Evolution of the Continental Crust in the Proterozoic Eastern Ghats Belt, India and new constraints for Rodinia reconstruction: implications from Sm–Nd, Rb–Sr and Pb–Pb isotopes

Precambrian Research 112 (2001) 183– 210 www.elsevier.com/locate/precamres

Evolution of the Continental Crust in the Proterozoic Eastern Ghats Belt, India and new constraints for Rodinia reconstruction: implications from Sm–Nd, Rb–Sr and Pb–Pb isotopes Karen Rickers a,b,*, Klaus Mezger b, Michael M. Raith a a

Mineralogisch-Petrologisches Institut, Uni6ersita¨t Bonn, Poppelsdorfer Schloß, 53115 Bonn, Germany b Institut fu¨r Mineralogie, Uni6ersita¨t Mu¨nster, Corrensstr. 24, 48149 Mu¨nster, Germany Received 28 April 2000; accepted 16 February 2001

Abstract For this study Nd, Sr and Pb isotope compositions were analyzed for ortho- and paragneisses from the Eastern Ghats Belt of India in order to determine its crust formation and crustal evolution. This belt represents a Proterozoic orogen that extends along the east coast of Peninsular India and forms part of the mobile belts in East Gondwana and Rodinia. The Eastern Ghats Belt was affected by Mesoproterozoic granulite facies metamorphism in the western segment (Western Charnockite Zone) and a Grenvillian regional-scale high-grade event in the central and eastern segments (Western Khondalite Zone, Charnockite Migmatite Zone and Eastern Khondalite Zone) as well as a local Pan-African overprint. The results of the isotope studies are used for the large-scale reconstruction of the indo-antarctic part of the Rodinia supercontinent. Based on Nd model ages and Pb isotope ratios from leached feldspars four crustal domains can be distinguished in the Eastern Ghats Belt. These domains can in part be correlated with the lithological division of the belt: (1) The Western Charnockite Zone south of the Godavari Graben is characterized by Nd model ages between 2.3 and 2.5 Ga for orthogneisses and 2.6 and 2.8 Ga for metasediments (Domain 1). The Pb isotopes are primitive indicating reworking of dominantly Archean and mixing with minor Proterozoic material; (2) North of Godavari Graben Nd model ages for orthogneisses are significantly higher with values ranging from 3.2 to 3.9 Ga. The Pb isotopes are strongly retarded; (3) The north-eastern parts of the Charnockite Migmatite Zone and Western Khondalite Zone form a distinct and almost homogeneous crustal domain (Domain 3) with Nd model ages between 1.8 and 2.2 Ga; (4) Between the isotopically homogeneous terranes stretches a broad transition zone (Domain 2) enclosing parts of the Western Khondalite Zone, Charnockite Migmatite Zone and Eastern Khondalite Zone. The Nd model ages for metasediments (2.1– 2.5 Ga) are younger than paragneiss ages of the adjoining Western Charnockite Zone. The Nd model ages for orthogneisses (1.8– 3.2 Ga) display a large spread, which is consistent with the Pb isotope signatures that indicate mixing of Archean with Proterozoic material.  Figure A.1 and Table A.1 can be found on the journal’s website, http://www.elsevier.com/locate/precamres * Corresponding author. Present address: HASYLAB/DESY, Notkestr. 85, 22603 Hamburg, Germany. E-mail addresses: [email protected] (K. Rickers), [email protected] (K. Mezger), [email protected] (M.M. Raith).

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The border zone between Domain 3 and the Archean Eastern Indian Craton forms a second transition zone (Domain 4) characterized by metasediments with Nd model ages between 2.2 and 2.8 Ga and orthogneisses with model ages around 3.2 Ga. Reworking of Archean crustal material is most intense along the border zones of the belt and ‘juvenile’ material is more dominant away from the orogenic front. This scenario is indicative of an active continental margin setting for the two Proterozoic episodes of orogenesis in the Eastern Ghats Belt. A correlation of Domain 3 with the Rayner Complex and the Prydz Bay region, Antarctica, for the early crustal evolution is supported by the similarity of the isotope signatures. The Napier Complex is very different to the Eastern Ghats Belt and an early joint evolution of these terranes is ruled out on the basis of the Pb –Pb and Sm –Nd systematics. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Eastern Ghats Belt; Rodinia reconstruction; Sr –Nd– Pb isotopes; Crustal evolution; Model ages

1. Introduction The understanding of crustal growth and plate tectonic processes early in the history of the Earth is linked with the accurate portrayal of the reconstruction of the genesis of ancient continents. While the reconstruction of Gondwana has attained a lot of attention in the last several years (Rogers et al., 1995; Unrug, 1996), the nature of its precursor, Rodinia, just recently entered the focus of interest (Powell et al., 1993, 1994; Wareham et al., 1998; Weil et al., 1998). The Rodinia supercontinent is the precursor of all later supercontinents. Its break-up started in the late Proterozoic and some parts remained together until the Mesozoic break-up of Pangea. The evolution of the continental crust of Rodinia is a key to the understanding of Proterozoic geodynamic processes. In East India, the Eastern Ghats Belt (EGB) has frequently been correlated with Enderby Land in Antarctica. Correlations with Antarctica are based on lithologies documenting a similar P–T evolution (Sengupta et al., 1999), the exposure of enderbites and sapphirine granulites and the fit of the coastlines and structural constraints (Hofmann, 1996). Little is known, however, about the Gondwana and pre-Gondwana evolution of the EGB. Just recently, a first regional determination of mineral ages in the EGB indicated the similarity of thermal histories of the eastern part of the EGB in India with the Rayner Complex and the Prydz Bay region in East Antarctica (Mezger and Cosca, 1999) for the time interval between 1 Ga and 500 Ma. The final break-up of Gondwana in the area of East

Antarctica and East India occurred ca. 160 Ma ago (Powell et al., 1988). This study focuses on pre-metamorphic processes using a combination of Sm–Nd and Rb –Sr whole rock systematics and common Pb signatures of leached feldspars. Such a study can provide information on the timing of different evolutionary stages that is essential to understand the processes that led to the generation and evolution of the crust in the EGB on a large scale. Regional Sm–Nd, Rb –Sr and Pb –Pb isotope mapping of the EGB of India helps to distinguish crustal blocks in the EGB and will aid when comparing the crustal evolution of the belt with potentially adjacent terranes in Rodinia. The combination of Sm–Nd and Rb –Sr crustal residence ages with Pb signatures of leached feldspars is a powerful tool for the discrimination of old reworked crust from juvenile crust and is thus essential for the understanding of crustal genesis and evolution (Doe, 1967; Moorbath et al., 1969; Zartman, 1974; Nelson and DePaolo, 1985; DePaolo, 1988; Wooden and Mueller, 1988). The major fractionation of Sm and Nd occurs during the formation of melts in the upper mantle. Later magmatic, metamorphic or sedimentary processes involving rocks derived from these melts generally do not significantly modify the Sm/Nd ratios. Neodymium isotope ratios thus allow the calculation of model crustal residence ages (e.g. Nelson and DePaolo, 1985; Arndt and Goldstein, 1987; DePaolo, 1988). From Sm–Nd isotope studies alone, however, it is not always certain whether these ages are real differentiation ages or mean

K. Rickers et al. / Precambrian Research 112 (2001) 183–210

crustal residence ages resulting from mixtures of old crustal material and juvenile, mantle-derived material (Arndt and Goldstein, 1987). In contrast to Sm/Nd, the ratios of Rb/Sr, U/Pb and Th/Pb can be fractionated significantly during crustal processes such as partial melting, hydrothermal alteration and diagenesis. The interpretation of the Rb –Sr systematics in polycyclic orogens is complicated by the fact that the Rb/Sr ratios are readily modified, particularly by partial melting processes. However, in combination with Nd and Pb isotopes the Sr isotopes provide insights into the timing of crustal processes that cannot be readily identified with Nd isotopes. Feldspars, and in particular K-feldspar, are characterized by extremely low U/Pb and Th/Pb ratios and thus represent the Pb isotopic composition of the whole rock at the time of last isotopic homogenization (e.g. Ludwig and Silver, 1977; Garie´ py and Alle`gre, 1985; Housh et al., 1989; Chamberlain and Bowring, 1990; DeWolf and Mezger 1994). K-feldspar is a suitable mineral to determine the initial Pb isotopic compositions because of its sufficiently high closure temperature for Pb (Cherniak, 1992), its high Pb concentrations and its common occurrence in (almost) all typical crustal rock types. When comparing Nd model ages with Pb isotope ratios, it becomes clear whether Nd model ages represent mixing ages or real crustal residence ages (e.g. Arndt and Todt, 1994; DeWolf and Mezger, 1994). Leaching of feldspars was applied in order to ensure that only common Pb incorporated during the last homogenization of the grains with the whole rock was analyzed (Ludwig and Silver, 1977; Tilton et al., 1981; Mezger et al., 1989). Late-stage radiogenic Pb, a product of the decay of minor amounts of U, Th and Rd incorporated in the feldspar, sits on inter-lattice sites and is removed preferentially from the crystal by the leaching procedure with aqua regia and HF. For the Pb isotope study, K-feldspar was preferred due to its higher Pb concentration while plagioclase was only separated in the rare cases when no K-feldspar was present. Wherever possible, feldspars were separated from the same rocks that were also used for crustal residence age determinations. This allows the direct comparison of the

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Pb isotope composition with the Sr and Nd isotope characteristics at the time of last Pb homogenization, which is known from dating of magmatic and metamorphic minerals. The combination of the different isotope methods characterized by different behaviour can be used to unravel more precisely processes that led to crust formation and evolution in the EGB.

2. Geological setting The Eastern Ghats Belt is a Proterozoic orogen exposed along the east coast of Peninsular India. It forms part of the mobile belt systems within East Antarctica and East India. The belt is bordered by the Bay of Bengal to the east and by Archean cratons to the west. These are from south to north, the Dharwar Craton, the Bastar Craton and the Eastern Indian Craton (Fig. 1). According to the most recent geological map of the Eastern Ghats Belt of India (Ramakrishnan et al., 1998), four major lithological units can be distinguished, all aligned parallel to the Archean cratons (Fig. 1): the westernmost unit, the Western Charnockite Zone (WCZ), consists of an association of basic granulites, enderbites and charnockites all with enclaves of metasedimentary migmatites. Late-stage pegmatites cross-cut all structures. The second unit, adjoining to the east is the Western Khondalite Zone (WKZ) composed of mainly garnet– biotite–sillimanite gneisses, calc silicate rocks and quartzites but also includes enderbitic and charnockitic intrusions hosting enclaves of supracrustal xenoliths. Latestage pegmatites are common. The easternmost unit, the Eastern Khondalite Zone (EKZ) is similar in composition to the WKZ. The Charnockite Migmatite Zone (CMZ), enclosed between the two distinct khondalite zones, is the most heterogeneous domain of the Eastern Ghats Belt and is typically composed of garnet-bearing diatexitic migmatites and leptynites, bands and rafts of khondalitic granulites as well as calc silicate rocks. This association of intensely migmatized supracrustal rocks is intruded by voluminous Stype granitoids many with characteristic feldspar megacrysts. Late-stage pegmatites are common.

