CHAPTER - 3

MINERALOGY

Petrography of BIF:

The BIF of g r a n u l i t e g n e i s s b e l t s a r e w e a k l y b a n d e d m a g n e t i t e -

quartzi tes . Magnetite quar tz i tes a r e generally coarse t o f ine grained, hard,

compact and a r e dark grey t o black in colour. The weathered sur faces a r e

ye l low t o brown in c o l o u r d u e t o t h e p r e s e n c e of l i m o n i t e . T h e y a r e

generally found a s long narrow bands persisting over lengths of 5-14 k m with

a w i d t h of 5-20 m. C o m m o n l y t h e s e bands vary in number f rom o n e t o

t h r e e in t h e t w o iron o r e deposits of t h e presen t study. Two var ie t ies of

magneti te-quartzi tes a r e recognised in t h e field. The most common var ie ty

is banded (PI. 1, Fig.a) showing gneissic t e x t u r e whereas t h e o t h e r devoid of

banding and is m a s s i v e e x h i b i t i n g g r a n u l i t i c t e x t u r e (PI. 1, Fig.b). T h e

thickness of t h e individual bands varies from 1-20 Inm. The c o a r s e banded

var ie ty is t h e most abundant magneti te-quartzi te . The grain s i z e is c o a r s e

a t t h e c res t s of m a j o r folds than in t h e limbs. Towards t h e c losure of t h e

m a j o r folds t h e rock is sheared and contains lenses or s t r e a k s of magnet i te .

Fine grained magneti te-quartzi tes a r e developed in r e g i o n s of c r u s h i n g a n d

faulting. At some places magnet i te is concent ra ted a s lenses or p o c k e t s of

varying sizes ranging from 60-150 c m in length and 10-50 c m in width. Thin

sect ion studies of magneti te-quartzi tes shows t h e presence of m a g n e t i t e a n d

q u a r t z p r e d o m i n a n t l y w i t h minor a m o u n t s o f g r u n e r i t e , h y p e r s t h e n e ,

h e d e n b e r g i t e , e u l i t e , g a r n e t , c h l o r i t e , a n d a p a t i t e (PI. I, Fig.c). Modal

analysis was carr ied ou t on 45 thin sect ions of magneti te-quartzi tes . Their

r a n g e s a n d a v e r a g e s a r e g i v e n in Table-2. T h e d i f f e r e n t a s s e m b l a g e s

observed in BIF are:

mt-qz-cpx-opx-apat

mt-qz-cpx-opx+hbl

In BIF t h e iron oxide minerals occur mostly a s elongated, rounded or

irregular grains Iorrning bands with granoblastic t e x t u r e (PI. I, Fig.b). The

association of iron oxide and quartz varies f r o m g r a n u l i t i c t o wel l b a n d e d

v a r i e t i e s w i t h a l t e r n a t i n g l a y e r s of q u a r t z a n d m a g n e t i t e (PI. 2, Fig.a).

Quar tz occurs a s i r r e g u l a r or e l o n g a t e d gra ins . Q u a r t z g r a i n s in c l o s e

association with t h e iron oxide a r e usually fine grained (PI. 2, Fig.a) and a

l i t t l e d i s t n c e a w a y f rom t h e iron oxide bands t h e y a r e c o a r s e g r a i n e d .

Q u a r t z shows s t r a i n , c r u s h i n g , o r deformat iona l e f f e c t s with a sheaf l ike

appearance (PI. I , Fig.e) or e x h i b i t i n g undulose e x t i n c t i o n (PI. 1, F ig .e )

suggesting t h a t the rocks have suffered considerable shearing stress. Q u a r t z

grains clearly shows euhedral and polygonal ou t l i n e s in o n e s e c t i o n which

may be due t o recrystallization a t granuli te facies metamorphism. In s o m e

sect ions t h e deformational e f f e c t s a r e seen by f laser t e x t u r e with e longa ted

p l a t e s of q u a r t z a n d t h e d e v e l o p m e n t of b l a s t o m y l o n i t i c t e x t u r e . Fine

closely spaced parallel cracks filled with iron oxide (PI. 2, Fig.d) a r e presen t

in q u a r t z g r a i n s a n d t h e development of iron oxide along them is noticed.

Lines of iron oxide blebs a r e found in quar tz grains. lron oxide is also found

along t h e margins of quartz grains within t h e bands of iron oxide. The veins

of iron oxide appears t o be supplying t h e i ron o x i d e , c u t a c r o s s s e v e r a l

~fohartz grains. lron oxide grains a r e intimately interconnected.

