CHAPTER -2

Geological Framework

GEOLOGICAL FRAMEWORK

2.1 Regional Geology of Dharwar Craton

A craton consists of old continental crust that was formed during Archean and

has attained stability by 2500 Ma ago. Some of the cratons expose large areas of

greenstone-granite terranes which enable us to study the process of continental crust

formation during the Archean. Cratons form part of a 'Shield' which is principally a

tectonically stable continental mass that predominantly comprises of Precambrian

rocks.

The Indian Peninsular Shield is such a stable continental mass among several

others on the Earth. The Peninsular India consists of the following shields and they

are 1) Southern Indian Shield, 2) Northwest Indian Shield, 3) Central Indian Shield

and 4) Eastern Indian Shield. Each of the above shields is characterized by one or

more cratons and mobile belts usually with tectonized boundaries (Radhaknshna,

1990). The Southern Indian Shield consists of 1) Dhanvar Craton, 2) Eastern Ghat

Mobile Belt, 3) Southern Granulite Terrane and 4) Proterozoic Sedimentary Basins.

During the early Cretaceous the Southern Indian Shield was part of the Gondwana

Supercontinent and was extending to the present-day Africa to the west and Australia

to the east (Naqvi and Rogers, 1987).

The Dhanvar Craton covers an area of 238,000 km' lying between the

latitudes N 12" 0' to N 18' 0' and longitudes E 74' 0' to E 80" 0' (Fig. 2.1). The

Dharwar craton occupies the northern part of the Southern Indian Peninsular Shield

and is bound by the Deccan Traps towards north and the high-grade granulite terrains

to the south and east. The Dhanvar craton is divided into an eastern and a western

domain with reference to N-S trending belt of granitoid intrusives known as "Closepet

Granites" (Viswanatha and Ramakrishnan, 198 1 ; Allen et al., 1 986). Subsequently,

Chadwick et al. (1992) suggested that a N-S sinistral shear zone that forms the eastern

boundary of the Chitradurga Schist Belt marks the boundary between eastern and

western parts of the Dharwar Craton.

The western Dhanvar craton is characterized by the presence of bimodal

volcanics, volcano-clastic and chemical sediments with the sedimentary component

dominating over the volcanics. Chitradurga Schist Belt, Bababudan Group and

Shimoga Schist Belt, to name a few, form part of the western Dharwar craton. The

eastern Dhanvar craton is interspersed with several schist belts such as the Kolar,

Rarnagiri and Hutti schist belts. which are surrounded by Iate .kchean granitoid

rocks. The schist belts of the eastern Dhanvar craton are made up of basaltic rocks

subordinated with chemical sediments. Unlike their \vestern counterparts. clastic

sediments such as, argillite and greywacke, are found only in negligible quantity in

the schist belts of the eastern Dharwar craton. A clear indication of a gneissic

basement for the supracrustals of Dharwar Group as encountered in the western

Dharwar craton (Chadwick et al.. 1981a & b and Chadwick et al.. 1985 a & b) is not

reported in the eastern Dhanvar craton.

Granitoid gneiss 8 intrusives

11 I 1 1 Closepet granitoids

Eastern Dharwar supracrustals

I 1 Western Dhawar supracrustals

Fig. 2.1 Geological map of the Dharwar Craton, southern India (modified after Chadwick et al., 1996). The Dharwar Craton is bounded by the Deccan Traps, Granulite Terrane and Cuddapah Basin. The eastern and western parts of the Dhanvar Craton are considered to have had distinct evolutionary histories. Ba - Bababudan, Sh - Shimoga, Ch - Chitradurga, Hu - Hutti, KO - Kolar, Ra - Rarnagiri and Sa - Sandur Schist Belts. M - Mylonitized zone on the eastern margin of the Chitradurga Schist Belt.

Gold mineralization in the schist belts of the eastern Dhan\.ar craton is another

characteristic feature, for e.g. Kolar, Ramagiri and Hutti schist belts are well, kno \~n

for Au-mineralization (Siva Siddaiah and Rajamani, 1989; Siva Siddaiah er crZ., 1994;

Giritharan and Rajamani, 1998, 2001). The geology of major schist belts of eastern

Dhanvar craton and their surrounding areas are discussed in the following paragraphs.

