Geol. Mag. 138 (4), 2001, pp. 495–498. Printed in the United Kingdom © 2001 Cambridge University Press

495

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Sm–Nd and Rb–Sr dating of amphibolite from the Nellore–
Khammam schist belt, SE India: constraints on the collisionof the Eastern Ghats terrane and Dharwar–Bastar craton
T. OKUDAIRA*†, T. HAMAMOTO‡, B. HARI PRASAD* & RAJNEESH KUMAR** Department of Geosciences, Osaka City University, Osaka 558-8585, Japan‡ Graduate School of Science, Niigata University, Niigata 950-2181, Japan(Received 7 November 2000; accepted 18 April 2001)

Abstract
The Nellore–Khammam schist belt, SE India, is sandwichedin between the Proterozoic Eastern Ghats terrane and theArchaean Dharwar–Bastar craton. We report Sm–Nd and
Rb–Sr mineral isochron ages of amphibolite from the schist
belt. The Sm–Nd and Rb–Sr ages are 824 ± 43 Ma and481 ± 16 Ma, respectively. The Sm–Nd age indicates the timing of peak metamorphism, whereas the Rb–Sr age indicates
the Pan-African thermal overprint. The peak metamorphism was related to collision of the Eastern Ghats terranewith the Dharwar-Bastar craton, which occurred during
early Neoproterozoic time.
1. Introduction
The Nellore–Khammam schist belt forms a linear belt thatruns parallel to the east coast of India (Fig. 1). As the
Nellore–Khammam schist belt is considered to be equivalent
to the Sargur schist belt (protolith age of c. 3.3–2.5 Ga:
Peucat et al. 1995) within the late Archaean Dharwar–
Bastar craton, it is known as an Archaean greenstone belt in
India (Ramam & Murty, 1997). This Archaean greenstone
belt was metamorphosed in late Mesoproterozoic to early
Neoproterozoic time (Ghosh et al. 1994), and thus transformed into a Proterozoic schist belt. The Nellore–
Khammam schist belt has a length of about 600 km, withvariable width from 30 to 130 km, and is bounded by the
Mesoproterozoic Eastern Ghats granulite terrane to the eastand by Mesoproterozoic basin sediments (Cuddapah Supergroup and Pakhal Supergroup) overlying the Archaean
Dharwar–Bastar craton to the west (Ramam & Murty, 1997;
Babu, 1998; Mezger & Cosca, 1999). The terrane boundaries
between the Nellore–Khammam schist belt and Eastern
Ghats terrane, and between the Nellore–Khammam schist
belt and Cuddapah Supergroup, have been considered to be
E-dipping low-angle fault systems (Ramam & Murty, 1997).
The structural features of the last phase of deformation in
the Nellore–Khammam schist belt are largely consistent
with those of the Eastern Ghats terrane and Dharwar–
Bastar craton (Rajneesh Kumar, Okudaira & Yoshida,2000). This fact indicates that the deformation in the† Author for correspondence: oku@sci.osaka-cu.ac.jp

Nellore–Khammam schist belt was at least partly simultaneous to that in the Eastern Ghats terrane and Dharwar–
Bastar craton. According to Chetty & Murthy (1994), thecollisional tectonics in the Eastern Ghats terrane operated
by NW-directed thrusting under a SE–NW compressional
stress regime. Because the Eastern Ghats terrane collided
with the Dharwar–Bastar craton in late Mesoproterozoic toearly Neoproterozoic time (Lakshminarayana, 1997) and the
Nellore–Khammam schist belt is sandwiched between them,
the Nellore–Khammam schist belt is a key element forunderstanding the crustal evolution of Peninsular India atthat time.
Geochronological data for metamorphic rocks in the
Nellore–Khammam schist belt are limited. Almost all of thedata were obtained by K-Ar analyses, such as the 536 ± 14
Ma fuchsite age (Sarkar et al. 1964), and the 989 ± 23 Ma
whole rock age and 806 Ma muscovite age (Ghosh et al.
1994). Because a closure temperature for the K–Ar system islower than amphibolite-facies conditions and there is no
detailed petrography for the dated samples, the previously
published data cannot be utilized for understanding the evolution of the Nellore–Khammam schist belt. Therefore,
radiogenic isotope dating with high closure temperatures,such as Rb–Sr and Sm–Nd dating of garnet and hornblende,is necessary. In this study, we report Sm–Nd and Rb–Sr mineral isochron ages for the high-pressure amphibolites of the
Nellore–Khammam schist belt in the Khammam district, SE
India, and discuss the evolution of the schist belt. Mineralabbreviations used in this paper are those of Kretz (1983).
2. Geological backgroundAs shown in Figure 1, the Nellore–Khammam schist belt,
has been divided into an upper and a lower structural unit
(Vasudevan & Rao, 1975; Narayana Rao, 1983; Ramam &Murty, 1997). The upper unit is dominated by metasediments that occur in the southwestern part of the belt; thelower unit consists mainly of metabasaltic rocks distributedin the eastern part. These units are juxtaposed along suspected low-angle faults that dip to the east (Ramam &Murty, 1997). The rocks of the upper unit are considered to
be derived from trench sediments (Raman & Murty, 1997).
Within the Nellore–Khammam schist belt, an ophiolitic
melange has been reported (Reddy, Narayana & Shanti

