Petrology, geochemistry and magnetic properties of Sadara sill: Evidence of rift related magmatism...

Petrology, geochemistry and magnetic properties of Sadara sill:

Evidence of rift related magmatism from Kutch basin, northwest India

Arijit Ray a,*, S.K. Patil b, D.K. Paul a, S.K. Biswas c, Brindaban Das a, N.C. Pant d

a Department of Geology, Presidency College, College Street, Kolkata 700073, Indiab Magnetic Observatory, IIG, Alibag 402201, Raigad, Maharashtra, India

c Department of Earth Science, IIT, Bombay, Powai, Mumbai, Maharashtra, Indiad Geological Survey of India, Faridabad, India

Received 21 July 2004; received in revised form 29 August 2005; accepted 9 September 2005

Abstract

Mafic volcanic rocks of the Mesozoic Kutch basin represent the earliest phase of Deccan volcanic activity. An olivine-clinopyroxene-

plagioclase-phyric undersaturated basalt occurs as a sill near Sadara in the Pachham upland, Northern Kutch. The Sadara sill is deformed and

emplaced along faults. The sill is alkaline in character and is transitional between basalt and basanite. Compared to primitive mantle, the Sadara

sill is enriched in Sr, Ba, Pb and LREE but depleted in Nb, Cr, Y, Cs and Lu. Fractional crystallization of olivine and clinopyroxene from an

alkaline mafic melt generated by low degree partial melting of mantle peridotite can explain the observed chemical variation in the sill.

IRM and L-F test experiments and mineral analyses show titano-magnetite as the major remanence carrying magnetic mineral. AF and thermal

demagnetizations of the Sadara sill yielded a mean ChRM direction as DZ315.68, IZK43.08 (a95Z9.78; kZ25.38) and the corresponding VGP

at 258S; 114.68E (dp/dmZ6.588/11.68). The Sadara sill pole is significantly different from those of the Deccan (65 Ma) and the Rajmahal Traps

(118 Ma) but is close to the Cretaceous poles of 85–91 Ma rock units from southern India. This suggests a pre-Deccan age for the sill.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: Deccan volcanism; Kutch (India); Geochemistry; Petrogenesis; Palaeomagnetism

1. Introduction

Deccan Trap is one of the well-studied Large Igneous

Provinces. It is generally agreed that the bulk of the Deccan

Volcanic rocks erupted around 65G1 Ma (Vandamme et al.,

1991; Pande et al., 1988; Pande, 2002) and that the earliest

Deccan volcanism occurred in the northwestern part of the

Province (Mahoney, 1988; Basu et al., 1993; Courtillot et al.,

2000). The role of the Reunion Plume (Duncan and Richards,

1991) in the generation of Deccan magma has been accepted by

most of the Deccan researchers.

The east–west trending Kutch Rift Basin is located in the

western margin of India. The rift extends from the western

continental shelf to the Radhanpur-Barmer arch (Fig. 1).Middle

Jurassic to early Cretaceous rift-fill sediments are covered by the

Deccan trap in the south and west. Magmatic rocks of the Kutch

Rift Basin have so far been identified as tholeiites and alkali

1367-9120/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jseaes.2005.09.006

* Corresponding author.

E-mail address: [email protected] (A. Ray).

basalts (De, 1964; Krishnamurthy et al., 1999). Some of these

alkali basalts and one tholeiite sample have been dated by Pande

et al. (1988). The 64 to 67 Ma age of alkali basalts and tholeiites

ofKutch compareswell with the age ofDeccan eruption.Mantle

xenolith bearing alkali basalts are fairly common in the central

part (Karmalkar et al., 2000; Karmalkar and Sarma, 2003;

Krishnamurthy et al., 1989) while tholeiites have been reported

from the southern part (Biswas, 1993).Magmatic rocks from the

northern part are relatively less studied. Magmatic rocks

occurring to the north of the Kutch Rift Basin are petrologically

different from Deccan tholeiite and alkali basalt. Their mode of

emplacement in most cases are fault controlled. Sadara sill,

intruding the Goradongar Formation, is a 7 km long body which

was emplaced along faults (Fig. 2). This sill has also been

affected by folding. The sill is made up of phenocryst charged

melanocratic basaltic rocks, geochemically different from

Deccan tholeiite. The present paper attempts to characterize

the Sadara sill from petrological, geochemical and paleomag-

netic points of view and explores its temporal relation with the

main Deccan eruption.

Journal of Asian Earth Sciences 27 (2006) 907–921

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Fig. 1. Regional tectonic map of Kutch (after Biswas, 1980) with the inset map of India showing the location of Kutch. IBF, Island Belt Fault; KMF, Kutch Mainland

Fault; NPF, Nagar Parkar Fault; KHF, Katrol Hill Fault; NKF, North Kathiawar Fault; SWF, South Wagad Fault; GDF, Goradongar Fault; GF, Gedi Fault; NPU,

Nagar Parkar Uplift; PU, Pachham Uplift; BU, Bela Uplift; KU, Khadir Uplift; CU, Chorar Uplift; WU, Wagad Uplift.

A. Ray et al. / Journal of Asian Earth Sciences 27 (2006) 907–921908

2. Geological setting

The Kutch Basin, located in the western margin of the

Indian craton, is a pericratonic rift basin (Fig. 1; Biswas, 2002).

The Nagar Parkar Fault (NPF) bounds the rift on the north and

the North Kathiawar fault (NKF) limits it to the south. The

Precambrian Nagar Parkar Ridge and the Deccan Trap-covered

Saurashtra (Kathiawar) plateau, are the northern and southern

shoulders, respectively. The rift basin is characterized by

intrabasins, tilted uplifts and intervening half-grabens (Fig. 1).

In the northern part of the rift basin, four uplifts are present

from west to east, namely the Pachham Uplift (PU), the Khadir

Uplift (KU), the Bela Uplift (BU) and the Chorar Uplift (CU)

constituting a linear belt called the ‘Island Belt Uplift’ (IBU).

Island Belt Uplifts are followed southward by theWagad Uplift

(WU) and the Kutch Mainland Uplift (KMU). The uplifts are

bounded by faults along their northern margins except for the

Wagad Uplift, which is bounded by a fault along its southern

edge. The Kutch Mainland Fault (KMF) is the master fault for

KMU, the Island Belt Fault (IBF) for the IBU, and the South

Wagad Fault (SWF) for the WU.

A unique feature of the Kutch basin is a first order basement

high called the Median High (MH) across the uplifts and the

half-grabens. The MH divides the KMU symmetrically and the

PU is situated on it. Both KMU and PU are affected by

subsidiary faults uplifting their southern parts—the Katrol Hill

fault in KMU and the Goradongar Fault (GDF) in PU. The

uplifts are hilly regions where rocks outcrop but the intervening

grabens and half-grabens are extensive mud and salt flats.

Mesozoic rocks (Middle Jurassic to Early Cretaceous) are

exposed in the uplifts while the Tertiary rocks occur in the

bordering peripheral plains. The faulted edges of the uplifts are

narrow deformation zones with folds and faults. The PU

consists of Kaladongar Hills along its northern faulted margin

(marked by IBF) and Goradongar Hills along the GDF in the

southern part which are also zones of folding and igneous

activitiy.

