Vol.2, No.10, 1085-1089 (2010) Natural Science
http://dx.doi.org/10.4236/ns.2010.210135
New experimental constraints: implications for
petrogenesis of charnockite of dioritic composition
Rajib Kar1, Samarendra Bhattacharya2*
1Jagannath Kishore College, Purulia, India;
2Indian Statistical Institute, Kolkata, India; *Corresponding Author:
Received 13 August 2010; revised 16 September 2010; accepted 21 September 2010.
ABSTRACT
Hornblende-dehydration melting experiments at
high temperatures (> 950oC) indicate change of
melt composition from tonalite/granodiorite to
quartz-diorite; clinopyroxene instead of hornbl-
ende as the residual phase and change in melt-
ing reaction from peritectic hornblende-dehydr-
ation to eutectic clinopyroxene-orthopyroxene-
plagioclase. In the light of these experimental
results, petrogenesis of a charnockite pluton of
homogeneous dioritic composition in the East-
ern Ghats Belt, India, can be explained as melt-
ing at high-temperatures (> 950oC). Negative Sr
and Eu anomalies further indicate plagioclase
as a major residual phase, consistent with melt-
ing at high-temperatures (> 950oC).
Keywords: Dioritic charnockite; Residual
clinopyroxene; Residual plagioclase; Eutectic
melting
1. INTRODUCTION
It is quite common that large-scale charnockitic bod-
ies are of variable composition from tonalite to granodi-
orite, and sometimes even dioritic composition is report-
ed [1]. On the other hand, petrogenesis of massif-type
charnockites have been variously described: a) mantle-
derived and differentiated melt [2]; b) high-temperature
melting of dry granulite facies rocks [3]; c) more mafic
varieties as mantle-derived melts [4]; d) product of
hornblende-dehydration melting in the deep crust [5].
New melting experiments provide constraints on the
petrogenesis of charnockitic rocks of dioritic composi-
tion. From the Jenapore area in the Eastern Ghats Belt,
India, charnockite-massif was described as the product
of hornblende-dehydration melting under granulite fa-
cies conditions, and with residual hornblende. There the
two-pyroxene granulites occur as minor patches and
bands and were explained as peritectic segregates [5]. A
stock-like body of charnockite (pluton) occurs in the
same locale, a few kilometer to the south (Lat: 20o46' N;
Long: 86o05' E). In contrast to the charnockite-massif, it
is of more mafic and homogeneous composition at the
outcrop-scale and commonly has both orthopyroxene
and clinopyroxene.
In the present communication we present geochemical
data from the charnockite pluton and in the light of new
experimental constraints explain its origin by melting at
high-temperatures (≥ 950oC).
2. EXPERIMENTAL CONSTRAINTS
The selected results of the hornblende-dehydration
melting experiments is presented in Figure 1. The melts
of 900oC and 925oC are tonalitic (normative Qtz / Plag >
0.25) and those above 950oC are quartz dioritic (norma-
tive Qtz / Plag < 0.25) in composition. The melt compo-
sition changes from corundum normative to diopside nor-
mative when temperature increases from 925oC to 950oC.
Also there is gradual decrease of plagioclase proportion
with temperature rise. Moreover, the orthopyroxene and
clinopyroxene are subequal in proportion at 900oC, and
orthopyroxene gradually decreases in proportion to a
trace amount at 1100oC. These results suggest that the
nature of melting reaction changes from hornblende
breakdown reaction at 925oC to eutectic clinopyroxene-
orthopyroxene-plagioclase melting reaction at 950oC [6].
3. CHARNOCKITE PLUTON
The charnockite pluton at Jenapore is a relatively ho-
mogeneous body of two-pyroxene granulite, unlike those
occurring as minor bands and patches within the massif-
type charnockite described from the area to the north [5].
Also as distinct from those within massif-type charnoc-
kite garnet is absent, while clinopyroxene is much more
abundant than orthopyroxene (Table 1).

R. Kar et al. / Natural Science 2 (2010) 1085-1089
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Figure 1. Mass proportions in melting experiments at 8 kbar.
Table 1. Modal mineralogy of the Charnockite pluton at Jenapore, Eastern Ghats Belt.
Sample JN35A 2.J.95 2.J.82 2.J.90A JN 194A 2.J.50A JN 35F
Quartz 4.3 6.2 10.4 6.3 5.7 8.1 8.5
K-feldspar 4.2 3.1 2.5 3.2 2.7 1.8 2.1
Plagioclase 25 24.3 22.4 22.6 23.4 21.5 23
Orthopyroxene 13.3 14.5 16.4 18.3 17.4 14.6 15.3
Clinopyroxene 41.2 42 38.5 38.6 39.2 41 39.3
Hornblende 2.5 3.1 1.5 1.7 3.4 4.2 3.8
Biotite 5.4 3.7 5.2 4.6 3.9 4.5 5.3
Opaque 2.2 3.1 1.5 3.4 2.7 3.2 2.5
Accessory 1.2 Trace 1.1 0.7 1.4 0.4 0.3
3.1. Geochemistry
3.1.1. Analytical Procedure
Both major and minor oxides as well as trace elements
were analyzed by ICP-MS at the Australian Geological
Survey Organization, Canberra. At AGSO the sample
preparation for ICP-MS has been based on a method out-
lined in Jenner et al., 1990 [7]. However, some refrac-
tory elements like Zr have been problematic and to over-
come this problem, a new method has been introduced.
