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Journal qf Southeast Asian Earth Sciences. Vol. 14, NOS 314. pp. 209-219, 1996 Copyright Q 1996 Elsevier Science Ltd

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Lithology and tectonothermal history of the Precambrian rocks of Orissa along the eastern coast of India

N. K. Mahalik Department of Geology, Utkal University, Bhubaneswar, 751 004, India

(Received 23 November 1995; accepted for publication 17 May 1996)

Abstract-Orissa possesses an excellent record of geological history spanning most of the geologic time from Archaean to the Quaternary. It has most of the typical lithologies and many tectonothermal events preserved in the various rock groups. Three distinct crustal blocks could be identified: two cratonic blocks and a mobile belt separated from each other by deep-seated regional fault boundaries. These are the north Orissa craton (NOC), the west Orissa craton (WOC) and the Eastern Ghats granulite belt (EGB). The fault boundaries separating them are identified as the north Orissa boundary fault (NOBF) and the west Orissa boundary fault (WOBF). The NOBF fault running along Mahanadi Valley could be termed the ‘Mahanadi rift’. The NOC contains extensive occurrences of low-grade folded banded iron formations (BIFs), granite intrusives and undeformed volcano-sedimentary assemblages belonging to the Archaean to early Proterozoic times. They are succeeded by medium-grade folded Proterozoic limestone-bearing sequences. The WOC craton is underlain by extensive occurrences of Archaean granites and undeformed Proterozoic limestone-bearing platform sediments. Small occurrences of Archaean BIFs and greenstones have also been noticed. The EGB consists of high-grade granulite-facies rocks such as the khondalites, charnockites, basic granulites, migmatites and augen gneisses. It has been considered as a ‘mobile belt’ during the middle Proterozoic Era. A distinct early Neoproterozoic (cu. 1000 Ma) chamockite event has been recorded in this belt. Close to the faulted boundaries of the crustal blocks are seen occurrences of anorthosites, alkaline rocks and chromite-bearing ultramafics. The rocks in each crustal block show evidence of multiphase tectonothermal history. Similarity of lithology, tectonothermal events and major rift features, e.g. the Mahanadi rift, place Orissa close to Eastern Antarctica. Copyright 0 1996 Elsevier Science Ltd

Introduction

The State of Orissa occupying a land area of 155,707 km* lies along the eastern coast of India (17”48’-23”34’N and 81”24’-87”29’E) and forms a part of the Indian Peninsular shield (Fig. 1). It is a unique region with an excellent record of geological history spanning from the Archaeans to the Quaternaries. It is important for its rich mineral resources such as bauxite, coal, chromite, iron ore, manganese ore, limestone, dolomite, gemstone, graphite and beach placers.

Many workers, since the middle of the nineteenth century, have worked and contributed to the different aspects of geology and mineral development in Orissa. Mention may be made of the following works, by no means a complete list: Jones (1934); Krishnan (1937); Dunn ( 1940); Dunn and Dey ( 1942); Crookshank ( 1963); Dutta (1963); Chatterji et al. (1964); Prasad Rao et al. (1964); Iyengar and Anand Alwar (1965); Kanungo and Mahalik (1967, 1972, 1975); Banerjee (1968); Murthy et al. (1971); Banerji (1974); Majumdar (1978); Basu et al. (1981); Sarkar et al. (1981); Halden et al. (1982); Acharya (1984); Mahalik (1984, 1986, 1987, 1994, 1995a, b); Kaila et al. (1987); Murti (1987); Aftalion et al. (1988); Saha et al. (1988); Nanda and Pati (1989); Paul et al. (1990); Chetty and Murthy (1993); Panda et al. (1993); Saha (1994); Chetty (1995). Recently, the Society of Geoscientists and Allied Technologists brought out an informative book on the geology and mineral resources of Orissa (SGAT, 1995).

About 80% of the land surface is occupied by the Precambrians, 5% coal-bearing Gondwanas (Upper Carboniferous to early Cretaceous) and 15% coastal Tertiaries and Quaternaries.

