10

CHAPTER-II

Geology of Southern Granulite Terrane

2.1. Introduction

This chapter provides a summary of geological and tectonic aspects of Southern

Granulite Terrane (SGT), which hosts varied lithologies of high grade rocks like

charnockites, khondalites and other granulite facies metamorphic rocks with granitic

intrusions of different ages. The SGT extending from 80 to 110 N latitude of India (Tamil

Nadu, Karnataka, and Kerala states) is one of few terranes in the world that has preserved

Archaean and Proterozoic crust with extensive high-grade granulite facies rocks (Fig 1). The

transitional boundary between the high-grade and low-grade terranes is demarcated by well

known ‘‘Fermor line’’ representing a fundamental crustal discontinuity along which there

must have been considerable vertical displacement (Fermor, 1936). This line separates the

exposed charnockitic region from non-charnockitic region. The SGT is believed to be of

lower crustal origin through a complex evolutionary history with multiple deformations,

anatexis, intrusions and polyphase metamorphic events. More than 80% of the terrane is

covered by varied lithologies of Archaean and Proterozoic age groups namely,

Sathyamangalam Group (>3200 Ma), layered mafic and ultramafic complexes, Bhavani

Group (~3000 Ma), Kolar Group (~2900 Ma), Khondalite Group, Charnockite Group (~2600

Ma) and Migmatitic complex (2200-2250Ma) etc. The Proterozoic rocks include younger

granulites/charnockies, granites, alkali syenites, corbonatites, mafic and ultramafic intrusives

mostly occurring in and around the Cauvery suture/ shear zone (CSZ) (GSI, 2006).

11

Fig 2.1: Geological frame work of Southern India (after Santosh and Sajeev, 2006)

2.1.1. Sathyamangalam Group

The Sathyamangalam group of rocks is considered to be equivalents of ‘Sargurs’ of

Dharwar craton exposed in the central and northwestern part of the Indian Peninsula. The

12

group consists of quartzite ±fuchsite ± kyanite ± sillimanite and banded iron formation,

sillimanite schist ± garnet, kyanite- schist, corundum bearing mica schist and talc-tremolite

schist; calc granulite, crystalline limestone/marble, ortho-and para amphibolite

(Gopalakrishnan, et al., 1975).

2.1.2. Layered mafic and ultramafic complexes

In the proximity of Sathyamangalam group, ultrabasic rock sequence of dunite,

peridotite, websterite, garnetiferous gabbro, gabbroic anorthosite occur near Mettupalayam

(NW part) and other areas. They generally occur as enclaves within the migmatitic gneisses

as a part of the dismembered sequence. Large volume of garnetiferous gabbro and

hornblendic anorthosite with chromitite layers as well as small lenses of eclogitic rocks are

the characteristic features of this suite (Gopalakrishnan, 1994b). The well known Sittampundi

anorthosite yielded 3000-2900Ma by Sm-Nd systematics (Bhaskara Rao et al., 1996).

Sittampundi layered anorthosites define two populations : older yield a Concordia age of

2541± 13 Ma, younger belonging to a high-grade metamorphic event at 2461± 15 Ma by

zircon LA-ICPMS U–Pband Hf isotope data (Ram Mohan et al. 2013). Similar complexes

also occur as small bodies, lenses and bands, in the northeastern sector of the SGT around

Cuddalore, Vellore, Tiruvannamalai and Dharmapuri areas. They consist of dunite, peridotite,

hypersthenite, enstatite, augitite, hornblendite, websterite, gabbro and anorthosite (GSI,

2006). These are also considered to be intrusive into precursor rocks of Charnockite group

and later deformed and metamorphosed along with the host rocks under granulite facies

conditions (Sugavanam et. al., 1978).

