PLEASE SCROLL DOWN FOR ARTICLE This article was downloaded by: [Mir, Akhtar R.] On: 5 September 2010 Access details: Access Details: [subscription number 926604424] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37- 41 Mortimer Street, London W1T 3JH, UK International Geology Review Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t902953900 Geochemistry, petrogenesis and tectonic significance of the Newer Dolerites from the Singhbhum Orissa craton, eastern Indian shield Akhtar R. Mir a ; Shabber H. Alvi a ; V. Balaram b a Department of Geology, Aligarh Muslim University, Aligarh, India b National Geophysical Research Institute, Hyderabad, India First published on: 22 April 2009 To cite this Article Mir, Akhtar R. , Alvi, Shabber H. and Balaram, V.(2011) 'Geochemistry, petrogenesis and tectonic significance of the Newer Dolerites from the Singhbhum Orissa craton, eastern Indian shield', International Geology Review, 53: 1, 46 — 60, First published on: 22 April 2009 (iFirst) To link to this Article: DOI: 10.1080/00206810902900053 URL: http://dx.doi.org/10.1080/00206810902900053 Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
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PLEASE SCROLL DOWN FOR ARTICLE
This article was downloaded by: [Mir, Akhtar R.]On: 5 September 2010Access details: Access Details: [subscription number 926604424]Publisher Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
International Geology ReviewPublication details, including instructions for authors and subscription information:http://www.informaworld.com/smpp/title~content=t902953900
Geochemistry, petrogenesis and tectonic significance of the NewerDolerites from the Singhbhum Orissa craton, eastern Indian shieldAkhtar R. Mira; Shabber H. Alvia; V. Balaramb
a Department of Geology, Aligarh Muslim University, Aligarh, India b National Geophysical ResearchInstitute, Hyderabad, India
First published on: 22 April 2009
To cite this Article Mir, Akhtar R. , Alvi, Shabber H. and Balaram, V.(2011) 'Geochemistry, petrogenesis and tectonicsignificance of the Newer Dolerites from the Singhbhum Orissa craton, eastern Indian shield', International GeologyReview, 53: 1, 46 — 60, First published on: 22 April 2009 (iFirst)To link to this Article: DOI: 10.1080/00206810902900053URL: http://dx.doi.org/10.1080/00206810902900053
Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf
This article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.
Geochemistry, petrogenesis and tectonic significance of the NewerDolerites from the Singhbhum Orissa craton, eastern Indian shield
Akhtar R. Mira*, Shabber H. Alvia and V. Balaramb
aDepartment of Geology, Aligarh Muslim University, Aligarh 202002, India; bNational GeophysicalResearch Institute, Hyderabad 500007, India
(Accepted 16 March 2009)
The Singhbhum Orissa craton, eastern India contains rocks as old as 3.6Ga. The NewerDolerites occur in two distinct orientations (NE/SW and NW/SE) in the SinghbhumGranitoid Complex (SBGC). These dikes are mostly tholeiites and quartz-normativedolerites associated with subordinate norites. We recognize three geochemical groupsof the Newer Dolerites that were emplaced in the SBGC. Group I dikes contain lowerSiO2 (,53.29%) and higher Mg #, Ni and Cr than group II dikes. Group III dikes havehigher SiO2 than groups I and II. A few investigated samples show boniniticgeochemical features. They have high-MgO (.8%), high-SiO2 (.52%) and low-TiO2
(#0.5%) bulk-rock compositions. The main feature of the Newer Dolerite spidergramsis enrichment in the large-ion lithophile elements (LILE, e.g. Rb, K and Ba) relative tohigh field-strength elements (HFSE), resulting in high LILE/HFSE ratios. Thesegeochemical characteristics suggest that the Newer Dolerites are subduction related.High La/Ta ratios (21–66) support a non-plume source. Therefore, we conclude thatthe Newer Dolerites show geochemical signatures similar to those of back-arc basalts.