Iron oxide bands conta in aggregates of pyroxene in magnet i te-quar tz i tes

forming aug{en t ex tu re (PI. I, Fig.d & PI. 2, Fig.e). Hypers thene occurs in

t h e form of anhedral t o subhedral grains with dis t inct c leavages o f t w o se t s

with one se t being dominant (PI. 3, Fig.c). Q u a r t z and euhedral gra ins of

magnet i te a r e enclosed with in hypersthene grains showing a t ype o f s ieve

t ex tu re (PI. 1, Fig.1). I-lypersthene occurs a s inclusions within magne t i t e (PI.

2, Fig.e). It is moderate t o strongly pleochroic: x-pink, y-yellow, and z-pale

green. The opt ic axial angle varies f rom 60 t o 65'. Birefringence ( Y- d)

ranges from 0.016 t o 0.018. These properties indicate t h a t t h e mineral i s

fer rohypers thene which is found in di f ferent degrees of a l t e r a t ion giving r ise

t o hornblende, ac t inol i te , and chlor i te . A dusty iron oxide re leased due t o

a l t e r a t i o n of h y p e r s t h e n e o c c u p i e s a t s o m e p l a c e s a long t h e c racks a n d

c leavages giving a re t icula te pa t t e rn (PI. 2, Fig.c).

Minor proportion of grunerite is found in magne t i t e quar t iz i tes which

occurs a s colourless grains. It has 2v(-) varying f rom 75' t o 80' a n d Z A C

varies from 12' t o 15'. The B i r e f r i n g e n c e r a n g e s f r o m 0.019 t o 0.021.

H e d e n b e r g i t e i s d i s t i ngu i shed f rom hypers thene by i t s weak pleochroism in

shades of green x = sea green, y : bluish g r e e n , and z = p a l e ye l lowish

g r e e n . The o p t i c a x i a l a n g l e v a r i e s f r o m 52' t o 58' with Z A C ranging

between 40' and 50'. Birefringence var ies f rom 0.025 t o 0.027. Eul i te i s

f o u n d in s o m e s e c t i o n s in a s s o c i a t i o n with magne t i t e and qua r t z which i s

pleochroic - x = c la re t red , y = yellow and z = sea green. The 2v(-) r anges

between 15 t o 18' and birefringence varies f rom 0.017 t o 0.019.

Deep red, pink, or brown ga rne t is found occasionally in s o m e sect ions

with g r a i n s of va ry lng s i z e a n d s h a p e . T h e s e g r a l n s a r e isotropic a n d

exhibits high relief and a r e charac te r i sed by number of c racks . A p a t i t e is

found a s a n acccssory in few sec t ions which is in t h e f o r m o f smal l needle

l i k e p r i s m a t i c c r y s t a l s (PI. 2, Fig.b & PI. 3, F i g . a ) a n d occasionally a s

rounded grains. Mostly t h e y a r e a r ranged paral lel t o t h e bands a n d a t s o m e

places these protrude f rom iron oxide grains (PI. 3, Fig.a).

O r e microscopic study of BIF:

Under re f lec ted light magnet i te is g rey in colour with brownish t inge.

The d e g r e e of mart i t isat ion is general ly low, a l t h o u g h t h e s u r f a c e s a m p l e s

s h o w m u c h a l t e r a t i o n t o h e m a t i t e (PI . 3, Fig .d) c h a r a c t e r i s e d by t h e

rep lacement a n d re l ic t textures. The a l t e r a t i o n is m o s t pronounced at t h e

margins and along 11 1 planes in magnet i te grains, with progressive ox ida t ion

t h e elongated h e m a t i t e l amel lae broaden a n d only residual a r e a s of m a g n e t i t e

remains. Magneti te showing cubic part ing planes which is ind ica ted by m e a n s

of t r i a n g u l a r p i t s (P1. 3, Fig.e). In t h e f i n a l s t a g e s of r e p l a c e m e n t n o

m a g n e t i t e is l e I t but i t s previous presence is ind ica ted by a Widmans ta t ten

l i k e t e x t u r e in h e m a t i t e . R e f l e c t a n c e of m a g n e t i t e in a i r in g r e e n l igh t

ranges from 20.4 t o 21.2 percent a n d i t is posi t ive t o e t c h reac t ion wi th 50%

HN03, 50% HCI, aquaregia, and SnC12-HCI s o l u t i o n . V i c k e r ' s h a r d n e s s of

magnet i te ranges from 600 t o 620 a t 50 g m s load and g o e s gradua l ly down

at higher loads which is in a c c o r d a n c e with t h e observa t ion of Nakhla (1956).