The present study area is located on the southeastern corner

of Goradongar Hills at the eastern end of the folded zones

(Figs. 1 and 2). A mafic sill is exposed in the eastern extremity

of Goradongar Hills, north of the village Sadara

(23845 014.7 00N, 69853 057 00E). The sill is exposed on the flank

of the hills at several places (Fig. 2) due to repetition by cross

faults, which are conjugate to the master faults. The sill, here

named the Sadara sill, is 10–15 m thick. It intruded into the

upper part of the Goradongar Formation (Bathonian-Callo-

vian), between the Raimalro Limestone and Gadaputa

Sandstone members of the Pachham Group (Fig. 2b; Biswas,

Fig. 2. a: Geological Map of the Sadara sill within Goradongar Hill, Pachham island, Kutch, Gujarat. b: Stratigraphic position of the Sadara sill within the

Goradongar formation.

A. Ray et al. / Journal of Asian Earth Sciences 27 (2006) 907–921 909

1977). The Sadara sill occurs mainly within a syncline between

the Ganiapur and Kank Hill faults (Fig. 2a). The occurrence

between the faults and its displacements suggests syntectonic

intrusion. Thus it appears that its occurrence is controlled by

pre-existing faults (Biswas, 1980; 2002). It was emplaced on its

downthrown side between the soft sandstone and the hard

limestone beds, which was later folded into a syncline over the

downthrown block during subsequent tectonic episodes. The

later generation faults are responsible for repeated occurrences

of the sill. About 15 km north of the sill, an igneous plug

covering an area of about 3.5 km2 occurs at Nir Wandh

(23854 058 00N, 69856 019 00E) at the northeastern corner of the PU.

The plug is exposed at the western plunge of the Kaladongar

anticline. On the eastern plunge of the same fold, another

igneous plug known as the Kuran plug occurs (Fig. 1). A

swarm of closely spaced mafic dykes occurs at the core of

the anticline between the Kuran and Nir Wandh plugs. The

proximity of the Sadara sill to the area of igneous activity in

the Kaladongar area suggests that the sill may be related to the

same igneous episode.

3. Analytical methods

3.1. Chemical analysis

Mineral analyses were carried out using a CAMECA SX51

Electron Microprobe at the Geological Survey of India

Laboratories at Faridabad. Operating conditions were 15 KV

accelerating voltage and 12 nA beam current, and a beam size of

1 mm. Natural mineral standards were used for all the elements

except Mn and Ti for which metal standards were used.

Fresh samples that do not contain any inclusions were

ground to less than 200 mesh in a Pulverisette rock grinder

using tungsten carbide vials. International rock standard, JB-2

and BHVO were analysed along with the samples to check the

accuracy of analysis.

Table 1

Modal composition of the Sadara sill, Kutch, Gujarat, India

Attributes P/3.1 P/3.2 P/3.7 P/3.5

Olivine phenocryst 17.2 7.3 17.8 21.8

Clinopyroxene phenocryst 19.9 22 13.8 23.6

OlivineCpyroxene

micro-phenocryst

4.5 4 1.0 4.3

Plagioclase phenocryst 15.2 19 26.6 16.1

OlivineCpyroxene in

ground-mass

7.2 9.8 6.3 6.0

Plagioclase in ground-mass 23.8 30.5 17.8 20.3

Opaque in ground-mass 3.4 2.9 6.7 2.7

Remaining fine grained

ground-mass

8.8 4.5 10.0 5.2

TOTAL 100.00 100.00 100.00 100.00

Percentage ground-mass 43.2 47.7 40.8 34.3

Percentage phenocryst 56.8 52.3 59.2 65.7

Olivine % in phenocryst 30.3 14.0 30.0 33.2

Clinopyroxene % in

phenocryst

35.0 42.0 23.3 35.9

Plagioclase % in phenocryst 26.8 36.3 44.9 24.4

Fig. 3. (a) and (b): Photomicrographs of olivine, clinopyroxene and plagioclase

phenocrysts set in a groundmass consisting of plagioclase, olivine,

clinopyroxene and opaque minerals. Length of the bar is 1 mm.


Major element analysis was carried out using X-ray

Fluorescence Spectrometry and the trace elements by

ICP-MS at the National Geophysical Research Institute,

Hyderabad. The precisions for major and trace elements are

indicated in Tables 6 and 7. Total iron was measured as Fe2O3,

which was then converted to FeO and divided between FeO

and Fe2O3. 85% of the total iron was allotted as FeO and the

remaining 15% as Fe2O3. Major and trace element compo-

sitions of International standards BHVO-1 and JB 2,

respectively, analysed during the present analysis are also

given in Tables 6 and 7 along with their certified results.

3.2. Magnetic analysis

Magnetic compass oriented block samples were collected

for palaeomagnetic and rock magnetic studies. The samples

were cored to prepare standard sized (2.5 cm!2.2 cm)

samples. Natural remanent magnetization (NRM), magnetic

susceptibility and low field AMS measurements were carried

out prior to the demagnetizations. A JR-5A Spinner magnet-

ometer was used for NRM measurements and MS-2B was used

for susceptibility measurements. AMS measurements on 22

samples were done using KLY-2 Kappabridge (M/S Agico,

Czech Republic). The Molspin Pulse magnetizer (UK,

maximum field of 1T) has been used for isothermal remanent

magnetization experiments.

4. Petrography

The Sadara sill is dark coloured, inequigranular with

phenocrysts of olivine, pyroxene and plagioclase set in a

mesostasis of finer grained olivine, clinopyroxene and

plagioclase with opaque minerals and glass (Fig. 3a and b).

All three constituent minerals show bimodal size distribution.

The modal composition of the Sadara sill (Table 1) shows the

abundances of the major constituent phases as phenocryst and

groundmass. Among the phenocrysts, olivine (14–33%),

clinopyroxene (23–42%) and plagioclase (24–45%) constitute

the assemblage. The phenocrystal phases together comprise

almost 60% (52–65%) by volume of the rock. There are size

variation of phenocrystal grains; in most samples, the olivine

phenocrysts vary in size from 3 to 8 mm, clinopyroxene from 3

to 5 mm and the plagioclase phenocrysts from 2 to 4 mm.

Olivine and clinopyroxene also occur in the size range of 1 to

3 mm and are considered microphenocrysts in the modal

analyses (Table 1). The groundmass grains of all the

constituent minerals are anhedral. Groundmass olivine varies

in size from 0.1 to 0.3 mm while groundmass pyroxene and

plagioclase are of nearly equal size, varying between 0.1 and

0.4 mm. The rocks display various types of texture including

porphyritic; ophitic-subophitic and intergranular. Glomeropor-

phyritic texture is common where plagioclase grains form

aggregates. Intergranular texture with plagioclase microlite

enclosing clinopyroxene is also present.

