Vol.2, No.10, 1079-1084 (2010) Natural Science
Copyright © 2010 SciRes. OPEN ACCESS
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: samar.bhattacharya@gmail.com.
Received 15 July 2010; revised 18 August 2010; accepted 23 August 2010.
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
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
Copyright © 2010 SciRes. OPEN ACCESS
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-
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
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.
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).
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.
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
Copyright © 2010 SciRes. OPEN ACCESS
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
Copyright © 2010 SciRes. OPEN ACCESS
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.
Eu / Eu
Figure 3. Rb / Sr versus Eu / Eu* plot of the charnockites in
the Eastern Ghats Granulite belt. Symbols as in Figure 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
(Gd / Yb)
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
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).
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].
S. Bhattacharya et al. / Natural Science 2 (2010) 1079-1084
Copyright © 2010 SciRes. OPEN ACCESS
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.
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
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-
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