Secular evolution of continental crust: recorded from massif-type charnockites of Eastern Ghats belt, India

doi:10.4236/ns.2010.210134

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Vol.2, No.10, 1079-1084 (2010) Natural Science

http://dx.doi.org/10.4236/ns.2010.210134

Secular evolution of continental crust: recorded from

massif-type charnockites of Eastern Ghats belt, India

Samarendra Bhattacharya1, Ashwini Kumar Chaudhary2*

1Indian Statistical Institute, Kolkata, India;

2Indian Institute of Technology, Roorkee, India; *Corresponding Author: [email protected].

Received 15 July 2010; revised 18 August 2010; accepted 23 August 2010.

ABSTRACT

It is reasonably well established that the Earth

has substantially cooled from the Archean to

the present and hence the sites, rates and pro-

cesses of crust formation must have changed

through geologic time. Archean and Proterozoic

granitic rocks are the principal record of such

changes. Massif-type charnockites in the East-

ern Ghats granulite belt, India, of Archean and

Proterozoic ages mirror the changing condi-

tions and/or processes of continental crust for-

mation. Though both can be explained by de-

hydration melting of mafic rocks, the conditions

differ. Potasium and rubidium rich Proterozoic

charnockites have significant negative Eu ano-

maly indicating melting at shallow depths in the

stability field of plagioclase. In contrast, sodium

and strontium rich Archean charnockites with

less LREE enrichment and less depletion in Eu

indicate melting at greater depths in the stability

field of garnet or amphibole.

Keywords: Secular changes; Continental crust;

Massif-charnockites; Eastern Ghats

1. INTRODUCTION

The continental crust comprising byouant quartzo-

feldspathic materials are difficult to destroy by subduc-

tion and hence can be considered as the principal record

of crustal evolution through geologic time. New conti-

nental crust may form magmatically from underlying

mantle. However, mantle melting products are predomi-

nantly basaltic, whereas continental crust is andesitic

which can not be extracted directly from melting of man-

tle-peridotite. Continental crust formation therefore re-

quires a second stage/or event of fractional crystalliza-

tion [1] or remelting of basaltic magma [2]. Although there

remains considerable debate on the processes of crust

formation in the Archean compared to those operating in

the later period (post-Archean), significant differences in

key geochemical features have been documented between

Archean and later granitic rocks [3-5]. Moreover, tec-

tonic setting for Archean magmatism as exemplified by

TTG remains unresolved. Partial melting may have taken

place in subducted slabs [6,7] or in underplated basalt

beneath thickened crust or oceanic plateau [8]. TTG

suites of Archean greenstone belts are taken as the Ar-

chean continental crust, while large varieties of Protero-

zoic granitic plutons represent the Proterozoic continen-

tal crust (cf. Table 3 in [2]). These authors have pre-

sented extensive discussion on these differences from a

Granitic perspective and their possible implications on

the changing processes and or conditions of crust forma-

tion from Archean to Proterozoic times.

Eastern Ghats granulite belt, India, comprises massif-

type charnockite as a major component in the regional

granulite terrane, and there is unambiguous evidence of

different generation of such charnockites. Archaean char-

nockites have been described from northern margin against

Singhbhum craton and western margin against Bastar

craton [9-11]. Some of the massif-type charnockite

suites in the central part of the granulite belt record only

Proterozoic ages [12,13]. Although, some workers have

described magmatic charnockites from the Eastern Ghats,

presumably as mantle-derived melt [14], it is difficult to

postulate silicic melts directly from mantle-melting. On

the other hand, some workers consider enderbitic char-

nockites of the Eastern Ghats belt as metamorphosed

igneous precursors and commonly describe them as

“now enderbite” [15]. Here again, the question of felsic

igneous rocks directly derived from mantle remains un-

resolved. Dehydration melting experiments have demon-

strated that silicic melts of tonalitic, granodioritic and

granitic compositions are produced at 8-10 Kbar, and ≥

850℃ from mafic rocks [16-18]. The massif-type char-

nockites in the Eastern Ghats belt are of variable compo-

sition and P-T conditions of granulite facies metamor-

phism are comparable to the experimental constraints as

mentioned above [19,20]. Thus a remelting of mantle-

S. Bhattacharya et al. / Natural Science 2 (2010) 1079-1084

1080

derived melt or hydrated amphibolite under granulite

facies conditions could be the favored model for the

massif-type charnockites of the Eastern Ghats belt [20-

22].