A compilation of available geochronological data is given in Table 2.1.

2.2 Geology of the Hutti Schist Belt and its Environs:

Hutti is located at about 200 km southwest of Hyderabad, 350 krn north of

Bangalore and 80 km west of Raichur in the Karnataka State (Fig. 2.1). The Hutti

Schist Belt also known as Hutti-Maski Schist Belt is a hook shaped belt (Fig. 2.2)

dominated by metamorphosed basalts, basaltic andesites, dacites, rhyolites, and quartz

porphyry in the order of decreasing abundance as exposed at the surface. Banded

Fenuginous Quartzite, greywacke, argillite, garnetiferous mica schist, andalusite

schist and garnet-cordierite schist also occur as thin impersistent bands within the

metavolcanics. Details of lithology, structure, metamorphism and petrogenesis are

discussed in Ramaswamy and Krishnamurthy (1968), Roy (1979), Anantha Iyer and

Vasudev (1979), Roy and Biswas (1982), Riaz Ulla et nl. (1996), Basir and

Balakrishnan (1999) and Vasudev et al. (2000).

The schist belt has two arms, one trending north - south (N-S arm) and the

other trending nearly west-northwest - east-southeast (referred to as E-W arm). The

mafic metavolcanics are characterized by pillow structures which are observed at

several places (e.g. Madrainakota, Chinchargi, Wandali and Maski) indicating that

they were originally laid under submarine conditions. Felsic volcanics show sharp but

conformable contact with the mafic units and are mostly confined to the eastern arm

of the schist belt. A thin unit of felsic volcanics is also encountered at the eastern

margin of the N-S ann of the schist belt near the village Gajalagatta.

Three distinct phases of folding of the rocks of the Hutti Schist Belt have been

reported by Roy (1979). The first phase of folding deformatioxi (FI) is characterized

by the development of microscopic and mesoscopic isoclinal folds and of penetrative

planar feature defined by the preferred orientation of chlorite, biotite and hornblende.

F2 folding is accompanied by the development of crenulation cleavage or fracture

cleavage and minor folds and show clear evidences of superposition of F1 folds.

Table 2.1 Ages of the schistose and granitoid rocks of the Kolar, Ramagiri and Hutti areas, Dhanvar Craton, south India.

Pb-Pb

age (Ma)

2732 * 155

12844

2746 5 64

_

' 2561 + 24

U-Pb

zircon

age ma)

2631 * 6.5 2610=k 10 2551 5 2.5

>3 140 2532*3

2707 + 18 12650 5 7 2613 4Z 6

252851

2580 =k 31

Reference

Hanson et a/. 1986 Balakrishnan et al. 1990 Balakrislman et al. 1990

Krogstad et a?. 199 1 Krogstad ef al. 1991 Krogstad et al. 1991 Krogstad ef a/. 1991 Krogstad et al. 1991 Krogstad ef al. 199 1 Krogstad et al. 1991

Zachariah et ul. 1995 Zachariah ef al. 1995

Balalu-ishnan et al. 1999 Balakrishnan et a/. 1999 Balakrishnan cf al. 1999 Balakrishnan et a/. 1999 Balakrishnan et al. 1999 Balakr-ishnan ct a/. 1999 Balakrishnan et ul. 1999

Vasudev et a/. 2000 Vasudev et 01. 2000

U-Pb

titanite

age Wa)

2552 + 1

25174~2 2501 + 8

2545 + 1 2614 + 4 2595 * 1 25164~3 2510 * 2 2468 * 4

i

Unit

Kolar area: Kolar amphibolites Central supracrustals Western anphibolitcs Champion Gneiss Dod Gneiss Dosa Gneiss Patna Granite Banded Gneiss. Kambha Gneiss Agmatite Gneiss

Ramagiri area: Central block metabasalts Unaltered dike Central prong Chenna Gneisses Ramagiri Complex