The Precambrians are very complex lithologically, genetically and tectonothermally. They occur in three distinct tectonic terrains (Mahalik, 1995b): the north Orissa craton (NOC), the west Orissa craton (WOC) and the Eastern Ghats granulite belt (EGB), separated from each other by deep-seated regional fault boundaries (Fig. 2). The NOC and the WOC are typical banded iron formation (BIF)-greenstone-granite terrains overlain by Proterozoic carbonate-bearing sequences, while the EGB is a high-grade granulite- facies terrain believed to be a ‘mobile belt’ during the middle Proterozoic (Geological Survey of India, 1994).

Several geoscientists are now engaged in correlation studies between India and Antarctica (Yoshida and Santosh, 1995) with respect to their spatial position during the Precambrian Gondwana Assembly. Pre- cambrians in other parts of southern India are now extensively studied for understanding the possible link of India with Antarctica. The Precambrians of Orissa, especially the EGB, which has a better potential for such linkage, is not yet studied from this angle. The present work tries to bring out the salient features of the Precambrians of Orissa in the three crustal blocks, which could guide future work in Orissa for an Antarctica linkage.

209

210 N. K. Mahalik

INDEX MAP

29

LEGEND

MG - Mahanadi Grakn GG - Godavari Grabrn

NOBF - North Orissa Boundary Fault WOBF -West Orissa Boundary Fault

0 -Bihar Orissa Craton @ - Eastrrnghats Granulitr Belt. @ -Central lndion Crof?

Fig. 1. Index map of India showing Orissa State and the tectonic divisions.

Lithologic association

The lithologic associations in the three different tectonic blocks, the NOC, the WOC and the EGB, are discussed separately to bring out their differences. Figure 3 is a map of the various lithologies of the state prepared by Mahalik (1984, 1986, 1995b) based on study of satellite imageries, field traverses along key sectors and from existing literature from various sources.

North Orissa craton (NOC)

It is a fault-bounded pear-shaped craton (Mahalik, 1986), with the north Orissa boundary fault (NOBF) (Mahalik, 1994) along its south, the arcuate Singhbhum thrust (Dunn, 1937) to its north and the Kamakhyana- gar-Nilgiri fault to its east. It forms a major part of the Singhbhum craton. The NOC consists of several major lithologic groups of sedimentary and igneous origin, ranging in age from the Archaeans to the younger Proterozoic. The most important aspect of the craton is its association with several sequences of BIF, interwoven with several major granite intrusives (Singhbhum granite, Bonai granite) and volcano-sedimentary se- quences such as the Simlipal group. These Archaean assemblages were succeeded by Proterozoic sequences formed both in geosynclinal (Darjing and Gangpur groups, Mahalik, 1987) and in platform environments (Kolhan group).

Basu et al. (1981) and Saha et al. (1988) believe that the NOC preserves fragments of the oldest crust with an

age of 3775 Ma represented by the older metamorphic group (OMG) and the older metamorphic tonalite gneiss (OMTG) of Champua area in Keonjhar district. However, this has been questioned by Moorbath et al.. (1986) and Moorbath and Taylor (1988), who believe these rocks to be around 3.27-3.52 Ga. Basu in Sharma et al. (1994) has changed his earlier opinion on the age of the OMG and the OMTG (Basu et al., 1981). The result of his subsequent study indicates that the protoliths of the OMG amphibolites are 3.3 Ga; therefore, the OMTG, which intrudes into the associated amphibolites, cannot be any older than 3.3 Ga. The Singhbhum, Bonai and Nilgiri granites have been dated by various methods showing ages of around 2900-3200 Ma (Saha, 1994). Besides the above granitic bodies, which are soda-rich, a potash-rich granite gneiss occurs extensively around the Pallahara-Kamakhyanagar area described by Mahalik (1994, 1995a) as ‘Palkam gneiss’ and by Saha (1994) as ‘Pallahara granite gneiss’. It is found to be intrusive into the granulite-fades rocks around Rengali dam (Mahalik, 1995a) and forms the basement of the BIFs of the Pallahara-Deogarh area. This deserves geochronological investigation because of its stratigraphic relation to the rocks of both the NOC and the EGB.