L i m o n i t e a n d G o e t h i t e a r e f o u n d a s a l t e r a t i o n products. T h e m a g n e t i t e -

q u a r t z i t e conta in mainly magnet i te without a n y i lmeni te with i t s a l t e r a t i o n

products .

4 X - r a y d i f r a c t o g r a r n s of m a g n e t i c c o n c e n t r a t e s o f magnet i te -quar tz i te

1

s n o w s t h e p r e s e n c e of o x i d e p h a s e s in t h e f o r m of m a g n e t i t e o r i t s

a l t e r a t i o n p r o d u c t s . T h e r e a r e n o su lphide , s i l i ca te and carbonate phase

minerals. Petrographic study of BIF indicates t h e presence of magnet i te or

i t s a l t e r a t i o n p r o d u c t s wi thout any i lmenite. Only t h e oxide minerals a r e

present. The BIF belongs t o oxide facies of James (1954) since t h e r e is no

iron si l icate, carbonate, or sulphide in BIF. The sedimentary c h a r a c t e r s of

BIF if any w e r e no t p r e s e r v e d b e c u a s e i t was s u b j e c t e d t o high g r a d e

metamorphism t o granuli te facies. Recrystallization and re format ion of iron

and s i l i c a bands were o b s e r v e d . The i n t r u s i o n of m e t a v o i c a n i c s a n d

metamorphism t o granulite facies metamorphism causes recrys.tallization and

reformation o i iron and s i l i c a and t h e r e w a s no p r e s e r v a t i o n of p r i m a r y

s t r u c t u r e s . The c e l l d imens ion of m a g n e t i t e r a n g e s f r o m 8.3929A0 to

8.3957~'. The hemati te , goethi te , and lirnor>ite phases a r c obscrvcd a s nri

a l te red products.

Table - 2: Modal analysis of banded iron formations of Kan jamalai-Godumalai a r e a s of Tamil Nadu with range and average

Mineral Range Average

Irorl o r e

Quar tz

Hypersthene

Hedenbergite

Gruneri te

Garnet

Eulite

Apati te

Petrography of granulites:

The granulites of Kan jamalai-Godumalai a reas a r e pyroxene granulites

and a r e t h e most i n t i m a t e l y a s s o c i a t e d rocks wi th magnetite-quartzites.

They a r e grey to dark grey in colour, coarse t o fine grained, and generally

massive. They exhibit granulitic and granoblastic t ex tures (PI. 4, Fig.c & e

arrd PI. 5, pip.^). T W O v a r i e t i e s of pyroxene g r a n u l i t e s a r e o b s e r v e d ( i )

Garnetiferous pyroxene granulites and (ii) pyroxene granulites. Thin sect ion

studies show the presence of minerals - pyroxene (opx & cpx), plagioclase,

hornblende , b i o t i t e , g a r n e t , c h l o r i t e , q u a r t z , a p a t i t e and opaques. The

opaques a r e in minor proportion. The different assemblages are:

opx-cpx-plag+opehbl-opaques

opx-cpx-plag+hblsarnet-opaques

opx-cpx-plag~hbl~arnet+biotite-opaques

opx-cpx-hbl-plagsar+biotite-opaques

opx-cpx-hbl-plag+biotite2z-apatite-opaques

opx-cpx-plag+qz-gar-hbl+bio;ichl+apat-opaques

The orthopyroxene (EnGO FsllO) occurs a s subidioblastic t o xenoblast ic

grains. It is strongly pleochroic in shades of green t o pink x = pink, y =

yellowish green and z = pale green. It has -2v, 52-64' ranging in d i f fe ren t

sections. Hypersthene is completely or partly replaced by hornblende, b io t i t e

or act inoti te . Relict pyroxene grains a r e found in hornblende around which

there is fibrous amphibole (PI. 4, Fig.a & d). Due t o conversion of pyroxene

t o hornblende release of iron oxide is noticed (PI. 4, Fig.b & PI. 5, Fig.a).

