Mafic xenoliths in Proterozoic kimberlites from Eastern Dharwar Craton, India:
Mineralogy and P–T regime
S.C. Patel a,*, S. Ravi b, Y. Anilkumar a, A. Naik c, S.S. Thakur d, J.K. Pati e, S.S. Nayak b
a
Department of Earth Sciences, Indian Institute of Technology, Powai, Mumbai, Maharashtra 400 076, India
Geological Survey of India, Bandlaguda Complex, Hyderabad 500 068, India
c
Department of Earth Sciences, Sambalpur University, Burla, Orissa 768 019, India
d
Wadia Institute of Himalayan Geology, 33 General Mahadev Singh Road, Dehra Dun 248 001, India
e
Department of Earth and Planetary Sciences, University of Allahabad, Allahabad 211 002, India
b
a b s t r a c t
Keywords:
Eclogite
Garnet pyroxenite
Kimberlite
Xenolith
Mafic xenoliths of garnet pyroxenite and eclogite from the Wajrakarur, Narayanpet and Raichur kimberlite fields in the Archaean Eastern Dharwar Craton (EDC) of southern India have been studied. The composition of clinopyroxene shows transition from omphacite (3–6 wt% Na2O) in eclogites to Ca pyroxene
(<3 wt% Na2O) in garnet pyroxenites. Some of the xenoliths have additional phases such as kyanite, enstatite, chromian spinel or rutile as discrete grains. Clinopyroxene in a rutile eclogite has an XMg value of
0.70, which is unusually low compared to the XMg range of 0.91–0.97 for all other samples. Garnet in
the rutile eclogite is also highly iron-rich with an end member composition of Prp26.5Alm52.5Grs14.7Adr5.1TiAdr0.3Sps1.0Uv0.1. Garnets in several xenoliths are Cr-rich with up to 8 mol% knorringite component.
Geothermobarometric calculations in Cr-rich xenoliths yield different P–T ranges for eclogites and garnet
pyroxenites with average P–T conditions of 36 kbar and 1080 °C, and 27 kbar and 830 °C, respectively.
The calculated P–T ranges approximate to a 45 mW m 2 model geotherm, which is on the higher side
of the typical range of xenolith/xenocryst geotherms (35–45 mW m 2) for several Archaean cratons in
the world. This indicates that the EDC was hotter than many other shield regions of the world in the
mid-Proterozoic period when kimberlites intruded the craton. Textural and mineral chemical characteristics of the mafic xenoliths favour a magmatic cumulate process for their origin as opposed to subducted
and metamorphosed oceanic crust.
1. Introduction
Deep-seated xenoliths in kimberlites are of great interest to
geologists for providing direct samplings of material from the
upper mantle. Ultramafic rocks (lherzolite, harzburgite, wehrlite,
dunite and pyroxenite) are the most common mantle-derived
xenoliths, whereas mafic xenoliths (garnet pyroxenite and eclogite) are less common. Garnet pyroxenite and eclogite are essentially biminerallic garnet-clinopyroxene rocks, but their
nomenclature is confused because there is often a compositional
transition between them. The term ‘eclogite’ is normally used
when the clinopyroxene is a Ca–Na pyroxene (omphacite),
whereas the rock with Ca pyroxene is called ‘garnet pyroxenite’.
The IUGS Subcommission for the nomenclature of metamorphic
rocks (SCMR) recently recommended the crystal-chemical classification of clinopyroxene by Morimoto et al. (1988) to be used for
defining omphacite (Desmons and Smulikowski, 2004 and upgrades). Following this scheme, Patel et al. (2006) identified eclog-
ite xenoliths from three kimberlite pipes of the Wajrakarur
Kimberlite Field (WKF) in the Eastern Dharwar Craton of southern
India (Fig. 1). In the present contribution we examine the mineralogical characteristics of garnet pyroxenite xenoliths from a number
of pipes of WKF, and a few pipes of the adjacent Narayanpet Kimberlite Field (NKF) and Raichur Kimberlite Field (RKF). Some newly
found eclogite xenoliths in the pipes are also included in the study.
A xenolith geotherm has been derived for the region based on
quantitative geothermobarometry.
2. Geology of Dharwar craton
The Archaean Dharwar craton is a typical granite-greenstone
terrane with a gneissic basement of tonalite-trondhjemite-granodiorite (TTG) composition known as Peninsular Gneisses (Naqvi
and Rogers, 1987). The craton is bounded in the east by the Proterozoic Eastern Ghats Mobile Belt, in the northeast by the Archaean
Bastar craton, and is covered in the northwest by the CretaceousTertiary lava flows of the Deccan Traps (Fig. 1) A striking feature
of the craton is the N–S to NW–SE trending, 400 km long and
20–30 km wide cluster of plutons known as the Closepet Granite,
337
Fig. 1. Generalised geological map of southern India modified after Drury et al. (1984) and Geological Survey of India (1998) showing kimberlite and lamproite fields in the
Eastern Dharwar Craton. BC = Bastar Craton; EDC = Eastern Dharwar Craton; KLF = Krishna lamproite field; NKF = Narayanpet kimberlite field; NLF = Nallamalai lamproite
field; TKF = Tungabhadra kimberlite field; WDC = Western Dharwar Craton; WKF = Wajrakarur kimberlite field.
