Elemental and Nd–Sr–Pb isotope geochemistry of flows and
dikes from the Tapi rift, Deccan flood basalt province, India
D. Chandrasekharam
b
a,)
, J.J. Mahoney b, H.C. Sheth
a,1
, R.A. Duncan
c
a
Department of Earth Sciences, Indian Institute of Technology, Powai, Bombay 400 076, India
Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, UniÕersity of Hawaii, Honolulu, HI 96822,
USA
c
College of Ocean and Atmospheric Sciences, Oregon State UniÕersity, CorÕallis, OR 97331, USA
Abstract
The Deccan Traps are a large rift-associated continental flood basalt province in India, parts of which have been studied
extensively in terms of geochemistry, palaeomagnetism and stratigraphy. However, the basalts of the Tapi rift in the central
part of the province have been little-studied thus far. Two ENE–WSW-trending tectonic inliers of the Deccan basalts in this
region, forming ridges rising from younger alluvium, are made up of basalt flows profusely intruded by basaltic dikes. Both
of these ridges lie along a single lineament, although they are not physically continuous. The flows are aphyric,
plagioclase-phyric and giant-plagioclase basalts, and the dikes are aphyric or plagioclase-phyric. We consider the two inliers
to have been originally continuous, from the presence of bouldery remnants of a major dioritic gabbro dike along both.
Samples of this dike from both ridges have previously yielded typical Deccan ages of 65.6 " 0.5 Ma and 65.6 " 0.6 Ma by
the 40Ar– 39Ar incremental heating technique. Initial 87Srr86 Sr ratios and ´ NdŽ t . values, and present-day Pb isotopic ratios of
most dikes indicate that they are isotopically similar to lavas of the Mahabaleshwar and Panhala Formations of the Western
Ghats, about 450 km to the south. Their mantle-normalized trace element patterns have small Pb and Ba peaks. One dike has
a strong Bushe Formation affinity and a Nd–Sr isotopic composition more extreme than that of any other Deccan rock yet
sampled, with ´ NdŽ t . s y20.2 and Ž87Srr86 Sr. t s 0.72315. Its mantle-normalized element pattern shows large Pb, Th and U
peaks and large Nb–Ta troughs. Its elemental and isotopic chemistry reflects substantial continental contamination. The
flows cut by the Mahabaleshwar-type dikes are isotopically similar to the Poladpur Formation lavas of the Western Ghats.
Their mantle-normalized element patterns show modest peaks at Rb, Ba and Pb and rather low Nb and Ta relative to La,
indicating that they have been contaminated to intermediate degrees. The mantle-normalized element patterns of all the flows
and dikes show enrichment in the light rare-earth elements, with small or no Eu anomalies. The entire flow-dike sequence is
similar to the Wai Subgroup of the Western Ghats, in terms of its elemental and isotopic chemistry and stratigraphic
relationships. Wai Subgroup-like lavas Ži.e., some of the younger magma types originally identified from the southern part of
the Western Ghats. are previously known from the central, northern and northeastern Deccan, and many have been thought
to be far-travelled flows erupted in the southwestern Deccan. Although at least the dikes, and probably the giant plagioclase
basalt flows of our study area, are locally generated and emplaced, our new data extend the known outcrop area of these
112
widespread magma types substantially, and these magma types indeed appear to have a nearly province-wide distribution.
Keywords: Deccan; basalt; Tapi; rift; isotopes; geochemistry
1. Introduction
The Deccan flood basalt province covers today an
area of 500,000 km2 in western and central India
ŽFig. 1. after erosion and excluding the substantial
area downfaulted into the Arabian Sea to the west
Že.g., Mahoney, 1988.. The combined stratigraphic
thickness of these lavas in the well-studied Western
Ghats region Žboxed area in Fig. 1. is ; 3000 m; no
statistically significant age difference exists between
lavas from the base and the top of this sequence, and
its formal mean age is 67.5 " 0.3 Ma ŽDuncan and
Pyle, 1988.. This immense volcanic outburst is generally ascribed to the birth of the Reunion
hotspot
´
beneath the northerly drifting Indian continent in the
Late Cretaceous Že.g., Morgan, 1981.. The stratigraphic framework of the Western Ghats region is
known from detailed field, chemical, isotopic and
palaeomagnetic studies Že.g., Cox and Hawkesworth,
1985; Beane et al., 1986; Lightfoot and Hawkesworth, 1988; Mahoney, 1988 and references
therein; Subbarao, 1988 and references therein;
Lightfoot et al., 1990; Peng et al., 1994., and a
geological map is also available ŽSubbarao and
Hooper, 1988.. On the basis of geochemical characteristics and field markers, the Western Ghats stratigraphy has been divided into three subgroups and 11
formations ŽTable 1.. All these lavas are thought to
have been erupted from a group of randomly oriented apparent feeder dikes in a region near Igatpuri
ŽHooper, 1990., which is considered to be the center
of an enormous shield volcano, and the lavas are
nearly horizontal with regional southerly dips of
- 0.58 in the region south of Igatpuri ŽBeane et al.,
1986; Devey and Lightfoot, 1986.. North of Igatpuri,
however, the dips become slightly northerly and
northeasterly ŽSubbarao et al., 1994..