2.1.3. Bhavani Group

The rocks belonging to Bhavani Group occur around Bhavani town (north of Palar

River) in the form of typical exposures of peninsular gneissic group of rocks. The gneissic

rocks also occur extending from Kerala border in the west through parts of Coimbatore,

13

Erode, Salem, Namakkal, Tiruchirapalli and Perambalur districts towards the east coast. The

rock types include fissile mica gneiss, quartzo-felspathic gneiss, augen gneiss, hornblende

gneiss, hornblende-biotite gneiss, biotite gneiss, granitic gneiss and pink migmatite (GSI,

2006).

2.1.4. Khondalite and Charnockite Groups

The khondalite and charnockite group of rocks (also equivalents of Eastern Ghats

Super group) and their reworked equivalents are the dominant variety of rocks in the SGT.

The Khondalite Group essentially consists of rocks of sedimentary parentage such as

quartzite and garnet-sillimanite gneiss ± graphite ± cordierite (metapelites) and occur mostly

to the south of Palghat-Cauvery Shear Zone (PCSZ). They are often interfolded and inter

banded with mafic granulite/amphibolite and charnockite. The Charnockite group,

comprising charnockite (hypersthene bearing granite), two-pyroxene granulite, banded

quartz-magnetite granulite/banded magnetite quartzite and thin pink quartzo-feldspathic

granulite, are extensively developed in the north-eastern sector of the SGT. They are also

well exposed in many prominent hill ranges such as Pallavaram-Chengleput, Javadi,

Shevaroy, Chitteri, Kalrayan, Kollimalai, Pachchamalai, Nilgiris, Kodaikanal, Palani,

Sirumalai, Varushanad, Gasthiarmalai and Hills around Nagercoil and are described as

charnockitic massifs (Fig. 2.2). The pyroxene granulites of Charnockite Group are considered

to represent mafic volcanics, while the banded magnetite quartzites indicates a volcanic

exhalative origin and the pink granulite is interpreted to represent the associated acid

volcanics (Gopalakrishnan et al., 1976, Suganvanam et al., 1978). The geochronological data

available for charnockites of the terrane show different ages ranging from ca 3000Ma to

550Ma. These data indicate that the charnockites occurring north of Palghat-Cauvery

Lineament (PCL)/Noyil-Cauvery rivers show prominently Neoarchaean ages of 3000-

2600Ma, while those occurring south of PCL yield younger ages of 550 Ma. However, in

14

recent years several mixed ages from 2200Ma to 600 Ma have been described (Thomson et

al., 2006; Plavsa et al., 2012). The charnockite interbands, which are rich in diopside, are

considered to be metamorphosed sediments, while mafic granulites/amphibolites probably

represent mafic volcanics (Gopalakrishnan et al., 1976; GSI, 2006). Based on U-Pb Zircon

dating, Ghosh et al. (1998) described that the Khondalite Group in Palaiyam area is younger

to the Mid-Archaean Sathyamangalam Group. However, the garnet-sillimanite gneiss from

adjoining Kerala has given Rb/Sr whole rock age of 3065 ± 75 Ma (Crawford, 1969). Recent

Nd-isotope studies have yielded model Nd ages (TDM) ranging from 2.60 Ga to 1.34 Ga

indicating a Palaeoproterozoic age for the Khondalite Group in Kerala (Harris et al., 1994).

Fig.2.2. Shaded relief image of southern India showing the distribution of major charnockite massifs (after Rajesh, 2004); TB-Trivandrum Block; NB-Northern Block

15

2.1.5. Migmatite Complex

The SGT has witnessed two major periods of granitic activity, initially during

Neoarchaean to Palaeoproterozoic and another during Neoproterozoic period. The older

granitic emplacement event are restricted to the northern part of Terrane, north of Palghat-

Cauvery Shear Zone (PCSZ), while the younger Pan-African event is widespread in the

region south of PCSZ. The time of emplacement of these granitoids has been well

constrained by Rb-Sr isotopic systematics (Nathan et al., 2001). The Neoarchaean granitoids

were developed mostly to the north of the CSZ viz Tiruttani, Sholingar, Bisanattam, Ebbari

and Krishnagiri (Ca 2500 Ma) (Krogstad and Hanson, 1988; GSI 1991), while

Paleoproterozoic granitoids are recognized around Gingee, Tiruvannamalai and Tirukovilur