Gangam Complex

Hutti area: Palkanmardi conglomerate Kavital granodiorite 150 km south-east of Hutti

Sm-Nd

age (Ma)

2690 5 140

2696 * 136

2454 5 100

Rock Type

Komatiites Massive tholeiitic amphibolites High-Mg amphibolites Jnclusion Granodiorite gneiss Granodiorite gneiss Granodiorite Granodiorite gneiss Granodiorite gneiss Granodiorite gneiss

Amphibole-phyric metabasalts Diabase dike P yroclastic Granodiorite gneiss Granodiorite Quartz-diorite Granodiorite Monzodiorite Post-kinematic granite

Granodiorite clast Granodiorite

Chapter 2 Geologica! Frume~t.oi*k

H-5

Quartz PorphyryIAcid volcanic

Palkanmardi Mixtite

Eastern Granitoids

Fig. 2.2 Geological map of the Hutti Schist Belt and the surrounding granitoids (modified from Srikantia, 1995). Sample locations H-1 and H-7 are from the northern granitoids. Sample location H-2 is from the western granitoids. Sample location H-5 and H-9 are from the eastern granitoids. The schist belt is hook shaped and is classified into E-W and N-S anns based on the metamorphic mineral assemblage. Palkanrnardi Polymictic Conglomerate is exposed in the northern part of the E-W arm of the schist belt. Felsic volcanic rocks are found intercalated with the schist belt rocks at several places.

Clzupter 2 Geological f i u n ~ e ~ r ~ o r k

The F2 folds plunge about 50" towards NW to iLyW and the regional h%W-SSE

trend of the N-S arm of the Hutti Schist Belt is considered to represent the axial trend

of F2 folds. Roy (1979) also considered that the hinge of the hook shaped belt as the

axial trace of FZ antiform. Intrusion of granitoid veins and growth of metamorphic

minerals along the schistosity planes (S1) that were developed during F1 folding has

been reported. Therefore, these veins are considered to predate F2 folding. F3 folds,

which have a distinct ENE-WSW trend, are associated with the development of kink

bands or chevron folds. Innumerable minor faults, fractures, shears and eiz echelon

folds have been recognized in different parts of the schist belt. A major fault known as

Oakley's Fault has also been identified in the Hutti mine area (Ramaswamy and

Krishnamurthy, 1968; Raju, 1978; Roy, 1979 and 1991 and Raju and Sharma, 1991).

Based on mineral assemblages and mineral chemistry a metamorphic grade

transitional between greenschist and amphibolite facies has been reported for the

Hutti metabasalts by Riaz Ulla et al., (1996).

Giritharan and Rajamani (1998) studied the geochemistry of the tholeiitic

metavolcanics in the Hutti Schist Belt. Based on geology, mineralogy and

geochemistry of the Hutti metabasalts, Giritharan and Rajamani (1 998) suggested that

the metavolcanics in the southern part (N-S arm) of the Hutti belt have undergone

variable extents of low-grade alteration, while the metavolcanics in the northern part

(E-W arnl) of the belt are not noticeably affected by alteration processes. The

amphibolites are uniform in composition with about 48 wt% silica and 15 wt%

alumina. The high alumina content of the amphiboles and the relatively lower

abundance of plagioclase indicate that the amphibolites are of pargasitic composition.

The Hutti metavolcanics have high TiOz and iron content.

The trace element concentrations of the Hutti metabasalts are similar to those

comnlonly observed in tholeiitic basalts of the Archean greenstone belts. The Hutti

metabasalts are characterized by flat to moderately LREE-enriched patterns (Fig.

2.3a) that are generally smooth except for positive and negative Eu anomalies in some

samples. They have CeN/YbN values falling in the range 1.07 to 2.98. Giritharan and

Rajamani (1998) further suggested the derivation of the metabasalts from melt-

metasomatized mantle sources by different extents of partial melting probably in an

island-arc tectonic setting.