Recent and ongoing work on the central part of
the province to the east of Igatpuri has traced the
Thakurvadi, Khandala and Poladpur Formations
ŽFms.. for several hundred kilometers Žkm. ŽSubbarao et al., 1994, 1998; Peng, 1998.. The Poladpur,
Ambenali and Mahabaleshwar Fms. also extend into
the southeastern Deccan for hundreds of km from
their type sections in the southwest Že.g., Mitchell
and Widdowson, 1991.. Lavas isotopically and
chemically resembling the Ambenali, Poladpur,
Khandala and Bushe Fms. have been described in the
Mhow, Jabalpur and Chikaldara areas in the northeastern Deccan ŽFig. 1., but because of their generally systematically higher Pb isotopic ratios, most
appear to have been erupted from different feeder
vents than the Western Ghats lavas ŽPeng et al.,
1998..
Three major rift zones cross the Deccan province:
the Narmada–Tapi rift zone Žwith the Satpura horst
in between., the Cambay rift, and the West Coast rift
belt ŽFig. 1.. They are marked by strong structural
disturbance and faulting, high positive gravity
anomalies, high heat flow, recent earthquake epicenters, possible Deccan magma chambers and intrusive
bodies, regional dike swarms, and varied magmatism, which have led various workers Že.g., Sheth
and Chandrasekharam, 1997a,b. to suggest that these
rift belts had developed their own magmatic systems
independent of the Western Ghats.
The stratigraphic status of the Deccan basalts of
the Tapi rift region, between the Western Ghats and
the Satpura range, has remained unknown and hardly
any information has been published on their geochemistry or palaeomagnetism. This region shows an
extensive cover of post-Deccan alluvium of the Tapi
River, from which a few ridges of the basalts rise,
with confused field relationships. These ridges are
either tectonic inliers or regional dikes ŽRavishanker,
1987; Guha, 1995; Sheth, 1998..
Two such ENE–WSW-trending ridges in the
western part of the Tapi rift are made up of similar
sequences of flows intruded by dikes, and the detailed field, chemical and isotopic studies we have
113
carried out on them form the subject of the present
paper. The first ridge Žnamed here the Shahada
ridge. begins 7 km south of Shahada town Ž21850X N,
74830X E; see Fig. 2a.. The other, Nandarde ridge, is
seen at Nandarde village, 20 km north of Shirpur
Ž21820X N, 74850X E..
2. Field geology
2.1. Shahada ridge
The Shahada ridge runs parallel to the Shahada–
Shirpur road for about 6 km, in an ENE–WSW
direction, parallel to the Narmada–Tapi regional tec-
Fig. 1. Map of western India, modified from Peng et al. Ž1994., showing the Deccan province Žwhite., the major rift zones traversing it
Žmodified from Hooper, 1990., and important localities mentioned in the paper. The area between Igatpuri and Amboli Žvertical box. is the
Western Ghats region.
114
Table 1
Formation stratigraphy of the Deccan basalts in the Western Ghats
ŽSubbarao and Hooper, 1988.
Group
Subgroup
Formation
Deccan Basalt
Wai
Desur
Panhala
Mahabaleshwar
Ambenali
Poladpur
Bushe
Khandala
Bhimashankar
Thakurvadi
Neral
Igatpuri
Jawhar
Lonavala
Kalsubai
physical continuity between the ridges is absent. The
flow stratigraphic sequences for the two ridges are
also not exactly the same.
tonic trend. At the Shirpur–Dondaicha road bifurcation ŽFig. 2a., the road-cut exposes a plagioclasephyric basalt flow in which a large multiple basaltic
dike has been intruded. This dike is made up of three
columnar rows, each ; 6 m wide Žthe central row is
an individual dike which has been intruded along the
median plane of the earlier dike, splitting it in two..
This central dike has been weathered to large Ž2–3
m. rounded boulders, which are scattered along the
base of the ridge, its slopes and top. To the east,
these boulders are exposed along the ridge top for its
full length. Many small basaltic dikes are seen all
along the ridge. The stratigraphic succession of the
basalts constituting the ridge, obtained on an eastward, downhill traverse, is shown in Fig. 2b.
2.2. Nandarde ridge
The Nandarde ridge is similar to the Shahada
ridge, and is made up of an aphyric basalt flow and a
giant plagioclase basalt ŽGPB. flow ŽFig. 2c.. Many
small dikes are present. They contain fragments of
the GPB at the ridge top. This GPB flow is widely
exposed to the north of Nandarde village as well.
The bouldery dike at the Shahada ridge is also seen
along this ridge.
Interestingly, the Shahada and Nandarde ridges
seem to lie along a single lineament Ždashed line in
Fig. 2a., parallel to the regional tectonic trend and
therefore reflecting some prominent structural control, although because of widespread alluvial cover,
Fig. 2. Ža. Map of the study area showing the locations of the
Shahada and Nandarde ridges. N.H. 3 is National Highway 3.
Area falls on Survey of India toposheet No. 46K Ž1:250,000
scale.. Žb. Flow stratigraphy of the Shahada ridge. Žc. Flow
stratigraphy of the Nandarde ridge. Heights are in meters above
mean sea level.