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The relative ages of the major rock types may be deduced from field relations: the oldest components in all zones are the supracrustal rocks that were multiply intruded by magmatic rocks (enderbites, charnockites, S-type granitoids, leptynites, pegmatites). The intruding magmas are either mantle-derived melts (basic granulites, enderbites and charnockites with crustal contribution), crustderived melts (S-type granitoids) or melts derived from granulite facies supracrustal rocks (leptynites, pegmatites, several generations of leucosomes).

Fig. 1. Simplified geological map of the Eastern Ghats Belt of India modified after Ramakrishnan et al. (1998) showing the sample locations with abbreviated location numbers (dots, the prefix KR is not used in the map). Additionally, sample locations of previous studies are shown as well: triangle, Paul et al. (1990); inverted triangles, Shaw et al. (1997); black squares, Krause (1998); white cross in black squares, Osanai et al. (1999); black cross in white square, Yoshida et al. (1999). The lineaments are largely based on Chetty (1995).

The mineral assemblages of supracrustal rocks as well as orthogneisses record granulite facies conditions and in some parts of the EGB, ultrahigh-temperature (UHT) conditions (e.g. Lal et al., 1987; Kamineni and Rao, 1988; Dasgupta et al., 1991, 1994; Shaw and Arima, 1996; Mohan et al., 1997; Rickers et al., 1998; Sengupta et al., 1999). The details of P– T evolution remain a matter of debate, with apparent evidence for both clockwise and anticlockwise oriented paths (for a compilation see Sengupta et al., 1999). Recent publications discuss a polyphase metamorphic evolution of the granulites (Dasgupta et al., 1995; Bhowmik, 1997; Rickers et al., 1998). The results of recent mineral dating revealed contrasting and polyphase metamorphic histories in different parts of the belt: the oldest tectonothermal event documented is a granulite facies metamorphism at 1.6–1.4 Ga (Simmat and Raith, 1998; Mezger and Cosca, 1999; Simmat, in preparation). In the southern part of the WCZ this is the major high-grade metamorphism and so far there is no evidence of notable metamorphic imprints during later episodes as documented by chemical dating of monazites from paragneisses and orthogneisses (Simmat and Raith, 1998; Simmat, in preparation) as well as U–Pb dating of monazite and allanite from late-stage pegmatites cross-cutting all structures (Mezger and Cosca, 1999). Middle Proterozoic ages (1.4 Ga) are also documented in the EKZ and CMZ by upper intercepts of discordant U– Pb zircon data for high-MgAl granulites (Jarick, 2000) but the geological meaning of these data is still a matter of debate. The prominent UHT metamorphism in the WKZ, the CMZ and the EKZ has been dated with the common Pb method at 1.1 Ga (Jarick, 2000). Mineral age data indicate a subsequent pervasive metamorphism in the WKZ, CMZ and EKZ at somewhat lower pressures and temperatures during the Grenvillian event (1000–960 Ma) (Simmat and Raith, 1998; Mezger and Cosca, 1999; Simmat, in preparation). This regional highgrade metamorphic event was followed by S-type granitoid intrusions (until 940 Ma) in the CMZ (Paul et al., 1990; Kovach et al., 1997; Krause, 1998) at the end of the Grenvillian orogeny. Latestage pegmatites indicate a Pan-African overprint (Mezger and Cosca, 1999).

K. Rickers et al. / Precambrian Research 112 (2001) 183–210

Several prominent Graben structures and lineaments have been recognized in the EGB (e.g. Chetty, 1995). The Godavari Graben, the Mahanadi Graben, the Angul– Dhenkanal and Nagavali–Vamsahdara lineaments transect the whole belt perpendicular to its general strike and have been correlated with Graben structures and shear zones in East Antarctica (e.g. Chetty, 1995; Hofmann, 1996; Sengupta et al., 1999). Another important lineament in the EGB is the Sileru lineament which separates the WCZ from the eastern units (Fig. 1). In order to obtain a representative overview of the entire belt, more than 250 samples from all major lithological units were collected. To elucidate the change of model age characteristics with increasing distance to the craton, profiles perpendicular to the general strike across the lithological units of the belt were sampled. For the isotope study, 40 representative paragneisses and orthogneisses were selected from the sample set (Fig. 1).

3. Analytical methods For the isotope analysis ca. 100 mg of whole rock sample powder were digested after adding a Rb – Sr and a Sm – Nd tracer. The tracers were optimized based on the concentrations of Rb, Sr, Sm and Nd determined by XRF analysis. The dissolution of whole rock samples was achieved in four steps. Samples were first treated with 1– 2 ml of a concentrated mixture of HF and HNO3 in a closed 15 ml Savilex® beaker on a hot plate (110°C) for a minimum of 15 h. Afterwards, sample size was reduced by drying the sample. In the second step, concentrated HF/HNO3 was added again and the vial was placed inside a Parr® bomb. The samples were digested for 2– 5 days in an oven at 210°C. After completion of this step a few drops of HClO4 were added and the samples were dried down again on a hot plate at temperatures around 180°C. After this step total dissolution was achieved in 6 N HCl. The sample was dried down again and then dissolved in 3 ml 2.5 N HCl. One third was used to separate Rb, Sr and REE on columns filled with ca. 15 ml

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DOWEX AG 50Wx8® (200–400 mesh) resin. Sm and Nd were separated in a second step using 5 ml columns filled with HDEHP (Richard et al., 1976). All isotope measurements were performed at the Zentrallaboratorium fu¨ r Geochronologie at the Universita¨ t Mu¨ nster. Rubidium concentrations were analyzed on a single collector mass spectrometer (NBS Teledyne SS1290), Sr, Sm, Nd and common Pb were analyzed on a multicollector VG sector 54 mass spectrometer fitted with nine cups. Rubidium was measured on double Tafilaments with the sample being loaded with water on the side filament. Replicate analyses of an internal whole rock standard yield a reproducibility of 0.3% indicating the minimum error for 85 Rb/87Rb ratios. Within-run reproducibility is much better with an average 2| of 0.11%. The mean results for the measured standard deviates from the original value so that a correction of measured ratios with a factor of 0.9939 (lab mean) was performed. Rubidium blank concentrations are always below 20 pg. Strontium was measured on Ta-single central filaments in the dynamic mode. The sample was loaded with H3PO4. Total procedural blank concentrations are negligible with less than 50 pg. Replicate measurements of NBS 987 standard yielded a reproducibility of 0.004% with a mean 87Sr/86Sr value of 0.7102659 28. Within-run reproducibility is in the same range with an average 2| of 0.0033%. Fractionation was corrected by normalizing ratios to 86 Sr/88Sr=0.1194. Neodymium and Sm were loaded with HCl on Re-side-filaments. Measurements were performed with triple filaments in the static mode for Sm and in the dynamic mode for Nd. Total blanks are below 120 pg and 70 pg for Nd and Sm, respectively, and are thus negligible. Sm/Nd ratios were determined to a precision of about 0.3%. Eleven runs of La Jolla standard yielded a reproducibility of 0.0044% for 143Nd/144Nd ratios with a mean value of 0.5118539 22. Within-run reproducibility is better than 0.003% at the 2| confidence level. Fractionation was corrected by normalizing the isotope ratios to 146Nd/144Nd = 0.7219 for Nd and 147Sm/152Sm= 0.560795 and 149Sm/152Sm = 0.516825 for Sm.

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For feldspar separation, hand-size samples were crushed in a steel jaw and sieved. Due to the size of feldspars observed in thin section, the fraction of 125–250 mm was taken. The magnetic fraction was separated from feldspar and quartz by using a Frantz® magnetic separator at the Zentrallaboratorium fu¨ r Geochronologie, Universita¨ t Mu¨ nster. Feldspars and quartz were then separated using a heavy liquid (mixture of bromoform and acetone). In a last step, 20– 50 mg of inclusionfree feldspar grains were handpicked under the binocular microscope. The feldspar sample was leached in three steps: first in dilute aqua regia in a 7 ml Savilex® screw-top beaker on a hot plate (100°C) for 10 h. Next, the grains were leached for 30 min on a hot plate (70°C) in dilute HF in order to remove Pb situated on inter-lattice sites as well as fine-grained alteration phases. After that, the samples were leached a third time in a slightly stronger HF on a hot plate (70°C) for 5–10 min. The time of leaching depended on the rate of reduction of feldspar grains. Care has to be taken in leaching plagioclase as its dissolution is not clearly visible because CaF2 forms. Between each leaching step, the grains were washed three times in distilled water. In the last step, they were dissolved completely in concentrated HF on a hot plate (100°C) and then dried. After conversion to the bromide form with 1 ml of 1 N HBr, Pb was separated by using anion-exchange HBr– HCl chemistry. Lead was loaded with phosphoric acid and silicagel (Cameron et al., 1969) on single Refilaments and was measured in the temperature range of 1250–1450°C in the static mode. Lead isotope ratios were corrected for mass fractionation using the results for repeated analyses of the NBS standard SRM 982 during the course of this study. The fractionation factor was 0.13% per amu. Standard measurements were performed identically to sample measurements. The reproducibility of the standard was 0.04% for the 207Pb/ 206 Pb ratio. Mean within-run reproducibility is much better with typical 2s of 0.005% for the same ratio. The standard reproducibility thus forms the largest error for Pb isotope measurements. Since the feldspars contain high concentrations of Pb, the contribution from the analytical procedures is negligible.

4. Results and discussion The Rb – Sr, Sm–Nd and common Pb data are listed in Tables 1 and 2. The data include duplicates for whole rock analyses as well as replicate Pb mass spectrometer analyses.

4.1. Sample characterization and estimation of crustal residence ages For the determination of Sm–Nd and Rb – Sr crustal residence ages, it is essential to select samples that have Sm–Nd and Rb –Sr ratios significantly different from the mantle such that their Sr and Nd isotope ratios show an evolution that is distinct from the evolution of these ratios in the mantle. Neodymium and Sr crustal residence ages are calculated from the intersection of the 143Nd/ 144 Nd and 87Sr/86Sr evolution lines of the sample with the mantle evolution line. For the composition of the mantle, several models have been proposed (e.g. DePaolo, 1981; Goldstein et al., 1984; Liew and Hofmann, 1988). In this study, we use the depleted mantle model of Goldstein et al. (1984) for the Nd model age determination. This model assumes a linear change of the mantle 143 Nd/144Nd ratio from CHUR (0.512638) at 4.6 Ga to a present value of 0.51316 with a 147Sm/ 144 Nd value of 0.214. The choice of a different mantle model would shift the absolute Nd model ages but would not modify the interpretation of the age patterns presented here because differences in ages remain approximately the same. The ortho- and paragneisses from the EGB display typical crustal 147Sm/144Nd ratios of 0.08– 0.14 (Taylor and McLennan, 1985) with a mean value of 0.11. Evolved rocks that have 147Sm/ 144 Nd higher than 0.12 may have undergone a fractionation of the Sm/Nd ratios due to partial melting or metasomatism. As a result of this disturbance these samples tend to yield unrealistically old model ages. Therefore samples with 147 Sm/144Nd ratios higher than 0.12, have been corrected for metamorphic disturbance using the equation proposed by Milisenda et al. (1994), modified for a pre-metamorphic 147Sm/144Nd ratio of 0.11 which is the mean value for the samples from the EGB.

