On the Geology of the Eastern Ghats of Orissa and Andhra Pradesh, India

ON THE GEOLOGY OF T H E EASTERN GHATS OF ORISSA AND ANDHRA PRADESH, INDIA P.K.BANERJ1 ABSTRACT The Eastern Ghats granulite - gneiss tract of Orissa and Andhra with Pradesh, dominantly composed of the khondalite group moderately extensive charnockites, granites, migmatites and local pegmatites, besides a few anorthosites-alkaline syenite complexes, covers approximately 52000 Sq. km. Serious problems have been faced in systematic lithofacies mapping owing to lack of marker bands, absence of unconformities, the thick weathered crust and limited

accessibility.

mapping

of this tract has not been very successful in spatial and

temporal

As a result, first-generation

discrimination/classification

systematic

of the granites and

mig-

matites in relation t o the associated granulites. This horst-shaped tract carries a clear impress of three cycles of acid igneous plutonism, deformation and prograde/retrograde metamorphism and more than two phases of basic igneous magmatism.

There is extensive reactivation along its margins by rifting, local emplacement of mafic/ultramafic complexes and then collision with the lying t o

Gorumahisani and equivalent lron Ore Group sequences its north and west. Deep inside the Eastern Ghats, the

orogenic cycles were not interleaved with sedimentation; and there is a recognisable contrast in their metamorphic P.T from the granulite-gneiss beyond

the

terrane

of South Andhra Pradesh and Tamil

Proterozoic t o Recent Godavari

-

Krishna

Nadu

aulacogen,

where charnockites become dominant at the expense of khondalites: high P.T granulite-facies assemblages become moze extensive and border abundant: and even the metamorphic impress of the (reactivated) zone assumes a higher grade. One

suspects one or more fundamental mid-Proterozoic event(s) of differential uplift between the North and the South Eastern Ghats, possibly coinciding with Pakhal sedimentation, deformation and igneous (granitic and alkaline) intrusions.

39 2

INTRODUCTION First-generation surface geological mapping of a major part of the Eastern Ghats on a 1:63,360 scale has been carried out by the Geological Survey of India during the period 1945 - 1985. Coincidentally,

this period witnessed a dramatic change in the analysis

and understanding of granites, migmatites and granulites following new researches in the advanced countries on mineral equilibria, structural analysis and geothermometric and barometric indicators in co-existing mineral phases. But the universal phenomenon of a variable lag effect between new research find6 in the laboratories and orthodox mapping techniques adopted by the geologists could not be overcome in the Eastern Ghat8 during the first-generation mapping programme. Industrial developments during the last 40 years

have

accessible

.

made

the

Eastern

Ghats

terrain

relatively

more

The present paper highlights some of the practical problems of systematic mapping of the Eastern Ghats, briefly reviews the existing information and makes a case for taking up second-generation mapping of selected sectors of this area along modern 1 ines

.

a 8

REACTIWTED AND GRINlTiSED FRlNGE CHARNOCKITES 8 UHONDALITES Cu

0

GRAPHITE

0

Mn

Cr

Figure 1. Generalized geological map of the Eastern Ghats and some associated mineral deposits.

39 3

The Eastern Ghats have an overall NE-SW trend from the southern bank of the Brahmani river in Orissa to the Nilgiri Mountains of Tamil Nadu, with a pronounced break between the Godavari and Krishna rivers of Andhra Pradesh (Fig.1). The area to the north and

of is

the Godavari river comprises the Eastern Ghats the

central theme of this paper. The area south

(North) of

the

Krishna river from Vijaywada onwards forms a distinctly different geomorphic, geologic ensemble and is hereafter referred to as the Eastern Ghats (South). The geology of the Eastern Ghats (North) was established in broad outline by the traverses by V.Bal1, T.L. Walker, C . S . Middlemiss, H. Crookshank, M.S. Krishnan, P.K. Ghosh (in adjoining Bastar dt. of M.P.) and others. (Pascoe, 1950). During the last four decades, systematic geological mapping of almost the whole Eastern Ghats (North) terrain on a scale of 1:63,360 has been undertaken by Geological Survey of India. Ground observations

on

lithology and structure have

been

supplemented

sporadically with modern aids like aerial photos, satellite imageries, isotope geochronology, etc. Some interesting data have been collected, but this high grade terrain remains comparatively i l l understood. STRATIGRAPHY On the basis of systematic geological mapping and some isotope geochronologic data, the stratigraphic arrangement of the rocks is generally believed to be as follows: U.Proterozoic (900 - 550 m.y)

Porphyritic gneiss, granite, pegmatite and quartz-vein, dolerites.

