geology and geochemistry of pachmarhi dykes and … · geology and geochemistry of pachmarhi dykes...

ORIGINAL PAPER

Geology and geochemistry of Pachmarhi dykes and sills, SatpuraGondwana Basin, central India: problems of dyke-sill-flowcorrelations in the Deccan Traps

Hetu C. Sheth Æ Jyotiranjan S. Ray Æ Ranjini Ray Æ Loyc Vanderkluysen ÆJohn J. Mahoney Æ Alok Kumar Æ Anil D. Shukla Æ Partha Das ÆSubhrashis Adhikari Æ Bikashkali Jana

Received: 8 September 2008 / Accepted: 22 January 2009

� Springer-Verlag 2009

Abstract Many tholeiitic dyke-sill intrusions of the Late

Cretaceous Deccan Traps continental flood basalt province

are exposed in the Satpura Gondwana Basin around

Pachmarhi, central India. We present field, petrographic,

major and trace element, and Sr–Nd–Pb isotope data on

these intrusions and identify individual dykes and sills that

chemically closely match several stratigraphically defined

formations in the southwestern Deccan (Western Ghats).

Some of these formations have also been identified more

recently in the northern and northeastern Deccan. How-

ever, the Pachmarhi intrusions are significantly more

evolved (lower Mg numbers and higher TiO2 contents)

than many Deccan basalts, with isotopic signatures gen-

erally different from those of the chemically similar lava

formations, indicating that most are not feeders to previ-

ously characterized flows. They appear to be products of

mixing between Deccan basalt magmas and partial melts of

Precambrian Indian amphibolites, as proposed previously

for several Deccan basalt lavas of the lower Western Ghats

stratigraphy. Broad chemical and isotopic similarities of

several Pachmarhi intrusions to the northern and north-

eastern Deccan lavas indicate petrogenetic relationships.

Distances these lava flows would have had to cover, if they

originated in the Pachmarhi area, range from 150 to

350 km. The Pachmarhi data enlarge the hitherto known

chemical and isotopic range of the Deccan flood basalt

magmas. This study highlights the problems and ambigu-

ities in dyke-sill-flow correlations even with extensive

geochemical fingerprinting.

Keywords Volcanism � Continental flood basalt �Dyke swarm � Deccan Traps � Gondwana

Introduction

The *66-million-year-old Deccan Traps continental flood

basalt (CFB) province (Fig. 1), with a present-day extent of

500,000 km2, has been studied extensively in terms of

geochemistry, palaeomagnetism, and lava stratigraphy. The

basalt pile is best developed in the Western Ghats region in

the southwestern part of the province, where it has been

divided into three subgroups and 11 formations with a

maximum stratigraphic thickness of *3.4 km (e.g. Cox

and Hawkesworth 1985; Beane et al. 1986; Khadri et al.

1988; Subbarao et al. 1988; Lightfoot et al. 1990)

(Table 1). Arguably, the Deccan lava flows were largely

fed by dykes in swarms, as shown, for example, for the

Columbia River CFB province (e.g. Swanson et al. 1975).

The feeder dykes may be identified with combined field,

petrographic, geochemical (major and trace element and

isotopic) and magnetic polarity studies of individual dyke

swarms, followed by statistical comparisons of the geo-

chemical data to well-characterized lava packages (e.g.

Bondre et al. 2006; Vanderkluysen et al. 2006).

Communicated by T. L. Grove.

H. C. Sheth (&) � R. Ray � P. Das � S. Adhikari � B. Jana

Department of Earth Sciences, Indian Institute of Technology

Bombay (IITB), Powai, Mumbai 400076, India

e-mail: [email protected]

J. S. Ray � A. Kumar � A. D. Shukla

Planetary and Geosciences Division, Physical Research

Laboratory (PRL), Navrangpura, Ahmedabad 380009, India

L. Vanderkluysen � J. J. Mahoney

Department of Geology and Geophysics, School of Ocean and

Earth Science and Technology (SOEST), University of Hawaii,

Honolulu, HI 96822, USA

123

Contrib Mineral Petrol

DOI 10.1007/s00410-009-0387-4

The Deccan province has three major zones of well

developed mafic dyke swarms (Fig. 1) (Auden 1949;

Deshmukh and Sehgal 1988). One is the region of the

Narmada and Tapi River valleys with the Satpura

Mountain Range in between, containing many hundreds

of tholeiitic and alkalic dykes with a general ENE-WSW

strike (e.g. Bhattacharji et al. 1996, 2004; Melluso et al.

1999; Ray et al. 2007). The second major zone of dykes

is the Konkan coastal plain, between the Arabian Sea to

the west and the Western Ghats escarpment to the east.

Here too tholeiitic and alkalic dykes are abundant, and

have a general NNW-SSE strike, parallel to the western

Indian rifted margin (e.g. Viswanathan and Chandrase-

kharam 1976; Sheth 1998). The third important zone of

dykes is the region in the Western Ghats northeast of

Mumbai. The almost exclusively tholeiitic dykes here do

not, as a group, show a strong preferred orientation as in

the other two areas (Beane et al. 1986), and yet, indi-

vidual swarms in this large region do show preferred

orientations, and probably fed some of the younger

Tamia

Matkuli

ChiloundJhirpa Anhoni

Devkhoh

Kandadhana

1119

Chhindwara 35 km

Mahulijir 464

455

674604

652Satdhara

Singanama

510,11,12

8,94

7

3

875

1

2

13

15

14

16

PMS1

PMS2

TAMIA SCARP

PACHMARHI

SCARP

Denwa gorge

Denwa River

78o 15' 78o 30' 78o 45'

22o 25'

22o 35'

22o 30'

22o 40'

22o 45'

22o 20'

Chakhla-Delakhari sill not mapped further west

saucer-shaped sill

Bijori Fm. (Late Permian)

Pachmarhi Fm. (Early Triassic)

Denwa Fm. (Late Lower & Middle Triassic)

Bagra Fm. (Jurassic)

Alluvium (Quaternary-Recent)

Talchir Fm. (Permian)

Precambrianbasement

Deccan dykes and sills

Deccan lavas (Late Cretaceous)

16o

20o

72o80o

Jabalpur

ARABIAN SEA 200 km

Mhow

ToranmalChikaldara

Mahabaleshwar

Saurashtra

Igatpuri

Sagar

Western Ghats

Mumbai

Kachchh

Pavagadh

MalwaPlateau

Nagpur

ChhindwaraPachmarhi

Mandla Lobe

DeccanPlateau

Dudhi River

Barwani

Low

er

Gon

dwan

aU

pper

G

ondw

ana

DhuleNandurbar

Sangamner

INDIA

5 km

521

623

6

695

883

886706

Banjarigurhi

UmariaBori

Denwa River

924

Pipardhar

Chakhla

400

608

547451

613Amadeh

506

Chakhla-Delakharisill

PIPARIYA

Sitadongri

Delakhari

498

9481041

1128

Bariam

Pagara

Ranikhera

PACHMARHI

17,18Dhupgarh

Mahadeo Chauragarh

Pachmarhi plateau

1352

1330 1308

966

887

613

Fig. 1 Map of the Pachmarhi

area showing the geological and

main geographical-

topographical features, and

localities. The map has been

compiled with inputs from

Crookshank (1936), Ghosh et al.

(2006) and the present work.

Elevations are in metres.

Numbers from 1 to 18 in

boldface represent dyke samples

(the letters ‘PMD’ have been

removed from each to avoid

cluttering). Inset map shows the

outcrop of the Deccan flood

basalts (shaded), important

localities discussed in the text,

and, schematically, the three

major dyke swarms in the

Deccan province (e.g.

Deshmukh and Sehgal 1988)


123

stratigraphic formations of the Western Ghats sequence

(Bondre et al. 2006).

The present study documents the field relations, petro-

graphic characteristics, and major and trace element and

Sr–Nd–Pb isotope geochemistry of Deccan dykes and sills

outcropping around the town of Pachmarhi (Panchmadhi),

central India (Fig. 1). These intrusions are an extension of

the Narmada–Satpura–Tapi dyke swarm. On the basis of

the geochemistry of these intrusions, we evaluate their role

as feeders to the Deccan lavas, particularly the lavas

forming some thick sections in the northern and north-

eastern Deccan Traps (the Toranmal–Mhow–Chikaldara–

Jabalpur areas, Fig. 1).

Field work and samples

Regional geology

In central India the Deccan lavas overlie the extensive

Vindhyan (Mid-Late Proterozoic) and Gondwana (Car-

boniferous to Lower Cretaceous) sedimentary basins,

locally developed Late Cretaceous sediments (the Bagh

and Lameta Formations), as well as Archaean and Prote-

rozoic igneous and metamorphic rocks (e.g. Choubey 1971;

Acharyya 2003). The sub-Trap Gondwana Basin is buried

in the subsurface over much of the region but has been

imaged by seismic profiling (Kaila 1988; Sridhar and

Tewari 2001). It is spectacularly exposed in the Pachmarhi

area as a result of post-Deccan tectonic uplift and stripping

of the lava cover (Sheth 2007). Chakraborty and Ghosh

(2005) claim that faults bound the Satpura Gondwana

Basin to its north and south, and that many syn-sedimen-

tary faults cut the sedimentary pile. Pachmarhi town sits

on a *1,000 m thick lens of north-dipping (*5�) Early

Triassic Pachmarhi Sandstone (Fig. 1), underlain by the

Permian Bijori Formation (dominantly shales). The Pac-

hmarhi sandstone is overlain by the Early-Mid Triassic

Denwa Formation (sandstones, shales, clays) and the

Jurassic Bagra Formation (conglomerates). According to

Venkatakrishnan (1984, 1987), multiple Late Cretaceous

planation surfaces in the region were raised and warped as

a result of major post-Deccan uplift that also produced

the long, ENE-WSW-trending, free-face, 280-m-high

Pachmarhi Scarp (Fig. 1). Rivers such as the Denwa and

Dudhi originate well to the south of the Scarp, several

hundred metres below the Pachmarhi plateau, and yet flow

north through the plateau, cutting deep gorges (Fig. 1),

suggesting that they existed before plateau uplift.

To the south, at Tamia, another high scarp (Fig. 2a)

exposes Gondwana sandstones and shales that are capped

by three Deccan basalt flows, and lower elevations expose

a large mafic intrusion, the Chakhla–Delakhari sill

(Crookshank 1936; Sen 1980, 1983). Many Deccan Trap

dykes and other intrusions can be found intruding the

Gondwana sequence, as in the gorges of the Denwa

(Fig. 2b–d). Alexander (1981) reported a K–Ar age of 61 ±

2 Ma for an unspecified dyke from this area. Sen and

Cohen (1994) reported 40Ar–39Ar ages of 66.1 ± 0.3 and

65.5 ± 0.3 Ma (2r errors) for two samples of the sill, and

noted that intrusion and volcanism in this region of the

Deccan were contemporaneous with the main Deccan

eruptions in the Western Ghats.