Table 1 The Sm–Nd and Rb–Sr whole rock isotope data of ortho- and paragneisses from the Eastern Ghats Belt of India and samples from the adjoining Indian craton mNd (Tm)

TDM (Ga)

916 914 925 926 924 926

0.0804 0.1232 0.0913 0.1874 0.1241 0.0858

2.5 2.5 1 2.5 0.5 2.5

−1.9 −4.9 −3.3 −5.4 −16.2 2.4

2.9 3.3 1.7 5.4 4.3 2.6

0.511418 0.511444 0.511632 0.511572 0.511347 0.511218 0.511203 0.511535 0.511471 0.511263 0.511208

917 929 922 919 916 913 911 921 913 927 920

0.1153 0.1148 0.1409 0.1336 0.1142 0.0844 0.0876 0.1235 0.1089 0.0907 0.0929

1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6 1.6

−7.1 −6.5 −8.2 −7.9 −8.3 −4.7 −5.6 −6.5 −4.8 −5.1 −6.6

2.7 2.6 3.2 3.0 2.8 2.3 2.3 2.7 2.4 2.3 2.4

Domain 1, WCZ, north of Goda6ari Graben R22 Enderbite 8.220 1.631 R23 Enderbite 11.33 1.969

0.511190 0.510317

925 929

0.1200 0.1050

2.5 2.5

−3.6 −15.9

3.2 3.9

Domain 2 KR8-1 KR27-2 KR31-1 KR65-2 KR29-2 dKR29-2 KR58-15 KR35-1 KR61-5 KR62-1

Khondalite Khondalite Metapelite Khondalite Enderbite Enderbite Charnockite Charnockite Enderbite Enderbite

93.18 65.24 34.99 114.1 60.70 70.03 58.28 59.61 99.51 51.19

17.26 11.36 5.635 16.72 11.09 12.19 10.99 12.94 15.58 10.28

0.511558 0.511546 0.511271 0.511374 0.511185 0.511195 0.511448 0.511574 0.511328 0.511639

917 919 926 921 929 915 918 923 918 925

0.1119 0.1052 0.0973 0.0886 0.1104 0.1052 0.1140 0.1312 0.0947 0.1213

1 1 1 1 1 1 1 1 1 1

−10.2 −9.6 −14.0 −10.8 −17.3 −16.5 −12.7 −12.4 −12.5 −9.9

2.4 2.3 2.5 2.2 2.9 2.7 2.6 2.9 2.3 2.5

Domain 3 KR91-2 DKR91-2 KR92-1 KR69-1 KR81-1 KR85-1 KR88-1 KR89-1 KR90-1 KR93-1

Khondalite Khondalite Khondatite Charnockite Charnockite Granitoid Charnockite Charnockite Charnockite Leptynite

47.93 50.52 79.97 56.51 44.59 68.08 33.91 47.04 80.22 42.08

8.290 8.760 14.22 10.34 8.631 12.16 6.585 9.538 14.05 8.720

0.511616 0.511640 0.511753 0.511767 0.511848 0.511729 0.511904 0.512023 0.511760 0.511875

918 925 924 918 924 919 918 922 923 917

0.1045 0.1048 0.1074 0.1106 0.1170 0.1080 0.1174 0.1226 0.1059 0.1252

1 1 1 1 1 1 1 1 1 1

−8.2 −7.7 −5.9 −6.0 −5.2 −6.4 −4.2 −2.5 −5.5 −5.8

2.1 2.1 2.0 2.0 2.1 2.1 2.0 1.9 2.0 2.2

Domain 4 AN3-2 KR82-1 KR37-1 KR76-1

Khondalite Khondalite Leptynite Enderbite

142.8 31.93 43.33 14.14

26.31 5.518 7.466 2.004

0.511663 0.511281 0.511146 0.510427

913 920 99 926

0.1114 0.1044 0.1041 0.0856

1 1 1 1

−8.1 −14.7 −17.3 −29.0

2.2 2.6 2.8 3.2

10.42 14.22 0.5331 10.43 1.846 4.159

0.510622 0.511177 0.511780 0.512212 0.510616 0.510933

WCZ, south of Goda6ari Graben migm. Gn 58.10 11.09 migm. Gn 111.2 21.11 Khondalite 16.05 3.741 Khondalite 19.43 4.294 Metapelite 43.02 8.129 Enderbite 68.24 9.535 Enderbite 60.48 8.771 Enderbite 92.23 18.84 Charnockite 85.57 15.41 Charnockite 57.42 8.620 Charnockite 42.07 6.469

Craton KR42-1 KR78-1 100 112 KR5-1 KR47-1

Granite tonal. Gn late granite Granite tonal. Gn Granite

Domain 1, KR43-2 KR7-2R KR13-1 dKRl3-1 KR16-1 KR1-3 dKR1-3 KR3-1 KR11-1 KR24-1 KR50-8

Nd (ppm)

78.29 69.76 3.529 33.64 8.986 29.29

Nd/144Nd

Sm/144Nd

TDM,corr. (Ga)

Sr (ppm)

Rb (ppm)

87

3.2

330.2 46.69 665.8

55.5 200 3.82

0.722171 1.257206 0.707579

919 927 926

436.4 216.4

0.982 194

0.715260 0.765979

920 921

47.13 44.31 99.26 112.5 64.36 273.9 264.8 25.82 91.34 112.5 137.6

128 154 112 153 116 91.9 93.1 39.8 188 137 103

0.926567 0.993634 0.802797 0.802812 0.867035 0.732701 0.732605 0.835188 0.867118 0.800304 0.772443

19.47 93.35 246.0

84.0 136 62.4

153.7 163.7 53.16 72.31 303.8 204.9

3.2 3.7

2.7 2.7

2.6

2.5 2.3

1.8 2.0

Sr/86Sr

87

Rb/86Sr

87

0.4867 13.03 0.016589

Sr/86SrT m

TDM (Ga)

TDM,corr. (Ga)

0.705 0.786 0.707

3.2 2.9 −14.9

2.1

0.007 2.610

0.715 0.672

−28.9 1.7

936 930 934 926 929 921 927 921 922 920 921

8.040 10.35 3.304 3.973 5.277 0.9731 1.019 4.517 6.053 3.562 2.176

0.742 0.756 0.727 0.712 0.746 0.710 0.709 0.731 0.728 0.718 0.722

2.0 2.0 2.1 1.8 2.2 2.3 2.2 2.1 1.9 1.9 2.3

0.909018 0.797996 0.747109

928 925 922

12.72 4.257 0.7364

0.727 0.737 0.737

1.1 1.6 4.5

73.5 61.8 68.8 292 172 83.7

0.774095 0.774059 0.805562 1.019515 0.775701 0.739730

923 924 938 929 922 922

1.393 1.098 3.778 12.05 1.644 1.185

0.754 0.758 0.752 0.847 0.752 0.723

3.7 4.7 1.9 1.8 3.2 2.3

61.22 54.02 91.39 30.46 166.9 165.1 35.77 61.05 153.4 46.12

77.4 76.8 171 363 55.0 38.7 444 323 209 276

0.808604 0.807665 0.818531 1.353536 0.724334 0.732486 1.367155 0.976415 0.783650 1.056145

935 929 925 936 922 924 935 933 925 929

3.694 4.152 5.472 36.64 0.9554 0.6800 38.18 15.73 3.971 17.88

0.756 0.748 0.740 0.830 0.711 0.723 0.821 0.751 0.727 0.800

2.0 1.8 1.5 1.2 1.7 3.3 1.2 1.2 1.5 1.4

12.53 285.0 242.8

90.7 137 9.34

1.049562 0.761261 0.708918

929 937 927

21.64 1.396 0.1113

0.740 0.741 0.707

1.1 3.0 7.1

3.1

2.1

189

Tm (Ga)

143

Rock type

K. Rickers et al. / Precambrian Research 112 (2001) 183–210

147

Sm (ppm)

Sample

K. Rickers et al. / Precambrian Research 112 (2001) 183–210

190

Table 2 The Pb–Pb isotopes of leached feldspars of ortho- and paragneisses from the Eastern Ghats Belt of India and samples from the adjoining Indian cratona Rock type

Mineral

206

207

Granite tonal. Gneis

Kfs Plg Plg Kfs Kfs Plg kfs

15.234 17.064 17.056 17.340 14.123 20.160 17.388

15.582 15.656 15.609 15.474 13.697 16.317 15.873

35.703 36.308 36.294 37.140 30.810 42.648 40.627

Enderbite Charnockite Charnockite

kfs kfs kfs kfs kfs kfs kfs plg plg plg kfs kfs

16.911 16.686 16.656 16.754 16.706 16.550 17.099 17.103 17.094 25.580 17.068 16.632

15.739 15.686 15.615 15.639 15.683 15.536 15.833 15.740 15.689 17.029 15.764 15.675

37.000 36.459 36.382 36.377 36.369 36.675 37.413 37.069 37.020 47.051 37.290 36.388

Domain 1, WCZ, north of Goda6ari Graben R22 R23

Enderbite Enderbite

kfs kfs

15.890 13.416

16.043 14.940

35.493 33.306

Domain 2 KR27-2 KR31-1 KR29-2 KR35-1 R21

Khondalite Metapelite Enderbite Charnockite Granitoid

kfs plg kfs kfs kfs

17.994 17.809 19.302 18.450 17.166

15.959 15.880 16.172 15.962 15.450

38.522 40.638 41.585 38.962 37.123

Khondalite

kfs kfs kfs kfs plg kfs kfs kfs kfs kfs

17.718 17.696 17.501 18.052 17.762 17.603 17.866 17.662 17.859 17.856

15.700 15.646 15.706 15.775 15.774 15.710 15.713 15.718 15.734 15.693

37.770 37.746 38.545 38.022 38.026 37.948 37.809 37.678 37.787 37.790

kfs kfs plg kfs

23.333 19.550 13.242 17.386

16.256 16.180 14.456 15.623

41.658 38.334 33.347 37.604

Sample Craton KR42-1 KR78-1 rKR78-1 100 112 KR5-1 KR47-1 Domain 1, WCZ, south of Goda6ari Graben KR43-1 KR7-2R rKR7-2R KR13-1 KR51-4 KR15-3 KR53-3 KR1-3 rKR1-3 KR3-1 KR11-1 KR50-5

Domain 3 KR91-2 rKR91-2 KR92-1 KR69-1 KR81-2 KR85-1 KR88-1 KR89-1 KR93-1 rKR93-1 Domain 4 KR82-1 KR37-1 KR76-1 KR74-1 a

Ratios are corrected for fractionation.