115
3. Petrography
All samples were studied in thin section on a
Leitz-Laborlux petrological microscope. The minerals of the bouldery dike ŽSH43, SH50. are mainly
plagioclase and clinopyroxene, with some subhedral
sieve-textured opaques, orthoclase and minor secondary hornblende. These rocks have a moderately
coarse grain size Ž3–5 mm. and subophitic texture.
The anorthite contents of the plagioclases were determined using a three-axis universal stage Žand later
a JEOL energy-dispersive X-ray analyser. at IITB.
Most plagioclase compositions fall towards the sodic
end of the series, in the oligoclase ŽAn 10 – 30 . range.
Some labradorite grains are also present, however.
The oligoclases contain many inclusions of prismatic
apatite. The clinopyroxenes have optic axial angles
Ž2V. consistent with diopsidic augite. The modal
compositions of the two dike samples are as follows:
SH43 ŽShahada.: 43% plagioclase, 44% clinopyroxene, 8% opaques, 4% orthoclase and 1% secondary
hornblende; SH50 ŽNandarde.: 51% plagioclase, 36%
clinopyroxene, 9% opaques and 4% orthoclase. Because of the evolved plagioclase compositions, Sheth
et al. Ž1997. termed these rocks dioritic gabbros
Žoligoclase or andesine are typical of diorites.. Their
normative composition is quartz-bearing soda-gabbro. The other dikes and the flows appear to be
ordinary tholeiitic basalts in thin section, with fine
grain size and porphyritic–subophitic textures. Plagioclase is the most common phenocryst phase.
However, the dike SH51 is unique in containing
numerous clusters of well-crystallized, microscopic
orthopyroxene Žferro-enstatite.. Each cluster contains
hundreds of individual orthopyroxene grains, and
such a phenomenon previously has not been described in any other Deccan rock except an E–Wtrending, 40-km-long dike near Dhule ŽKeshav et al.,
1998; Sheth, 1998.. The origin of these clusters is
presently being investigated ŽChandrasekharam et
al., in prep...
4. Geochronometry
Radiometric dating of the Shahada and Nandarde
bouldery dike by the 40Ar– 39Ar incremental heating
technique at Oregon State University yielded good
plateau and isochron ages of ; 66 Ma. The Shahada
ridge sample ŽSH43. was dated at 65.6 " 0.5 Ma and
the Nandarde ridge sample ŽSH50. at 65.6 " 0.6 Ma
Žfive-step weighted mean plateau ages, 1 s .. Complete 40Ar– 39Ar results with plateau age and isochron
age plots can be found in the work of Sheth et al.
Ž1997..
5. Petrochemistry
5.1. Analytical methods
Major-element compositions of selected flow and
dike samples, determined by inductively coupled
plasma-atomic emission spectrometry ŽICP-AES. at
the Regional Sophisticated Instrumentation Center,
IITB, are reported in Table 2. Trace-element data,
also reported in Table 2, were obtained using inductively coupled plasma-mass spectrometry ŽICP-MS.
at the University of Hawaii, where the isotope dilution and isotope ratio measurements were also performed. Sample preparation and mass spectrometric
techniques for measurement of Pb, Nd and Sr isotopic ratios and isotope dilution abundances of Pb,
Nd, Sm, Sr and Rb are closely similar to methods
described by Mahoney et al. Ž1991.. The results,
standard reference values and estimated analytical
uncertainties are presented in Table 3. ICP-AES
major and trace element data for the dioritic gabbro
bouldery dike samples ŽSH43, SH50. can be found
in the work of Sheth et al. Ž1997..
5.2. Discussion
The isotopic ratios for four dike samples, including the bouldery dike samples ŽSH43, SH50, SH41
and SH51, where t s 66 Ma., are closely similar to
each other. Their ´ Nd Ž t . values are slightly positive
Žq1.4 to q1.7., and on plots of ´ Nd Ž t . vs. Ž87 Srr86
Sr. t and ´ Nd Ž t . vs. present-day 206 Pbr204 Pb, data
for these dikes plot within the isotopic arrays of the
Mahabaleshwar and Panhala Fms. of the Western
Ghats ŽFig. 3a and b.. In addition, plots of presentday Pb isotopic ratios ŽFig. 4a–c. show that the data
for the dikes plot precisely within the Mahabaleshwar Fm. field.
116
Table 2
Major- and trace-element compositions of the Shahada ridge–Nandarde ridge basalts
Ž1. Major-element data were measured by ICP-AES at IITB. Data for KC-11 standard provide an idea of accuracy. Ž2. Mg No. is molecular,
assuming that 85% of the total iron is in the FeO form; thus Mg No.s wt.% MgOrwMgO q 0.85FeOŽT.x = 100. Ž3. LOI is percentage loss
in weight of sample powder on ignition at 9008C for ; 2 h. Ž4. Trace elements were measured by ICP-MS at UH. Data for the USGS
standard BHVO-1 provide an idea of accuracy. The reference values for KC-11 are those recommended by Walsh Ž1982., and the BHVO-1
reference values are from a literature-based compilation Žmainly Govindaraju, 1989..