dated 2.51 Ga (Friend and Nutman, 1991). Sediments of Meso- to
Neoproterozoic intracratonic sedimentary basins such as the Cuddapah basin unconformably overlie the granite – greenstone terrane. The craton is divided into two sub-provinces – Eastern
Dharwar Craton (EDC) and Western Dharwar Craton (WDC) with
Chitradurga Boundary Fault located along the eastern margin of
the Chitradurga schist belt as the boundary between them (Swami
Nath et al., 1976; Drury et al., 1984; Chadwick et al., 2000). Some
workers believe that the Closepet Granite, which is located
50 km east of the Chitradurga Boundary Fault represents the
boundary between the EDC and WDC (Naqvi and Rogers, 1987;
Gupta et al., 2003; Moyen et al., 2003). Although the actual boundary between the two cratonic blocks remains debatable there are
notable differences in lithology and metamorphism of the two
blocks. The WDC is dominantly occupied by TTG gneisses (3.0–
3.4 Ga) with minor schist belts of Sargur age (3.0–3.3 Ga), major
schist belts of Dharwar age (2.9–2.6 Ga) containing predominant
platformal sediments, and a few Late Archaean granitoid plutons
dated in the range of 2.60–2.65 Ga (Jayananda et al., 2006 and references therein). On the other hand the EDC is characterised by
voluminous Late Archaean granitoids (2. 51–2.75 Ga) (the ‘‘Dharwar batholith” of Chadwick et al., 1996, 2000) with minor TTG
gneisses and thin volcanics-dominated schist belts of Dharwar
age. The schist belts in the craton are metamorphosed to greenschist to amphibolite facies regional metamorphism. The profusion
of granitoids in the EDC is responsible for low pressure regional
metamorphism (andalusite-sillimanite type) in this block in contrast to the intermediate pressure regional metamorphism (kyanite-sillimanite type) in the WDC.
2.1. Kimberlite fields
Kimberlites discovered in southern India till now are restricted
to the EDC and are distributed in four fields, viz. WKF, NKF, RKF,
and Tungabhadra Kimberlite Field (TKF) (Fig. 1). The WKF contains
28 kimberlite pipes spread over four clusters, namely Wajrakarur
338
(13 pipes; P1–P13), Chigicherla (5 pipes; CC1–CC5), Kalyandurg (6
pipes; KL1–KL6) and Timmasamudram (4 pipes, TK1–TK4) (Nayak
and Kudari, 1999; Srinivas Choudary et al., 2007) (Fig. 2). There are
30 pipes in the NKF in four clusters, which are Narayanpet (10
pipes; NK1–NK10), Maddur (11 pipes; MK1–MK11), Bhima (BK1–
BK3) and Kotakonda (6 pipes; KK1–KK6) (Rao et al., 1998). The
RKF has 6 pipes out of which 3 pipes (SK1–SK3) occur in Siddanpalli cluster, and other three pipes (RK1–RK3) are dispersed (Sridhar
et al., 2004). The TKF is the most recently discovered kimberlite
field, which has two pipes (MNK1 and MNK2) (Ravi et al.,
2007b). Most of the WKF pipes are diamondiferous, but the NKF
and RKF pipes have not yet been proved to be diamondiferous
(Neelakantam, 2001). Detailed studies on RKF and TKF kimberlites
are yet to be attempted. In addition to the four kimberlite fields
there are two lamproite fields in the EDC, viz. Krishna Lamproite
Field and Nallamalai Lamproite Field.
Available radiometric ages for the kimberlites of WKF range
from 840 to 1150 Ma, whereas those of NKF range from 1080 to
1400 Ma (Anil Kumar et al., 1993; Chalapathi Rao et al., 1996,
1999). Chalapathi Rao et al. (1999) have suggested that the
emplacements of Kotakonda kimberlite in the NKF, and Chelima
lamproite in the NLF were contemporaneous (1400 Ma) and that
these pipes are older than the WKF kimberlites (1090 Ma). However, the older phlogopite K–Ar and Ar–Ar ages reported by Chala-
pathi Rao et al. (1999) are not borne out by the phlogopite Rb–Sr
isochron ages of Anil Kumar et al. (2001). Therefore, it can be safely
concluded that the kimberlites of EDC erupted episodically close to
1090 Ma.
3. Mafic xenoliths
Xenolith samples include hand specimens (several centimeters
in size) collected from pits and boreholes, and subrounded mineral
aggregates, also termed nodules (2–5 mm across) in heavy mineral
concentrates (Ravi et al., 2007a). In most kimberlites of EDC, mafic
xenoliths are greatly subordinate in numbers to ultramafic xenoliths (Ganguly and Bhattacharya, 1987; Nehru and Reddy, 1989).
However, the KL2 pipe of WKF is unusual as eclogites constitute
more than 95% of the xenolith population (Rao et al., 2001). In both
garnet pyroxenite and eclogite xenoliths, garnet and clinopyroxene
together constitute P80 vol%. Some mafic xenoliths are observed
to contain additional phases such as kyanite, enstatite, chromian
spinel and rutile as discrete grains which constitute 620 vol% of
the rock (Table 1). The mineral abbreviations used in this paper
are: adr, andradite; alm, almandine; cpx, clinopyroxene; grs, grossular; grt, garnet; ilm, ilmenite; kn, knorringite; omp, omphacite;
prp, pyrope; rt, rutile; sps, spessartine; uv, uvarovite.
Fig. 2. Geological sketch map of Wajrakarur kimberlite field modified from Nayak and Kudari (1999).
339
Table 1
Mafic xenoliths in kimberlite pipes of EDC
Xenolith type
Kimberlite
pipe
Hand specimens from pits and
boreholes
Nodules from
heavy mineral
concentrate
Kyanite
eclogite
KL2
–
Enstatite
eclogite
Rutile eclogite
Chromian
spinel
eclogite
Biminerallic
eclogite
KL2
KL2SL1 (omphacite totally
altered), KL2SL5, KL2SL12,
KL2MUP, KL2-7 (garnet-free
part)
–
P3
CC4
P3Xe/86
–
–
CC4N8b
KL2
–
KL2N9b, KL2N9c
P2
P10
MK8
P12
–
–
–
P2N4a, P2N5a
P10N7b, P10N7c
MK8N1a
P12N6b
P12
–
P12N6c
P2
–
P2N5b, P2N5c
P3
NK3
P3MXe/86, P3Xe11/86
–
RK3
–
–
NK3N2a,
NK3N2b
RK3N5a,
RK3N8b
Enstatite
garnet
pyroxenite
Chromian
spinelgarnet
pyroxenite
Biminerallic
garnet
pyroxenite
KL2N9a
Pipes KL2, P2, P3, P10, P12, CC4 are in WKF, pipes MK8 and NK3 in NKF, and pipe
RK3 in RKF.