(2254 Ma; Balasubrahmanyan et al., 1979). The rocks of the Khondalite and Charnockite

groups have been subjected to regional migmatisation and retrogression with influx of

quartzo-feldspathic material resulting in the formation of different types of gneisses such as

biotite gneiss, hornblende gneiss, augen gneiss, garnetiferous biotite gneiss, garnetiferous

quartzo-feldspathic gneiss depending upon the parent rock (GSI, 2006). These rocks are

grouped under migmatite complex. The formation of these gneissic rocks took place in

different stages of formation from meta-texites to diatexites (Gopalakrishnan et al., 1976).

These rocks have also experienced multiple deformations and polymetamorphism with

concomitant anatexis giving rise to a range of migmatites. The Migmatite Complex mostly

belongs to Archaean age and a few belong to Paleoproterozoic age i.e. 2250- 2100 Ma,

(Crawford 1969; 2250 Ma, GSI, 1978, Balasubramanyan et.al., 1979).

2.1.6. Alkaline magmatism

The SGT has witnessed two events of alkaline magmatism, during Paleoproterozoic

period and Neoproterozic periods. In the northern part of the SGT, several syenite-

carbonatite, nepheline-syenite, theralite and camptonite bodies have intruded in a concordant

16

linear fashion with in the country rocks around Pikkili and Hogenakkal areas. Along this

NNE-SSW trending lineament, some pyroxenite, syenite and carbonatite plutons are also

reported around Hogenakkal area. The Hogenakkal and Pikkili Syenite intrusives range in

age from 1994 Ma and 2371Ma (Natarajan et al., 1994; NGRI, 1994). The alkaline related

plutonism was also extensive around Vellore, Dharmapuri and Salem districts, where a

number of ultramafic-syenite-carbonatite bodies of Elagiri, Koratti, Samalpatti and

Pakkanadu occur in the form of a NNE-SSW trending zone extending for about 200km from

Gudiyattam in the north to Bhavani in the south (GSI, 2006). A number of smaller

ultramafic-syenite-carbonatite bodies also occur along sub-parallel NNE-SSW trending shear

zones on both sides of the main zone of alkaline activity. The ultramafics also occur in large

proportion around the Samalpatti pluton, Koratti pluton, Pakkanadu pluton and they carry

large chunks of ilmeno-rutiles at many places. Small carbonatite bodies of both sovite

Benstonite (barium bearing carbonatite) and beforsite composition occur within the

Samalpatti pluton. In Salem district, many ultramafic bodies are also well known for hosting

the famous magnesite deposits (G.S.I, 2006). Reddy et al., (1995) described an age of

808±18Ma for the Salem ultramafic complex.

2.1.7. Mafic Dykes

A number of mafic dyke swarms traverse the northern part of the SGT and intrude the

charnockite and migmatite group of rocks with varying trends from WNW-ESE and NNE-

SSW and rarely N-S and NNW-SSE. In the central part of terrane, they trend in ENE-WNW

to NE-SW. The textural characteristics of dykes show dolerite, gabbroic/basaltic in nature.

Petrochemical studies (Krishna Rao and Nathan, 1999) indicate that the majority of these

dykes are quartz normative tholeiites, while olivine-dolerite dykes show basaltic-komatiite

chemistry. K-Ar isotopic ages of these mafic dykes are interpreted to be around 1700Ma

(Radhakrishna and Mathew Joseph, 1993; Sarkar and Mallick, 1995).