The Hutti Schist Belt is surrounded on all sides by granitoid rocks. They are

usually granodiorites, quartz-diorites and granites (Srikantia, 1995; Basir and

Balakrishnan, 1999). Srikantia (1995) recognized two distinct phases of ganitoid

intrusions: a) The older grey coloured phase, designated as Kavital Granitoid, is a

major pluton to the southeast of the belt. This granitoid is medium to coarse-grained

and porphyritic at places with enclaves of the schist belt rocks. b) The younger pink

coloured phase of granitoid, designated as Yelagatti Granite, is exposed along the

northern rnargin of the schist belt (Fig. 2.2).

Based on geochemical characteristics, Basir (2000) divided the granitoid

rocks surrounding the Hutti Schist Belt into four categories, viz. monzodiorite, quartz-

monzodiorite, granodiorite and granite. Basir (2000) also inferred that the western,

eastern and northern granodiorites had different petrogenesis based on geochemical

modeling. Basir and Balakrishnan (1999) found that the granodiorites occurring to the

east and west of the Hutti Schist Belt have LREE-enriched patterns relative to the

Chondrite (Fig. 2.3b).

Basir (2000) suggested that the western granodiorites were derived by

fractional crystallization processes including liquid immiscibility from quartz-

monzodiorite. The magmas represented by the eastern granodiorites were derived by

partial melting of intermediate rocks, such as, quartz-monzodiorites. Partial melting,

under hydrous conditions, of LILE and LREE enriched composite sources in the

mantle wedge gave rise to quartz-monzodiorite magmas. Where as, those of the

northern granodiorites were derived by low extents of partial melting of island-arc

basalts under low pressure conditions. Magmas that represent granites (sensu stricto)

having about 74% SiOz occurring as small outcrops around the Hutti Schist Belt had

been derived by partial melting of granodiorites under hydrous conditions (Basir,

2000).

The Palkanmardi unit occumng in the northeastern part of the Hutti Schist

Belt is unique in that it consists of polymictic conglomerates and clasts derived from

various lithologies including granitoids, metabasalts and banded iron formation (BIF).

These clasts are set in a dark grey matrix and controversies over origin of these

conglomerates still exist. This unit is much similar to the "Champion Gneiss" that has

been reported from the eastern part of the Kolar Schist Belt (Viswanatha and

Ramakrishnan, 198 1 ; Rajamani et al., 1985).

1000

b Uti Granodiorite

-B- Ankasdoddi Granodiorite

Fig. 2.3 a) Chondrite normalized REE patterns of Hutti metatholeiites showing flat and LREE enriched patterns (Giritharan and Rajamani, 1998). The equivalent sample locations in the present work are given in Appendix I. b) Chondrite normalized REE patterns for some granodiorites from the Hutti area occurring east and west of the schist belt (Basir and Balakrishnan, 1999)- The granodiorites are LREE-enriched relative to the Chondrite.

Krishna Murthy (1934) made the earliest description of the Palkanmardi

conglomerate as autoclastic. He regarded the matrix as quartz-diorite and the boulders

of plutonic rocks as relics of intrusive layers disrupted by deformation and heating.

Ramaswamy and Krishnamurthy (1968) described the conglomerate as sedimentary

based on the rounding of the granitic pebbles. Roy (1979) and later Roy and Biswas

(1982) interpreted it as neither a genuine sedimentary conglomerate nor an

agglomerate. They suggested that the well-rounded nature of granodiorite pebbles and

Cllapfer- 2 Geological Franzework

gavels are indicative of a sedimentary environment and the angular to sub-angular

fragments of arnphibolite are indicative of their tuffaceous or pyroclastic nature. The

character of the matrix indicates it to be an admixture of volcanic (basaltic) and

sedimentary (arkosic) material. Srikantia (1995) proposed, on the basis of clast shape,

poor sorting and coarse matrix, a glacio-fluvial origin for the Palkanmardi

conglomerate.

Geochronological data on the rocks of the Hutti area are sparse. A granodiorite

sampled near Gooty, 150 krn south-west of Hutti, believed to be an equivalent of

Kavital granitoids has given a SHRIMP age of 2561 * 21 Ma (Vasudev et al. 2000).

