Contrasting magmatic signatures in the Rairakhol and alkaline complexes, Eastern Ghats belt,

S Bhattacharya1,∗ and MBasei2 1Indian Statistical Institute, Kolkata 700 108, India. 2Geoscience Institute, S˜ao Paulo University, 05508-080, SP Brazil. ∗e-mail: [email protected]

The relation between alkaline magmatism and tectonism has been a contentious issue, parti- cularly for the Precambrian continental regions. Alkaline complexes at the southwestern margin of Eastern Ghats belt, India, have been interpreted as rift-valley magmatism. However, those complexes occurring in granulite ensemble in the interior segments of the Eastern Ghats belt could not possibly be related to the rift-system, assumed for the western margin of the Eastern Ghats belt. Koraput complex was emplaced in a pull-apart structure, dominated by magmatic fabrics and geochemically similar to a fractionated alkaline complex, compatible with an alkali- basalt series. Rairakhol complex, on the other hand, shows dominantly solid-state deformation fabrics and geochemically similar to a fractionated calc-alkaline suite. Isotopic data for the Kora- put complex indicate ca. 917 Ma alkaline magmatism from a depleted mantle source and post- crystalline thermal overprint at ca. 745 Ma, also recorded from sheared metapelitic country rocks. The calc-alkaline magmatism of the Rairakhol complex occurred around 938 Ma, from an enriched mantle source, closely following Grenvillian granulite facies imprint in the charnockitic country rocks.

1. Introduction Hampshire, Canada, do not appear to be asso- ciated with rift valleys, but form elongate belts While continental flood basalts are more common that may trace the path of moving continent over along successful branches of rift system, alka- mantle plume (Foland and Foul 1977). line magmatism predominates along failed arms The alkaline complexes of Rairakhol and Kora- normally preserved within continents (Philpotts put both occur in granulite ensemble and far from 1990). The alkaline province at the southwest the western cratonic margin of the Eastern Ghats margin of the Eastern Ghats granulite terrain is belt, India. For such alkaline complexes within the generally interpreted as a rift-valley magmatism, high-grade Eastern Ghats belt, it is difficult to though no in-depth study of the structural setting postulate a model of rift-valley magmatism. has yet been undertaken (Leelanandam 1998). In this communication, we present isotopic Moreover, all these complexes are not of the same data for the two complexes and their immedi- age. Except for the modern rift systems such as ate country rocks in order to reveal the possible that of east Africa, the relation between tectonism link between tectonism and magmatism. Addi- and magmatism has not been precisely determined. tionally, these data may shed new light on On the other hand, some continental alkaline com- the possible tectonic model for the two alkaline plexes, such as those of White Mountains in New complexes.

Keywords. Alkaline magmatism; granulite ensemble; mantle sources; moving continent.