17

2.1.8. Ultrabasic/basic rocks

The anorthosites and ultramafic rocks occur in the southern part of the terrane

(Oddanchatram, Kadavur and Tirunelveli), south of the PCSZ indicating that they were

possibly emplaced within a Palaeoproterozoic crust (GSI, 2006). The host charnockite for the

Oddanchatram anorthosite has been dated as 550Ma (Bartlett et.al., 1995; Jayananda et.al.,

1995), and Oddanchatram anorthosite yielded ca.600Ma (Ghosh et.al., 1998) suggesting its

ambiguity of the host and the intrusive. The Kadavur anorthosite complex of hornblende

gabbro body has been reported as ca. 600Ma from K-Ar mineral age by Balasubramanyan

and Sarkar, 1981. Recently Teale et al., (2011) documented U-Pb ages of 829 ± 14Ma for the

metamorphic zircons from Kadavur gabbro anorthosite complex and the metmorphic zircons

from the surrounding quartzite yielded 843± 23.

2.2. Metamorphism

The SGT broadly witnessed two granulite facies metamorphic events: Neoarchaean

(ca. 2.5 Ga) cratonic part in the north and a Neoproterozoic (ca 1.0–0.55 Ga) terrane in the

south. The Charnockites from Pallavaram and Shevroy hill massifs show quartz- K-feldspar-

plagioclase feldspar- clinopyroxene- garnet- hornblende- biotite- apatite- zircon- magnetite-

ileminte- rutile- pyrope (Rajesh and Santosh, 2004). However, the mafic charnockites from

the Shevroy hill massif has a dominant mineralogy of plagioclase- clinopyroxene-

orthopyroxene- hornblende- magnetite- ilemenite- rutile- K-feldspar- biotite- apatite- zircon.

The non-garnetiferous enderbites from both the Biligirirangan and Nilgiri hill massifs have a

similar mineralogy of quartz- plagioclase- feldspar- clinopyroxene- garnet- hornblende-

biotite- apatite- zircon- magnetite- ilmenite- rutile- pyrope. The felsic type Cardamom hill

massif has a dominant mineralogy of quartz- K-feldspar- plagioclase feldspar-

orthopyroxene- magnetite- ilmeinite- apatite, - hornblende- biotite while the intermediate

type Cardamom hill massif has mineralogy of quartz- plagioclase- K-feldspar-

18

orthopyroxene- clinopyroxene- hornblende- biotite- magnetite- rutile- zircon- apatite. The

Nagercoil massif charnockites show a common mineral assemblage of quartz- plagioclase-

K-feldspar- orthopyroxene- clinopyroxene- hornblende- biotite-magnetite- rutile- zircon-

apatite- garnet (Rajesh and Santosh, 2004). The Nilgiri block was metamorphosed under

medium-high pressure granulite facies conditions (6-10 kbar; Harris et al 1982). The ages of

high-grade metamorphism for the Nilagiri massif charnockites are ca. 2496 ± 15Ma

(Jayananda and Peucat 1996). The geochemical characters of the charnockitic massifs in the

Madurai Block were interpretedto show strong calc-alkaline affinity (Chacko et al., 1992-

Cardamom Hill Charnockites; Thomson et al., 2006) and also described (Rajesh, 2007) a

subduction related origin for the charnockites of the Cardamom Hill. Whereas, Ishii et al.

(2006) who described P-T estimates up to 8.5-9 kbars and 940-1040°C from cordierite and

orthopyroxene-bearing ultra high temperature (UHT) granulites within the AKSZ of

Trivandrum block. The metamorphism along Kerala Khondalite Block (KKB) is

characterized by a clockwise P-T path with post peak isobaric cooling followed by isothermal

decompression (Santosh, 1987; Fonarev et al., 2000; Cenki et al., 2002, 2004). Some studies

(Braun et al., 1996; Chacko et al., 1996; Satish Kumar and Harley, 1998; Nandakumar and

Harley, 2000; Cenki et al., 2002) indicate that the P-T estimation from KKB represents the

highest-grade assemblages record temperatures in the range of 8400-10700C and pressures up

to 9.5 kbar.