Dike
Flow
Flow
GPB
Dike
Dike
GPB
Reference
Measured
SH41
SH44
SH45
SH47
SH49
SH51
SH52
KC-11
KC-11
55.55
1.10
16.51
8.71
0.14
3.99
6.86
3.30
2.15
0.32
54.31
1.24
15.29
10.45
0.15
4.56
7.33
3.51
2.01
0.29
BHVO-1
45
121
32
317
133
27
136
21
9.7
19
1.16
1.08
0.41
2.05
15.8
37.8
5.40
24.8
6.10
1.98
6.40
0.96
5.20
0.99
2.40
0.33
2.02
0.29
BHVO-1
46
123
30
317
137
24
133
26
8.6
17
1.11
1.11
0.39
2.09
15.9
38.3
5.38
24.6
5.72
1.89
6.06
0.89
5.12
0.91
2.36
0.29
1.89
0.26
wt.%
SiO 2
TiO 2
Al 2 O 3
FeOŽT.
MnO
MgO
CaO
Na 2 O
K 2O
P2 O5
LOI
Total
Mg No.
51.88
3.47
13.21
14.15
0.23
5.10
8.20
1.52
0.36
0.40
1.64
100.16
43
49.67
1.82
16.06
10.67
0.18
5.48
8.95
1.95
0.13
0.21
3.70
98.82
51
50.24
2.47
14.17
12.57
0.20
5.66
8.23
2.56
0.74
0.27
4.52
101.63
48
48.22
2.96
17.41
12.86
0.19
4.43
8.64
2.36
0.42
0.30
3.08
100.87
42
52.29
0.62
16.73
9.23
0.19
7.15
10.38
1.61
0.49
0.13
1.74
100.56
62
50.56
3.15
14.14
14.49
0.24
6.20
8.38
2.13
0.63
0.45
0.50
100.37
47
48.70
3.25
10.90
15.11
0.25
4.23
9.26
3.11
1.58
0.30
2.18
98.87
37
ppm
Co
Ni
Sc
V
Ba
Y
Cu
Ga
Rb
Nb
Ta
Th
U
Pb
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
46
54
41
388
192
49.9
241
30.3
14
21.2
1.21
1.69
0.29
3.20
21.9
53.5
7.22
32.9
8.75
2.34
9.81
1.55
8.76
1.75
4.27
0.61
3.78
0.57
42
61
28
283
233
22.5
144
30.8
9
10.3
0.7
1.52
0.19
2.05
15.2
33.0
4.55
20.3
4.88
1.61
5.43
0.78
4.48
0.84
1.99
0.26
1.79
0.28
44
45
43
327
205
29.8
191
27.0
24
13.7
0.7
1.56
0.27
2.60
20.2
46.6
6.08
26.3
6.72
1.94
7.02
1.05
5.54
1.06
2.39
0.33
2.13
0.28
42
69
50
352
158
34.1
290
30.6
11
20.6
1.32
1.94
0.25
2.60
21.1
45.5
6.3
28.2
6.85
2.20
7.50
1.21
6.24
1.24
3.14
0.41
2.48
0.37
46
105
37
222
206
24.3
101
22.9
15.6
5.4
0.28
4.35
1.15
5.85
15.3
30.6
3.5
13.0
3.19
0.82
3.46
0.59
3.78
0.77
2.19
0.33
2.16
0.32
51
101
39
363
266
43
239
33.3
13
16.3
0.98
2.48
0.46
4.24
25.1
55.8
7.57
33.1
8.17
2.33
9.10
1.41
8.20
1.56
3.72
0.54
3.57
0.50
49
104
31
342
285
42
693
31.9
52
14.8
0.80
2.10
0.49
3.20
21.5
50.6
6.70
30.0
7.63
2.33
8.73
1.35
7.45
1.52
3.67
0.54
3.26
0.46
117
Table 3
Isotope dilution trace-element abundances and Nd–Sr–Pb isotopic data
Ž1. Rb, Nd, Sm, Pb and Sr abundances were measured by isotope dilution. Analytical uncertainty on Nd and Sm abundances is - 0.2%, on
Sr - 0.5%, on Rb ; 1%, and on Pb - 1%. Small acid-washed chips of the samples were used for this Žas part of the procedure for isotope
ratio determinations; e.g., see Peng and Mahoney, 1995., and hence, the isotope dilution abundance values are not strictly representative of
the bulk rocks, although they are close to the bulk rock values.
Ž2. ´ Nd Ž t . and Ž87 Srr86 Sr. t are the initial ratios age-corrected to 66 Ma. Pb isotopic ratios are present-day values.
Ž3. Analytical uncertainty on ´ Nd is better than "0.2 units; on 87 Srr86 Sr better than 0.00002; on 206 Pbr204 Pb and 207 Pbr204 Pb better than
"0.012, and on 208 Pbr204 Pb better than "0.038.
Ž4. Data are reported relative to the following standard values: for NBS981 Pb, to the values of Todt et al. Ž1984.; for NBS987 Sr,
87
Srr86 Sr s 0.71024 " 0.00002; for La Jolla Nd, 143 Ndr144 Nd s 0.511845 " 0.000010.
Sample
Rb ppm
Nd ppm
Sm ppm
Pb ppm
Sr ppm
´ Nd Ž t .
