New experimental constraints: implications for pe-trogenesis of charnockite of dioritic composition

doi:10.4236/ns.2010.210135

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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: [email protected].

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

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

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

0.1

0.01

0.001

Nb Sr

Zn Ni

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

1000

100

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.

R. Kar et al. / Natural Science 2 (2010) 1085-1089

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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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