ORIGINAL PAPER

Geochemistry of the mafic dykes in parts of the Singhbhumgranitoid complex: petrogenesis and tectonic setting

Akhtar R. Mir & Shabber H. Alvi & V. Balaram

Received: 23 April 2009 /Accepted: 22 December 2009 /Published online: 26 February 2010# Saudi Society for Geosciences 2010

Abstract Singhbhum granitoid complex has been intrudedby numerous mafic dykes trending in different directions.The studied dykes were originated from subalkalinemagma, ranging in composition from basalt throughbasaltic-andesite to andesite. In the present work, thestudied dykes are divided into two groups on the bases ofabundance of SiO2, MgO, Al2O3, Ni, and Cr. The first onehas higher Mg# than that of group II. These dykes haveenriched incompatible trace element patterns. These areparticularly enriched in the light rare earth elements andlarge ion lithophile elements with depleted high fieldstrength elements (Nb, P, Ti). High Ba/Nb and Sr/P ratiosof present mafic dykes are the indications of subductionsignature. The geochemical characters of the back arcextension basalt tectonic setting is suggested for the studieddykes. Higher PM-normalized Th/Nb, Rb/Nb, and Ba/Nbratios of studied samples support their non-plume source.

Keywords Mafic dyke . LREE . LILE .

Singhbhum granitoid complex . Proterozoic age

Introduction

Mafic dykes constitute a common expression of crustalextension and represent a major avenue by which basaltic

magma is transferred from the mantle to the upper crust,a process which has operated episodically throughoutthe last 3.0 Ga of the earth’s history. Mafic dykes providea window into the composition and evolution of thesubcontinental mantle and, hence, the tectonic evolutionof the overlying continental crust (Tarney 1992). Theyprovide valuable information regarding global geodynamicprocesses (Halls 1982).

The Indian shield is full of mafic dykes of differentorientations. Mafic dykes of the Indian shield have beenstudied since the mid-nineteenth century (Clarke 1869).Most of the mafic dyke swarms of the Indian shield are ofProterozoic age. It has been considered that among all theother cratonic areas of India (viz., Bundalkhand, Bastar,Dharwar, Singhbhum Orissa craton, etc.), the mafic dykesin the Singhbhum Orissa craton are of much younger age(Devaraju 1995). In this paper, the mafic dykes in parts ofSinghbhum Orissa craton are studied to understand theirnature, geochemical characteristics, and genesis.

Geological background

The eastern Indian shield comprises the Chotanagpurgranite-gneiss complex in the north, the Singhbhum Orissacraton in the south, and ENE–WSW trending Singhbhummobile belt between them (Fig. 1a). The Singhbhum Orissacraton is roughly a triangular-shaped region, bounded bythe arcuate Singhbhum shear zone in the north, the Sukindathrust in the south, relatively high-grade metamorphicSatpura belt in the northwest, and the Eastern GhatsGranulite belt in the southwest. A major part of this cratonis occupied by the Singhbhum granitoid complex of 3.2–2.8 Ga (Moorbath et al. 1986) (Fig. 1b after Saha 1994).The Singhbhum granitoid complex is composed of more

A. R. Mir (*) : S. H. AlviDepartment of Geology, Aligarh Muslim University,Aligarh 202002, Indiae-mail: mirakhtar.r@gmail.com

V. BalaramNational Geophysical Research Institute,Hyderabad 500007, India

Arab J Geosci (2011) 4:933–943DOI 10.1007/s12517-010-0121-6

than 12 separate magmatic bodies, which have beenemplaced during two major phases of magmatism (Saha1994). The older metamorphic group (OMG) is the oldestrecognized unit in this craton, having the age of 3.3 Ga(Sharma et al. 1994). OMG occur as large enclaves rangingin size from a few square meters to several hundred squarekilometers. Large bodies of tonalitic and granodioriticcompositions (older metamorphic tonalite gneiss) areemplaced into the OMG volcanosedimentary formations.Banded iron formation (BIF) belts skirt the Singhbhumgranite complex in the east, south, and west. Major onesamong these are Gorumahisani–Badamphar in the East,Tomka–Daiteri in the South and Southwest, and Noamundi–Koira in the West. The rocks of the BIF Group overlyunconformably the Singhbhum granitoid complex. Thestabilization of this craton is attested by the intrusion ofspectacular set of reticulating dyke swarms know as Newerdolerite dykes (Dunn and Dey 1942; Roy et al. 2004) mainlyconcentrated in the southern part of Singhbhum shear zone.Published K-Ar dates of these dykes give a long range from2,144 to 950 Ma (Sarkar and Saha 1977; Mallick and Sarkar1994) indicating them to be of definite Proterozoic age. Themafic dykes in the studied area display mainly four distinctdirections, viz., N–S, NNE–SSW, NNW–SSE, and E–Wamong which NNE–SSWand NNW–SSE are most dominanttrend directions.