M. Proterozoic ( 1 6 0 0 - 900 m.y)

Anorthosite, quartz mangerite, gran i te , nephe 1ine s yen i te , gabbro and norite.

Archaean to L. Proterozoic

Migmatites, biotite granite gneiss, pegmatite and quartz vein. Charnockite (acid to basic) suite; Khondalite group (metamorphosed arenaceous, argillaceous and carbonate sediments and some intercalated volcanics)

.

394

Detailed

lithofacies

mapping of the khondalites

has

been possible during the programme of mapping on the 1:63,360. Outcrops are few except on hill crests cuttings.

Similarly,

the

contact between the

nowhere scale of and road

charnockites

and

khondalites is obscure; nowhere is any acid or intermediate charnockite seen to be cutting through the metasediments of the khondalite group. The original discordance, i f any, has possibly been

transposed by repeated folding.

The alternative

possibilty

that the charnockites were the basement over which the Khondalite group sediments accumulated, is rather thin. Quartzites are the dominant facies of the khondalite group in many areas of the Eastern

Ghats

(North);

but

these d o

not

carry

any

rutile,a

characteristic accessory mineral of the charnockites; nor d o the khondalites include any basal or intraformational conglomerate with pebbles or boulders of charnockite. The bulk chemistry of khondalite S . S . (quartz t garnet t sillimanite t graphite) is unlike any modern magmatic rocks. In fact, Walker (1902) had commented upon its possible sedimentary analogue, a well-leached clay, mixed calc-silicate

with a little quartz. The association granulites and manganiferous quartzites is

with also

suggestive of a sedimentary parentage. The

charnockite

suite ranges from

alaskite/leucogranodiorite

through hypersthene granite, enderbite (with antiperthitic plagioclase and mesoperthitic microperthite), quartz syenite and diorite t o hypersthene granulite, the basic member frequently as xenolithic enclaves in acid charnockite.

occurring The basic

charnockites also occur as small bodies within khondalites; but the abundance of the basic enclaves is extremely erratic. It may be more than a coincidence that the acid charnockites in both the northern and southern Eastern Ghats appear to have whole-rock Rb-Sr dates of around 2695 Ma. (Perraju et al., 1979) to 2600 Ma. (Vinogradov et al., 1864) and compare well with the Rb/Sr isochrons of 2500 8 0 Ma. for the Madras city and Nilgiri Hills charnockites (Crawford., 1969). Recent detexminations of whole rock Rb-Sr isochrons and lower intercept data of U-Pb in zircon from the charnockite of Kabbaldurga, Karnataka (Haith et al., 1988) are also in conformity with a n estimate of about 2500 Ma. as a major phase of charnockitization of diverse types of hypozonal plutonic assemblages near the Archaean-Proterozoic boundary.

39 5

The intrusive/infiltrative nature of the granites migmatites towards both charnockites and khondalites is

and well

displayed in various outcrops and road cuttings in Sambalpur, Dhenkanal, Ganjam and other districts. All stages of conversion of khondalite S . S . (Banerji, 1982b) into migmatites are exposed lending credence to the concept of anatectic derivation of some of these migmatites from pelitic assemblages (Bose and Sanyal, 1987). Anastomosing veins of leucogranite in acid charnockites (quartz t K-feldspar noted.

plagioclase

t

t

orthopyroxene) are

also

frequently

But the crucial issue, which has s o far baffled the geologist mapping hundreds of square kilometers every year (mostly on the basis problem

of

of migmatites.