Most of the olivine phenocryst grains are euhedral to

subhedral. Some of the olivine grains have been altered to

serpentine along cracks and some of the olivine phenocrysts

commonly show reaction embayment. Opaque inclusions are

also present within some olivine grains. Most of the larger

olivine grains are zoned. Pyroxene phenocrysts form short

prismatic euhedral grains, while all plagioclase phenocrysts

occur as subhedral laths. Pyroxene in the rock is mainly augite

Table 2

Chemical composition of Olivine from the Sadara sill, Kutch, Gujarat, India

Oxide Core Rim Core Core Core Core Gm Core Rim Core Core Rim Core Gm* Gm

SiO2 40.51 35.14 40.19 37.84 38.64 39.97 36.25 40.32 35.56 40.32 39.29 36.26 37.39 37.22 38.71

Al2O3 0.08 0.10 0.06 0.07 0.02 0.09 0.01 0.02 0.02 0.07 0.00 0.06 0.05 0.07 0.08

TiO2 0.00 0.02 0.00 0.00 0.02 0.04 0.00 0.00 0.07 0.00 0.00 0.05 0.00 0.02 0.06

FeO 10.91 33.88 12.36 18.92 20.19 10.33 28.97 14.45 37.55 13.15 15.11 30.97 27.64 28.63 23.89

MgO 48.41 28.35 47.9 40.91 39.28 47.49 31.9 44.25 25.74 44.53 43.05 30.53 33.68 31.93 36.15

MnO 0.17 0.58 0.19 0.28 0.45 0.00 0.58 0.22 0.84 0.20 0.27 0.62 0.37 0.44 0.42

CaO 0.30 0.56 0.25 0.39 0.27 0.25 0.43 0.20 0.48 0.24 0.33 0.52 0.37 0.43 0.40

Cr2O3 0.16 0.10 0.01 0.08 0.00 0.05 0.03 0.00 0.00 0.08 0.00 0.01 0.03 0.07 0.00

NiO 0.00 0.00 0.00 0.08 0.10 0.35 0.24 0.30 0.14 0.31 0.28 0.06 0.18 0.09 0.12

Na2O 0.00 0.05 0.06 0.01 0.01 0.01 0.03 0.00 0.05 0.02 0.01 0.00 0.00 0.04 0.00

K2O 0.00 0.00 0.01 0.00 0.00 0.02 0.00 0.03 0.03 0.00 0.01 0.00 0.00 0.00 0.00

Total 100.54 98.78 101.03 98.58 98.98 98.6 98.44 99.79 100.48 98.92 98.35 99.08 99.71 98.94 99.83

Cation on the basis 4 (O)

Si 0.995 0.988 0.988 0.987 1.007 0.998 0.996 1.012 0.999 1.015 1.006 0.999 1.014 1.012 1.016

Al 0.002 0.003 0.002 0.002 0.000 0.003 0.000 0.000 0.001 0.002 0.000 0.002 0.002 0.002 0.003

Ti 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.001 0.001 0.00 0.001

FeI2 0.224 0.797 0.254 0.413 0.440 0.216 0.666 0.303 0.882 0.277 0.324 0.713 0.702 0.651 0.524

Mg 1.772 1.189 1.755 1.590 1.525 1.767 1.307 1.656 1.078 1.671 1.643 1.253 1.234 1.294 1.414

Mn 0.004 0.014 0.004 0.006 0.010 0.000 0.010 0.005 0.020 0.004 0.006 0.015 0.014 0.010 0.009

Ca 0.008 0.017 0.007 0.011 0.007 0.010 0.010 0.005 0.014 0.006 0.009 0.015 0.015 0.013 0.011

Cr 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.002 0.000

Ni 0.000 0.000 0.000 0.002 0.002 0.010 0.010 0.006 0.003 0.006 0.006 0.001 0.001 0.002 0.003

Total 3.005 3.008 3.010 3.013 2.991 3.004 2.999 2.987 2.999 2.983 2.994 2.999 2.983 2.986 2.981

Fa 11.160 39.510 12.570 20.430 22.190 10.840 33.310 15.41 44.23 14.13 16.33 35.73 35.70 33.08 26.770

Fo 88.240 58.590 86.880 78.170 76.560 88.500 64.750 83.32 53.32 84.99 82.46 62.01 62.71 65.12 71.600

Table 3

Chemical composition of Plagioclase Feldspar from the Sadara sill, Kutch,

Gujarat, India

Oxide Core Core Rim Core Groun-

dmass

SiO2 47.53 46.54 54.88 49.05 52.6 51.52 51.4

TiO2 0 0.04 0.1 0.11 0.19 0.11 0.05

Al2O3 32.39 32.6 27.34 30.5 29.44 30.1 29.95


(titanaugite) and salite as evidenced by their pleochroic

character (feebly pleochroic from light yellow to green) and

z^c values around 35–408. Pyroxene and plagioclase pheno-

cryst grains are distinctly zoned. Reaction structures and

zoning are common in the pyroxenes. Pyroxene phenocrysts

are frequently twinned. The symmetric extinction angle of

plagioclase varies between 30 and 408 indicating labradorite to

bytownite compositions.

Cr2O3 0 0 0 0.12 0.01 0 0.04

FeO 0.58 0.33 0.34 0.44 0.71 0.63 0.73

NiO 0.09 0 0.02 0 0 0.01 0.05

MnO 0 0.09 0 0.06 0.02 0 0

MgO 0.07 0.04 0 0 0.05 0.03 0.06

CaO 16.41 16.69 9.88 0.1 12.06 13.64 12.99

BaO 0 0.03 0.09 15.06 0.01 0 0.07

Na2O 2.21 1.92 4.33 2.68 3.8 3.6 3.9

K2O 0.1 0.09 1.85 0.16 0.15 0.14 0.12

Total 99.38 98.37 98.83 98.28 99.04 99.78 99.36


Si 2.200 2.176 2.514 2.285 2.402 2.349 2.355

Ti 0.000 0.002 0.004 0.004 0.007 0.004 0.002

Al 1.770 1.797 1.476 1.675 1.584 1.618 1.617

Fe3 0.020 0.013 0.013 0.017 0.027 0.024 0.028

Fe2 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Mn 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Mg 0.010 0.003 0.000 0.007 0.004 0.002 0.004

Ba 0.000 0.001 0.002 0.000 0.000 0.000 0.001

Ca 0.813 0.836 0.485 0.752 0.590 0.660 0.638

Na 0.198 0.174 0.384 0.242 0.336 0.318 0.346

K 0.006 0.006 0.108 0.009 0.009 0.008 0.007

Cations 5.017 5.008 4.986 4.991 4.959 4.983 4.998

Or 0.006 0.006 0.111 0.009 0.010 0.008 0.007

Ab 0.195 0.171 0.393 0.241 0.359 0.323 0.349

An 0.799 0.823 0.496 0.750 0.631 0.669 0.644

4.1. Mineral chemistry

Thecompositionsof cores and rimsofphenocrysts, groundmass

grains and the opaque mineral are given in Tables 2–5.

(a) Olivine: It is themost dominant phenocryst. The core and rim

compositions of olivine phenocryst and groundmass grains

are given in Table 2. The forsterite content of the core varies

between76and88molepercentwhile the rim is iron rich.The

forsterite content in the rims is around 65 mole percent. The

forsterite content in the groundmass olivine (Fo55–65) is also

similar to that found in the rims of the phenocrysts.

