU instrumentthe UK) at CSIR-NGRI, Hyderabad. The pow-dered samples were digested by a close vesseldigestion method. Each sample powder of 0.5 gmwas added with 10 ml acid solution of HF andHNO3 (in 7:3 ratio) into a Savillex vial and thenheated at 150�C for 48 hrs. Later, one or twodrops of perchloric acid (HClO4) was mixed andthen again heated approximately for 1 hr at150�C until all the liquid was dried up. Then, a20-ml solution with an equal proportion of HNO3

and Millipore water was added to the residue andthen heated for 50–55 min at 80�C, so that allthe suspended particles are dissolved completely.This solution was then taken into 250 ml stan-dard Cask with a 10 ml solution with an equalproportion of HNO3 and Millipore water. Fivemillilitre Rhodium (Rh) was added as an internal

Figure 2. (a–c) Field photographs of E–W oriented maBcdykes from the Chotanagpur Gneissic Complex.


standard and diluted to 250 ml by adding Milli-pore water. Further, a part of 5 ml of this dilutedsolution was again diluted to 50 ml by addingMillipore water, and the Bnal solution was takeninto the Eppendorf tube for trace elements andREE analysis. BHVO-1 was used as referencematerial for trace and rare earth elements withrefrence standard deviation (RSD) value close to2%. The major elements, trace elements, andREE concentrations along with LOIs are reportedin table 2.

4.2 Results

All the studied maBc dyke samples have a low LOI\1%. Dyke samples have significant variations inMg# –(Mg/(Mg2+ + Fe2+)˝, from 21.13 to 68.5.Based on Mg#, the dykes are classiBed into twogroups, viz., low Mg# (21–50) and high Mg#(64–69) (Bgure 4a). Generally, wide variations inmajor oxide chemistry are observed, which alsoreCect in their CIPW norm calculation (table 2).CGC-1, CGC-6, -10 and -11, are olivine normative;

b

c

e

a

d

f

Figure 3. Photomicrographs of studied maBc dyke samples from the CGC: Images (a–e), mineralogically are almost similar asthey consist of clinopyroxene, orthopyroxene plagioclase, and Fe-rich groundmass, but there is a continuous decrease in grainsize, indicating cooling of magma at different depth; (f) Presence of hornblende show evidence of uralitisation (deutericalteration) in the response of hydrothermal process.


Table

2.Major

andtraceelem

entconcentrationof

representative

samples

from

theCGC

dykes.

Sample

no.

CGC-1

(Type-2)

CGC-2

(Type-1)

CGC-3

(Type-1)

CGC-4

(Type-1)

CGC-5

(Type-1)

CGC-6

(Type-1)

CGC-7

(Type-1)

CGC-8

(Type-1)

Latitude

24�030 03.66100

24�010 3.72100

24�010 3.72100

23�570 43.16400

23�570 53.76600

23�550 04.29000

23�580 27.32000

24�030 03.66100

Longitude

85�540 36.57500

85�580 12.92300

85�580 12.92300

86�040 18.61700

86�040 15.16800

86�060 35.30800

86�000 31.90600

85�540 36.57500

Trend

E–W

E–W

E–W

E–W

E–W

E–W

E–W

E–W

Major

oxides

(wt.%)

SiO

245.48

49.08

51.39

54.03

50.76

46.99

49.79

48.97

TiO

20.19

0.98

1.12

1.48

0.91

0.66

0.68

0.74

Al 2O

311.54

12.27

11.81

11.66

13.15

11.06

11.70

12.41

Fe 2O

3T

9.34

14.98

13.76

13.35

16.38

11.54

12.47

12.23

MnO

0.16

0.19

0.18

0.18

0.19

0.22

0.193

0.19

MgO

19.06

7.09

7.89

6.51

5.11

13.54

10.37

9.71

CaO

12.76

10.73

10.03

9.63

9.79

13.13

10.81

11.12

Na2O

0.99

1.77

1.91

2.09

2.16

1.03

2.00

1.78

K2O

0.10

0.63

0.59

0.50

0.60

0.53

0.89

0.90

P2O

50.06

0.36

0.43

0.58

0.332

0.3

0.14

0.14

Total

99.71

98.13

99.14

100.04

99.42

99.04

99.08

98.11

Mg#

63.74

28.99

33.08

29.6

21.19

50.29

41.75

40.64

LOI

0.5

0.3

0.4

0.4

0.35

0.01

0.18

0.55

CIP

Wnorm

Q9.62

11.33

16.19

12.9

3.48

3.88

Or

0.59

3.78

3.49

2.95

3.6

3.19

5.26

5.38

Plg

35.13

38.66

38.14

38.71

42.73

32.74

37.31

38.31

Di

28.16

19.11

17.05

14.51

15.29

29.49

23.95

22.76

Hy

6.19

8.83

11.75

9.49

5.64

19.6

14.72

13.66

Ol

19.78

0.32

Il0.34

0.43

0.41

0.41

0.41

0.49

0.41

0.43

Hm

9.35

14.99

13.76

13.35

16.39

11.54

12.48

12.24

Ap

0.14

0.86

11.34

0.76

0.7

0.35

0.35

Spn

0.02

1.88

2.22

3.11

1.73

0.98

1.14

1.29

Analyte

Sc

27.1816

40.97

42.50

35.45

34.35

42.67

41.15

23.16

V90.41

292.88

187.57

250.26

345.93

238.08

251.82

231.94

Cr

498.18

254.68

1250.47

153.43

237.64

508.58

297.66

229.90

Co

52.75

46.67

35.34

29.73

52.44

41.80

36.80

36.25

Ni

210.47

75.40

142.25

45.55

61.22

106.83

83.17

71.03

Cu

59.95

66.46

91.69

31.07

61.70

18.61

56.76

46.90

Zn

149.27

221.19

92.67

118.05

121.93

101.19

171.15

125.16

Ga

14.90

19.72

11.98

13.51

23.28

15.62

13.27

13.76

Rb

6.04

9.41

7.48

10.18

9.70

8.76

25.96

18.24


Table

2.(C

ontinued.)

Sample

no.