observations

is

the

stratigraphic ordering of the different granites Unlike the surrounding greenstone terranes there

of outcrops and hand specimens),

and are

no easily recognisable (and mappable) cycles of sedimentary and/or volcanic sequences starting with unconformities marking the onset of the different stratigraphic sequences. For example, around the Chilka Lake of Orissa, there are khondalites and a garnetiferous granite

gneiss, which is intruded by anorthosite (Rb/Sr

isochron

age 1404 2 89 Ma.). Besides there is a garnetiferous quartz mangerite bearing subangular inclusions of anorthosites (Sarkar et al., 1981). One has to be very careful to decipher the subtleties contained within such "similar looking" rocks. To cite another example, south of Phulbani town at the heart of the E. Ghats province, the charnockites are retrograded to garnetiferous, amphiboleand biotite-bearing rocks by tectono-metamorphic differentiation faintly

(Kamenev, 1982) and later metasomatism. They show

pleochroic

orthopyroxene with rims either of

aggregates of garnet chlorite. These rocks

or of uralite, were intruded by

dactylitic

red-brown biotite and a porphyritic granite

gneiss (quartz t microperthite + garnet t brown biotite t zircon). Careful Pb-Pb dating of zircon, apparently infiltrated into these charnockites, yielded a good isochron of about 800 Ma. (D.K.Pau1, Pers. Comm.). Without the benefit of such isotope geochronologic signposts, the granites associated with these charnockites could hardly be suspected to be upper Proterozoic. In fact, I had sampled these rocks with the Proterozoic to Archaean dates.

hope

of

getting

some

Early

396

Such Perraju

upper

Proterozoic

granites have also been

reported

by

et al., (1979) from the Srikakulam area of Andhra Pradesh

on the basis of Rb-Sr dating. Such repeated stratigraphically availability

granite emplacements, many of which cannot classified in space (at the present level

and speed of dependable isotope geochronologic

be of

data

in India) have also been a source of considerable uncertainty in tectonic and metallogenic modelling. A regional-scale understanding of the controls and geometry of magmatism in space and time is lacking.

Hundreds of careful geochronologic measurements will before the granitic suites of this area can be

needed

be

correlated

and arranged sequentially over the whole region. STRUCTURAL FEATURES geologist engaged in systematic mapping is equally in delineating regional fold forms. There are no

The capped

bands within manganiferous

the khondalite group except for calc silicates and zones which occur only at a few localities as small

The charnockites are massive looking. On top of it, there

strips.

is

handimarker

a thick lateritic cover at many places. Consequently, structu-

ral analysis is on either mesoscopic and microscopic scale or on the scale of satellite imagery (1:250,000). Lineaments identified in black and white Landsat imageries of Band 5 and I are quite abundant

in

this tract. These are generally oriented

in

NE-SW,

NW-SE and E-W directions. Many of these are possibly expressions of regional-scale fold trends, which appear on mesoscopic scale as tightly rocks

appressed,

isoclinal and coaxially refolded

systems

in

of the khondalite group and in the migmatites. The dominant

foliation is axial planar to these isoclinal folds and is also parallel/subparallel to the axial planes of intrafolial folds, which are generally believed to be F1, e.g., near Angul (Halden et al., 1982). Other open fold forms are also noticed around plutonic emplacements,

e.g.,

of

anorthosites

and

porphyritic

granites

around Chilka Lake, Mandibisi (Koraput), Kottauru (Visakhapatnam), etc. Elsewhere, these open fold forms are subdued and do not show up in Landsat imageries, although in the field, they appear to be quite pervasive. The porphyritic granites are intermittently characterised on mesoscopic scale by swirls of feldspar porphyroblasts sub-horizontally oriented, indicating viscous, lateral flowage of the rock

39 7

mass

during

semisolid emplacement. It would be naive

to

expect

that all these porphyritic granitoids with such flowage features are of the same age ( U . Proterozoic). But no further stratigraphic refinement is possible with the available data. Besides

the

lineaments, along

fold-parallel

lineaments,

there

are

other

which appear to cut across the regional folds,

e.g.,

the Tel, the Vansadhara, etc. These appear to define

major

fracture/fault zones (Prudhivi Raju and Vaidyanadhan, 1981). Discrete mylonite zones, parallel to such features, have been mapped locally (e.g., P.PerraJu in Puri district, Orissa). The northern, western and southern margins of the Eastern Ghats (North) are intruded by granophyres, K-granites, alkaline syenites, gabbros and anorthosites, ultramafic rocks, etc., and carry

distinct and overlapping dislocation zones. The "so-called"

Mahanadi

trend

is a part of this domain, wrapped around

by

the

greenstone-granite provinces of the Gorumahisani and Bengpal groups. The western margin was described by Crookshank (19381, who noted albitization and crushing. Road cuttings to the power house of the upper clearly.