Keywords: Newer Dolerites; Precambrian crustal evolution; X-ray fluorescence;fractional crystallization
Introduction
Mafic magmatism within the cratons has received special attention because the early earth
was more capable of producing higher temperature primitive magmas. In post-Archaean
times, mafic magmatism was not only influenced by progressive secular compositional
variation of mantle sources but also by the onset of plate tectonics (Condie 1990). Mafic
dikes have played an important role in the crustal evolutionary processes (Tarney 1992).
They occur in a variety of geologic and tectonic settings and their detailed study in space
and time helps in various ways to understand several geological events. Mafic magmatism
in the Singhbhum Orissa craton ranges from 3.3Ga (as oldest ‘enclaves’ in the craton) to
about approximately 1.0 Ga as the Newer Dolerites (Boss 2009).
Geological setting
The Indian continental lithosphere consists of four Archaean cratons, viz. The Dharwar,
Bastar, Singhbhum and Bundelkhand (Radhakrishna and Naqvi 1986; Leelanandam et al.
ISSN 0020-6814 print/ISSN 1938-2839 online
q 2011 Taylor & Francis
DOI: 10.1080/00206810902900053
http://www.informaworld.com
*Corresponding author. Email: [email protected]
International Geology Review
Vol. 53, No. 1, 10 January 2011, 46–60
Downloaded By: [Mir, Akhtar R.] At: 06:11 5 September 2010
2006; Mondal et al. 2006; Mondal 2009) (Figure 1(a)). The Singhbhum Orissa craton
records a long history of crustal formation from mid-Archaean to Mesoproterozoic time
(Saha et al. 1988; Saha 1994). Therefore, it provides a characteristic area for the study of
different stages of the Precambrian crustal evolution. This part of the Indian shield
comprises the Chotanagpur granite-gneiss complex in the north, the Singhbhum Orissa
craton in the south and E–W trending Singhbhum orogenic belt (Ghosh et al. 2005)
between these two. The major portion of this shield consists of 2.8–3.2Ga old granitoids
and banded iron formations, generally referred to as the Singhbhum Granitoid Complex
(SBGC). The SBGC is transected by several dikes of mafic to acidic compositions,
collectively known as the Newer Dolerites (Dunn 1940; Saha 1994) (Figure 1(b)). Newer
Dolerites mark the last major intrusive phase that affected the Singhbhum Orissa craton
(Dunn 1940; Dunn and Dey 1942; Saha 1994).
On the basis of K–Ar radiometric dating, Sarkar and Saha (1983) suggested that the
Newer Dolerite intrusions continued intermittently up to 950Ma. Palaeomagnetic results
have shown that these hypabyssal rocks represent manifestations of at least three
magmatic episodes (Verma and Prasad 1974). Mallick and Sarkar (1994) suggested three
phases of mafic intrusions viz. at 2100, 1500 and 1100Ma.
Petrography
Macroscopically, the Newer Dolerites are mainly massive and their colour varies from
black to greyish green. Under the microscope, they are characterized by ophitic to
subophitic textures. Plagioclase occurs both as phenocryst and groundmass (typically
andesite to labrodorite). Euhedral to subhedral tabular plagioclase phenocrysts display
lamellar twinning. The main mafic minerals are clinopyroxene and orthopyroxene.
Common alteration products are epidote and sericite. Opaque phases are seen as small
discrete grains within the matrix.
Geochemistry
Analytical techniques
Out of several Newer Dolerite samples collected from Rairangpur and Jashipur areas
(Figure 1(b)), least altered/fresh samples were selected for the whole-rock major and trace
Figure 1. (a) Map showing different crustal provinces constituting the Precambrian Indian cratons(after Naqvi and Rogers 1987). (b) Simplified geological map of SBGC around Rairangpur andJashipur (after Saha 1994).