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Figure 1. Generalized geological map around the Nellore-Khammam schist belt (Okudaira, Hari Prasad & Rajneesh Kumar,
2000 and references therein). Location of the amphibolite samples studied here is indicated.
Kumar, 1994). In the lower unit, subduction-related magmatism in different settings such as oceanic-island arc, continental margin or back arc has been well recognized, andaffinities to within-plate basalt and N-MORB are absent(Satyanarayana, Dhana Raju & Kanungo, 1994;
Sarvothaman, 1995, Hari Prasad et al. 2000). The metamorphic grade generally increases from greenschist facies toupper amphibolite facies, from west to east (Vasudevan &Rao, 1975; Ramam & Murty, 1997; Babu, 1998; Moeen,1998). Almost all of the previously reported P–T data plot
within the amphibolite-facies field of Spear (1993), and they
are located near the boundary of the phase transition fromkyanite to sillimanite (Okudaira, Hari Prasad & Rajneesh
Kumar, 2000 and references therein).
3. Petrography and geochronology
In the Khammam district (latitude 17˚17’ to 17˚24’ N, longitude 80˚25 to 80˚29’ E), amphibolites predominate with subordinate amounts of granites and anorthosites. Based on
their mineralogy, the amphibolites studied here can bedivided into three rock types: garnet amphibolite, Grt–Cpx
amphibolite and Cpx–Ttn amphibolite. The mineral assemblages of the three rock types differ as follows: garnet
amphibolite – Pl + Hbl + Grt + Qtz; Grt–Cpx amphibolite –Pl + Hbl + Grt + Cpx + Qtz; Cpx–Ttn amphibolite – Pl +Hbl + Cpx + Ttn + Qtz. They show granoblastic microstructures, indicating that they did not undergo intense retrograde
metamorphism and deformation. The minerals do not show
distinct compositional zoning. Plagioclase in the garnet
amphibolite has Xan = 0.22–0.27; in the Grt–Cpx amphibo-lite Xan = 0.30–0.38; and in the Cpx–Ttn amphibolite
Xan = 0.55–0.77. Amphiboles are calcic to subcalcic and fallin the range of tremolitic to pargasitic hornblende. The average values of Mg/(Mg+Fe) of the amphiboles are approximately 0.47 for the garnet amphibolite, 0.6 for the Grt–Cpx
amphibolite, and 0.6 for the Cpx–Ttn amphibolite. Garnet ispresent in the garnet amphibolite and the Grt–Cpx amphibolite, and is grossular–almandine of fairly constant composition in both rock types (garnet amphibolite: Xalm = 0.52,
Xgrs = 0.33, Xprp = 0.11; Grt–Cpx amphibolite: Xalm = 0.51,
Xgrs = 0.31, Xprp = 0.17). Clinopyroxene is diopside(XMg ≈ 0.75 for the garnet amphibolite; 0.73 for the Cpx–Ttn
amphibolite). Based on these mineral chemistries, Hari
Prasad et al. (2000) reported P–T conditions in the range
650–750 °C and 0.8–1.2 GPa.
In this study one garnet amphibolite (sample 970227-5),
two Grt–Ttn amphibolites (samples 980101-2 and 9801014), and one Grt–Cpx amphibolite (sample 980112-24) wereanalysed for Sm–Nd and Rb–Sr isotopic analyses; theGrt–Cpx amphibolite sample was used for mineral isochron
dating. Experimental procedures used were those of Kagami
et al. (1987), Kagami, Yokose & Honma (1989) andHamamoto, Osanai & Kagami (1999). Isotope ratios were
measured with a thermal ionization mass spectrometerequipped with five Faraday cups (MAT-261) and with nine
Faraday cups (MAT-262) at Niigata University, operated instatic mode. Sr and Nd isotope ratios were carefully measured by several runs on the same filament for each sample.
The 87Sr/86Sr and 143Nd/144Nd ratios with lowest 2σ values
among several runs are shown in Table 1. The 87Sr/86Sr ratios
during the present study are reported relative to 0.710242