Table 1 shows the regional lava stratigraphy of the

Western Ghats region in the southwestern part of the

Deccan province. Peng et al. (1998) identified thick

sequences of lavas geochemically similar to those of the

Western Ghats (the Khandala, Poladpur and Ambenali

Formations) in the northeastern Deccan, around Chikaldara

and Jabalpur, and found Khandala- and Poladpur-like lavas

in the northern Deccan (around Mhow), where Ambenali-

type compositions are absent. The lavas are in the south-

western stratigraphic order as well. However, Peng et al.

(1998) found that the Poladpur-like lavas, in particular, had

systematically higher 206Pb/204Pb ratios than their chemi-

cally similar southwestern counterparts, and inferred that

Table 1 Stratigraphy of the

Deccan flood basalts in the

Western Ghats, with formation

thicknesses, magnetic polarity,

and Sr isotopic values

(at 66 Ma)

a The Desur is considered by

many as a ‘‘Unit’’ of the Panhala

Formation itself. Table based on

Subbarao and Hooper (1988),

Peng et al. (1994), and

references therein

N normal magnetic polarity,

R reverse magnetic polarity

Group Sub-group Formation Magnetic polarity 87Sr/86Sr(66 Ma)

Deccan Basalt Wai Desura (*100 m) N 0.7072–0.7080

Panhala ([175 m) N 0.7046–0.7055

Mahabaleshwar (280 m) N 0.7040–0.7055

Ambenali (500 m) R 0.7038–0.7044

Poladpur (375 m) R 0.7053–0.7110

Lonavala Bushe (325 m) R 0.7078–0.7200

Khandala (140 m) R 0.7071–0.7124

Kalsubai Bhimashankar (140 m) R 0.7067–0.7076

Thakurvadi (650 m) R 0.7067–0.7112

Neral (100 m) R 0.7062–0.7104

Jawhar-Igatpuri ([700 m) R 0.7085–0.7128


123

they erupted from different vents. Chandrasekharam et al.

(1999) and Mahoney et al. (2000) found that some thin,

discontinuously exposed lava sections in the Tapi River

valley, and the 870 m-thick Toranmal section in the Sat-

pura Range, were composed of several Poladpur-type lavas

cut by a few broadly Bushe-like or Mahabaleshwar-like

dykes; the latter would be consistent with the southwestern

Deccan stratigraphy. They did not find Ambenali-like lavas

or dykes.

The Pachmarhi dykes

Prior to the field work, we measured the lengths and trends

of 55 dykes on six survey of India topographic sheets

(1:50,000 scale and 20 m contour interval) covering an

area of 14,500 km2 between latitudes 22�150 N and 22�450

N and longitudes 78�150 E and 79�000 E. The dykes were

identified in the toposheets as narrow linear ridges, with

equal slopes on either side, often extending for several

kilometres, or as linearly aligned segments. However,

because of the highly dissected terrain (relief [1 km) and

few roads, field work and sampling were restricted to the

area shown in Fig. 1, which covers parts of four toposheets

(nos. 55 J/6, 7, 10, 11).

The dykes we sampled are typical dolerites and basalts,

undeformed and relatively fresh. Almost all are vertical or

very steeply dipping (PMD11, 85�), but PMD7 dips north

at about 45�. Sample names in this study represent both

individual samples and whole dykes or sills. Of course, no

one sample can fully represent a large dyke or sill, where

mineral proportions and some aspects of chemical com-

position can change over relatively small distances (cf.

Bondre et al. 2006). Some dykes form prominent ridges

(PMD14), whereas others have quite low relief (PMD4, 5,

15). Many show blocky jointing (PMD6, 10, 12, 17).

Several are well exposed along the Denwa River valley

intruding the Gondwana sandstones and clays (PMD7–

PMD12), sometimes with straight chilled margins of

tachylite. Nowhere were the dykes seen to pass into flows

and no dyke tips (terminations) were seen. Almost all of

the dykes, with the exception of PMD6, are simple, single

intrusions. PMD6 is highly weathered in outcrop and seems

to have multiple fine-grained dykes at its margin.

Most dykes strike ENE-WSW to E-W with a normal

distribution on a strike-frequency histogram (Fig. 3a) and a

strike of N73.9�. Only one dyke (PMD17), near Dhupgarh

summit, is N-S trending. Dykes PMD10 and PMD12,

exposed on the Denwa River bed, are highly sinuous. We

did not see any dykes occupying faults, so their general

ENE-WSW trend shows the regional minimum compres-

sive stress (r3) direction to have been *NNW-SSE during

emplacement (cf. Pollard 1987; Gudmundsson and Mari-

noni 2002; Ray et al. 2007). Dyke lengths vary from \1–

19.1 km (PMD2), and the length distribution of the dykes

follows a negative power law (Fig. 3b), as is typical (Gu-

dmundsson 1995). The mean, mode and median lengths are

3.2, 1.3 and 2.1 km, respectively. Measured thicknesses

range from 2 to *30 m and can be variable along strike.

Fig. 2 Field photographs.

a The Tamia scarp and Deccan

basalt lava flows overlying

Gondwana sandstone. b Dyke

PMD7 along Denwa River bed,

dipping roughly north (left) and

with columns perpendicular to

its contacts. c Dykes PMD8 and

PMD9 on the Denwa River bed.

d The 28–34 m wide Satdhara

dyke PMD11


123

For example, PMD11 in the Denwa River bed is 28 m wide

near its western end but 34 m near its eastern end where it

is cross-cut by PMD12. A plot of dyke strike versus length

(Fig. 3c) and a plot of dispersion from the mean strike

versus length (Fig. 3d) show that most of the longer dykes

lie close to mean strike direction and the smaller dykes

exhibit more dispersion. This shows that the larger

dykes better represent the direction of the time-averaged r3

(cf. Ray et al. 2007).

The sills and Tamia lavas

The Chakhla–Delakhari sill is the most extensive Deccan

Trap dolerite intrusion in the Pachmarhi area (Fig. 1) and

has a maximum thickness of 200 m (Crookshank 1936;

Sen 1980, 1983). Crookshank (1936) considered the sill a

multiple intrusion and a feeder to the lowest and uppermost

of the three lava flows exposed at Tamia. According to

Sen (1980), the sill thins westwards and merges with a

composite dyke composed of dolerite and granodiorite-

porphyry, whereas to the east it passes into a Tamia lava

flow. Mineral chemical and whole-rock geochemical data

for the sill and Tamia lavas were presented by Sen (1980,

1983) and Sen and Cohen (1994) who described its highly

fractionated, Fe-rich composition and internal mineralogi-

cal zoning. Isotopic data have not been available so far. We

sampled the lower chilled margin of the sill exposed at

Umaria village (sample PMS1).

An intrusion with a curious semicircular outcrop pat-

tern—especially striking in Google earth images—is

noticeable in the eastern part of our study area around

Kandadhana hamlet (Fig. 1). Its long dimension is 7 km

and it rises *300 m above surrounding ground. We

believe that this intrusion is a saucer-shaped sill, a recently

recognized category of sill intrusions with flat floors and

steeply dipping (or vertical) rims. An excellent example is

the Golden Valley sill complex in the Karoo flood basalt

province, where the largest sill is 19 km in diameter

(Galerne et al. 2008). Saucer-shaped sills are typically

emplaced at shallow depths in thick, horizontal or gently

dipping, relatively undeformed sedimentary sequences

(Malthe-Sørensen et al. 2004; Goulty and Schofield 2008;

Polteau et al. 2008), which is exactly the case around

Pachmarhi. They are fed by dykes or other sills. Crook-

shank’s (1936) map shows an apparent continuation of the

Kandadhana intrusion with the Chakhla–Delakhari sill

towards the east, suggesting the former to be a satellite

intrusion (at a higher structural level, as the regional dip is

due north) of the latter. At Kandadhana gently north-dip-

ping sandstones form low ground, and coarse dolerite of

the intrusion forms the arcuate rim (sample PMS2 taken

Nu

mb

er o

f d

ykes

Nu

mb

er o

f d

ykes

25

20

15

10

5

0

25

20

15

10

5

0

30

0 20 40 60 80 100 120 140 160 180

0 2 4 6 8 10 12 14 16 18 20

Strike (Nxo)

Length (km)

y = 31.97x -1.2469

R2 = 0.9672

Length (km)

Length (km)

Str

ike

(Nxo )

Mean strike (N73.9o)

0 2 4 6 8 10 12 14 16 18 200

20

40

60

80

100

120

140

05 10 15 20

20

40

60

-20

-40

Dis

per

sio

n (

deg)

(a)

(b)

(c)

(d)

Fig. 3 Plots of structural data for Deccan dykes around Pachmarhi.

a Histogram showing the distribution of 55 strike data. b Frequency

histogram of dyke lengths showing a typical power law distribution

(of the general form y = axk). The exponent k is negative (-1.2469)

and a, the constant of proportionality, is 31.97. Squared Pearson

correlation coefficient R2 = 0.967. c The variation in length as a

function of dyke trend, indicating that the longest dykes are close to

mean trend. d Dispersion from mean trend as a function of length.

Vertical lines connect the maximum and minimum dispersion (blackcircles) observed for a particular length, and thus define the range of

dispersion observed in all dykes of a given length. Single black circlesindicate only one dyke of that particular length


123

here). Because of dense forest cover we could not see their

contact, or the relationships between PMS2 and nearby

dykes such as PMD3 or PMD16.

Petrography

The dykes are all dolerites or basalts, with plagioclase,

clinopyroxene, sieve-textured opaques and sometimes

olivine as the principal minerals (Fig. 4). Several are

medium grained, aphyric, with ophitic texture (e.g., PMD1,

Fig. 4a). Others are plagioclase-phyric, the plagioc-

lases zoned (Fig. 4b, c), and some show altered olivine

phenocrysts (Fig. 4d). Sample PMD3 is a plagioclase-

megaporphyritic basalt. Sill sample PMS1 (lower chilled

margin of Chakhla–Delakhari sill) is relatively fine-grained,

whereas PMS2 (from the Kandadhana intrusion) is coarse

dolerite.

Based on petrographic studies, Sen (1980) divided the

Chakhla–Delakhari sill into five distinct zones. From bot-

tom upwards they are: (1) the lower chilled zone (up to 8 m

thick), with an overall fine grain size and abundant inter-

stitial glass, (2) the 27-m-thick olivine-rich zone, (3) the

70-m-thick, coarse-grained, olivine-poor central zone, (4)

the 55-m-thick upper zone, with two chemically and

morphologically contrasting groups of olivine and abun-

dant interstitial granophyric intergrowths, and (5) the 10–

25 m thick upper chilled zone with abundant interstitial

glass. Sen (1980, 1983) reported much petrographic and

mineral chemical data, and found strong chemical varia-

tions and zoning in the olivine, plagioclase, and pyroxenes

(augite, subcalcic augite and pigeonite) throughout the sill.

He showed extensive gravity settling of olivine, and also to

a lesser extent of plagioclase, and considered periodic

recharge of the sill by small new batches of differentiated,

Fe-rich liquids as likely. The upper zone also shows peg-

matitic lenses (100 9 30 cm) made up of large tabular

plagioclase crystals, long ferroaugite blades and coarse

titanomagnetite and ilmenite, along with interstitial

granophyre, apatite, and quartz.