International Geology Review 47
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element analyses. Major elements were analysed at the National Geophysical Research
Institute, Hyderabad, by X-ray fluorescence using fused pellets. The pellets were backed
with boric acid. Trace elements including REE were determined by inductively coupled
plasma–mass spectrometry (ICP-MS; Perkin Elmer, Sciex ELAN DRC II) at the National
Geophysical Research Institute, Hyderabad. The procedure, precision and detection limits
are the same as given by Balaram et al. (1996).
Elemental mobility
Limited growth of secondary minerals such as epidote and sericite have been considered as
in situ changes formed by interaction with late magmatic fluids and thus are considered
isochemical (Pollard et al. 1983; Taylor and Pollard 1988). Large-ion lithophile elements
(LILE) except Th are generally considered as mobile during secondary processes. Th is
believed as immobile or less mobile even during high degrees of alteration (Lafleche et al.
1992). The effect of alteration on LILE concentrations can be assessed using Rb/Sr ratios.
Rb/Sr ratio in least altered basaltic rocks is very low (0.007) and very high (8) in
highly altered mafic rocks (Lafleche et al. 1992). Therefore, the low values of Rb/Sr ratio
(0.05–0.48) in investigated Newer Dolerites do not indicate any major effect of post
igneous processes on primary concentrations of LILE. In addition, the pattern shown by
incompatible elements in a primitive mantle (PM) normalized diagram suggest that they are
least altered. It is generally agreed that transition metals (e.g. Cr and Ni), rare earth elements
(REEs), high field-strength elements (HFSEs) as well as Th and Ti are relatively immobile
during low-temperature alteration (Staudigel et al. 1996). Therefore, in the present study,
trace and rare earth elements are largely used for important petrogenetic interpretations.
Geochemical characteristics
Geochemical data given in Table 1 may be classified into three distinct groups on the basis
of SiO2, Al2O3, MgO, CaO, K2O, Ni, and Cr abundances. In groups I, II and III, Mg # ranges
as (64–77), (43–65) and (62–72), respectively. The analytical data of the Newer Dolerites
shows the chemical variation from basalt to dacite (Figure 2; after Le Bas 2000). Subalkaline
and alkaline magma series may be effectively discriminated by ratios of various
incompatible elements (Ti, P, Zr, Y, and Nb), especially by Nb/Y ratio (Pearce and Cann
1973). Studied Newer Dolerites have Nb/Y ratios less than 0.7, indicating their subalkaline
affinity (Pearce and Gale 1977). On the AFM diagram (Figure 3), Group I dikes show an
Mg-rich tholeiite character while Group II dikes have a Fe-tholeiite basaltic composition and
Group III dikes plot in calc-alkaline field. The wide range ofMg # (43–77), Ni (9–125), and
Cr (31–733) contents suggest that the Newer Dolerites experience fractional crystallization
(Baragar et al. 1995; Hou et al. 2001; Peng et al. 2007). Two samples D4M and D5M show
geochemical features such as MgO .8%, SiO2 . 52%, and TiO2 # 0.5% similar to
boninitic rocks (Crawford et al. 1989; Hall and Hughes 1990; Le Bas 2000; Smithies 2002).
Petrogenesis
Horan et al. (1987) observed in a Ce versus Nd plot that the mantle derived melts affected by
progressive crustal contamination or AFC lie along lines intersecting the Nd axis and do not
pass through the origin. In the Ce versus Nd diagram (Figure 4), the Newer Dolerites follow
the trend line passing through the origin and occupy the place between calculated 2 and 10%
partial melting of PM (Sun and McDonough 1989), which indicates that the source of the
Newer Dolerites was enriched in light rare earth elements (LREE) before melting or the
A.R. Mir et al.48
Downloaded By: [Mir, Akhtar R.] At: 06:11 5 September 2010
Table1.
Majorelem
entdata,CIPW
norm
ativecompositions,andtraceelem
entdataincludingrare
earthelem
ents(REEs)ofNew
erDoleritesfrom
Singhbhum
Orissacraton,easternIndia.