2.2.1. Ultra high temperature metamorphism and eclogite facies rocks

Extreme crustal metamorphism at T=900-11500C and P= 7-13 kbar occur in some

areas of the SGT, which are generally designated as ultra-high temperature (UHT)

metamorphic rocks (Harley, 1998; 2004). There have been several reports of granulite facies

assemblages, such as orthopyroxene-bearing granulites, sapphirine-bearing pelites, and calc-

silicate rocks, which suggest high to ultra high temperature metamorphism (e.g., Chacko et

19

al., 1996; Raith et al., 1997; Satish Kumar and Harley, 1998). In the eastern part of the CSZ

and in Madurai Granulite Block several workers (e.g., Shimpo et al., 2006; Collins et al.,

2007b; Clark et al., 2009; Tsunogae and Santosh, 2006; Nishimiya et al., 2010) have

documented the evidence for a prograde high-pressure (HP) event and subsequent ultra high-

temperature (UHT) metamorphism along a clockwise path based on geochemical and

petrological studies. Fluid inclusion studies of some metamorphic rocks within the CSZ

include those on Mg-Al-rich granulites (Ohayama et al., 2008) and mafic granulites

(Nishimiya et al., 2008; Santosh et al., 2010) show the occurrence of abundant CO2 rich

inclusions in Mg-Al-rich rocks and retrogressed eclogites (high-pressure granulites). Similar

primary fluid inclusions with markedly low-density CO2 have also been reported from some

of UHT terranes elsewhere such as from the Trivandrum Block (Fonarev et al., 2001) and

Madurai Block (Tsunogae et al., 2008). In a recent study, Nishimiya et al. (2010) reported

equilibrium sapphirine + quartz assemblage from Panangad area within the PCSZ providing

unequivocal evidence for extreme crustal metamorphism at UHT conditions associated with

the collisional assembly of the Gondwana supercontinent in the Neoproterozoic-Cambrian

period. The results indicate peak UHT conditions of 940-990°C at 7-8 kbar followed by a

retrograde event of 600-700°C along a clockwise P-T path (Santosh et al., 2009a). Recently

Sajeev et al. (2009) described regtrogressed eclogites from the well known Sittampundi

anorthosite complex with in the garnet gabbro layer in anorthosite. The P-T conditions

represent that garnet-rutile-melt was the peak metamorphic assemblage that eclogites

developed at ca 20 kbar and above 10000C.

2.3. Tectonic bocks in SGT

The Southern Granulite Terrane has a complex evolutionary history from the

Paleoarchaean to Neoproterozoic (3500–550 Ma) with repeated multiple deformations,

anatexis, intrusions and polyphase metamorphism (Bartlett et al., 1998; Bhaskar Rao et al.,

20

2003). The SGT comprises juxtaposed crustal blocks, which are fragmented and

dismembered (Chetty et al., 2006; and the references there in). These blocks are separated by

two major shear zones namely the Cauvery Shear Zone (CSZ) and Achankovil Shear Zone

(AKSZ). These shear zones separate distinct geological domains in terms of lithology,

structural style, and crust forming age of the basement gneisses (Bhaskar Rao et al., 2003).

Essentially, the CSZ divides the SGT into discrete tectonic blocks, viz., the Archaean

granulite blocks to the north and the Neoproterozoic granulite blocks to the south (CSZ,).

Fig.2.3: Regional tectonic framework of CSZ showing various Shear zones within it. The present two study areas are marked as red ellipses. WDC-Western Dharwar Craton, EDC-Eastern Dharwar Craton, Tz- Transition Zone, CSZ-Cauvery Suture Zone, Mo-Moyer Shear Zone, Bh-Bhavani Shear Zone, PCSZ-Palghat Cauvery Shear Zone and MSZ- Mettur Shear Zone (Modified after Chetty et al., 2003).