(b) Plagioclase: The compositions of the cores and rims of

plagioclase phenocrysts and the composition of groundmass

plagioclase are given inTable 3. The core of the plagioclase is

usually Ca rich (An80–82) compared to the rim (usually An50–

70). Groundmass plagioclase grains have a rather uniform

composition of An63–67.(c) Clinopyroxene: Clinopyroxene is less abundant than olivine

in the phenocryst assemblage, but pyroxene ismore abundant

as a groundmass constituent. The composition of the

Table 4

Chemical composition of Pyroxene from the Sadara sill, Kutch, Gujarat, India

Oxide Core Rim Core Rim Core Rim Core Rim Gm Gm

SiO2 48.31 48.34 48.31 46.83 47.66 47.18 48.9 47.04 47.02 45.71

TiO2 1.43 1.98 0.9 2.19 1.5 3.05 1.57 2.99 2.63 3.22

Al2O3 5.44 3.5 5.94 5.41 7.36 5.38 5.75 6.05 6.14 6.76

Cr2O3 0.35 0.02 0.43 0.09 0.39 0 0.53 0.05 0.08 0.06

FeO 5.77 8.59 5.23 7.47 6.04 8.98 5.93 8.24 8.05 8.78

NiO 0 0 0 0 0 0 0 0 0.03 0.19

MnO 0.05 0.16 0.15 0.13 0.17 0.12 0.06 0.17 0 0.22

MgO 14.69 12.82 14.77 13.33 13.35 11.18 13.81 11.73 12.22 11.42

CaO 22.17 22.16 22.01 22.06 21.87 22.2 23.51 22.15 22.72 22.4

BaO 0 0.02 0.06 0.1 0 0 0 0 0.04 0

Na2O 0.32 0.54 0.34 0.44 0.6 0.59 0.4 0.58 0.49 0.42

K2O 0 0.01 0.01 0.02 0.01 0.04 0.03 0.01 0.02 0.03

Total 98.53 98.14 98.15 98.07 98.95 98.72 100.49 99.01 99.44 99.21


Si 1.802 1.880 1.804 1.773 1.774 1.796 1.797 1.778 1.765 1.730

Al iv 0.198 0.157 0.196 0.227 0.226 0.204 0.203 0.222 0.235 0.270

Al vi 0.041 0.000 0.066 0.014 0.097 0.037 0.045 0.047 0.036 0.031

Ti 0.040 0.057 0.025 0.062 0.042 0.087 0.043 0.085 0.074 0.092

Fe3 0.088 0.088 0.091 0.118 0.077 0.038 0.088 0.047 0.085 0.087

Cr 0.010 0.001 0.013 0.003 0.011 0.000 0.015 0.001 0.002 0.002

Mg 0.817 0.727 0.822 0.752 0.740 0.634 0.756 0.661 0.684 0.644

Ni 0.000 0.000 0.000 0.000 0.000 0.000 0.002 0.000 0.000 0.000

Fe2 0.092 0.180 0.072 0.118 0.111 0.248 0.094 0.214 0.167 0.191

Mn 0.002 0.005 0.005 0.004 0.005 0.004 0.002 0.005 0.000 0.007

Ca 0.886 0.903 0.881 0.895 0.872 0.906 0.925 0.897 0.914 0.908

Na 0.023 0.040 0.025 0.032 0.043 0.044 0.029 0.042 0.036 0.031

K 0.000 0.000 0.000 0.001 0.001 0.002 0.002 0.001 0.001 0.002

Cations 3.999 4.038 4.000 3.999 3.999 4.000 4.001 4.000 3.999 3.999

Wo 47.053 47.576 47.213 47.531 48.444 49.617 49.651 49.313 49.405 49.617

En 43.388 38.303 44.051 39.936 41.111 34.721 40.580 36.339 36.973 35.191

Fs 9.559 14.120 8.735 12.533 10.444 15.663 9.769 14.349 13.622 15.191


pyroxene phenocrysts varies between Wo50.65En35.47Fs13.88and Wo52.12En42.57Fs5.31 (Table 4). The core is slightly Mg

rich (En43) compared to the rim (En35). Groundmass

pyroxenes are relatively iron rich (around Fs10).

(d) Opaque minerals: The composition of the opaque minerals,

mostly Fe–Ti oxide (titano-magnetite) is given in Table 5.

These occur as anhedral grains in the groundmass.

5. Geochemistry

Major and trace element compositions of seven Sadara whole

rock samples along with CIPW norms are shown in Tables 6

and 7. All seven samples are similar in terms of their major

element composition, but in terms of trace element compositions

define two distinct groups. The Sadara sill is silica under-saturated

(around 45% silica) basaltic rock with moderately high MgO.

MgO contents range between 9.2 and 10.7 wt% with mg number

varying between 64 and 68. The CaO and FeO contents of the

rocks are higher compared to continental flood basalt (Wilson,

1989) and TiO2 is around 2%. All the samples have Na2O/K2O

ratios around 3. Normative values indicate presence of nepheline,

forsteritic olivine and wollastonitic and enstatitic pyroxene along

with 40% plagioclase (Ab and An in nearly equal proportions).

On the basis of total alkali vs. silica (Fig. 4 after Le Bas, 2000) the

Sadara sill is classified as ‘transitional basalt’ transcending the

boundary between basalt and basanite-tephrite.

The LOI data of the Sadara samples vary from 1.0 to 1.3%.

Petrographic evidence of alteration is not conspicuous, although a

possibility exists that the abundances of Sr, Ba and Rb may vary

due to alteration. In view of the evidence cited above, we tend to

interpret that the concentration of the elements in the Sadara

samples have not significantly changed. The inter-element

relationship of some trace elemental abundances and ratios are

plotted in Fig. 5. Two samples (nos. P/3.1 and P/ 3.7) are different

from the others in having lower Ba/Zr values of 2.18 and 2.24

compared to Ba/Zr values of 4.09 to 4.65 in the other samples.