Considering charnockite-massifs as products of partial

melting in the deep crust under granulite facies condi-

tions, U-Pb ages of zircons in them can be taken as rep-

resenting this deep crustal anatexis, while the Nd-model

ages could provide the mantle-derivation ages of their

protoliths.

In this communiqué, we present selected geochemical

and isotopic data for two sets of massif-type char-

nockites of the Eastern Ghats belt, of Archean and Pro-

terozoic ages respectively. These data could mirror the

differences in some key geochemical features of the con-

tinental crust. These distinctive features may also pro-

vide some useful constraints on changing processes of

crust formation from Archean to Proterozoic.

2. GEOLOGICAL SETTING

The Eastern Ghats granulite belt skirting the eastern

coast of India is bounded by granite-greenstone belts of

Singhbhum and Bastar cratons to the north and west

respectively (Figure 1). The granulite lithologies record

polyphase deformation and possible multiple granulite

facies imprints [23-26]. Massif-type charnockite is a

Figure 1. Generalized geological map of the Eastern Ghats

Granulite belt, India.

major component in this regional granulite terrane and

occurs in different crustal domains [10]. The charnoc-

kite-massifs considered here occur in the Archean do-

mains around Jenapore & Jaypur and Proterozoic do-

mains around Sunki, Paderu and Naraseraopet (see loca-

tions in Figure 1).

3. GEOCHEMICAL SIGNATURES

Bulk composition was determined by XRF spectrome-

try at National Geophysical Research Institute, Hydera-

bad and Operating condition for XRF machine was 20/

40 KV for Major oxides, nominal analysis time was 300

seconds for all major oxides. For the XRF analysis the

overall accuracy (% relative standard deviation) for ma-

jor and minor oxides are given as less than 5%. The av-

erage precision is reported as better than 1.5%. For ICP–

MS analysis at Institute Instrumentation Centre, Indian

Institute of Technology, Roorkee, the average precision

were 4.1% RSD.

The analytical data are given in Table 1. Compared to

the Archean charnockites the Proterozoic charnockites

are potash-rich, with high K2O / Na2O ratios (Figure 2)

and this is consistent with the compositions of granitic

rocks of the two periods, as described in Kemp and

Hawkesworth, 2004 [2]. Compared with the Archean char-

nockites the Proterozoic charnockites are rubidium-rich

with high Rb / Sr ratios (average 1.01, n = 11: Protero-

zoic and average 0.18, n = 7: Archean). The lower Rb /

Sr ratios in the Archean charnockites reflect elevated Sr

in the Archean than in the Proterozoic charnockites (Fig-

ure 3). However, Sr / Nd and Nb / La ratios are variable

in both sets (Figure 4); though lower Nb / La ratios in

many samples from Archean could reflect different pro-

cesses in the Archean [2]. Greater fractionation of HREE,

extending to higher (Gd / Yb)N ratios in the Archean

charnockites is consistent with those in Archean green-

stone belts (Figure 5). REE patterns are distinctive, pri-

marily in the significant Eu depletion in the Proterozoic

charnockites and much less Eu depletion in the Archean

charnockites. Significant Eu-depletion coupled with Sr-

depletion is characteristic of the Proterozoic charnockites

compared with those of the Archean charnockites (Fi-

gure 6). Archean charnockites show relatively less en-

richment in LREE, much less Eu-depletion and greater

fractionation of HREE, compared to those in the Pro-

terozoic charnockites.