late Granite Granite tonal. Gneis Granite migm. Gneis migm. Gneis Khondalite Khondalite Hbl-syenite Syenite Enderbite

Khondalite Charnockite Charnockite Granitoid Charnockite Charnockite Leptynite

Khondalite Leptynite Enderbite Granitoid

Pb/204Pb

Pb/204Pb

208

Pb/204Pb

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For the calculation of the Sr crustal residence ages a depleted mantle model was used that assumes a linear evolution of the 87Sr/86Sr ratio from the BABI initial (0.69898) at 4.6 Ga to a modern ratio of 0.702 (Faure, 1986) and a 87Rb/86Sr ratio of 0.0459. The samples from the EGB display highly variable 87Rb/86Sr ratios of 0.111– 38.3 (Table 1) with a mean of 7.09 which is much higher than the average 87Rb/86Sr ratio of the continental crust of 0.4098 (Faure, 1986). Corrections for Sr model ages similar to the correction procedure of Nd model ages are only meaningful for the one sample (KR76-1) from the EGB, that displays a lower 87Rb/86Sr ratio (0.111) than the mean continental crust. The high Rb/Sr ratios of samples from the EGB (0.234–12.399; not included: KR76-1) do not indicate Rb depletion which is commonly reported from high-grade terranes (e.g. Hebei Province, China: Jahn and Zhang, 1984; Tanzania: Appel, 1996). Trace element patterns show that Rb has not been depleted with respect to Sr and K (Rickers, unpublished data). Compared to the average compositions of the upper and lower crust, Sr displays a significant depletion which results in the high Rb/Sr ratios. In the discussion of crustal residence ages it always has to be kept in mind that sediments and intrusive rocks store different age information. Metasediments derived from several sources of different primary ages can provide average crustal residence ages of the provenance area whereas orthogneisses may provide age information of the source region of melts, i.e. a deeper crustal level. In the EGB the magmatic precursors of the orthogneisses intruded the metasediments and are thus geologically always younger. Combining model ages of both rock types thus contributes to the understanding of the age distribution within the crust at the time of intrusion.

191

lian units (WKZ, CMZ, EKZ), 147Sm/144Nd ratios overlap for ortho- and paragneisses with 0.09–0.13 and 0.09–0.11, respectively (Table 1). Nd model ages display a regional distribution pattern which is shown in a simplified geological map of the Eastern Ghats Belt (Fig. 2). Four age groups can be distinguished, all of which display much older crustal residence ages than the known metamorphic events. A complex and detailed premetamorphic history is evident from the combined age constraints of metasediments and orthogneisses. The first age group is characterized by Nd model ages between 2.9 and 3.9 Ga. This group is repre-

4.2. Sm –Nd systematics Sm–Nd isotopes show a weak correlation between provenance area and rock type with the parent/daughter ratio: in the Mesoproterozoic metamorphic unit (WCZ), 147Sm/144Nd ratios range from 0.08 to 0.11 for orthogneisses and 0.11–0.14 for paragneisses, whereas in the Grenvil-

Fig. 2. Distribution of Nd model ages (calculated after Goldstein et al., 1984) in the Eastern Ghats Belt of India. Data from this study and previous studies are combined in this diagram (Paul et al., 1990; Shaw et al., 1997; Krause, 1998; Osanai et al., 1999; Yoshida et al., 1999). The age distribution pattern allows the distinction of two homogeneous and two heterogeneous domains in the Eastern Ghats Belt.

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sented by granitoids from the Dharwar and Eastern Indian cratons and orthogneisses from the EGB of India. The occurrence of middle to early Archean orthogneisses in the EGB is restricted to the central part of the belt and to the border zone of the EGB with the craton. The second age group is characterized by Nd model ages of 2.5–2.9 Ga. This group is represented by orthogneisses and metasediments from the EGB. The distribution of late Archean orthogneisses in the EGB (2.5– 2.9 Ga) is restricted to the central part of the belt whereas metasediments with Archean model ages (2.5– 2.9 Ga) occur south of the Godavari Graben in the WCZ, in the central part of the EGB and north of Chilka Lake. The third group has Nd model ages between 2.2 and 2.5 Ga. Orthogneisses in this age range are concentrated in the WCZ south of the Godavari Graben but occur as well in the central part of the EGB. Metasediments of this age group are restricted to the Western Khondalite Zone (WKZ). The fourth age group is the youngest, with crustal residence ages between 1.8 and 2.2 Ga. For both metasediments and orthogneisses, this age group is limited to the central and northern parts of the belt. The distribution pattern of Nd model ages allows the distinction of several crustal domains in the EGB, which resemble only in part the lithological units suggested by Ramakrishnan et al. (1998). The first crustal domain (Domain 1) coincides with the WCZ south of the Godavari Graben, which is characterized by very homogeneous Nd model ages. With one exception (KR3-1: 2.6 Ga) crustal residence ages for orthogneisses range from 2.3 to 2.5 Ga and metasediments are older at 2.6– 2.8 Ga. The metasediments have similar model ages to the adjoining Dharwar craton granitoids and were probably derived from this craton without incorporation of any more juvenile material. The model age is thus a mean crustal residence age of the provenance areas. The intrusion age of the orthogneisses in the WCZ is currently not known. The narrow range in model ages indicates either the derivation of melts from a homogeneous crustal block or a complete homogenization of components during melting and mixing. Without the knowledge of the intrusion ages of the orthogneisses, no statement

can be made as to whether Nd model ages indicate the time of extraction from the mantle or a mixing age which might be the result of complex assimilation fractional crystallization (AFC) processes that led to the formation of the orthogneisses. North of Godavari Graben, enderbitic orthogneisses of the WCZ are characterized by distinctly higher Nd model ages of 3.2–3.9 Ga. The high Nd model ages along the border with the Archean craton indicates reworking of Archean crust without addition of juvenile material. A second domain (Domain 3) showing homogeneous Nd model ages is situated in the northern part of the EGB west of Chilka Lake. The SW and NE limits of this domain more or less coincide with the Nagavali– Vamsahdara and Mahanadi lineaments. Here Nd model ages are the same for orthogneisses and metasediments with values between 1.8 and 2.2 Ga. No zircon U–Pb data exist for orthogneisses from this part of the EGB. If the intrusion ages for the magmatic precursors of the orthogneisses are also Grenvillian, as was shown for similar rocks further south (Paul et al., 1990; Krause, 1998), Nd model ages would signify average crustal residence ages. Sm–Nd data from other authors (Paul et al., 1990; Shaw et al., 1997; Krause, 1998) (Table A.1, published in the Precambrian Research data repository: www.elsevier.com/locate/precamres) in addition to the data presented here, display a heterogeneous age distribution pattern for the domain (Domain 2) situated between the homogeneous crustal blocks described above, which is bordered to the west by the Sileru lineament and to the east by the Nagavali–Vamsahdara lineaments. Metasediments have Nd model ages of 2.1– 2.5 Ga. These ages are younger than those of paragneisses from the WCZ. In contrast to the rather homogeneous age distribution for the metasediments, orthogneisses have highly variable Nd model ages of 1.8–3.2 Ga. In the central part of the domain, there is a decrease in orthogneiss Nd model ages from dominantly Archean in the area just north of the Godavari Graben towards the north-east. The variable crustal residence ages of the broad domain reflect an extremely inhomogeneous isotopic composition, which might be the result of incomplete mixing of different amalgamated crustal segments or the reworking of pre-

K. Rickers et al. / Precambrian Research 112 (2001) 183–210

existing rather homogeneous crustal material coupled with the addition of variable amounts of juvenile crust increasing north-eastwards. A distinct fourth domain (Domain 4) separates Domain 3 from the Eastern Indian Craton in the north: metasediments display Nd model ages of 2.2 – 2.8 Ga and orthogneisses are indistinguishable in Nd model ages from the craton. Similar to Sri Lanka (Milisenda et al., 1988, 1994), Nd model age provinces do not follow strictly the lithological units. The WCZ consists of a single lithological unit and seems to be a distinct crustal domain as well. In contrast, Domain 3 includes several lithological units. Domain 3, however, is situated north of the Nagavali– Vamsahdara lineaments described by Chetty (1995). The evolution of the domains defined by Nd model age distribution patterns will be discussed in more detail below in combination with the results of the Pb isotope study. The first significant conclusion from Nd isotope systematics alone with respect to the geodynamic evolution of the EGB, however, emerges from the much higher crustal residence ages compared to metamorphic events. The metamorphic processes therefore probably took place without major additions of juvenile crust.

4.3. Rb – Sr systematics As in the Sm–Nd system, a major fractionation between Rb and Sr takes place during the formation of basaltic melts in the mantle. This allows the calculation of crustal residence ages. The interpretation of Sr crustal residence ages in highgrade metamorphic rocks is often difficult, because Rb and Sr are generally fractionated by crustal processes. The very different chemical behaviour of Rb and Sr due to the different ionic radii and different charges commonly results in a parent/daughter fractionation during high-grade metamorphism, partial melting, fluid interaction and alteration. The entire Eastern Ghats Belt was affected by at least one granulite facies event and parts of the belt are polymetamorphic. Strontium isotope data for the granulite facies rocks from the Eastern Ghats Belt thus have to be interpreted cautiously. In most samples, Sr crustal residence

193

Fig. 3. Plot of Sr model ages (calculated from the depleted mantle model) vs Nd model ages (calculated after Goldstein et al., 1984) for ortho- and paragneisses of the Eastern Ghats Belt of India. There is a strong discrepancy of the model ages of the two isotope systems with Sr model ages being mostly younger than Nd model ages. This indicates the disturbance of the Rb – Sr system. Symbols as in Fig. 2.

ages differ significantly from Nd model ages (Fig. 3). Most Sr model ages are younger, but much older ages can also be observed. The regional distribution pattern for Sr crustal residence ages is most homogeneous for granulites from the WCZ south of the Godavari Graben (Domain 1) with ages ranging between 1.9 and 2.3 Ga. No age difference is evident for metasediments and orthogneisses, as is the case for the Nd model ages in that area. In Domain 1, Sr model ages are younger than Nd model ages by 100–900 million years (Fig. 3). For the rest of the EGB, the Sr model age distribution is extremely heterogeneous. The variance of Sr model ages indicates a disturbance in the Rb–Sr isotope system. Sr initials at the time of the known metamorphic events are characterized by a large spread indicating that the change in isotope ratios was caused either by the granulite facies metamorphism or by an earlier fractionation event. The fact that Sr model ages are younger than Nd model ages points to an increase of the Rb/Sr ratio at some point in time. The time of this differentiation is probably indicated by the slope of the correlation lines in Fig. 4 to be discussed below. In an isochron diagram (Fig. 4), the Sr data define two distinct trends with data from the

194

K. Rickers et al. / Precambrian Research 112 (2001) 183–210

WCZ plotting on one and the data from the eastern units of the EGB on another trend. The trends are defined by metasediments and orthogneisses alike and no differing behaviour of these rock types can be observed. The slope of the correlation line for the samples from the WCZ corresponds to an age of 1.929 0.13 Ga. The Sr initial is low, at 0.7059 0.010 and the MSWD is high, at 191. The trend for the samples from the other units of the EGB gives an age of 1.1590.08 Ga with an initial 87Sr/86Sr of 0.734 9 0.054 and a MSWD of 7560. Such correlation lines have been interpreted in the past as the age of metamorphism (Long, 1964; Moorbarth et al., 1972; Faure, 1986; De Maesshalck et al., 1990). However, in recent years, such a simplified interpretation has been questioned (Buhl, 1987; Hofmann, 1993). It is more likely that these isochrons provide some average age that integrates the differentiation processes from the time of crust formation to the final modification of the rocks and does not date one specific event (see also Rickers, 2000 for a detailed discussion of this issue).