J. Earth Syst. Sci. 119, No. 2, April 2010, pp. 175–181 © Indian Academy of Sciences 175 176 S Bhattacharya and M Basei

2. Geological setting Kunavaram complex of the Prakasam province, Upadhyay and Raith (2006) described an intru- 2.1 Eastern Ghats belt sion age of 1384 ± 63 Ma (the upper intercept) and a metamorphic imprint of 632 ± 62 Ma (the The high-grade Eastern Ghats belt along the lower intercept). From Ongole domain, several east coast of India, and bounded by Singhbhum complexes were dated between 1263 and 1352 Ma, and Bastar cratons to the north and west by SHRIMP data on zircons (Upadhyay et al respectively, is described as a compressional 2006b; Upadhyay and Raith 2006; Vijaykumar orogen (Bhattacharya 1997; Bhattacharya and et al 2007). In a recent publication, Vijaykumar Sen 2003). The lithological make-up of this and Leelanandam (2008), suggested that alkaline belt could be described in terms of three rocks and carbonatites (ARCs) of the Prakasam broad groups, namely, metapelitic granulites, province, dated between 1.5 and 1.35 Ga, represent charnockite-enderbite gneisses and associated rifting along the eastern margin of thickened arc mafic granulites and migmatitic gneisses. Addition- crust. These authors further suggested conver- ally, a transition zone along significant length of the sion of ARCs into DARCs (deformed alkaline western margin was also described (Ramakrishnan rocks and carbonatites) either during Grenvil- et al 1998). Anorthosite and alkaline complexes lian or Pan-African orogeny. Although, Grenvillian are other important rock types in this high-grade thermal event and related granulite facies meta- belt. Polyphase deformation and complex, possi- morphism is dominant in the northern Eastern bly multiple granulite facies records are charac- Ghats belt, southern Eastern Ghats belt, adjoin- teristic of this regional granulite terrane. Recently ing the Prakasam province, recorded a granulite published Sm-Nd isotopic data suggest a num- facies event at 1.6 Ga (Mezger and Cosca 1999; ber of crustal domains or provinces with unre- Bhattacharya et al 2010). These data would sug- lated pre-metamorphic history (Rickers et al 2001). gest a post-granulite intrusion for the alkaline com- Although dominant and pervasive granulite facies plexes of the Prakasam province. On the other event is of Grenvillian age, older granulite facies hand, some alkaline complexes in the northern imprints as well as later thermal overprints are Eastern Ghats belt occur in granulite ensemble, far recorded from several crustal domains (Mezger and from the western margin. Two such complexes of Cosca 1999; Bhattacharya et al 2001, 2002, 2003; Koraput and Rairakhol are considered here. Some Bose et al 2008; Simmat and Raith 2008). More- published ages from these complexes also indi- over, north and south of the Godavari Graben, cate post-intrusion granulite facies metamorphism Eastern Ghats belt is commonly described as sepa- (Nanda et al 2008). rate crustal segments (Mezger and Cosca 1999). One significant difference between the north and south segments relates to the nature of the cratonic 3. Field-setting and petrological margin. background The northern segment is marked by a crustal scale shear zone at the western margin and Koraput complex was emplaced in a pull-apart granite gneisses of the Bastar craton occur in the structure and flanked by mylonitic shear zones immediate vicinity (Bhattacharya 2004). South of developed in the metapelitic country rocks. This the Godavari Graben (Ongole domain), on the body shows distinct intrusive contact, domi- other hand, Nellore schist belt and Nallamalai nated by magmatic fabrics, no tectonic folds fold belt are sandwiched between the Eastern and only occasionally thin shear bands that are Ghats belt and the Eastern Dharwar craton interpreted as syntectonic fabric with that in (Bhattacharya and Saha 2008). the metapelitic country rocks (Bhattacharya and Kar 2004). Sodic amphibole, acmitic clinopyro- 2.2 Alkaline complexes xene, annite-rich biotite and sodic plagioclase are typical minerals of an alkaline complex. Bulk Several alkaline complexes have been reported from chemistry follows a tholeiitic trend with early Fe- the high-grade Eastern Ghats belt (Leelanandam enrichment. Feldspar fractionation is also evident 1998). Many of these complexes occur at or near in this complex (Bhattacharya and Kar 2005). the southwest margin of the Eastern Ghats belt High concentration of HFS elements is compa- and commonly described as Prakasam alkaline rable to that in the within-plate basic sodic province, although all are not of the same age. magma (Sun and McDonough 1989). However, For the complex, at the northwestern Gupta et al (2005) described some metamorphic margin of the Eastern Ghats, Upadhyay et al imprints, while Nanda et al (2008) suggested post- (2006a) described intrusion age of 1480 ± 17 Ma, intrusion granulite facies metamorphism to be SHRIMP data from zircon interior. From the 0.70–0.87 Ga, based on U-Pb zircon