2.3.1. Cauvery suture zone

A crustal-scale (suture) shear zone system divides the Southern granulite terrane

(SGT) of southern India into discrete tectonic blocks (Drury and Holt, 1980; Gopalakrishnan

et al., 1990; Chetty, 1996). The most prominent of these is an east–west trending zone of

intense planar fabrics separating the northern late Archaean granulite block(s) from the late

Neoproterozoic Madurai granulite block (MGB), which is well known as the Cauvery shear

21

(suture) zone system (CSZ). The geology and the age relationships of the CSZ have been

reviewed recently by Bhaskar Rao et al. (2003), Chetty et al. (2003), Jain et al. (2003),

Mukhopadhay et al. (2003) and Ramakishanan (2003). The CSZ has been variously described

as: (i) a collision zone and cryptic suture, evident from the occurrence of remnants of

probable ophiolitic sequence (Gopalakrishnan, 1994a), (ii) dextral shear zone as exemplified

by the deflection of north-south Archean fabrics to near east-west disposition along the

MBSZ (Drury et al., 1984), (iii) an analogue of the central part of Limpopo mobile belt

(Ramakrishnan, 1993), (iv) the Archean-Proterozoic Terrane boundary (Harris et al., 1994),

(v) a zone of Paleo- and Neoproterozoic reworking of Archean crust (Bhaskar Rao et al.,

1996), and (vi) a Neoproterozoic dextral-ductile transpressive tectonic zone (Meissner et

al,2002; Chetty et al., 2003, Chetty and Bhaskar Rao, 2006). The CSZ is considered as the

trace of the Cambrian suture zone of Gondwana (Collins et al., 2007a).

Based on the U-Pb SHRIMP ages from charnockites of Salem-Madras Block, Clark et

al., (2009) described the magmatism at ca.2530 Ma age and subsequent high-grade

metamorphism and partial melting at ca. 2480 Ma. Available geochronological data on the

protoliths of the Salem Block indicate Meso- to Neoachaean rocks stretching as far south as

the PCSZ with metamorphic ages indicating a granulite facies event in the latest Archaean to

earliest Proterozoic (Peucat et al., 1993).

The CSZ has been interpreted as a dextral transcurrent shear zone and its crustal

architecture modelled in terms of a regional ‘flower structure’ (Chetty et al., 2003; Chetty and

Bhaskar Rao, 2006) typical of collisional orogens (Roure et al. 1989). It has been emphasized

that the CSZ essentially forms a zone of late Neoproterozoic reworking of Archaean

protoliths. The history of crustal reworking here is considered contemporaneous with the Pan

African orogenic processes linked to the amalgamation of continental fragments in eastern

Gondwana (Meert, 2003). Although, there is no consensus on the correlations of CSZ with

22

the shear zones in other Gondwana fragments, a recent view favours its correlation with the

Betsimisaraka suture zone in the eastern Madagascar (Collins and Windley, 2002), while

earlier models linked it to the sinistral Ranatsora shear zone of southern Madagascar to the

west and the boundary between Napier complex and Rayner complex of east Antarctica

(Chetty, 1995; Harris, 1997; Janardhan, 1999). All the geological and geophysical studies

across the CSZ described in the literature suggest that the CSZ comprises a complex tectonic

zone of dismembered and imbricated crustal blocks.

Several major shear zones have been delineated from Landsat interpretation from the

SGT (Drury and Holt, 1980). They include: (1) the Moyar-Attur Shear Zone, (2) the Bhavani

Shear Zone, (3) the Palghat-Cauvery Shear Zone (PCSZ), and (4) the Achankovil Shear

Zone. The most important shear zones of the region are the Moyar-Bhavani (MBSZ), the

Palghat-Cauvery (PCSZ) and the Achankovil (AKSZ) shear zones. The MBSZ branches into

several curvilinear shear zones in the NE-SW direction. Prominent among them is the Mettur

Shear Zone (MTSZ). Based on satellite data interpretation and subsequent ground follow-up,

Chetty et al. (2003) termed the network of these crustal-scale shear zones as the Cauvery