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R A P I D C O M M U N I C AT I O NTable 1. Trace element concentrations and isotopic data for Sm–Nd system and Rb–Sr system
Garnetamphibolite
970227-5
Whole rock

980101-2
Whole rock

980101-4
Whole rock

Whole rock

Felsic fraction

Hbl

Grt

Sm (ppm)
Nd (ppm)147
Sm/144Nd143
Nd/144Nd
2σ error
TDM (Ga)

3.96
18.8
0.12770.511236
0.000011
3.37

4.77
23.1
0.12470.511017
0.000013
3.62

8.17
40.2
0.12280.511015
0.000014
3.55

1.19
4.51
0.15880.511817
0.000015
–

0.117
0.613
0.11540.511614
0.000014
–

1.13
2.54
0.26810.512383
0.000014
–

0.48
0.72
0.40320.513166
0.000015–

Rb (ppm)
Sr (ppm)87
Rb/86Sr87
Sr/86Sr
2σ error

9.51
227
0.12150.712345
0.000011

0.837
496
0.004884
0.710796
0.000017

0.948
573
0.004784
0.710632
0.000014

2.86
78.5
0.10550.711373
0.000014

1.33
329
0.011710.710679
0.000014

5.54
43.5
0.3686
0.713132
0.000013

0.223
2.74
0.23520.712254
0.000015

Cpx–Ttn amphibolite
Grt–Cpx amphibolite 980112-24

Nd isotopes normalized to 146Nd/144Nd = 0.7219. Depleted mantle values for Nd are (143Nd/144Nd)today = 0.51315, (147Sm/144Nd)today = 0.2137(parameters are those of Peucat et al. 1988). Sr isotopes normalized to 86Sr/88Sr = 0.1194.

for NBS987 (Kagami, Yokose & Honma, 1989) and143
Nd/144Nd ratios are reported relatively to GeologicalSociety of Japan standard JNdi-1 = 0.512116 (Tanaka et al.
2000). During the measurement, the isotope ratios of87
Sr/86Sr for NBS987 and of 143Nd/144Nd for JNdi-1 were0.710231 ± 0.000014 and 0.512107 ± 0.000012, respectively.
Rb, Sr, Sm and Nd concentrations were measured using87
Rb–84Sr and 149Sm–150Nd mixed spikes, respectively.
Blanks for the whole procedures during this study were
Rb < 0.25 ng, Sr < 0.52 ng, Sm < 0.025 ng, and Nd < 0.22 ng.
Rb–Sr and Sm–Nd isochron ages were calculated using
the equation of York (1966), with the following decay constants: λ87Rb = 1.42 × 10-11 yr-1 (Steiger & Jäger, 1977) andλ147Sm = 6.54 × 10-12 yr-1 (Lugmair & Marti, 1978). To calculate the internal 2σ errors, the external errors were estimatedto be 87Rb/86Sr 0.5%, 87Sr/86Sr 0.01%, 147Sm/144Nd 0.1% and143
Nd/144Nd 0.01%. Garnet, hornblende, felsic fraction andwhole rock for the Grt–Cpx amphibolite sample 980112-24
yields a Sm–Nd isochron age of 824 ± 43 Ma with an initial143
Nd/144Nd ratio of 0.510967 ± 0.000075 (Fig. 2a). The
Rb–Sr isochron age from the same sample is 481 ± 16 Mawith an initial 87Sr/86Sr ratio of 0.710624 ± 0.000051 (Fig.
2b). The depleted mantle model ages (TDM) of the amphibolites with 147Sm/144Nd < 0.15 range from 3.37 to 3.62 Ga,
supporting the idea that the protolith of the Nellore–
Khammam schist belt is equivalent to that of the Archaean
Sargur schist belt.