A SHRIMP U-Pb age of 2576 * 12 Ma has also been reported by Vasudev et al.

(2000) on zircons separated from a deformed clast of granodiorite in the Palkanmardi

unit. Based on this age information the above authors (ibid.) suggested that the

granodiorite clasts from the Palkanmardi unit could have been derived from Kavital

granitoid. However, because of uncertainties involved in origin of the Palkanrnardi

unit, equivalence of Kavital granodiorite to Uti granodiorite and absence of age data

on granitoids surrounding the Hutti Schist Belt, the significance of these ages

(Vasudev et al. 2000) is not clear.

In a con~panion study carried out by Schmidt (2003) and Schmidt et al.

(2004), biotites from the mineralized zones of the Hutti Schist Belt have given Rb-Sr

isochron ages of 2373 * 23 Ma and 2298 rir 23 Ma while biotite separates from a

sample of Kavital granitoids and a granodiorite sample east of Gajalagatta have given

Rb-Sr isochron ages of 2298 i 22 Ma and 2348 k 23 respectively.

Detailed account of Au-mineralization in the Hutti Schist Belt has been given

by several workers (Biswas et al., 1985; Naganna, 1987; Biswas, 1990; Curtis and

Radhakrishna, 1995; Giritharan and Rajamani, 1998 & 2001; Sangurmath, 2001; Pal

and Mishra, 2002; Pal, 2003; Pandalai et al., 2003; Kolb et al., 2004; Mishra et al.,

2005). The gold deposit of Hutti, of gold-quartz-sulfide reef type, is situated along

the northwestern margin of the Hutti-Maski schist belt (Pal and Mishra, 2002). Nine

mineralized NNW-SSE trending parallel shear zones have been identified within a

pile of metamorphosed basic volcanic rocks (Curtis and Radhakrishna, 1995).

Auriferous quartz veins are preferentially developed in these shear zones within

amphibolites. Geochemistry of the host rock, ore veins, wall-rock alteration zone and

unaltered zones has been discussed by Giritharan and Rajamani (2001) and Pal and

Mishra (2002).

2.3 Geology of the Kolar Schist Belt and its Environs:

The Kolar Schist Belt (KSB) is located 80 krn east of Bangalore in the

Karnataka State (Fig. 2.1). The N-S trending Kolar Schist Belt is 80 km in length

attaining a maximum width of 6 krn in the central parts. The schist belt is made up of

amphibolites, chiefly tholeiites but also komatiites, Banded Iron Formation (BIF) and

felsic and graphitic schists (Fig. 2.4). Details of lithology, metamorphism, structure,

geochemistry, geocluonology and petrogenesis are described in Narayanaswamy et al.

(1960), Ziauddin (1975), Anantha Iyer and Vasudev (1979), Viswanatha and

Ramakrishnan (1981), Rajamani et al. (1981), Granath and Rajamani (1982),

Balakrishnan and Rajamani (1987), Mukhopadhyay (1988), Mukhopadhyay and

Haimanot (1 989) and Balakrishnan et al. (1 990).

Two distinct types of amphibolites have been recognized based on MgO

content. The more abundant one is the low K-tholeiitic type consisting predominantly

of calcic amphiboles and plagioclase. Based on textural criteria the tholeiitic

amphibolites were referred to as schistose, the granular and the massive amphibolites.

The less abundant fibrous variety has higher MgO content having aluminium-rich

hornblende and aluminium-poor actinolite with geochemical characteristics similar to

komatiites and komatiitic basalts. On the basis of mineralogy and mineral chemistry

of calcic amphiboles, Rajamani et al. (1981) suggested that these rocks have

undergone middle to upper amphibolite facies, low pressure metamorphism.