The BIFs of north Orissa are the extensively developed supracrustals encircling the Singhbhum granite complex. There exist different opinions on the stages of development of these supracrustals and their relation to the granite intrusives. Saha and co-workers (Saha, 1994), Jones (1934) and Dunn (1940) believe that

Lithology and tectonothermal history, Orissa

- 19-

LEGEND

- Matmadi Rift (Graben)

- Godawi Graben

NOBF- Narth Orissa Boundary Rult - lee

WO8F- West Orissa Baundary Fall? 8CALL

0 100 KM. “,o

I I

es0 es- 970

Fig. 2. Broad geological features of Orissa and surrounding states along the east coast of India. Locality index: 1, Keonjhar; 2, Sambalpur; 3, Raipur; 4, Bolangir; 5, Bhubaneswar; 6, Koraput; 7, Visakhapatnam;

8, Rajamundri; 9, Jagdalpur.

all the BIFs were formed as a single assemblage during the Archaean underlain and/or intruded by the different phases of Singhbhum granites. According to Saha (1994) their age of formation could be between 3.3 and 3.1 Ga. The second view (Iyengar and Banerjee, 1971; Banerji, 1974, 1975, 1980; Iyengar and Murthy, 1982) classify the BIFs into two distinct age groups, older (Gorumahisani group) and younger (Noamundi group). Yet there is a third view presented by Prasad Rao et al. (1964) and Acharya (1976, 1984), who classify the BIFs into three distinct stratigraphic formations, the oldest around Pallahara and Gorumahisani, the intermediate at Daitari and the youngest at the Joda-Koida region. The youngest one contains rich deposits of iron and manganese ores and forms a horseshoe-shaped synclinal structure (Jones, 1934).

There are extensive volcano-sedimentary sequences, as seen in the Simlipal and Keonjhar plateaus. These are

considered equivalent to the Dhanjoris and younger to BIF by Saha and his associates, while others (Prasad Rao et al., 1964; Iyengar and Banerjee, 1971; Banerji, 1974) found them sandwiched between BIFs. The extensively occurring lavas (designated as Malangtoli Lava by Saha, 1994) between Malangtoli and Pallahara underlie the undeformed Kolhan sequence of the area. The age of volcanics and the volcano-sedimentary sequences in the NOC is not definitely known. Saha (1994) describes them as early Proterozoic in age.

In contrast to the BIF-greenstone sequences that evolved during the Archaean, there are other important metasedimentary assemblages which formed much later in Proterozoic basins. They are devoid of iron formations but detrital fragments of BIF and granites are observed in them, indicating their younger age. The important lithogroups are the Darjing, Gangpur and the Kolhan groups, characterised by their calcareous

212 N. K. Mahalik

Lithology and tectonothermal history, Orissa 213

carbonaceous association (Mahalik, 1987). While the Darjing and Gangpur rocks were folded and met- amorphosed (staurolite grade), the Kolhans were least deformed and metamorphosed. The age of the Kolhans could be 2000 Ma, as suggested by Saha (1994).

Among the Proterozoic intrusives, mention may be made of the Mayurbhanja granite, dated as 2084 f 70 Ma (Iyengar et al., 198la, b) and the gabbro-anorthosites occurring along the northeastern edge of the craton. Dolerite dykes in small to large exposures, and designated as the ‘Newer Dolerites’, are found extensively in the Singhbhum granite body. They show two important trends, NE-SW and NW-SE. They vary in their age from 1080 to as much as 2000 Ma (Saha, 1994). Along the southeastern margin of the craton, close to the NOBF, important chromite-bearing ultramafic rocks are observed intrusive into the BIF and later folded into a synform (Banerjee, 1972). The age of the ultramafic rocks are not known with certainty, but they are older than some phases of the Singhbhum granite and the dolerites.

From the above account it is noticed that the NOC consists of varied lithologies, most typically the BIFs, granite intrusives, volcano-sedimentary assemblages belonging to the Archaean age and the Proterozoic calcareous-carbonaceous metasedimentaries.