CGC-1

(Type-2)

CGC-2

(Type-1)

CGC-3

(Type-1)

CGC-4

(Type-1)

CGC-5

(Type-1)

CGC-6

(Type-1)

CGC-7

(Type-1)

CGC-8

(Type-1)

Sr

184.02

233.06

169.40

275.94

296.61

248.19

154.91

120.17

Y12.28

28.77

13.41

27.50

29.92

17.74

20.91

14.39

Zr

150.26

185.13

100.27

76.36

227.93

140.33

86.47

113.12

Nb

3.99

6.95

2.97

6.37

8.28

5.03

4.30

4.93

Cs

0.11

0.14

0.08

0.06

0.15

0.12

0.08

0.07

Ba

81.76

138.33

65.79

99.64

179.94

87.10

97.06

82.33

La

7.63

14.08

5.77

10.39

16.42

8.97

6.92

5.69

Ce

17.45

34.11

13.83

26.30

39.15

21.50

16.99

13.48

Pr

2.26

4.73

1.89

3.82

5.33

2.95

2.39

1.82

Nd

8.86

20.14

7.92

17.69

22.46

12.40

10.56

7.88

Sm

1.92

4.76

1.95

4.68

5.30

2.94

2.83

2.12

Eu

0.52

1.53

0.65

1.55

1.56

0.86

1.03

0.71

Gd

2.47

5.36

2.39

4.71

5.84

3.42

3.36

2.64

Tb

0.32

0.83

0.37

0.81

0.86

0.51

0.59

0.42

Dy

1.93

4.81

2.24

4.85

5.05

2.94

3.60

2.59

Ho

0.40

0.99

0.47

0.97

1.05

0.61

0.75

0.54

Er

1.16

2.60

1.23

2.42

2.74

1.62

1.90

1.35

Tm

0.17

0.37

0.17

0.33

0.40

0.23

0.26

0.19

Yb

1.14

2.40

1.11

2.05

2.57

1.48

1.61

1.17

Lu

0.16

0.35

0.16

0.29

0.37

0.21

0.23

0.17

Hf

3.62

4.44

2.45

1.89

5.57

3.48

2.16

2.82

Ta

0.46

0.57

0.30

0.43

0.70

0.39

0.33

0.41

Pb

2.87

4.01

2.34

2.17

2.58

2.35

2.78

3.01

Th

0.75

1.08

0.52

0.60

1.41

0.75

0.51

0.58

U0.33

0.42

0.22

0.19

0.54

0.32

0.21

0.27

Zr/Y

12.23

6.43

7.47

2.77

7.61

7.91

4.13

7.86

Dy/Yb

1.13

1.34

1.34

1.58

1.31

1.33

1.49

1.47

Tb/Yb

1.29

1.57

1.54

1.81

1.53

1.58

1.67

1.64

Dy/Dy*

0.39

0.51

0.57

0.68

0.47

0.49

0.73

0.66


Table

2.(C

ontinued.)

Sample

no.

CGC-10(T

ype-2)

CGC-11(T

ype-2)

CGC-12(T

ype-1)

CGC-13(T

ype-1)

CGC-14(T

ype-1)

DNC*

Latitude

24�010 3.72100

23�570 43.16400

23�570 53.76600

23�550 04.29000

23�580 27.32000

Longitude

85�580 12.92300

86�040 18.61700

86�040 15.16800

86�060 35.30800

86�00’31.90600

Trend

E–W

E–W

E–W

E–W

E–W

Major

oxides

(wt.%)

SiO

247.32

47.85

49.30

47.56

49.45

47.08

TiO

20.724

0.57

0.90

0.90

0.88

0.48

AL2O

38.50

7.62

10.74

14.40

12.13

18.22

Fe 2O

3T

9.43

8.09

10.57

12.97

10.30

9.91

MnO

0.178

0.164

0.18

0.19

0.17

0.15

MgO

19.68

20.42

12.59

8.71

11.46

10.08

CaO

13.22

12.86

12.16

12.34

12.51

11.22

Na2O

0.93

0.92

1.32

1.76

1.51

1.79

K2O

0.15

0.13

0.46

0.29

0.41

0.28

P2O

50.08

0.09

0.17

0.15

0.14

0.08

Total

100.25

98.75

98.44

99.31

99.01

99.9

Mg#

64.27

68.52

50.65

36.66

48.95

LOI

0.5

0.55

0.45

0.05

0.8

DNC*

CIP

Wnorm

calculation

s

Q3.51

3.58

3.41

Or

0.95

0.77

2.78

1.71

2.42

Plg

26.44

24.09

33.19

45.49

37.89

Di

34.73

35.44

27.07

21.22

26.17

Hy

12.58

15.7

18.81

11.86

16.41

Ol

14.27

13.15

Il0.39

0.34

0.39

0.41

0.39

Hm

9.44

8.09

10.58

12.98

10.31

Ap

0.21

0.23

0.39

0.37

0.35

Sp

1.27

0.96

1.74

1.68

1.66


Table

2.(C

ontinued.)

Analyte

CGC-10(T

ype-2)

CGC-11(T

ype-2)

CGC-12(T

ype-1)

CGC-13(T

ype-1)

CGC-14(T

ype-1)

BHVO-1

(CV)

BHVO-1

(OV)

Sc

47.16

30.33

41.60

44.96

38.81

31.8

32.40

V201.87

136.43

226.13

261.16

250.14

317

320.70

Cr

1300.20

1145.63

692.00

339.90

312.45

289

292.23

Co(59)