The amphibolites occurring in the western part of the belt have light rare earth

element (LREE)-depleted characteristics and those in the eastern part have LREE-

enriched characteristics (Rajamani et al., 1985). Rajamani et al. (1985, 1989) showed

on the basis of REE and trace element studies that the tholeiitic and the komatiitic

amphibolites could not have been derived by different extents of partial melting of

similar mantle sources. Further based on trace element signatures they showed that

the tholeiites could not have been derived from magmas similar to those parallel to the

komatiitic rocks by fractional crystallization. Rajamani et al. (1985) suggested that

the komatiitic amphibolites were derived by partial melting of mantle sources at

pressures exceeding 5 GPa (50 kbars) while the magma represented by tholeiitic

amphibolites were derived by partial melting of Fe-enriched mantle sources at

pressures less than 3 GPa (Rajamani et nl., 1989). The komatiitic and tholeiitic

amphibolites occurring in the western part were considered to have derived from

LREE depleted sources while those in the eastern part were considered to have

derived from LREE enriched sources.

76"15' 76'20'

Patna i

Iron Formatio

Fig. 2.4 Geological map of the central part of the Kolar Schist Belt and the surrounding granitoids after Balakrishnan and Rajamani, (1987). Inset shows the outline of the entire belt. Sample locations marked as L-1, K- 1 and K-2 are from the western Dod-Dosa Gneissic Comples. Sample locations L-3, K-3 and K-5 are from the eastern Kambha Gneisses. Location K-4 represents the Patna Granite.

Pb and Nd isotopic characteristics of the komatiites and tholeiites are distinct

and confirmed the suggestion that their magmas were derived from different mantle

sources (Balakrishnan et al., 1990). The western tholeiitic amphibolites have given a

whole-rock Pb-Pb age of 2732 * 155 Ma and Sm-Nd age of 2696 5 136 Ma

(Balakrishnan et al., 1990). These ages also agree with an Rb-Sr isochron age of 2700

* 40 Ma for the eastern amphibolites reported by Walker et al. (1 989).

CIzupter 2 Geologicnl Frainework

Balakrishnan and Rajamani (1987) have studied the granitoid rocks

surrounding the Kolar Schist Belt. They have classified them into two primary types;

(i) bodies of quartz-monzodiorites to granodiorites named as Dod and Dosa gneisses

found west of the schist belt and (ii) migmatitic gneisses of granodiorite composition

known as Kambha Gneisses occurring to east of the schist belt. At several outcrops

the Kambha Gneisses show migmatitic structure as seen near villages Satu and

Tippanapalli. A foliated granodiorite pluton referred to as the Patna Granite also crops

out to the northwest of the Kolar schist belt.

Krogstad et al. (1989) and Krogstad et al. (1991) have shown that the Dod and

Dosa gneisses intruded at 2631 i 6.5 and 2610 k 10 Ma respectively. The less

deformed Patna Granite intruded at 2551 2.5 Ma, while the Kambha Gneisses give

an age of 2532 h 3 Ma. The titanite U-Pb age for the Patna Granite is 2552 h 1 Ma

while that for the Kambha Gneisses is 2517 $. 2 Ma. Based on these data the Kolar

Schist Belt is thought as an Archean suture representing the boundary between at least

two distinct Late Archean gneissic terranes. Due to the absence of any intrusive or

cross-cutting relationship of the younger dating granitoids with the schist belt,

presence of sheared rocks at margin of the schist belt and dichotomy of cooling

history of granitoids occurring on either side of the schist belt, Krogstad et al. (1989)

suggested that the two gneiss terranes were juxtaposed after 25 17 i 2 Ma, the titanite

age for the Kambha Gneisses, but before 2420 i 12 Ma which is the 40Ar-39h

muscovite plateau age from sheared gneisses occurring close to the western contact

between the western granitoids and the Kolar Schist Belt (Krogstad, 1989). An

internlediate to felsic metavolcanic andlor hypabyssal unit is present within the schist

belt and is in tectonic contact with the eastern Kambha Gneisses.

