Comparison of the Indian and Nubian-Arabian Shields

COMPARISON OF THE INDIAN AND NUBIAN-ARABIAN SHIELDS JOHN J . W .

ROGERS

ABSTRACT The

Indian

(IND) and

Nubian-Arabian (NAS) shields

have

a

similar outcrop area but are otherwise quite different. Precambrian rocks of IND evolved over a period of nearly 3000 Ma., whereas virtually all of the rocks of the NAS were formed between 1000 and 500 Ma. ago. Lithologic suites of the NAS are principally alkaline

volcanic/sedimentary

batholiths,

subduction complexes

and

and most of the older suites of the

calc shield

in or near intra-oceanic arcs. The earliest rocks of the IND are mantle-derived tonalitic-trondhjemitic gneisses and mafic/ ultramafic volcanic/sedimentary belts. Recognizable belts of formed

compressional deformation characterize the middle Proterozoic of IND, and some of them resulted in production of large granulitefacies terrains. The IND does not generally contain identifiable volcanic/sedimentary subduction occurred assemblage

of

rocks.

or in

calc-alkaline subduction suites, and if the older IND, it produced a different

Ophiolite/melange suture zones are

easily

recognized in the NAS, but possible sutures in the IND must be inferred from thrust faults separating different lithologic terrains.

Both

IND and NAS are similar in their

later

tectonic

development, including: production of alkali granites; subsidence of thick, partly deformed basins on recently formed crust; and ultimate development of platformal cover sequences of clastic sediments. The greater depth of exposure of the IND, containing exposed granulite, does not explain the lithologic difference from the NAS. Both seismic and gravity data indicate that the NAS contains denser, more mafic rocks than the IND at all levels throughout the crust.

It

is possible that future, post-stabilization

processes

will change the composition of the NAS to that of the IND, but the hypothesis is not testable with present data.

224

INTRODUCTION This and

paper

compares the general features of the Indian

Nubian-Arabian

(NAS)

shields,

emphasizing

both

(IND) the

similarities and contrast in their development. The NAS consists of a Nubian part, west of the present Red Sea, and an Arabian part, to the east.

A

500 KM

bells

rochi

Figure 1. Generalized map of the Indian shield. The mapped Precambrian suites correspond broadly to the suites listed in Table 1, although some of the rocks counted f o r Table 1 cannot be shown at the scale of this figure. The nomenclature of the various areas and suites corresponds to maps in Naqvi and Hoqers, ( 1 9 8 7 ) . Symbols used for Proterozoic supracrustal rocks in orogenic belts include; 1 - Delhi Supergroup; 2 - Singhbhum orogenic belt; and 3 - Sausar, Sakoli, and Dongargarh suites of the Bhandara craton. Granulite terrains include: CH - Chotanagpur; EG - Eastern Ghats; and SO - Southern granulite terrain, separated from the Dharwar craton (ED and WD) by a non-tectonic transition zone. Archaean gneissic terrains with infolded supracrustal rocks include: AR Aravalli; BU - Bundelkhand; SI - Singhbhum; BH - Bhandara; ED Eastern Dharwar; WD - Western Dharwar. Rift valleys include; N Narmada; S - Son; M - Mahanadi; and G - Godavari. Major thrusts include: 1 - unnamed thrust in Western Dhrwar craton, 2 - Eastern Ghats front (numbered at two places); 3 - Sukinda; 4 - Singhbhum (Copper Belt); 5 - thrust south of Son valley; and 6 - Great Boundary fault. The map does not show platformal sedimentary (cover) sequences, principally middle to late Proterozoic but p o s sibly ranging from the late Archaean to the lower Paleozoic.

225

Both the Indian (Fig. 1 ) and Nubian-Arabian shields (Fig. 2 ) are fragments of larger terrains. The Indian shield rifted trom Gondwanaland, and an unknown extent has been subducted under the Himalayas. The Nubian-Arabian shield apparently has an exposed western border aqainst older rocks in North Africa, although the exact location of the contact is controversial (Kies et a1.,1985; Vail, 1985; Kroner et al., 1987a, 1987b). Precambrian

