Evolution of granulite blocks of southern India and their relation to the East Gondwana continent

VI

Evolution of granulite blocks of southern India and their relation to the East Gondwana continent A. S. JANARDHAN

and

Department of Geology, Manasa Gangotri,

K. FRANCIS

of

composition,

with

enclaves

granulites

and

minor

amounts

lithologies.

The

The South

Indian

shield

cratonic ages

Peninsular

areas,

nuclei

and

and

different

divided

the

distinct separated

by

the

ages

whilst

the

essentially small

major

in addition

to

granulite

of the

besides

the

grade

entire

Neoproterozoic

article terrains

granulite

events

different

granulite

its

of

on

intruded the

by

with

Kodaikanal

ranges

is on the timing have

assembly,

position

to

rise

using

to the

explain

the

shield

and

Indian

with

of peak

given and

southern

a

respect

to

East

of the northern

The northern

block

of the

Biligiri

blocks

a common Ma.

fringe

Biligiri Rangan

These the

Rangan,

and

major late

Dharwar

the

Though

aspects,

they granulite

Archaean craton

Hill

Nilgiris

they

are

have

all

event

at

granulite

to its south.

IBRI Hills

The BR Hills that

constitute

of

are

Karnataka

the Granulite

regions.

in several

undergone

the

BR Hills

now,

there

granulite

continuity can

established

melting

from

5.5 to

(Hansen

et al.,

1995).

terrain

Thus,

represents

Dharwar

of juvenile,

the

craton.

charnockites

et al.,

granulite

be observed

and Janardhan, of the

across

to the

in pressures

granulite

Nilgiri granulite

was

The

derived

hydrated

oceanic

1994).

terrain

granulite

mostly

the

southern made

parts up

of

terrain

as syn-accretionary the

ages

are

between

lithologies

the

BR Hills

kbar zone,

centre

decrease southerly BR Hills. recent

of in

The

it decreases

the

massif.

pressure

increase The above publications

of two

is in

of BIF and

are found shear

from

are encountered and

enclaves

the

along,

zone,

which

Nilgiri

continuity

terrains.

The

essentially

strips

pelites Moyar

is no structural

shear the

of, the

granulite

of 10.2

Small

bearing

north

Ma.

are

as

event

Metasedimentary

are scarce.

garnet-kyanite

2510

with

granulites.

1989),

granulite

terrain

to granitic,

pyroxene

the

to

this

classified

et a/.,

and

2600 of

trondhjemitic

has been

(Peucat

of protoliths

charnockites

There

block

comprises

Salem-Madras

ca 2500

the

Until

BR Hills

increase

(Janardhan

two

different

in the

margin

seperates

and

from

an older

terrains

for the acid

or just

Gondwana.

ranges

facies

Shankara

The Nilgiri

evolutron

ages,

terrains,

of terrane

relative

with

the

which

composite

Granulites

of

and lithological

been

from partial

part for

Sm-Nd

ages

region.

BR Hill

crust

granulite methods,

lithologies.

of different

emphasis

concept

of

and

as Ma,

is ca 2500

peak

concordia,

et a/. , 1995).

has

protolith

swathes

been

deals

of SGT. The emphasis

present

or the

contains

post-

1989)

by various

evidence

terrain

reactrvated

event

and monazite

area

and a gradual 1984;

as ca 3400

for the

zircon

Hills

intrusions.

The present of granulite

has

of

of BR

et al.,

are

obtained

amphibolite

the

charnockites

abundant

the

protoliths

rock

in this

8 kbar

Bilgiri

block,

metasedimentary

block

a

margin,

(SGT),

intercept

grade

Salem-Madras

southern grade

composition,

granulite

and

terrain

granulite

The

the

lower

A structural

with

southern

has been

event

craton,

belt,

its

The

granulite

two

(PCSZ).

event

pyroxene

classified (Peucat

Ma date

(Mahabaleswar

trending

granulite

Nilgiri

terrains.