2.4 Geology of tlie Ramagiri Schist Belt and its Environs:

Rarnagiri is situated about 170 km north of Bangalore and 30 krn west of

Dharmavaram in the Andhra Pradesh State (Fig. 2.1). The north-south trending

Ramagiri Schist Belt is trident shaped, 2-4 km wide and extends discontinuously for

80 km (Fig. 2.5). It consists of three arms of metavolcanic rocks, eastern, central and

western arms that spread out northward, dominated by amphibolites, pillowed

metabasalts, intensely folded greenstones, sheared metagabbros, phyllites, chlorite

schists, mafic and felsic tuffaceous rocks and ferruginous quartzites. Details of

lithology, structure, geochemistry, geochronology and petrogenesis are given in

Chupter 2 Geological Framework

Krishnarnurthy ( 1 960): Ghosh el al. (1 970). Chaudhuri (1 986), Zachariah (1 992),

Zachariah et al. (1 993, Zachariah et a3 (1 9961, Balakrishnan et al. (1 999) and Mishra

and Rajarnani (1 999).

2 - e, kttapal l i N 9 Kuntimaddi

d Cherlopalle

I

I

0 Eastern Block Western Block

n Central Block 0 Gangampalli Complex 0 Rarnaglri Complex

I U C h e n n a Gne~sses i mserpen t in~ te

77730'

Fig. 2.5 Geological map of the Rarnagiri Schist Belt and the surrounding granitoids (modified from Ghosh et al., 1970). Sample locations R-1 and R-3 are from the western Gangam Complex. Sample location R-4 is fiom the central Ramagiri Complex. Sample location R-5 is fiom the eastern Chenna Gneisses. The schist belt is shown as three prongs.

The western arm is made up of LREE-depleted metabasalts, serpentinites and

Banded Ferruginous Quartzite (BFQ). The central arm is about 2 krn wide and is

fh-ther divided into three distinct blocks, eastern, central and western blocks, with

tectonic contacts. The eastern arm and the eastern block of the central arm have

LREE-depleted low-K tholeiitic amphibolites and minor BFQ. The central block

consists of greenschist facies, mafic to felsic volcanic rocks, tuffs, pyroclastics,

volcanogenic sediments and BFQ. The western block of the central arm consists of

fine-grained, LREE-depleted metabasalts that show pillow structure at places.

Cllnpter 2 Geological Framework

Zachariah et rrl. (1996) obtained a Pb-Pb isochron age of 2746 i 64 Ma for a

set of sampies from the central block of the central arm. A pyroclastic fi-om the central

arm has given a U-Pb zircon age of 2707 It 18 Ma (Balakrishnan et al. 1999). One of

the E-W trending unaltered dikes cutting across the three blocks in the belt and the

surrounding granitoid gneisses gave a Sm-Nd age of 2454 * 100 Ma which may be a

minimum for the time of block assembly (Zachariah et al. 1996).

The Ramagiri Schist Belt is surrounded by granitoid rocks on all sides.

Granitoid rocks referred to as the Gangam Complex crop out on the western part of

the Ramagiri schist belt and the Chenna Gneisses to the east of the belt. The Ramagiri

Complex separates the central and western arms of the schist belt. The Chenna

Gneisses are well foliated and granodioritic and at places migrnatized. Monzodiorite

predominates in the Gangam Complex and the Ramagiri Complex consists of quartz-

diorite, granodiorite and granite in the decreasing order of abundance. Balakrishnan et

al. (1999) obtained U-Pb titanite ages of 2614 * 4 Ma and 2595 i 1 Ma for Ramagiri

Complex and 2516 * 3 Ma and 2510 k 2 Ma for western Gangam Complex

granitoids. The U-Pb zircon ages of 2613 * 6 Ma for the Rarnagiri Complex

granitoids and 2528 i 1 Ma for the Gangam granitoids are indistinguishable from

their respective titanite ages. A 2650 rt 7 Ma U-Pb zircon age for the eastern Chenna

Gneisses is suggestive of the time of crystallization of zircons, while U-Pb titanite

ages of 2545 + 1 Ma is interpreted as the time of migrnatization of Chenna Gneisses.

A small unit of post-kinematic granite vein, intrusive into the Gangam Complex has

yielded a titanite age of 2468 * 4 Ma.

Based on the above data Balakrishnan et al. (1999) suggested that the schist

belt, consisting of highly sheared rocks, could represent a terrane boundary between

the three disparate granitoid terranes juxtaposed after 2516 and before 2468 Ma.