Ž87 Srr86 Sr. t
206
Pbr204 Pb
207
Pbr204 Pb
208
Pbr204 Pb
Dike
Dike
Flow
Flow
Dike
Dike
Dike
Flow
SH41
SH43
SH44
SH45
SH49
SH50
SH51
SH52
13.0
33.76
8.794
3.33
206.5
q1.5
0.70474
17.595
15.395
38.227
23.5
32.03
8.305
2.74
212.6
q1.4
0.70481
17.483
15.366
38.144
7.07
19.55
4.723
2.09
300.4
y3.9
0.70782
18.423
15.527
38.874
20.2
22.83
5.558
2.07
328.8
y3.7
0.70747
18.485
15.495
38.923
14.9
13.20
2.927
5.63
142.2
y20.2
0.72315
20.935
16.032
41.401
34.2
71.14
10.96
2.75
230.5
q1.5
0.70474
17.578
15.379
38.326
13.5
33.98
8.822
3.42
210.7
q1.7
0.70471
17.607
15.394
38.208
55.6
24.28
6.402
3.14
261.4
y2.1
0.70697
18.506
15.567
39.331
On the other hand, isotopic data points for the
flows SH44, SH45 and SH52 ŽGPB. all fall within
the Poladpur Fm. field ŽFigs. 3 and 4.. The dike
SH49, which intrudes a lower, unanalyzed flow
ŽSH48, Fig. 2c. has ´ Nd Ž t . s y20.2 and a corresponding Ž87 Srr86 Sr. t s 0.72315; these values are
the lowest and highest, respectively, yet known for
any Deccan rock. The Bushe Fm. of the Western
Ghats is the most crustally contaminated formation
known in the Deccan, and the dike SH49 has a
strong Bushe Fm. affinity, but has Nd–Sr isotopic
values even more extreme than known Bushe Fm.
lavas. Interestingly, this dike has the highest Mg No.
Ž62. of all samples in this study; the combination of
high Mg No. and high Sr isotopic ratios has been
noted previously among lavas of the Bushe Fm.
Že.g., Mahoney et al., 1982; Cox and Hawkesworth,
1984, 1985; Lightfoot and Hawkesworth, 1988;
Lightfoot et al., 1990., and has been explained on
the premise that more primitive Žand thus hotter.
mantle magmas can experience greater amounts of
crustal contamination during ascent than do more
evolved, cooler magmas Že.g., O’Hara, 1978; Patchett, 1980; Huppert and Sparks, 1985; Mahoney,
1988.. Significant crustal contamination has thus
probably caused the highly radiogenic Sr isotopic
and unradiogenic Nd isotopic composition of this
dike Žnote also its relatively high SiO 2 with very low
TiO 2 and FeOŽT., features typical of Bushe lavas
and of many other crustally contaminated basalts..
Huppert and Sparks Ž1985. modelled magma ascent
in dikes, and noted that at low flow rates in narrow
dikes, flow conditions should be laminar, and hence,
magma should solidify against dike walls, preventing
subsequent batches from contamination. However,
basaltic magmas can ascend turbulently if flow rates
are sufficiently high, and under turbulent conditions,
the products of partial melting of the country rocks
are continuously swept away by the magma, thus
allowing further contamination to continue. The dike
width above which magma flow is expected to be
turbulent is ; 3 m. Note that the outcrop width of
the SH49 dike in the field is ; 15 m. This E–Wtrending dike, and the flow it intrudes ŽSH48., are
found 4 km south of Boradi ŽFig. 2a., just to the
eastern side of the Shirpur–Boradi road. They are
not seen at the Nandarde ridge itself.
Fig. 5 shows the primitive-mantle-normalized
trace element patterns of the Shahada and Nandarde
ridge flows and dikes. The patterns of the isotopi-
118
Fig. 3. Ža. Nd–Sr and Žb. Nd–Pb isotopic plots for the Shahada and Nandarde ridge flows and dikes. Formation fields are after Peng et al.
Ž1994..
cally Mahabaleshwar-like dikes SH41 and SH51 have
modest Pb and Ba peaks and are enriched in the light
rare-earth elements relative to the heavy ones, with
small negative Eu anomalies resulting from plagio-
clase fractionation ŽFig. 5a.. The pattern for the
isotopically Bushe-like dike SH49 shows large peaks
at Pb, Th and U, and a large trough at Nb–Ta, as
expected from crustal contamination, and is light
119
rare-earth-enriched with a negative Eu anomaly ŽFig.
5a.. The patterns for the flows ŽFig. 5b. have peaks
at Pb, Rb and Ba, troughs at Nb–Ta, and light
rare-earth enrichment with very small Eu troughs.
These flows appear to have been contaminated to
intermediate degrees, just like the Poladpur Fm.
lavas of the Western Ghats.
6. A regional perspective
To briefly restate our main results, most dikes of
the Shahada and Nandarde ridges are isotopically
and chemically similar to lavas of the Mahabaleshwar Fm. of the Western Ghats Žsouthwestern Deccan., and the flows to the Poladpur Fm. The whole
assemblage of rocks is therefore similar to the Wai
Subgroup. The presence of Wai Subgroup lavas,
dominantly belonging to the Poladpur Fm., has also
been reported in the Buldana and Lonar areas of the
central Deccan ŽFig. 1., 150–200 km SSE of our
area, ŽSubbarao et al., 1994, 1996, 1998; Peng,
1998..