Petrography

The studied dykes are greenish to black in color. They arehard and massive, medium to coarse-grained. They showporpheritic and ophitic to subophitic textures betweenplagioclases and clinopyroxene. Plagioclases occur asphenocrysts as well as in the groundmass ranging incompositions from andesite to labrodorite. Euhedral tosubhedral tabular plagioclase phenocrysts display lamellartwinning. Plagioclase crystals are often saussuritised. Themafic minerals are mainly composed of clino- andorthopyroxene. Augite is the dominant clinopyroxene.Some piegonite crystals showing carlsbad twining are alsopresent. Intergrowth of K-feldspar with quartz is alsopresent. Common alteration products are epidote andsericite. Opaque minerals are seen as small discrete crystalswithin the groundmass.

Geochemistry

Analytical techniques

Ten fresh samples are collected from the mafic dykes in theChaibasa and Haludpukur areas (Fig. 1b). These sampleswere selected for the whole-rock major oxides and some

Fig. 1 a Simplified geological map of Eastern Indian Shield (after Sarkar 1982). b Simplified geological map of Singhbhum Orissa craton aroundChaibasa and Haludpukur areas (after Saha 1994)

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trace element analyses. Whole-rock major elemental analyseswere carried out at National Geophysical Research Institute,Hyderabad by X-ray fluorescence (Philips Magi X PROmodel PW 2540 sequential X-ray spectrometer) techniques.Trace elements including rare earth elements (REE) weredetermined by inductively coupled plasma–mass spectro-metry techniques using Perkin Elmer, Sciex ELAN DRC-IIsystem at NGRI, Hyderabad. The precision of ICP-MS datais <5% RSD for all the REE (Balaram and GnaneshwaraRao 2003).

Elemental mobility

It is most important to know the effect of post-igneousalteration processes on the rock chemistry before theinterpretation of given geochemical data. Limited growthof secondary minerals such as epidote and sericite has beenconsidered as in situ changes formed by interaction withlate magmatic fluids and thus are considered isochemical(Pollard et al. 1983; Taylor and Pollard 1988). Binary

variation diagrams between MgO and other major oxidesare illustrated in (Fig. 2) to evaluate the alteration affects onthe major oxides compositions of the investigated dykes.These samples show normal crystallization trends in thesediagrams. This observation rules out possibility of mobilizationof these elements and suggests that these oxides reflectmagmatic features. Thus, these major oxides can be used forclassification purpose.

During low-grade metamorphism, Zr is considered as arelatively immobile element (Floyd and Lees 1973; Winchesterand Floyd 1977; Macdonald et al. 1988). Hence, Zr can beused as a parameter for evaluating the elemental mobility andalso to understand the differentiation of magma. Figure 3shows binary plots of Zr against Rb and Nd and Nd plottedagainst La. These plots demonstrate magmatic trends withpositive relationships between Nb–Zr, Rb–Zr, Nd–Zr, andNd–La. Thus, it appears that trace elements are least disturbedby post-crystallization processes and may represent primarycharacteristics. The effect of alteration on large ion lithophileelement (LILE) concentrations can be assessed using Rb/Srratios. The low values of Rb/Sr ratio (0.05 to 0.35) in theinvestigated samples do not indicate any major effect of post-igneous processes on the primary concentrations of LILE(Lafleche et al. 1992).