These also have lower Ba/Nb ratios and Sr content. This reflects a

lower Ba and Sr content in these two samples compared to the rest

(Table 7). Among the trace elements, Rb/Nb, Ba/Zr and LREE

tend to increase with contamination by or addition of LILE-rich,

Nb-deficient continental crust, but Sr decreases in the liquid with

fractional crystallization (Chatterjee and Bhattacharji, 2001). In

the mg no vs. Zr diagram (Fig. 5a), all seven samples show

continuous variation with a very gentle negative slope indicating a

slow rate of increase of Zr during progressive differentiation. In a

plot of Ba/Zr against Sr (Fig. 5b), a positive correlation (rZ0.94)

is seen suggesting a role for fractional crystallization. In the

SREE vs. Sr plot (Fig. 5c), a positive correlation is clear,

indicating the plagioclase fractionation. The Ba/Nb vs. Sr plot

Table 5

Chemical composition of Opaque minerals from the Sadara sill, Kutch, Gujarat,

India

Oxide

SiO2 0.35 0.08 0 0.04 0.05 0.02 0.08

TiO2 20.87 26.36 25.96 25.07 27.24 26.54 27.66

Al2O3 1.08 2.19 2.29 1.31 2.1 2.66 1.69

Cr2O3 0.02 0.12 0 0 0.06 0.09 0.01

FeO 69.4 65.84 64.49 64.26 64.39 63.68 63.72

NiO 0 0.25 – – 0.06 0.12 0.05

CoO – – – – 0.02 0.08 0.25

MnO 0.98 0.91 0.77 0.93 0.71 0.68 0.76

MgO 0.18 0.75 1.96 0.85 1.52 1.97 1.45

CaO 0.21 0.12 0 0.14 0.02 0.03 0

BaO 0.05 0.15 0.07 0.13 – – –

Na2O 0 0.02 0 0.01 0.01 0.11 0

K2O 0.09 0 0 0.01 0 0 0.03

Total 93.23 96.79 95.54 92.75 96.18 95.98 95.7


Si 0.014 0.003 0.000 0.002 0.002 0.001 0.003

Ti 0.612 0.746 0.735 0.740 0.770 0.748 0.790

Al 0.050 0.097 0.102 0.061 0.093 0.117 0.076

Cr 0.001 0.004 0.000 0.000 0.002 0.003 0.000

V 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Fe3 0.706 0.402 0.429 0.457 0.362 0.382 0.338

Fe2 1.558 1.670 1.600 1.653 1.662 1.614 1.685

Mn 0.032 0.029 0.025 0.031 0.023 0.022 0.024

Mg 0.010 0.042 0.110 0.050 0.085 0.110 0.082

Ni 0.000 0.008 0.000 0.000 0.002 0.004 0.001

Ca 0.009 0.005 0.000 0.006 0.000 0.000 0.000

K 0.004 0.000 0.000 0.001 0.000 0.000 0.000

Total 2.996 3.006 3.001 3.001 3.001 3.001 2.999


(Fig. 5d) reflects a positive correlation also indicating dual effects

of fractional crystallization and assimilation. A gentle positive

slope is observed between SREE and Nb/Y (Fig. 5e) suggesting a

decrease in Nb/Y with differentiation. Also Nb/Y ratios tend to

decrease with increasing depletion of garnet in the mantle source

during partial melting (R.W. Kent personal communication). A

positive correlation in the variation of Cr and Ni (Fig. 5f) indicates

simultaneous fractionation of olivine and clinopyroxene from the

parent melt. Interestingly, SREE (except Tm and Tb) and Ni are

inversely correlated (rZK0.89), possibly reflecting removal of

olivine from the melt. In Fig. 5b, d and f involving parameters like

Ba/Zr, Ba/Nb, Sr, Cr and Ni, two samples (Sample Nos. P 3.1 and

P 3.7) are different from the others. Fig. 5a–f show bivariate

geochemical relations where the average chemical composition of

Deccan tholeiites (Thompson et al., 1983) are distinctly different

from the Sadara sill, but the average chemical composition of

alkali basalts of Kutch (our unpublished data) are either grouped

together with or lies close to the Sadara samples.

The SREE in the Sadara samples vary from 160 to 187 ppm

and (La/Lu)n varies between 10.99 and 11.82. There is no

discernible Eu anomaly in any of the samples. In a primitive

mantle normalized diagram (Fig. 6), all the elements show 10 to

100 times enrichment over primitive mantle. Of these elements,

Sr, Nb, Ta, Ba, Th, La and Pb show positive highs (greater amount

of enrichment) whereas K, Rb, Nd, Ce, Sm, Y and Yb show

negative dips. The primitive mantle normalized patterns of the

Sadara sill and OIB (OIB data from Sun and McDonough, 1989)

are broadly similar (Fig. 6). Sadara samples show positive spikes

for Ba, Pb and Zr and negative dips for Rb compared to OIB. The

trace element abundances of alkali basalt of the Kutch Mainland

(our unpublished data) are also plotted for comparison. On a

chondrite-normalized (Chondrite data from Evensen et al., 1978)

diagram (Fig. 7), the samples show enrichment of the LREE over

HREE. On this diagram, the LREE pattern of the Sadara samples

show an overall enrichment compared to Deccan tholeiite (data

from Thompson et al., 1983) but depletion for HREE. The REE

patterns of Sadara samples and Kutch alkali basalts (our

unpublished data) are very similar.

6. Rock magnetic studies

6.1. Paleomagnetism

Magnetic susceptibilities were observed in the range of 8 to

16!10K3 SI, with a mean of 12.08!10K3 SI units; the NRM

intensities were in the range of 1.61 to 9.90 A/m (meanZ5.12 A/m). Koenigsberger ratios (QZNRM/kH) were calculated

and were found to be in the range of 3.5 to 25.1 with a mean of

3.5, indicating the suitability of the rocks for paleomagnetic

investigations.

The NRM directions of 26 samples were distributed in the first

and fourth quadrants of the stereonet with shallow to intermediate

positive inclinations indicating strong overprints of the present

earth field (PEF) directions. Out of 26 samples, 18 samples

representing all the block samples were subjected to stepwise AF

demagnetizations in the fields of 2.5, 5.0, 7.5, 10.0, 15.0, 20.0,

25.0, 30.0,40.0, 60.0, 80.0 and 100.0 mT fields for isolating the

characteristic remanent component (ChRM) in the samples.

Another 6 samples were used for thermal demagnetizations by

applying the thermal steps at 50, 100, 150, 200, 250, 300, 350,

400, 450, 500, 560, 585 and 600 8C. The Molspin (UK) AF

demagnetizer and MAVACS system from Geofyzika, Brno

(Prihoda et al., 1989) were used for AF and thermal

demagnetizations, respectively.

During the AF demagnetizations, a low coercivity com-

ponent of viscous origin having mostly PEF directions was

erased by the application of a 40 mT AF field. Further

demagnetization steps, particularly within the steps of 45–

80 mT, yielded north-westerly declinations along with shallow

to intermediate negative inclinations, representing the high

coercivity ChRM component in the samples. A low blocking

temperature component containing the viscous remanence was

removed by the application of thermal demagnetization steps at

around 250–400 8C and the ChRM component was recovered

by the application of 4508–600 8C thermal steps. The maximum

available 100 mT AF field could not demagnetize some of the

samples. It was also noticed during the thermal demagnetiza-

tions that there was still a 5% NRM intensity component

remaining even after the application of 600 8C, indicating some

haematite component in the samples. From the thermal

demagnetization spectra (intensity decay curves), it could be

observed that titano-magnetite was the major remanence carrier

with a minor influence of haematite. Fig. 8a and b represent the

behavior of the specimens during AF and thermal demagnetiza-

tions, respectively.

Table 6

Major element composition and CIPW norms of the Sadara sill, Kutch, Gujarat, India

Oxides/

Sample No.