500 kilometers 1

I

Figure 2 . Generalized map of Nubian-Arabian shield. The Red Sea has been closed by removal of 105 km left-lateral displacement along the Aqaba-Dead Sea shear zone and 6 degrees of counterclockwise rotation of Africa. The Arabian shield is shown in its present north-south orientation. Major suture zones are named following the terminology of Johnson et al. ( 1 9 8 7 ) for the Arabian shield and Vail (1985) for the Nubian shield; symbols are: 1 - Ar Rayn; 2 - Haliban; 3 - Nabitah (numbered at three places) ; 4 Yanbu; 5 - Bir Umq; 6 - Afaf; 7 - Sol Hamid; 8 - Nakasib; 9 Baraka. Possible correlations of the shear zones across the Red Sea are shown by question marks. The major terrains o f the Arabian shield are named following the terminology of Johnson et al. ( 1 9 8 7 ) ; symbols are: A -. Midyan; B .- Hijaz; C - Jiddah/Taif; D Asir; E - Afif; F - Ad Dawadimi; G - Ar Rayn; H - Nabalah/ Najran, Fault "a" shows the present location of the Tertiary Aqaba-Dead Sea left-lateral shear zone; fault "b" is shown as two of the many segments of the left-lateral Najd fault system of late Proterozoic aqe. Volcanic and sedimentary suites in successor basins and crustal downwarp basins (terminology of Johnson et al., 1 9 8 7 ) are not shown.

226

outcrops

in

northeastern

reconnaissance sion

of

the

seaboard

of

Africa extend south t o

the

Ethiopian

b u t t h e s o u t h e r n areas have been e x p l o r e d o n l y i n

plateau basalts,

s e e Mohr, 1979), a n d t h e y may b e a n

(e.g.,

Mozambique b e l t t h a t o c c u p i e s much o f Africa.

The

northern

and

eastern

the sides

exteneastern of

the

Nubian-Arabian s h i e l d are covered by t h i c k Phanerozoic s e d i m e n t a r y sequences. have

Remarkably,

nearly

b o t h t h e I n d i a n and Nubian-Arabian s h i e l d s

area

same

the

of

present

surface

exposure,

a p p r o x i m a t e l y one m i l l i o n s q . km. The

geophysical

shields depths Kaila

1)

p r o p e r t i e s of t h e I n d i a n

show s i g n i f i c a n t d i f f e r e n c e s ,

Nubian-Arabian MOHO

A d e e p seismic s u r v e y by

a r e 35 t o 40 km i n b o t h s h i e l d s . e t al,

and

despite the fact that

(1979) a c r o s s t h e D h a r w a r c r a t o n (ED a n d WD i n

Fig.

o f t h e I n d i a n s h i e l d y i e l d e d a v e r a g e v e l o c i t y v a r i a t i o n s shown

i n Fig.

3.

teleseismic 1987).

These v e l o c i t i e s are c o n s i s t e n t w i t h measurements from data

( s u m m a r i z e d i n C h a p t e r 1 o f Naqvi

Unfortunately,

granulite differences

terrain

(SO

between

it

the in and

profile

did

l),

Fig.

and

not

and

extend

possible

t h e Dharwar c r a t o n

have

Rogers, into

the

geophysical not

been

determined. P-WAVE VELOCITY 0

5

6

IN

7

KM/SEC 0

10

5 z

20

1

t W

n

30

40

F i g u r e 3 . C o m p a r i s o n of P-wave v e l o c i t i e s i n : t h e D h a r w a r c r a t o n of the I n d i a n s h i e l d ; e a s t e r n p a r t of t h e Nubian-Arabian shield ( A f i f t e r r a i n ) ; a n d w e s t e r n p a r t of t h e A r a b i a n p o r t i o n of the Nubian-Arabian shield. I n d i a n d a t a f r o m Kaila e t a l . (1979); A r a b i a n d a t a f r o m Mooney e t a l . (1985) a n d G e t t i n g s e t al. (1986).

227

A

refraction

survey

across

the

Arabian

portion

of

Nubian-Arabian shield (Fig. 2 ) produced different profiles in the Afif region, possibly underlain by a crust, Mooney

the

velocity thin old

and in the more mafic western parts of the shield (Fig. 3 ; et al., 1985; Gettings et al., 1986). Particularly in the

west, the NAS shows higher velocities (higher crustal densities) throughout the entire thickness of the shield down to the MOHO. These

high

below)

velocities confirm geologic

observations

(discussed

that the western part of the NAS consists almost wholly of

late Proterozoic, subduction-generated, supracrustal batholithic suites formed originally on oceanic crust. Gravity the

and

data (Bowin et al., 1981) confirm the seismic data for

Indian and Nubian-Arabian shields.