Subhedral t o anhedral grains of quartz in minor proportion a r e presen t within

hypersthene. In some sect ions small prismatic grains of a p a t i t e a r e presen t

within hypersthene which a r e showing very high rel ief (PI. 4, Fig.c). A rim

of clinopyroxene around hypersthene is also observed. Hornblende is found t o

bc dcvclol~cd 1ro111 bol l \ l~ypcrsllicllc n ~ i d cliliol,yroxe~~c (1'1. 4, 1:ig. a & dl.

P l a g i o c l a s e observed in large quantities occurring a s ind~vidua l grains

or a s small inclusions in pyroxene (PI. 5, Fig.c). Plagioclase grains a r e f resh

and show good cleavage and twinning. It encloses pyroxenes a t some places

(PI. 5, Fig.d). Both twinned and untwinned grains of plagioclase a r e present .

The composition of plagioclase is andesine (40 t o 50 An) t o labradori te (60 t o

6 8 An). Plagioclase and pyroxenes exhibit polygonal outl ines in some sec t ions

( P I . 5 , F i g . f ) . P l a g i o c l a s e e n c l o s e s b o t h o r t h o a n d c l i n o p y r o x e n e s .

Pyroxene grains also conta in p lag ioc lase . D e v e l o p m e n t of h o r n b l e n d e or

biot i te is observed in pyroxenes. In some granuli tes exclusively t h e presence

of t w o pyroxenes with minor proportion of plagioclase is observed.

C a r n e t i f e r o u s g r a n u l i t e s c o n t a i n g a r n e t in s i g n i f i c a n t p r o p o r t i o n s

t o g e t h e r with pyroxene , p lag ioc lase , and quar tz (PI. 4, Fig.f). It exhib i t s

s y m p l e c t i c t e x t u r e (PI. 4, Fig.f). G a r n e t c o n t a i n s e u h e d r a l g r a i n s of

p l a g i o c l a s e and q u a r t z . At t h e c o n t a c t of garne t and pyroxene t h e r e i s

development of hornblende a n d b i o t i t e (PI. 5, Fig.b). G a r n e t m a y h a v e

d e v e l o p e d due t o in te rac t ion between pyroxene, plagioclase, and iron oxide

during metamorphism. R i m of g a r n e t s o v e r p y r o x e n e a t p y r o x e n e a n d

plagioclase in te r face is observed.

Green t o bluish green secondary hornblendes a r e found t o b e developed

due t o retrogression and they occur a s p a r t i a l m a n t l e s or c o r o n a s a r o u n d

hypersthene (PI. 4, Fig.a & d). The pleochroism of hornblende is d i s t inc t ive

in shades of green. Textural ev idence shows t h e h o r n b l e n d e s a r e f o r m e d

l a t e r t h a n h y p e r s t h e n e a n d a r e s e e n t o b e d e r i v e d f rom pyroxene. The

coronas or par t ia l mant les o f secondary hornblende may b e fo rmed subsequent

t o t h e m a i n s t a g e o f me tamorph i sm. Hornblende a l so man t l e s b io t i t e and

opaque grains (PI. 5, Fig-a). Besides occurr ing a s narrow grains hornblende is

a l s o i n t e r g r o w n w i t h b i o t i t e , p l a g i o c l a s e , a n d i r o n o x i d e n e a r t h e

or thopyroxcne grains. The widespread mantling 01 Irypcrsll\cne, b io t i t e , and

opaque g r a i n s b y h o r n b l e n d e i n d i c a t e s t h a t i t is a l a t e r phase. In some

sect ions where hornblende is predominant in addi t ion t o qua r t z and opaques,

ac t ino l i t e is found t o b e developing f rom hornblende. Minute pr ismat ic , and

needle shaped grains of a p a t i t e a r e present in high proportions. They a l so

occur within qua r t z grains.

The typical clinopyroxene is aug i t e which is cha rac t e r i s ed by p r i sma t i c

c leavages . It is euhedral t o subhedral in na tu re and shows a l t e r a t i o n to a

fibrous mater ia l along t h e cracks . It has +2v = 58', Z A C var ies f rom 40 t o

43'. Concentra t ion of iron oxide along t h e borders of t h e grains, suggest ing

expulsion of iron oxide t o t h e borders have observed cha rac t e r i s t i ca l ly , s o m e

t imes t h e iron oxide forms a s coronas around plagioclase a n d pyroxene (PI. 4,

Fig.b and PI. 5, Fig. a & e ) .