southern

special

zone

2500

trondhjemitic

of two

the age of the granulite The

is no definite

blocks

the Dharwar

- towards

Hills

Using

into

east-west

comprises

component

The

shield

of their

Sivasamudram

(1984)

to

been

granulites

garnet/whole

varying

1986).

a granite-greenstone

Rangan

acid

of

and Southern

shear

block

many into

Drury

Indian

Palghat-Cauvery Northern

belts

Naqvi,

Northern

like

divided

approach,

South

blocks:

been

mobile

(Radhakrishna

a slightly

shield,

has

have

accretionary

Ma.

other

tonalitic

metasedimentary granulites

ANT0

of Mysore,

University Mysore-

charnockites

terrain.

between highest along

the

6 kbar This

these

pressures Moyar

towards

north-south

contrast

to

the

in pressure gradient of the evidence detailed in several (Janardhan

1989;

Raith

et

a/., 1990; Janardhan et al., 1994) clearly suggest that the protoliths of the Nilgiri granulites were accreted to the Dharwar craton at ca 2600 Ma and both the terrains underwent the dominant granulite event at ca 2510 Ma. Dunai and Touret (1993) have dated the fluid

VII

inclusions in the core of garnet at 2500 Ma., which coincides with the age data obtained by other methods. The protolith of the Nilgiri hill charnockites were derived from a juvenile source. This conclusion is based on eNd values of -1.5 to + 1.5 and Sr initial values ranging from 0.698 to 0.707, with a clustering around 0.702 at 2500 Ma (Peucat et al., 1989; 1993; Janardhan et al., 1994; Taylor and Janardhan, unpubl. data). The heat source for the late Archaean granulite metamorphism could have come from a deep seated mantle plume and the fluids from alkali basalts associated with the plume activity. It is interesting to note that carbonatites of Hognekal (located towards the northeast of the Nilgiri massif) give ages of 2400 Ma, indicating that the crust had stabilized by this time. The slight negative eNd values of the Hognekal carbonatites (Anil Kumar et a/., 1990; pers. comm.) indicate an enriched mantle source. The eNd values of the Sittampundi anorthosite complex and some components of the Closepet granite show positive eNd values (Bhaskar Rao et al., in press; Jayananda et al., 1995a). These data suggest strongly that the mantle beneath this region changed its composition from depleted to enriched at around 2500 Ma. This can be cited as evidence for the mantle plume and connected igneous activity. Salem granulite terrain Detailed accounts of granulites of this region are found in the works of Rameshwar Rao et al. (1990, 1991) and Hansen et al. (1995). Peucat et al. (1993) has classified these as syn-accretionary granulites, formed at 2500 Ma. They add that the protoliths were derived from a juvenile source (eNd values range from -0.5 to +3.5 and Sr initial values ranging between 0.701-0.703). Thus, there is a striking similarity between the Nilgiri and Salem granulite blocks. Granulite ranges of the southern block The southern block or the southern granulite terrain (SGT), as it was termed originally, covers the entire region south of the Palghat-Cauvery shear and includes the Kodaikanal, Varushanad ranges and also the terrain south of the Achankovil shear (ASZ), known in recent years as the Kerala Khondalite belt (KKB). The entire terrain underwent a dominant granulite metamorphism during Pan-African times. In this note, the focus is on the Eastern

Kodaikanal ranges. These hills are chiefly composed of charnockite with swathes of metapelites, talc-silicates and quartzites. Pelitic granulites form an important constituent, interbanded with garnet-bearing two-pyroxene granulite bodies. Anorthosites of massif type viz., the Oddanchatram anf Kadavur bodies, intruded the domal and basinal structures, in this area alone. Charnockites are locally garnetlocalities south of bearing, in some Oddanchatram. Alkaline granites, syenites and carbonatites occur as late intrusive bodies. Granites of Pan-African age occur as sheets and as domal intrusions. Metamorphic