Suture Zone (CSZ). They divided the region between the Biligiri Rangan and Kodaikanal

high-grade charnockite massifs (see Fig. 2.3) into the Moyar-Bhavani shear zone (MBSZ),

the Chennimalai-Noyil shear zone (CNSZ), the Dharapuram shear zone (DSZ), the Devattur-

Kallimandayam shear zone (DKSZ), the Karur-Oddanchatram shear zone (KOSZ). All the

shear zones of the CSZ exhibit dextral strike-slip movement with a maximum lateral

displacement of ~ 80 km (Drury et al., 1984; Chetty et al., 2003). The maximum width of the

shear zones is around 20 km.

2.3.2. Moyar-Attur Shear Zone (MASZ)

It is an E-W trending wide shear zone extending for about 200 km from east of Attur

through Bhavani, Salem, Bhavanisagar, Moyar, and south of Gundlupet in the west. Ghosh et

23

al. (2004) described the kinematic history in and around the shear zone (MASZ). The

structural fabric within the MASZ represents a flattening type of deformation. The MASZ

links with the Bhavani Shear Zone near Bhavanisagar and with the Cauvery Bhavani Shear

Zone, just west of Bhavani. A number of charnockite massifs occur adjacent to the MASZ.

Coorg Massif, the Biligirirangan Hill Massif and the Shevaroy Hills Massif occur from east

to west to the north of the MASZ, while Nilgiri Hill Massif and the Kollimalai Hill Massif

occur to the south of MASZ. The structural studies by Ghosh et al., (2004) from the Salem

area, the Bhavani area, and the Bhavanisagar area indicate that the regional strike of the

lithologic and tectonic fabrics (S0 and S1, respectively) in north and south of the MASZ is N-

S to NNE-SSW. Inter layered BIF, mafic granulites and TTG gneisses occur along the shear

zone, which are highly deformed and mylonitized. Minor folds and lineations, two periods of

folding were recognized in the shear zone (Ghosh et al., 2004). The lineations are subparallel

to the axes of sub-vertical to moderately east plunging F2 folds, suggesting sub vertical

extension along the shear planes.

2.3.3. Bhavani Shear Zone (BSZ)

The Bhavani Shear Zone is a NE-SW trending shear zone marking the southern

boundary of the Nilgiri massif that occurs at the western part of the CSZ. Similar to the

MASZ, the BSZ is also characterized by a zone of intense mylonitic fabrics, in turn

overprinted by late brittle to brittle ductile structures with sheared gneisses of steeply dipping

with sparse sub-vertical stretching lineations (Ghosh et al., 2004).

2.3.4. Cauvery-Bhavani Shear Zone

It is about 100 km long and 10 km wide shear zone that follows the NW-SE course of

the Cauvery river from south of Namakkal to just west of Bhavani town, where it joins the

MASZ. This shear zone also marks a prominent metamorphic boundary between

predominantly granulite facies rocks to the north east and the retrogressed equivalents in the

24

amphibolite facies to the south west. Around Namakkal area, the general strike of the

lithological units and their gneissic fabric trends from NE-SW to NW-SE along the shear

zone. This change in strike is in part due to a major late E-W trending fold (F2), (Ghosh et

al., 2004) described as the Namakkal Fold well developed in the Nainarmalai Hill and the

Saruva Malai Hill, respectively. The limbs of the F2 Namakkal fold preserve early kilometer-

scale Z-shaped folds (F1) within an early dextral shear zone (S1). Two pyroxene granulites,

granitic gneiss, amphibolites, BIF and metacherts are the major lithologies which are strongly

deformed and trending NE-SW in the north to the E-W and NW-SE in the south is related to

four deformational episodes.