West Orissa craton (WOC)

The WOC constitutes the eastern fringe of the central Indian shield and is bounded by the Mahanadi rift on the north, the Godavari rift on the south and the EGB to the east (Figs 1 and 2). It is a least-studied area and mostly occupied by extensive occurrences of granite gneisses and granites with scattered exposure of supracrustals of Archaean to Proterozoic ages. It is very similar to the NOC or Dharwar craton in its lithological content. The gneisses and granites of the adjoining areas (Bastar) have been dated (Sarkar et al., 1990) and ages of 3.0-2.6 Ga have been recorded. The gneisses and associated pegmatites contain gemstone deposits. The Archaean supracrustals consists of the Bengpal and BIF-bearing Bailadila suite of rocks (Crookshank, 1963). The Bailadila suite contains extensive deposits of iron ore in adjoining Madhya Pradesh and are equivalent of the BIF suite of rocks of the Joda- Noamundi area of the NOC.

During the Proterozoic, undisturbed shales, sand- stones and limestones were deposited in Proterozoic sedimentary basins (e.g. Chhatisgarh and Indravati basins) under platform condition (Geological Society of India, 1987).

Outcrops of Eastern Ghats granulites occur within the WOC as a detached thrust sheet (Rath et al., 1994) in parts of the Kalahandi and Bolangir districts. The rocks consist of khondalites, calcgranulites and crystalline limestones. These rocks host important deposits of bauxite and graphite (Acharya and Dash, 1984). The Khariar nepheline syenites and Bolangir anorthosites that occur within this thrust sheet are discussed below. The isotopic data of Sarkar et al. (1990) suggest a temporal similarity of WOC with the rocks of the NOC and Dharwar craton.

SEAES 4!i 4 H

Eastern Ghats granulite belt (EGB)

The EGB is a belt of high-grade metamorphic rocks with thrust along its western margin as confirmed by deep seismic sounding (Kaila and Bhatia, 1981). This thrust (Fig. 2) may be named as the ‘west Orissa boundary fault’ (WOBF). Another thrust defines its northern margin, the NOBF. It is along these thrust contacts the Eastern Ghats mobile belt was accreted to the cratonic blocks to the north and west.

The EGB consists predominantly of chamockites and migmatitic gneisses with a large expanse of supracrustal rocks classified as khondalites. Large areas of EGB close to the fault boundaries are covered by coarse-grained augen gneisses. Exposures of anorthosites and nepheline syenites are also found along the fault boundaries of this belt, such as the Bolangir and Chilika anorthosites and the nepheline syenites of Koraput, Khariar, Redhakhol and Samal (Bose, 1970; Sahu, 1976, 1980; Sarkar et al., 1981; Panda et al., 1993). Small intrusives of carbonate rocks (carbonatite) are observed in parts of the Koraput and Kalahandi districts (Swain, 1994).

The major lithologies in the EGB are khondalites and chamockites. They are found either together in interlayered sequences or independently in large continuous exposures. On a regional scale, certain spatial distribution characteristics are noticeable in the distribution of chamockites and khondalites (Fig. 3). They occur in alternating sequences, e.g. (1) the western-boundary chamockites, (2) the west-central khondalites, (3) the east-central chamockite-migmatite- khondalites and (4) the coastal khondalites. As distinct from these sectoral distribution charnockites, khon- dalites and migmatites also occur in the Mahanadi graben area, which truncates all the above suites

The western-boundary charnockites and the west- central khondalites are affected by several faults and shears. The western charnockite is truncated by the Nagavalli shear. The west-central khondalite is pushed northwestwards across the western-boundary fault into the WOC. Khondalites in this sector form bauxite-bearing plateaus, e.g. the Panchpatmali, Baphlimali and Gandhamardan plateaus. Khondalites in other sectors do not contain bauxite or even bauxite-forming minerals. The east-central sector is typically a chamockite-migmatite complex containing bands of khondalite. Patchy occurrences of charnockite are developed in the gneisses in this sector. The fourth sector is the coastal khondalites, which run parallel to the coast in a NE-SW direction, and is associated with chamockites and leptynites. Eastern Ghats rocks in the Mahanadi graben are bounded by the NOBF and Mahanadi shear and form a westerly plunging syncline. More work is needed to differentiate the charnockites and khondalites of these different sectors.

