Significance of Wollastonite- and Scapolite-bearing assemblages from the Kerala Khondalite belt, southern India

XI

well

in the

Bay

suggestions

distensional the

Bengal

somewhat

for a systematic of the alkaline

speculative.

tectonics

more

renders

Society

these

This

calls

study of the geological setting rocks and the incidence of

Gondwana

throw

of

in the other

configuration

members

that

may

light on the possibilities

of

then

discussed

India

43,

27,

41 l-41

history

and

145-I

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Sinha-Roy, S. et al. 1995. Geological Society India Memoir 31, 63-69. Soman, K. et al. 1996. Journal Geological Society India

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Santosh, M. et al. 1989. Santosh, M. et al. 1994.

15.

Geological

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3-24.

extensional

tectonics

Significance The

Precambrian

ancestry

of

western and eastern coasts, at present, if established evaluation/studies, constraining

has

the

Gondwana.

role

on the

rather speculative by more detailed

great

thermal

The

rifting

implication

evolution

of

mantle

in

of

and the break

been emphasised tectonics and

plumes

considered to the generation

has

be an important mechanism in of asthenospheric upwelling.

weakening

prerequisite up takes had

the

before

place,

lithosphere

is possibly

large scale continental

attained

a

break

continental

blocks

is true with several

The role of fluids

two

carbonic

al.,

1980)

and Valley,

large thickness.

other aspects

As

of continental

petrogenesis

models

of granulite

metamorphism

and vapour-free

models

of such

in granulite

end-member

Vi.?.,

thickness

of dilatory tectonics and is needed to break up

Japan

1984).

Evidence

inclusion

and stable

in support

isotope

studies,

Adirondack

Mountains,

cited as primary

against

paucity

of wollastonite-bearing

Possibly

the Precambrian

history

part in determining

lithosphere

will break

may

where

have

an

a thickened

Indian

in support

granulite

The

June

K. 1994. Geological (Abstract) pp6-9.

1994

Survey

India,

The

Hinz, 1981. Geologisch Jahrbuch 22, 3-28. Kaila, K. L. Krishna, V. G. and Mall, D. M. 1981. Tectonophysics 76, 99-130. Krishnan, M. S. 1953. Memoir Geological Survey India

81,

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Krishnamurthy, Special

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Leelanandam,

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C. Abstracts pp28-29. Ratnakar, J. and Leenandam,

upper

Minerals

Divison

Survey

India,

biotite

Geological

gneiss

to

throughout detailed convincing derived findings

this study

of

facies

numerous and has

been

recent

for the

carbonic

fluids

(e.g.

Santosh

of

granulites) a theme

years,

of

yielding

role of externally

in inducing et

of garnet-

patch-type

anhydrous

terrain

to

grade.

“incipient

transformation vein-

India. a vast

metamorphosed

granulite

in the

evidence

forms

and

orthopyroxene-bearing

reactions C. 1989.

(local

assemblages

belt

sequence

occurrence

charnockites”

regime

the recent

belt of southern

Khondalite

amphibolite

in the

granulite

of wollastonite-bearing

Kerala

of

has been held

This article summarises

metasedimentary Gopalakrishnan,

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On the other hand, the

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of thermal

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M. SATISH-KUMAR

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by many workers. decompression

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The general conclusions derived from the study of talc-silicate rocks from the Kerala Khondalite belt are the following: i) the occurrence of wollastonite-bearing assemblages does not necessarily restrict the possibility of CO, influx for charnockite formation in the terrain, but a pervasive influx of carbonic fluids in the terrain is unsupported;

Yokohama

of granulites

talc-silicate granulites,

National

Yokohama,

Japan

of Earth’s

Interior,

Tottori,

A suite

grossular-quartz

(Satish-Kumar

for Study

the

anorthite

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Institute,

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as closed

there

resulting

813-819.

coronas between wollastonite and anorthite, and formation of grossular from the anorthitecalcite-quartz. All these reactions indicate an a/.

S. L. and

Mineralogy

grossular-bearing into

fluids,

post-peak

associated

textures

of scapolite

cooling

-728.

Petrology

textures

during

dehydration

assemblages, breakdown and

retrograde

influx

initial regime,

of carbonic

S. L. and Buick, 693

Harley,

the classic locality, Nuliyam at the southern margin of KKB, Harley and Santosh (1995) showed that the wollastonite bearing assemblages

an

of charnockites and also partially the wollastonite bearing

Satish-Kumar,

and quartz,

symplectites (Satish-Kumar

these

a post-peak

and

wollastonite

down

press).

influx

peak

56-58.

grossular-absent

KKB

breaking

indicate

local

Lamb,

Important

breaking

in

underwent

the

terrain;

assemblages.

The

textures

textures.

Harley

assemblages

terrain

in the

REFERENCES

information on the pressure-temperature-fluid histories of the talc-silicate rocks (cf Harley Buick,

conditions

composition

during

was

of meters. are comprised

charnockites.

granoblastic

great

the

fluid

existed

the

grossular in a few assemblages are

peak metamorphic have

metamorphic iii)

controlled

vapor-free)

wollastonite-scapolite-clino-

assemblages with The grossular-absent

commonly

an internally

belt

with

structural trends of the associated and varying in thickness from less than

meter

ii)

(mostly

before decompression; and iv) during the decompressional

in press).

in the Kerala

and layers

to Mineralogically,

a

of

has led to the re-evaluation

as bands

regional gneisses,

Kerala

part

the source and role of CO, (&Harley 1995; Satish-Kumar et al., 1995, occur

the

southern

(metapelites,

quartzites

rocks, mafic opx granulites

Eastern

Ghats show

three

belt of Peninsular that

deformation

distinct

types

the

of folding.

relations among thermobarometric maximum the

metapelites, condition

of which

deformation. might

are exposed part of the

India.

giving

of 950°C

to three textural

phases and suggest a at 9 kbar

metamorphism

has been correlated peak

at least

rise

Petrography,

The

Structural

suffered

different calculations

temperature

conditions

second

area

events,

and

(two-pyroxene) (charnockite),

leptynites and sillimanite granites) in Rayagada, the north-central studies

Misasa,

Japan

for

of

the

with

the

metamorphic

have been in a higher

P-T regime.