Lavas isotopically and chemically indistinguishable from the Ambenali Fm. constitute the top parts
of sequences south of Jabalpur and north of
Chikaldara in the northeastern Deccan ŽFig. 1.. They
are underlain by flows chemically resembling the
Poladpur Fm., and at Chikaldara, several flows similar to the Khandala Fm. Lavas with broadly Poladpur- and Khandala-like elemental compositions are
also abundant in a thick section to the south of
Mhow, and thus appear to be widespread across the
northeastern Deccan ŽPeng, 1998; Peng et al., 1998..
However, most of the northeastern Poladpur- and
Khandala-like basalts are different from the chemically similar southwestern basalts in having consistently higher 206 Pbr204 Pb. As the general stratigraphic order in the Chikaldara and Jabalpur areas is
crudely the same as that in the southwestern Deccan
Ži.e., Ambenali-like lavas overlying chemically Poladpur-like lavas, which in turn overlie chemically
Khandala-like lavas., Peng et al. Ž1998. argued that
they are petrogenetically related to the southwestern
Deccan basalts but erupted from different feeder
vents; that is, the northeastern basalts are not just
far-travelled lavas erupted from the same dikes as
the southwestern ones. The locations of the feeder
dikes for the northeastern flows are unknown; many
may be relatively near the sections studied. Thus,
overall, Wai Subgroup-like lavas seem to be
widespread over the central and northern Deccan,
and are by no means confined to the southern region
of the Western Ghats.
However, with regards to the basalts of our study
area, their chemical and isotopic similarities with the
Wai Subgroup basalts do not necessarily mean that
the entire Wai Subgroup Žas defined from the Western Ghats. physically extends into our area. This
statement derives from the following observations.
Ž1. Several GPBs are present in our area Žour unpublished data; SH47 and SH52 are only two examples;
Sheth, 1998.. In the Western Ghats, GPBs are confined to the oldest ŽKalsubai. subgroup only, and are
conspicuously absent from the younger ŽLonavala
and Wai. subgroups ŽHooper et al., 1988.. Therefore,
the widespread occurrence of GPBs in the Wai Subgroup-like sequences in the Shahada–Nandarde region strongly suggests that this region has had an
evolutionary history at least partly independent of
the Western Ghats. Highly plagioclase-phyric flows
and GPBs are thought to have had relatively local
eruptive vents because of their expected high viscosity Že.g., Mahoney, 1988.. Ž2. The relatively dike-rich
Narmada–Satpura–Tapi region could have been an
important source area for the Deccan eruptions, noting the important role of rifting and lithospheric
extension Že.g., Sheth and Chandrasekharam,
1997a,b.. Ž3. A few cases of dikes and sills passing
into flows have actually been demonstrated or inferred for the Narmada–Satpura region, based on
field, geochemical, palaeomagnetic and geochronometric studies Že.g., Crookshank, 1936; Subbarao et
al., 1988; Bhattacharji et al., 1994; Sen and Cohen,
1994..
The present study, supplemented by some recent
studies ŽDeshmukh et al., 1996; Yedekar et al., 1996;
Peng et al., 1998., strongly suggests that some of the
common magma types in the Deccan are much more
widespread than generally believed previously, and
our findings extend the outcrop areas of these geographically widespread and volumetrically abundant
magma types substantially. However, the Ambenali
Fm., which separates the Poladpur and Mahabaleshwar Fms. in the Western Ghats, is completely absent
in our study area, and around Mhow, but Ambenali-
120
121
Fig. 5. Primitive-mantle-normalized incompatible trace-element patterns of Ža. dikes and Žb. flows from the Shahada and Nandarde ridges.
Normalizing values are from Sun and McDonough Ž1989..
type lavas form the top parts of more distant sections
in the northeastern Deccan, and may have flowed
consistently in a northeastern direction ŽPeng et al.,
1998..
7. Conclusions
Flows and dikes of the Shahada–Shirpur region
of the Tapi rift in the Deccan flood basalt province
are closely similar to the Wai Subgroup of the
Western Ghats, in terms of their elemental and isotopic chemistry. Wai Subgroup-like lavas are previously known from the central, northern and northeastern Deccan, and many have been thought to be
far-travelled flows erupted along the Western Ghats.
On the other hand, at least the dikes, and probably
also the GPB flows of our study area have been
emplaced from relatively local feeders distributed
along this rift zone. However, our new data extend
Fig. 4. Plots of Ža. 207 Pbr204 Pb vs. 206 Pbr204 Pb, Žb. 208 Pbr204 Pb vs. 207 Pbr204 Pb, and Žc. 208 Pbr204 Pb vs. 206 Pbr204 Pb, for the Shahada
and Nandarde ridge flows and dikes. Formation fields in Ža. and Žb. are after Mahoney Ž1988., and those in Žc. are after Peng et al. Ž1994..
122
the known outcrop area of the many widespread
Deccan magma types substantially, and these magma
types indeed appear to have a nearly province-wide
distribution.
Acknowledgements
Thanks are due to Dr. Surendra Pal Verma for his
kind invitation to contribute this paper. We also
acknowledge insightful reviews from Peter Hooper,
Ray Kent and Wendy Bohrson, and financial support
from the Department of Science and Technology,
Govt. of India Žto DC and HCS., and the U.S.