4.4. Pb isotopes from feldspars The Pb isotope data obtained from feldspar analyses are presented in Table 2. The results allow a classification into four distinct crustal domains in the EGB, which basically follow the Nd model age distribution pattern. The data from Archean (TDMNd \ 2.5 Ga) and younger samples

are presented separately in two 207Pb/204Pb versus 206 Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams with the Pb evolution curve taken from Stacey and Kramers (1975) (S & K) as well as relevant geochrons based on single-stage evolution with the S & K value at 4.6 Ga as starting point (Fig. 5a–b). The 1.5 Ga isochron starting at the S & K value at 3.7 Ga is also plotted. Samples with extreme Pb compositions (112, KR3-1, KR82-1) are not presented in the diagrams. The groups defined by Sm–Nd systematics are plotted as fields.

4.4.1. Relationship of the Pb isotopes to geochrons, rele6ant isochrons and Nd model ages For the discussion of the Pb isotopes, the samples are divided into two major groups, one with Nd model ages\ 2.5 Ga and one with agesB 2.6 Ga. Samples with Archean Nd model ages display highly variable Pb isotope ratios in both the 207 Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams (Fig. 5a) which stresses the diversity of evolutionary histories of the Archean material in the EGB and the adjoining cratons. With one exception (KR76-1) all samples plot above the S & K curve in the 207Pb/204Pb versus 206Pb/204Pb diagram. A classification can be made based on the 207Pb/206Pb ratios. The first group is characterized by high 207Pb/ 206 Pb ratios which are consistent with the Archean Nd model ages of the samples. Samples from the craton and all samples from the northern part of

Fig. 4. ‘Regional Rb – Sr-isochrones’ for: (a) the Western Charnockite Zone indicating an age of 1.92 Ga; and (b) the eastern units of the Eastern Ghats Belt with an age of 1.15 Ga. Symbols as in Fig. 2.

K. Rickers et al. / Precambrian Research 112 (2001) 183–210

195

Fig. 5. Lead isotope signatures of ortho- and paragneisses from the Eastern Ghats Belt and samples from the adjoining craton. The groups defined by Sm– Nd systematics are plotted as grey fields. Samples which are mentioned in the text with the sample number are marked with the sample number without the prefix KR. Lead signatures of samples with an Archean Nd model age are plotted in (a), while signatures of younger samples are shown in (b). All samples from the craton are represented in (a). Samples from which Nd model ages have not been determined are also plotted in (b) and are marked with an X. Lead isotope signatures show a similar distribution pattern as Nd model ages and are consistent with a division of the EGB into three domains. Samples from Domain 4 display highly variable Pb signatures and cannot be combined into one group. The Stacey and Kramers (1975) evolution curve as well as relevant geochrons and one isochron are plotted for comparison. The numbers along the S & K curve are ages in Ga.

the WCZ plot to the left of the 2.0 Ga geochron and the S & K curve in the 207Pb/204Pb versus 206 Pb/204Pb diagram (Fig. 5a). The samples from the WCZ (enderbites) probably were thermally overprinted at ca. 1 Ga, however, to maintain such primitive Pb isotope ratios, they must have experienced a reduction in v (238U/204Pb), most likely as a result of an early high-grade metamorphism around 2.5 Ga. Metasediments from the WCZ south of the Godavari Graben (Domain 1) and two samples from the craton form a cluster with intermediate 207 Pb/206Pb ratios, plotting between the 1.5 Ga and 1.0 Ga geochrons above the S & K curve (Fig. 5a). The isotopic compositions of the samples from the cratons— both situated close to the

border of the craton with the EGB— probably indicate post Archean resetting of the Pb system. The Archean signature of the samples is supported by clearly distinct 208Pb/204Pb ratios. Metasediments from the southern part of the WCZ cluster in a tight array close to the 1.5 Ga isochron. This age represents the time of last homogenization of Pb and is consistent with the metamorphic age of 1.4–1.6 Ga determined by mineral dating for this unit of the EGB (Simmat and Raith, 1998; Mezger and Cosca, 1999; Simmat, in preparation). Crustal differentiation ages based on U–Pb fractionation that are consistent with a last homogenization of the feldspars at 1.5 Ga and the observed Pb signatures range between 2.75 and 3.5 Ga and are slightly higher than Nd

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model ages. 208Pb/204Pb versus 206Pb/204Pb systematics for the metasediments from the WCZ document no Th/U fractionation during crustal evolution with all data points plotting close to the S & K curve (Fig. 5a). The third group of samples with Archean Nd model ages plots to the right of the 1.0 Ga geochron and above the S & K curve. Samples from Domains 2 and 4, one sample from the craton (KR5-1) close to the WCZ and one sample from the WCZ (KR3-1) close to the border to the craton are included in this group. All samples are characterized by low 207Pb/206Pb ratios which clearly indicate a last homogenization of the Pb system in post Archean time. For the samples from the eastern part of the belt, a last homogenization around 1 Ga indicated by the geochron is consistent with U–Pb mineral ages dating the last high-grade regional metamorphic event in the northern part of the EGB (Simmat and Raith, 1998; Mezger and Cosca, 1999; Jarick, 2000; Simmat, in preparation). The sample from the WCZ (KR3-1) close to the craton is characterized by Pan-African (513948 Ga) rims on monazites (Simmat, unpublished data) which might date the time of last homogenization of the Pb system in this sample. The samples with Proterozoic Nd model ages (Domains 1 –3) form two groups in the 207Pb/ 204 Pb versus 206Pb/204Pb diagram (Fig. 5b) where all samples plot close to the S & K evolution curve and preclude any (early) Th/U fractionation. The orthogneisses from Domain 1 cluster between the 1.5 Ga and 1.0 Ga geochrons around the 1.5 Ga isochron and overlap with the metasediments from Domain 1. The age of 1.5 Ga is consistent with the last reported high-grade metamorphic event in Domain 1 (Simmat and Raith, 1998; Mezger and Cosca, 1999; Simmat, in preparation). Crustal differentiation ages that are consistent with a last homogenization of the feldspars at 1.5 Ga and the observed Pb signatures range between 2.75 and 3.5 Ga and are significantly higher than the Proterozoic Nd model ages and indicate either reworking of ensialic crust or the derivation of the orthogneiss material around 2.4 Ga from a source characterized by higher v-values compared to the S & K evolution curve.

The second group of Proterozoic samples plots to the right of the 1.0 Ga geochron and includes all samples from Domain 3 and two metasediments from Domain 2. The time of last highgrade metamorphism in that area is 1 Ga. The low 207Pb/204Pb ratios probably are consistent with the derivation from Proterozoic material.

4.4.2. Implications for the e6olution of the Archean cratons All samples from the border zones between the adjoining cratons and the Eastern Ghats Belt display Pb isotopic signatures that are quite distinctive when compared to the undisturbed craton and the mobile belt. Two samples south of Godavari Graben are characterized by clearly distinct Pb signatures (207Pb/206Pb B0.83) indicating a particular crustal evolution (Fig. 5a). Both samples were collected from within 10 km of the border of the WCZ with the Dharwar Craton (Fig. 1). Sample KR3-1 (WCZ) exhibits PanAfrican rims on monazites (Simmat, unpublished data). This event probably is not restricted to the WCZ but also took place locally — presumably close to the contact zone — in the craton. This allows the modeling of a secondary evolution until 0.5 Ga for both samples. As the Archean WCZ sample is most likely reworked cratonic gneiss material, the starting point of the secondary isochron is ca. 2.5 Ga, the time of metamorphism in the Dharwar Craton. Lead signatures may be explained by an evolution from 2.5 until 0.5 Ga with a v of 14 (KR5-1) and 34 (KR3-1) and a s of 4.26–5.5. The high v-value for KR3-1 must be the result of some early U/Pb fractionation due to Pb loss. Lead loss is also documented by the comparatively low Pb concentration in the sample (5 ppm). The high s-value is a result of early Th/U fractionation. Until now, the meaning of the Pan-African activation in this part of the EGB is not understood: it might either represent the age of a tectonic amalgamation of the WCZ and the craton or a local reactivation of the contact zone. Another sample (KR47-1) situated in the Dharwar Craton close to the WCZ is characterized by Pb signatures that document a post Archean homogenization which might be related to a reacti-

K. Rickers et al. / Precambrian Research 112 (2001) 183–210

vation of the border zone most probably due to the granulite facies metamorphism in the EGB. High s-values of this sample indicate an Archean U/Th fractionation. Post Archean resetting in the craton is also evident in the border zone of the Eastern Indian Craton and the EGB (KR78-1).

4.4.3. E6olution of the Western Charnockite Zone The Pb isotopic compositions of feldspars from the WCZ indicate a distinct petrogenesis for the WCZ situated south and north of the Godavari Graben: feldspars separated from enderbites from the northern part have retarded Pb signatures and are consistent with a last homogenization in the Archean and a mantle-derivation before 3 Ga, which is also indicated by Nd model ages (3.2 and 3.9 Ga). So far, no reliable age information based on minerals exists for this part of the EGB but Pb systematics show no evidence for the 1.6 Ga high-grade event recorded in the southern segment. If reworking during the Proterozoic formation of the EGB took place it must have been at low-grade conditions that did not affect the feldspar Pb signature or the v must have been very low (v B1) from the Archean until the Mesoproterozoic which is extremely unlikely for these rocks. The Pb systematics thus strongly exclude post Archean homogenization and therefore suggest an ultimate Archean evolution. The ortho- and paragneisses from the southern part of the WCZ are clearly different in both — Nd model ages and Pb signatures — compared to the orthogneisses from the northern part. The scatter of Pb isotopic compositions around the 1.5 Ga isochron is consistent with the metamorphic age of ca. 1.6 Ga (Simmat and Raith, 1998; Mezger and Cosca, 1999; Simmat, in preparation). The linear array of the data set, however, may be due to a cogenetic development in a certain time span or to mixing. In the first case, the slope of the line may contain age information and Nd model ages are constant, whereas in the latter, 207Pb/204Pb ratios correlate with Nd model ages as a result of the composition of the mixing components. In the southern part of the WCZ, the linear array for ortho- and paragneisses is accidental because Nd model ages for I-type or-