Khondalite is the most prominent supracrustal in the EGB. It is associated with minor amounts of quartzite, calcgranulite and crystalline limestone. It could be diverse in nature. Divakar Rao et al. (1994) have recorded geochemical difference in the khondalites. They classify them into high-silica and low-silica types and believe them to form in two depositional stages. While many believe the khondalites to form by deposition in sedimentary basins, Dash et al. (1987) thought them as a metamorphosed deeply weathered soil profile. The ages of the khondalites are not known with certainty.

214 N. K. Mahalik

Perraju et al. (1979) found 3090 Ma and Vinogradov et al. (1964) 2600 Ma for Orissa khondalites. As has been stated earlier, the west-central khondalites host important bauxite deposits, while khondalites in other areas do not contain any bauxite deposits or bauxite-forming minerals.

Charnockites constitute another important lithogroup and are variable in nature. They occur as massive independent bodies with large aerial extension, as an interbedded sequence with khondalites and in patchy occurrences of small dimensions in other host rocks. An interesting interbanded charnockite and leptynite rock has been observed a few kilometres northwest of Chilika lake along the road from Khurda to Berhampur (Kar, 1989). The bands of charnockite are measurable from as much as a metre to fractions of centimetre. It is observed that the chamockite is becoming transformed into leptynite, and all stages of transformation of chamockite into leptynite are observable. These are chamockites ‘in breaking’ and deserve further examin- ation. Bhattacharya et al. (1993, 1994) believe that the charnockites of the Chilika area are older than the leptynites. In the Angul area, in the Mahanadi graben, on the other hand, the leptynites have been chamockitised as reported by Park and Dash (1984) and Aftalion et al. (1988) indicating the younger age of chamockites. In many areas chamockites occur as neosomes in small patchy occurrences in migmatitic complex whose palaeosome has undergone recrystallisation under upper amphibolite-granulite facies condition (Halden et al., 1982). These are the chamockites ‘in making’.

The Angul chamockites are dated between 1100 and 950 Ma (Aftalion et al., 1988) while Paul et al. (1990) suggest a major charnockite event between 1170 and 950 Ma for the chamockites of the Phulbani area. It may be pointed out that the charnockites of the Visakhapatnam area in Andhra Pradesh have a compar- able date, 979 Ma (Grew and Manton, 1986). Recently, chamockite of the Tikri area in Koraput has been dated by Sarkar and Nanda (1994) at 950 Ma. All these dates are similar to those found in Sri Lanka (Kroner et al., 1987) and the Rayner complex of East Antarctica (Grew and Manton, 1986). In contrast, the age of chamockites in South India is 2600 Ma (Crawford, 1969).

Some of the anorthosite and nepheline syenite bodies occurring along fault boundaries have been dated. The Chilika anorthosite is believed to have been intruded during 1300-1400Ma (Sarkar et al., 1981) and the nepheline syenites were emplaced during two episodes, 1400 and 856 Ma (Sarkar et al., 1994a, b).

The EGB is believed to be distinct from the south Indian granulite belt (Drury et al., 1984). The south Indian granulites evolved much earlier in early Proterozoic, while the granulites of the EGB evolved in late Proterozoic (Grew and Manton, 1986; Aftalion et al., 1988).

Structural features The most important structural features of the

Precambrians of Orissa are several folded belts with their typical tectonic trends, regional faults and lineaments, circular features and graben structure, as shown in Fig. 4. The regional structural features separating the three crustal blocks are the NOBF and the WOBF.