Geochemical characteristics and magma type

Geochemical data of the two groups of the mafic dykes isgiven in Table 1. MgO content in the first group varies from

Fig. 2 Binary plots of a MgO vs SiO2, b MgO vs TiO2, c MgO vsK2O + Na2O, d MgO vs CaO, e MgO vs Fe2O3, and f MgO vs CaO/Al2O3 for the mafic dykes

Fig. 3 Binary plots of a Zr vs Rb, b Zr vs Nd, c Zr vs Nb, and d Lavs Nd for the mafic dykes

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Table 1 Major and trace element data including rare earth elements (REE) of the mafic dykes from Singhbhum granitoid complex, Eastern India

Sample no. Group I Group II

D16C D20C D23C D2C D5C D6C D8C D12C D17C D19C

SiO2 53.87 51.09 50.63 55.86 52.78 53.62 56.69 51.05 57.61 52.98

TiO2 0.27 0.27 0.52 0.29 1.43 0.9 0.86 0.83 0.47 0.99

Al2O3 10.35 9.90 10.36 12.32 10.45 10.5 11.38 11.26 10.42 11.64

Fe2O3 8.25 9.99 11.8 8.56 16.67 12.52 12.19 15.13 10.94 12

MgO 16.51 18.11 18.11 12.16 8.19 12.34 8.13 10.83 10.35 12.52

CaO 8.78 9.01 6.05 7.75 7.13 7.87 7.06 7.88 7.21 6.69

Na2O 1.34 1.10 1.54 2.2 2.27 1.39 2.90 2.12 2.04 2.25

K2O 0.48 0.33 0.81 0.7 0.78 0.57 0.59 0.66 0.79 0.64

MnO 0.14 0.16 0.16 0.13 0.19 0.17 0.15 0.2 0.15 0.16

P2O5 0.02 0.02 0.03 0.02 0.1 0.11 0.05 0.03 0.03 0.12

Mg# 80 78 75 74 49 66 57 59 65 67

Ni 43 47 73 39 19 33 36 23 31 26

Cr 225 301 524 115 93 226 86 75 186 188

Co 49 52 49 43 48 48 50 56 50 42

V 128 168 132 125 266 205 172 255 157 191

Sc 27 24 19 24 29 29 19 39 27 27

Pb 7 7 6 8 7 8 7 8 8 8

Zn 57 91 66 59 98 82 77 86 80 74

Cu 22 94 65 76 135 84 65 95 66 82

Ga 9 13 10 13 15 13 15 14 13 12

Rb 20 30 30 28 36 25 17 35 39 34

Sr 126 155 175 193 160 216 330 215 112 146

Ba 117 169 184 174 190 164 195 153 143 120

Zr 43 56 44 42 89 68 57 85 52 64

Nb 1.86 4.30 3.14 2.66 6.82 5.11 3.44 3.67 7.33 4.45

Ta 0.12 0.27 0.20 0.17 0.43 0.32 0.22 0.23 0.46 0.28

Y 13 24 17 14 37 23 18 22 26 22

U 0.09 0.19 0.15 0.15 0.18 0.10 0.12 0.06 0.33 0.11

Th 1.06 2.23 1.48 1.63 1.60 1.25 1.04 0.67 3.85 1.14

Hf 1.10 1.44 1.13 1.08 2.28 1.74 1.46 2.18 1.33 1.64

Cs 0.52 0.53 1.02 0.75 4.04 0.81 0.86 1.06 0.84 0.62

La 5.38 11.47 8.20 7.98 11.07 9.16 9.81 4.44 17.56 9.02

Ce 12.27 26.39 18.67 17.46 28.01 22.39 23.20 11.40 38.69 22.00

Pr 1.26 2.74 1.93 1.71 3.23 2.53 2.57 1.38 3.70 2.47

Nd 6.53 14.26 10.01 8.58 18.68 13.90 14.13 8.40 17.52 13.71

Ce 12.27 26.39 18.67 17.46 28.01 22.39 23.20 11.40 38.69 22.00

Sm 1.53 3.26 2.26 1.82 4.71 3.29 3.32 2.35 3.68 3.22

Eu 0.44 0.98 0.69 0.61 1.56 1.11 1.19 1.03 0.79 1.05

Gd 1.93 4.00 2.82 2.27 6.02 4.18 3.81 3.31 4.45 4.00

Tb 0.34 0.67 0.47 0.39 1.03 0.67 0.58 0.57 0.72 0.67

Dy 1.95 3.70 2.53 2.11 5.76 3.70 2.91 3.39 3.91 3.57

Ho 0.42 0.80 0.55 0.47 1.23 0.78 0.60 0.74 0.85 0.73

Er 1.33 2.48 1.68 1.47 3.84 2.38 1.72 2.36 2.72 2.28