conditions

Estimations of peak and post-peak metamorphic conditions have been made for pelites, calcsilicates, garnet-bearing pyroxene granulite bodies, xenoliths of country rocks within, and at the borders of, the anorthosite body, and rare garnetiferous charnockite. The pelitic assemblages include garnet-biotite-sillimanitecordierite-K-feldspar-quartz. In addition, sapphirine bearing assemblages with sillimaniteorthopyroxene-perthitic K-feldspar-cordierite are common as for example at Perumal Malai (Sivasubramanian et al., 1991, 1992); Panrimalai (Grew, 1984) and Ganguvarpatti (Muthuswami, 1949; Mohan and Windley, 1993). Grew (I 982) gave a brief account of such occurrences. Pelitic assemblages also exhibit a spine1 + quartz association. The above assemblages are characteristic of high temperatures between 800-900°C. Pressure estimates deduced from co-existing mineral assemblages of pelites, charnockites and pyroxene granulites range between 6-9 kbar. The high pressures of 9 kbar are from garnet-bearing assemblages developed at the contacts of the Oddanchatram anorthosite with the pelites, pyroxene granulites and country rock xenoliths. Garnet-bearing charnockite also give pressures close to 8.2 kbar (Sivasubramanian et a/. , 1991; Wiebe and Janardhan, 1993). The retrogressive assemblages of cordierite-garnet and GRAIL give low pressures of 5 kbar. In general, the postpeak P-T-t path is dominated by late isothermal decompression with high temperatures. Based on thermobarometric estimates and reaction textures like relict kyanite embayed in cordierite-spine1 symplectite in pelitic granulites of the Trivandrum region, Chacko et al. (1987) and Frost and Chacko (1989) suggest a clockwise path, with pressures ranging between 5-8.5 kbar.

VIII

Age

constraints

a cooling

age

gneisses Pro tolith ages

rim

ages

of

(Bartlett The SGT is distinctive granulite

terrain

younger

give

1995b).

This

recorded

ages.

Protolith

dates

2100

Ma

obtained

single

by e Nd values

550

(Harris

et

al.,

earlier

nucleation

and model charnockites

well

Ma

and

above

similar

Wiebe

Eastern

(Srikantappa granulites

and

Eastern the

Janardhan

Ghat

northern

Hills.

Their

1100

out

clustering

of

trend

continues

margin

of the

observation

that

westwards eastern was

presence

of

Oddanchatram-Kadavur

which

are similar

Proterozoic

described Chilka

from lake

elsewhere lake

massif-type the

anorthosite

anorthosite,

anorthosite have

of age

Hence,

the

as signatures

of

the

et al. (1995) of

metapelites

ca

Ma)

and

Oddanchatram 400

seen body

Ma. BIF

also,

the

around can

Eastern

Ghat

have

1300-1400

from

the

with the Chilka

1300-I

BIF bands

anorthosite

Nd ages

(viz.,

so far in any part of the

Oddanchatram Bartlett

the

bodies

1300

of

be around

not been reported

SGT.

ca

By analogy

the

could

Ghats

for

Trivandrum

Ma (Pan-African)

the

terrains

with

the

late

data

obtained

analysis

of

granulite

gives

from

charnockite

corresponds

an age

to the Pan-African

Ma age has been

obtained

two

model

cordierite

ages

pyroxene

event.

from

rock

Ma,

which

has not shown any retrogression. Garnet in the two-pyroxene granulite body, and garnet from the Oddanchatram down symplectically plagioclase uplift. Thus, obtained cooling

anorthosite body has broken to orthopyroxene-calcic

assemblages during isothermal the ages of 435 (and 340 Ma ?)

from

such

garnets

may

ages or the ages of uplift

al., 1995b).

SGT as event

of

seem

to

the

formed granulite of the just ages

below

(Dunai

and

intrusive

further

south.