4. ISOTOPIC SIGNATURES

Mantle-derivation ages for the charnockite suites were

determined by Sm-Nd isotopic analysis of whole rocks

by Thermal Ionisation Mass Spectrometry at Indian In-

stitute of Technology, Roorkee. Detail analytical proce-

S. Bhattacharya et al. / Natural Science 2 (2010) 1079-1084

108

1081

Table 1. Selected oxides, trace element and isotopic data of massif-type charnockites in EGB.

S. Bhattacharya et al. / Natural Science 2 (2010) 1079-1084

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5 6

Figure 2. K

2O versus Na2O plot of the charnockites in the

Eastern Ghats Granulite belt. Archean charnockites: solid sym-

bols; Proterozoic charnockites: open symbols.

0.1

0.01

Rb/Sr

0.1

Eu / Eu

Figure 3. Rb / Sr versus Eu / Eu* plot of the charnockites in

the Eastern Ghats Granulite belt. Symbols as in Figure 2.

1.2

0.8

0.6

0.4

0.2

Nb / La

Sr / Nd

0 10

50 60

Figure 4. Nb / La versus Sr / Nd plot of the charnockites in the

Eastern Ghats Granulite belt. Symbols as in Figure 2.

dure is given in Bhattacharya et al., 2010 [27]. Measured

ratios for isotopic composition are normalized to 146Nd /

144Nd = 0.7219 for Nd. The measured ratio of 143Nd /

144Nd for Ames Nd Standard was 0.512138 ± 4 (quoted

0.1

(Gd / Yb)

0.1

Eu / Eu

Figure 5. (Gd / Yb)N versus Eu / Eu* plot of the charnockites

in the Eastern Ghats Granulite belt. Symbols as in Figure 2.

Sample / Chondrite

10000

1000

100

La Ce Nd Sm

Figure 6. Chondrite normalized REE plot of the charnockites

in the Eastern Ghats Granulite belt.

value 0.512138).

Mantle-derivation ages (TDM) for the Proterozoic char-

nockites vary between 2.3 and 2.8 Ga (Naraseraopet),

between 2.3 and 2.4 Ga (Paderu) and between 2.8 and

3.1 Ga (Sunki) and those for the Archean charnockites

vary between 3.3 and 3.5 Ga (Jenapore) and between 3.4

and 3.5 Ga (Jaypur) respectively (Table 1).

5. DISCUSSIONS

High Rb / Sr ratios and significant negative Eu-ano-

malies in the Proterozoic charnockites indicate residual

plagioclase. The implication is that intracrustal melting

occurred at shallow depths, in the stability field of pla-

gioclase. In contrast, low Rb / Sr ratios, indicating ele-

vated Sr, and lack of significant negative Eu-anomalies

in the Archean charnockites are indicative of intracrustal

melting at greater depths in the stability field of garnet or

amphibole [2].

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Large discrepancies between crystallization ages (of

anatectic charnockitic melt), given by U-Pb zircon ages

reported in the literature (Table 1) and mantle derivation

ages given by TDM for the Proterozoic charnockites con-

firm that older crustal material was present within the

source regions of the charnockitic magma [28]. This is in

contrast to the relatively little time gap between crystal-

lization ages and Nd-model ages for the Archean char-

nockites, similar to those observed in Archean TTGs.

6. CONCLUSIONS

These differences in the geochemical and isotopic sig-

natures between Archean and Proterozoic charnockites

reflect different conditions of crust formation rather than

different processes: both can be explained by dehydra-

tion melting of hydrated basalt or amphibolite under

granulite facies conditions; but Proterozoic charnockites

at shallower depths (in plagioclase-stability field) than

Archean charnockites (in garnet or amphibole-stability

field).

7. ACKNOWLEDGEMENTS

Indian Statistical Institute, Kolkata provided the infrastructural fa-

cilities. Financial support was provided by the Department of Science

and Technology, Government of India, in the form of a research pro-

ject.

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