Tm 0.24 0.43 0.28 0.24 0.63 0.41 0.29 0.39 0.48 0.38

Yb 1.24 2.34 1.54 1.33 3.43 2.03 1.44 2.09 2.54 1.96

Lu 0.19 0.34 0.22 0.20 0.50 0.29 0.21 0.31 0.38 0.28

CaO/Al2O3 0.85 0.91 0.58 0.63 0.68 0.75 0.62 0.70 0.69 0.57

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18.11 to 16.51 whereas the second group has lower MgOcontents than the first one and ranging from 12.52 to 8.13.Al2O3 contents in group I range from 9.90 to 10.36, and ingroup II, it shows a variation from 8.13 to 12.52. Group Isamples possess higher Mg#, Ni, and Cr than that of groupII (Table 1). The analytical data of mafic dykes show thechemical variation from basalt through basaltic andesite toandesite (Fig. 4 after Le Bas 2000). Nb/Y ratio less than 0.7of the studied mafic dykes indicate their subalkaline affinity

(Pearce and Gale 1977). Trace elements appear to be betterdiscriminant than the major elements because instead ofshowing differences in primary composition, they are verysensitive to crystal fractionation. YTC (Y = Y + Zr, Ti =TiO2%×100, C = Cr) ternary discriminated diagram afterDavies et al. (1979) is constructed to show the originaldifferentiation trend of a magmatic suite because Y andZr are enriched progressively during fractionation likeNa2O and K2O. TiO2 behaves very much like iron content

Table 1 (continued)

Sample no. Group I Group II

D16C D20C D23C D2C D5C D6C D8C D12C D17C D19C

Rb/Sr 0.16 0.19 0.17 0.15 0.23 0.12 0.05 0.16 0.35 0.23

Nb/Y 0.14 0.18 0.18 0.19 0.18 0.22 0.19 0.17 0.28 0.20

La/Nb 2.89 2.67 2.61 3.00 1.62 1.79 2.85 1.21 2.40 2.03

Th/Yb 0.85 0.95 0.96 1.23 0.47 0.62 0.72 0.32 1.52 0.58

Nb/La 0.35 0.37 0.38 0.33 0.62 0.56 0.35 0.83 0.42 0.49

Nb/Ce 0.15 0.16 0.17 0.15 0.24 0.23 0.15 0.32 0.19 0.20

Ti/Y 125 67 183 124 232 235 286 226 108 270

Ti/Zr 38 29 71 41 96 79 90 59 54 93

Th/Yb 0.85 0.95 0.96 1.23 0.47 0.62 0.72 0.32 1.52 0.58

Ta/Yb 0.10 0.12 0.13 0.13 0.13 0.16 0.15 0.11 0.18 0.14

La/Yb 4.34 4.90 5.32 6.00 3.23 4.51 6.81 2.12 6.91 4.60

La/Sm 3.52 3.52 3.63 4.38 2.35 2.78 2.95 1.89 4.77 2.80

Th/Nb 0.57 0.52 0.47 0.61 0.23 0.24 0.30 0.18 0.53 0.26

Rb/Nb 10.75 6.98 9.55 10.53 5.28 4.89 4.94 9.54 5.32 7.64

Ba/Nb 63 39 59 65 28 32 57 42 20 27

(La/Yb)n 1.03 4.21 1.36 3.77 10.69 4.65 1.93 1.48 8.24 2.92

(La/Sm)n 2.21 2.43 0.94 2.27 3.02 3.17 1.31 0.85 3.33 2.42

Fig. 4 Total alkali silica classi-fication diagram for the maficdykes (after Le Bas 2000)

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being enriched in tholeiite suites and decreasing system-atically in both calc-alkaline and magnesian suites and Crfollows MgO. On YTC (Fig. 5) diagram, the studiedsamples of mafic dykes of group II plot along the tholeiiteline and that of group I plot along the high-Magnesian suitetrend line.