Sen and Cohen (1994) have provided petrographic

descriptions of the Tamia lavas. The lowermost Tamia flow

is a coarse dolerite with phenocrysts of olivine, plagioclase,

and clinopyroxene. Pigeonite is also found as inclusions in

plagioclase. The middle and upper flows are fine-grained

and glass-rich, the former containing plagioclase mega-

crysts up to 3 cm long and the latter containing phenocrysts

of olivine and plagioclase. The olivines are often altered to

iddingsite.

Analytical methods, and general chemical-petrological

characteristics

Small chips of the least altered material were cleaned with

distilled water and powdered in a Retsch stainless steel

planetary ball mill. Between two samples the mill was

operated with a small amount of the second sample so as to

Fig. 4 Photomicrographs of

some of the Pachmarhi dyke

samples showing typical

textures (see text). a Sample

PMD1, crossed polars, view

1.5 mm wide. b PMD12,

crossed polars, view 6 mm

wide. c PMD16, crossed polars,

view 1.5 mm. d PMD6, over the

polarizer, view 1.5 mm. Mineral

grains are marked as ol(olivine), cpx (clinopyroxene),

pl (plagioclase), and ox (Fe–Ti

oxide)


123

precontaminate it. The samples were analyzed at the

PLANEX Facility of the Indian Space Research Organi-

zation (ISRO), located at the Physical Research

Laboratory, Ahmedabad. The major elements were ana-

lyzed on pressed powder pellets by X-ray fluorescence

(XRF) spectrometry (Axios, from Panalytical Limited,

following the methods of Norrish and Chapell 1977). The

trace elements were analyzed on sample solutions by

inductively coupled plasma mass spectrometry (ICPMS,

Thermoelectron X-Series II, following the methods of Jain

and Neal 1996 and Neal 2001). The XRF and ICPMS data

are reported, together with information on accuracy and

precision of the measurements, and LOI (weight loss on

ignition to 1,000�C) values in Table 2. For calibrating the

instruments, international rock standards such as AGV-1,

BIR-1, G-2, GSP-1, STM-1 and W-2 were used, whereas

others (BHVO-2 and BCR-2) were analyzed as unknowns

along with the samples.

Bulk rock Sr and Nd isotopic analyses were also carried

out at the Physical Research Laboratory. Sample powders

were dissolved using a standard HF–HNO3–HCl dissolu-

tion procedure for silicate rocks. Sr separation was done by

conventional cation exchange chemistry and Nd was sep-

arated from other REE (rare earth elements) using Ln

specific resin from eichrom with dilute HCl as the elutant.

The isotope ratio measurements were carried out in static

multi-collection mode on an ISOPROBE-T mass spec-

trometer. Pb isotopic analyses were performed at the

University of Hawaii following Mahoney et al. (1991). The

isotopic data and information on the accuracy and precision

of the isotopic analyses are reported in Table 3.

We used the SINCLAS program (Verma et al. 2002) for

LOI-free major oxide data, CIPW norms, and a standard-

ized rock name following the IUGS nomenclature (Le Bas

et al. 1986). Table 2 shows the measured major and trace

element compositions of the samples and the rock names

obtained with SINCLAS. As noted by Middlemost (1989),

the Fe2O3/FeO ratio increases with oxidation (indirectly,

degree of differentiation) and weathering, and SINCLAS

incorporates an option for using Fe2O3/FeO ratios

depending on rock type, given by Middlemost (1989), to

split the total iron. A value of 0.20 was recommended by

Middlemost for fresh basalts. All samples are classified as

subalkalic basalt, like the overwhelming majority of

Western Ghats and northeastern Deccan lavas (e.g. Beane

et al. 1986; Beane 1988; Peng et al. 1998; Sheth 2005;

Fig. 5). The rocks are fairly or moderately evolved, with

MgO contents ranging from 3.48 (PMS2) to 7.26 (PMD5)

wt.% and Mg numbers (Mg#) ranging from 36.1 (PMS2) to

54.2 (PMD9), where Mg# = [atomic Mg/(Mg?Fe2?)] 9

100. All rocks are quartz-normative, the normative quartz

ranging between 1.15 (PMD5) and 7.10 (PMS2) wt.%. The

LOI values provide an idea about the level of sub-aerial

alteration suffered by the rocks, and are low to moderate

(0.50–1.86) except for PMD9 (5.47 wt.%). We also pro-

cessed the data of Sen and Cohen (1994) for the Chakhla–

Delakhari sill and Tamia lavas through SINCLAS. LOI

values are not available. All except one of their 14 samples

are subalkalic basalt, the exception being a basaltic

andesite (D34). Mg# ranges from 38.7 to 52.2.

Geochemical correlations with the southwestern

Deccan lavas

Our objective is to establish geochemical correlations, or

the lack thereof, of the Pachmarhi dykes and sills to the

Deccan lava Formations, and thus to evaluate whether

these intrusions could represent feeders to any of the well-

studied, main lava packages in the province. Previous

studies of lava piles exposed in parts of the Deccan lacking

an established stratigraphy have employed several tools in

order to aid comparisons with the southwestern Deccan

formations (Peng et al. 1998; Mahoney et al. 2000; Sheth

et al. 2004; Sheth and Melluso 2008). These include binary

discriminant diagrams using major and trace elements and

element ratios, multivariate statistical methods (particu-

larly discriminant function analysis), normalized multi-

element patterns, and Sr–Nd–Pb isotopic ratios. The same

tools have been used to systematically correlate mafic dyke

swarms to particular Deccan lava packages or sequences

(Bondre et al. 2006; Vanderkluysen et al. 2006).

Binary diagrams and discriminant function analysis

Binary diagrams of alteration-resistant elements such as

Nb, Zr, Ba, and Y, and their ratios insensitive to crystal

fractionation (e.g., Nb/Zr), were seen to have no utility in

correlating the Pachmarhi rocks with any specific south-

western Deccan formations, because of the substantial

overlap in the compositional characteristics of several

formations, and because the Pachmarhi rocks’ data plot in

these areas of overlap (not shown).

Discriminant function analysis (DFA) was performed

in order to quantitatively evaluate chemical affinities of

the Pachmarhi dykes and sills and the Tamia lavas to

individual southwestern Deccan formations. DFA was

performed twice, first using several major (excluding

Na2O, K2O, total iron and MnO) and trace elements (Ni,

Sc, V, Ba, Rb, Sr, Zr, Y, and Nb) as the discriminating

variables, and again using just these trace elements. The

methodology used was exactly that used in previous similar

studies (Peng et al. 1998; Mahoney et al. 2000; Sheth et al.

2004; Bondre et al. 2006). The formation matches (tabu-

lated results available from the authors) obtained with the

two runs are different in all cases except for three samples


123

Table 2 Major oxide (XRF) and trace element (ICPMS) data for Pachmarhi dykes and sills

PMD1

dyke

Basalt,

subalk

PMD2

dyke

Basalt,

subalk

PMD3

dyke

Basalt,

subalk

PMD4

dyke

Basalt,

subalk

PMD5

dyke

Basalt,

subalk

PMD6

dyke

Basalt,

subalk

PMD7

dyke

Basalt,

subalk

PMD8

dyke

Basalt,

subalk

PMD9

dyke

Basalt,

subalk

PMD10

dyke

Basalt,

subalk

PMD11

dyke

Basalt,

subalk

PMD12

dyke

Basalt,

subalk

wt.%

SiO2 49.08 50.64 48.46 49.01 48.23 49.56 49.41 50.55 48.61 48.71 50.47 48.4

TiO2 3.00 3.12 3.27 2.17 2.72 3.08 2.10 2.63 2.81 2.73 2.51 2.68

Al2O3 12.23 12.81 12.78 12.06 12.33 12.01 12.34 12.47 12.46 12.08 12.65 12.01

Fe2O3T 16.66 15.61 15.24 16.19 15.59 17.46 15.53 16.91 13.71 15.30 16.14 16.4

MnO 0.21 0.19 0.19 0.20 0.20 0.21 0.20 0.22 0.17 0.28 0.22 0.21

MgO 5.95 5.92 6.03 6.52 7.12 6.69 6.80 5.80 6.93 6.06 5.62 6.56

CaO 9.58 9.81 10.01 10.63 10.59 10.09 11.17 9.94 11.44 10.20 10.33 10.11

Na2O 2.03 2.07 2.02 1.96 2.04 2.00 1.89 2.14 1.47 1.71 2.07 1.74

K2O 0.57 0.78 0.53 0.35 0.37 0.37 0.30 0.58 0.16 0.35 0.48 0.47

P2O5 0.36 0.42 0.38 0.18 0.26 0.26 0.20 0.26 0.26 0.28 0.25 0.25

Total 99.67 101.37 98.91 99.27 99.45 101.73 99.94 101.50 98.02 97.70 100.74 98.83

LOI 0.44 0.90 0.67 1.02 1.05 1.77 1.01 1.24 5.47 1.86 1.39 1.78

Mg# 45.5 47.0 48.0 48.5 51.6 47.2 50.6 44.5 54.2 48.1 44.9 48.3

ppm

Sc 38.79 35.24 33.90 36.49 32.84 35.94 38.48 32.12 38.96 35.83 34.57 35.51

V 493.30 401.60 479.00 368.90 408.80 452.20 409.90 372.00 446.00 415.90 424.40 416.00

Cr 540.40 511.50 521.30 573.20 503.80 495.70 550.50 510.90 237.30 319.60 524.50 370.60

Co 57.92 49.62 47.94 45.09 50.50 53.97 51.27 45.34 47.73 50.24 49.89 50.65

Ni 66.66 59.59 85.12 58.05 88.37 61.09 74.15 48.38 70.45 69.16 55.04 72.51

Rb 17.11 24.65 14.66 8.17 8.30 10.96 6.77 15.17 2.06 7.44 14.31 13.83

Sr 216.10 213.50 238.40 167.70 217.50 241.20 175.00 161.90 184.90 175.50 185.60 190.30

Y 34.63 38.69 34.19 27.82 28.61 32.27 27.83 30.41 30.25 29.28 32.24 29.43

Zr 181.60 227.40 191.80 114.10 153.30 171.40 121.50 141.00 161.10 154.70 158.80 162.50