data. Nanda Contrasting magmatic signatures in Rairakhol and Koraput alkaline complexes 177 et al (2008) also argued that the host gneisses was 0.511857 ± 0.000046 (2σ) and the laboratory suffered an earlier granulite facies event. On the blanks for the chemical procedure during the other hand, Vijaykumar and Leelanandam (2008) period of analysis yielded maximum values of 0.4 ng considered the Koraput complex as DARC, a reac- for Nd and 0.7 ng for Sm. tivated and exhumed high-grade terrain of the For the U-Pb analytical work, the measured craton. ratios of the NBS 982 standard were 204Pb/206Pb = The Rairakhol complex also occurs in a granulite 0.02732±0.00003; 207Pb/206Pb = 0.46656±0.00003 ensemble and mylonitic shear zones are commonly and 208Pb/206Pb = 0.99783 ± 0.00005. The labora- observed at the margins. Earlier, Panda et al tory blanks for the chemical procedure during the (1993, 1998) considered a rift setting, north of period of the analysis yielded maximum values of but not far from the Godavari Graben. However, 15 pg for Pb and 2 pg for U. Swain (2005) showed that this complex mostly record solid-state deformation fabrics: gneissic and 4.2 Isotopic results mineral-defined foliation and folded gneissic foli- ation and folded boudins. Some mylonitic shear The U-Pb zircon analytical data for the complexes bands are also observed in this complex (Swain and their neighbouring country rocks are given in 2005). Calcic amphibole and pyroxene, phlogopite- table 1, and whole rock Rb-Sr & Sm-Nd data are rich biotite and calcic plagioclase are the char- given in table 2. acteristic minerals and on this ground could be For the Rairakhol complex, U-Pb isotopic data described as calc-alkaline rather than alkaline. This for zircon from nepheline syenite shows an upper is also consistent with a calc-alkaline trend in their intercept age of 946 ± 6 Ma with MSWD of 7.6, bulk composition (Bhattacharya et al 2004). Large when regressed through origin (figure 1). However, positive Nb anomaly is the characteristic of this one concordant age of 938.8 ± 3.1Ma,withMSWD complex and could reflect crustal contamination of 1.5, may be taken as the precise magmatic age. 207 206 (Condie et al 1987; Saunders et al 1992). Rath et al It is also important to note that Pb/ Pb age of (1998) and Panda et al (1998) described pegma- the zircon fractions vary between 931 and 950 Ma toidal and ‘younger’ intrusives within ‘older gneis- (table 1). sic members’, suggesting more than one episode The neighbouring charnockitic gneiss records a of nepheline syenite intrusion. The only isotopic concordant age of 1006 ± 4.8Ma, with MSWD of data reported from this complex so far, is Rb-Sr 0.17 (figure 2 and table 1). whole rock isochron of 1413 ± 23 Ma by Sarkar et al Whole rock Rb-Sr and Sm-Nd isotopic data for (2000). the Rairakhol complex show negative fSm and pos- itive fRb values, indicating a source that were enrichedinRbanddepletedinSm.Alsocrustal 4. Isotopic study residence age given by TDM between 1.6 and 1.7 Ga. Negative epsilon values (calculated at 938 Ma) indi- U-Pb isotopic analysis of zircons from the two cate that they were either derived from or assimi- complexes and their immediate country rocks were lated older crustal rocks. Or in other words, their undertaken to date magma emplacement/crystal- Sm/Nd had been lowered originally when they were lization and their relation to the thermal imprints separated from mantle. Initial Sr and Nd values in the country rocks. Rb-Sr and Sm-Nd isotopic calculated at 938 Ma are 0.70820 and 0.70902, and compositions of both felsic and mafic assemblages 0.511209 and 0.511142, respectively. These values of the two complexes were determined to under- indicate an enriched composition of their mantle stand the nature of the magma sources. source. For the Koraput complex discordia diagram for 4.1 Analytical procedure zircon from nepheline syenite shows an upper intercept age 917 Ma and lower intercept age of The isotopic analyses were performed at the 745 Ma, with MSWD of 0.093 (figure 3). Here the Centre of Research in Geochronology, S˜ao Paulo 207Pb/206Pb age of zircon fractions vary between University, using a VG-354 multi-collector mass 774 and 820 Ma and if these are considered as spectrometer. the minimum crystallization age, the 917 Ma upper For the Rb-Sr analytical work, the measured intercept age may be taken as the intrusion age, ratio of 87Sr/86Sr obtained for NBS 987 standard while the lower intercept age of 745 Ma may be was 0.710254 ± 22 (2σ) and the laboratory blank considered as a later thermal overprint. In fact this for the chemical procedure during the period of age is also recorded from U-Pb isotopic data of zir- analysis yielded maximum value of 4 ng for Sr. cons of the metapelitic country rocks around the For the Sm-Nd analytical work, the measured Koraput complex (figure 4). Here lower intercept ratio of 143Nd/144Nd obtained for La Jolla standard age of 746 Ma and upper intercept age of 2244 Ma 178 S Bhattacharya and M Basei