North Orissa boundary fault (NOBF)

This regional fault structure separates the NOC from the WOC and the EGB (Figs 1 and 2). A small section of it on the eastern end was identified earlier by Prasad Rao et al. (1964) as the Sukinda thrust. Later, Mahalik (1984) extended it westwards across the Orissa border into Madhya Pradesh and termed it the ‘NOBF’. It is clearly identified in the satellite imageries as a linear break. It runs for hundreds of kilometres from central India in the west up to the eastern coast of India. A coal-bearing Gondwana group of rocks are found along this fault. It is hidden beneath the Mahanadi deltaic sediments (Kaila et al., 1987). As it runs along the Mahanadi river valley, it is also termed the ‘Mahanadi rift’. At different sections across this fault, abrupt contact of the NOC and EGB could be seen (Mahalik, 1994). Recently this fault zone is being correlated with the Lambert rift of East Antarctica (Federov et al., 1982; Hofmann, 1996).

West Orissa boundary fault (WOBF)

The ‘WOBF’ is the name coined here for the fault separating the WOC and EGB (Fig. 4). It has been termed earlier as the ‘Sileru shear’, as it extends into Andhra Pradesh along the Sileru river valley (Chetty and Murthy, 1993). From Orissa it runs southerly through Andhra Pradesh up to Madras, separating the EGB from the cratonic blocks on the west. Kaila and Bhatia (1981) have confirmed it as a thrust by deep seismic sounding studies.

Besides the above regional fault features occurring along the boundary of the crustal blocks, there are several other features found within each crustal block. They are discussed here briefly.

North Orissa craton (NOC)

Several faults have been identified within this craton (Fig. 4). These are the Sarapalli-Redhakhol fault, the Malyagiri-Kamakhyanagar fault, the Sambalpur- Sonepur fault and the Kamakhyanagar-Nilgiri fault. Some of them have been described by Mahalik (1994).

The major folds in the NOC consist of the northwesterly plunging Malyagiri fold, the westerly plunging Deogarh and Daitari folds and the northnorth- easterly plunging horseshoe syncline, all formed in BIFs. These relate to Archaean events. During the Proterozoic a major synclinorium structure was formed in the Darjing-Gangpur sequence (Mahalik, 1987).

Intersecting dolerite dykes with NW-SE and NE-SW trends are extensively seen within the Singhbhum granite body. Several circular features have also been identified and discussed by Mahalik (1995b). The NOBF has hosted a major graben structure that provided shelter for the Gondwana sediments of the Upper Carboniferous to early Cretaceous period. This graben is termed the ‘Gondwana’ graben or ‘Mahanadi graben’.

West Orissa craton (WOC)

Besides the boundary faults to the north and east, there are other faults within this craton, e.g. the Nawapara-Khariar fault, along which the Khariar nepheline syenites were emplaced. A segment of the EGB

Lithology and tectonothermal history, Orissa

SYMBOLS USE0

Fig. 4. Structural and tectonic features of Orissa..

has been pushed into this craton by thrusting, something which needs proper investigation.

Easternghats granulite belt (EGB)

The EGB shows very complicated tectonics with many faults and shears. As discussed earlier, the NOBF and WOBF truncate it against the cratonic blocks. It also meets the coastal sediments along a well-defined NE-SW lineament. It could be a fault with down throw to the east favouring the formation of a coastal deltaic basin. Other shears cutting across the belt are the Nagavalli, Vansdhara and Mahanadi shears, named after rivers of the same names. Chetty (1955) shows the Nagavalli- Vansdhara shears as the possible extension of the boundary between the Napier and Rayner complexes of East Antarctica. Three important tectonic trends are noticeable in the EGB, differentiated in satellite imageries. These are the predominant NE-SW trend (Eastern Ghats trend), the NW-SE trend cutting across the NE-SW trend, and the E-W (Mahanadi) trend truncating the earlier two. As stated earlier, the EGB has been thrusted into the surrounding cratons, e.g. the Gandhmardan assemblage within the WOC and

the Rengali assemblage within the NOC (Mahalik, 1995a).