36.46

32.81

47.36

49.74

37.67

45

45.67

Ni

169.08

180.46

110.75

93.59

86.53

121

121.78

Cu

123.33

111.94

74.33

75.56

71.20

136

136.75

Zn

198.40

206.93

213.92

104.52

142.07

105

105.85

Ga

10.16

8.36

19.13

19.35

13.24

21

21.44

Rb

9.24

8.41

8.69

7.30

5.57

11

11.11

Sr

132.16

101.62

179.74

165.05

137.80

403

410.10

Y12.10

8.46

24.99

27.64

19.82

27.6

28.03

Zr

70.89

70.10

180.23

168.97

89.34

179

182.43

Nb

2.26

2.16

6.32

6.30

4.30

19

19.25

Cs

0.06

0.05

0.12

0.13

0.06

0.13

0.13

Ba

54.00

51.19

115.72

119.26

85.87

139

140.61

La

4.57

3.75

12.46

11.30

6.62

15.8

15.91

Ce

10.99

8.89

29.61

27.14

16.28

39

39.26

Pr

1.51

1.20

4.02

3.73

2.29

5.7

5.76

Nd

6.44

4.99

16.48

15.80

10.16

25.2

25.53

Sm

1.72

1.27

3.95

3.95

2.73

6.2

6.21

Eu

0.63

0.40

1.04

1.25

0.99

2.06

2.08

Gd

2.06

1.60

4.63

4.80

3.24

6.4

6.42

Tb

0.33

0.24

0.72

0.76

0.57

0.96

0.95

Dy

2.10

1.44

4.30

4.69

3.47

5.2

5.22

Ho

0.43

0.30

0.88

0.97

0.72

0.99

1.00

Er

1.10

0.80

2.34

2.55

1.81

2.4

2.41

Tm

0.15

0.11

0.33

0.36

0.25

0.33

0.32

Yb

0.98

0.72

2.10

2.33

1.59

2.02

1.99

Lu

0.14

0.10

0.30

0.33

0.23

0.291

0.29

Hf

1.72

1.71

4.49

4.23

2.22

4.38

4.39

Ta

0.20

0.21

0.53

0.56

0.42

1.23

1.21

Pb

3.32

3.57

3.75

2.51

2.18

2.6

2.60

Th

0.39

0.35

0.99

0.95

0.49

1.08

1.07

U0.16

0.15

0.41

0.39

0.21

0.42

0.41

Zr/Y

5.85

8.28

7.21

6.11

4.50

Dy/Yb

1.43

1.32

1.37

1.34

1.46

Tb/Yb

1.55

1.53

1.57

1.49

1.63

Dy/Dy*

0.66

0.56

0.52

0.60

0.73

Q=

Quartz,

Or=

Orthoclase,Plg

=Plagioclase,Di=

Diopside,Hy=

Hypersthene,

Ol=

Olivine,Il=

Ilmenite,

Hm=

Hem

atite,Ap=

Apatite,Spn=

Spinel,T=

standsfortotal

FeasFe 2O

3,Mg#=

Mg/(M

g+Fe)

9100,Dy/Dy*=Dy/–(La+Sm)/2˝.


20 40 60 800.4

0.8

1.2

1.6

2 eCaO /Al2O3

Pl

Ol

Cpx

Mg#20 40 60 80

1

2

3

4Na2O+K2O (wt%) d

Mg#20 40 60 80

8

12

16

20Fe2O3 t (wt%) f

Mg#

20 40 60 800

0.2

0.4

0.6 P2O5 (wt%)

Mg #

g

20 40 60 804

6

8

10

12

14Nb/Th

Mg#

Fractional crystallization

Crustal

conta

miantio

n

h

20 40 60 800

400

800

1200

1600

2000

Mg#

Cr (ppm)

ol

cpx

1 10 100 1000 100001

10

100

1000

Cr (ppm)

V (ppm)

ol

cpx

k

0 1 2 3 4 50

0.4

0.8

1.2

1.6TiO2 (wt%)

Fe2 O

3 / MgO

I

j

40 42 44 46 48 50 52 54 56 58 6010

20

30

40

50

70

80

Mg#

high Mg#

low Mg#

40 600

4

8

12

16

Low Mg#

High Mg#

NGRI lab standard

fiodite

TephriteOl<10%BasaniteOl>10%

phono-tephrite

tephri-phonolite

phonolite

trachy-andesite

basaltictrachy-andesitetrachy-

basalt

trachiteQ<20%trachydaciteQ>20%

BasaltBasalticandesite andesite dacite

Na2 O

K2O

(wt%

)

Alkaline

Subalkaline

b

SiO 2 (wt%)

a

Picrobasalt

60

50

+

SiO 2 (wt%)

FeOT

N a2 O+K

2 O MgO

Calc-Alk

D

A

ABBD

AT

ABTFB

BT

PrimtiveEvolved

Tholeiite

chigh Mg# dykes low Mg# dykes

Figure 4. Rock, series classiBcation and geochemical variation diagrams for the CGC maBc dykes: (a) atomic Mg number vs.silica plot; note dotted lines discriminate low Mg# low SiO2, high Mg# low SiO2; note orange circle and green square stand forand low Mg# low and high Mg#, respectively. (b) Total alkali–silica (TAS) diagram; note dotted red line discriminate betweenalkaline and sub alkaline basalt Beld (Irvine and Baragar 1971). (c) Triangular AFM plot, discriminate between tholeiitic andcalc-alkaline magma series based on oxygen fugacity (Irvine and Baragar 1971). (d–k) Geochemical variations plot for CGCdykes; (d) Mg# vs. Na2O+K2O, (e) Mg# vs. CaO/Al2O3, (f) Mg# vs. Fe2O3t, (g) Mg# vs. P2O5, (h) Mg# vs. Nb/Th, (i)Mg# vs. Cr, (j) Fe2O3/MgO vs. TiO2, and (k) Cr vs. V. Abbrevations. A: andesite, B: basalt, D: dacite, AB: basaltic andesite,AT: tholeiitic andesite, BT: tholeiitic basalt, FB: ferro-basalt, ABT: tholeiitic basaltic andesite.


however, CGC-6 has significantly less concentra-tion (0.32%) of olivine in comparison to the otherthree samples (13–19% olivine), whereas rest of thesamples are quartz normative (table 2).The CGC dykes are mainly classiBed as basalt,

as they contain 45–54 wt.% SiO2 and 1.05–7.20wt.% Na2O + K2O on TAS diagram. However, onesample (CGC-4) is of basaltic andesitic composi-tion (Bgure 4b; Irvine and Baragar 1971). All thestudied samples show an aDnity of tholeiitic natureon the AFM diagram (Bgure 4c; Irvine and Baragar1971). Further, low Mg# dykes have varied TiO2

(0.42–1.1 wt.%), P2O5 (0.12–0.58 wt.%), Fe2O3

(10.57–16.38 wt.%), moderately varied CaO(9.63–13.13 wt.%), and limited content of MnO(0.16–0.18 wt.%), Al2O3 (11–13.1 wt.%). Similarly,high Mg# dykes have varied TiO2 (0.19–0.72wt.%), P2O5 (0.063–0.095 wt.%), and limitedMnO (0.16–0.17 wt.%), CaO (12.76–13.22 wt.%),but they show moderate variations in Al2O3