GroupI
GroupII
GroupIII
Sam
ple
D4M
D17M
D22M
D23M
D3M
D5M
D7M
D8M
D9M
D14M
D16M
D20M
D19M
D21M
SiO
253.29
45.88
52.13
51.54
53.08
58.79
52.84
53.59
52.37
53.09
51.89
54.61
67.73
68.05
TiO
20.5
0.31
1.46
0.5
1.4
0.44
1.55
1.6
1.3
2.24
1.33
0.92
0.45
0.38
Al 2
O3
9.65
9.55
10.41
10.71
10
11.66
11.43
11.4
11.79
10.64
10.84
10.70
11.12
10.55
Fe 2
O3
12.59
13.22
14.25
12.34
17.23
10.04
15.71
15.52
14.23
17.05
16.17
14.12
6.94
6.09
MgO
15.25
21.91
12.82
15.41
8.54
9.39
7.82
6.99
8.59
6.61
9.65
9.03
5.60
7.96
CaO
6.48
7.52
5.86
6.98
7.42
6.81
7.60
7.69
8.08
6.72
7.21
7.69
4.11
2.62
Na 2
O1.31
0.83
2.43
1.48
1.75
2.11
2.33
2.48
2.75
2.33
2.27
2.29
2.87
2.98
K2O
0.72
0.58
0.43
0.84
0.29
0.58
0.43
0.42
0.48
0.74
0.39
0.44
1.05
1.24
MnO
0.17
0.18
0.17
0.17
0.21
0.14
0.18
0.18
0.15
0.19
0.2
0.17
0.11
0.1
P2O
50.03
0.02
0.05
0.03
0.07
0.03
0.12
0.13
0.26
0.39
0.05
0.04
0.02
0.02
CIPW
norm
sQuartz
9.32
7.53
4.97
17.20
18.59
13.81
15.04
9.47
16.66
11.22
14.16
31.19
28.79
Orthoclase
4.25
3.43
2.54
4.96
1.71
3.43
2.54
2.48
2.84
4.37
2.30
2.60
6.21
7.33
Albite
11.08
7.02
20.56
12.52
14.81
17.85
19.72
20.98
23.27
19.72
19.21
19.38
24.29
25.22
Anorthite
18.32
20.62
16.23
20.10
18.57
20.63
19.46
18.73
18.41
16.39
18.24
17.62
14.36
11.75
Diopside
9.77
12.60
6.31
10.32
10.69
9.32
9.94
10.67
12.49
5.72
10.40
13.80
3.71
0.15
Hypersthene
33.45
27.64
29.01
33.60
16.32
19.07
14.87
12.47
15.61
13.81
19.22
16.09
12.23
19.76
Olivine
14.78
Ilmenite
0.36
0.39
0.36
0.36
0.45
0.30
0.39
0.39
0.32
0.41
0.43
0.36
0.24
0.21
Sphene
0.76
0.26
3.11
0.76
2.86
0.69
3.31
3.43
2.78
4.97
2.71
1.79
0.80
0.66
Apatite
0.07
0.05
0.12
0.07
0.16
0.07
0.28
0.30
0.60
0.90
0.12
0.09
0.05
0.05
Hem
atite
12.59
13.22
14.25
12.34
17.23
10.04
15.71
15.52
14.23
17.05
16.17
14.12
6.94
6.09
Ni
44
125
69
67
18
30
24
22
918
25
32
17
43
Cr
389
733
255
462
83
147
92
81
31
42
105
74
86
173
International Geology Review 49
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Table
1–continued
GroupI
GroupII
GroupIII
Sam
ple
D4M
D17M
D22M
D23M
D3M
D5M
D7M
D8M
D9M
D14M
D16M
D20M
D19M
D21M
Co
58
74
57
60
54
46
59
62
44
59
59
63
31
23
V176
141
298
171
271
146
304
309
326
399
288
250
107
71
Sc
28
33
23
27
37
24
32
32
16
32
33
32
13
7Pb
43
54
44
44
44
44
56
Zn
68
59
80
65
118
67
118
106
97
123