Hand specimens of eclogite from the KL2 pipe are characterized
by honey brown to dark brown garnets of 1–5 mm size, which are
distributed in a pale greenish grey to white matrix. The matrix is a
hydrous Ca–Al silicate derived by secondary alteration of omphacite. Some samples show mineralogical banding on 1–3 cm scale,
with transition from kyanite eclogite to kyanite-free eclogite. The
latter is commonly marked by layering from garnet-rich to garnet-poor bands, with conspicuous graded layering in a few samples
(Ravi et al., 2007a) (Fig. 3a). In hand specimens of both garnet
pyroxenite and eclogite which do not show gross inhomogeneities,
the amount of garnet and clinopyroxene is 30–50 vol% each.
Despite extensive secondary alteration the original outlines of
omphacite grains in the matrix are recognizable in thin sections.
The matrix comprises anhedral to interstitial grains of omphacite
of 0.5–4 mm size in which subhedral to rounded grains of garnet
are set. There is often a patchy distribution of the phases in the
rocks. In omphacite-rich portions straight or curved grain boundaries and 120° angles at many triple junctions can bee seen. Garnet
commonly shows semi-opaque, kelyphitic alteration rim, which
consists of an aggregate of fine-grained phlogopite, K-feldspar
and hydrous Cal-Al silicate. In samples where omphacite in the
matrix is completely altered, fresh omphacite is occasionally preserved as subhedral to subrounded inclusions in garnet. Microfracturing is very common in all types of xenoliths (Fig. 3b). All the
nodules of eclogite and garnet pyroxenite from heavy mineral concentrate of different pipes are medium grained (1–5 mm size) with
pink garnet and green omphacite or Ca pyroxene as the principal
minerals. Modal proportions of garnet and clinopyroxene in the
nodules vary widely, but the nodules are clearly related to other
rocks in the mafic xenolith suite.
In some of the eclogite xenoliths garnet grains are characterized
by microscopic triangular arrays of exsolution needles of rutile
Fig. 3. Photograph (a) of hand specimen of biminerallic eclogite from KL2 pipe, and photomicrographs (b–d) of rutile eclogite from P3 pipe of WKF under plane polarized
light. (a) Graded layering is defined by decrease (white arrow) in the size of garnet grains (dark grey) in a garnet-rich layer. Omphacite is completely altered to grey to white
hydrous Ca–Al silicate. Small divisions in scale are millimeters. (b) Omphacite-garnet assemblage with numerous fractures. (c) Fine exsolution needles of rutile in garnet. (d)
Discrete grain of rutile with exsolution lamellae of ilmenite.
340
Table 2
Microprobe analyses of omphacite, Ca pyroxene and enstatite (n = number of points; blank = not analysed)
Omphacite
Cr-spinel
eclogite
Biminerallic
eclogite
Enstatite garnet
pyroxenite
Cr-spinel garnet
pyroxenite
Biminerallic garnet pyroxenite
P3Xe/86
n=4
CC4N8b
MK8N1a
P12N6b
P12N6c
P2N5b
P2N5c
P3MXe/86
n=6
P3Xe11/86
n=6
NK3N2a
NK3N2b
RK3N5a
RK3N8b
P12N6b
51.27
0.10
4.49
3.41
3.03
0.07
14.44
0.02
18.95
3.15
0.05
98.98
54.87
0.10
3.88
3.95
1.80
0.07
14.60
0.05
15.77
3.34
0.01
98.44
53.84
0.12
3.70
2.77
1.69
0.06
14.85
0.04
19.91
2.82
0.00
99.80
54.60
0.14
3.77
2.92
1.86
0.09
15.08
0.08
18.95
2.64
0.00
100.13
54.94
0.15
4.88
0.66
3.40
0.07
15.35
0.08
18.94
2.81
0.02
101.30
54.04
0.04
5.70
0.25
1.59
0.06
14.42
0.09
20.72
2.14
0.08
99.13
53.79
0.00
3.60
1.08
2.80
0.12
15.83
0.06
19.39
2.54
0.00
99.21
54.12
0.10
7.12
0.14
1.00
0.02
14.16
0.12
20.69
2.46
0.01
99.94
53.11
0.20
2.67
0.17
3.21
0.08
16.30
0.07
24.45
0.90
0.00
101.16
54.22
0.13
3.39
0.16
2.28
0.06
15.69
0.13
23.05
0.97
0.00
100.08
53.40
0.17
2.62
2.40
0.00
16.63
52.58
0.12
3.71
0.15
2.60
0.00
16.71
22.82
0.99
0.00
99.03
22.07
1.23
0.00
99.17
55.56
0.02
1.29
0.46
4.66
0.16
35.08
0.04
0.64
0.12
0.00
98.03
55.61
SiO2
0.21
TiO2
8.71
Al2O3
0.40
Cr2O3
FeO
7.35
MnO
0.05
MgO
8.19
NiO
0.03
CaO
13.42
6.20
Na2O
0.00
K2O
Total
100.17
Cations per 6 oxygens
Si
2.007
Ti
0.006
0.000
AlIV,a
0.371
AlVI
Cr
0.011
2+
0.222
Fe
Mn
0.002
Mg
0.441
Ni
0.001
Ca
0.519
Na
0.434
K
0.000
Total
4.014
Mg/(Mg + Fe)
a
IV
AI = 2-Si.