4. Discussion
The closure temperature ranges of the Sm–Nd system in garnet and hornblende are 600-850 and 700-800 °C, respectively, whereas the Rb–Sr system in garnet and hornblende
has a much lower closure temperature (≈ 530 °C) than that of
the Sm–Nd system (e.g. Burton & O’Nions, 1990). The
Rb–Sr whole rock and mineral isotope data define a welldefined isochron (Fig. 2b). In the amphibolites analysed
here, the lack of compositional and microstructural changes
from retrograde metamorphism suggests that thermal overprint was not sufficiently intense to modify the mineralchemistry, and that the assemblage formed at the peak metamorphism of 650–750 °C. The peak P–T condition of theamphibolites is close to the closure temperature for the
Sm–Nd system and is higher than that of the Rb–Sr system.
The analysed minerals equilibrated during a Neoproterozoic

peak event (824 ± 43 Ma), and show no evidence for being
re-equilibrated during an Early Palaeozoic event (481 ± 16Ma). The 481 ± 16 Ma Rb–Sr age may represent the time
when the post-peak, retrograde P–T path passed through theclosure temperature of the Rb–Sr system in Ordovician time.
Alternatively, this Rb–Sr age can be interpreted to be theresult of redistribution and re-equilibration of Rb and Sr
caused by thermal overprint. In the Eastern Ghats granulite
terrane, a c. 500 Ma Pan-African thermal event has been
well recognized (Shaw et al. 1997), and hence the Ordovician
Rb–Sr age for the Grt–Cpx amphibolite may correspond toa Pan-African thermal overprint.
The P–T condition of c. 700 °C and 1 GPa for the amphibolites lies on the line with dT/dP ≈ 0.7 °C/MPa, assuming
linear geotherm. This is equivalent to a geothermal gradientof c. 20 °C/km, assuming a rock density of 2750 kg/m3. Sucha geothermal gradient corresponds to that of a high- tointermediate-pressure metamorphic belt (Spear, 1993). If themetamorphism with a geothermal gradient of c. 20 °C/km
resulted from the collision of the Eastern Ghats terrane with
the Dharwar–Bastar craton, the high-pressure condition
must have occurred during or following the collision. In conclusion, the Nellore–Khammam schist belt appears to represent the footwall or western foreland of an Neoproterozoic
orogen, during which the Eastern Ghats terrane accreted to
the Dharwar–Bastar craton. During this orogeny, the late
Archaean protoliths of the Nellore–Khammam schist belt
were tectonically reworked and metamorphosed under highpressure amphibolite-facies condition, as indicated by a824 ± 43 Ma Sm–Nd mineral isochron age for an amphibolite. Much later, both the Nellore–Khammam schist belt andthe Eastern Ghats terrane underwent a thermal event in
Early Palaeozoic times as indicated by a 481 ± 16 Ma Rb–Sr
isochron age for the same mineral fraction.
Acknowledgements. We thank H. Kagami for his valuablesuggestions and helpful support for isotopic analysis at
Niigata University. We would like to express thanks to
M. Yoshida, R. S. Divi, and A. T. Rao for their encouragement and support. We thank J. J. Peucat, H. M. Rajesh, and
L. Ratschbacher for their comments on the early versionof the manuscript. Constructive reviews by T. Argles and
M. Timmerman improved the submitted manuscript and are
greatly acknowledged. The third and fourth authors thankthe Ministry of Education, Science and Culture (Japan) for
financial support. This paper contributes to IGCP-368.

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Figure 2. Sm–Nd and Rb–Sr isochron diagrams of Grt–Cpx
amphibolite sample 980112-24 from the Khammam district.
(a) 143Nd/144Nd v. 147Sm/144Nd diagram. (b) 87Sr/86Sr v.
87
Rb/86Sr diagram. The errors are smaller than the symbols.

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