Development of secondary hornblende f rom g ranu l i t e f ac i e s rocks h a v e

been repor ted by Sen a n d Oliver (1981) f rom Mann region of Austra l ia . I t is

found t o be common phenomena in granul i te f ac i e s t e r r a ins . T h e n a t u r e o f

r eac t ions t h a t produce secondary h o r n b l e n d e s is u n c e r t a i n . T h e p l a u s i b l e

r e a c t a n t s a r e hypers thene a n d plagioclase:

Orthopyroxene+ plagioclase+ wa te r ---> hornblende+ m a g n e t i t e + plagioclase.

The involvement of ionic reac t ions in t h e format ion of secondary hornblendes

its wel l its scc 'o~i t l~~ry rr~ilrcrals liltc b iu l i l c atid gart icl is a l so possible.

T h e d e v e l o p m e n t of s e c o n d a t y hornblendes may be regarded a s t h e

manifestat ion of retrogression of granuli te fac ies r o c k s which is c o m m o n l y

noticed in high grade metamorphic terrains. The f i r s t s t a g e of retrogression

is rep lacement of o r t h o p y r o x e n e by a p a l e g r e e n f i b r o u s c u m m i n g t o n i t e

oriented parallel t o t h e orthopyroxene cleavage. Clinopyroxene is r immed by

act inoli te . As r e t r o g r e s s i o n p r o c e e d s o r t h o p y r o x e n e a n d c u m m i n g t o n i t e

d i s a p p e a r a n d c l i n o p y r o x e n e i s r i m m e d by a b l u e g r e e n h o r n b l e n d e .

Hornblende and plagioclase have highly irregular grain boundaries but c lear ly

show pseudomorphism and polygonal outlines of t h e grains.

S e v e r a l t y p e s of h o r n b l e n d e grains occur: (a) grains surrounded w i t h

opaque iron oxide dust believed t o have formed granuli te fac ies hornblende by

t h e exsolution of opaque iron oxide, or (b) subhedral hornblende gra ins with

q u a r t z i n c l u s i o n s which m a y b e d e v e l o p e d f r o m c l i n o p y r o x e n e , o r ( c )

recrystal l ized euhedral t o subhedral grains of hornblende. The t r a n s f o r m a t i o n

of pyroxene t o amphibole is accompanied by a d e c r e a s e in a n o r t h i t e c o n t e n t

of plagioclase from An50-75 t o An25-25. Retrogressed m a f i c rocks h a v e less

p l a g i o c l a s e . B i o t i t e m a y o v e r g r o w o p a q u e oxides a t s o m e places. T h e

c o n v e r s i o n of p y r o x e n e t o h o r n b l e n d e in t h e g r a n u l i t e s l e a d i n g t o t h e

d e v e l o p m e n t of r i m s or c o r o n a s o f h o r n b l e n d e a round pyroxene, probably

sugges ts t h a t t h e granuli tes have been re t rograded during l a t e r d e f o r m a t i o n

or during the i r formation when conditions were riot to ta l ly dry.

Based on t h e m i n e r a l a s s e m b l a g e , i t is sugges ted t h a t retrogression

may have occurred a t t e m p e r a t u r e between 650-550'~. The re t rogress ion may

have caused t h e influx of l a r g e v o l u m e s of d o m i n a n t l y h y d r o u s f l u i d , a s

suggested by Beach (1980). The source of fluids may b e (a ) der ived f rom

degassing of lower c rus ta l rocks undergoing progressive metamorphism or (b)

d i r e c t l y i r o m m a n t l e or ( c ) d e r i v e d f r o m t h e d e w a t e r i n g of subducting

oceanic crust . Whatever may be u l t imate source of fluids, retrogression of

A r c h a e a n g r a n u l i t e s under c o n d i t i o n s appropr ia te t o t h e middle c r u s t is a

common phenomena (Sills, 1983).

Retrograde hornblendes commonly occur in granul i te fac ies rocks a n d

they a r e lighter in colour than green or brown original hornblendes. In t h e

present study t h e tex tura l fea tures s h o w s e c o n d a r y h o r n b l e n d e s a r e b e i n g

d e r i v e d f r o m pyroxenes . In rocks of basic composit ion par t ia l mant les o r

coronas o f secondary h o r n b l e n d e s a r e f o u n d a r o u n d o r t h o p y r o x e n e . T h e

h o r n b l e n d e a l s o m a n t l e s b i o t i t e and opaque grains. Besides occur r ing a s

narrow rims hornblende is also intergrown with biot i te , feldspar, and iron o r e

near orthopyroxene grains. Hornblende occurs mos t commonly a t t h e i n t e r

f a c e s b e t w e e n p l a g i o c l a s e a n d h y p e r s t h e n e g r a i n s . T h e m a n t l i n g o f

hypersthene, biot i te , and opaque grains by hornblende ind ica tes t h a t i t is a

la te r phase.