The new method involves digesting pieces of the lithium
tetraborate/lithium metaborate fusions that have been pre-
pared and run for XRF major element analysis. Appro-
ximately 100 micrograms of chips from the smashed discs
are weighed accurately into Savillex Teflon vessels. Five
milliliters of internal standard, one milliliter of HF and
five milliliters of HNO3 are then added. The vessels are
sealed and heated for twelve hours at 120oC on a timed
hotplate, such that cooled samples are ready the follow-
ing morning. The digests are then transferred to volu-
metric flasks and made up to volume ready for the ICP-
MS. The precision can be assessed from the Zr analysis
(Table 2).
3.1.2. Results
The analytical data for the seven samples from the
charnockite-pluton is presented in Table 3. In the Qz-Or-
Pl diagram six (6) of the seven (7) analyzed samples plot
in the field of Qz-diorite, while one sample plots in the
field of Qz-monzodiorite (Figure 2). Normative quartz:
plagioclase ratios vary between 0.02 and 0.15 and all the
samples are diopside normative, varying between 6.4 and
11.7. All these features are compatible with the new ex-
perimental constraints indicating high temperature melt-
ing (≥ 950oC) in mafic rocks. Moreover, these composi-
tional characteristics (homogeneous) suggest a change of
melting reaction from peritectic to eutectic, as in the re-
cent melting experiment [6].
The incompatible elements like K, Rb & Ba are en-
riched, while Ti and base metals like Cr & Ni are depleted
Table 2. Comparative Zr analysis in ppm.
Standards ICP-MS
old
ICP-MS
new
AGSO
XRF
Recommended
[8]
W-2 78 95 93 94
BIR-1 15 15 15 15.5
DNC-1 36 37 36 41
QLO-1 171 189 188 185
BHVO-1 151 176 175 179
AGV-1 205 235 235 227

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Table 3. Composition of the charnockite pluton of Jenapore, Eastern Ghats, India.
Area Jenapore
Sample JN35A 2.J.95 2.J.82 2.J.90A JN 194A 2.J.50A JN 35F
SiO2 49.69 52.03 54.42 52.19 52.74 52.87 53.05
TiO2 2.9 1.72 1.53 1.34 0.97 1.11 1.71
Al2O3 13.85 15.53 15.09 15.74 16.58 16.63 15.29
Fe2O3 2.38 1.36 1.75 1.08 1.46 0.75 1.71
FeO 13.64 9.44 9.18 8.85 8.01 7.78 8.8
MnO 0.22 0.16 0.16 0.15 0.14 0.13 0.15
MgO 3.75 5.5 4.75 6.46 6.94 6.07 5.16
CaO 7.9 8.19 7.28 8.48 8.51 8.68 8.05
Na2O 1.14 2.78 2.96 2.73 2.3 2.26 2.97
K2O 2.07 1.51 1.56 1.26 1.06 1.87 1.79
P2O5 0.75 0.42 0.37 0.28 0.21 0.3 0.41
LOI 1.54 1.28 0.86 1.37 0.98 1.47 0.81
Total 99.83 99.92 99.91 99.93 99.9 99.92 99.9
Trace elements in ppm
Cr 9 119 105 185 98 205 78
Ni 8 50.5 33.5 37.5 20 49 24
Ni 8 50.5 33.5 37.5 20 49 24
Sc 45 31.5 31.5 33.5 32 31 34
V 263 193 162 170 167 160 184
Cu 36 26 26 20 22 22 19
Zn 176 120 127 104 97 89 114
Zn 176 120 127 104 97 89 114
Ti 17400 10320 9180 8040 5820 1660 10260
K 8588 6265 6472 5228 4338 7759 7427
Rb 48 54 54.5 34 40 56 49
Ba 1527 727 734 736 376 1066 1076
Sr 341 327 296 325 314 376 324
Zr 329 255 226 177 117 197 257
Nb 35.3 25.3 24.8 15.5 11.6 17 24.1
Th 2.46 1.98 1.51 2.83 8.84 1.49 3.72
U 0.41 0.3 0.48 0.32 0.56 0.27 0.5
La 92.7 55.3 44.9 47.5 56 56.2 64.2
Ce 234 120 97.2 97 115 115 135
Pr 22.7 13.1 11 10.4 11.8 12.2 14.8
Nd 87.7 50.3 43 39.4 42.8 45.2 55.7
Sm 15.5 9.45 8.23 7.06 7.55 7.29 9.85
Eu 3.87 2.26 2.13 1.99 1.6 2.21 2.58
Gd 15 8.92 8.33 7.05 6.96 7.14 9.2
Tb 2.28 1.37 1.32 1.11 1.06 1.07 1.43
Dy 12.9 7.83 7.5 6.32 5.98 5.96 8.01
Ho 2.8 1.69 1.64 1.4 1.29 1.28 1.76
Er 8.11 4.74 4.74 4.09 3.63 3.64 5.07
Yb 6.88 4.07 3.99 3.57 3.2 3.22 4.33
Lu 1.01 0.59 0.59 0.52 0.47 0.46 0.65
∑ REE 505.45 279.62 234.57 227.41 257.34 260.87 312.58
(La/Sm)N 3.76 3.68 3.43 4.23 4.67 4.85 4.10
(Gd/Lu)N 1.85 1.88 1.76 1.69 1.84 1.93 1.76
Eu/Eu* 0.19 0.19 0.19 0.21 0.17 0.23 0.20
(Figure 3). These features suggest a melt character for
these dioritic charnockites. However, Zn is significantly
enriched and could be related to clinopyroxene as a ma-
jor phase, which commonly contains trace amounts of
Zn. Similar degrees of enrichment in Rb & Sr relative to
primitive mantle is consistent with partial melting in ma-

R. Kar et al. / Natural Science 2 (2010) 1085-1089
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Figure 2. Normative Qz-Or-Pl diagram for the charnockites.