120
78
56
45
Cu
75
50
123
78
85
61
147
149
71
101
106
130
77
59
Ga
13
921
13
15
15
17
18
18
19
16
15
16
15
Rb
42
27
29
36
21
26
20
20
45
49
21
24
53
55
Sr
103
219
547
149
116
244
133
136
483
158
147
181
222
115
Ba
186
179
130
151
59
273
159
72
37
228
79
108
175
343
Zr
59
47
99
57
90
49
101
103
109
133
96
83
36
24
Nb
3.55
0.92
6.17
2.74
5.26
3.60
6.84
7.25
1.84
10.91
3.36
2.47
4.90
3.25
Ta
0.22
0.06
0.39
0.17
0.33
0.23
0.43
0.45
0.12
0.68
0.21
0.15
0.31
0.20
Y18
13
27
19
36
19
43
44
18
62
33
18
15
9U
0.43
0.05
0.31
0.10
0.09
0.25
0.16
0.16
0.02
0.25
0.05
0.08
0.63
0.41
Th
3.67
0.49
3.68
1.36
1.73
2.68
1.74
1.86
0.16
2.87
0.52
0.95
3.16
3.30
Hf
1.51
1.21
2.54
1.46
2.31
1.26
2.59
2.64
2.79
3.41
2.46
2.13
0.92
0.62
Cs
4.15
0.78
0.71
0.81
1.99
1.59
1.55
1.71
2.81
2.95
0.75
1.60
1.02
0.73
La
10.99
3.13
15.27
8.56
7.05
12.52
12.53
13.06
7.86
22.43
4.81
5.96
9.19
12.44
Ce
24.34
7.02
36.70
19.55
18.49
28.12
31.84
33.34
23.00
56.38
13.64
14.55
20.35
25.21
Pr
2.48
0.73
4.21
2.03
2.25
2.88
3.71
3.89
3.00
6.48
1.77
1.63
2.02
2.44
Nd
12.14
3.88
23.60
10.80
13.77
14.84
21.35
22.33
18.95
37.12
11.46
9.54
10.14
12.19
Ce
24.34
7.02
36.70
19.55
18.49
28.12
31.84
33.34
23.00
56.38
13.64
14.55
20.35
25.21
Sm
2.60
0.87
5.60
2.54
3.88
2.97
5.50
5.81
4.44
8.97
3.46
2.33
2.21
2.31
Eu
0.75
0.38
1.77
0.88
1.31
0.93
1.83
1.91
1.74
2.86
1.38
0.95
0.75
0.87
Gd
3.09
1.27
6.18
3.10
5.35
3.43
6.89
7.11
4.98
10.83
4.66
3.08
2.65
2.60
Tb
0.49
0.25
0.95
0.54
0.98
0.55
1.21
1.25
0.68
1.80
0.87
0.52
0.40
0.34
Dy
2.71
1.65
4.71
2.94
5.52
2.86
6.66
6.87
3.29
9.53
5.05
2.85
2.14
1.52
Ho
0.57
0.41
0.92
0.64
1.20
0.62
1.42
1.48
0.64
2.02
1.10
0.59
0.46
0.29
Er
1.85
1.48
2.66
1.90
3.81
1.98
4.44
4.56
1.82
6.27
3.46
1.89
1.46
0.86
A.R. Mir et al.50
Downloaded By: [Mir, Akhtar R.] At: 06:11 5 September 2010
Table
1–continued
GroupI
GroupII
GroupIII
Sam
ple
D4M
D17M
D22M
D23M
D3M
D5M
D7M
D8M
D9M
D14M
D16M
D20M
D19M
D21M
Tm
0.31
0.27
0.42
0.34
0.65
0.34
0.74
0.79
0.26
1.04
0.59
0.31
0.25
0.14
Yb
1.70
1.59
2.15
1.76
3.53
1.84
4.08
4.27
1.26
5.56
3.08
1.70
1.43
0.70
Lu
0.26
0.25
0.31
0.25
0.52
0.26
0.61
0.64
0.18
0.82
0.46
0.24
0.21
0.11
Elementalratios
Rb/Sr
0.41
0.12
0.05
0.24
0.18
0.11
0.15
0.15
0.09
0.31
0.14
0.13
0.24
0.48
Nb/Y
0.20
0.07
0.23
0.14
0.15
0.19
0.16