The

the late Archean SGT.

has only

that

the Nilgiri

southern

extension the

terrain, the

to

domes

possibility.

event.

represent (Jayananda

the

of

anorthosite between

some

(e.g.

the

tilt

estimates event region, of high

800-SOOOC. from

Dasgupta of

is another

This

the Eastern

et al., 1990; occurence

of

Kodaikanal

Pan-African

recorded

He

Ma of

Tiruchengodu)

characteristic

The local

for

in the Kodaikanal

around

plutons

taken

ca 670

by barometric

(Sengupta

1995).

position

have

The late cooling

and

to this

those

as well

al.,

present could

extent

assemblages with

Salem

expulsion

The southward

is supported

or a later

originally the

temperatures

be related

et

of

the

and could

compares

massif

time.

1993),

PCSZ,

local significance.

its

closure

from

to the

Pan-African

and to some

has mineral

fundamental

sideways

BR hills

granite

Nilgiri

ranges

and

by SGT,

granulites

Compared

shear

Touret,

this

fringing

Ma

of

during

belt

550

Nilgiri

place

Ma)

and

terrain

south

(2500

(locally)

likely

the

arrangement succeeded

Proterozoic

It is quite

India

to the south,

respectively,

Ghats

which

The 550

garnet

give et al.,

the

granulite

of

Pelitic

also

that

the

Archean

Ma

the

garnet-whole ca 550

ASZ

in Peninsular

1100

orogeny.

formation

and of

1992).

geochronologic

craton

temperatures Sm-Nd

of

Ma

ages

age (Santosh

suggesting

the Dharwar

support

region.

granulite

and cooling et a/.,

The SGT, particularly 550

give

at ca 550

vicinity

a spatial

be taken

recorded

Ma

charnockite

Discussion

PCSZ separates

aspects

anorthosite

with

age.

the Achankovil

in many

Eastern

in the world.

along on

anorthosites, to

the

Kodaikanal

based

in agreement

has undergone

Granulite

stated

are

(Choudary

all strongly

granites

of

brought

Ma) signatures

(1993)

meta

zircon

for

539 f 20 Ma metamorphic a whole

Madurai

Ma

1985)

in the

for

the Kober

metamorphism

et al., Ma

Ma

The Ponmudi

of

Pan-African

et a/. , 1994).

Ghat fca

1995)

data.

440-460

+55

547 k 14

ages

exhibit Possible

425

et al., 19851,

et al.,

19921,

Nd

The distinctiveness

has been

at

of

zircon

the

zircon

1987)

the

in the

ages

with

from

et a/. , 1992).

values

and

Ma for Ponmudi

the SGT as a whole TDM

the

Archaean

(Jayananda

Ma (Buhl,

2740-2240

(Choudary

from

is in agreement

age of ca 1970

late

aspects

obtained

method

the

in several

protolith

charnockites

ages

from

of

(Hansen

massif

comparison

components

of the

SGT

with the Eastern Ghat. However, the SGT exhibits evidence of the widespread 550 Ma old Pan-African event, leading to the formation of