On average, the NAS shows

free-air anomalies in the range of 0 to t25 mgal in western Arabia and the Nubian part of the shield. Free-air gravity anomalies in the IND are in the range of 0 to -20 mgal for most of the unrifted parts

of

the

shield,

with the granulite area (SO

in

Fiq.

1)

anomalies of approximately - 2 5 to -50 mgal. Thus, both and gravity data indicate a denser crust in much of the

showing seismic

Nubian-Arabian shield than in the Indian shield. Average

depths

of

erosion obviously vary enormously

in

the

shields. In the Nubian-Arabian shield, however, the maximum metamorphic grade of any rock suite is amphibolite facies, mostly in the area of major batholiths. A large proportion of the NAS rocks

in

their

shield are in greenschist

facies.

Greenschist-

-facies rocks occur in the Indian shield but they are primarily in cover suites not shown in Fig. 1. Conversely, large areas of the IND contain granulite-facies rocks, in some of which equilibration pressures

of 6 to 10 kb have been measured (summary in Naqvi

and

Rogers, 1987). Thus, exposure depths in the Indian shield are probably in the range of 5 to 25 km, whereas most rocks exposed in the Nubian-Arabian shield probably were never buried deeper than 10 to 15 km. The

most

Nubian-Arabian Indian shield

striking

difference

between

the

Indian

shields is the ranqe of ages of formation. contains abundant areas o t Archaean gneisses

and The and

associated, generally high-grade, supracrustal rocks (Radhakrishna and Naqvi, 1986), and ages as high as 3400 distributed (summary in Naqvi and Rogers, 1987).

Ma. are widely Platformal cover

228

sequences, deformed only on some margins, began to form over broad areas of the shield in the middle Proterozoic, and limited evidence that

on

the

the ages of orogenic activity and rifting

indicates

Indian shield had become a unified, relatively

stable,

block by about 1500 Ma. ago. Conversely, the Nubian-Arabian shield shows largely indirect P b and Nd isotopic evidence for rocks as old as Archaean or early Proterozoic (Stacey and Stoeser, 1983; Stacey and Hedge, 1984).

Rocks of this age apparently occur

at depth in the eastern part of the shield, and one exposed body in the Afif terrain has an age of 1600 to 1700 Ma. (Stacey and Hedge, 1984). All measured ages in the Nubian-Arabian shield west of the Afif terrain are in the range of 1000 to 500 Ma., both for supracrustal and batholithic suites (Jackson and Hamsay, 1980; Marzouki et al., 1982; Klemenic, 1985; Stern and Hedge, 1985; Stoeser and Camp, 1985). Undeformed cover sediments began to form in the early Phanerozoic in the NAS. The abundances of various major rock suites in the Indian and Nubian-Arabian shields are summarized in Table 1. Each shield is discussed briefly below before further comparison is made. LITHOLOGIES IN THE INDIAN SHIELD The major lithologic suites of the Indian shield are identified in Fig. 1 and Table 1. An extensive bibliography f o r the following discussion was provided by Naqvi and Rogers (198'7), and only a few summary papers are cited here. Old Archaean gneissic complexes, commonly with intricately infolded mafic supracrustal suites, have been recognized in at least four parts of the Indian shield - the Dharwar craton (Pichamuthu

and

S r inivasan,

1983; Radhakrishna 1983); the Singhbhum nucleus (Sarkar, 1982); parts of the Aravalli-Delhi area (Naha and Roy, 1983; Sen, 198.3); and probably the Bundelkhand area (Sharma, 1983). In addition, it seems likely that the Bhandara region contains an extensive Archaean craton (Radhakrishna and Naqvi,

1986).

The gneisses are commonly in

amphibolite

facies

and consist of the typical tonalite-trondhjemite ("gray gneiss") suite found in many shields. Most of them appear originally to have been mantle-derived magmatic rocks. The mafic

supracrustal suites infolded with the gneisses are to

ultramafic,

consisting

of

komatiites

to

mostly basalts,

229

quartz-poor metasediments, some silicic volcanic rocks (forming a bimodal igneous assemblage), and minor chert, carbonate and other sediments. later

These assemblages have ages from at least 3,400 Ma. to

in the Archaean.