0.16
0.10
0.18
0.10
0.14
0.33
0.36
Zr/TiO
2118
152
68
114
64
111
65
64
84
59
72
90
80
63
Zr/Y
3.28
3.62
3.67
3.00
2.50
2.58
2.35
2.34
6.06
2.15
2.91
4.61
2.40
2.67
La/Nb
3.10
3.40
2.47
3.12
1.34
3.48
1.83
1.80
4.27
2.06
1.43
2.41
1.88
3.83
Ce/Nb
6.86
7.63
5.95
7.14
3.52
7.81
4.65
4.60
12.50
5.17
4.06
5.89
4.15
7.76
(La/Yb)n
4.37
1.33
4.80
3.29
1.35
4.60
2.08
2.07
4.22
2.73
1.06
2.37
4.34
12.01
(Gd/Yb)n
1.47
0.65
2.33
1.43
1.23
1.51
1.37
1.35
3.20
1.58
1.23
1.47
1.50
3.01
(La/Sm)n
2.66
2.26
1.72
2.12
1.14
2.65
1.43
1.41
1.11
1.57
0.88
1.61
2.62
3.39
Sm/Yb
1.53
0.55
2.60
1.44
1.10
1.61
1.35
1.36
3.52
1.61
1.12
1.37
1.55
3.30
La/Ta
49.95
52.17
39.15
50.35
21.36
54.43
29.14
29.02
65.50
32.99
22.90
39.73
29.65
62.20
Th/Yb
2.16
0.31
1.71
0.77
0.49
1.46
0.43
0.44
0.13
0.52
0.17
0.56
2.21
4.71
Ta/Yb
0.13
0.04
0.18
0.10
0.09
0.13
0.11
0.11
0.10
0.12
0.07
0.09
0.22
0.29
Ba/Nb
52
195
21
55
11
76
23
10
20
21
24
44
36
106
Sr/P
0.79
2.51
2.51
1.14
0.38
1.86
0.25
0.24
0.43
0.09
0.67
1.04
2.54
1.32
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magmas were contaminated before fractional crystallization. The incompatible element
ratios such as Zr/TiO2, Zr/Y, La/Nb and Ce/Nb are not affected by processes of fractional
crystallization and partial melting (Condie 1990), thus the values of such ratios observed in
the Newer Dolerites (Table 1) appear to be source derived.
Fractionation of plagioclase and olivine cannot alter the Zr/Y ratios, but fractionation
of amphibole and clinopyroxene may raise the ratio with increasing Zr content
(Floyd 1993; Ahmad et al. 1999; Kumar and Ahmad 2007). Thus, the positive correlation
between Zr/Y and Zr (Figure 5) shown by the Newer Dolerites indicate fractionation of
clinopyroxene. However, the positive correlation between Zr/Y and Zr can also be
produced by the decreasing degree of partial melting. Two calculated partial melting
curves (Drury 1983) corresponding to two sets of source mineralogy (curve I: olivine
60%þ orthopyroxene 20%þ clinopyroxene 10%þ plagioclase 10%; curve II: olivine
Figure 2. TAS (SiO2 versus total alkalies) diagram for the Newer Dolerites (after Le Bas 2000).
Figure 3. AFM (K2O þ Na2O 2 FeO* 2 MgO) triangular diagram (after Irvine and Baragar1971) for the Newer Dolerites.