197

thogneisses (enderbitic to charnockitic) and metasediments are variable and do not correlate with 207Pb/204Pb ratios. For that reason and because of field relationships with supracrustal xenoliths hosted in orthogneisses, it is likely that ortho- and paragneisses in the WCZ are derived from independent and different sources. In order to evaluate the generation of orthogneisses both, mixing processes and derivation from an enriched homogeneous source have to be considered. The I-type enderbites and I-type charnockites are characterized by a narrow range of Nd model ages between 2.3 and 2.5 Ga and similar Pb signatures. The intrusion age of the orthogneisses is not known, whereas the age of the last metamorphism is known from monazite dating to vary between 1.4 and 1.6 Ga (Simmat and Raith, 1998; Simmat, in preparation). Taking these constraints into account, three possibilities for the formation of the enderbites and charnockites arise: (1) the enderbites were differentiated from the mantle at an unknown time between 1.5 and 2.4 Ga and underwent mixing with Archean material either directly at the time of differentiation or at 1.5 Ga at the latest. In this case the evolution of the orthogneisses would have had to be along a secondary isochron from the time of differentiation until 1.5 Ga; (2) Enderbites and charnockites follow a one-stage evolution according to the Stacey and Kramers model from 3.7 to 1.5 Ga. The 1.5 Ga isochron is then a real isochron. The differentiation age of 3.7 Ga, yet, is much older than Nd model ages determined for the orthogneisses; (3) The enderbites and charnockites differentiated at 2.4 Ga from a radiogenic source. The composition of that source must have had a higher 207Pb/204Pb ratio than the S & K curve. The Zartman and Doe (1981) (Z and D) upper crust curve would be a plausible source to generate the orthogneisses. Another possibility is derivation from an enriched mantle that evolved with a v= 10.5 from the 3.7 Ga S & K value until 2.4 Ga. The second stage v-values (necessary to generate the Pb isotopic compositions) scatter between typical crustal values of 11 and 13.5. However, no magmatic activity at 2.4 Ga has been documented in the WCZ and a mantle source with a v-value higher than 10

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seems extremely unlikely. The first model is consistent with the scatter around the 1.5 Ga isochron, the metamorphic age, and the homogeneous Nd model ages of the orthogneissses and is thus favoured. The Archean material that became assimilated is most probably derived from the adjoining (Dharwar) craton. Since the age of the intrusion of the orthogneisses is not known, no timing of the mixing process can be deduced. The last closure with respect to the Pb system of the mixed material must have taken place with the cooling below the closure temperature of feldspar (at ca. 1.5 Ga) after the last metamorphism at ca. 1.6 Ga. This allows modeling of mixing of

Fig. 6.

Archean crust with early Proterozoic mantlederived basalts at 1.5 Ga (Fig. 6a). Mean Nd and Pb concentrations from basaltic material from the WCZ and adjoining cratonic material have been used to model the generation of the orthogneisses. Mean Pb isotope signatures for the cratonic source have been corrected for homogenization at 1.5 Ga. For the basaltic source model compositions of the Zartman and Doe (1981) mantle at 1.5 Ga have been used. Due to the lack of U–Pb zircon ages dating the mantle differentiation of the orthogneisses, the minimum age of 1.9 Ga (Sr–isochron) was assumed to represent the Nd model age of the basalts. Fig. 6a shows that mixing of 40% of the Archean crust and 60% of basaltic material reduces the Archean model age to 2.4 Ga and partly preserves the Archean Pb signature. This modeling shows that the Pb and Nd isotopic data of the orthogneisses can be consistent with mixing of Archean crust and mantle derived basalts at 1.5 Ga. Therefore a model may be advanced in which mantle-derived basalts were added to the crust and assimilated as much as 40% of pre-existing Archean crust before they finally crystallized in the crust at 1.5 Ga. The high proportion of Archean crust that had to be incorporated into Fig. 6. Model for the generation of Domain 1: (a) model evolution of the orthogneisses: mixing line between a hypothetical continental crust and mantle derived basalts. The isotope compositions for the craton are mean Pb signatures of our data recalculated with a v of 10 until 1.5 Ga and mean Nd model ages, the mantle signatures are the Zartman and Doe (1981) model mantle Pb composition at 1.5 Ga and the Nd model age is the minimum model age of 1.9 Ga. Concentrations are mean concentrations from samples from the Craton and basalts from the WCZ from Rickers (unpublished data) as follows: craton: Nd =50 ppm, Pb=40 ppm; mantle derived basalt: Nd = 20 ppm, Pb= 10 ppm; (b) mixing line between granitoids and greenstone belts from the Dharwar Craton. For the felsic end member, the same as in (a) was assumed, the isotopic composition of the greenstone end member are recalculated with a typical v of 8.2 for greenstone material (Krogstad et al., 1995) from 2.5 until 1.5 Ga from the model Pb signature of the mantle according to Zartman and Doe (1981). The concentration of the basic end member are mean values of late Archean greenstone belts (Taylor and McLennan, 1985) and are as follows: Nd = 17 ppm, Pb =39 ppm. The composition of the paragneisses in the Western Charnockite Zone may be explained by a 3:7 – 5:5 mixture of the two end members.

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the basic material cannot be explained by a simple assimilation process. This model implies remelting in the crust, that results in the homogenization of the basic and felsic end members or a complex AFC process. Metasediments in the WCZ are characterized by lower 207Pb/204Pb versus 206Pb/204Pb ratios and Nd model ages between 2.6 and 2.8 Ga and therefore must have been derived from an independent and different source. The 207Pb/204Pb ratios are still more radiogenic than expected for 2.7 Ga old material that differentiated from the S & K mantle. A suitable mantle source to explain the high 207Pb/204Pb ratios evolved with a v of 10 from 3.7 Ga until the time of the differentiation of the precursors of the paragneisses. The second stage v-values then range from 12 to 13.5. Another, more likely explanation for the high 207Pb/ 204 Pb ratios, that avoids the unusual mantle composition with a v of 10 and takes into account the sedimentary nature of the paragneisses, is mixing of detrital material comprising an Archean component with high v-values and a low v-component (Fig. 6b). The last homogenization with respect to the Pb isotope system took place at the time of metamorphism. The low v-component could either be juvenile basaltic material or detrital matter from Archean greenstone belts from the adjoining craton; a probable source for the high v-component are Archean granitoids from the craton. The modeling of the mixing of basic and felsic end members from the craton is rather hypothetical as end member compositions are not known exactly. For the felsic end member, mean Nd and Pb concentrations of granitic rocks from the craton have been used and their mean Pb signatures recalculated to 1.5 Ga. The Nd and Pb concentrations of the basic greenstone material are mean concentrations for late Archean greenstone belts according to Taylor and McLennan (1985). The Pb signature of the basic greenstone material was recalculated from a model differentiation age of 2.5 to 1.5 Ga with a typical v-value for greenstone belts of 8.2 (Krogstad et al., 1995). Fig. 6b shows that the range in 207Pb/204Pb ratios for the metasediments from the southern WCZ may very well be explained by a mixture of 50– 70% of Archean granitoid material with 50– 30%

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of basic greenstone material. According to studies of Taylor and McLennan (1985) a 1:1 mixture of the mafic and felsic end members of the Archean bimodal igneous suite represents a mean composition of the Archean upper crust. The metasediments in the WCZ are thus interpreted to originate from the gneiss–granitoid–greenstone belts from the adjoining Dharwar Craton without further addition of juvenile material. The Pb signatures of S-type orthogneisses in the WCZ, a Hbl –syenite (KR15-3) and a syenite (KR53-3) with unknown Nd model ages fall within the group of metasediments. Therefore, the formation of these rocks might be explained by mixing of old crust with juvenile mantle at 1.5 Ga.

4.4.4. E6olution of the Western Khondalite Zone, the Charnockite–Migmatite Zone and the Eastern Khondalite Zone Published Pb and Nd data from the lithological units to the east of the WCZ, help to characterize this part of the EGB. The knowledge that the intrusive age of the felsic rocks is Grenvillian (Paul et al., 1990; Kovach et al., 1997; Shaw et al., 1997; Krause, 1998) facilitates understanding of the evolution of the eastern region which is the main part of the EGB. 4.4.4.1. Comparison with published Pb isotope data and e6olution of the two isotopically heterogeneous units (Domains 2 and 4). Earlier studies of Pb isotopes on leached feldspars from S-type granitoids (Krause, 1998) and metasediments (Mezger and Cosca, 1999) complement the data presented here from the north-eastern part of the EGB (Fig. 7). The results are similar to those presented here for this terrane but pronounced radiogenic signatures are more common. The data are consistent with the proposed division of the northern part of the EGB into a homogeneous Domain 3 with lower 207Pb/204Pb ratios and an inhomogeneous Domain 2 with very radiogenic and highly variable isotope ratios. In both studies, the authors discuss the admixture of a prominent Archean component to explain the elevated 207Pb/204Pb ratios. Krause (1998) proposed a two-stage development of a model Archean crustal source capa-

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ble of explaining the genesis of the S-type granitoids characterized by high 207Pb/204Pb versus 206 Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb ratios. According to this model the Archean crust differentiated from the mantle between 3.75 and 3.6 Ga and evolved with a v typical for the crust and a high s, until a first event of partial anatexis between 2.7 and 1.7 Ga, after which the melt fraction evolved with an upper crustal v according to Zartman and Doe (1981). The granitoids were derived from that source through renewed anatexis during the Grenvillian orogeny. This

Fig. 7. Lead isotope results of previous studies on metasediments (Mezger and Cosca, 1999) and S-type granitoids (Krause, 1998) from the eastern units of the EGB in comparison to our groups. The complementary data support the division of the eastern EGB into a homogeneous Domain 3 and a heterogeneous Domain 2.

model has been proposed for S-type granitoids but may in principle also be applied to the metasediments. The key is that the Archean Pb system re-opened during the Grenvillian orogeny. In the case of the granitoids, the re-opening occurred due to partial melting in the granulite facies, whereas for the para- and orthogneisses it is related to granulite facies metamorphism. For the three samples (two paragneisses: KR31-1, KR27-2 and one orthogneiss: KR29-2) from Domain 2, which are characterized by extremely radiogenic compositions, we can model an evolution from a fictive but typical Archean component at 2.5 until 1 Ga with a v of 12 and 18 and s of 7 and 5.5, respectively. Two samples with Archean Nd model ages from Domain 4, displaying low 207Pb/206Pb ratios (207Pb/206Pb B0.83), one leptynite (KR37-1) and one metasediment (KR82-1), have also been modeled according to Krause (1998): the Pb signatures may be explained by a secondary evolution from 2.5 until 1 Ga with a v of 20 (KR37-1) and 60 (KR82-1) and s of 3.1 and 2.4, respectively. The exceptionally high v of 60 may have been caused by an early Pb loss that is also evident from whole rock analysis (Pb B 3 ppm). The low s-values most probably indicate early gain of U. The time span from 2.5 until 1 Ga is compatible with the interpretation of the reworking of an Archean crust during Grenvillian granulite facies metamorphism at ca. 960 Ma documented by monazite ages (Mezger and Cosca, 1999; Jarick, 2000; Simmat, in preparation).