P/3.1 P/3.2 P/3.3 P/3.4 P/3.5 P/3.6 P/3.7 Standard

(BHVOK

1) Litera-

ture value

Observed

Value

(NGRI)

Precision %

SiO2 45.29 45.52 45.94 45.41 46.76 46.35 46.75 49.94 49.89 0.93

TiO2 1.87 1.88 1.87 2.01 1.85 2.03 1.8 2.71 2.67 1.86

Al2O3 13.04 12.64 13.14 13.14 12.94 13.61 12.41 13.80 13.71 0.57

Fe2O3 1.80 1.77 1.74 1.82 1.71 1.75 1.75 12.23 12.19 0.03

FeO 9.20 9.03 8.88 9.31 8.75 8.95 8.94 – – –

MnO 0.16 0.17 0.17 0.17 0.16 0.17 0.16 0.16 0.11 0.32

MgO 9.23 10.72 9.87 10.11 9.67 9.01 9.66 7.23 7.12 2.37

CaO 11.57 12.15 11.91 11.93 12.04 11.76 11.13 11.41 11.33 1.67

Na2O 2.74 2.99 3.05 3.1 2.98 3.13 2.54 2.26 2.21 0.59

K2O 0.99 0.96 0.99 0.97 1.03 1.11 0.94 0.52 0.11 2.36

P2O5 0.52 0.51 0.58 0.52 0.57 0.62 0.53 0.27 0.21 2.78

LOI 1.00 1.19 1.23 1.30 1.12 1.20 1.17 – – –

Total 97.41 99.53 99.37 99.79 99.56 99.69 97.78 100 99.55

mg no. 64 68 67 66 66.5 64.4 66

Na2OCK2O

3.73 3.95 4.04 4.07 4.01 4.24 3.48

CIPW norms of Sadara sill, Kutch

Normative

mineral

P/ 3.1 P/ 3.2 P/ 3.3 P/ 3.4 P/ 3.5 P/ 3.6 P/ 3.7

Q (S) 0 0 0 0 0 0 0

Or (KAS6) 6.01 5.72 5.91 5.76 6.13 6.6 5.7

Ab (NAS6) 21.23 16.47 19.23 17.27 20.84 20.69 21.99

An (CAS2) 20.86 18.33 19.38 19.13 18.97 19.88 20.14

Lc(KAS4) 0 0 0 0 0 0 0

Ne(NAS2) 1.38 4.86 3.67 4.91 2.43 3.2 0

Di wo(CS) 14.59 16.43 15.37 15.56 15.76 14.66 13.89

Di en(MS) 12.57 14.16 13.25 13.42 13.59 12.64 11.97

Ol fo(M2S) 7.78 8.97 8.15 8.38 7.51 7 3.79

mt(FF) 0.54 0.56 0.56 0.56 0.53 0.56 0.54

he(F) 11.97 11.49 11.33 11.84 11.14 11.37 11.61

il(FT) 0 0 0 0 0 0 0

ap(CP) 1.17 1.12 1.28 1.14 1.25 1.36 1.19

Total 98.08 98.11 98.12 97.98 98.14 97.96 98.16


Principal component analysis (PCA) was applied to obtain the

ChRM component from the AF and thermal demagnetization data

sets. Fisher (1953) statistics was applied to find the mean ChRM

direction which was noticed as DZ315.68, IZK438 (a95Z9.78;

kZ25.38). The pole position corresponding to the mean ChRM

was calculated as 26.78N; 298.68E (dp/dmZ6.768/11.58) and the

palaeo-latitude at 20.98S. The NRM and ChRM directions of the

studied samples are shown in Fig. 9.

6.2. Rock magnetism

The Molspin Pulse magnetizer, having a maximum field of

1 T, was used for the acquisition of the isothermal remanence.

The samples were magnetized by increasing fields in the steps

from 50 to 1000 mT, and the intensity of isothermal remanent

magnetization was measured after each step using the Molspin

Spinner magnetometer. Reverse fields were also applied on the

fields to reduce the magnetization to zero and for further

saturation in the opposite direction. Fig. 10 shows the response

of IRM experiments on the samples of this study. The IRM

saturations at the fields of 200 mT indicate that titano-

magnetite was the major remanence carrying magnetic mineral

in the samples, as noted from thermal demagnetization

intensity decay curves. The coercivity of remanence was

noticed around 20–30 mT, indicating the magnetic mineral

grain size as SD to PSD type (Cisowski, 1981).

For the Lowrie-Fuller (1971) test experiments (Fig. 11), the

representative samples were AF demagnetized with increasing

fields reaching 100 mT and the remnant intensities were

measured after each step, as was in the routine AF

demagnetizations. Then they were saturated by applying the

1 T field. Following the saturation, once again they were

subjected to AF demagnetizations similar to those in the NRM

demagnetizations. The relative intensity decay pattern between

NRM and SIRM demagnetization curves indicates the grain

size of the magnetic mineral in the samples. From Fig. 11 it

can seen that the NRM decay pattern is harder than those of the

SIRM decay pattern, indicating the dominance of SD type

magnetic minerals in the samples.

Lowrie-Fuller test and IRM data suggest that the major

remanence carrying magnetic mineral was SD to PSD type titano-

magnetite in the samples. These results are supported by electron

probe analyses of the opaque phases as discussed in an earlier

section.

Table 7

Trace element abundances (in ppm) of the Sadara sill, Kutch, Gujarat, India

Elements P/3.1 P/3.2 P/3.3 P/3.4 P/3.5 P/3.6 P/3.7 JB2 Measured JB2 Certified

Ba 388.68 687.76 710.59 737.73 800.69 788.51 390.53 205.73 208

Rb 16.04 15.12 15.91 15.39 16.58 18.65 15.34 6.26 6.2

Sr 546.24 667.41 712.12 710.52 803.18 761.66 528.51 176.98 178

Y 24.84 24.21 25.05 24.75 25.2 26.46 24.38 24.52 24.9

Zr 178.26 164.22 173.9 170.33 172.27 185.96 174.49 51.21 51.4

Nb 45.42 42.97 46.38 46.59 45.05 51.36 44.34 0.96 0.8

Th 4.68 3.98 4.16 4.28 3.98 4.68 4.59 0.32 0.33

Pb 21.75 4.75 3.78 4.21 3.76 4.43 19.63 5.38 5.4

Ga 17.24 22.16 24.44 23.36 25.7 26.03 17.06 16.77 17

Zn 152.38 126.35 119.83 122.09 119.21 125.15 148.51 107.65 110

Cu 99.53 101.33 98.03 105.16 101.75 107.57 95.75 223.28 227

Ni 213.66 188.86 192.85 177.65 187.67 157.51 220.64 14.92 14.2

V 309.58 297.16 301.59 306.65 309.887 290.25 288.72 590.62 578

Cr 535.13 159.66 144.05 133.24 152.13 126.75 538.51 28.84 27.4

Hf 3.68 3.9 3.73 3.98 3.77 4.09 3.51 1.39 1.42

Cs 0.33 0.33 0.31 0.32 0.39 0.41 0.34 0.87 0.9

Sc 34.58 38.2 36.4 35.73 38.52 34.68 34.8 55.53 54.4

Ta 3.68 4.45 4.21 4.67 4.33 4.98 3.84 0.21 0.2

Co 76.95 72.46 73.74 74.65 80.79 66.37 73.72 39.63 39.8

U 0.67 0.74 0.76 0.77 0.74 0.86 0.66 0.14 0.16

Rare earth element abundances (in ppm) of Sadara intrusive, Kutch, Gujarat.