National Science Foundation Žto JM..
References
Beane, J.E., Turner, C.A., Hooper, P.R., Subbarao, K.V., Walsh,
J.N., 1986. Stratigraphy, composition and form of the Deccan
basalts, Western Ghats, India. Bull. Volcanol. 48, 61–83.
Bhattacharji, S., Chatterjee, N., Wampler, J.M., Gazi, M., 1994.
Mafic dikes in Deccan volcanics-indicator of India intraplate
rifting, crustal extension and Deccan flood basalt volcanism.
In: Subbarao, K.V. ŽEd.., Volcanism. Wiley Eastern, pp.
253–276.
Chandrasekharam, D., Vaselli, O., Sheth, H.C., Keshav, S., in
prep. Petrogenetic significance of orthopyroxene crystals in
the Tapi rift dikes.
Cox, K.G., Hawkesworth, C.J., 1984. Relative contribution of
crust and mantle to flood basalt magmatism, Mahabaleshwar
area, Deccan Traps. Philos. Trans. R. Soc. London, Ser. A
310, 627–641.
Cox, K.G., Hawkesworth, C.J., 1985. Geochemical stratigraphy of
the Deccan Traps at Mahabaleshwar, Western Ghats, India,
with implications for open system magmatic processes. J.
Petrol. 26, 355–377.
Crookshank, H., 1936. Geology of the northern slopes of the
Satpuras between Morand and Sher Rivers. Mem. Geol. Surv.
Ind. 66.
Deshmukh, S.S., Sano, T., Nair, K.K.K., 1996. Geology and
chemical stratigraphy of the Deccan basalts of Chikaldara and
Behramghat sections from the eastern part of the Deccan Trap
province, India. In: Deshmukh, S.S., Nair, K.K.K. ŽEds..,
Deccan Basalts. Gondwana Geol. Mag. Spl., Vol. 2, pp. 1–22.
Devey, C.W., Lightfoot, P.C., 1986. Volcanological and tectonic
control of stratigraphy and structure in the western Deccan
Traps. Bull. Volcanol. 48, 195–207.
Duncan, R.A., Pyle, D.G., 1988. Rapid eruption of the Deccan
flood basalts at the CretaceousrTertiary boundary. Nature
333, 841–843.
Govindaraju, K., 1989. Compilation of working values and sample
descriptions for 272 geostandards. Geostand. Newsl. 13, 1–
113.
Guha, S., 1995. Tectonic framework and evolution of the Tapi
basin — an intracratonic half-graben from west-central India.
Ind. Miner. 49, 61–78.
Hooper, P.R., 1990. The timing of crustal extension and the
eruption of continental flood basalts. Nature 345, 246–249.
Hooper, P.R., Subbarao, K.V., Beane, J.E., 1988. The giant
plagioclase basalts ŽGPBs. of the Western Ghats, Deccan
Traps. In: Subbarao, K.V. ŽEds.., Deccan Flood Basalts. Mem.
Geol. Soc. Ind., 10, pp. 135–144.
Huppert, H.E., Sparks, R.S.J., 1985. Cooling and contamination of
mafic and ultramafic magmas during ascent through the continental crust. Earth Planet. Sci. Lett. 74, 371–386.
Keshav, S., Sheth, H.C., Chandrasekharam, D., 1998. Field geology, petrography, and orthopyroxene clusters of the DhuleParola dike, Tapi valley, central Deccan basalt province. Curr.
Sci. 74, 252–254.
Lightfoot, P., Hawkesworth, C., 1988. Origin of Deccan Trap
lavas: evidence from combined trace element and Sr-, Nd- and
Pb-isotopic studies. Earth Planet. Sci. Lett. 91, 89–104.
Lightfoot, P.C., Hawkesworth, C.J., Devey, C.W., Rogers, N.W.,
van Calsteren, P.W.C., 1990. Source and differentiation of
Deccan Trap lavas: implications of geochemical and mineral
chemical variations. J. Petrol. 31, 1165–1200.
Mahoney, J.J., 1988. Deccan Traps. In: Macdougall, J.D. ŽEd..,
Continental Flood Basalts. Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 151–194.
Mahoney, J.J., Macdougall, J.D., Lugmair, G.W., Murali, A.V.,
Sankar Das, M., Gopalan, K., 1982. Origin of the Deccan Trap
flows at Mahabaleshwar inferred from Nd and Sr isotopic and
chemical evidence. Earth Planet. Sci. Lett. 60, 47–60.
Mahoney, J.J., Nicollet, C., Dupuy, C., 1991. Madagascar basalts:
tracking oceanic and continental sources. Earth Planet. Sci.
Lett. 104, 350–363.
Mitchell, C., Widdowson, M., 1991. A geological map of the
southern Deccan Traps, India and its structural implications. J.
Geol. Soc. ŽLondon. 148, 495–505.
Morgan, W.J., 1981. Hotspot tracks and the opening of the
Atlantic and Indian Oceans. In: Emiliani, C. ŽEd.., The Sea,
Vol. 7. Wiley, New York, pp. 443–487.