A.R. Mir et al.52
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60%þ orthopyroxene 20%þ clinopyroxene 10%þ garnet 10%) for Archaean mantle
source (Sun and Nesbitt 1977) are drawn in the diagram. Most of the Newer Dolerites plot
along curve II indicating olivine, orthopyroxene, clinopyroxene, and garnet as the
dominant mineralogy of the source. Some samples of group II show a sub-horizontal shift
away from the partial melting curve, indicating fractional crystallization of clinopyroxene
^ olivine ^ plagioclase (Floyd 1993; Ahmad et al. 1999; Kumar and Ahmad 2007).
Figure 4. Nd versus Ce diagram for the Newer Dolerites.
Figure 5. Zr/Y versus Zr diagram for the Newer Dolerites. Melting curves (I and II) are after Drury(1983) and vectors for fractional crystallization are after Floyd (1993).
International Geology Review 53
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The high (La/Yb)n and (Gd/Yb)n (Table 1) in combination with the relatively low
HREE abundance of the Newer Dolerites suggest that they may have formed by low
degrees of partial melting of a garnet bearing source (Deniel 1998; Xu et al. 2001).
The Sm/Yb ratio can be used to estimate the depth of melting because it is insensitive to the
effect of fractional crystallization (e.g. McKenzie and O’Nions 1991; Zi et al. 2008).
The high Sm/Yb ratio (0.55–3.52) and the depletion of heavy rare earth elements
(HREEs) in the Newer Dolerites indicate a magma origin involving smaller degrees of
melting of a garnet bearing mantle source.
Subduction processes enrich Th with respect to Ta and consequently increas Th/Yb
relative to Ta/Yb, as subduction components in general carry Th but not Ta or Yb. Crustal
contaminationmay also increaseTh/Yb relative toTa/Yb because of higher abundances ofTh
relative to Ta in continental crust (Farahat 2006). In the Th/Yb versus Ta/Yb diagram
(Figure 6), the Newer Dolerites plot above themantle array suggests that the Newer Dolerites
were derived from a source region metasomatized by subduction processes (Farahat 2006).
The REE patterns of the Newer Dolerites show enrichment of LREE and a more or less
flat sub-parallel pattern of HREE (Figure 7). Group I and II dikes are moderately
fractionated with their (La/Yb)n ranging as (1.33–4.80) and (1.06–4.60), respectively.
Whereas Group III dikes are highly fractionated (La/Yb ¼ 4.34–12.01). The Newer
Dolerites on Primitive Mantle-normalized multi-element patterns (Figure 8) show negative
Nb, Ta, P, and Ti anomalies. Observed Ba, Nb, Ta, Sr, P, and Ti depletions and selective Zr
enrichment on the spidergrams are similar to that found in back-arc extension basalts
(Saunders and Tarney 1984; Saunders and Tarney 1991) and island arc settings (Holm
1985). Two alternative processes could explain the negative Nb anomaly observed in the
Newer Dolerites: (i) subduction related enrichment of lithospheric mantle (Kepezhinkas
et al. 1997) and (ii) chemical interaction between lithospheric mantle and asthenosphere-
derived magma having incompatible elements but little Nb (Arndt and Christensen 1992;
Patchett et al. 1994). High La/Nb (1.34–4.27) and La/Ta (21.36–65.50) ratios of the Newer
Dolerites support the first possibility as the cause for Nb anomalies in the Newer Dolerites.
Tectonic setting
The Ti/1000 versus V discriminant diagram (Shervais 1982) is used to identify the palaeo-
tectonic environment for the Newer Dolerites. In this diagram, the samples plot in Arc
Figure 6. Th/Yb versus Ta/Yb diagram for the Newer Dolerites (after Pearce 1982). Vectors: S,subduction related metasomatism; C, crustal contamination; W, within plate component and F,fractional crystallization.
A.R. Mir et al.54
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tholeiite and mid ocean ridge basalt (MORB) and back arc basalt (BAB) fields (Figure 9).