An exception to the mafic composition

is

shown by the Aravalli Supergroup of the Aravalli area, which is siliceous, phosphatic, and at least partly platformal (Roy and Paliwal,

1981).

because of Complex.

'The Aravalli suite is included in this

its intimate intermingling with the

Banded

category Gneissic

Three broad areas of granulite-iacies rocks are shown on Figure All

1.

of

them

(charnockites,

contain

khondalites,

a

mixture

of

high-grade

mafic granulites, etc.)

-grade suites, primarily in amphibolite facies.

and

suites lower-

Both prograde and

retrograde relationships have been shown between these facies (e.g., Janardhan et al., 1982; Chacko et al., 198'7). 'The dominant composition of the areas is silicic, with mafic rocks forming less than 10% of the assemblages. The southern granulite area is commonly regarded as an extension of the Dharwar craton, difiering largely in level of exposure, and the contact between the Dharwar craton

and

tectonic

the

granulite

discontinuity

area is a

formed

about 2 5 0 0 Ma.

1960; Gopalakrishna et al., 1986). The region lithologically similar to the separated

gradational ago

zone

without

(Pichamuthu,

Eastern Ghats is a broad southern granulites but

from amphibolite-facies Archaean rocks on the west by a

major thrust (the Eastern Ghats front; Crookshank, 1938; Kaila and Bhatia, 1981). The poorly known Chotanaspur terrain (Ghose, 1 9 6 3 ) also contains abundant granulite-facies rocks and is separated on the south from the Singhbhum nucleus by the Singhbhum mobile belt and associated thrusts. Movement on both the Eastern Ghats front and the Singhbhum thrust appears to have occurred in the middle Proterozoic (summary in Rogers, 1986), and the metamorphism in these two terrains may have corresponding ages (see summary geochronologic information for the Eastern Ghats in Chapter 5

of of

Naqvi and Rogers, 1987). The granulite terrains contain numerous anorthositic suites, which are apparently related to the granulite met amo r ph ism

.

Highly occur belt

in

deformed

suites of early to middle

at least three places.

Rocks of the

Proterozoic Singhbhum

rocks mobile

are a typical flysch (geosynclinal) suite compressed between

230

the Chotanagpur area and Singhbhum nucleus (Sarkar, 1982); ophiolites have been proposed to occur as part of the melange in the belt. The Delhi suite, which may extend into the late Proterozoic,

contains

a more platformal

sedimentary

assemblage

(Sant et al., 1980; Singh, 1982). Deformation may have been related to movement on the Great Boundary fault and formation o f granulitic rocks in other parts of the Aravalli belt. Several suites of mostly platformal sediments in the Bhandara area have been deformed and metamorphosed, particularly along the Satpura trend south of the Narmada and Son rifts (Narayanaswamy et al., 1963). Identifiable

batholithic

suites showing

calcalkaline

trends

from gabbro to granite apparently are scarce in the Indian shield. Late Archaean/early Proterozoic potassic granites, generally without cogenetic mafic rocks, are common in several areas, particularly

in the eastern part of the Dharwar craton

(Closepet

and related suites). The middle Proterozoic thrusting (subduction related?) does not seem to have produced calc alkaline batholiths. Platformal numerous relatively

sediments

(not shown in Fig. 1 ) began to

places in the shield a s the underlying basements stable

(e.g.,

Chanda

and

Bhattacharyya,

form

at

became 1982;

Srivastava et al., 1983; Meijerink et al., 1984). These suites consist primarily of fluvial to shallow-water clastic sediments and carbonates; those suites as old as middle Proterozoic have minor basaltic flows near their base. Some suites may be as youns as lower Palaeozoic. The late Archaean Dharwar sedimentary assemblages of the Western Dharwar craton (Naqvi, 1985, and references cited therein) are somewhat arbitrarily included in this

sedimentary suite.

The Dharwar basins apparently range over

an age of several hundred Ma., and some are intensely compressed, but the suite as a whole does not appear to have been part of a major orogenic belt. LITHOLOGIES IN THE NUBIAN-ARABIAN SHIELD Rock types in the Nubian-Arabian shield are summarized Figure 2 and Table 1. Many of them are different from rocks

in in

the Indian shield and the last section of this paper discusses the significance of this difference.

231

The parts

major lithologic assemblage in both the Arabian and Nubian of

the shield consists of volcanic and

associated

with

the

sedimentary

development of intra-oceanic

rocks

island

arcs.