4.4.4.2. E6olution of Domain 3 in the northern EGB. Pb isotopes on leached feldspars for Domain 3 form an extremely narrow array in both the 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagram (Fig. 5b). In contrast to the samples from Domains 2 and 4, samples from Domain 3 plot close to the S & K curve in the two diagrams. Isotopic signatures for khondalites, charnockites and porphyritic S-type granitoids are the same, implying the derivation from a homogeneous source. The Nd model ages ranging from 1.8 to 2.2 Ga for metasediments and orthogneisses point to the rapid evolution of the crust. The metamorphism known at ca. 1 Ga

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suggests a last homogenization of feldspars around that time. Crustal differentiation ages that are consistent with the Pb signature and a last homogenization of the Pb system at 1 Ga are only slightly higher than Nd model ages (1.8– 2.2 Ga). The Pb characteristics thus point to the incorporation of mostly early Proterozoic and minor or no Archean material, as indicated by Nd model ages. The homogeneous Pb isotopic compositions indicate either perfect mixing or no mixing and a homogeneous source. The Nd systematics are equally homogeneous as Pb signatures and therefore the differentiation of the crustal material in Domain 3 from a 1.8 to 2.2 Ga old homogeneous source is the most likely scenario.

4.4.4.3. Comparison of Domains 2 and 3. In order to evaluate the relation of pre-existing crust, within Domains 2 and 3, several scenarios of mixing have been modeled (Fig. 8). The observed isotope systematics of both Domains 2 and 3 may be explained by the reworking of pre-existing crust and the addition of juvenile material at the time of last orogeny. However, the different characteristics may not be explained by variable additions of juvenile crust to one homogeneous Archean crust. Rather, crusts of different composition must have been involved in the generation of Domains 2 and 3. For Domain 2, compositions similar to the crust of the adjoining Dharwar Craton are suitable (Fig. 8). A general decrease of the Archean signature toward the north-east is observed. The inhomogeneous regional distribution pattern of the Pb–Pb and Sm – Nd (207Pb/ 204 Pb =15.450–16.172 and TDMNd =2.3 – 2.9 Ga) isotope systematics of orthogneisses in Domain 2 is conformable with variable proportions of juvenile and ensialic material. The granitoids studied by Krause (1998) form a linkage to Domain 3 for which the reworking of a Proterozoic crust similar to Domain 1 (A in Fig. 8) and a terrane with lower 207Pb/204Pb characteristics and even younger Nd model ages (B in Fig. 8) is appropriate. The pre-existing crust cannot be further specified. According to this model, Domain 2 is made up of rocks that were derived from inhomogeneous Archean crustal material, whereas the rocks of Domain 3 represent reworked, more homoge-

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Fig. 8. mNd(1Ga) vs 207Pb/204Pb diagram for ortho-, paragneisses and granitoids from the eastern units of the EGB and samples from the craton with all Pb signatures corrected for a last homogenization at 1 Ga. Granitoid data are from Krause (1998). Mixing lines have been calculated between a model depleted mantle end member at 1 Ga and four different crustal end members. The isotopic compositions of the crust are: (A) mean Pb and Nd signatures from WCZ samples recalculated with a v of 10 from 1.6 to 1 Ga; (B) idealized Pb (207Pb/ 204 Pb=15.70) and Nd (mNd(1Ga) = −10) signatures for a Proterozoic component; (C) mean Pb and Nd signatures from the craton recalculated with a v of 10 from 2.5 to 1 Ga; and (D) Pb and Nd compositions of sample KR 42-1 recalculated with a v of 10 from 2.5 to 1 Ga. The isotopic composition of the mantle is the model composition at 1 Ga according to Zartman and Doe (1981) for the Pb system and according to Goldstein et al. (1984) for the Sm – Nd system. Concentrations for the end members are as follows: (A) 27 ppm Pb, 40 ppm Nd; (B) 27 ppm Pb, 40 ppm Nd; (C) 20 ppm Pb, 40 ppm Nd; (D) 31 ppm Pb, 78 ppm Nd, depleted mantle: 3 ppm Pb, 10 ppm Nd.

neous Proterozoic material. For both terranes, the isotope systematics favour an active continental margin setting. Two models may be envisioned supporting this scenario: either Domain 2 and Domain 3 are unrelated or they represent distinct parts of one orogen. In the latter case, Domain 3 represents a position far away from the orogenic front whereas Domain 2, which records incomplete reworking of the crust, is typical for the orogenic front. The transition between Domain 2 and Domain 3 is not gradual but rather sharp and may be of tectonic origin. This hypothesis is supported by coincidence with a system of lineaments described by Chetty (1995). Assuming that the lineaments in fact are shear zones, which still is to be evaluated by detailed field work, Raith (unpublished data)

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suggested westward thrusting of the north-eastern part of the EGB onto the Eastern Indian Craton. If valid, Domain 2 might represent a western and Domain 3 a more juvenile eastern segment of the EGB. With Domain 3 representing more juvenile additions and Domain 2 a more Archean component, crustal growth in the EGB during the Grenvillian orogeny obviously took place in a north-eastwards direction. It is apparent from the above discussion that different crustal domains exist in the EGB and they can be defined by the combination of Sm– Nd whole rock and common Pb isotope data on feldspars. Juvenile material has been added to the crust to a variable extent, but most pre-existing material has been reworked during the two major tectonothermal events (at ca. 1.6 and 1 Ga) in the EGB. Crustal growth in the EGB thus occurred through continent –continent collision processes.

been corrected for metamorphic disturbance assuming a pre-metamorphic 147Sm/144Nd ratio of 0.11. The Nd crustal residence age results are illustrated in a simplified paleogeographic reconstruction of East Gondwana (Fig. 9). The Napier Complex is the oldest crustal domain in Antarctica with Nd model ages between 3.2 and 4.2 Ga (DePaolo et al., 1982; McCulloch and Black, 1984; Black et al., 1986; Black and McCulloch, 1987; Owada et al., 1994) (Fig. 10) and has no counterpart in the EGB. The Rayner Complex, enclosing the Napier Complex toward the southwest, records clearly younger Nd model ages between 1.4 and 2.4 Ga (Black and McCulloch, 1987). The Mawson Coast area, stretching

4.5. Crustal age domains in East Gondwana and East Rodinia In the discussion of the reconstruction of Rodinia and Gondwana, it is instructive to compare the isotopic compositions of the EGB with potentially adjacent terranes in the supercontinents. As Pb data are scarce and Rb– Sr is highly sensitive to fractionation, Sm– Nd isotope data presented in this study are compared with data from east Antarctica, Peninsular India and Sri Lanka. A compilation of data from the literature on Precambrian rocks in central Gondwana (Mo¨ ller et al., 1998) has been used for this purpose. The data set has been extended with results from localities situated to the north in Antarctica (Rauer Group, Prydz Bay Region, Northern Prince Charles Mountains, Vestfold Hill Complex), with data from the Indian cratons adjoining the EGB and with recently published data (Table A.1 and Fig. A.1, published on the Else6ier web page: www.elsevier.com/locate/precamres). Following Mo¨ ller et al. (1998) all Nd model ages were calculated using the depleted mantle model of Goldstein et al. (1984) and Nd isotopes were renormalized to a 146Nd/144Nd ratio of 0.7219 where necessary. Nd model ages of samples displaying a 147Sm/144Nd ratio higher than 0.12 have

Fig. 9. Reconstruction of East Gondwana (compiled by M. Raith) with a compilation of Nd model ages recalculated according to Goldstein et al. (1984). Abbreviations in the map are as follows: EIC, Eastern Indian Craton; BC, Bastar Craton; DC, Dharwar Craton; CB, Cuddapah Basin; N, Nilgiri Hills; B, Biligirirangan Hills; MC, Madras Granulite Complex and Shevaroy Hills; MB, Madurai Block; KKB, Kerala Khondalite Belt; EGB, Eastern Ghats Belt; AC, Androyen Complex; V, Vohibory Terrane; WC, Wanni Complex; HC, Highland Complex; VC, Vijayan Complex; VHC, Vestfold Hills Complex; RGr, Rauer Group; PB, Prydz Bay; PCM, Prince Charles Mountains; MwC, Mawson Coast; RC, Rayner Complex; NpC, Napier Complex; POC, Prince Olaf Complex; OSG, Ongul-Skallen Group; YBC, Yamato Belgica Complex; MR, Mahanadi Rift; GR, Godavari Graben; LR, Lambert Rift; EER, East Enderby Rift.

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Fig. 10. Histograms of apparent Nd crustal residence ages from the Eastern Ghats Belt and nearby regions in East Gondwana. Sm– Nd data are recalculated after Goldstein et al. (1984) and corrected for 147Sm/144Nd ratios higher than 0.12 as described in the text. (a) Eastern Ghats Belt, this study; (b) Eastern Ghats Belt, previous studies (Paul et al., 1990; Shaw et al., 1997; Krause, 1998; Osanai et al., 1999; Yoshida et al., 1999); (c) Indian cratons: Dharwar Craton (this study; Peucat et al., 1993; Jayananda et al., 1995a; Kumar et al., 1996; Pandey et al., 1997; Jayananda et al., 2000), Eastern Indian Craton (this study; Basu et al., 1981; Sharma et al., 1994; Sengupta et al., 1996); (d) Antarctica, Enderby Land: Napier Complex (DePaolo et al., 1982; McCulloch and Black, 1984; Black et al., 1986; Black and McCulloch, 1987; Owada et al., 1994), Rayner Complex (Black et al., 1986), Mawson Coast (Young et al., 1997); (e) Antarctica towards the east of Enderby Land: Prydz Bay Region (Black et al., 1986; Hensen and Zhou, 1995), Northern Prince Charles Mountains (Zhao et al., 1997), Vestfold Hill Complex (Black et al., 1991); (f) Pensinsular India, South India, northern part: Madras Granulite Complex (Bernard-Griffiths et al., 1987), Cuddapah Supergroup (Zachariah et al., 1999), Shevaroy and Biligiriangan Hills (Peucat et al., 1989; Kumar et al., 1998; Schleicher et al., 1998), Nilgiri Hills (Peucat et al., 1989; Raith et al., 1999); (g) Pensinsular India, South India, southern part: Palghat-Cauvery shear zone (Harris et al., 1994; Bhaskar Rao et al., 1996), Madurai Block (Harris et al., 1994; Brandon and Meen, 1995; Jayananda et al., 1995b; Bartlett et al., 1998), Kerala Khondalite Belt (Choudhary et al., 1992; Harris et al., 1994; Brandon and Meen, 1995; Unnikrishnan et al., 1995; Bartlett et al., 1998); (h) Sri Lanka (Kagami et al., 1990; Milisenda et al., 1994, 1988).