(7.62–11.54 wt.%), and Fe2O3 (7.62–9.34 wt.%)(table 2). To examine the differentiation trend,some major oxides, minor oxides, and trace ele-ments are plotted against Mg# (Bgure 4d–k).Commonly, in all cases, the fractionation trendsare well deBned.The chondrite normalised rare earth elements

(REEs) pattern for low Mg#, and high Mg# dykesare characterised by enrichment of LREE(Bgure 5). The La and Yb range for low Mg# dykesvaries between 24–70 and 9–17, respectively,whereas for high Mg# dykes, it varies between20–40 and 6–12, respectively (Bgure 5 and table 2).Eu and Gd show minor negative and positiveanomalies, respectively. The primitive mantlenormalised spider diagram show enrichment forhighly incompatible elements in both low Mg# and

high Mg# dykes, with variations among the sam-ples as well as in groups (Bgure 6a, b and table 2).The low Mg# dykes mark with contrasting anddistinct pattern in comparison to high Mg# dykes,and display strong negative anomaly for Ti, amoderate negative anomaly for Nb and Th, a lessmoderate negative anomaly for Sr and positiveanomaly for Rb, and K. Whereas, majority ofsamples show negative anomaly for P, but sampleCGC-2, 3, and 4 are marked with positiveP-anomaly. In contrast, low Mg# dykes, particu-larly one sample (CGC-1), showed positive anom-aly for Th, Hf, and Zr and strong negative anomalyfor Ti and moderate for U, K, and P, while othertwo samples (CGC-10,11) showed moderate posi-tive anomaly for Th and Zr and negative anomalyfor K.

5. Discussion

5.1 Crustal contamination

Possibility of crustal contamination is least in thedykes unless the samples are collected from dykemargin (Gill and Bridgwater 1979; Kalsbeek andTaylor 1986; Tarney and Weaver 1987). To assessthis possibility, we have used several trace elementratio plots (Bgure 7a–c). Overlapping SiO2 rangefor some selective samples against variable Mg#(Bgure 4a), minor Zr positive anomaly and nega-tive Sr anomaly (Bgure 6) suggest the crustalcontamination from upper crust as shown in rocksof eastern Dharwar craton by Pandey and Chala-pathi Rao (2019). Enrichment of Rb and K in a lowMg# sample (e.g., CGC-2 and CGC-5 with Mg#28.99 and 21.19, respectively) is due to the accu-mulation of amphibole (Bgure 3f) in the response of

1

10

100

La ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

OIB

E-MORB

Sam

ple/

Chon

drite

low Mg# dykeshigh Mg# dykes

Figure 5. The chondrite normalised Rare earth element spider diagram for low Mg#, and high Mg#. Chondritic, OIB, andE-MORB values are taken from Sun and McDonough (1989).


hydrothermal alteration. On La/Nb vs. Th/Nbdiagram (Zamboni et al. 2016, Bgure 7a), all thesamples closely lie to the line that representsrock/magma–Cuid interaction, corroborates thepresence of Cuid related enrichment in trace ele-ments in the mantle source regions. Or, alterna-tively it is possible that the presence of traceelements enrichment could be the resulatant ofcrustal contamination. Negative Nb anomaly (lowMg# dykes; Bgure 6b) indicates the involvement ofcrustal material and likely points that magmawould have got contaminated before the emplace-ment and hence indicating the probable chemicalheterogeneity within the mantle (Dostal et al. 1986;Sun and McDonough 1989). The anomalouslyenriched concentration of Th and U in the conti-nental crust can be used to trace crustal input intomantle-derived material. Th/Nb vs. Nb/U plot(Rudnick and Gao 2003; Bgure 7b), particularly tothose of high Mg# low SiO2 (high Mg# dykes),shows contamination with lower crustal material(Th/NbPM = 2; Nb/U= 25 for the lower conti-nental crust; Bgure 7b). On Th/Yb vs. Nb/Yb plot(Pearce 2008; Bgure 7c), samples of both groupsplot above MORB-OIB array, between E-MORBand OIB region, but having close aDnity to OIB,implying crustal input into the dyke magma en-route to a shallower depth. On Rb/Nb vs. Ba/Thratio plot (Weaver 1991), dyke samples plot close

to enrich mantle 1 Beld with close aDnity to con-tinental crust suggesting incorporation of sedi-mentary components into dykes derived fromcontinental crust. Similarily, on Ba/Nb vs. Ba/Laratio plot, dyke samples plot close to enrich mantle1 (Bgure 7e) but on the contrary, do not have closeaDnity to continental crust, it is because of therelatively less incompatiable nature and also lowconcentration of La in continental crust in com-parison to Th. Consequently, based on the abovediscussion, a reasonable inference can be made thatstudied dyke samples may have been aAected bycrustal contamination during their emplacement.

5.2 Magmatic evolution and the role of crystalfractionation

The extreme variability of Mg#, a negative cor-relation of Na2O + K2O and Al2O3 against Mg#,Ni (61–180), Cr (153–1300) variability preclude thepossibility that the studied dyke samples areequivalent to primary magma composition. Minornegative and positive anomalies for Eu and Gdindicate removal and accumulation of plagioclaseand hornblende, respectively. On the CaO/Al2O3

plot (Bgure 4e), the concave uptrend for low Mg#dykes implies clinopyroxene and plagioclase frac-tionation. In contrast, the near-vertical trend forhigh Mg# dykes mainly indicates plagioclasefractionation with no or very less clinopyroxenefractionation. The negative trend of Fe2O3 againstMg# (Bgure 4f) is due to the accumulation of iron-rich minerals. The low Mg# dykes distinctivelyshow an almost similar trend for P2O5 (Bgure 4g),possibly ascribed to varied apatite concentration inthe dyke magmas. Although apatite is not a ubiq-uitous feature within maBc magmas, it occurs as aminor phase (e.g., Bushveld and Stillwater com-plex in south Africa; Piccoli and Candela 2002). OnTiO2 vs. Fe2O3/MgO (Bgure 4j), the concentrationof both the oxides is relatively rich in log Mg#dykes indicating more fractionated nature of thedykes compared to high Mg# dykes. However,some overlapping for TiO2 among selected samplesof low Mg# dykes (CGC-6, CGC-7, CGC-8, andCGC-9) and high Mg# dykes (CGC-10 and CGC-11) is observed. Figure 4(e, i, j, and l) indicatedtowards olivine and clinopyroxene fractionation indykes.Elemental index or element ratio plots seem to

be more accurate in comparison to oxides tounderstand the fractionation trend as the element