Several fold closures are noticeable in the EGB, such as the Gandhmardan closure in WOC and the Kapilas closure in the Mahanadi graben. The NE-SW-trending foliation of khondalites show distinct refolding. One example of refolding of earlier foliation is well observed at Chhatia in Cuttack. Regional fold closures are not seen in the coastal khondalite belt. As much as four deformational phases have been identified in the migmatite complex of the Angul area (Halden et al., 1982). Bhattacharya et al. (1993, 1994) have dealt on the structural setting of Chilika lake granulite-migmatite- anothosite suite, with an emphasis on the time relation of charnockite. Three folding episodes were identified. They hve shown that chamockites are older than leptynites and anorthosites developed after the second fold movement in the area.

Metamorphic events

The cratonic blocks usually show low-grade metamor- phism, while the EGB shows complex polymetamorphic

216 N. K. Mahalik

evolution in granulite-facies condition. In the NOC, the majority of the BIF, granite intrusives and volcano- sedimentary assemblages show greenschist-facies meta- morphism. Only one member of the BIF occurring between Malyagiri and Kamakhyanagar (Fig. 3) are metamorphosed up to kyanite grade, as exhibited by the extensive occurrence of very coarse garnet-staurolite- kyanite schists along with coarse granular quartzites and amphibolites. This is supposed to be the oldest BIF of the NOC. The folded Proterozoic metasedimentary sequences of north Orissa, represented by the Gangpur and Darjing groups, contain garnetiferrous staurolite schists testifying their metamorphism under lower amphibolite facies (Kanungo and Mahalik, 1972; Mahalik, 1987). The WOC mostly shows low-grade metamorphic condition. Its tectonothermal and geochronological events are broadly similar to the NOC. The EGB, on the other hand, shows metamorphism under upper amphibolite to granulite-facies condition represented by khondalite+harnockite migmatite com- plex. The time sequence of deformation and metamor- phism are very complex. Aftalion et al. (1988) and Bowes and Dash (1992) have presented a sequence of structural, metamorphic and igneous events for the Eastern Ghats rocks of the Angul area. Deformation and metamorphism (D,-M,) started during late Ar- chaean (?) at 6-7 kbar and 650-700°C followed by DrMz under 5-6 kbar and 600-650°C during cu. 1.1 Ga, DrM3 under 4-5 kbar and 600-65O”C and closing with localised charnockitisation at ca. 0.96 Ga and granite pegmatite during cu. 0.96-0.85 Ga. Das- gupta (1995) has shown that the Eastern Ghats rocks in Andhra Pradesh were metamorphosed under an extreme P-T condition (8-9 kbar and 950-1000°C) and an anticlockwise P-T trajectory comprising a prograde path of high dT/dP followed sequentially by near-iso- baric cooling and near-isothermal decompression. Mukherjee (1989, 1994) and Mukherjee et al. (1986) have dealt with the thermal tectonic history of the Eastern Ghats based on their study of Bolangir anorthosites intrusive into the surrounding granulites. According to them, the initial prograde metamorphism of the granulites, the later mantle derived igneous massif invasion and the final P-T convergence of the granulite metamorphism have resulted in a composite overturned (anti-clockwise) Y-shaped P-T-r configuration. It is suggested that this configuration is characteristic of Eastern Ghats-type metamorphic belts seen elsewhere in Archaean-Proterozoic continental segments. According to Mukherjee (1989), the Eastern Ghats-type belts have anorthosite intrusive cores acting as ‘exotic’ heat sources, resulting in characteristic anticlockwise over- turned and forked Y-shaped P-T-t loops. Sen et al. (1995) have reported a multi-stage P-T record from their study on the metapelitic and chamockite granulites around Chilika lake. The different P-T stages are a high-T decompression from above 10 kbar to 8 kbar around llOO”C, isobaric cooling from 830-67O”C, a second decompression to 6 kbar and a third decompres- sion to 5 kbar at 650°C.

Evolution of the Precambrians

The three Precambrian crustal blocks in Orissa have widely variable lithologic and tectonothermal histories

and it is difficult to attempt a common time sequence of events for the three crustal blocks. From the available geochronologic data, middle to late Archaean events have been identified in all the blocks. In the NOC, the OMG and the OMTG are believed to be the oldest, followed by a sequence of granite intrusions, deposition of BIFs and volcano-sedimentary rocks all belonging to the Archaeans. They were followed by rocks of the Proterozoic represented by the Dajing-Gangpur metasediments, gabbro-anorthosites, Newer Dolerites and chromite-bearing ultramatics. Iyengar and Murthy (1982) present a series of mineral and rock dates ranging from 3500 to 850 Ma for rocks of the Singhbhum-Orissa craton.