Older suites, with ages in the range of 900 to 700 Ma., commonly contain bimodal volcanic assemblages (basalt-rhyolite or spilite--keratophyre), and younger suites tend to contain calc alkaline

volcanic assemblages (Stern, 1981; Roobol et al., 1983).

Associated

sedimentary

rocks

formed in a variety

of

settings,

including marginal basin, fore-arc basin, back-arc basin, etc. (Camp, 1984; Clark, 1985; Stoeser and Camp, 1985; Johnson et al., 1987; tionary

Kroner et al., 1987a).

Melange and other suites of

accre-

prisms are also common (A1 Shanti and Gass, 1983; Ries et

al., 1983). In the central part of the Nubian-Arabian shield the arc suites formed on oceanic crust and show some tendency to become younger towards the northwest, indicating progressive accretion in that direction (Jackson and Ramsay, 1980; Stoeser and Camp,

1985).

Much

of

the subduction

apparently

was

directed

downward toward the southeast. Volcanic/sedimentary assemblages not appear to be significantly different in the Afif area, possibly underlain by thin continental sial, from those in the

do

western part of Arabia, formed on oceanic crust. Subduction zones in the Afif area, however, were apparently oriented north-south. In

addition

assemblages

near

to

volcaniclastic

suites,

some

sedimentary

the western edge of the shield appear

to

have

been deposited as continental-margin sediments by erosion of older rocks to the west (Kroner et al., 1987a). Old zircons in some Egyptian suites (Dixon, 1981) probably also were derived from this terrain. Calc-alkaline batholiths constitute a major part of the Nubian-Arabian shield (Neary et al. , 1976; Dixon, 1981; Marzouki et al., 1982; Jackson et al., 1984; Jackson, 1986; Stoeser, 1986). Older suites, close to 900 Ma. old, are generally more mafic (with tonalites and diorites) than younger suites, which consist mainly of granodiorites, adamellites, and granites formed closer to '700 Ma. ago. The batholithic suites are associated northwestward building of the island-arc terrain and closure

with also

the with

along the north-south suture zones in the eastern part o f

the shield.

232

The suture zones shown in Figure 2 are intensely deformed belts that probably have undergone shearing in a variety of directions. Rock suites along the zones include melanges of dominantly oceanic rocks, some o f which are ophiolites or fragments of ophiolites (Frisch and A1 Shanti, 1977; A1 Shanti and Gass, 1983; Ries et al., 1983; Coleman, 1984; Kroner, 1985; Stoeser and Camp, 1985). Correlation of several of these zones has been proposed across the present Red Sea, giving a coherence to both the Nubian and Arabian parts of the shield (Stoeser and Camp, 1985; Vail, 1985). A suite of alkali-rich granites (includinq the alkali feldspar granite of Stoeser, 1986) is broadly distributed throughout the Nubian-Arabian

shield.

These rocks form separate plutons, do not

have significant amounts of cogenetic mafic rocks, are undeformed, and appear to represent magmatic activity at the time of stabilization of the shield, about 600 to 550 Ma. ago (Rogers et al., 1978; Greenberg, 1981; Stern and Hedge, 1985). Near the end of major compressive activity in the Nubian-Arabian shield, several areas accumulated thick sequences consisting predominantly of sedimentary with some volcanic rocks. These areas are categorized as successor basins, formed above suture

zones,

or basins of

crustal downwarp that extended

over

broad areas (Johnson et al., 1987). These suites show some coinpressive deformation and may be transitional between the older subduction-zone

assemblages

and

younger ,

Phanerozoic

cover

sequences. The late Precambrian Najd fault system is primarily strike-slip (Moore, 1979; Stern, 1985). Stern et al. (1984) proposed that the NAS

underwent

significant

extension

related

to

strike-slip

movement. The alkali granites of Egypt are associated with volcanic and sedimentary suites that may be rift related, and dike swarms are abundant at numerous places in the shield. Nevertheless, identifiable rift valleys of Najd age have not been found,

and

many

writers describe the evolution of

Arabian shield solely as a result (e.g., Shackleton, 1986).

of

the

compressional

Nubianmovements

Relatively undeformed platform sequences began to cover the NAS about 600 to 500 Ma. ago, possibly slightly older in the eastern part of the shield (McClure, 1980; Uabbagh and Rogers, 1983). These rocks are predominantly fluvial and shallow-water clastic