Secondary a m p h i b o l e s p r o d u c e d by r e t r o g r e s s i o n f r o m p y r o x e n e i n

g r a n u l i t e f a c i e s r o c k s , a r e d i s t i n c t i v e in th ie r op t ica l c h a r a c t e r s a n d a r e

repor ted from d i f fe ren t g r a n u l i t e f a c i e s r o c k s ( H i m m e l b e r g a n d P h i n n e y ,

1957; Drury, 1973; Sen and Ray, 1971; Sen and Oliver , 1981; Sills, 1983; Ja in

et al., 1991). G r a n u l i t e s m a i n l y c o n t a i n s p y r o x e n e a n d p l a g i o c l a s e o f

gabbroic composition. Hornblende is found a s a n a l te ra t ion product. In f e w

sect ions exclusively orthopyroxene and c l i n o p y r o x e n e a r e p r e s e n t . A f e w

sect ions show t h e presence of garnet-pyroxene-plagioclase-iron o r e and quar tz

(PI. 4, Fig.f). At t h e c o n t a c t of garne t and pyroxene t h e r e is development

of hornblende and biotite. Quar tz grains present within pyroxene grains show

a type of poikiloblastic texture.

The granulite which exclusively contains hornblende and plagioclase in

v e r y high propor t ion c o n t a i n s minor proportion of orthopyroxene, iron ore,

a p a t i t e and biotite. The orthopyroxene is typically of hypersthene (En35-F~45)

composition and shows pleochroism from pink t o green with a n a v e r a g e of 2v

(-58 t o 60'). Iron ore occurs a s g r a i n s a s s o c i a t e d w i t h h y p e r s t h e n e a n d

hornblende and s m a l l p a r t i c l e s a long t h e c l e a v a g e s . H o r n b l e n d e s h o w s

pleochroism in shades of green a s x = yellowish green, y = brownish green

and z = dark green. Z A c varies from 16 t o 18' and i t s 2v ranges from 74'

t o 78'. Hornblende grains shows perfect lineation a t several places. Bioti te

is seen along t h e cleavages and cracks of hornblende. It is reddish brown

when i t is associated with iron ore and shows p l e o c h r o i s m f r o m y e l l o w t o

brown. Apati te occurs a s slender prisms enclosed in o ther minerals in these

granulites.

Pyroxene and plagioclase In granulites e x h i b ~ t polygonal outl ines (PI. 5,

Fig.f) which is c h a r a c t e r ~ s t i c o f g ranul i tes derived f r o m rocks of igneous

wig in (Katz, 1978). Since t h e pyroxene granuli tes conta ins hypers thene t h e y

may be t e rmed as g r a n o l ~ t e s (Wtnkler, 1977) and it c a n be in[crred t h a t t h e y

suffered granulite facies metamorphism. In f e w c a s e s r e t r o g r e s s i o n t o o k

p lace resulting in t h e development of secondary hornblende. The r n i n e r a o g y

and t e x t u r e of t h e rocks suggests t h a t t h e r o c k s s u f f e r e d g r a n u l i t e f a c i e s

m e t a m o r p h i s m a t a t e m p e r a t u r e of 6 5 0 - 7 5 0 ' ~ . F r o m t h e mineralogical

c h a r a c t e r s described above i t can b e inferred t h a t t h e paren t m a t e r i a l for

t h e granuli tes may b e gabbroic in composition.

O r e microscopic studies of granulites:

Opaque minerals present in pyroxene granuli tes a r e magnet i te , i lmenite,

c h a l c o p y r i t e , g o e t h i t e a n d mar t i t e . Magneti te is greenish brown in colour

and is isotropic. Ref lec t iv i ty in g r e e n l i g h t r a n g e s f r o m 19 .8 t o 20.6%.