Kolab dam, near Jeypore, expose

this

crush

zone

The northern margin of the Eastern Ghats (North) is characterised by impersistent blastomylonites and crush zones. There is a

motley crowd of specialised facies in this area: K-granites and

pegmatites (Dhenkanal

with beryl (Sambalpur ), district),

chromiferous ultramaf ic masses

impersistent

sulphide

ores

(Adas),

magnetite-bearing granophyre (Sukinda), etc. This sector has recently been described by Banerji et al., (1987). Early Proterozoic rifting followed by collision appcars to have dominated the growth of this remobilized sector, prompting this author to compare a part of its evolution with the northern Red Sea rift (Banerji and Chatterjee, 1988). The southern margin is defined by the Godavari graben which has been a depocenter for upper Proterozoic, Gondwana and Meso-Cenozoic sediments beginning with the Pakhal Basin (Rao, 1987). Repeated reactivation on diversely oriented surfaces, locally orthogonal to the graben margins, has been recorded (Sastri, 1980; Rao, 1987). Recent

offshore

wells of Oil India, off the

Mahanadi

delta,

39 8

show that rocks of this province continue below the continental shelf sediments of the Bay of Bengal. Late Mesozoic to Cenozoic reactivation is attested to by a network of faults sequences of volcanic rocks (Baishya et al., 1986). In

totality,

appears

therefore,

the Eastern Ghats

and

(North)

thick

province

to be a prominent horst, bounded on all sides by a

large

number of faults. Ductile to brittle state dislocations along these faults of various ages were superposed on polycyclic ductile state folding movements, accompanying magmatic and metamorphicmetasomatic processes. METAMORPHISM The like

khondalites and migmatites bear high-grade index sillimanite,

kyanite and cordierite and reflect,

minerals by

their

different textural characters, cycles of high PT (Banerji, 1982b; Dash, 1982). Inclusions of spinel, sapphirine and sillimanite in garnet, and sillimanite and sapphirine rims around spinel are locally observed. Other index associations viz., hypersthene t sillimanite,

cordierite t garnet t sillimanite t biotite t quartz

ilmenite k spinel 2 plagioclase corundum anothophyllite hypersthene, sapphirine t sillimanite t sapphirine t spinel t

sillimanite, reflect

kornerupine

high

temperature

t spinel, wollastonite

and high-to

+

calcite,

medium-pressure

t

etc.

environ-

ments. In the charnockitic suite, partial alteration of the pyroxenes to hornblende and/or to biotite, the presence of dactylitic rims of garnet around pyroxenes, and symplectitic intergrowth of felspars suggest retrogression to hornblende granulite/almandine

amphibolite facies. This retrogression was of

varying intensity from place to place. Consequent has map

upon such repeated metamorphism and deformation, it

not been possible to prepare an accurate metamorphic facies of this terrain either. In south-west Orissa and north Andhra

Pradesh, comprising the districts of Koraput, Visakhapatnam Srikakulam and Prakasam, the higher grade assemblages are more abundant and the retrograded variants occupy less area than in the northern

parts

viz.,

the districts of Ganjam, the Kalahandi, Bolangir, Sambalpur, Dhenkanal, Puri and Cuttack. It may be more than a concidence that the latter are more closely traversed by a network of fracture lineaments and rectangular drainage courses.

399

ALKALINE AND MAFIC-ULTRAMAFIC MAGMATISM ALONG THE MARGINS The

present

boundary of the Eastern Ghats (North) provides

a

classic igneous

example of recurrent deep-seated tectonism and related activity. A number of alkaline and mafic to ultramafic

igneous western

masses have been identified along the northern and the reactivated margins with characteristic ore deposits of

Cr,

Ni,

Cu, Zr, apatite, REE, etc. But these intrusive masses the margins of the Eastern Ghats (North) province viz., the

along

ultramafic masses around Asurbandh and Moulabhanj Parbat, the Bolangir and Kalahandi anorthosites, Koraput syenite, Borra