Chondrite-normalized REE and primitive mantle-normalized incompatible multi-element patterns areshown in Fig. 6. These diagrams show the enrichedpatterns for incompatible trace elements especially for thelight rare earth elements (LREEs; Fig. 6a) and LILEs(Fig. 6b). Figure 6a also shows more or less flatsubparallel pattern of heavy rare earth elements. Group Idykes are moderately fractionated with their (La/Yb)nranging from 1.35 to 4.60, whereas group II is highlyfractionated with their (La/Yb)n ranging from 1.44 to12.01. The normalization of the studied mafic dykesamples against Primitive Mantle (Fig. 6b) show negativeNb, P, and Ti anomalies. Observed Ba, Nb, Sr, P, Tidepletions, and selective Zr enrichment on the spider-grams are similar to that found in back-arc extensionbasalts (Saunders and Tarney 1984, 1991) and island arcsettings (Holm 1985). Two alternative processes could

Group I

1

10

100

La

Ce Pr

Nd

Sm Eu

Gd

Tb Dy

Ho Er

Tm Yb

Lu

Sam

ple

/Ch

on

dri

te

D16C D20C D23C

Group II

1

10

100

La

Ce Pr

Nd

Sm Eu

Gd

Tb Dy

Ho Er

Tm Yb

Lu

Sam

ple

/Ch

on

dri

te

D2C D5C D6C D8CD12C D17C D19C

Group I

0.01

0.10

1.00

10.00

100.00

Rb Ba Th Nb La Ce Sr Nd P Zr Ti Y

Sam

ple

/PM

D16C D20C D23C

Group II

0

1

10

100

Rb Ba Th Nb La Ce Sr Nd P Zr Ti Y

Sam

ple

/PM

D2C D5C D6C D8CD12C D17C D19C

a b

Fig. 6 a Chondrite normalized REE plots for the mafic dykes. Normalizing values are after Taylor and Mclennan (1985). b Primitive mantlenormalized multi-element pattern for the mafic dykes. Normalizing values are after Sun and McDonough (1989)

Fig. 5 a YTC (Y = Y + Zr, Ti=TiO2%×100, C = Cr) ternary diagram(after Davies et al. 1979) for the mafic dykes

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explain the negative Nb anomaly observed in the studieddykes: (1) subduction-related enrichment of lithosphericmantle (Kepezhinskas et al. 1997) and (2) chemical interac-tion between lithospheric mantle and asthenosphere-derivedmagma having incompatible elements but little Nb (Arndtand Christensen 1992; Patchett et al. 1994). High La/Nb(1.21–3.00) ratios of mafic dykes suggest in support of thefirst possibility as the cause for Nb anomalies in the presentsamples. Observed shallow negative anomalies for Sr and Euin few of the studied samples (Fig. 6a) may indicatefractionation of feldspar during evolution of the melt (Tarneyand Jones 1994).

Petrogenesis

Fractional crystallization and crustal contamination

To evaluate the evolution of the MD, various binarydiagrams (Fig. 2) have been plotted taking MgO as thereference oxide because of its wide range of values andimportant behavior during fractional crystallization. Thestudied dykes show increasing trend in SiO2, K2O + Na2O,Fe2O3, and TiO2 with the decrease of MgO and decreasingtrend in CaO and CaO/Al2O3 with the decrease of MgO(Fig. 2). These variations are consistent with fractionalcrystallization of a mafic magma (Robinson et al. 1999;Ahijado et al. 2001). A positive correlation between CaO,CaO/Al2O3, and MgO supports the fractionation of clino-pyroxene. During plagioclase removal, CaO/Al2O3 ratioincreases whereas it remains constant during olivine frac-tionation (Dungan and Rnodes 1978). Therefore, the highCaO/Al2O3 ratio (greater than 0.9) in the MD might haveundergone plagioclase fractionation. Thus, the geochemicaldata of studied dykes are consistent with clinopyroxene andplagioclase being the main fractionating phases.

Fractional crystallization associated with crustal contam-ination is an important process during magmatic evolution(De Paolo 1981) and may modify both elemental andisotopic compositions. Crustal materials are rich in LILEs,K2O, and Na2O and depleted in P2O5 and TiO2. Therefore,crustal contamination will increase K2O, Na2O, and LILEsand decrease P2O5 and TiO2. Low concentrations andnarrow ranges of K2O and Na2O in the investigatedsamples suggest in favor of their minimal crustal contam-ination (Table 1). Crustal components are also rich in Thand Pb. Thus, low Th, Pb, and low Th/Yb ratio of studieddykes supports against crustal contamination (Table 1). Thevalues of Nb/La (<0.5) and Nb/Ce (<0.21) of the MD arenot only very low compared to those of primitive mantle(PM 1.02 and 0.40, respectively, Taylor and McLennan1985; 1.04 and 0.40, respectively, Sun and McDonough1989) but are also lower than average bulk crust (0.69 and

0.33, respectively) and average lower crust (0.83 and 0.39,respectively, Taylor and Mc Lennan 1985). Such lowervalues are not expected to be produced by processes ofcontamination by an average crustal component. Thus,these trace element characteristics may have been obtaineddue to LREEs–LILEs-enriched source characteristics withdepletion of high field strength elements (HFSEs such asNb (Weaver and Tarney 1983; Ahmad and Tarney 1994)).