Nb 16.15 20.80 18.36 8.17 12.89 14.96 7.35 9.04 12.14 11.92 10.10 12.22

Ba 310.7 217.9 224.6 85.44 145.1 512.5 91.16 110 104.2 118.7 125.6 157

La 17.46 24.18 19.13 9.42 13.55 16.00 10.30 13.29 15.48 14.67 14.67 14.76

Ce 39.73 52.55 42.54 21.67 31.57 36.51 24.10 29.86 34.65 33.35 33.37 33.50

Pr 5.29 6.75 5.59 3.07 4.31 4.89 3.31 3.98 4.58 4.40 4.42 4.42

Nd 25.77 31.10 26.40 15.18 21.13 23.72 16.40 19.18 21.54 20.80 21.28 21.05

Sm 6.73 7.61 6.65 4.31 5.59 6.13 4.60 4.99 5.45 5.29 5.70 5.26

Eu 2.19 2.33 2.20 1.53 1.88 2.03 1.61 1.69 1.77 1.76 1.85 1.71

Gd 7.51 8.32 7.39 5.01 6.13 7.03 5.20 5.75 5.96 5.87 6.36 5.77

Tb 1.15 1.27 1.14 0.82 0.95 1.07 0.87 0.91 0.94 0.94 1.00 0.90

Dy 6.91 7.56 6.73 5.07 5.72 6.35 5.24 5.58 5.75 5.75 6.11 5.58

Ho 1.37 1.48 1.33 1.01 1.11 1.26 1.06 1.14 1.19 1.15 1.23 1.14

Er 3.60 3.91 3.53 2.76 2.87 3.34 2.90 3.02 3.17 3.14 3.33 3.04

Tm 0.50 0.55 0.49 0.39 0.39 0.46 0.40 0.42 0.45 0.43 0.46 0.42

Yb 3.50 3.86 3.36 2.69 2.71 3.15 2.79 2.92 3.13 3.09 3.19 3.04

Lu 0.46 0.52 0.44 0.37 0.36 0.43 0.38 0.40 0.43 0.41 0.43 0.42

Hf 4.69 5.76 4.83 2.98 3.92 4.39 3.16 3.52 3.93 3.92 4.08 3.89

Ta 0.94 1.22 1.07 0.48 0.75 0.87 0.42 0.50 0.70 0.69 0.56 0.70

Pb 1.72 2.80 1.86 1.16 0.97 1.53 1.49 1.59 1.65 1.61 2.25 1.97

Th 1.99 3.19 2.06 1.29 1.40 1.84 1.32 1.90 1.64 1.62 2.18 1.61

U 0.40 0.67 0.56 0.28 0.33 0.52 0.35 0.58 0.40 0.34 0.38 0.41


123

Table 2 continued

PMD13

dyke

Basalt,

subalk

PMD14

dyke

Basalt,

subalk

PMD15

dyke

Basalt,

subalk

PMD16

dyke

Basalt,

subalk

PMD17

dyke

Basalt,

subalk

PMD18

dyke

Basalt,

subalk

PMS1

sill

Basalt,

subalk

PMS2

sill

Basalt,

subalk

Ref.

Std.

BCR-2

±2r Meas.

Std.

BCR-2

±2r

Wt.%

SiO2 48.94 48.63 49.14 50.16 49.49 49.08 51.13 49.90 54.10 1.60 54.89 0.70

TiO2 2.90 2.53 2.31 2.20 2.23 2.75 3.13 2.83 2.30 0.10 2.15 0.17

Al2O3 12.14 11.80 12.87 12.80 12.33 12.14 12.80 14.37 13.50 0.40 13.47 0.09

Fe2O3T 17.26 16.23 15.12 16.02 16.50 16.69 16.88 13.87 13.80 0.40 13.53 0.08

MnO 0.20 0.20 0.19 0.20 0.21 0.21 0.21 0.16 0.20 0.02 0.19 0.01

MgO 4.46 5.59 7.15 6.30 7.00 6.63 5.89 3.36 3.59 0.10 3.22 0.18

CaO 9.67 10.03 10.93 10.56 10.40 10.24 10.03 9.70 7.12 0.22 6.81 0.07

Na2O 2.05 1.90 2.00 2.13 2.04 1.81 2.32 2.34 3.16 0.22 2.93 0.11

K2O 0.61 0.49 0.34 0.41 0.50 0.38 0.56 0.75 1.79 0.10 1.87 0.05

P2O5 0.25 0.27 0.22 0.21 0.20 0.27 0.31 0.44 0.35 0.04 0.38 0.10

Total 98.48 97.67 100.27 100.99 100.90 100.20 103.26 97.72 99.91 99.34

LOI 1.18 0.74 0.85 0.58 0.50 1.62 0.43 1.58

Mg# 37.7 44.6 52.5 47.9 49.8 48.1 44.9 36.1

BHVO-2 ±2r BHVO-2 ±2r

ppm

Sc 31.89 35.34 31.96 33.78 33.64 34.33 34.80 22.32 31.0 2 31 2

V 512.80 397.80 354.30 390.50 397.80 403.20 443.00 305.50 329.0 18 330 10

Cr 349.30 384.40 505.50 517.90 684.30 512.90 437.40 337.10 285.0 28 281 10

Co 45.13 46.08 44.33 46.79 51.52 48.13 49.39 30.03 47.0 4 46 2

Ni 37.42 46.71 67.30 57.40 69.66 67.58 46.19 30.20 112.0 18 110 4

Rb 16.12 12.12 8.38 9.72 13.16 8.58 13.20 18.45 10.1 1.2 10.1 0.2

Sr 172.90 173.40 171.40 175.60 168.40 173.30 189.40 224.80 382.0 20 376 12

Y 31.67 29.97 26.07 26.92 25.49 28.69 30.08 34.05 23.0 2 22.8 0.8

Zr 157.40 145.80 120.40 122.80 128.10 155.30 167.90 206.50 160.0 16 159 6

Nb 10.18 10.62 10.25 7.32 7.72 11.94 13.92 18.19 16.4 0.14 16.4 0.4

Ba 165.5 125.4 70.14 106.3 102.6 122 165.1 261.4 128.0 8 124 4

La 14.68 14.00 11.16 10.17 11.28 14.41 14.98 20.79 15.6 0.12 15.5 0.6

Ce 33.29 31.94 25.21 23.97 25.82 32.64 34.30 46.45 37.0 2 36.6 1.2

Pr 4.38 4.24 3.37 3.26 3.45 4.29 4.57 5.99 5.0 0.6 4.96 0.12

Nd 21.17 20.22 16.34 15.92 16.56 20.38 21.95 28.02 24.0 2 23.8 0.8

Sm 5.54 5.28 4.42 4.48 4.42 5.08 5.67 6.89 5.8 1.0 5.8 0.2

Eu 1.86 1.72 1.51 1.56 1.48 1.66 1.86 2.21 2.0 0.2 1.96 0.06

Gd 6.35 5.83 4.96 5.13 4.91 5.62 6.24 7.41 5.9 0.8 5.82 0.22

Tb 1.00 0.94 0.82 0.84 0.78 0.89 0.98 1.13 0.86 0.06 0.85 0.04

Dy 6.18 5.71 4.95 5.19 4.81 5.38 5.87 6.77 4.9 0.8 4.82 0.2

Ho 1.23 1.15 1.01 1.05 0.95 1.10 1.17 1.34 0.91 0.12 0.9 0.02

Er 3.31 3.12 2.67 2.81 2.61 3.01 3.05 3.48 2.3 0.2 2.29 0.10

Tm 0.45 0.44 0.37 0.38 0.36 0.41 0.43 0.47 0.3 0.10 0.3 0.02

Yb 3.15 3.10 2.56 2.69 2.54 2.98 2.96 3.29 2.0 0.4 2.0 0.2

Lu 0.43 0.42 0.35 0.36 0.34 0.41 0.40 0.44 0.26 0.08 0.26 0.02

Hf 4.01 3.85 3.10 3.14 3.10 3.82 4.10 5.00 4.1 0.8 4.1 0.2

Ta 0.57 0.60 0.55 0.42 0.42 0.68 0.80 1.03 0.94 0.14 0.95 0.04

Pb 1.96 2.10 1.77 1.51 2.01 1.91 1.75 2.35 1.4 0.4 1.70 0.06

Th 2.15 1.61 1.40 1.32 1.68 1.59 1.78 2.32 1.18 0.18 1.17 0.04

U 0.60 0.42 0.28 0.33 0.36 0.34 0.38 0.64 0.44 0.06 0.45 0.24

Samples PMD8, PMD9, PMD6, PMD12 and PMS1 are petrographically basalts, PMD3 is a plagioclase-megaporphyritic basalt; all other samples are dolerites.

Full field and petrographic descriptions are available from the authors. Fe2O3T is total iron measured as Fe2O3. Mg# (Mg number) = [atomic Mg/

(Mg?Fe2?)] 9 100. ‘BCR-2 meas.’ is average of eight measurements, and ‘BHVO-2 meas.’ is average of six measurements. Reference values for BCR-2 are

from Wilson (1997), and reference values for BHVO-2 are from Gao et al. (2002) and Weis et al. (2005)


123

of the lower Tamia flow, which are classified with the

Poladpur, and sample 6A from the middle flow, classified

with the Jawhar–Igatpuri. For most samples, in both runs,

the Mahalanobis distances to nearest formation centroids

are 10 or more, and the highest is 86.6. Thus, the DFA not

only is inconclusive but also underscores the significant

dissimilarity of the Pachmarhi intrusions to the Western

Ghats sequence.

Multielement patterns

To further evaluate their stratigraphic affinities, the

Pachmarhi dyke and sill data were compared with data for

individual flows or members from various southwestern

Deccan formations using primitive-mantle-normalized

multielement patterns. Several southwestern formations

have distinct multielement patterns; for example, the

Ambenali and Poladpur can be distinguished on the basis of

the former’s lack of significant Pb peaks. Little-altered

samples of the Mahabaleshwar and Poladpur can be distin-

guished from each other by the former’s normalized K lower

than normalized Nb and Ta. The Poladpur and Khandala,

both considerably contaminated by continental lithosphere,

show sizeable Pb peaks and sometimes Nb–Ta troughs rel-

ative to Th and La. They can be mutually distinguished from

the overall slope of the pattern from left to right; the Poladpur

being almost flat and the Khandala considerably steeper.

Patterns of dykes PMD1, 2, 3, 6 and 9 are strongly like

those of the Mahabaleshwar Formation, though somewhat

less ‘‘enriched’’ in La, Ce, Nb, Ta, and elements from Sm

to Tm than the Mahabaleshwar (Fig. 6a). Dykes PMD10,

12 and 18, and the Chakhla–Delakhari sill sample PMS1

also resemble the Mahabaleshwar Formation in their mul-

tielement patterns (Fig. 6b). All these rocks systematically

have higher normalized Nb and Ta compared to normalized

K, though alteration-related K loss–or gain–renders K

relationships unreliable.

On the other hand, dykes PMD4, 7 and 15 are distinctly

Poladpur-like (Fig. 6c). A typical Khandala pattern is

shown for comparison in Fig. 6c and is notable for its

steeper slope than the Poladpur’s. Dykes PMD8, 11, 13, 14

and 17, as well as sill sample PMS2, show some resem-

blance to Khandala Formation lavas in their multielement

patterns (Fig. 6d, e). Note the distinct Nb–Ta troughs

in these patterns as well as their steep slopes. Finally,

dyke PMD5 has some key characteristics of Ambenali

Formation lavas in its multielement pattern; note the

characteristic absence of a sizeable Pb peak and the lack of

a Nb–Ta trough, which contrast with the Poladpur pattern

(Fig. 6f).