(71, Cheruvakonda olivine syenite, etc., did not suffer any high

syenite-carbonatite Kunavaram metamorphic

alteration

or

any

strong

regional

gabbro-norite, granulite-grade scale

folding

movement after their emplacement within the charnockite tract, although in selected sectors (e.g., near Moulabhanj Parbat, Orissa), they were intruded by granophyric rocks. This habitat is in sharp contrast to similar emplacements within the Eastern Ghats (South) tract, south of the Krishna river. Recent studies by this author (Banerji, in press) suggest that the Kondapalle chromiferous orthopyroxenite, norite and anorthosite folding

in the Eastern Ghats (South) province suffered strong

movements. In the Rama quarry of Kondapalle (sunk by

the

Ferro- Alloys Corporation), chromitite-silicate interbands show both tightly appressed, reclined folds as well as open and upright folds. Further south in the Sittampundi field also, the chromiferous pyroxenites and norites (retrograded) have suffered intense folding and brecciation and occur as isolated rafts with a style of folding and shearing different from the tightly folded and boud inaged host anorthosite, emplaced within garnet-hypersthene

granulite and garnet-corundum diorite

gneiss.

Partial retrogression of all these rocks is witnessed by the sporadic formation of large prismatic crystals of zoisite-epidote within Sittampundi anorthosite, e.g., near Nallkavundan-Palliyan 78O02'E: 581, development of hornblende with release of (11°19'N: opaque granules from garnet in the garnet-hypersthene granulite, development of amphiboles, chlorite and magnetite in the chromitite suite, etc. The geological habitat of the Sittampundi mass appears to be similar to the Oddanchatram anorthosite (Wiebe and

Janardhan,

1988)

of

the

Tamilnadu

and

the

Kurihundi

400

anorthosites of Karnataka (Ramakrishnan et al., 1978) although this author is in complete agreement with Ashwal (1988) that the similarity among these anorthosites does not imply ,la distinct anorthosite

event".

A

comparative

geological

and

geochemical

re-study of these anorthosite masses of South lndia associated with granulites, granitic gneisses and greenstones might be useful in unravelling their similarities and contrasts with East lndian (Chilka, Bankura, Kalahandi, Bolangir, Nausahi, Kumardubi, etc.) anorthosites and related rocks. CONTRASTS BETWEEN EASTERN GHATS (NORTH) AND EASTERN GHATS (SOUTH) The contrast in the time and level of emplacement of chromite-rich maf ic-ultramaf ic bodies vis-a-vis folding movements between the

Eastern Ghats (North) and the Eastern Ghats (South) provinces

is also reflected in the overall rock assemblages.

the

From Vijaywada south and southwestward up to the Nilgiri hills, khondalite-charnockite association of the Eastern Ghats

(South) possibly reflects a more complex and initially higher metamorphic PT than the rocks of the Eastern Ghats (North). Dhanaraju's

for

(1977) estimate of 600'

-

7OO0C and a Pload of 6-7 Kb

the temperature of crystallisation of orthopyroxene in

rocks

of the Chipurupalle-Razam area, together with the recent work of Murthy and Nirmal Charan (1988) on the cordierite gneisses near Guntur, suggest a general range of 600° - ,750' and Plead of 6 - I Kb for metamorphism of the rocks of the Eastern Ghats (north). estimates

- 84OoC and a

Plead

of 9 - 10 Kb for the Madras granulites by Weaver et al., (1978) and of 700' - 920°C and a of 8 . 8 - 11.3 Kb for the Oddanchatram anorthosites and However,

of 720'

Plead

charnockites

(Wiebe

and

Janardhan, 1988) suggest a

higher

P,T

environment preserved in some assemblages, although the later cycle of charnockite formation around Kabbaldurga at 75OoC and Plead of 5.5 Kb is generally believed to be a shallower event (Raith et al., 1988), possibly related to localised higher geothermal gradients. The relatively lower and simpler P-T history of the northern sector is also borne out by the three following observations: 1. The

transitional

nature

of the contact between

the

Eastern

Ghats gneisses and the Bonai granite off Riamal, Sambalpur district, (Banerji et al., 1987); not far from the contact with