O’Hara, M.J., 1978. Thermal history of magmas: the low pressure
reference point. Philos. Trans. R. Soc. London, Ser. A 288,
627–629.
Patchett, P.J., 1980. Thermal effects of basalt on continental crust
and crustal contamination of magmas. Nature 283, 559–561.
Peng, Z.X., 1998. Geochemical studies of the northwestern, central and northeastern Deccan Traps, India: sources, petrogenesis, and relations to southwestern Deccan stratigraphy. PhD
dissertation, University of Hawaii, USA.
Peng, Z.X., Mahoney, J.J., 1995. Drillhole lavas from the northwestern Deccan Traps, and the evolution of Reunion
hotspot
´
mantle. Earth Planet. Sci. Lett. 134, 169–185.
Peng, Z.X., Mahoney, J., Hooper, P., Harris, C., Beane, J., 1994.
A role for lower continental crust in flood basalt genesis?
Isotopic and incompatible element study of the lower six
123
formations of the western Deccan Traps. Geochim. Cosmochim. Acta 58, 267–288.
Peng, Z.X., Mahoney, J.J., Hooper, P.R., Macdougall, J.D., Krishnamurthy, P., 1998. Basalts of the northeastern Deccan Traps,
India: isotopic and elemental geochemistry and relation to
southwestern Deccan stratigraphy. J. Geophys. Res., in press.
Ravishanker, 1987. Neotectonic activity along the Tapi–Satpura
lineament in central India. Ind. Miner. 41, 19–30.
Sen, G., Cohen, T.H., 1994. Deccan intrusion, crustal extension,
doming, and the size of the Deccan-Reunion
plume head. In:
´
Subbarao, K.V. ŽEd.., Volcanism. Wiley Eastern, pp. 201–216.
Sheth, H.C., 1998. Geochemistry, petrogenesis, stratigraphy and
structure of Deccan flood basalts of the western Satpura–Tapi
region, India. PhD dissertation, Indian Institute of Technology,
Bombay, India.
Sheth, H.C., Chandrasekharam, D., 1997a. Plume–rift interaction
in the Deccan volcanic province. Phys. Earth Planet. Inter. 99,
179–187.
Sheth, H.C., Chandrasekharam, D., 1997b. Early alkaline magmatism in the Deccan Traps: implications for plume incubation
and lithospheric rifting. Phys. Earth Planet. Inter. 104, 371–
376.
Sheth, H.C., Duncan, R.A., Chandrasekharam, D., Mahoney, J.J.,
1997. Deccan Trap dioritic gabbros from the western
Satpura–Tapi region. Curr. Sci. 72, 755–757.
Subbarao, K.V. ŽEd.., 1988. Deccan Flood Basalts. Mem. Geol.
Soc. Ind., 10, xxivq393 pp.
Subbarao, K.V., Hooper, P.R., 1988. Reconnaissance map of the
Deccan Basalt Group in the Western Ghats, India. In: Subbarao, K.V. ŽEd.., Deccan Flood Basalts. Mem. Geol. Soc.
Ind., 10 Ženclosure..
Subbarao, K.V., Ramasubba Reddy, N., Prasad, C.V.R.K., 1988.
Geochemistry and palaeomagnetism of dykes from Man-
daleswar region, Deccan basalt province. In: Subbarao, K.V.
ŽEd.., Deccan Flood Basalts. Mem. Geol. Soc. Ind., 10, pp.
225–233.
Subbarao, K.V., Chandrasekharam, D., Navaneethakrishnan, P.,
Hooper, P.R., 1994. Stratigraphy and structure of parts of the
central Deccan basalt province: eruptive models. In: Subbarao,
K.V. ŽEd.., Volcanism. Wiley Eastern, pp. 321–332.
Subbarao, K.V., Peng, Z.X., Mahoney, J.J., Hooper, P.R., Navaneethakrishnan, P., 1996. Current studies on the geochemistry
of the Deccan Traps from the central region. Deccan ’96 Intl.
Conf. Abstracts, 31.
Subbarao, K.V., Peng, Z.X., Mahoney, J.J., Hooper, P.R., Navaneethakrishnan, P., 1998. Chemical and isotopic stratigraphy
of the Central Deccan basalt province with implications for
regional structure. In: Subbarao, K.V. ŽEd.., Mem. Geol. Soc.
Ind., submitted.
Sun, S.-s., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A.D., Norry, M.J. ŽEds..,
Magmatism in the Ocean Basins. Geol. Soc. Spec. Publ., 42,
pp. 313–345.
Todt, W., Cliff, R.A., Hanser, A., Hofmann, A.W., 1984. 202 Pb
q205 Pb double spike for lead isotopic analyses. Terra Cognita
4, 209.
Walsh, J.N., 1982. Whole rock analysis by inductively coupled
plasma spectrometry. Inst. Min. Metall., pp. 79–91.
Yedekar, D.B., Aramaki, S., Fujii, T., Sano, T., 1996. Geochemical signature and stratigraphy of the Chhindwara–Jabalpur–
Seoni–Mandla sector of the Eastern Deccan volcanic province
and problems of its correlation. In: Deshmukh, S.S., Nair,
K.K.K. ŽEds.., Deccan Basalts. Gondwana Geol. Mag. Spl.,
Vol. 2, pp. 49–68.