11

10

Rb Ba Th U K Nb La Ce Sr Nd P Hf Zr Sm Ti Tb Y

Sam

ple/

Prim

ie

man

tle

1000.1

1

10

100

OIB

E-MORB

a

b

high mg# dykes

low mg# dykes

Figure 6. The Primitive normalised multi-element spiderdiagram for (a) low Mg# and (b) high Mg#. The primitivemantle, OIB, and E-MORB values are taken from Sun andMcDonough (1989).


ba

0 2 4 6 80

0.4

0.8

1.2

1.6

2

La / Nb

Th/N

b

UM

0.1 1 10 1000.01

0.1

1

10

N-MORB

E-MORB

OIB

Nb/Yb

Cru

stal

inpu

t

MORB-OIB array

Th/Y

b

depleted mantle

enriched mantle

Rock /Magma-Fluid interaction

Cont

amin

atio

n by

mel

tc

Si/K

0 50 100 150 200 250 3000

100

200

300

400

slope= 1.4035intercept= -11.128r2 =.9965

{.5(F

e+M

g)+2

( Ca+

3Na)

}/k

Si/K

plagio

clase

& ol

ivine

fracti

onati

on

g

0 50 100 150 200 250 3000

100

200

300

400

[.25A

l+(M

g+Fe

)+1.

5Ca+

2.75

Na]

/K

Si+1.5Ti/K


ol+cp

x+plg

fracti

onati

on

h

0 50 100 150 200 250 3000

100

200

300

400


{.5( F

e+M

g)+2

( Ca+

3Na)

} / k

plagio

clase

& ol

ivine

fracti

onati

on

f

high mg# dykeslow mg# dykes

0.1 1 10

40

80

120

160

200

EMII

Rb/Nb

Ba/

Th

EMI

HIMU N-MORB

Primordial mantle

Continental crust

d

1 10 100

20

5

10

15

25

30

EMII

Ba/Nb

Ba/

La EMI

HIMUN-MORB

Primordial mantle

Continental crust

e

bb

0 0.4 0.8 1.2 1.6 20

10

20

30

40

50

Increasing degree of crustal contamination

OIB and N-MORB

Th/Nb

Nb/

U LCC

UCC

Figure 7. Trace element diagrams to trace crustal input and Pearce element ratio diagrams: (a) La/Nb vs. Th/Nb plot (afterZamboni et al. 2016), discriminate melt vs. Cuid component; (b) primitive normalised Th/Nb vs. Nb/U plot compared with thevalue of lower continental crust and upper continental crust (Sun and MacDonough 1989; Rudnick and Gao 2003); (c) Nb/Yb vs.Th/Yb plot (Pearce 2008); (d, e and f) Pearce element ratio diagrams for olivine, plagioclase, and clinopyroxene fractionation(Russel and Nicholls 1988; Nicholls and Russel 2016). Abbreviations. r: correlation coefBcient, UM: upper mantle, DMM:depleted mantle melt, LCC: lower continental crust, UCC: upper continental crust, OIB: ocean island basalt, E-NORM: enrichedmid-oceanic ridge basalt, MORB: normal mid-oceanic ridge basalt.


concentration is a more sensitive parameter for thefractionation (Russel and Nicholls 1988). There-fore, a better way to model the fractionation sys-tematics is by Pearce element ratio plot (Russeland Nicholls 1988; Nicholls and Russel 2016),which are specifically designed to check the frac-tionation of any particular mineral or minerals. Inthis plot ratio, stoichiometric proportions of cer-tain major elements are plotted along the ordinateaxis with respect to conserved elements (e.g., K, P,Zr) plotted along the abscissa. The denominator ofthe ratio is always conserved elements that remainto be unchanged during magma evolution andreduces the eAect of closer problems so that thefractionation pattern can be understood in absoluteterms. Olivine, plagioclase, and clinopyroxene arepossible phases that have been fractionated(Bgure 7d–f). It is interesting to note that highMg# dykes should have plotted somewhere nearthe origin of the fractionation trend line due to highMg#, but instead, they plot towards the end of thetrend line. Therefore, it could be misinterpreted interms of fractionation order. However, this reversepattern is due to the more rigorous interaction ofMg# enriched samples with crustal material thatincreases the silica concentration in low Mg#dykes in comparison to those of high Mg# dykes.

5.3 Partial melting conditions and mantlesource

The slope of the REE pattern is a useful qualitativecriterion to understand the degree of melting(Fram and Lesher 1993; Hirschmann et al. 1998),which in turn, can comment on the source (en-riched or depleted?). A low degree of partialmelting produces an inclined pattern (high LREE/HREE), indicative of enriched mantle source(OIB). In contrast, Cat pattern (LREE/HREEclose to 1) produces a high degree of partial meltingand suggests magma derivation from the depletedsource. With different REE patterns and REEcontent (Bgure 5a and b), the two categories ofdyke samples likely represent their genesis fromdistinct chemical sources or by different degrees ofpartial melting of a single source. Similarly, sig-nificant variations in Zr/Y (4.13–12.23; table 2)can not only be exempliBed by evolutionarymagma processes (e.g., fractional crystallisationand partial melting), but also could be ascribed todistinct chemical source or varying degree of par-tial melting. Hence, in the light of many arguments

as discussed above, it is vital to know the validityof these arguments. In the subsequent paragraph,we discuss the trace element modelling based ondata of this study, to decipher the source charac-terises for the CGC dykes.Trace element distribution in an igneous rock

system is an important tool to evaluate its sourceand melting conditions. Although, according toSaccani et al. (2018), it is not entirely possible toquantify the melting processes as the compositionof mantle source is not well understood or cannotbe modelled in absolute terms; however, semi-quantitative modelling of trace elements providesreasonable understanding about the melting con-ditions. Modal or batch (equilibrium) meltingapproach is used here to constrain the partialmelting conditions for the dykes. Figure 8(a and b)shows the melting curve for both groups of dykes.It is inferred that low Mg# dykes were derived dueto the variable degree of melting that ranges from3% to 15% partial melting of garnet-spinel peri-dotite with source composition assuming primitivemantle. The garnet–spinel source consisted of