Ca pyroxene Enstatite
Rutile
eclogite
0.67
Enstatite garnet
pyroxenite
1.897
0.003
0.103
0.093
0.100
0.094
0.002
0.796
0.001
0.751
0.226
0.002
4.068
1.996
0.003
0.004
0.162
0.114
0.055
0.002
0.792
0.001
0.615
0.236
0.000
3.980
1.953
0.003
0.047
0.111
0.079
0.051
0.002
0.803
0.001
0.774
0.198
0.00
4.022
1.967
0.004
0.033
0.127
0.083
0.056
0.003
0.810
0.002
0.731
0.184
0.000
4.000
1.958
0.004
0.042
0.163
0.019
0.101
0.002
0.815
0.002
0.723
0.194
0.001
4.024
1.956
0.001
0.044
0.199
0.007
0.048
0.002
0.778
0.003
0.803
0.150
0.004
3.995
1.961
0.000
0.039
0.116
0.031
0.085
0.004
0.860
0.002
0.758
0.180
0.000
4.036
1.936
0.003
0.064
0.236
0.004
0.030
0.001
0.755
0.003
0.793
0.171
0.000
3.996
1.924
0.005
0.076
0.038
0.005
0.097
0.002
0.880
0.002
0.949
0.063
0.000
4.041
1.961
0.004
0.039
0.105
0.005
0.069
0.002
0.846
0.004
0.893
0.068
0.000
3.996
1.955
0.005
0.045
0.068
0.073
0.000
0.908
1.924
0.003
0.076
0.084
0.004
0.080
0.000
0.911
0.895
0.070
0.000
4.019
0.865
0.087
0.000
4.034
1.948
0.001
0.052
0.001
0.013
0.137
0.005
1.833
0.001
0.024
0.008
0.000
4.023
0.89
0.94
0.94
0.94
0.89
0.94
0.91
0.96
0.90
0.92
0.93
0.92
0.93
Table 3
Microprobe analyses of garnet (n = number of points; blank = not analysed)
Rutile
eclogite
Cr-spinel
eclogite
Biminerallic
eclogite
Enstatite garnet
pyroxenite
Cr-spinel garnet
pyroxenite
Biminerallic garnet pyroxenite
P3Xe/86 n = 3
CC4N8b
MK8N1a
P12N6b
P12N6c
P2N5b
P2N5c
P3MXe/ 86
n=4
P3Xe11/ 86
n=6
NK3N2a
NK3N2b
RK3N5a
RK3N8b
42.17
0.12
20.55
5.71
6.11
0.31
21.24
0.02
4.53
0.06
100.82
41.45
0.01
20.73
4.68
8.32
0.56
22.13
0.01
2.94
0.08
100.91
41.71
0.06
21.32
3.54
7.37
0.35
21.01
0.01
4.76
0.05
100.18
41.19
0.08
24.25
0.61
11.04
0.27
19.62
0.04
3.96
0.05
101.11
41.39
0.07
23.91
0.26
6.93
0.30
16.38
0.00
10.40
0.03
99.67
41.00
0.38
21.22
1.76
10.22
0.54
20.21
0.05
4.27
0.07
99.72
41.65
0.04
23.67
0.22
7.78
0.16
16.28
0.00
9.33
0.17
99.30
39.91
0.05
23.96
0.38
13.88
0.45
16.21
0.04
5.81
0.03
100.72
41.10
0.03
24.38
0.24
12.38
0.34
16.37
0.04
6.22
0.01
101.11
39.94
0.04
23.82
40.27
0.03
23.05
12.56
0.20
18.22
11.98
0.36
17.84
4.87
6.03
99.65
99.56
39.34
41.28
SiO2
TiO2
0.08
0.27
16.53
Al2O3 20.65
0.02
8.30
Cr2O3
FeO
24.95
8.04
MnO
0.42
0.38
MgO
6.63
18.00
NiO
0.02
0.00
CaO
7.00
8.37
0.05
0.41
Na2O
Total
99.16
101.58
Cations per 12 oxygens
Si
3.065
2.997
Ti
0.005
0.015
Al
1.896
1.414
Cr
0.001
0.476
Fe
1.626
0.488
Mn
0.028
0.023
Mg
0.770
1.948
Ni
0.001
0.000
Ca
0.584
0.651
Na
0.008
0.058
Total
7.984
8.070
End member percentage
Adr
5.1
4.9
TiAdr
0.3
0.7
Uv
0.1
16.0
Grs
14.7
0.0
Kn
0.0
7.7
Prp
26.5
57.0
Sps
1.0
0.8
Alm
52.5
12.9
2.984
0.006
1.714
0.319
0.362
0.019
2.241
0.001
0.343
0.008
7.997
0.0
0.3
11.3
0.0
4.9
70.7
0.6
12.2
2.947
0.001
1.737
0.263
0.495
0.034
2.346
0.001
0.224
0.011
8.059
2.6
0.0
4.7
0.0
8.2
68.8
1.1
14.5
2.973
0.003
1.791
0.200
0.439
0.021
2.233
0.001
0.364
0.007
8.032
1.6
0.1
9.9
0.3
0.0
73.8
0.7
13.4
2.922
0.004
2.028
0.034
0.655
0.016
2.075
0.002
0.301
0.007
8.044
0.6
0.2
1.7
7.4
0.0
68.4
0.5
21.1
2.972
0.004
2.024
0.015
0.416
0.018
1.754
0.000
0.800
0.004
8.007
0.0
0.2
0.8
25.8
0.0
58.7
0.6
13.9
2.964
0.021
1.808
0.101
0.618
0.033
2.178
0.003
0.331
0.010
8.067
5.2
1.0
4.6
0.0
0.4
71.0
1.1
16.8
3.001
0.002
2.010
0.013
0.469
0.010
1.749
0.000
0.720
0.024
7.998
0.0
0.1
0.7
23.7
0.0
59.3
0.3
15.9
2.900
0.003
2.052
0.022
0.843
0.028
1.756
0.002
0.452
0.004
8.062
1.1
0.1
1.1
12.4
0.0
57.5
0.9
26.8
2.947
0.002
2.060
0.014
0.742
0.021
1.750
0.002
0.478
0.001
8.017
0.0
0.1
0.7
15.2
0.0
58.5
0.7
24.8
2.903
0.002
2.040
2.934
0.002
1.979
0.763
0.012
1.974
0.730
0.022
1.938
0.379
0.471
8.073
8.076
2.7
0.1
0.0
9.6
0.0
64.2
0.4
23.0
4.1
0.1
0.0
11.1
0.0
63.0
0.7
21.0
341
342
Table 4
Microprobe analyses of chromian spinel, rutile and ilmenite (blank = not analysed)
SiO2
TiO2
Al2O3
Cr2O3
V2O5
Fe2O3b
FeO
MnO
MgO
CaO
Total
Oxygens
Si
Ti
Al
Cr
V
Fe3+
Fe2+
Mn
Mg
Ca
Total
Fe/(Fe + Mg)
Cr/(Cr + Al)
Cr-spinel
eclogite
Cr-spinel garnet
pyroxenite
Rutile eclogite
CC4N8b
P12N6c
P3Xe/86
Cr-spinel
Cr-spinel
Rutile
0.14
2.66
32.70
29.49
0.11
0.47
14.98
54.05
4.40
15.18
0.31
15.75
0.14
100.77
4
0.004
0.058
1.112
0.673
2.39
15.25
0.26
12.63
0.00
100.14
4
0.003
0.011
0.560
1.357
0.096
0.366
0.008
0.678
0.004
2.999
0.35
0.38
0.057
0.405
0.007
0.598
0.000
2.998
0.40
0.71
0.00
97.69
0.22
0.03
0.57
0.12
0.01
0.01
98.65
2
0.000
0.990
0.003
0.000
0.005
0.001
0.000
0.000
0.999
Ilmenite
L1a
L2
0.00
51.95
0.61
0.00
0.31
0.00
53.15
2.75
0.02
0.32
43.94
0.00
1.47
42.04
0.05
1.28
98.28
3
0.000
0.988
0.018
0.000
0.005
99.61
3
0.000
0.979
0.079
0.000
0.005
0.929
0.000
0.055
0.861
0.001
0.047
1.995
0.94
1.972
0.95
a
L1 and L2 are two different lamellae exsolved from rutile.