Vicker's hardness a t 500gms load ranges f rom 580-610. It shows +ve t o e t c h

reac t ion with 50% HCI, H F and aquaregia. It is found a s smal l p la tes or a s

i rregular grains, commonly contains i lmenite in d i f fe ren t forms. It is a l so

found along t h e si l icate grain margins. In few c a s e s m a g n e t i t e is peppered

h e r e and t h e r e within t h e s i l i ca te grains. Mart i t isat ion general ly low e x c e p t

in a f e w grains. Alterat ion of m a g n e t i t e t o g o e t h i t e i s f o u n d a l o n g t h e

b o r d e r of t h e g r a i n s d u e t o hydration. Some t i m e s rep lacement proceeds

i n t o t h e grains leaving re l ic t magneti te . I lmenite is found in t h e form of

thick and elongated rods occupying t h e oc tahedra l part ing planes commonly

and ra re ly cubic parting planes (PI. 3, Fig.f). I lmenite is pinkish brown in

c o l o u r a n d h a s l o w e r r e f l e c t i v i t y t h a n m a g n e t i t e . It s h o w s s t r o n g

bireflectance, and anisotropism. The re f lec t iv i ty in green l ight r a n g e s f r o m

18.5 t o 18.7%. It is -ve t o a l l e t c h reagents . H e m a t i t e is l ight g r e y wi th

bluish t i n g e and feeably anisotropic. H e m a t i t e shows r e d i n t e r n a l re f lec t ions

accasionally and t h e re f lec t iv i ty ranges f rom 24 t o 26% in g r e e n light.

Two generat ions of i lmeni te a r e noticed in s o m e o r e mounts. In t h e s e

cases o n e or t w o s e t s of long thin rods of i lmeri i te occupying t h e o c t a h e d r a l

parting planes Iorrns t h e f irs t generat ion. Among these o n e s e t is p rominent

and thicker than t h e other . Very fine droplets or shor t worm or lense l ike

g r a i n s of i l m e n i t e of s e c o n d g e n e r a t i o n a r e n o t i c e d , exhibit ing emulsion

tex ture . Ilmenite lamellae occurring in cubic p a r t i n g p l a n e s a r e v e r y f i n e

a n d l l ~ o s c ill o c t a l ~ c d r a l part ing planes a r e cortrsc and rnair>tain a cons tan t

angle of 60-120' with one another producir~g a widrnarrstatlen pat tern. The

i l m e n i t e l a r n e l l a e h a v e w e l l d e f i n e d margins against magnet i te and ra re ly

d i sc re te grains of i lmenite a r e noticed.

All these features suggests t h a t i lmenite l a m e l l a e w e r e f o r m e d a s a

consequence of exsolution caused by oxidation or cooling. It is bel ieved by

s o m e investigators t h a t t i t anomagnet i te contains magnet i te and spinel in solid

s o l u t i o n and t h e o x i d a t i o n of which causes t h e development of exsolut ion

blebs of i lmenite along t h e crystallographic p lanes of m a g n e t i t e . A n o t h e r

way of accounting for exsolution is based on geochemical principle t h a t fair ly

la rge amourlts of t i tanium will be in solid solution with in magnet i te a t high

t e m p e r a t u r e which g e t s exsolved a s i lmenite a t low tempera ture .

Working group of G.S.I., N.G.R.I., and A.M.D. studied t h e geology and

g e o c h e m i s t r y of granuli te t e r ra in of North Arcot, Pal lavaram a n d a d j a c e n t

a r e a s . A c c o r d i n g t o t h e work ing g r o u p t h e p y r o x e n e g r a n u l i t e s a r e

m e t a g a b b r o s whi le hornblendi tes a r e basic/ul trabasic volcanic flows. They

g a v e P-T conditions of metamorphism a s 800-900' and 9-12 k bars pressure.

A c c o r d i n g t o t h e m t h e P-T c o n d i t i o n s of equilibration of t h e assemblages

cor respor~d t o a depth burfial of 20-30 krns and this cor robora tes w i t h t h e

d a t a f rom o ther Shield a r e a s of t h e world. Weaver e t al. (1982) found t h a t

t h e granuli tes of Madras display t h e tholei i t ic c h a r a c t e r and g a v e t h e P - T

condi t ions o f metamorphism a s 8 0 0 - 9 0 0 ' ~ and 9-11 k bars pressure.

According to them the P-T conditions of equilibration of the assemblages

correspond to a depth burfial of 20-30 kms and this corroborates with the

data from other Shield areas of the world. In view of the above the

pyroxene granulites of the area studied are considered as metavolcanic rocks

I,aving the characters of tholeiites which have suffered grarlulite facies

metamorphism.