Mantle source

The similarity of spider-gram patterns (Fig. 6b) for the twogroups of the studied dykes probably suggests that theyhave been generated from the same or similar sources andhave undergone similar petrogenetic history. Two calculatedpartial melting curves (Drury 1983) corresponding to two setsof mineralogical sources (curve I olivine 60% + orthopyr-oxene 20% + clinopyroxene 10% + plagioclase 10%; curveII olivine 60% + orthopyroxene 20% + clinopyroxene 10% +garnet 10%) for mantle source are shown in the binary Zr–Zr/Y diagram of Sun and Nesbitt (1977) (Fig. 7). Most of theMD plot along curve II indicating olivine, orthopyroxene,clinopyroxene, and garnet as the dominant mineralogy of thesource. Sub-horizontal shift of few samples away from thepartial melting curve indicates fractional crystallization ofclinopyroxene ± olivine ± plagioclase (Floyd 1993; Ahmadet al. 1999).

Fig. 7 Zr/Yversus Zr diagram for the mafic dykes.Melting curves (I andII) are after Drury (1983) and vectors for fractional crystallization areafter Floyd (1993)

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Incompatible element ratio Sm/Yb can trace the presence orabsence of residual garnet and can be used to estimate the depthof melting because it is insensitive to the effect of fractionalcrystallization (McKenzie andO’Nions 1991). Garnet has highdistribution coefficient for Yb (Dgarnet/melt=6.6) relative to Sm(Dgarnet/melt=0.25) (Johnson 1994). Yb will be held in thesource region during low degrees of partial melting but can be

released in the melt when all the garnet is exhausted. The highLa/Sm and Sm/Yb ratios greater than 1 in studied dykesindicate a magma origin involving smaller degrees of meltingof a garnet-bearing mantle source. Presence of garnet in thesource of the MD is also supported by the La/Ybpm vs Lapmplot (Fig. 8). In this diagram (Fig. 8), the majority of theinvestigated samples plot along the melting curve of lherzolitewith residual garnet in the source.

Modification of the mantle source by subduction component

The bulk partition coefficients for Zr, Ti, and Y duringpartial melting at upper mantle conditions, especially whengarnet is a residual phase, are in the following order:DZr<DTi<DY (Pearce and Norry 1979). Hergt et al. (1991)stated that the abundances and ratios of Ti, Zr, and Yare broadly similar in many mantle- and crustal-derivedmelts. Thus, their ratios in basaltic rocks either representthe source characteristics or relatively large amounts(∼25–30%) of assimilation of granite and/or sediments.According to Hergt et al. (1991), melts generated by partialmelting of normal mid-ocean ridge basalt (N-MORB)source will have much higher Ti/Y ratios. The studiedsamples have lower Ti/Y and Ti/Zr ratios as compared toMORB (Fig. 9). This may be expected because of theinfluence of the subduction components (slab-released fluidswith high LILE–LREE/HFSE ratios (Brenan et al. 1994)) inthe development of the chemical nature of the subcontinentallithospheric source(s) and greater stability of the Ti-bearingphases in such sources (Saunders et al. 1980; Brenan etal. 1994).

The high Ba/Nb ratios ranging from 20 to 119 and high Sr/Pratios ranging from 0.28 to 2.21 of MD are the indications ofsubduction signature (e.g., Davidson 1996; Borg et al. 1997).