Fe2O3 in chromian spinel recalculated following the method of Barnes and
Roeder (2001). All iron assumed as Fe2O3 in rutile and as FeO in ilmenite.
b
which are 2–10 lm thick and 10–200 lm long (Fig. 3c). These needles always show inclined extinction. Recent experiments have
shown that Ti solubility in garnet depends on P–T conditions.
Zhang et al. (2003) reported increasing solubility of TiO2 (0.8–
4.5 wt%) in garnet with increasing P and T in the experimental conditions of 50–150 kbar and 1000–1400 °C. On the other hand
Kawasaki and Motoyoshi (2007) observed that TiO2 content of garnet increases with temperature and decreases with pressure in the
P–T range of 7–20 kbar and 850–1300 °C. The results of these studies show that rutile exsolution in garnet can be the result of
decompression and/or cooling.
Discrete grains of rutile in an eclogite xenolith show two sets of
oriented ilmenite lamellae which are nearly perpendicular to each
other and uniformly distributed throughout the rutile (Fig. 3d). The
lamellae are 0.5–5 lm thick and 20–200 lm long, are most likely
the result of a primary exsolution phenomenon (e.g. Rudnick
et al., 2000). The limit for solid solution of ilmenite in rutile is
7 wt% at 1050 °C (Basta, 1959). Liu et al. (2004) reported exsolution
of ilmenite from rutie in eclogite and attributed it to decompression from a pressure greater than 60–70 kbar. Zhao et al. (1999)
highlighted the role of oxygen fugacity in rutile-ilmenite assemblages in the mantle. However, the relative effects of pressure,
temperature and oxygen fugacity, and of other constituents, such
as Al and V on the solubility of FeTiO3 in TiO2 are unknown.
Standards include both natural and synthetic minerals and data
reduction was done using the ZAF correction procedure.
Mineral chemistry of one sample each of rutile eclogite, Cr-spinel eclogite, biminerallic eclogite, enstatite garnet pyroxenite and
Cr-spinel garnet pyroxenite, and several samples of biminerallic
garnet pyroxenite from the P3, P12 and CC4 pipes of WKF, MK8
and NK3 pipes of NKF, and RK3 pipe of RKF are given in Tables
2–4. The mineral chemistry of other xenoliths listed in Table 1 such
as kyanite-, enstatite- and biminerallic eclogites from KL2, P2 and
P10 pipes of WKF can be found in Patel et al. (2006). The primary
minerals in the mafic xenoliths do not show discernible zoning
within individual grains or significant compositional variation
among grains in the same xenolith.
4.1. Clinopyroxene
The clinopyroxenes generally have low total iron content
(<3.5 wt%) except for the rutile eclogite sample P3Xe/86, in which
the value is 7.4 wt% FeO (Table 2). In order to derive a clinopyroxene formula from a chemical analysis, it is desirable to have Fe2+
and Fe3+ values. In microprobe analyses, only total iron is determined from which Fe2+ and Fe3+ values can be calculated from stoichiometry. However, for clinopyroxenes with low total iron
content such as those in the present study, the calculation is very
sensitive to analytical error, especially of SiO2 due to its major
abundance and +4 charge (e.g. Sobolev et al., 1999). It was therefore decided to choose the most iron-rich clinopyroxene as a reference for the calculation of Fe3+/Fetot ratio, and then use this ratio to
calculate Fe2+ and Fe3+ contents in clinopyroxenes of all other samples. The most iron-rich clinopyroxene occurs in the rutile eclogite
which gives a Fe3+/Fetot ratio of 0.17. This is closely comparable to
the published values of Fe3+/Fetot ratio for eclogitic clinopyroxenes
based on different analytical methods such as Mössbauer spectroscopy (0.08–0.14, McCammon et al., 1998), Mössbauer milliprobe
spectroscopy (0.22–0.23, Sobolev et al., 1999) and micro-XANES
analysis (0.25–0.30, Schmid et al., 2003). After calculation of Fe2+
and Fe3+ contents in all clinopyroxenes using this ratio, their chemistry is plotted in a triangular diagram with components Ca–Mg–Fe
Q (Wo, En, Fs)
Biminerallic
eclogite
Kyanite eclogite
N = 25
Quad
Enstatite eclogite
80
80
omphacite
aegirine-augite
Rutile eclogite
Chromian spinel
eclogite
Biminerallic
garnet pyroxenite
Chromian spinelgarnet pyroxenite
Enstatite-garnet
pyroxenite
20
20
aegirine
jadeite
4. Mineral chemistry
NaAlSi2O6 (Jd)
Chemical compositions of minerals were determined by JEOLJXA-8600M electron microprobes (EMP) at the Indian Institute of
Technology, Roorkee, and the Geological Survey of India, Hyderabad. The operating parameters were: acceleration voltage of
15 kV, probe currents of 20–50 nA, and beam diameter of 2 lm.