La 32.13 33.84 34.37 36.22 33.07 38.08 31.48 2.37 2.37

Ce 64.82 67.36 68.08 72 65.47 75.42 63.6 6.73 6.77

Pr 6.25 6.46 6.53 6.91 6.25 7.15 6.09 0.93 0.96

Nd 34.65 33.39 33.08 35.54 32.33 36.72 33.92 6.96 6.7

Sm 6.14 6.52 6.5 6.83 6.39 7.02 5.77 2.23 2.25

Eu 2 2.15 2.11 2.21 2.08 2.26 2 0.85 0.86

Gd 6.68 6.87 6.71 7.13 6.62 7.36 6.69 3.16 3.2

Tb 0.93 0.98 0.96 1.01 0.95 1.03 0.92 0.61 0.62

Dy 4.44 4.63 4.6 4.87 4.52 4.92 4.43 3.75 3.66

Ho 0.84 0.87 0.86 0.91 0.85 0.92 0.85 0.82 0.81

Er 2.59 2.57 2.54 2.69 2.53 2.76 2.5 2.61 2.63

Tm 0.37 0.4 0.4 0.41 0.39 0.44 0.38 0.45 0.45

Yb 1.97 2.09 2.06 2.15 2.03 2.26 1.91 2.51 2.51

Lu 0.28 0.31 0.31 0.33 0.31 0.34 0.28 0.37 0.39

SREE 164.09 168.44 169.11 179.21 163.79 186.68 160.82

A.Rayet

al./JournalofAsia

nEarth

Scien

ces27(2006)907–921

915

Fig. 4. Na2OCK2O vs. SiO2 Diagram (Le Bas, 2000) showing plots of Sadara

samples.


7. Petrogenesis

The Sadara sill is petrographically different from alkali basalts

of the Kutch Mainland in having olivine and clinopyroxene

Fig. 5. Bivariate diagrams using trace elements and selected trace element ratios sho

average Deccan tholeiite (Thompson et al., 1983) and the average of seven samples o

SREE vs. Sr; d, Ba/Nb vs. Sr; e, SREE vs. Nb/Y and f, Cr vs. Ni.

phenocrysts up to 60% (Table 1) compared to only about 15%

olivine phenocrysts in the alkali basalt (our unpublished data).

Coarse olivine and clinopyroxene crystals and their modal

proportions indicate a long period of residence of these phases

in the magma chamber prior to emplacement. The Mg rich (Fo76–

88) cores of olivine phenocrysts indicate a possible equilibration

with a basaltic liquid having MgO content around 12% (Roeder

and Emslie, 1970; Garcia, 1996; Revillon et al., 1999) that

corresponds to the observed MgO content of Sadara samples.

Similarly, the core of the plagioclase phenocrysts (An80–82) could

have been formed under equilibrium from this Ca rich mafic

parental melt. It is reasonable to assume that the parent magma

was a transitional-alkali basalt having high values of MgO and

CaO. The composition of pyroxene cores support this inferred

parent melt composition. Fractional crystallization of olivine,

clinopyroxene and plagioclase from the parent melt would cause

depletion in MgO and CaO content and concomitant enrichment

wing inter-element variation among Sadara samples and their comparison with

f Kutch Alkali basalt (our unpublished data). a, mg no vs. Zr; b, Ba/Zr vs. Sr; c,

Fig. 6. Primitive mantle normalized spidergram of Sadara sill and OIB (data of

Primitive mantle and OIB taken from Sun and McDonough, 1989).


in FeO in the residual melt. Fractionation of olivine, clinopyrox-

ene and Ca-plagioclase also caused depletion in Ni, Cr and Sr in

the melt. A positive correlation between Ba/Zr and Zr, Cr and Ni

and negative correlation between total REE and Ni suggest a role

for fractional crystallization of olivine and clinopyroxene. The

depletion of MgO in the olivine rims and CaO in the rims of

plagioclase phenocrysts indicate theMg and Ca poor nature of the

magma during the final stage of crystallization before emplace-

ment. The alkali and magnesium enriched and LREE fractionated

pattern shown by the Sadara sill is similar to undersaturated

transitional basaltic rocks from Boina Centre, Afar Rift, Ethiopia

(Wilson, 1989 p. 347).

Based on Ba/Zr and Ba/Nb values, the Sadara samples can be

classified into two groups—one having low Ba/Zr (around 2.2)

and the other having high Ba/Zr (around 4.5). From the observed

geochemical variations in the Sadara samples, fractional crystal-

lization of olivine, clinopyroxene and calcic plagioclase from an

alkaline, LILE and LREE enriched mafic melt prior to

emplacement seems plausible. The parent alkaline melt might

have been generated at 20 kb pressure (in presence of 1.9%water)

by nearly 6% partial melting of garnet peridotite mantle (Mysen

and Kushiro, 1977). The Sadara sill, being transitional between

basalt and basanite, requires a slightly greater degree of partial

melting.

Fig. 7. Chondrite normalized REE plots of the Sadara sill, average of Deccan

trap tholeiites and average of Kutch alkali basalts (data of Deccan tholeiite from

Thompson et al., 1983; data of Chondrite from Evensen et al., 1978; data

of alkali basalt from our unpublished data).

8. Discussion

In the overall geological context of the Deccan Volcanic

Province, the Kutch region is important for several reasons. There

is a significant volume of alkali basalt with or without ultramafic

xenoliths. From north to south, Kutch exposes gabbroic rocks and

alkali basalts with phenocrysts of olivine, plagioclase and

clinopyroxene such as at Sadara of the present study. A

differentiated plutonic complex occurs around Nir Wandh in

Pachham Island to the east (Biswas, 1993). Further south, alkali

basalt plugs intrude the Mesozoic sedimentary succession around

Bhuj. Tholeiitic flows corresponding to the Deccan volcanic cycle

occur along the coast as the southernmost onland east-west

trending belt. Thus there is a petrological zonation from north to

south in Kutch.

An increased interest has been shown in tholeiitic flows of

Anjar in the eastern part of Kutch (Courtillot et al., 2000) because

of the presence of a Cretaceous-Tertiary succession. Palaeomag-

netic and 40Ar–39Ar age determinations on the tholeiites of Anjar

indicate an age of 64–67 Ma, in conformity with the accepted age

of eruption for the bulk of Deccan volcanism. However, Basu

et al. (1993) reported the oldest 40Ar–39Ar age of Deccan eruption

(68.3 Ma) from the northwestern part of Gujarat and indicated

that the Reunion plume-related volcanism was initiated from

western Kutch. But age data for the basalts of the northernmost

Pachham belt is not available.