The La/Yb ratio (La (LREE) is a highly incompatible element and Yb (HREE) is a less
incompatible element; both are relatively fluid immobile elements) is an excellent
indicator of degree of enrichment (high value of La/Yb for enriched sources) or degree of
melting (lower value of La/Yb for higher degree of melting) of mantle sources (Verma
2006). Similarly, the La/Sm ratio is also an indicator of mantle sources and their degree of
melting. Rift rocks show high (La/Yb)cn and (La/Sm)cn values whereas arc rocks show
Group I
1
10
100
La Ce Pr
Nd
Sm Eu
Gd
Tb
Dy
Ho Er
Tm Yb Lu
La Ce Pr
Nd
Sm Eu
Gd
Tb
Dy
Ho Er
Tm Yb Lu
La Ce Pr
Nd
Sm Eu
Gd
Tb
Dy
Ho Er
Tm Yb Lu
Sam
ple/
Cho
ndrit
e
1
10
100
Sam
ple/
Cho
ndrit
e
1
10
100
Sam
ple/
Cho
ndrit
e
Group II
Group III
D4M D17M D22M D23M
D3M D5M D7M D8M
D9M D14M D16M D20M
D19M D21M
Figure 7. Chondrite normalized REE plots for the Newer Dolerites. Normalizing values are afterTaylor and McLennan (1985).
International Geology Review 55
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low values of both parameters (Verma 2006). Therefore, the low values of both
parameters, (La/Yb)cn ,12.01 and (La/Sm)cn , 3.39 in the Newer Dolerites indicate
the arc relation. A high Ba/Nb ratio (Ba, a LILE, is fluid mobile and therefore indicative of
slab; and Nb, a HFSE, is relatively fluid immobile and therefore indicative of mantle
source) is considered an excellent subduction signal (Verma 2006). Similarly, a high Sr/P
ratio (Sr, a LILE, is fluid mobile, indicative of slab; and P, a HFSE, is fluid immobile,
indicative of mantle) is also an excellent subduction signal (e.g. Davidson 1996; Borg et al.
1997; Verma 2006). Therefore, the high Ba/Nb ratios ranging from 10 to 195 and high Sr/P
ratios ranging from 0.09 to 2.54 of the Newer Dolerites are the indications of subduction
signature. Plume derived melts have generally a low La/Ta ratio (8–15) (Zi et al. 2008).
Group I
0
1
10
100
Rb Ba Th Nb Ta La Ce Sr Nd P Zr Ti Y
Rb Ba Th Nb Ta La Ce Sr Nd P Zr Ti Y
Rb Ba Th Nb Ta La Ce Sr Nd P Zr Ti Y
Sam
ple/
PM
1
10
100
Sam
ple/
PM
0
1
10
100
Sam
ple/
PM
D4M D17M D22M D23M
Group II
D3M D5M D7M D8MD9M D14M D16M D20M
Group III
D19M D21M
Figure 8. Primitive mantle normalized multi-element pattern for Newer Dolerites. Normalizingvalues are after Sun and McDonough (1989).
A.R. Mir et al.56
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The high La/Ta ratio (21–66) in the Newer Dolerites supports their non-plume source.
Instead, the source region was modified by fluids and melts derived from a subducted slab.
Conclusion
The Newer Dolerites show a compositional range from basalt to dacite. They possess
enriched LREE-LILE and depleted HFSE characteristics, and are characterized by
moderately to highly fractionated spidergrams with prominent negative Nb and Ti
anomalies. LILE and LREE enrichment and depletion of Ti are similar to subduction-
related arc basalts. High La/Ta ratios (21–66) in the Newer Dolerites support their
non-plume source. Therefore, on the basis of the analytical data, it is concluded that the
Newer Dolerites show geochemical signatures similar to those of back-arc extension
basaltic rocks.
Acknowledgements
We thank the chairman, Department of Geology, Aligarh Muslim University for providing facilitiesto carry out the work. We are highly thankful to the Director, NGRI, for permission to analyse thesesamples. Thanks are due to Mohd Yousuf, Mushtaq Ahmad, Zahoor-ul-Islam, and Bilal Ahmad forfruitful discussion and suggestions. Constructive comments and valuable suggestions from ananonymous reviewer are duly acknowledged.
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