50
3+
NaFe Si2O6 (Ae)
Fig. 4. Clinopyroxene compositions plotted on end-member triangular diagram of
Morimoto et al. (1988). 12 Omphacite compositions are from Patel et al. (2006) for
KL2, P2 and P10 pipes of WKF; 3 omphacite compositions and 10 quadrilateral
pyroxene compositions are from this study. Q = quadrilateral pyroxene; Wo = wollastonite; En = enstatite; Fs = ferrosilite; Jd = jadeite; Ae = aegirine.
343
are relatively the most magnesian, and garnets of kyanite eclogite
are reltively the most calcic in composition.
CaO
Biminerallic
eclogite
4.3. Chromian spinel, rutile and ilmenite
Mg
O
Kyanite eclogite
30%
Enstatite eclogite
Rutile eclogite
Mg
O
Chromian spinel
eclogite
Chromian spinel in an eclogite has XFe (=Fe2+/Mg + Fe2+) value of
0.35, and XCr (=Cr/Cr + Al) value of 0.38 (Table 4). In a garnet pyroxenite chromian spinel has XFe = 0.38 and XCr = 0.71. Discrete rutile
grains in rutile eclogite, and ilmenite lamellae exsolved from them
are somewhat aluminous and V-rich. Al2O3 content of rutile is
0.2 wt%, whereas that in ilmenite ranges from 0.6 to 2.8 wt%.
V2O5 contents are 0.6 wt% and 0.3 wt% in rutile and ilmenite,
respectively. MgO content of ilmenite is up to 1.5 wt%.
55%
Biminerallic
garnet pyroxenite
Chromian spinelgarnet pyroxenite
Enstatite-garnet
pyroxenite
C
B
A
FeO+MnO
5. Geothermobarometry
MgO
Fig. 5. Garnet compositions on the CaO–(FeO + MnO)–MgO diagram of Coleman et
al. (1965).
pyroxene, jadeite and aegirine (Fig. 4). Clinopyroxene compositions
for eclogites of KL2, P2 and P10 pipes of WKF are taken from Patel
et al. (2006) and plotted in this figure for comparison. Clinopyroxene falling in the omphacite field is classified as omphacite,
whereas that falling in the Quad field is Ca pyroxene. It can be seen
that there is a compositional transition from omphacite (in eclogite) to Ca pyroxene (in garnet pyroxenite). Jadeite component in
omphacite is up to 45 mol% which is very unusual for deep-seated
xenoliths world-wide (Sobolev et al., 1999).
The XMg (=Mg/Mg + Fe2+) value of all clinopyroxenes except one
falls in the range of 0.91–0.96. The exception is the iron-rich
omphacite in the rutile eclogite which has an XMg value of 0.70.
TiO2 content in all clinopyroxenes is <0.2 wt%. MnO and K2O contents are invariably low (60.1 wt%). Cr2O3 content is mostly below
1 wt%; but several clinopyroxenes have 3–4 wt% Cr2O3, which is
less than that in associated garnet.
4.2. Garnet
Garnets in the xenoliths of garnet pyroxenite and eclogite have
wide variations in Ca, Fe and Mg (Table 3). They contain only small
concentrations of Mn (0.2–0.6 wt% MnO) and Ti (0.01–0.4 wt%
TiO2), and are virtually devoid of Ni (<0.05 wt% NiO). Na2O content
is mostly below 0.1 wt%, although in one analysis it is as high as
0.41 wt%. Cr2O3 content in most garnets is below 1 wt%, although
in a few samples it is high (1.8–8.3 wt%). End member calculations
following the method of Sobolev et al. (1973) for Cr-rich garnets
give the following values (in mol%) for all samples except rutile
eclogite: pyrope (57–74), almandine (12–27), grossular (0–26),
spessartine (61), andradite (0–5), Ti-andradite (61), uvarovite
(0–16) and knorringite (0–8). The garnet of rutile eclogite is highly
iron-rich with end member composition of Prp26.5Alm52.5Grs14.7
Adr5.1TiAdr0.3Sps1.0Uv0.1.
The chemistry of all garnets is summarised in the CaO–
(FeO + MnO)–MgO ternary plot after Coleman et al. (1965) along
with the data points for eclogites of KL2, P2 and P10 pipes of
WKF from Patel et al. (2006) (Fig. 5). Garnets of all samples except
rutile eclogite of P3 pipe and kyanite eclogites of KL2 pipe fall in
the Group A field. Garnet of rutile eclogite belongs to Group C,
whereas garnets of kyanite eclogite fall in the fields of Group B
and C. Garnets of enstatite eclogite and enstatite garnet pyroxenite
Geobarometry of eclogites and garnet pyroxenetites has been a
problem world-wide because of the high thermodynamic variance
of the assemblages. Nimis and Taylor (2000) formulated a Cr-inclinopyroxene barometer which is applicable to clinopyroxenes
with Cr2O3 contents between 0.5 and 5 wt% and with sufficient calcium to form CaCr-Tschermak’s (CaCrVIAlIVSiO6) component. These
compositional criteria are satisfied by only a few samples of eclogite and garnet pyroxenite. P–T conditions for these samples have
been deduced from the intersection of the Cr-in-clinopyroxene
geobarometer with the garnet-clinopyroxene Fe–Mg exchange
geothermometer.
The application of Fe3+ corrections in the temperature calculations in geothermobarometry has long been a subject of controversy. As demonstrated by Canil and O’Neill (1996), Sobolev et al.
(1999) and Proyer et al. (2004), the errors introduced by estimating
the Fe3+ content of clinopyroxenes from EMP analyses are large and
often unacceptable for geothermobarometry. Sobolev et al. (1999)
studied the effects of various Fe3+/Fetot values for clinopyroxene
and garnet on calculated temperatures, and found that for clinopyroxene, T decreases with increasing Fe3+/Fetot whereas for garnet, T
increases with increasing Fe3+/Fetot. They concluded that due to
this compensation effects between garnet and clinopyroxene the
Fe3+ corrections in EMP analyses do not greatly affect temperature
estimates in eclogites. Therefore, in the present study temperatures have been calculated assuming Fe2+ = Fetot for both clinopyroxene and garnet.