40 1

the

Eastern

Ghats

the

Bonai

qranite

bears

enclaves

of

khondalites, the latter infiltrated by K-granite and granophyre at the margin. 2. The absence of any field evidence in the E. Ghats (North) suggesting development of charnockites from granites, unlike such evidence in the southern sector, e.g., from the Kabbal and Krishnagiri areas (Pichamuthu, 1961; Ramiengar et al., 1978; Janardhan et al., 1982). This contrast suggests that the metamorphic cycles following various episodes of granite emplacement in the Eastern Ghats (North) did not again reach appropriate P,T,X for their conversion into charnockites. 3. The khondalite group of rocks is much less abundant in the Eastern Ghats (South), where high grade charnockites and gneisses dominate. The the

Krishna-Godavari graben, lying in between the northern and southern

provinces,

bears

clastic

sediments

from

(U.

Proterozoic onwards). Repeated reactivation o f this graben appears to

have begun in late Proterozoic times and the metamorphic event

within

Pakhal sequence o f this graben could

the

near-isochronous Ghats.

with

the

conceivably

young peqmatites across

the

be

Eastern

MINERAL DEPOSITS IN THE EASTERN GHATS Compared

to

the B.I.F. provinces to its north and west,

this

terrain is comparatively poor in known mineral resources. This may partly be due to incomplete investigation. Dense vegetation and abundant colluvium on the hill slopes and fairly thick alluvial or laterite cover in the plains and plateaus render a visual search for mineral deposits in the course of systematic mapping an accidental event; certain significant indications of a higher mineral potential are obtained now and then, e.g.: ( 1 ) the discovery

of

a large graphite deposit at Tumudibandh,

Kalahandi

district, Orissa by boulder trains in a first-order nullah in a densely forested, almost inaccessible part of the Eastern Ghats (North) during the late forties/early fifties and ( 2 ) the discovery of a small copper sulphide deposit at Adas, Yambalpur district,

Orissa,

in the early eighties (Banerji et

al.,

1988)

prompted by an accidental misinterpretation of a laterite saprolite as gossan in the seventies. Perceptive, concept-oriented geophysical

and geochemical surveys in this tract would

probably

40 2

lead to new mineral discoveries. An airborne survey has been undertaken over a part of this area. It is hoped that ground follow-up geophysical and geochemical search will prove rewarding. Economically important mineral occurrences s o far known include high- to medium-grade graphite (Loth flaky and massive varieties), low-

to

medium-grade

and

generally

high-phosphorus

(due

to

apatite) manganese ore (mainly pyrolusite and psilomelane), local enrichments of apatite-magnetite and sporadic occurrences of semi-precious

emerald,

bery1,zircon and garnet. There

are

also

reports of sapphire. The graphite deposits occur a s bedded to massive deposits grey quartzites of the khondalite group, e.g., in Tumudibandh Orissa

(which

concentrated

is

the largest single deposit in the

palaeosomes

from

graphite-rich

State);

schists

of

i in as the

Khondalite group in migmatite terranes, e.g., in Bolangir and Sambalpur districts; and as clots of pure, coarsely flaky graphite in pegmatites. The graphite reserves are huge, although large single deposits are exceptions. At a few places, graphite deposits are

associated

with sulphides, e.g., at Adas, Sambalpur

dt.

In

relatively flat terrains with extensive soil/laterite/alluvium cover, ground geophysical exploration has proved useful in Bolangir district and elsewhere. The small (1-2 million tonnes) manganese-ore deposits of Visakhapatnam, Srikakulam, Vizianagaram and Koraput districts have been explored and studied in detail by GSI officers. According to Krishna

Rao

et

al.,

(19821,

manganese

silicate,

oxide

and

carbonate minerals form the protores in the A.P. sector and form a part of the khondalite group. The deposits occur within banded quartzites and at the contact of calc-granulite and quartzite. In the Koraput Dt, the ore bodies are associated with ferruginous and cherty quartzites. On supergene alteration these have given to economic ore bodies.

rise

The precious and semiprecious stones (aquamarine, yellow beryl, amethyst, sapphire, etc) are mostly associated with aegirineaugite/nepheline syenites and zoned pegmatites along the western and northwestern reactivated border of the Eastern Ghats. Fresh and locally large garnet crystals are common in some granites. The fluor-apatite and magnetite-bearing veins in Kasipatnam, southeast of the Borra pyroxenite-syenite-carbonatite(?)

occurence in Andhra

403

Pradesh, appear to lie in extension of the reactivated fringe. The currently disputed (Rao and Narendra, 1986) Borra carbonatite ( ? ) carries bastnaesite - a REE-rich mineral - in small quantities (Ramam and Viswanathan, 1977). The deposit is high grade (P205

:

"39%) but erratic. Besides these, local (e.g., near Palasama in Sambalpur district) low-grade (40-70%) sillimanite deposits have been located by GSI within sillimanite-quartz schists, with or without garnet

and

Eastern

magnetite.