Table 1. U-Pb isotopic data for zircons from alkaline complexes and country rocks.

Pb U 207Pb/206Pb Sample (ppm) (ppm) 206Pb/204Pb 206Pb/238U 207Pb/235U 207Pb/206Pb (Ma) RAS173 Nepheline syenite, Rairakhol complex A 89.0 595.4 1987.1 0.157282 ± 22 1.52401 ± 35 0.070276 ± 262 936 C 80.9 531.8 1264.3 0.15675 ± 91.51858 ± 132 0.070264 ± 363 936 B 80.4 529.8 1190.0 0.157755 ± 24 1.53857 ± 209 0.070735 ± 291 950 D 118.8 778.3 2516.6 0.15876 ± 71.54427 ± 127 0.070547 ± 187 944 E 114.3 794.0 1801.1 0.151283 ± 76 1.4738 ± 65 0.070655 ± 218 947 F 98.8 681.0 1673.7 0.150573 ± 43 1.4555 ± 69 0.070107 ± 275 931

RAS49 Charnockitic gneiss, Rairakhol A 75.2 425.5 868.4 0.169355 ± 34 1.6981 ± 146 0.072722 ± 241 1006 B 109.7 669.0 3104.0 0.172511 ± 23 1.76616 ± 158 0.074253 ± 421 1048 C 85.5 490.1 512.7 0.174465 ± 15 1.78434 ± 237 0.074177 ± 288 1046

K68 Nepheline syenite, Koraput complex A 15.4 82.6 1303 0.133737 ± 18 1.224810 ± 322 0.066423 ± 175 820 B 8.0 41.0 361 0.127960 ± 22 1.1488 ± 243 0.065114 ± 163 778 C 13.2 84.1 617 0.126671 ± 97 1.135030 ± 164 0.064987 ± 298 774 D 10.4 50.2 303 0.128562 ± 49 1.166410 ± 259 0.065802 ± 247 800

KH6 Metapelitic granulite, Koraput A 46.5 340.5 869 0.135209 ± 49 1.263590 ± 146 0.067780 ± 238 862 B 43.9 269.7 3170 0.155010 ± 113 1.858450 ± 248 0.086954 ± 152 1360 C 36.1 270.9 1509 0.134697 ± 67 1.368650 ± 348 0.073694 ± 244 1033 D 51.2 398.1 3113 0.129969 ± 28 1.282470 ± 224 0.071566 ± 183 974 E 36.7 272.4 1791 0.135615 ± 145 1.347160 ± 349 0.072046 ± 257 987

are recorded with MSWD of 3.3. The upper inter- 5. Discussion cept age of 2244 Ma, could define protolith age of the khondalites; incidentally, similar protolith ages Both the Rairakhol and Koraput complexes occur for the paragneisses of the Eastern Ghats belt, in granulite ensemble and far from the cratonic by Sm-Nd systematic have been described earlier margin. The alkaline complexes of the Prakasam (Rickers et al 2001). province occur in a plate margin setting and hence Whole rock Rb-Sr and Sm-Nd isotopic data amenable to explanation by rift-valley magma- for the Koraput complex show negative fSm and tism. Vijaykumar and Leelanandam (2008) pro- positive fRb values, indicating a source that posed that ARCs of the Prakasam province, aged wereenrichedinRbanddepletedinSm.Also between 1.5 and 1.35 Ga represent rifting along eastern margin of the thickened arc crust; while mantle derivation age given by TDM is ca. 1.0 Ga. Positive epsilon values (calculated at 917 Ma) conversion to the DARCs may be related to either indicate that the rocks were derived from resi- the Grenvillian or Pan-African orogeny. Two com- dual solids in the mantle reservoir, after magma plexes of the present study occur in an intra-plate setting and hence not easily explainable by rift- had been withdrawn at an earlier time. Initial Sr valley magmatism. Incidentally, high concentra- and Nd values calculated at 917 Ma are 0.70252 and tion of HFS elements in the Koraput complex is 0.70306, and 0.511698 and 0.511652, respectively. also compatible with the within-plate basic sodic These values further indicate a depleted mantle magma (Sun and McDonough 1989). Again the iso- as their source. However, alkali enrichment in this topic data for these two complexes indicate broadly complex is apparently contrary to the depleted syntectonic intrusion with granulite facies meta- mantle source, as indicated by the initial Sr and Nd morphism (dominant Grenvillian in the northern values as also the positive epsilon values. But this Eastern Ghats belt), in a compressional setting. alkali enrichment can be explained in terms of a Hence these are unlikely the older ARCs later con- later thermal imprint at ∼ 745 Ma (lower intercept verted to DARCs, as envisaged by Vijaykumar and in U-Pb discordia (figure 3)). Leelanandam (2008). For such continental plutonic Contrasting magmatic signatures in Rairakhol and Koraput alkaline complexes 179 (938/917) i (938/917) Sr i Nd . 36 0.511209 0.511142 0.70820 0.70902 917) . . / 5 4 − −

(938 Figure 1. Concordia diagram for zircons from nepheline ε syenite, Rairakhol complex. DM T (Ga) (Ma) (Ma) (Ma) Rb f 5043 8.88 0.48 1.7 1.0 4.7 0.511698 0.70252 51 1.69 1.0 4.2 0.511652 0.70306 50 5.33 1.6 . . . . Sm 0 0 0 0 f − − − − Nd 144 Nd/ 143

Figure 2. Concordia diagram for zircons from charnockitic Nd gneiss, Rairakhol. 144 Sm/

147 complexes an alternative could be plate movement over a rising plume, as envisaged for the alkaline

Sr intrusions of the White Mountains (Foland and 86 Faul 1977).