There are several calibrations of the garnet-clinopyroxene geothermometer and the most widely used ones for eclogites and garnet pyroxenites are those by Ellis and Green (1979), Powell (1985),
Krogh (1988) and Krogh Ravna (2000). For the xenoliths under
study the calibrations of Krogh (1988) and Krogh Ravna (2000)
yield similar temperatures. The temperatures obtained from the
calibrations of Ellis and Green (1979) and Powell (1985) are also
similar, but significantly higher than those from Krogh (1988)
and Krogh Ravna (2000) for most of the xenoliths (Table 5). For a
given calibration, eclogites record higher P–T conditions than garnet pyroxenites. The Cr-spinel eclgoite sample CC4N8b yields
anomalously high temperatures by all the four calibrations of the
garnet-clinopyroxene geothermometer and most likely are an artefact due to the high Cr and Ca contents of garnet in this samples.
Enstatite is present in two samples, one of which is a garnet
pyroxenite (sample P12N6b) and the other is an eclogite (sample
KL2N9a). The presence of enstatite allows calculation of pressure
from the Al-in-orthopyroxene (coexisting with garnet) geobarometer. Brey and Kohler (1990) gave a calibration of this geobarometer along with a calibration of the two-pyroxene geothermometer.
P–T values obtained from the simultaneous solution of the Al-inorthopyroxene geobarometer and either two-pyroxene geothermmeter or garnet-clinopyroxene geothermometer for the two sam-
344
Table 5
Results of geothermobarometry for the mafic xenoliths from the kimberlite pipes of EDC. Mineral analyses used for sample KL2N9a, P10N7b and P10N7c are taken from Patel et al.
(2006) and those for other samples is from this study
Sample no.
Remark
T (EG)
P (NT)
T (Powell)
P (NT)
T (Krogh)
P (NT)
T (KR)
P (NT)
T (BK)
P (NT)
T (BK)
P (BK)
T (EG)
P (BK)
T (Powell)
P (BK)
T (Krogh)
P (BK)
T (KR)
P (BK)
P12N6b
Enstatite garnet pyroxenite
830
22.1
682
14.3
635
12.0
Biminerallic garnet pyroxenite
P12N6c
Cr-spinel garnet pyroxenite
MK8N1a
Biminerallic eclogite
P10N7c
Biminerallic eclogite
P10N7b
Biminerallic eclogite
CC4N8b
Cr-spinel eclogite
741
27.1
895
33.1
869
23.3
885
29.6
1041
35.5
1094
33.4
1140
40.1
1265
38.7
858
23.6
P2N5b
730
26.8
889
32.9
899
24.5
915
30.4
1036
35.3
1085
33.2
1128
39.7
1323
40.4
681
14.3
Biminerallic garnet pyroxenite
859
30.5
976
36.4
995
28.5
981
32.3
1093
37.0
1141
34.8
1157
40.7
1269
38.8
696
25.9
P3MXe/86
885
31.3
998
37.3
1015
29.4
1001
32.8
1108
37.4
1153
35.2
1169
41.1
1272
38.9
KL2N9a
Enstatite eclogite (Garnet host and
exsolved ortho- and clinopyroxenes)
Enstatite eclogite (Omphacite host
and exsolved garnet and clinopyroxene)
833
27.8
867
36.4
1208
47.1
971
41.9
1192
46.2
943
40.4
1058
39.1
790
32.5
1225
48.1
845
35.3
KL2N9a
T in °C and P in kbar. P (NT) = Cr-in-cpx geobarometer of Nimis and Taylor (2000); P (BK) = Al-in-opx geobarometer of Brey and Kohler (1990). T (BK) = two-pyroxene
geothermometer of Brey and Kohler (1990); other temperatures are from garnet-clinopyroxene geothermometer of Ellis and Green, 1979 (EG), Powell (1985), Krogh (1988),
and Krogh Ravna, 2000 (KR).
ples are given in Table 5. The enstatite garnet pyroxenite satisfies
the criteria for Cr-in-clinopyroxene geobarometry in addition to
its suitability for the Al-in-orthopyroxene geobarometry. A comparison of pressures calculated from the two geobarometers in this
rock shows that the pressure yielded by the Al-in-orthopyroxene
geobarometer is invariably much less than that by the Cr-in-clinopyroxene geobarometer. Considering all the geothermobarometric
calculations it is seen that the equilibration pressures and temperatures of the xenoliths fall mostly in the ranges of 20–48 kbar and
700–1225 °C, respectively.
6. Xenolith geotherm
Continental geotherm for a given region is conventionally estimated on the basis of a number of parameters including surface
T (˚C)
600
800
1000
1200
1600
1400
P (NT) - T (BK)
10
0.85 Tm
0.90 Tm
P (NT) - T (Krogh)
P (NT) - T (KR)
P (NT) - T (EG)
20
P (NT) - T (Powell)
P (BK) - T (BK)
2
60 mW/m
P (Kbar)
30
P (BK) - T (Krogh)
P (BK) - T (KR)
40
Graph
ite
Diamo
nd
P (BK) - T (EG)
2
50 mW/m
P (BK) - T (Powell)
50
60
45 mW/m2
70
2
30 mW/m
2
40 mW/m
80
Fig. 6. Empirical geotherm (thick solid line) for mafic xenoliths. Conductive model
geotherms (dashed lines) for different surface heat flow values, and mantle solidii
(Tm) are from Pollack and Chapman (1977). Abbreviations are as in Table 5.
heat flow, radiogenic heat source distribution, variation of thermal
conductivity and mode of heat transfer within the lithosphere.
Generally several assumptions regarding these parameters are
made while calculating model geotherms which introduce significant error. However, independent estimates of pressures and temperatures obtained from mantle xenoliths allow the construction
of empirical temperature–depth curves (O’Reilly and Griffin, 2006
and references therein), which can be compared with model
geotherms.