Dykes PMD8 and PMD9 merge in the field (Fig. 2), but

were sampled away from the point of merger. Figure 6a, d

show that they are distinct geochemically, although

this may be alteration-related in part (with the biggest

Table 3 Sr, Nd and Pb isotope data for Pachmarhi dykes and sills

Sample (87Sr/86Sr)m87Rb/86Sr (87Sr/86Sr)t (143Nd/144Nd)m

147Sm/144Nd (143Nd/144Nd)t eNd(t) 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

PMD2 0.70548 0.334 0.70516 0.512716 0.144 0.512654 ?2.0 17.576 15.397 38.189

PMD3 0.70780 0.178 0.70763 0.512755 0.152 0.512689 ?2.7 17.929 15.439 38.351

PMD6 0.71602 0.131 0.71589 0.512768 0.156 0.512701 ?2.9 17.954 15.450 38.522

PMD7 0.70765 0.112 0.70754 0.512691 0.169 0.512618 ?1.3 19.157 15.673 39.632

PMD11 0.70751 0.223 0.70731 0.512664 0.162 0.512594 ?0.8 19.281 15.695 39.907

PMD13 0.70919 0.270 0.70894 0.512669 0.158 0.512601 ?0.9 – – –

PMD14 0.70733 0.202 0.70714 – 0.158 – – – – –

PMD17 0.70666 0.226 0.70644 0.512655 0.161 0.512585 ?0.6 19.223 15.690 39.789

PMS1 0.70491 0.202 0.70472 0.512779 0.156 0.512712 ?3.1 17.939 15.440 38.487

PMS2 0.70574 0.237 0.70552 – 0.149 – – – – –

The 143Nd/144Nd and 87Sr/86Sr values have been age-corrected to t = 66 Ma (subscript m indicates the measured value; subscript t is the age-

corrected value); Pb isotopic ratios are present-day values, as in most previous Deccan studies. For calculating eNd the following chondritic

average values are used: (143Nd/144Nd)p = 0.512638 (147Sm/144Nd)p = 0.1967 (143Nd/144Nd)66 Ma = 0.512553. 87Rb/86Sr and 147Sm/144Nd

isotopic ratios were calculated from the whole-rock Rb and Sr and Sm and Nd concentrations, respectively, measured by ICPMS (see Table 2).

Within-run 2r errors for the data were between 0.000010 and 0.000016 for 87Sr/86Sr and 0.000006 and 0.000010 for 143Nd/144Nd, corresponding

to a 2r uncertainty of 0.2 units on eNd. BHVO-2 analyzed for Sr and Nd isotopes gave values of 0.70345 and 0.512986, respectively, comparable

to those obtained by Weis et al. (2005) (0.703481 ± 0.000020 and 0.512983 ± 0.000010 on unleached sample; 2r errors). Sr and Nd isotope

ratios were corrected for fractionation using 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. The average values for NBS 987 Sr

and JNdi Nd analyzed over a period of 3 years are 87Sr/86Sr = 0.71024 ± 0.00002 and 143Nd/144Nd = 0.512100 ± 0.000010 (±0.2 eNd units) at

the 2r level of uncertainty. The value of 143Nd/144Nd = 0.512100 for JNdi corresponds to a value of 0.511843 for the widely used La Jolla Nd

standard (Tanaka et al. 2000). Pb isotopic data are reported relative to the NBS 981 Pb values of Todt et al. (1996). Reproducibility for NBS 981

(n = 44) is ±0.009 for 206Pb/204Pb, ±0.011 for 207Pb/204Pb, and ±0.028 for 208Pb/204Pb. Within-run 2r errors on the data were less than the

external uncertainties on this standard. Total procedural blanks for Pb were negligible at \30 pg


123

differences in Rb and K). Dykes PMD10 and PMD12 are

both highly sinuous and were suspected to be the same dyke

in the field; the former crops out south of the large dyke

PMD11, whereas PMD12 intrudes PMD11. Samples

PMD10 and 12 are both very similar geochemically and

Mahabaleshwar-like (Fig. 6b), whereas the dyke that they

intrude (PMD11) is strongly Khandala-like (Fig. 6d), con-

sistent with the southwestern Deccan stratigraphic order.

Dyke PMD2 is about 5 km north of the Chakhla–

Delakhari sill (Fig. 2), and is very similar geochemically to

it (Fig. 6a, b). PMD2 may be fed from the sill, or vice versa.

On the other hand, dyke PMD3 meets the Kandadhana

saucer-shaped sill (sample PMS2), and whereas PMD3 is

strongly Mahabaleshwar-like in its multielement pattern

(Fig. 6a), PMS2 is distinctly Khandala-like (Fig. 6e), and

no simple relationship is possible between the two. PMD13

and PMS2 are nearby and both similar to the Khandala

(Fig. 6d, e); however, dyke PMD16, which is much closer

to PMS2, is quite distinct and Poladpur-like (Fig. 6c).

We also plotted the data of Sen and Cohen (1994) for

the Chakhla–Delakhari sill and Tamia lavas on the same

diagrams to compare them to the southwestern formation

data. Sen and Cohen (1994) did not obtain clear, conclusive

matches of the data to particular formations with a Nb/Y

versus Rb/Y plot. Figure 6g shows that the sill samples

resemble the Mahabaleshwar Formation, as do the Tamia

lower and middle flow samples (Fig. 6h, i). The upper

flow’s affinity is somewhat ambiguous (Fig. 6j). Matches

obtained with the DFA were, however, different, even

between the two DFA runs.

Whereas many Pachmarhi intrusions have patterns that

correspond rather closely to patterns of the Mahabaleshwar,

Poladpur, and Khandala Formations (Fig. 6), one important

difference is that the patterns of many samples of this study,

as well as some of Sen and Cohen (1994), have pronounced

Ti peaks (relative to Eu and Gd) whereas the southwestern

Deccan patterns do not. In mafic volcanic rocks Ti is mainly

sequestered in titanomagnetite or ilmenite (e.g. Thy et al.

2008). We do not observe local accumulation of these oxide

minerals in these rocks under the microscope. A question is

whether we are seeing differing degrees of magma evolu-

tion or fractionation. A simple plot of Mg# with TiO2

(Fig. 7) shows that these parameters are inversely corre-

lated in the Deccan flood basalts (the typical tholeiitic trend

of iron enrichment, Carmichael 1964). The Pachmarhi

intrusions have significantly lower Mg# values (54.1–36.1)

and higher TiO2 contents than half the southwestern Deccan

lavas. Sen (1980, 1983) and Sen and Cohen (1994) have

shown that the Chakhla–Delakhari sill has a highly frac-

tionated composition and shows evidence for periodic

recharge and mixing with fresh small batches of Fe-rich

mafic liquids, as well as local Fe–Ti-oxide-rich pockets.

Their data for the sill and the Tamia lavas, plotted in Fig. 7,

bear this out. These rocks have significantly lower Mg#

values and significantly higher TiO2 contents than the

southwestern Deccan lavas, and a few of their samples have

normalized multielement patterns with large Ti peaks

relative to Eu and Gd.

However, the Ti peaks in the Pachmarhi intrusions’

patterns, as against non-existent Ti peaks (or Ti troughs)

observed in their closest southwestern Deccan lavas, can-

not be explained by the much more evolved compositions

of the Pachmarhi rocks, inasmuch as these rocks do not

show comparably elevated concentrations of the other

incompatible elements (like Nb and Zr). Also, whereas

even massive plagioclase fractionation, not reflected in the

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

46 47 48 49 50 51 52 53 54 55 56

SiO2 (wt.%)

Na 2O

+ K

2O (

wt.%

)

Western Ghats (B88)Chikaldara (P98)Mhow (P98)Jabalpur (P98)Toranmal (M00)C-D sill (S&C94)Tamia lavas (S&C94)PMD dykesPMS sills

Basalt

Basaltic andesite

Trachybasalt

Irvine & Baragar 1971

Macdonald & Katsura 1964Basaltic

trachyandesiteFig. 5 Total alkali-silica

diagram of Le Bas et al. (1986)

showing data for the Western

Ghats sequence (n = 624,

Beane 1988), basalts from the

Chikaldara (n = 33), Mhow

(n = 32) and Jabalpur (n = 18)

areas in the northern and

northeastern Deccan (Peng et al.

1998), the Toranmal section in

the northern Deccan (n = 27,

Mahoney et al. 2000), as well as

the Pachmarhi dykes and sills

and Tamia lavas (data of this

study and Sen and Cohen 1994).

All data are LOI-free values


123

Mg#, may cause Eu (and Sr) to drop rapidly relative to Ti,

this would not change the Ti/Gd ratios. The Ti peaks are

therefore not caused by the Pachmarhi rocks being more

evolved than many southwestern Deccan lavas. We discuss

this later.

Sr, Nd and Pb isotopic ratios have been by far the most

useful line of evidence in regional correlations among the

exposed sections in the Deccan province (e.g. Mahoney

et al. 2000), in correlating dykes or groups of dykes to flow

packages (e.g. Bondre et al. 2006; Vanderkluysen et al.

2006), as well as in identifying lithospheric contaminants

of the Deccan magmas. To these, therefore, we now turn.

Isotopic data: implications for geochemical correlations

and petrogenesis

Isotopic ratios of Sr, Nd and Pb are not affected by frac-

tional crystallization, and are known not to change

appreciably by moderate amounts of post-eruption sub-

aerial alteration (e.g. Mahoney et al. 2000). Additionally,

the southwestern Deccan stratigraphic formations are

mostly very well separated in Nd–Sr–Pb isotope space

(Fig. 8). The fields they define have been interpreted

as mixing arrays between Ambenali-like magmas and

continental lithospheric materials of various types (e.g.

2

10

70

Rb Ba Th U K Nb Ta La Ce Pr Pb Sr Nd P Zr Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu

Mahabaleshwar avg.PMD1 - adj. @ LuPMD2 - adj. @ LuPMD3 - adj. @ LuPMD6 - adj. @ LuPMD9 - adj. @ Lu

3

10


Mahabaleshwar avg.

PMD10 - adj. @ Lu

PMD12 - adj. @ Lu

PMD18 - adj. @ Lu

PMS1 - adj. @ Lu

4

10


Poladpur avg.

PMD4 - adj. @ Lu

PMD7 - adj. @ Lu

PMD15 - adj. @ Lu

PMD16 - adj. @ Lu

Khandala (Giravli, JEB128)

50

60

rock

/prim

itive

man

tlero

ck/p

rimiti

ve m

antle

rock

/prim

itive

man

tle

(a)

(b)

(c)

Fig. 6 Comparison of

primitive-mantle-normalized

multielement patterns

(normalizing values from Sun

and McDonough 1989) of the

Pachmarhi dykes and sills (data

of this study), and the Chakhla–

Delakhari sill (data of Sen and

Cohen 1994) with those of

selected southwestern Deccan

lavas, members, or formation

averages (main data sources are

Beane et al. 1986; Beane 1988;

P. R. Hooper and J.J. Mahoney,

unpublished data). Many

patterns are adjusted to the same

LuN value (where LuN is Lu

concentration normalized to that

estimated in primitive mantle)

for easy visual comparison and

to compensate for differing

degrees of fractionation


123

Mahoney et al. 1982; Lightfoot and Hawkesworth 1988;

Peng et al. 1994). Whereas the Sr–Nd isotopic arrays

suggested a simple scenario of mixing between an Ambe-

nali-like mantle end member and various continental

lithospheric or crustal end members, Peng et al. (1994)

concluded that two stages of mixing were required by the

Sr–Pb and Nd–Pb isotopic arrays. They proposed that the

first stage occurred between an Ambenali end member and

variable amounts of high-206Pb/204Pb continental litho-

sphere (possibly lithospheric mantle). This also produced

the ‘‘common signature’’ magmas with relatively restricted

isotopic (and chemical) variation, common to several of

the lower stratigraphic formations of the Western Ghats

stratigraphy. The elongated arrays for the southwestern

formations in the Sr–Pb and Nd–Pb plots were suggested to

represent a second stage of mixing between various first

stage mixing products (including the common signature)

and low-206Pb/204Pb material (possibly lower crust). Peng

et al. (1998) found that the northeastern Deccan lavas

around Jabalpur have systematically higher 206Pb/204Pb

ratios than their southwestern counterparts, such that they

define a field essentially aligned with the stage 1 mixing

array that was postulated by Peng et al. (1994). Peng et al.