Rock / PRIM
1000
100
10
1
0.1
0.01
0.001
Rb Ba Th K Nb Sr Zr Ti Y Zn Ni Cr
Figure 3. Multi-element spider diagram for the charnockites.
Normalizing values from Taylor and McLennan, 1985 [23].
fic rocks involved in the break down of hornblende and
plagioclase. However, unlike the tonalitic charnockites
(cf. Figure 9 in [5]), negative Sr anomaly in the dioritic
charnockites here implies plagioclase as a major residual
phase [9]. Zr contents between 117 & 329 are variable,
but most of the samples have near saturation concentra-
tion. This and relatively high Th (between 1.49 & 8.84
ppm) and U (between 0.27 & 0.56 ppm) suggest interac-
tion between melt and restitic zircon. Also unlike the
tonalitic charnockites, total REE contents are high, be-
tween 227 & 505 ppm, suggests near saturation concen-
tration. Relatively less HREE fractionation (Gd / Yb)N,
between 1.69 & 1.88 than LREE fractionation (La /
Sm)N, between 3.43 & 4.85, suggests melt-pyroxene
coexistence. Significant negative Eu anomaly is charac-
teristic of these charnockites of quartz-dioritic composi-
tion unlike those in the tonalitic charnockites and Ar-
chaean tonalites [5,10] suggests major residual plagio-
clase (Figure 4). This is also consistent with the signature
of negative Sr anomaly.
4. DISCUSSIONS
The Eastern Ghats Mobile Belt, along the east coast of
Rock / Chondrite
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
1000
100
10
1
Figure 4. Chondrite normalized REE diagram for the char-
nockites. Normalizing values from Taylor and McLennan, 1985
[23].
peninsular India, is commonly described as a collisional
orogen [11]. Extremely high temperatures (> 900oC)
have been recorded from different granulite lithologies
and from different parts of this regional granulite terrain
[12-16]. On the other hand, dehydration melting experi-
ments provided important constraints on the petrogenesis
of massif-type charnockitic rocks of tonalitic and grano-
dioritic compositions [17-20]. The latest experiments of
hornblende-dehydration melting at high-temperatures (≥
950oC), indicate changing melt composition from tonalite
/granodiorite to quartz-diorite, along with residual cli-
nopyroxene instead of hornblende [6]. In this context it
is important to note that this is the first report of char-
nockite pluton of dioritic composition in the Eastern
Ghats Belt. Erstwhile magmatic charnockite or their pro-
toliths are described as enderbite, of tonalitic composi-
tion [21-22]. The tonalitic to granodioritic charnockite-
massif of Jenapore was described as the product of horn-
blende-dehydration melting with residual hornblende &
or garnet by Kar et al. [5]. In the same locale a stock-like
body of charnockite, its quartz-dioritic composition with
residual clinopyroxene and plagioclase provide evidence
of high-temperatures (> 950oC). This is also consistent
with the proposed change in the melting reaction from
peritectic hornblende-dehydration melting to eutectic cli-
nopyroxene-orthopyroxene-plagioclase melting.
5. CONCLUSIONS
1) This is the first report of dioritic charnockite pluton
in the Eastern Ghats Belt.
2) Yet another evidence of Ultra-high temperature crus-
tal metamorphism in the Eastern Ghats Belt.
3) Negative Sr and Eu anomalies, unlike those of tona-
litic charnockites and Archaean tonalites, imply pla-
gioclase as a major residual phase.

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6. ACKNOWLEDGEMENTS
Melting experiments at the Petrological Laboratory of the Zurich In-
stitute was supported by a Swiss Federal Fellowship to RK. Analytical
data by ICP-MS were acquired by courtesy Dr. J.W. Sheraton.
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