geochemistry and petrogenesis of tholeiitic dykes from the
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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: dssarma@ngri.res.in
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 Page 2 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 Page 3 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)
160 Page 4 of 24 J. Earth Syst. Sci. (2021) 130:160
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.
J. Earth Syst. Sci. (2021) 130:160 Page 5 of 24 160
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.
160 Page 6 of 24 J. Earth Syst. Sci. (2021) 130:160
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.
J. Earth Syst. Sci. (2021) 130:160 Page 7 of 24 160
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
160 Page 8 of 24 J. Earth Syst. Sci. (2021) 130:160
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
J. Earth Syst. Sci. (2021) 130:160 Page 9 of 24 160
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
160 Page 10 of 24 J. Earth Syst. Sci. (2021) 130:160
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˝.
J. Earth Syst. Sci. (2021) 130:160 Page 11 of 24 160
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.
160 Page 12 of 24 J. Earth Syst. Sci. (2021) 130:160
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).
J. Earth Syst. Sci. (2021) 130:160 Page 13 of 24 160
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).
160 Page 14 of 24 J. Earth Syst. Sci. (2021) 130:160
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
slope= 1.2212intercept= -10.154r2 =.9947
ol+cp
x+plg
fracti
onati
on
h
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
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.
J. Earth Syst. Sci. (2021) 130:160 Page 15 of 24 160
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).
160 Page 16 of 24 J. Earth Syst. Sci. (2021) 130:160
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,
J. Earth Syst. Sci. (2021) 130:160 Page 17 of 24 160
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.
160 Page 18 of 24 J. Earth Syst. Sci. (2021) 130:160
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).
J. Earth Syst. Sci. (2021) 130:160 Page 19 of 24 160
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).
160 Page 20 of 24 J. Earth Syst. Sci. (2021) 130:160
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.
J. Earth Syst. Sci. (2021) 130:160 Page 21 of 24 160
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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