(1998) therefore concluded that the northeastern lavas as a

group had undergone less of the second stage mixing with

low-206Pb/204Pb continental lithospheric material, and thus

should have erupted from different vents than the south-

western lavas.

rock

/prim

itive

man

tlero

ck/p

rimiti

ve m

antle

4

10

70



Khandala (Giravli, CAT194)

PMD8 - adj. @ Lu JEB128

PMD11

PMD13

4

10

70



Khandala (Giravli, CAT194)

PMD14

PMD17 - adj. @ Lu JEB128

PMS2

(d)

(e)

3

10

40


rock

/prim

itive

man

tle Poladpur avg.

Ambenali avg.

PMD5 - adj. @ Lu Amb

(f)

Fig. 6 continued


123

3

10

100

Rb Ba Th U K Nb Ta La Ce Pr Pb Sr Nd P Zr SmEu Ti Gd Tb Dy Y Ho Er TmYb Lu

Mahabaleshwar avg.PMS1 (this study)D33A (C-D sill, S&C94)D43 (C-D sill, S&C94)D4 (C-D sill, S&C94)D34 (C-D sill, S&C94)D1A (C-D sill, S&C94)D5 (C-D sill, S&C94)

3

10

100


Mahabaleshwar avg.

60A (Tamia Flow 1, S&C94)

10B (Tamia Flow 1, S&C94)

97B (Tamia Flow 1, S&C94)

97C (Tamia Flow 1, S&C94)

Poladpur avg.


Ambenali avg.

3

10

100


Mahabaleshwar avg.

Poladpur avg.


14AA (Tamia Flow 2, S&C94)

14BB (Tamia Flow 2, S&C94)

6A (Tamia Flow 2, S&C94)

3

10

100


Mahabaleshwar avg.

Poladpur avg.


108 (Tamia Flow 3, S&C94)

Ambenali avg.

PMS1 (this study)

(g)

(h)

(j)

(i)

rock

/prim

itive

man

tlero

ck/p

rimiti

ve m

antle

rock

/prim

itive

man

tlero

ck/p

rimiti

ve m

antle

Fig. 6 continued


123

The isotopic compositions of most Pachmarhi intrusions

are consistent with the two-stage mixing model of Peng

et al. (1998), as Fig. 8a, b show. The initial eNd values

(eight samples) are all positive and show a very small range

(?0.6 to ?3.1). This strongly contrasts with the large range

of initial eNd values documented for the Deccan lavas (from

?7.5 to -20.2, e.g. Lightfoot and Hawkesworth 1988;

Peng et al. 1994; Chandrasekharam et al. 1999; Mahoney

et al. 2000).

The dyke PMD2 falls within the field for the Mahab-

aleshwar Formation of the southwestern Deccan in all three

isotope plots. The Chakhla–Delakhari sill PMS1 falls

within the area of overlap of the Ambenali and Mahab-

aleshwar formation fields, in all three plots. It is noteworthy

that the change from Ambenali to Mahabaleshwar magma is

transitional in the southwestern Deccan, with much isotopic

overlap, and the upper Ambenali lavas show some

Mahabaleshwar-like characteristics (Najafi et al. 1981;

Mahoney et al. 1982; Devey and Cox 1987). This may

explain how PMS1 can be more Mahabaleshwar-like than

Ambenali-like in its normalized multielement pattern

(Fig. 6). The proximity of the samples PMS1 and PMD2 in

the field (5 km) and their relative close similarity in mul-

tielement patterns as well as in isotope ratios, suggests that

both may be genetically related. PMS1 in particular does

not appear to be significantly contaminated by lithospheric

material.

Dyke samples PMD3 and PMD6 do not have clear

affinities with any southwestern Deccan formation. Both

are strongly Mahabaleshwar-like in their normalized mul-

tielement patterns (Fig. 6). Both lie in the Mahabaleshwar

Formation field in the Nd–Pb isotope plot (Fig. 8a), but in

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

30.0 35.0 40.0 45.0 50.0 55.0 60.0 65.0 70.0

Western Ghats (B88)Chikaldara (P98)Mhow (P98)Jabalpur (P98)Toranmal (M00)C-D sill (S&C94)Tamia lavas (S&C94)PMD dykesPMS sills

Mg#

TiO

2 (w

t.%)

Fig. 7 Mg number (Mg#)-TiO2 diagram for the Deccan flood basalts,

showing data for the Western Ghats (southwestern Deccan) sequence

(n = 624, Beane 1988), basalts from the Chikaldara (n = 33), Mhow

(n = 32) and Jabalpur (n = 18) areas in the northern and northeastern

Deccan (Peng et al. 1998), the Toranmal section in the northern

Deccan (n = 27, Mahoney et al. 2000), the Chakhla–Delakhari sill

(n = 6) and Tamia lavas (n = 8) (Sen and Cohen 1994) as well as the

Pachmarhi dykes (n = 18) and sills (n = 2) of this study. All data are

LOI-free values

(b)

(a)

(c)

Fig. 8 Isotopic data for the Pachmarhi dykes (D-samples) and

Chakhla–Delakhari sill (S1). The letters ‘PM’ have been removed

from the sample numbers to avoid cluttering. Fields are shown for

southwestern Deccan formations, lavas of the Jabalpur area, and

normal MORB (mid-ocean ridge basalts) from the Central Indian

Ridge (CIR) (from Peng et al. 1998). Also plotted are data for Mhow

(M-samples) and Chikaldara lavas (C-samples) from Peng et al.

(1998), as well as two lavas from Toranmal (SH-samples) from

Mahoney et al. (2000). Heavy arrows in (a) and (b) schematically

illustrate the two-stage model of contamination proposed by Peng

et al. (1994). The stippled area represents the ‘‘common signature’’

from which fields for several southwestern formations diverge and

which appears to have been an important stage 1 magma type in the

lower portion of the southwestern lava pile (Peng et al. 1994). Fields

for the Jawhar–Igatpuri, Thakurvadi and Panhala Formations have

been left off one or more panels to avoid cluttering. Analytical errors

on data points are smaller than the size of the symbols


123

the Sr–Pb isotope plot (Fig. 8b) PMD3 lies in the area of

overlap between the Poladpur and Khandala Formations,

whereas PMD6 lies outside of the Bushe Formation field,

with still lower 206Pb/204Pb. In the Sr–Nd isotope plot

(Fig. 8c) both are well outside any of the southwestern

fields, and PMD6 particularly has a highly unusual com-

position, with both high eNd and high 87Sr/86Sr. This dyke

is highly weathered in outcrop, with moderate LOI (1.77

wt.%). Assuming it was originally isotopically Mahab-

aleshwar-like, introduction of a Sr-rich secondary phase

(like calcite) with very high 87Sr/86Sr might explain its

characteristics, or a Rb-rich alteration phase (like mica)

would have to have developed early on and in highly vis-

ible amounts to raise the Sr isotopic ratio of the sample in

66 million years. Both mechanisms can be rejected based

on petrographic observations. We believe that the anoma-

lous Sr–Nd isotopic composition of sample PMD6 is a

primary feature, and at least one other intrusion in the

northeastern Deccan has broadly similar characteristics (J.

Mahoney, unpublished data). If PMD6 represents mixing

between an Ambenali magma and a high-Nd–Sr isotopic

ratio contaminant (hitherto unknown), the nearly flat mix-

ing curve requires closely similar Sr/Nd elemental ratios in

both end members. Or, this apparent flat mixing curve is

only part of a larger, strongly parabolic, mixing trend.

Much more Sr–Nd isotopic data on nearby Deccan intru-

sions and lavas and their basement rocks than presently

available in the literature are required to model PMD6 with

any confidence.

Dyke samples PMD7 (Denwa River), PMD11 (Satdhara

dyke) and PMD17 (Dhupgarh dyke) are the thickest dykes

encountered. Whereas PMD7 resembles the Poladpur

Formation in its normalized multielement pattern, the other

two resemble the Khandala (Fig. 6). Sr isotopic values for

the three are rather different (Fig. 8). All three lie at the

high-206Pb/204Pb end of the Poladpur Formation field, but

do not have 206Pb/204Pb ratios as high as the Jabalpur area

lavas, except that the Dhupgarh dyke PMD17, sampled at

an elevation close to 1,200 m above mean sea level, lies

within the field of the Jabalpur flows in the Sr–Nd isotope

plot. Whereas they are not exactly Jabalpur-like in all

isotopic plots, the three dykes could correspond to an

intermediate degree of the proposed stage 2 contamination

(more stage 2 contamination than the Jabalpur lavas and

their as yet unidentified feeders, but less stage 2 contami-

nation than the Poladpur Formation lavas).

We have only Sr isotope data for dyke PMD14 and the

sill sample PMS2, and only Sr–Nd isotope data for the

dyke PMD13, which place it well outside the southwestern

Deccan Formation fields and relatively near PMD7, 11 and

17 in the Sr–Nd isotope plot (Fig. 8c). The one chemically

Ambenali-like dyke PMD5 (Fig. 6f) was not analyzed for

isotopes.

Correlations with the northern and northeastern

Deccan lavas

The Pachmarhi dykes can be ruled out as feeders to the

Jabalpur area lavas, based on the combined Nd–Sr–Pb

isotopic characteristics. However, data for several Pac-

hmarhi dykes having higher 206Pb/204Pb ratios than the

southwestern Poladpur Formation (Fig. 8) plot in the same

region of isotopic space as do data for many lavas of the

Mhow and Chikaldara sections, as well as for two lavas

from the Toranmal section.

Furthermore, several Pachmarhi dyke samples that clo-

sely match specific Mhow, Chikaldara, or Toranmal lava

flow samples (Fig. 8) in isotopes also closely match these

flow samples in chemical composition (Fig. 9). Thus,

Toranmal flow SH102 and Pachmarhi dyke PMD11 have,

respectively (eNd)t = ?1.3 and ?0.8 (nearly overlapping

within analytical errors), and 206Pb/204Pb = 19.523 and

19.281 (outside analytical error but not greatly different).

The two are much more different in their initial Sr isotopic

ratios (0.70579 vs. 0.70731), and no relationship can be

claimed between the two. However, Fig. 9a shows a very

close chemical similarity in the more alteration-resistant

elements between the two. Figure 9b, c show other examples

of pairs of Pachmarhi intrusions and individual northern or

northeastern Deccan lavas with broadly similar chemical

characteristics (not the Jabalpur flow 25) and ‘‘decoupled’’

Nd–Pb, Sr–Pb, and especially Nd–Sr isotopic compositions.