granulite

facies

charnockites

interbedded granulite grade aspect has special significance granulite

belts of southern

et

the SGT with

The Rb-Sr ages of 550 + 15 Ma and

The protolith

the eastern

India,

and

lithologies. in correlating

other This the

and particularly

Gondwana

ages for charnockites,

fragments. timing

and

IX

conditions of granulite metamorphism and postpeak evolutionary trend of SGT show resemblance with that of Madagascar (Nicollet, 1990; Paquette et al., 1994 and references therein) and parts of the Eastern Antarctic Rayner complex and the Lutzow-Holm Bay area (Yoshida, 1995; Harley and Fitzsimons, 1995; Motoyoshi, 1990 and references therein). REFERENCES Anil Kumar et a/. 1990. International conference on geochronology, cosmochronology and isotope geology, Canberra, Australia. Abstract Volume 7. Bartlett, J. M. et al. 1995. Journal Geological Society India Memoir 34, 391-397. Bhaskar Rao, Y. J. et al. (In press). Contributions Mineralogy Petrology. Buhl, D. 1987. Unpubl. Ph. D. thesis, University of Munster, Germany. Chacko, T. et al. 1987. Journal Geology 95, 343358. Choudary, A. K. et al. 1992. Geological Magazine 129, 257-264. Dasgupta, S. et al. 1995. Journal Petrology 36, 435 461. Drury, S. A. 1984. Abstracts 28th International Geocongress 1, 420-42 1. Dunai, T. J. and Touret, J. L. R. 1993. Earth Planetary Science Letters 119, 271-281. Frost, B. R. and Chacko, T. 1989. Journal Geology 97, 435-450. Grew E. S. 1982. Journal Geological Society India 23, 469-505. Grew, E. S. 1984. Journal Geological Society India 25, 116-l 19. Hansen, E. C. et al. 1984. Archaean Geochemistry ~~162-181. Hansen, E. C. et al. 1985. EOS 66, 419-420. Hansen, E. C. et al. 1995. Journal Geology 103, pp. 629-65 1. Harley, S. L. and Fitzsimons, I. C. W. 1995. Geological Society India Memoir 34, 73-l 00. Harris, N. B. W. et al. 1994. Journal Geology 102, 135-l 50. Janardhan, A. S. 1989. Abstracts. In: Structure and dynamics of the Indian lithosphere pp90-91. NGRI, Hyderabad. Janardhan, A. S. et al. 1994. Journal Geological Society India 44, 27-40. Jayananda, M. et al. 1995a. Contributions Mineralogy Petrology 119, 314-329. Jayananda, M. et al. 1995b. Journal Geological Society India Memoir 34, 373-390. Mahabaleshwar, B. et al. 1995. Journal Geological Society India 45, 33-49. Mohan, A. and Windley, 8. F. 1993. Journal Metamorphic Geology 11) 867-878. Motoyoshi, Y. 1990. Interim report of Japan-Sri Lanka joint research ~~132-139. Muthuswamy, T. N. 1949. Proceedings Indian Academy Science 30(6), 295-301.

SEAES 1415%F

Nicollet, C. 1990. Granulites and crustal evolution pp291-310. Paquette, J. L. et al. 1994. Journal Geology 102, 523-538. Peucat, J. J. et al. 1989. Journal Geology 97, 537. 550. Peucat, J. J. et al. 1993. Journal Metamorphic Geology 11, 879-888. Radhakrishna, B. P. and Naqvi, S. M. 1986. Journal Geology 67, 145-l 66. Raith, M. et al. 1990. Granulites and crustalevolution pp339-366. Rameshwar Rao et al. 1990. Journal Geological Society India 35, 55-69. Rameshwar Rao et al. 1991. Journal Petrology 32, 539-554. Santosh, M. et al. 1992. Bulletin Indian Geological Association 25, l-25. Sengupta, P. etal. 1990. Journal Petrology 31 I 971996. Shankara, M. A. and Janardhan, A. S. 1995. Indian Mineralogist 29, 60-73. Sivasubramanian, P. et al. 1991 . Journal Geological Society India 38, 532-537. Sivasubramanian, P. et al. 1992. Journal Geological Society India 40, 287-290. Srikantappa, C. et al. 1985. Journal Geological Society India 26, 849-872. Wiebe, R. A. and Janardhan, A. S. 1993. Journal Geological Society India Memoir 25, 1 13-l 17. Yoshida. M. 1995. Journal Geological Society India 34, 25-45.

The configuration of the Indian Shield - Precambrian tectono-thermal events and constraints on the thermal history of Gondwana T. M. MAHADEVAN Sree Bagh, Ammankoil Road, Cochin-682

035,

India

The Indian shield largely owes its configuration to northeast trending faults along the east coast and the north-northwest faults along the west coast. The faulting has been sequential, the east coast faulting having been initiated in the Jurassic coast faulting in the Cretaceous (Krishnan, 1953). The structural history of the west coast is better understood than that of the east coast. The transition from continental to intermediate/oceanic crust is more abrupt and the west coast rift was the source region for the large Deccan continental basalt volcanism. The continental boundaries of the east coast extend well into the Bay of Bengal and the coast is covered over large segments by post-Jurassic sediments. The Rajmahal-Sylhet volcanism may and the west