233

sediments. Apparently the shield was rapidly uplifted and eroded near the beginning of the Phanerozoic, permitting sediments to form on terrain at the depth of alkali-granite emplacement shortly after the magmatism and shield stabilization. The

Nubian-Arabian shield does not contain any major areas

of

gneiss or granulite development. Local suites identified as gneiss apparently

formed

from

supracrustal rocks

at

slightly

higher

grades of metamorphism than was typical of most of the shields, and a few granulite-facies rocks are associated with high-temperature coritact zones around plutons. DISCUSSION The principal comparisons of the lndian shields are shown in Tables 1 to 3 . The

and

Nubian-Arabian

comparison of abundances of rock suites in the two shields

(Table

1)

is

clearly affected by somewhat

arbitrary

decisions

about the assignment of individual rock types to the various groups. This problem is particularly acute for the Indian shield where many analogues.

rocks As

do

not

discussed

have

easily

previously, two

identified

Phanerozoic

uncertain

assignments

concern Archaean supracrustal suites. One problem is the Aravalli Supergroup of the Aravalli-Delhi belt, which is here placed in the category of mafic supracrustal suites infolded into gneiss despite its tion

platformal lithology; the placement is based on its with

deformed

gneisses.

The second problem

associa-

the late Archaean Dharwar schist belts, which are here regarded as platforma1 cover belts and

is

sequences despite the intricate deformation of the possibility that one may represent a suture

(Naqvi,1985). Possibly both the Aravalli equivalent in the Indian shield to the volcanic assemblages of successor basins other crustal downwarp basins that have

some zone

and Dharwar suites are sedimentary and minor above suture zones and been proposed in the

Nubian-Arabian shield (Johnson et al., 198'1). Abundances of rocks in the Indian shield were obtained by point counting maps of individual cratons in Naqvi and Rogers The following identifications were used:

(1987).

1. Gneiss signifies rocks that are mostly Archaean and have a tonalitic to trondhjemitic composition. The category includes:

undesignated

sialic

suites of the Singhbhum nucleus; the

Banded

234

TABLE 1 Average

abundances

of

rock

suites

in

the

Indian

(IND) and

Nubian-Arabian (NAS) shields. IND 41%

Gneiss (mostly tonalite/trondhjemite) Granulite terrain (and associated anorthosite, etc.) Archaean mafic supracrustal rocks infolded with gneiss Archaean/early Proterozoic granite (Post-tectonic, potassic, alkali feldspar granite is approximately 1/3 of total) Supracrustal rocks of early to middle Proterozoic orogenic belts

30 7

11 10

N AS 44%

Supracrustal volcanic and sedimentary suites Calc alkaline plutonic rocks Post-tectonic, potassic, alkali feldspar granites Gneiss (in the Nubian part of the NAS)

Gneissic

Complex

Bundelkhand

of

the

Aravalli-Delhi

belt;

46

7 3

1/2

of

the

area, presumed to he continuous under Vindhyan cover;

and mapped gneissic terrains in the Bhandara and Dharwar cratons, assumed to he continuous under cover of late Dharwar supracrustal rocks and the Cuddapah basin. 2.

Granulite terrains include all rocks in areas that

contain

significant amounts of charnockite, khondalite, mafic granulite, anorthosite, and related suites. This figure includes lower grade (commonly amphibolite-facies) rocks interdistributed with the granulites. The areas designated include: the Chotanagpur area; the

Eastern

Ghats; and the granulite terrain of

southern

India

south of the Dharwar craton. 3. Supracrustal rocks infolded into Archaean gneisses include: the Older Metamorphic and Iron Ore suites of the Sinqhbhum nucleus;

the Aravalli Supergroup of the Aravalli-Delhi belt;

the

Sukma, Bengpal, and Bailadila suites of the Bhandara craton; and the older ("Sargur") suites of the Western and Eastern Dharwar cratons. The problems of placement of the Aravalli Supergroup and schists o f the late Archaean Dharwar belts of the Western Dharwar craton are discussed in the text. 4. Archaean/Early Proterozoic granites include: the major granite bodies o f the Singhbhum nucleus (Singhbhum, Mayurbhanj,