On the other hand, the Dhupgarh dyke PMD17 and the

Chikaldara flow C14 show a fairly close match in their

multielement patterns (Fig. 9d), and also in isotopic com-

position. Their respective values of eNd(t) = ?0.6 and ?1.0,

(87Sr/86Sr)t = 0.70644 and 0.70595, 206Pb/204Pb = 19.223

and 19.181, 207Pb/204Pb = 15.690 and 15.675, and208Pb/204Pb = 39.789 and 39.599. Overall, therefore, with a

few exceptions the individual Pachmarhi intrusions cannot

be correlated exactly or perfectly to specific northern or

northeastern lavas.

Petrogenesis

As Fig. 10a shows, the incompatible-element composi-

tions (Ti excepted) of several Pachmarhi dyke and sill

rocks are broadly consistent with the incorporation of a

small amount of typical Archaean (or Proterozoic) granitic

crust into Ambenali-type mafic magmas (the magma type

in the Deccan with the least continental lithospheric

influence, e.g. Mahoney et al. 1982). However, unlike the

Bushe lavas of the southwestern Deccan that are inferred

to have been contaminated by old, Rb-rich granitic upper

crust (e.g. Lightfoot et al. 1988, and Fig. 9b), the

Pachmarhi rocks are not significantly enriched in SiO2.


123

This, and a plot of Nb/Y versus Rb/Y (Fig. 10b) after

Peng et al. (1994), suggest the lower crust as a much more

likely contaminant for the Pachmarhi magmas. Here a

broad array defined by the Pachmarhi rocks points in the

same direction as the southwestern Deccan formations,

towards the proposed contaminants E3 (Indian basic

granulites) and E1 (Indian basic amphibolites), and the

Pachmarhi rocks can be considered mixtures between

Ambenali-type Deccan magmas, and these two lower

crustal rock types.

The exact mechanism of contamination by crust is

unlikely to be bulk mixing, because of the thermal

3

10

50


SH102 (Toranmal flow)

PMD11 (Pachmarhi dyke)

4

10

100


SH104 (Toranmal flow)

PMS1 (C-D sill)

2

10

Rb Ba K Nb Pb Sr Nd P Zr Sm Ti Y

M16A (Mhow flow)



25 (Jabalpur flow)

4

10

Rb Ba K Nb Pb Sr Nd P Zr Sm Ti Y

PMD17 (Dhupgarh dyke)

C14 (Chikaldara flow)

rock

/ pr

imiti

ve m

antle

60

40

(a)

(b)

(c)

(d)

rock

/ pr

imiti

ve m

antle

rock

/ pr

imiti

ve m

antle

rock

/prim

itive

man

tle

Fig. 9 Primitive-mantle-

normalized multielement

patterns (normalizing values of

Sun and McDonough 1989) for

Pachmarhi dyke and sill

samples compared to those of

isotopically broadly similar lava

flows from Toranmal, Mhow,

and Chikaldara sections. Data

sources for the lavas are Peng

et al. (1998) and Mahoney et al.

(2000)


123

demands it imposes on the assimilants (DePaolo 1981).

Two contrasting modes of magma contamination by

lithospheric materials, assimilation and fractional crystal-

lization (AFC, DePaolo 1981), and temperature-controlled

assimilation, have both been described from the Deccan

(Cox and Hawkesworth 1985; Mahoney et al. 2000; Sheth

and Ray 2002; Sheth and Melluso 2008; Chatterjee and

Bhattacharji 2008). The former produces a negative cor-

relation between an index of fractionation (such as Mg

Number or Sr content, the exact behaviour depending on

fractionation assemblage) and an index of contamination

(such as Ba/Zr, Ba/Ti, or 87Sr/86Sr ratios). In the other,

MgO or Mg#, an indirect measure of magma temperature,

is positively correlated with 87Sr/86Sr or similar measure of

contamination. In the Pachmarhi rocks there is no signifi-

cant correlation between 87Sr/86Sr ratios and Mg# (squared

Pearson correlation coefficient R2 = 0.013) or 87Sr/86Sr

ratios and Sr contents (R2 = 0.142) (not shown). This could

mean that AFC or temperature-controlled assimilation are

not viable mechanisms, or that the contamination was a

much more complicated process, possibly involving sev-

eral starting magmas undergoing mixing with multiple

lithospheric contaminants (Devey and Cox 1987).

Peng et al. (1994), in the absence of direct evidence on

the basement of the Western Ghats lava pile (such as

xenoliths or surface outcrops), compiled a dataset for

several Archaean basic amphibolites of western India

(Rajasthan) to model the elemental and isotopic composi-

tions of the common signature magmas which seem to have

been very important in the genesis of the lower formations.

They considered amphibolites to be much more likely

contaminants than granulites, because amphibolites require

2

4

10

60

Rb Ba Th U K Nb Ta La Ce Pr Pb Sr Nd P Zr SmEu Ti Gd Tb Dy Y Ho Er Tm Yb Lu

PMD10 (dyke)

PMD12 (dyke)

PMD18 (dyke)

PMS1 (C-D sill)

Amb. avg.

Amb. + ArchFelCrust (95:5)

rock

/ pr

imiti

ve m

antle

Khandala

Bushe

Jawhar-IgatpuriThaku

rvadi

Neral

Bhimashankar

Mahabaleshwar

Poladpur

Ambenali

Upper cont. crust

amphibolites

Khondalites

Collision granitesCharnockites

Collision granites

Indi

an b

asic

amph

ibol

ites

Lewisi

angr

anul

ites

Indi

an b

asic

gran

ulite

s

Indi

an b

asic

gran

ulite

s

Low

er c

ont.

crus

t

OIB

E-M

OR

B

N-M

OR

B

Lith

osph

eric

man

tle

N-MORB

0.5 1.0 1.5 2.00

0.20

0

0.40

0.60

0.80

Rb/Y

Nb

/Y

Reunion0.

140.

24

0.61

0.87 0.

91.

0

1.3

1.8

3.4

4.5

7.0

9.511

39

E2

E1

E3

(a)

(b)

PMD dykesPMS sills

Fig. 10 a Primitive mantle

normalized patterns of several

Pachmarhi dykes, compared to a

95:5 bulk mixture of average

Ambenali magma and average

Archaean felsic crust (Rudnick

and Fountain 1995). The

normalizing values are of Sun

and McDonough (1989). b Nb/

Y versus Rb/Y plot based on

Peng et al. (1994), showing the

fields of several lower

stratigraphic formations in the

Western Ghats, and the data for

the Pachmarhi dykes and sills.

The numbers shown along linesare the typical Rb/Nb ratios of

various continental and oceanic

rocks (references in Peng et al.

1994). E1, E2 and E3 are the

crustal contaminants inferred by

Peng et al. (1994) to have been

involved in producing the range

of chemical and isotopic

compositions seen in the lower

Western Ghats stratigraphy,

including the ‘‘common

signature’’ magmas


123

substantially lower melting temperatures or yield greater

melt fractions at given temperatures than granulites. They

were able to reproduce the chemical and isotopic compo-

sitions of the common signature magmas by 10–30% bulk

contamination of Ambenali and Reunion-type magmas by

a calculated 40% partial melt of the average Indian basic

amphibolite. Rushmer’s (1991) experimental work shows

that at 8 kb pressure (equivalent to mid-lower crustal

conditions), hornblende—a phase with a high Kd value for

Ti relative to, say, Sm, Zr, or Y (references in Peng et al.

1994)—begins to break down at about 925�C into moder-

ately siliceous melt (*55% SiO2) and an anhydrous

granulitic residue (clinopyroxene, orthopyroxene, plagio-

clase). Notably, whereas increasing modal amounts of

these residual phases, as well as ilmenite, are produced at

increasing temperatures up to *1,000�C, no residual

ilmenite accompanies these residual phases at *925–

950�C. This means Ti-rich liquids during the beginning

stages of hornblende dehydration melting (Rushmer 1991).

Bulk mixing of such amphibolite partial melts with Deccan

basalt magmas would generate mixtures with enrichments

in Ti but not in the other incompatible elements like Nb

and Zr, as well as high Ti/Eu and Ti/Gd ratios. Also, the

mixed liquids would retain their basalt, ferrobasalt or

basaltic andesite compositions (Fig. 5), and appear con-

siderably more fractionated, in terms of Mg# (Fig. 7), than

they really were (cf. Natland 2007).

Precambrian lower crustal rocks—granulites, amphibo-

lites, and gneisses—indeed outcrop a few tens of

kilometres south of Pachmarhi, where they define the

Central Indian Tectonic Zone, a Proterozoic fold belt

(Acharyya 2003). Despite much available data on the

mineral chemistry and metamorphic evolution of these

rocks (Bhowmik and Roy 2003; Bhowmik et al. 2005;

Bhowmik 2006), we have not found whole-rock geo-

chemical data for them with which to model their partial

melting. Nevertheless, we show in Fig. 11 that several of

the Pachmarhi dykes—specifically dykes PMD7, 11, and

17—are closely similar to the common signature lavas

from the lower levels of the Western Ghats stratigraphy,

and dyke PMD2 and sill PMS2 have a very strong simi-

larity to the individual common signature lava sample

SAM001 in their normalized multielement patterns,

including Ti, and with the exception of a trough at P for the

latter (data for many trace elements, including Eu and Gd,

are not available). Notably, dykes PMD7, 11, and 17 are

very close in all three isotopes (Fig. 8), and cover a very

small region in isotopic space not far from the common

signature lavas. Whereas we do not think this suggests that

the feeders of the common signature lavas are in the Pac-

hmarhi area, vast as the distance implied is (*600 km), the

result is significant in showing that (1) magmas closely

similar to the common signature magmas of the lower

Western Ghats stratigraphy were produced and emplaced

600 km to the northeast, (2) basic amphibolites such as

those used by Peng et al. (1994) are indeed a plausible

contaminant for the Pachmarhi intrusions, and (3) for

explaining closely similar geochemical signatures in Dec-

can lava sections separated by hundreds of kilometres,

independent parallel evolution is at least as viable and

attractive a hypothesis as long-distance surface transport

(cf. Sen and Cohen 1994; Sheth and Chandrasekharam

1997; Peng et al. 1998).

Discussion

The Chakhla–Delakhari sill: feeder or not?

Isotopic data on sample PMS1 from the lower chilled

margin of the Chakhla–Delakhari sill provide, for the first

time, information on the isotopic composition of the

starting magma that produced this sill, although we realize

that the sill requires much more thorough geochemical and

isotopic characterization. The PMS1 composition is

slightly lower in eNd than most Ambenali Formation lavas,

which are contaminated little by continental lithospheric

materials, and requires that the fractionation and contami-

nation that resulted in the PMS1 composition would have

occurred still earlier, before the sill was emplaced, proba-

bly in a deeper magma chamber.

The sill is intruded into the Gondwana sandstones and

shales underlying the Tamia lavas. Discriminant function

analysis classified samples of the lower flow at Tamia

dominantly with the Poladpur Formation, and those of the

middle flow with the Jawhar-Igatpuri and Poladpur For-

mations, whereas the lower and middle flows are best

matched with the Mahabaleshwar Formation on the basis

of normalized multielement patterns. The upper flow’s

affinities remain ambiguous with either tool. Isotopic data

are unfortunately unavailable for Tamia lavas. On the basis

of the multielement patterns of the lower two Tamia flows,

the combined Ambenali/Mahabaleshwar-type isotopic

composition of the sill’s lower chilled margin (PMS1), as

well as dyke PMD2, we suggest that the sill and this dyke

are potential feeders to the lower two Tamia flows.