In the WOC the granite gneisses are the oldest and range in age from 3000 to 2600 Ma. The BIFs that are similar to the BIF of the NOC must be of Archaean age. The WOC has, in all probability, a similar history as that of the NOC.

For the EGB, scattered dates ranging from 3 100 to 850 Ma are available. These are (1) a metamorphic event at 3100 Ma based on Rb-Sr whole-rock dates (Perraju et al., 1979) (2) an event of basic and felsic magmatism at 2900 Ma based on model Sm-Nd age (Paul ef al., 1990) (3) a metamorphic event at cu. 2600 Ma based on U-Pb zircon age (Vinogradov er al., 1964) and Rb-Sr whole-rock age (Perraju et al., 1979), (4) an igneous event of anorthosite intrusion at 1400 Ma (Sarkar et al., 1981), (5) a metamorphic and magmatic event at 900 + 100 Ma (Grew and Manton, 1986; Aftalion et al., 1988; Paul et al., 1990) and (6) a granite pegmatite event at 850 Ma (Bowes and Dash, 1992).

Thus, it is seen that rocks in the three crustal blocks of Orissa show a history of evolution beginning somewhere at least in the middle of the Archaean to the late middle Proterozoic. A Pan-African event in Orissa has not been reported so far.

Antarctica connection

Recent works by Yoshida and Santosh (1955) and Yoshida er al. (1992) show that India was a close neighbour of East Antarctica during the Neoprotero- zoic. Similarity in petrologic characters, tectonothermal events and geochronology have been establishing this relationship. There is large amount of research results pertaining to south Indian granulite terrain for a close match of the continents. The Eastern Ghats terrain is increasingly studied to find a similarity with Eastern Antarctica. Several workers, such as Federov et al. (1982), Grew and Manton (1986), Aftalion et al. (1988), Paul et al. (1990), Yoshida el al. (1992), Unnikrishnan- Warrier et al. (1993), Chetty (1995), Dasgupta (1995), have shown a positive relation of the EGB with the Napier and Rayner complexes of Enderby Land, East Antarctica. The Eastern Ghats of Orissa could be correlated with the Rayner complex on the basis of similar lithology and the Neoproterozoic thermal event (cu. 1000 Ma). The Mahanadi rift along the NOBF is now strongly believed to be related to the Lambert rift of East Antarctica (Federov et al., 1982; Hofmann, 1996). Chetty (1995) has shown that the Nagavalli- Vansdhara combined shear could be a continuation of the boundary shear between the Napier and Rayner

Lithology and tectonot :hermal history, Orissa

complexes extending into the Palaghat-Cauvery shear zone of South India. He feels that the Madurai granulite block-Rayner complex-Chilika block (in Orissa) forms a continuous Proterozoic granulite fragment, while the northern granulite belt (South IndiakNapier complex- Araku block (Andhra Pradesh) forms another contigu- ous Archaean granulite belt separated by a major shear zone. The EGB of Orissa has been less studied from this angle and deserves further attention to build up its past relation with the Rayner complex.

Conclusion

Precambrians of Orissa, occurring in vast areas, present geological history of a large time span from mid-Archaean to early Neoproterozoic. They occur in greenstone-granite cratonic blocks and granulite-facies mobile belt sutured between themselves along regional fault boundaries. They are characterised by a wide spectrum of lithologies, tectonics and thermal history. In the study of the Antarctica-India link up, the Eastern Ghats granulite belt of Orissa and the north Orissa boundary fault (Mahanadi rift) would provide great scope for further work.

Acknowledgements-The author is grateful to Prof. M. Yoshida and Dr M. Santosh for inviting him to write a review paper on Precambrians of Orissa. He is also thankful to Dr S. Dasgupta for a critical review of the original manuscript.

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