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from the northern part of the Rayner Complex to the north has nearly identical Nd model ages of 1.7 –2.4 Ga (Young et al., 1997). The Northern Prince Charles Mountains, situated in the eastern part of the Lu¨ tzow –Holm Complex, are characterized by similar Nd model ages of 1.6– 2.4 Ga (Zhao et al., 1997). Situated to the east, the Prydz Bay region has mainly 1.7– 2.3 Ga (with one exception of 3 Ga) Nd model ages (Black and McCulloch, 1987; Hensen and Zhou, 1995). Additional data have been published by Sheraton et al. (1984) but these have not been used here as recalculated model ages reach values of 5.2 Ga and therefore are unrealistic. The same considerations apply to the Sheraton et al. (1984) Rauer Group data (Nd model ages from 2.7 to 2.9 Ga). The Vestfold Hill Complex is characterized by Nd model ages of 2.7– 3.0 Ga (Black et al., 1991). In Peninsular India, the Eastern Indian Craton west of the EGB yields Nd model ages of mainly 3.1– 4.0 Ga (Basu et al., 1981; Sharma et al., 1994; Sengupta et al., 1996). No published Nd isotope data exist for the Bastar Craton. Ages in the Eastern Dharwar Craton are between 2.7 and 2.9 Ga (Pandey et al., 1997; Jayananda et al., 2000) in its eastern part and between 3.0 and 3.5 Ga in the western segment (Jayananda et al., 2000). The Western Dharwar Craton yields ages of 2.9– 3.4 Ga (Peucat et al., 1993; Jayananda et al., 1995a; Kumar et al., 1996). Nd model ages of the Cuddapah Supergroup, located to the west of the southern part of the WCZ, range between 2.6 and 2.9 Ga (Zachariah et al., 1999). The Madras Granulite Complex, south of the EGB, is characterized by Nd model ages around 2.8 Ga (BernardGriffiths et al., 1987), the Shevaroy Hills, the Biligirirangan Hills and the Nilgiri Hills yield similar Nd model ages of 2.6– 2.9 Ga (Peucat et al., 1989; Raith et al., 1999). The Madurai Block, situtated to the south of the Nilgiri, Biligirirangan and Shevaroy Hills, yields a wide range of Nd model ages from 1.8 to 3.1 Ga (Harris et al., 1994; Jayananda et al., 1995b; Brandon and Meen, 1995; Bartlett et al., 1998).

From the compilation of Nd model ages four different crustal domains are recognized: terranes characterized by: (1) early Archean (\ 2.9 Ga) Nd model ages (Napier Complex, Eastern Indian Craton, Western Dharwar Craton, western part of Eastern Dharwar Craton, Madurai Block, EGB: WCZ north of the Godavari Graben and parts of Domains 2 and 4); (2) late Archean Nd model ages (2.9–2.5 Ga; Vestfold Hill Complex, western Dharwar Craton, eastern part of Eastern Dharwar Craton, Cuddapah Supergroup, Madras Granulite Complex, Shevaroy, Biligirirangan and Nilgiri Hills, Madurai Block, EGB: metasediments of Domain 1 and Domain 4 and orthogneisses and metasediments from Domain 2); (3) early Proterozoic Nd model ages (2.2–2.5 Ga; Madurai Block, Rayner Complex, Mawson Coast, EGB: orthogneisses of Domain 1, metasediments and orthogneisses from Domain 2 and metasediments from Domain 4); (4) middle Proterozoic Nd model ages (1.4–2.2 Ga; Madurai Block, Rayner Complex, Mawson Coast, Prydz Bay region, Northern Prince Charles Mountains, EGB: metasediments and orthogneisses from Domains 2 and 3). In recent reconstructions of Gondwana, the Napier Complex is juxtaposed with the southern part of the EGB and the Rayner Complex with its northern part (Fig. 9) (Hofmann, 1996; Unrug, 1996; Sengupta et al., 1999). Our study, focussing on the pre-metamorphic evolution of the EGB, reveals a general decrease of Nd model ages toward the north-east in this mobile belt similar to that observed in east Antarctica and emphasizes the similarity of the eastern units of the EGB (especially Domain 3) with the Rayner Complex, the Mawson Coast area, the Prydz Bay region and the Northern Prince Charles Mountains. The Napier Complex, however, forms a distinct terrane without a counterpart in the EGB. This is consistent with the recent study on the East Antarctic-East Indian correlation based on mineral age dating (Mezger and Cosca, 1999). There, it is pointed out that the central and eastern zones of the EGB show a tectonothermal evolution similar to the Rayner Complex, whereas the Napier Complex

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forms a distinct and exotic crustal domain in East Gondwana, which cannot be correlated with the western units of the EGB exposed in its southern part. Our findings are also consistent with the most recent study of the Rodinian configuration of East Antarctica based on U– Pb zircon data (Fitzsimons, 2000). The new age data confirm the correlation of the Eastern Ghats Belt with the Rayner Complex and document that towards the east the Rauer Group is the first part of East Antarctica which belongs to a separate displaced block which became assembled into East Antarctica in the Pan-African. As a general conclusion, we propose the genesis of Domain 1 involved material from the Indian cratons in the west and Domain 3 can be correlated with the Proterozoic parts of East Antarctica in Rodinia. Domain 2 represents reworked Archean material and may be related to Domain 3. No statement can be made when the Napier Complex was assembled to East Rodinia. The latest possible time is the assembly of Gondwana.

5. Conclusions The combination of Nd crustal residence ages with Sr whole rock characteristics and Pb signatures of leached feldspars reveals a complex crustal composition of the EGB, that has previously not been recognized. The combination of the three isotope systems unravels a mosaic of discrete crustal segments with distinct geological and geochemical histories. The isotope data provide clear evidence that the rocks from all parts of the EGB are derived from mostly reworked Archean and early to middle Proterozoic crustal material; juvenile additions during the major orogenic cycle around 1 Ga are minor. Additionally, the isotope data allow the delineation of isotopically distinct segments each with its characteristic geologic history. The WCZ south of Godavari Graben adjoining directly to the craton is a distinct Proterozoic crustal block that seems to have formed from juvenile material around 2.3 Ga with addition of

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minor amounts of Archean material. The magmatic and high-grade tectono-metamorphic evolution of this segment ceased about 1.6 Ga ago. The intrusive ages of the precursors to the orthogneisses may be older than 1.9 Ga as indicated by the Sr isotope systematics. North of the Godavari Graben, the WCZ is characterized by Archean isotopic signatures that are particularly pronounced in the Pb–Pb and Sm–Nd systems. Both isotope systems indicate an Archean intrusive and metamorphic history for the enderbites. In this area there is no evidence for the incorporation of post Archean material during the evolution of this segment. Due to the lack of U–Pb zircon ages for these rocks the exact timing of the Archean events cannot be constrained. Although the Nd and Pb isotopes provide unambiguous evidence for Archean material in the WCZ, no old zircons have yet been found. However, this is probably mostly due to the scarcity of U–Pb zircon data for the whole EGB. The units adjoining to the east of the WCZ show a complex pattern of isotope signatures that cannot be correlated with the lithological units defined by Ramakrishnan et al. (1998). A homogeneous crustal domain can be identified in the northern part of the EGB, west of Chilka Lake (Domain 3). Lead and Nd isotopes indicate reworking of dominantly early Proterozoic (1.8–2.2 Ga) material at 1 Ga. Between the two fairly homogeneous crustal segments situated in the south and in the north of the EGB, there is a broad transition zone (Domain 2), characterized by generally decreasing orthogneiss Nd model ages toward the north-east. Lead isotopic compositions document the incorporation and reworking of a substantial Archean component and indicate variable sources for metasediments and orthogneisses from the different parts of the transition zone. The border between Domains 2 and 3 might be of tectonic origin as indicated by the Nagavali–Vamsahdara lineaments. If the northern part (Domain 3) represents a westward-overthrusted block as proposed by Raith (unpublished data), Domains 2 and 3 might represent two segments of the same conti-

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nental arc with Domain 3 documenting the setting located further away from the orogenic front. The similarity of the eastern units of the EGB with the Proterozoic terranes in East Antarctica is striking and supports their correlation in Rodinia. The early Archean Napier Complex has often been correlated with the EGB due to the lithological similarities, the occurrence of unusual UHT metamorphism and a similar P– T evolution (e.g. Sen et al., 1995; Sengupta et al., 1999). The results from this isotope study preclude involvement of the Napier Complex in any of the orogenic episodes recognized in the EGB. The early Archean Nd model ages (DePaolo et al., 1982; McCulloch and Black, 1984; Black et al., 1986; Black and McCulloch, 1987; Owada et al., 1994) and the late Archean time for the UHT metamorphism are strong evidence that this block forms an exotic terrane within the context of a reassembled Rodinia. The close proximity of the Napier Complex with the southern segment of the EGB in the reconstructed Rodinia continent would make this area the most likely sink for detritus with early to middle Archean Nd model ages from the Napier Complex. The lack of distinctly old Nd model ages and Pb isotope signatures in the southern part of the EGB, however, make the Napier Complex an unlikely provenance area for a significant amount of sedimentary material. One possible interpretation for this relationship is that the Napier Complex became attached to the area that now forms the EGB after the last sediments were deposited. Although the EGB and the Napier Complex have similar P– T evolutions and also evidence for unusual UHT metamorphism, the pronounced differences in their Nd model ages, Pb isotope signatures and metamorphic ages (Mezger and Cosca, 1999; Simmat, in preparation) provide strong evidence that the two terranes are not related. Their tectonothermal evolution took place in distinct places and at different times. The incorporation of juvenile material during the metamorphic processes around 1 Ga is minor, indicating that the EGB is an orogen consisting mainly of ensialic material. Especially along the border of the craton, old crustal material was reworked during the orogenic processes, while

model ages toward the east are slightly younger. The high proportion of reworked material does not support a long-term accretion of oceanic material. The geodynamic process for the EGB at 1.6 and 1 Ga, consistent with the isotopic signatures, is a continent–continent collision with only minor addition of juvenile material. The crustal growth process in the EGB was directed north-eastwards during the two Proterozoic orogenies.

Acknowledgements We thank A. Bhattacharya (Indian Institute of Technology) who introduced us into the EGB and S. Dasgupta (Jadavpur University, Calcutta) for his collaboration during two field trips and his help with their organisation. J.K. Nanda (Geological Survey of India) is thanked for his collaboration during further field work and the provision of two samples from the craton which were used in this study. We thank S. Hoernes (Universita¨ t Bonn) for the scientific support of the EGB project. Fruitful discussions with A. Bhattacharya, S. Dasgupta, J.K. Nanda and P. Sengupta (Jadavpur University, Calcutta) contributed to our understanding of the composition and evolution of the EGB. We sincerely thank J.J. Peucat and J. Percival for their careful and constructive reviews and A. Kro¨ ner for editorial handling. K.R. thanks the staff at the Zentrallaboratorium fu¨ r Geochronologie (ZLG) at Mu¨ nster, especially Heidi Baier, for the help with the isotope studies and Ralf Simmat (Mineralogisches Institut, Bonn) and Peter Appel (Institut fu¨ r Geowissenschaften, Kiel) for discussions. This research was financially supported by the Deutsche Forschungsgemeinschaft (DFG) through grants Ra205/20-1, 2. This paper is a contribution to the IGCP 368 and 440.

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