spinel-garnet peridotite

garnet peridotite

high Mg# low SiO2

1

2

1 - 3%2 - 8%3 - 12%4 - 15%

ol = 53%, opx= 25%, cpx= 15%, spn= 4%, grt=1

3

ol = 55%, opx= 20%, cpx= 22%, grt =3%

4

low mg# dykes

high mg# dykes (only CGC-10)

a

b

15%

1

10

1001

10

100

1

234

b

a

Figure 8. Semi-quantitative trace element modelling for CGCdykes. (a) The high Mg# dykes are derived by 15% of partialmelting within garnet stability Beld of peridotite mantle,consisted of ol=55%, opx=20%, cpx=22% and grt=3%. (b)Whereas, the high Mg# dykes are represented by variabledegree of partial melting (3–15%) within spinel–garnet con-taining peridotite mantle, consisted of ol=53%, opx=25%,cpx=15% spn=4% and grt=3% (for detailed discussion see thetext). (Abbreviations: ol: olivine, opx: orthopyroxene, cpx:clinopyroxene, spn: spinel and grt: garnet). Data used in traceelement modelling is taken from shellnutt et al. (2018).


olivine (53%), orthopyroxene (25%), clinopyroxene(15%), spinel (4%), and garnet (1%). Whereas, forhigh Mg# dykes, only one sample (CGC-10) wasselected to replicate the melting proportion due tomore contamination in other samples. This sampleis compatible with 15% partial melting within thegarnet peridotite consisting of olivine (55%),orthopyroxene (20%), clinopyroxene (22%), andgarnet (3%) assuming source composition primi-tive mantle. Other details used in the modelling aregiven in table 3.Varied fractionated nature of studied dyke

samples and wide variations in major oxides do notallow to make a direct comment on source chem-istry. But, high Mg# (63.7– 68.5), high Mg# dykesparticularly in two samples CGC-10 and CGC-11,show poor fractionation. Therefore, to some extent,these samples can be approximated to constrainthe primitive magma composition. The (Tb/Yb)Nratio [1.8 is representative of the garnet-freesource (Liao et al. 2019; Wang et al. 2002). Dykesin the present study have (Tb/Yb)N ratio1.29–1.81, pointing towards the genesis of thedykes’ magma within the garnet-rich source. It isalso possible that some dykes would have origi-nated from the spinel-garnet transition zone assome samples have (Tb/Yb)N close to 1.8 (e.g.,CGC-4 = 1.81 and CGC-7 = 1.67). The studieddyke samples show a close aDnity to E-MORB on(Gd/Yb)M vs. (La/Yb)M (Bgure 9a; Rogers et al.2018) and Th/Ta vs. La/Yb (Bgure 9b; Condie1989) variation plots. On (Gd/Yb)M vs. (La/Sm)Mplot (Bgure 9a, M stands for primitive normalised),all the samples plot in spinel Beld but remarkablycloser to garnet–spinel Beld boundary expect onesample (CGC-11), which plots in garnet composi-tion Beld. This phenomenon indicates that dykeswere generated either by the melting of spine-richmantle or melting from the spinel-garnet peridotitesource in the transition zone. The Dy/Dy* ratio(0.73–0.39) and intermediate-range of Dy/Yb ratio(1.69–2.23) possibly related to enriched source andderivation of magma from spinel–garnet transitionzone, respectively (Davidson et al. 2013, table 2).The TiO2/Yb vs. Nb/Yb plot (Bgure 10a) indicatesthe depth of melting where variance on TiO2/Yband Nb/Yb reCected in terms of garnet residue andsource, respectively (Pearce 2008). The distinctionbetween OIB and E-MORB is that OIB charac-terised by deeper melting with garnet residues,whereas E-MORB represented with shallowermelting with garnet. Accordingly, it can be inter-preted that the majority of the samples are

compatibles with shallower melting. Further, onRb/Na vs. Ba/Th and Ba/Nb vs. Ba/La plot(Bgure 7d and e; Weaver 1991) dykes have closeaDnity to enriched mantle-1 (EM-1) reservoirwhich also corroborate that the CGC dykes likelyto be derived from shallower melting event.

5.4 Tectonic settings

On the chondrite-normalised spider diagrams, theREE pattern for high Mg# dykes is consistent withE-MORB (Bgure 5a). However, certain REE, par-ticularly MREE and HREE, have a strong aDnityfor OIB (Bgure 5), as they display LREE enrich-ment (e.g., La/SmN=1.43–2.03 and La/YbN=2.98–4.56; table 2). The overall enrichment is15–70 and 10–25 times of chondrite abundance forLa and Yb, respectively. The low Mg# dykestypically consistent with average E-MORB pat-tern, with certain REE particularly HREE resem-ble OIB aDnity (Bgure 5). On (Gd/Yb)M vs. (La/Yb)M and Th/Ta vs. La/Yb variation plots bothtypes of dykes display close aDnity to E-MORB(Bgure 9a and b). On Nb/Y vs. Ti/Y plot(Bgure 10b), the majority of dyke samples plot inthe E-MORB Beld with a few plot away fromMORB Beld. Such behaviour can be attributed tocrustal contamination. On La/10–Y/15–Nb/8 tri-angular diagram (Bgure 10c; Cabanis and Lecolle1989), low Mg# dykes are compatible with theE-MORB except for one sample (CGC-6), whereashigh Mg# dykes still plot in the E-MORB Beld butclose to continental tholeiites and alkaline basaltBeld pointing towards more involvement of crustalcontamination.Various geodynamic models have been proposed

to understand the geological evolution of the CGC(Gupta and Basu 1982; Sarkar 1982; Mukhopad-hyay 1990; Mahato et al. 2008). Origin of the CGCis debatable, but it is widely regarded in the asso-ciation with Singhbhum Craton, where Singhbhumplate converges below the Chotanagpur regionresulting in the formation of Singhbhum MobileBelt (Sarkar 1982; Mahato et al. 2008). Accordingto Sarkar (1982), the three plate tectonic cycles,viz., Brst cycle (2000–1600 Ma), second cycle(1550–1170 Ma) and, third cycle (1000–850 Ma)describe the complete convergence of Singhbumand CGC. Accordingly, the evolution of thesemicrocontinental blocks led to the developmentof multiphase deformation, metamorphism, andsuperposed folding. The three-plate tectonic cycleswere intervened with the periods of dormancy,


Table3.Com

pilation

ofinform

ationusedin

geochemical

mod

elling.Values

ofdistribution

coefBcien

tsan

dsourcearetakenfrom

Shelln

uttet

al.(201

8)an

dthereferencestherein.