Palaeomagnetic investigations consisting of detailed AF and

thermal demagnetizations and rock magnetic experiments in the

present study indicate that the mean ChRM direction obtained

from the Sadara intrusive was of ‘primary’ origin. The obtained

VGP at 258S; 114.68E (dp/dmZ6.588/11.68) was compared

(Fig. 12) with those of the Deccan Super Pole (age: 65 Ma;

Vandamme et al., 1991), Rajmahal Traps (age: 118 Ma;

Poornachandra Rao and Mallikharjuna Rao, 1996), Central

Kerala leucogabbro Pole (age: 78–84 Ma; Radhakrishna et al.,

1994), Satyavedu Sandstone (age: 85 Ma; Mital et al., 1970),

Karnataka Cretaceous dyke (age: 90 Ma; Anil Kumar et al., 2001)

and St Mary Island Pole (age: 91 Ma; Torsvik et al., 2001), by

plotting all of them on an Apparent Polar Wander Path (APWP)

for Indo-Pakistan since the Late Palaeozoic as proposed by

Klootwijk (1984) (Fig. 12 and Table 8). From Fig. 12, it is

apparent that the Sadara Pole was distinctly different from those

of the Deccan Super Pole of 65 Ma and also from the Rajmahal

Traps of 118 Ma, but grouped well with the 78–85 Ma age rock

unit VGPs.

The transitional alkali basalts of the Pachhambelt are important

in order to understand the possible interaction of Jurassic rift-

related magmas and the Reunion hot spot to which the Deccan

eruption has been linked (Duncan andRichards, 1991). The olivine

composition of the Sadara sill (Fo76–88) is comparable with the

Fig. 8. (a) and (b) represent AF and thermal demagnetization results of the representative samples from the Sadara sill body. (i) Stereographic projections, (ii)

Normalized Intensity decay (J/Jmax) along with PTRM (hatched histograms, (J/(H for AF demagnetizations and (J/(T for thermal demagnetizations) and (iii)

Zijderveld diagram.

Fig. 9. Equal area plots of NRM directions and ChRM directions with (95confidence circle around the mean ChRM direction.


olivine composition of the Deccan picrite from Igatpuri-Triambak

(Krishnamurthy et al., 2000). The conspicuous chemical zoning in

the olivine and plagioclase of the Sadara sill reflects the

composition of the evolving host magma.

Considerable geophysical studies have been carried out in the

Kutch region following the devastating earthquake of 26 January

2001. The studies on aftershock data of this earthquake indicated

the presence of a large mafic igneous body with fluids in the lower

crust, close to the mantle, at depths of 35–40 km (Mandal et al.,

2001). Earlier, Raval (2001) suggested the presence of such a

mafic body during the Late Cretaceous Reunion hot spot activity.

The first order Median High (Fig. 1) may be a surface expression

of such a body. Thus it could be inferred that during the

extensional phase of rifting in Late Triassic-Early Jurassic time,

undersaturated transitional alkali basaltic melt from the upper

mantle intruded into the lithosphere and formed a magma

chamber in the lower crust. During subsequent tectonic episodes,

intrusions from this magma chamber invaded the upper crust and

the sedimentary succession along pre-existing deep faults mostly

as sills and dikes.

The fault-controlled emplacement of the Sadara sill in

Pachham Island might be one such magmatic event that

occurred earlier than, and unrelated to, the main phase of

Deccan eruption. The chemical zoning of the phenocrystal

Fig.10.IR

Macq

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-field

curvenorm

alizedtothehighest

applied

field

(1T).

Inthefigure,

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,theapplied

field

response

from

K40to

300mTwas

shown.

Fig.12.Figure

showing

thepresentstudy

pole(point1)

plottedwith

otherwelldefi

ned

Cretaceous

poles(Table

8).Forthe

clarityofthe

figure

andalso

forthe

comparisons,the

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ovalsfor

thepresent

pole(point

1),Deccan

Super

Pole

(point6)

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Rajm

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akistaniAPWPproposed

byKlootw

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Fig.11.Norm

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agnetizatio

nsfortheNRM

andSIRM

(L–Ftest)

Table 8

Some well-defined Paleo-poles from Indian subcontinent

Pole

No.

Rock unit Dec Inc A95 G.Lat(8N) G.Lon(8E) P.Lat

(8S)

P.Lon

(8E)

Dp Dm Age Ma Reference

1 Sadara intrusive 315.6 K43 9.78 23.79 69.8 K25 114.6 6.58 11.6 85 This study

2 Satyavedu Sandstone 321 K58 4.4 13.5 80 K26.1 113 4.8 6.5 85 Mital et al., 1970

3 Central Kerala Gabbro 307 K57 12 9.7 76.7 K21.6 119 12.7 17.5 78 Radhakrishna et al., 1994

4 St. Mary Island flows 301.2 K58.5 7.5 13.35 74.68 K14.2 118 8.2 11.1 91 Torsvik et al., 2001

5 Karnataka dyke 329 K54 3.3 12.8 77.1 K33.9 107.7 4.1 2.9 90 Anil Kumar et al., 2001

6 Deccan Super Pole 156.4 46.7 2.4 20 75 K36.9 101.3 2.4 2.4 65 Vandamme et al., 1991

7 Rajmahal Traps 316.4 K64 2.4 24.7 87.7 K9.4 118.6 3 3.7 118 Poornachandra Rao et al., 1996

A.Rayet

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phases and similarity in composition of phenocryst rims and

groundmass grains, along with the systematic increase in trace

element ratios like Rb/Nb and Ba/Zr, lead to the conclusion

that the original primitive character of the Sadara sill was

modified by progressive fractional crystallization of early-

formed olivine, clinopyroxene and plagioclase. The primitive

character of the sill is reflected by its Mg-rich composition

(Fo88) of olivine cores (Roeder and Emslie, 1970; Garcia,

1996).

9. Conclusions

Our present study of the sill occurring around Sadara in

Pachham Island indicates the occurrence of olivine-pyroxene-

plagioclase-phyric transitional alkali basalt that was emplaced

along a fault in the Kutch Rift Basin. Compared to primitive

mantle, the Sadara samples are enriched in Sr, Ba, Pb and LREE

but depleted in Nb, Cr, Y, Cs and Lu. Geochemically Sadara

samples are different from average Deccan tholeiites (Thompson

et al., 1983). The average alkali basalt of the Kutch mainland falls

on the trend defined by the Sadara samples in some geochemical

variation diagrams (Fig. 5b–d, f), although trace elements such as

Ni, Zr and Nb are higher in the former. The chemical composition

of the magmatic rocks was modified by fractional crystallization.

Palaeomagnetic data indicate an age of around 85 Ma for the sill.

It appears that the sill represents a rift related magmatic event that

occurred earlier than the Deccan eruption.

Acknowledgements

The authors thank P.K. Govil and V. Balaram of NGRI for

analytical assistance, Department of Science and Technology,

Government of India for financial support and H.N. Bhattachar-

yya for facilities at the Department of Geology, Presidency

College, Kolkata. Comments and suggestions by journal

reviewers P.R. Hooper and R.W. Kent and editorial suggestions

by Kevin Burke improved the presentation considerably.

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Petrology, geochemistry and magnetic properties of Sadara sill: Evidence of rift related magmatism...

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