Fig. 9 Ti/Y versus Ti/Zr forthe mafic dykes. Values forN-MORB, PM, and oceanisland basalts are after Sunand McDonough 1989;Low-Ti CFB are after Hergtet al. (1991); Granite, PATS,and TAC are after Taylorand McLennan 1985

Fig. 8 (La/Yb)pm vs (La)pm diagram for the mafic dykes. Partialmelting curves for a garnet lherzolite having the composition: olivine=0.59, orthopyroxene=0.15, clinopyroxene=0.20, and garnet=0.06; and aspinel lherzolite consisting of: olivine=0.60, orthopyroxen=0.20, clino-pyroxene=0.145, and spinel=0.055. Mineral/melt partition coefficientsfor La and Yb are from Arth (1976) and Irving (1978). Numbered ticksrepresent percentages of partial melting

940 Arab J Geosci (2011) 4:933–943

Subduction processes enriches Th with respect to Ta andconsequently increases Th/Yb relative to Ta/Yb, as subduc-tion components in general carry Th but not Ta or Yb. Crustalcontamination may also increase Th/Yb relative to Ta/Ybbecause of higher abundances of Th relative to Ta incontinental crust (Farahat 2006). In Th/Yb vs Ta/Yb diagrams(Fig. 10), the mafic dykes plot above the mantle arraysuggests that these dykes were derived from source regionmetasomatized by subduction processes.

Tectonic setting

Geochemistry of basaltic rocks is frequently used to discrim-inate tectonic settings; this is mainly because basaltic rocks areformed in almost every tectonic environment, and they arebelieved to be geochemically sensitive to the changes in platetectonic framework. To understand the paleotectonic environ-ment of the studied dykes, Ti/1,000 versus V discriminantdiagram after Shervais (1982) is used as shown in (Fig. 11).The studied samples plot in Arc tholeiite and MORB andBack arc basalt fields. The La/Yb ratio is an excellentindicator of degree of enrichment (high value of La/Ybfor enriched sources) or degree of melting (lower valueof La/Yb for higher degree of melting) of mantle sources(Verma 2006). Similarly, the La/Sm ratio is also anindicator of mantle sources and their degree of melting.Rift rocks show high (La/Yb)cn and (La/Sm)cn valueswhereas arc/back arc rocks show low values of bothparameters (Verma 2006). Therefore, the low values of bothparameters (La/Yb)cn<12.01 and (La/Sm)cn<3.39 in themafic dykes indicate the arc/back arc relation. The PM-normalized Th/Nb, Rb/Nb, and Ba/Nb ratios are always <1,distinct geochemical feature, for plume derived basaltic rocks

(Saunders et al. 1992). Therefore, higher PM-normalizedTh/Nb, Rb/Nb, and Ba/Nb ratios of studied samples (Table 1)support their derivation from non-plume source.

Conclusion

The studied mafic dykes show a compositional range frombasalt to andesite. These dykes intrude the Singhbhumgranitoid complex in different directions. These dykes wereoriginated from tholeiite–high magnesian magma. La/Smand Sm/Yb ratios greater than 1 in studied dykes indicatetheir garnet-bearing mantle source. Present samples haveolivine, orthopyroxene, clinopyroxene, and garnet as the

Fig. 10 Th/Yb versus Ta/Ybdiagram for the maficdykes (after Pearce 1982)

Fig. 11 Ti/100 vs V tectonic discrimination diagram (after Shervais1982) for the mafic dykes

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dominant mineralogy of their source. Th/Yb and Ta/Ybratios of the studied dykes suggest that their source regionwas metasomatized by subduction processes. The studiedsamples possess enriched LREE–LILE and depleted HFSEcharacteristics and are characterized by moderate to highlyfractionated spider grams with prominent negative Nb andTi anomalies. LILE and LREE enrichment and depletion ofTi are the geochemical characteristics of subduction-relatedarc/back arc basalts. Arc/back arc relation of studiedsamples is shown by low values of (La/Yb)cn <12.01 and(La/Sm)cn<3.39. Higher PM-normalized Th/Nb, Rb/Nb,and Ba/Nb ratios of studied samples rules out theirderivation from the plume source. Analytical data ofstudied mafic dykes show geochemical characteristicssimilar to those of back-arc extension basaltic rocks.

Acknowledgements The authors are grateful to the chairman,Department of Geology, Aligarh Muslim University for providingfacilities to carry out this work. We are highly thankful to Director,NGRI, for permission to analyze these samples. We are thankful to Dr.Gnaneshwar T. Rao and Dr. S. H. Jafri for their help and guidanceduring chemical analysis at NGRI, Hyderabad. Thanks are also due toDr. Talat Ahmad for fruitful discussion and suggestions. Constructivecomments and valuable suggestions from anonymous reviewer areduly acknowledged.

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