Distributioncoefficents

La

Ce

Nd

Sm

Eu

Tb

Dy

Er

Yb

Lu

ol

0.007

0.006

0.007

0.007

0.0068

0.01

0.013

0.011

0.049

0.0454

opx

0.0003

0.02

0.03

0.05

0.03

0.03

0.15

0.152

0.227

0.255

cpx

0.056

0.092

0.15

0.26

0.39

0.45

0.3

0.36

0.166

0.168

spn

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

grt

0.001

0.007

0.026

0.102

0.243

0.75

3.2

6.5

8.5

11

Concentrationoftrace

elem

ents

inthesource(C

O)

CO

0.648

1.675

1.25

0.406

0.154

0.099

0.674

0.438

0.441

0.0675

Type-1dykes

Bulk

distributioncoefficents

Do

0.012825

0.0226

0.03435

0.05585

0.07024

0.081

0.09015

0.09855

0.1091

0.11442

Degreeofpartialmelting

F3%

64.556949

52.741861

42.104577

31.052576

26.419731

22.87874

17.697309

13.912636

11.839537

10.668381

8%

29.811279

27.161657

23.932679

20.01257

18.078927

16.412612

13.695142

11.40913

10.00205

9.2336141

12%

20.661378

19.286692

17.386506

15.045374

13.76927

12.851297

11.936767

10.762154

10.047289

9.6828177

15%

17.000973

16.17718

14.918526

13.361874

12.537527

11.75961

10.401871

9.1132863

8.2167282

7.7705523

Type-2dykes

Bulk

distributioncoefficents

Do

0.01626

0.02775

0.04363

0.07411

0.10283

0.133

0.19915

0.31065

0.36387

0.44293

Degreeofpartialmelting

F15%

16.690029

15.76685

14.307146

12.458572

11.184123

10.062949

8.3110877

6.3917636

5.6481101

5.0475371

Equationusedin

calculationsis

equilibrium

orbatchmelting.CL/CO=1/F+(1�F)D;CL:Concentrationoftrace

elem

ents

intheliquid

phase,CO:Concentrationoftrace

elem

ents

inthesource,

F:Meltproportion,andD:Bulk

distributioncoefBcient.


during which upliftment, intrusions of maBc dykeswarms, erosion, and sedimentation took place. Inanother model, ensialic rifting was proposed tooccur between the Singhbhum and the CGC, inresponse to a mantle plume. This resulted in theemplacement of maBc magmatism and progressiverifting (e.g., incipient rifting), leading to the basininitiation. However, the dykes in the present caseare trending in E–W direction and does not showany signature of mantle plume. Thus, theemplacement of these dykes can be related to anepisode of passive extension without the involve-ment of a mantle plume. Further, the moreE-MORB aDnity of dykes in comparison to OIB,strongly empahsize the emplacement of the dykesinto rifting environment without the involvementof mantle plume.

6. Emplacement mechanism

In this section, we attempt to give a possible model(Bgure 11) for the emplacement of the studieddykes. The genesis of a dyke or a dyke swarm is afunction of several variables, including stress pat-tern, depth of magma, magma viscosity, the volu-metric Cow rate in the magma chamber, and dykedriving pressure (Becerril et al. 2013). Primarilydykes are the by-product of crustal extensionalevents and provide vital clues regarding crustalstabiliazation events, supercontinental assembly,dispersal, and crust-mantle interaction (Srivastavaet al. 2008). The maBc dykes are widespread and

episodic in precambrian continental crust repre-senting critical time and strain markers and con-sequently could provide possible palaeo-stressregime (Yellapa and Chetty 2008), if the dykessubsequenty have not been inCuenced by tectonicregime. The emplacement mechanism of dykes isa site-speciBc phenomena and largely controlledby tectonic regime of that particular area, e.g.,wheather the dykes are emplaced within tectonicplates or at divergent plate boundaries. Dykingevents in oceanic settings have less variations interms of geochemistry and mineralogy as thecomposition of oceanic crust more or less similar tomagma derived from deeper mantle reservoir.However, dykes in the continental settings geo-chemically and mineralogically are moreheterogenous because magma needs to travelthrough entire crustal thickness and consequentlymore often than not dykes show signatures ofcrustal contamination as is the case with thepresent dykes.The dykes represent the Blling of magma into

pre-existing vertical or sub-vertical fractures rela-ted to extensional stress within the crust (Gud-mundsson 2012). Moreover, there would have beenfurther initiation of new fractures or widening ofpre-existing fractures due to magma driving force.In rifting environment, for extensional fracturesuch as dykes maximum principal stress direction(r1) and minimum principle stress (r3) directionsare parallel and perpendicular to the strike direc-tion of fracture (Gudmundsson 2006, 2013),respectively. As a result, the orientation of dykes in

1 101

10

La/Yb0 1 2 3 4 5

1

2

3

4

(La / Sm)M

(Gd/

Yb) M

OIB

E-MORB

Garnet field

Spinel field

a

Th/T

a

OIBE-MORBPM

blow mg# dykes high mg# dykes

Figure 9. Mantle source diagrams for CGC dykes: (a) (Gd/Yb)M vs. (La/Sm)M variation diagram discriminate between spineland garnet mantle source (Rogers et al. 2018) (M= Primitive normalised); (b) Th/Ta vs. La/Yb variation plot (Condie 1989)showing E-MORB aDnity of CGC dykes. Primitive mantle (PM), E-MORB, and OIB values are taken from Sun andMcDonough (1989).