235

Nilgiri,

and Bonai); an estimate of the amount of granite in

the

Aravalli-Delhi belt older than the Erinpura and related suites; 1/2 of the Bundelkhand terrain, which was assumed to include the area overlain by the Vindhyan suite; the Dongargarh granite o f the Bhandara

area; and the Closepet and related potassic qranites

of

the Eastern Dharwar craton. Supracrustal rocks of Proterozoic mobile belts include: of the Singhbhum mobile belt; the Delhi Supergroup of the

5.

rocks

Aravalli-Delhi belt; and the Sausar, Sakoli, and Dongargarh suites of the Bhandara craton. Abundances of rocks in the Nubian-Arabian shield are a weighted average. alkali

Rocks feldspar

plutonic

suites

in the Arabian shield (weighting of two) granites, except

calc-alkaline

alkali-feldspar

plutonic granites

include

suite5 and

(all

alkaline

rocks), and supracrustal suites (from Stoesser, 1 9 8 6 ) . Rocks in the Nubian shield (weighting of one) are based on the tabulation of Rogers ( 1 9 7 8 ) for Egypt. The Nubian shield includes: alkali-feldspar suites

granites calc-alkaline

(alkali rocks

granites); plus

calc-alkaline

1/2

of

"gabbr o-d i or i te" ; supracr usta 1 suites assemblages p l u s 1/2 of rocks mapped a s gneiss

rocks

plutonic

mapped

as

(volcan ic/sed imentar y gabbro-diorite); and

.

Despite detailed problems of assignment o f rock suites in Table 1, the differences between the two shields are s o large that it is obvious that they are composed of different lithologic assemblages.

The

major distinctions are summarized in

Clearly the Indian shield is dominated by tonalite-trondh jemite composition, whereas

'Table 2 .

gneiss, mostly of the Nubian-Arabian

shield has very little gneiss and certainly no large terrains o f orthogneiss. Granulite-facies rocks occur only in the Indian shield.

Calc-alkaline

batholiths

are recognizable only

in

the

Nubian-Arabian shield. No lithologic suites in the Indian shield can be shown to be subduction related because o f their similarity to modern island-arc or continental-margin assemblages (with the possible exception of the Singhbhum belt; Pig. 1). I f rocks the Indian shield were formed by subduction, then conditions the

Archaean were sufficiently

the

igneous and sedimentary products of a subduction

were very different.

different

from the present

in in

that

environment

236

TABLE 2 . Comparison of the Indian (IND) and Nubian-Arabian (NAS) shields IND

NAS

Gneiss

abundant

rare; local

Calc alkaline volcanic suites

rare

Calc alkaline batholiths

not identified rare or absent abundant

abundant subduct i on-zone suites (both calc alkaline and primitive bimodal) abundant

Ophiolites Granulite and associated rocks (anorthosite, etc.) Sutures

Despite histories

the

probable but unproved

differences

in

rock

abundant in melanges absent clearly present

types,

the

generalized

of the Indian and Nubian-Arabian sheilds are remarkably

similar (Table 3 ) . The oldest rocks in most of the Nubian-Arabian shield were formed in an oceanic environment as the volcanic/ sedimentary assemblages of intra-oceanic island arcs. Similarly, the

oldest rocks in the Indian shield were mantle derived, either

meta-igneous rocks with initial isotopic ratios characteristic of the mantle or metasediments derived from a maf ic/ultramaf ic source. The Nubian-Arabian shield has clearly been assembled by suturing different blocks together, although it is likely that the blocks

were not formed very far from each other.

Although sutur--

ing is more problematical in the Indian shield, it seems likely that at least some disparate blocks were brought together about 1500 Ma. ago (Rogers, 1986); earlier (Archaean) sutures have also been proposed (Naqvi, 1985; Krogstad et al., 1986). Because the two shields are approximately the same size, and contain five to ten separate blocks dependinq on the method of countinq, the individual terrains in the two shields have about the same averaqe size

(100,000

sq.

km).