Notably, Crookshank (1936) reported that the sill was

the feeder of the lower and upper of the three Tamia flows.

Sen (1980) considered this questionable noting that oli-

vines in these two flows (Fo87–88) were much more

magnesian than the most Mg-rich olivine in the sill (Fo76).

Subsequently, Sen and Cohen (1994) concluded that at

least the upper flow had been fed by the sill. Our finding

that the sill and the dyke PMD2 are closest geochemically

to the lower and middle flows only adds to the existing

confusion. We note here that whereas a few samples of the


123

sill, analyzed by Sen and Cohen (1994), show Ti peaks in

their normalized multielement patterns as many of our

samples do, none of their Tamia flow samples show these

peaks. The sill is well documented to be mineralogically

(and hence chemically) heterogeneous, and to show strong

zoning in all mineral phases (Sen 1980, 1983) indicative of

mineral-melt disequilibrium and possibly recharge. We

believe that in this situation the field geological evidence of

Crookshank (1936) should be given absolute credence.

The Ambenali/Mahabaleshwar-type isotopic composi-

tion of sample PMS1 is more significant, however, when

the chemically Ambenali-like lavas forming parts of the

northeastern sections (Chikaldara and Jabalpur) are con-

sidered. A couple of these lavas also have Ambenali-type

Sr–Nd–Pb isotopic ratios, whereas most have higher

206Pb/204Pb. Southwestern Ambenali-type compositions per

se, or northeastern Ambenali like compositions, are absent

in the northern sections (Toranmal and Mhow). Neither are

dykes of these compositions reported in these areas. The

source areas of the northeastern Ambenali-like lavas have

therefore remained unknown. Sample PMS1, with ‘‘south-

western-type’’ Ambenali/Mahabaleshwar characteristics,

may represent magma that fed the few chemically and

isotopically Ambenali-like northeastern lavas, whereas the

high-206Pb/204Pb but otherwise Ambenali-like northeastern

lavas may have been affected by small amounts of

high-206Pb/204Pb lithospheric material (Peng et al. 1998).

This large sill is thus probably not simply a post-volcanic

intrusion, though the difficulty is that many dykes evidently

derived from it and analyzed by us do not appear to have fed

4

10

40

Rb Ba K Nb La Ce Pb Sr Nd P Zr Sm Ti Y

'common signature'

PMD 7

5

10

100


SAM001 (com. sign.)

PMD 2

PMS2

4

10

70


Average 'common signature'

PMD11

PMD17

Mix 3a (P1994)

rock

/ pr

imiti

ve m

antle

rock

/ pr

imiti

ve m

antle

rock

/ pr

imiti

ve m

antle

(a)

(b)

(c)

Fig. 11 a–c Primitive mantle

normalized patterns of

individual Pachmarhi rocks

compared to average ‘‘common

signature’’ pattern from the

Western Ghats, as well as to

sample SAM001 from the

Jawhar–Igatpuri Formation, one

of the common signature

samples. Mix 3a in (a) is a

pattern for a 20:40:40 bulk

mixture of a 40% partial melt of

Indian basic amphibolite,

Ambenali magma, and Reunion

magma as calculated by Peng

et al. (1994)


123

any of the hitherto characterized lavas, and, the dykes that

are required to transport Ambenali/Mahabaleshwar-type

magma stored in the sill and feed the equivalent north-

eastern lavas remain to be found.

The geochemical variability of Deccan flood basalt

magmas

The Pachmarhi dykes, although all subalkalic tholeiites,

show considerable geochemical variation, particularly in

the trace elements and isotopic compositions. We have

been able to identify distinct Poladpur-type, Khandala-

type, Mahabaleshwar-type, and Ambenali-type chemical

signatures in the 18 dykes we sampled. Based on these

signatures, several dykes might be correlated with the

corresponding stratigraphic formations in the northeastern

Deccan. However, as observed elsewhere in the Deccan

(Bondre et al. 2006; Vanderkluysen et al. 2006), the

isotopic signatures in most of these samples differ signifi-

cantly from the isotopic signatures of the chemically

closest lava formations, and many of the Pachmarhi dykes

and sills show substantial Ti peaks (relative to Eu and Gd)

in their primitive-mantle-normalized multielement pat-

terns. Noting the particularly unusual Nd–Sr isotopic

composition of dyke PMD6, it is apparent that the actual

range of chemical and particularly isotopic variations in

Deccan magmas is thus greater than observed in the

southwestern Deccan lavas alone.

The mismatches exist not just between chemical and

isotopic compositions, but also among different isotope

ratios. Whereas data for one dyke and sill sample each plot

within the isotopic fields for the Ambenali and Mahab-

aleshwar Formations, almost all other Pachmarhi intrusions

have mixed characteristics and do not consistently fall in

the same formation fields in all three isotopic plots. An

interesting outcome of this study is the realization that

whereas the high-206Pb/204Pb northern and northeastern

lavas (Toranmal, Mhow, Chikaldara and Jabalpur sections)

have Sr–Nd isotopic compositions essentially like those of

the southwestern lavas (Fig. 8), several Pachmarhi dykes

(e.g. PMD7, 11, and 17) have Pb isotopic ratios like those

of some of the northern-northeastern (excluding most

Jabalpur) lavas, and yet have Sr–Nd isotopic compositions

quite different from all hitherto analyzed Deccan flood

basalt (both southwestern, and northern-northeastern)

lavas. These dykes, as also PMD3 and PMD6, have sig-

nificantly higher Sr isotopic ratios for a given (eNd)t value

compared to all Deccan lavas.

Identifying feeder dykes: what to expect?

Under ideal circumstances, individual feeder dykes would

correlate with the lava flows they generated, in chemical

(barring local differences due to crystal settling, alteration

conditions, etc.) and isotopic characteristics, as well as

magnetic polarity. In the Columbia River CFB province,

individual dykes have been unambiguously correlated with

particular flows using these tools (e.g. Hooper 1997 and

references therein). The Columbia River province is much

younger (Miocene) than the Deccan and many of its basalts

much less weathered than the Deccan basalts, where

alteration is ubiquitous (Mahoney 1988; Baksi 2007). Also,

noting (1) possible lateral geochemical heterogeneity in

both major dykes and lava flows fed by them (e.g. Baker

et al. 1991), and (2) the likelihood that a single sample

from a long dyke will in most cases not represent the full

range of chemical and isotopic variation within it, we

suspect that Columbia-River-type correlations between

individual dykes and flows may be much more difficult to

achieve in the Deccan.

Thus, Bondre et al. (2006) find some dykes of the

Sangamner area in the Western Ghats (Fig. 1) to have close

chemical and Nd isotopic similarities to particular flow

packages, but to have small but significant Sr isotopic

differences outside of analytical error. Vanderkluysen et al.

(2006) find that about half the Western Ghats dykes studied

by them that closely resemble particular stratigraphic for-

mations chemically, have significantly different Sr–Nd–Pb

isotopic compositions from those of the same formations.

The vast scale of Deccan magmatism, the abundant

evidence for open system processes (e.g. Cox and

Hawkesworth 1985; Peng et al. 1994), and the extremely

complex geological history and heterogeneous lithology of

the basement (e.g., Ray et al. 2008 and references therein)

mean that the geochemical variability in the Deccan flood

basalt magmas and the scores of distinct geochemical

‘‘flavours’’—not necessarily stratigraphically controlled

(e.g. Mahoney et al. 2000)—are but natural.

If some of the northern or northeastern lavas were fed by

the Pachmarhi dykes or sills, what distances of lateral

magma transport in the crust or flow of lava on the surface

are required? Despite the potential of flood basalt flows to

cover large distances after eruption (e.g. Keszthelyi and

Self 1998) and proven examples of lava flows in the

Columbia River province having flowed several hundred

kilometres from their vents (Tolan et al. 1989), the distance

from Pachmarhi to Toranmal appears daunting, at 500 km.

However, Mhow is 350 km from Pachmarhi, and Chik-

aldara is only a modest 150 km. We therefore consider that

some of the lava flows forming the northern and north-

eastern sections (e.g. flow C14 from Chikaldara, flow

M16A from Mhow, and, more tentatively, flows SH102

and SH104 from Toranmal), having very close chemical

and isotopic relationships to specific Pachmarhi dykes, and

whose eruptive vents remain unidentified in any case, may

have originated in the Pachmarhi area.


123

Conclusions

Deccan dykes and sills that intrude the Satpura Gondwana

Basin around Pachmarhi, central India are subalkalic, like

the great majority of Deccan flood basalt lavas, but are

significantly more evolved (lower Mg#, higher TiO2

contents) than many Deccan basalts. They may be prod-

ucts of mixing between Deccan basalt magmas and partial

melts of basic Precambrian Indian amphibolites. An

evaluation of the dykes and sills as feeders to the Deccan

lavas, particularly the lavas of the eastern and northeast-

ern Deccan Traps, finds some strong chemical similarities

between particular dykes and sills on the one hand, and

lava flows on the other. However, only a few of these are

matched by Sr–Nd–Pb isotopic values. These Pachmarhi

dykes appear petrogenetically broadly related to some

lava flows of the northern and northeastern Deccan region

(Toranmal, Mhow, and Chikaldara sections), and if they

were feeders to these flows the distances of transport

implied are 150–350 km. The large Chakhla–Delakhari

sill and a nearby [10-km-long dyke may have fed some

of the northern-northeastern Ambenali- or Mahabalesh-

war-like lavas. The geochemical variability of the Deccan

flood basalts—both elemental and isotopic—is greater

than perceived in the Western Ghats region. This study

highlights both the unusual geochemical compositions of

Deccan dykes and sills around Pachmarhi, and the prob-

lems and ambiguities that can remain, even with extensive

elemental and isotopic fingerprinting, in the task of dyke-

sill-flow correlations in large and complex CFB provinces

such as the Deccan.

Acknowledgments Field work was supported by seed grant

03IR014 to Sheth from the Industrial Research and Consultancy

Centre (IRCC), IIT Bombay. We gratefully remember the late

Nandkumar Gavli for his valuable field assistance. We thank S.V.S.

Murty, Coordinator, PLANEX Programme (ISRO), for allowing use

of XRF and ICPMS facilities at the Physical Research Laboratory.

Pb isotopic work was supported by the US National Science Foun-

dation grant EAR02-29824 to Mahoney. We thank Andreas Bunger,

D. Chandrasekharam, Neil Goulty, Nitin Karmalkar, James Natland,

Kanchan Pande, Sverre Planke, Stephane Polteau, Sheila Seaman,

Gautam Sen, Sam Sethna, Surendra Pal Verma, and S. Viswanathan

for helpful discussions. The manuscript was considerably tightened

and improved by the thoughtful official reviews of Nilanjan Chat-

terjee and Gautam Sen, and the editorial handling of Timothy Grove.

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