Pollack and Chapman (1977) computed model conductive geotherms in the lithosphere for surface heat flow values in the range
of 30–150 mW m 2 which are shown in Fig. 6. All the calculated P–
T values from the xenoliths of eclogite and garnet pyroxenite in the
kimberlites of EDC are plotted in this figure. The P–T values are
scattered, but most of them fall between the 40 and 50 mW m 2
model geotherms, and some fall above the 50 mW m 2 model geotherm. An important constraint on geotherm is provided by the
diamondiferous nature of most of the studied kimberlite pipes. It
implies that the geotherm must intersect the graphite-diamond
transition curve below the mantle solidus. This condition is satisfied in Fig. 6 if the geotherm is 645 mW m 2. With this constraint
the P–T ranges obtained from the mafic xenoliths approximate to a
45 mW m 2 model geotherm. The P–T conditions of the xenoliths
show that they have equilibrated outside of the diamond stability
field. But since the kimberlites are diamondiferous it is obvious
that the transporting magmas must have originated at greater
depths than recorded by the xenoliths. P–T values for the sample
CC4N8b fall above the 0.85Tm mantle solidus because of the anomalously high temperature yielded by this sample.
Ganguly and Bhattacharya (1987) and Nehru and Reddy (1989)
calculated mantle geotherms using garnet-clinopyroxene Fe–Mg
exchange geothermometer of Råheim and Green (1974), and Al-in
orthopyroxene geoarometer of Lane and Ganguly (1980) and Perkins et al. (1981) on ultramafic xenoliths from the P3 pipe of
WKF. These geotherms are linear in nature, and a comparison with
model geotherms shows that they fall between 42 and 50 mW m 2
model geotherms. Thus the xenolith geotherm of 45 mW m 2 derived in the present study is broadly consistent with the findings
of Ganguly and Bhattacharya (1987) and Nehru and Reddy (1989).
345
It is well known that xenolith geotherms are strongly dependent on
the geothermobarometers used (Grutter and Moore, 2003). Nevertheless empirically constructed xenolith geotherms provide reliable constraints on geothermal models since they are
independent of the uncertainties of model geotherms (Cull et al.,
1991).
7. Discussion
7.1. Implication of xenolith geotherm on heat flow
Gupta et al. (1991) reported mean heat flow values of
40 ± 3.4 mW m 2 for the EDC and 31 ± 4.1 mW m 2 for the WDC.
Such heat flow variations in cratonic regions reflect variations in
radiogenic heat produced in the crustal column and heat conducted into the crust from the underlying mantle. Senthil Kumar
and Reddy (2004) measured K, U and Th abundance through
in situ gamma-ray spectrometry at numerous sites covering all
major rock formations of both EDC and WDC. From the crustal heat
contribution models they concluded that mantle heat flow of the
EDC is higher (17–24 mW m 2) relative to the WDC (7–
10 mW m 2).
The xenolith geotherm of 45 mW m 2 obtained in the present
study for the EDC is towards the higher side of the typical range
of xenolith/xenocryst geotherms (35–45 mW m 2) for several Archaean cratons in the world (Finnerty and Boyd, 1987; O’Reilly
and Griffin, 2006). This leads us to believe that the EDC was hotter
than many other shield regions of the world in the mid-Proterozoic
period when kimberlites intruded the craton. This can be attributed, following the present-day heat flow model of Senthil Kumar
and Reddy (2004), to high mantle heat flow beneath the EDC in the
mid-Proterozoic time.
7.2. Lithospheric thickness
The finding of formerly supersilicic garnet in an enstatite eclogite xenolith from the KL2 pipe of WKF led Patel et al. (2006) to suggest that the minimum peak pressure for the rock is 50 kbar since
supersilicic garnet is experimentally stable at pressures in excess
of 50 kbar (Ringwood and Major, 1971). This pressure translates
to a minimum lithospheric thickness of 150 km beneath the EDC
during the mid-Proterozoic period. This value is in agreement with
the result of several workers who have estimated lithospheric
thickness beneath the Dharwar craton using different methods
(Pandey and Agrawal, 1999 and references therein). Estimates
based on heat flow values include lithospheric thickness of
148 km (Pandey and Agrawal, 1999) and P200 km (Gupta et al.,
1991) for the Dharwar craton as a whole. From magnetotelluric
studies Gokarn et al. (1998) estimated a lithospheric thickness of
180 km for the craton. Based on geobarometric calculations in
ultramafic xenoliths from the P3 pipe of WKF, Ganguly and Bhattacharya (1987) concluded that the lithosphere was at least
185 km thick below the EDC during the mid-Proterozoic period.
7.3. Origin of mafic xenoliths
Mantle eclogites and garnet pyroxenites found in different parts
of the world represent a rather heterogeneous group of rocks because the wide range of possible solid solution in the garnet and
clinopyroxene structures can accommodate a variety of bulk compositions. Therefore, it is not reasonable to postulate a single origin
for all mafic xenoltihs. Two contrasting petrogeneses have been
postulated for the origin of mantle eclogites (review by Godard,
2001). They represent either (1) high pressure magmatic cumulates which occur as magma chambers or dykes within upper man-
tle (e.g. Schmickler et al., 2004) or (2) subducted and
metamorphosed oceanic crust (e.g. Barth et al., 2001). This question is still debated nowadays. Both types of eclogites may occur
in the same kimberlite.
Textural and mineral chemical characteristics favour a magmatic cumulate origin for the mafic xenoliths in the Dharwar kimberlites. Graded layering observed in hand specimens (Fig. 3a), and
microtextural features such as garnet necklace and garnet-kyanite
cluster (Patel et al., 2006) must have resulted from gravitative
accumulation of early-formed crystals of garnet. The composition
of clinopyroxene shows transition from omphacite in eclogites to
Ca pyroxene in garnet pyroxenites (Fig. 4). Such transition demonstrates the cogenetic relationship of eclogites and garnet pyroxenites, and favours a high pressure igneous origin of these rocks.
However, since eclogite xenoliths record higher pressure than garnet pyroxenite xenoliths (Table 5) the former must have been derived from a greater depth than the latter.
Acknowledgements
The Dy. D.G., Geological Survey of India (Southern Region) is
thanked for permission to SR to carry out research work at I.I.T.,
Bombay. Tamal Ghosh of IIT-Roorkee is thanked for help in EPMA
analyses. Constructive reviews by Kuo-Lung Wang and an anonymous reviewer were extremely helpful in improving the quality
of the paper.
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