Sr/ Again there are striking differences in geochemi- 87 cal and isotopic signatures between the Rairakhol and Koraput complexes, although the field setting Sr is more or less identical. 86 In the Koraput complex, mineralogy and chemis- Rb/ try are compatible with a fractionated alkali-basalt 87 series, while those of the Rairakhol complex can only be described as a syenite complex compatible with a calc-alkaline suite. Intrusion age-wise Rairakhol complex is defi- nitely older with a concordant age of 938 Ma, while Koraput complex is younger, even if the upper intercept age of 917 Ma is considered. This age difference, namely older intrusion in the north Whole rock Rb-Sr and Sm-Nd isotopic data for Rairakhol and Koraput alkaline complexes in the Eastern Ghats belt, India (Rairakhol) and younger one in the south (Kora- put) may be interpreted as northward moving plate over a rising plume. At the ground level the two Nepheline syeniteMafic syenite 9 Alk-1 6 0.806 Alk-2 0.1204 0.719824 0.70410 0.0977 0.1130 0.511743 0.512378 Nepheline syenite 6 Alk-1 0.2192 0.70593 0.0966 0.512253 Table 2. Mafic syenite 9 Alk-29 = Rairakhol suite; 6 = 0.517 Koraput suite. 0.715134 0.0986 0.511816 Rock type Sample complexes are around 180 km apart and this would 180 S Bhattacharya and M Basei

difference between intrusion age (938 Ma) and crustal residence ages (1.6–1.7 Ga) of the Rairakhol complex, apparently supports that contention. However, it is difficult to postulate melting of crustal rocks to produce nepheline norma- tive magma, unless alkali enrichment can be demonstrated as a post-intrusion phenomenon. Although, solid-state deformation fabrics dominate in this complex, no metamorphic changes or post- intrusion thermal imprints have been recorded in this complex, unlike that in the Koraput com- plex. Two other alternative interpretations may also be considered. Parental basic magma may have resided in a lower crustal chamber for a long period, until fractionation lowered their density to the point that the largely felsic magma could buoyantly rise through the crust (Philpotts 1990). Figure 3. Concordia diagram for zircons from nepheline However, for the Rairakhol complex this period is syenite, Koraput suite. appreciably long of the order of 500 Ma, and hence an unlikely proposition. In the other alternative interpretation, assimilation of older crustal rocks may be responsible for the negative epsilon values at the time of intrusion. A long crustal residence, in that case, may represent the older crustal rocks in this interpretation. Incidentally, large positive Nb anomaly, reported earlier (Bhattacharya et al 2004), also suggests crustal contamination. Thus our favoured interpretation for the Rairakhol com- plex is assimilation of older crustal rocks during intrusion. In relation to the granulite facies thermal imprints, dominated by the Grenvillian event (1.1 to 0.9 Ga), both the complexes are broadly syntectonic, although, later thermal imprints ∼ 745 Ma are recorded in the Koraput complex and sheared metapelitic country rocks. Around 800 Ma thermal overprints are recorded from several segments in some recent publications (Simmat and Figure 4. Concordia diagram for zircons from metapelitic Raith 2008), as also some unpublished data of granulite, Koraput. monazites with the authors.

Acknowledgements indicate horizontal movement of the continental plate at ∼ 9mmperyear. Isotopic analytical work was undertaken at the Sr-Nd isotopic signatures in the two complexes Geochronological Research Centre, University of are also distinct. While the Koraput complex has S˜ao Paulo, Brazil under a collaborative Research depleted mantle composition as its source, the Project C-91 and SB acknowledges support from Rairakhol complex has enriched mantle composi- the Centre. Indian Statistical Institute, Kolkata tion as its source. For the Koraput complex, post- is acknowledged for providing infrastructural intrusion thermal imprint, as in this presentation facilities. as also by others (Gupta et al 2005; Nanda et al 2008) can explain the apparent contradic- tion between a depleted mantle source and alkali References enriched compositions of the rocks. It was suggested in an earlier paper that Bhattacharya S 1997 Evolution of the Eastern Ghats granu- lite belt of India in a compressional tectonic regime and Rairakhol complex was the result of intra-crustal juxtaposition against Iron Ore Craton of Singhbhum by melting, on the evidence of negative epsilon oblique collision-transpression; Proc. Indian Acad. Sci. values (Bhattacharya et al 2004). The significant (Earth Planet. Sci.) 106 65–75. Contrasting magmatic signatures in Rairakhol and Koraput alkaline complexes 181

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MS received 8 July 2008; revised 15 January 2010; accepted 17 January 2010