Both

shields

contain

late-

to

post-orogenic potassic granites, although their abundance seems to be higher in the Nubian-Arabian shield. Both shields contain early sedimentary (and minor volcanic) cover sequences that

237

TABLE 3 A . Diagramatic history of the Indian shield ( I N D )

. . . . .Age . . . .in . . .Ma. . . . . . . . . . . . . . . . . . . . . 3000 . . . . . . . . . . . . .2000 . . . . . . . . . . . .1000 ......... Deposition of maf ic/ ultramfic supracrustal belts; tonal ite/ trondhjemite magmatism from the mantle and format i on of qne i s ses Granite magmatism Granulite format ion Formation and deformation of Proterozoic supracrustal rocks in orogenic belts

mostly post-tectonic --__

-

_ I

locally deformed P la tfor m sed imenta t i on

Subduction-related sedimentation and volcanism Calc-alkaline batholithic ma qma t i s m

mostly primitive volcanism

mostly ca lc -a lka 1 i ne volcanism

mostly tona 1i te

mostly adamel lite qranite I

Transcurrent faulting rifting ( ? ) , and post-tectonic granite magmatism locally deformed Platform sedimentation

_---_____________---____________________------------------

have undergone at least local deformation. Both shields ultimately became stable platforms and permitted deposition of extensive clastic suites that are either undeformed or deformed only locally. In short, both shields evolved by a similar sequence of

events from an oceanic terrain to a crust of typical continen-

tal thickness, although the western Nubian-Arabian shield crust appears to be more mafic than the crust of the Indian shield. important question is whether the Indian and Nubian-Arabian

An

shields really are lithologically different, despite their broadly similar histories, or whether they simply appear to be different because of erosional exposure to greater depth in the lndian shield. That is, i f the Nubian-Arabian shield was eroded another 5 to 10 km, locally exposing granulite-facies rocks, would it be the same as the Indian shield? There is no compelling reason why the Indian shield should have been eroded more than the Nubian-Arabian shield. that

For

it

so

example, the Nubian-Arabian shield is not

could

not

have

been

eroded

sufficiently

to

young expose

granulite-facies rocks. Nevertheless, the Indian shield generally does show higher-pressure assemblages than the Nubian-Arabian shield, and the possibility that this difference controls the differences in lithologies must be considered. Three

lines of evidence indicate that the differences

the Indian differences the entire

between

and Nubian-Arabian shields are caused by lithologic rather than by difference in exposure. One is that thickness of the NAS crust (at least in the western

part shows higher P-wave velocities than the Indian crust, and the indicated higher density is verified by higher free-air qravity values. Thus, the entire Nubian-Arabian shield is probably more mafic than the Indian shield. A second line of evidence is that calc-alkaline batholithic suites (gabbro to granite) should appear at

the level of exposure in the Indian shield if they had

formed

there; their absence indicates lack of production rather than removal by erosion of upper-level rocks. A third line of evidence is that greenschist-facies supracrustal suites in the Indian shield do not contain the calc-alkaline volcanic rocks found in the Nubian-Arabian shield island-arc suites, a difference not related to level of erosion. If Indian

the

Nubian-Arabian shield would not become similar to

shield

through further

erosion, would it become

the

similar

239

through time? That is, would further development of mostly silicic, gneissic, rocks in the Nubian-Arabian shield convert it to

the

lithology of the Indian shield?

This question cannot

be

answered with any certainity. Some of the gneissic suites in the Indian shield were originally magmas emplaced at very low pressures the

of

equilibration, and production of similar suites

now-stable Nubian-Arabian shield does not seem likely in

in the

future. Conversely, vvmaturingvland uplift of shields has been proposed on the basis of sedimentary assemblages formed on and around

shield

compressive

areas

deformation

after

they

became

stable

(Rogers et al., 1984).

It

to

further

is

possible

that the Nubian-Arabian shield will continue to undergo transformation to a more sialic crust during the next few hundred million

years.

The accuracy of this paper cannot be

ascertained

until that time. ACKNOWLEDGEMENTS Work

in

India was supported by two grants from Special Foreign

Currency

Foundation:

171281,

administered by S.M.Naqvi, and a grant in U.S. and Indian (EAR79-05723), administered by the writer.

Grant

National

Science currency

a

the

(INT78-

Work in

Nubian-Arabian (OIP75-07943)

shield was supported: in from the National Science

University

South Carolina; and in Saudi Arabia by funds

available

of

the

Egypt by a grant Foundation to the made

by Riyadh University to visit students in the field.

I

have received help from numerous people during these programs and would particularly like to thank E.J. Callahan, T. Chacko, M.E. Dabbagh,

J.K.

Greenberg, S.M. Hussain, S.H.Jafri,

J.R.

Monrad,

S.M. Naqvi, H. Narain, R.C. Newton, R. Ressetar, and P.T. Stroh. REFERENCES A1

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