Photomicrographs of BlF

PLATE - 1

a. BIF exhibits typical banding with magnet i te and quar tz . Polarised light x 56

b. BIF showing granulitic texture. Polarised l ight x 56

c. BIF conta in ing t h e minera l a s s e m b l a g e of hypers thene ,

garnet, chlorite and biotite.

Polarised light x 56

d. BIF showing a d e n texture with aggregates of pyroxene.

Polarised light x 46

e. BIF conta in ing m a g n e t i t e a n d q u a r t z . Q u a r t z s h o w i n g

sheaf like appearance.

Crossed Nicols x 56

f . E u h e d r a l t o s u b h e d r a l g r a i n s o f h y p e r s t h e n e within magnetite showing sieve texture.

Polarised l ight x 56

4 8

PLATE -1

Photomicrographs of BIF

PLATE - 2

a. Alternating layers of quartz and magnetite in B1F. Crossed Nicols x 56

b. Inclusions of apat i te within magnetite in BIF.

Polarised light x 56

c. Hypersthene showing a l tera t ion t o c h l o r i t e a n d b i o t i t e .

Iron oxide giving a re t icula te pa t t e rn in t h e pyroxene

grain.

Poarised light x 56

d. BIF containing magnetite and quartz. Fine closely spaced

cracks in quartz filled with iron oxide.

Crossed Nicols x 56

e. Inclusions of pyroxene within magneti te, bands of iron

oxide is also present.

Polarised light x 56

f. Small grains of iron oxide in quartz grains.

Crossed Nicols x 56

ii 9

PLATE- 2

Potomicrographs of BIF

PLATE - 3

a. Fine needles of apatite projecting from iron oxide grains

(LcSt cc~rtral ill-en).

Polarised light x 56

b. Pyroxene aggregates within iron oxide bands in B1F.

Polarised light x 56

c. 'Subhedral to anhedral pyroxenes within iron oxide in BIF. Polarised light x 56

d. Magnetite in BIF showing alteration to martite.

Reflected light, Nicols not crossed in oil x 600

e. Magnetite in BIF showing the cubic parting planes due to

alteration inditated by means of triangular pits.

Reflected light, Nicols not crossed in oil x 600

f . Magnetite (in pyroxene granulite) containing fine lamellae

of ilmenite.

Reflected light, Nicols not crossed in oil x 600

5 0

P L A T E - 3

Photomicrographs of Crarlulites

PLATE - 4

a. R i m s of hornblende around relict pyroxene. Plagioclase

containing inclusions of pyroxene. Polarised light x 56

b. Release of iron oxide formed arond the grains of clino-

pyroxene.

Polarised light x 56

c. Granulite exhibit ing granoblastic t e x t u r e with mineral

assemblage of orthopyroxene, clinopyroxene, hornblende

and prismatic grains of apatite.

Polarised light x 56

d. Rims of hornblende around hypersthene.

Polarised light x 56

e. G r a n u l i t e showing granoblastic t e x t u r e wi th minera l

a s semblage of o r thopyroxene , c l i n o p y r o x e n e a n d

plagioclase.

Polarised light x 56

f . Garnetiferous granulite with mineral assemblage of garnet

pyroxene arid plagioclase. Garnet shows sy r r~p iec t i c

texture. Development of hornblende and bioti te a t t h e

contact of garnet and pyroxene.

Polarised light x 56

5 1

PLATE - 4

PLATE - 5

a. C o n c e n t r a t i o n o f i r o n o x i d e a l o n g t h e b o r d e r s o f

plagioclase and pyroxene due t o recrystal i izat ion.

Polarised light x 56

b. Deve lopment of h o r n b l e n d e and b i o t i t e d u e t o r e t r o -

gressive metamorphism.

Nicols nearly crossed x 56

c. G r a n u l i t i c t e x t u r e w i t h m i n e r a l a s s e m b l a g e o f

o r t h o p y r o x e n e , c l i n o p y r o x e n e , p l a g i o c l a s e , q u a r t z ,

hornblende and iron oxide. Pyroxene conta ins plagioclase.

Polarised tight x 56

d. Pyroxene within plagioclase and plagioclase in pyroxene.

Polarised light x 56

e. Granulite c o n t a i n i n g o r t h o p y r o x e n e , c l i n o p y r o x e n e a n d

plagioclase, convers ion of p y r o x e n e t o hornblende a n d

concentration of iron oxide along t h e borders.

Polarised light x 56

f. Pyroxene and plagioclase showing polygonal outlines.

Polarised light x 56

5 2

PLATE - 5