the Beld can be used to establish the principalstress direction, namely, r1 (maximum), and r3(least). Ghosh and Chatterjee (2008) suggestedthat several intra-continental rift zones predomi-natly determines the nature of magmatic events inCGC. In the present case, dykes are oriented in theE–W direction, meaning that there was a local orregional scale build-up of the tensional stress Beldin the E–W direction leading to propagation ofcracks within the brittle part of the Earth’s crust(Bgure 11). Because the dykes do not show anyradiating pattern as all the dykes are radiating inE–W direction, it is possible that the dykes couldbe related to an episode of passive extension.Trace element modelling indicates that magmas

likely originated from spinel–garnet transition zoneand garnet stability Beld for low Mg# and highMg# dykes, respectively. The fractional crystalli-sation of olivine, clinopyroxene, and plagioclase(Bgures 4e, h, i, j, l and 7d–f) would have occurredin the magma chamber itself or while magmas werenavigating to surface or in both (Bgure 10; rect-angle a). Whereas, extreme enrichment of Fe–Tioxides (Bgure 4k) indicates shallow level fraction-ation (Bgure 10; square b). Once the primitivemagmas Blled the existing fractures, it is a likelyscenario that dykes were contaminated by crustalmaterial, as discussed in section 6.1 (Bgure 7a–c;trace element ratios plot). The high Mg# dykes arecomparatively more contaminated than low Mg#dykes. The possible reason would be the interactionof higher content of crustal material with thesedykes as they are more primitive and therefore,represent high-temperature conditions.Consequently, the dyke magma would have

released more latent heat to the country rocksleading more melt into dyke magma. Finally,evolved magmas were intruded into the fractures inthe upper or lower parts of the crust and progres-sively cooled, leading to E–W oriented dykingevent (Bgure 10). Finally, it is possible that thesedykes have undergone post-magmatic alteration(Bgure 10, point 1) which is evidenced by Beldstudies, presence of quartz veins and also thepresence of amphibole in photomicrographs provesthe above interpretation.

7. Conclusions

The E–W trending Chotanagpur Gneissic Complexdykes are tholeiitic basalts. Fractionation of oli-vine, clinopyroxene, plagioclase, and Fe–Ti oxides

a

0.1 1 10 1000.1

1

10

Nb/Yb (ppm)

TiO 2

/Yb

Tholeiitic Alkaline

OIB

MORB array (shallow melting)

E-MORBN-MORB

OIB array (deep melting)

Plume-Ridge interaction

Calc-alkaline basalt Continental

tholeiites

Alkaline basalt

E-MORB

1. Volcanic Arc Tholeiites2. Tholeiites + Calc - alkaline3. Back Arc Basalts4.N-MORB

1

23

4La/10 Nb

/8

Y/15c

0.01 0.1 1 10

100

1000

10000

Mid-Oceanic ridge E-MORB

within-plateVolcanic arc

AlkalineSubalkaline Transitional

Nb/Y (ppm)

Ti/Y

(ppm

)

b

low mg# dykes high mg# dykes

Figure 10. Tectonic discrimination diagrams for CGC dykes:

(a) TiO2/Yb vs. Nb/Yb plot (Pearce 2008) indicating the

depth of melting; (b) Nb/Y vs. Ti/Y plot for tectonic settings

discrimination; and (c) triangular La/10–Y/15–Nb/8 diagram

for tectonic settings discrimination (Cabanis and Lecolle

1989).


largely control the mineralogical and geochemicalvariability within the CGC dykes, further crustalcontamination also contributes to the chemicalvariability of these dykes. The CGC dykes areemplaced into intra-continental rifting environ-ment having close aDnity to E-MORB basalt.Trace element modelling constrains the source asspinel–garnet peridotites rich lithosphere mantlefor low Mg# dykes with 3–15% partial melting,whereas high Mg# dykes were derived by 15%partial melting within garnet rich peridotitelithosphere mantle.

Acknowledgements

The authors are grateful to Dr V M Tiwari,Director, CSIR-National Geophysical ResearchInstitute, Hyderabad for his permission to publishthese results. The UGC-JRF supported in the form

of Fellowship to RP and AP, and the GEOMETproject funds were utilised to conduct Beldwork. DrM Ram Mohan is thanked for suggestions andfruitful discussions during the preparation of theMS. Drs M Satyanarayanan and A Keshav Krishnaare acknowledged for their guidance in the geo-chemical analytical work. We sincerely acknowl-edge the constructive comments by the Journalreviwers which improved the quality of the manu-script. Editorial handling by Prof N V C Rao isgratefully acknowledged. This study forms a partof the doctoral thesis of RP. This is CSIR-NGRIcontribution no. NGRI/Lib/2020/ Pub-185.

Author statement

Rahul Patel: Carried out the Beldwork; samplepreparation for geochemistry; data interpretationand initial and Bnal draft preparation. Ravi Shan-kar: Contributed in Beldwork; data interpretation

Moho

Rifting

spinel peridotite containing lithospheric mantle

Crystallization in magma chamber ? or while magma rising to surface

a

Country rocks(Granites)

Shallow level crystallization (e.g. Fe-Ti oxides)

b

ContaminationContamination

Surface

Hydrothermal alteration

1

Magma chamber

Garnet-spinel transition zone Magma reservoir

garnet peridotite containg lithospheric mantle

Asthenosphere

Not to scaleσ 1σ

3

E-W orie

nted

dyking ev

entN

S

E

W

Country rocks(Granites)

Figure 11. A conceptual mechanism for dyke emplacement in the central part of CGC. Melting initiated in the spinel–garnettransition zone for low Mg# dykes and the garnet stability Beld for high Mg# dykes, form a deep-seated magma reservoir,magma then fed to the magma chamber, subsequently upon emplacement of magma into fractures, crystallisation occurs:(a) Either in magma chamber or when magma installed into fractures and (b) shallow level crystallisation. Magma whileascending to surface, variably contaminated with crustal inputs. Dykes composition, to some extent, was modiBed byhydrothermal alteration once they were exposed to the surface.


and write-up of the initial and Bnal version.D Srinivasa Sarma: Designed the study; guided theanalytical work, contributed in the write-up of theBnal version, and overall supervision of the work.Aurovinda Panda: Contributed in Beldwork; datainterpretation and write-up of the initial and Bnalversion.

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Corresponding editor: N V CHALAPATHI RAO


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