Ghats

The northern reactivated

frinqe

also bears small auriferous pyrite -

of

the

chalcopyrite

ore bodies in garnetiferous amphibolites at Adas, Sambalpur district (Banerji et al., 19881, and pods of high-to medium-grade chromite

associated

with serpentinites, e.q., at

Asurbandh

and

Maulabhanj Parbat (Banerji, 19'12). The most important mineral deposit in this tract is, however, an exogenetic blanket of red and brown bauxite, popularly known as the East Coast Bauxite deposits. This is associated with the highest

planation

surface,

which

shows up as

large

to

small

mesa-type plateaus in the western and southern part of the granulite tract. The deposits are essentially made up of gibbsite, haematite

and

geothite and are characterised by very low

silica

and titanium (Rao and Ramam, 1979). CONCLUSION The Eastern Ghats rock assemblages, north graben, record a complex sequence Godvar i magmatism

of of

the Krishnametamorphism,

and deformation. Isotope geochronologic measurements on

these rocks span almost the whole of the Proterozoic and possibly a part of the Archaean era. The available data are, however, inadequate and some are equivocal. But the absence of sedimentary and/or

volcanic sequences, alternating with the various cycles of

granite emplacement, is an observable fact in the field and so is the suspected contrast in metamorphic history between the blocks, north and south of the Krishna-Godavari graben. It appears that the khondalite group represents an Archaean clastic (and minor chemogenic)

sedimentary pile, which was dragged down to depths of

approx. 2 0 - 3 0 km from the surface by intense compressive stress reaching below or near the Conrad discontinuity. Subsequently, uplift of these rocks to surface levels did not upwarp the corresponding

M

discontinuity,

as is revealed by

extensive

negative

404

Bouguer gravity anomalies below the Eastern Ghats. This feature is comparable

to the gravity anomaly pattern in the granulites

from

Enderby Land, Antartica (Wellman and l'ingey, 1977). explanation for this feature (Banerji, 1982a) could underpinning

by

lateral

flow of

acidic

material

One be

accompanying

uplift of this early Proterozoic lower crust, althouqh what forces caused such underpinning, possibly in stages, and what relation, any,

if

this

mass

flowage

surrounding

greenstone

sedimentary

basins

had with

the

development

provinces and subsequent

of

development

(Cuddapah-style) are still in the

domain

the

of of

speculation. Alternatively, one has to agree with the interpretation of Wiebe and Janardhan (1988) and Leelanandam et al.,

(1988) that these granulites represent the lower parts of

Himalaya-type

a

collision zone, which attained an estimated crustal

thickness of about 75 Km during the early- to mid-Proterozoic era. In to

order to choose between these alternatives, it is necessary

map these rocks in detail with proper isotope

petrologic

and

geochronologic,

geochemical back up. This will provide us with

a

dynamic view of the evolution of the deeper crust and should prove to be of interest for the International Lithosphere Project. In an era when the superdeep borehole at Kola Peninsula has intersected granulites at depths close to the Conrad

already discon-

tinuity, the Eastern Ghats at our doorstep could provide an equally interesting 3D X T view, if a proper work proqramme could be executed. Needless to mention that such a programme, forming second-generation mapping along selected transects on a scale of 1:10,000

(10

Km- wide approx.) needs to have larqe-scale

inputs

from isotope geochronology and geochemistry, not to mention refraction seismic profiling, which together with airborne magnetic and regional gravity data will provide adequate constraints in modelling the evolution of the Eastern Ghats. ACKNOWLEDGEMENTS The S.K.

is published with the approval of the Director Geological Survey of India. I am also obliged to Shri.

paper

General,

Bandopadhyay

manuscript.

of

Wing

Hqrs., for repeated

typing

of

the

405

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