2.3.5. Palghat-Cauvery Shear Zone (PCSZ)

The PCSZ is an E-W trending shear zone along the Palghat-Cauvery Lineament

(following the Noyil and Cauvery rivers) and also the southern boundary of the CSZ

extending from the east coast to the west coast of India (Drury and Holt, 1980;

Ramakrishnan, 1993; GSI and ISRO, 1994). Regional trends of the structural fabric,

however, suggests that the PCSZ is a crescent shaped shear zone extending from Kazhikode

at the west coast to the east coast through Mallapuram and Palghat and southwest of

Namakkal, where it merges with the Cauvery-Bhavani Shear Zone. Thus the PCSZ, together

with the Bhavani Shear Zone and the Cauvery-Bhavani Shear Zone delineating the western

part of the CSZ with an overall dextral sense of movement. Horizontal sheets of granitic and

mafic granulite gneisses dominate in this part of the CSZ. In the Palghat Gap area, at least

four phases of granitic intrusions, each related to discrete shearing events, have been

identified (Ghosh et al., 2004).

2.3.6. Cauvery Suture Zone: a ‘flower structure’

A 100km wide corridor in the central part of the CSZ has been studied as a part of

geotransect recently. The structural studies reveal many E-W trending sub parallel shear

25

zones. From north to south, they are: (i) Moyar- Bhavani Shear Zone (MBSZ), marking the

northern boundary; (ii) Chennimalai-Noyil Shear Zone (CNSZ), (iii) Dharapuram Shear Zone

(DSZ), (iv) Devattur-Kallimandayam Shear Zone (DKSZ), and (v) Karur-Oddanchatram

Shear Zone (KOSZ) and all of them are dextrally displaced (Chetty and Bhaskar Rao, 2003).

These shear zones delineate distinct domains of contrasting geological characteristics. Chetty

and Bhaskar Rao, (2006) described that large-scale north-verging thrusts and related

structures accompanied by north–south shortening are dominant during D1, while D2 is

characterized by extensive dextral shearing, migmatisation and the emplacement of granitoids

and alkaline intrusive. Both gneissic foliation(S1) and mylonitic foliation(S2) show dip

values that vary consistently from north to south in the region; southward dipping in the

MBSZ and northward dipping in all the other shear zones south of it. The disposition,

regional geometry, dramatic variations in foliation fabrics, persistent dextral kinematic

indicators on all scales of observation, the apparent contemporaneous development of

mylonitic fabrics based on Rb–Sr mica ages (Meissner et al., 2002; Bhaskar Rao et al., 2003)

and the presence of possible convex upward reversed or thrust faults suggest that the CSZ

could reflect a crustal-scale flower structure displayed (Fig. 2.4.) (Chetty and Bhaskar Rao,

2006).

Fig. 2.4: A 3D cartoon shows crustal scale flower structure of CSZ (after Chetty and Bhaskar Rao, 2006).

26

2.3.7. Madurai granulite block

The Madurai Granulite Block occurs immediately south of the CSZ and is the largest

crustal block in southern India. This block comprises dominantly of charnockite massifs

intercalated with tonalitic/granodioritic gneisses and elongated narrow belts and slivers of

metasedimentary rocks including quartzites, metamorphosed carbonates, iron formations and

pelites, all suggesting an accretionary realm (Santosh et al., 2009a). A N-S trending Karur-

Kambam-Painavu-Trissur (KKPT) Shear Zone runs with in the MGB (Ghosh et al., 1998,

2004). The MGB can be lithologically divided into a western region and an eastern region;

Madurai Block in Kerala (MBK) and Madurai Block in Tamil Nadu (MBTN) (Cenki and

Kriegsman, 2005). The MBK is characterized by two different groups of hornblende-biotite

and orthopyroxene-biotite (charnockite) gneisses, one being quartz rich and the other feldspar

rich. While the eastern part, MBTN is composed of massive charnockites and enderbites with

heterogeneously distributed quartzites and calc silicate series of rocks (Cenki and Kriegsman,

2005).

2.3.8. Achankovil shear zone (AKSZ)

The AKSZ separates the Madurai Block from the Trivandrum Block (see Fig.1). The

AKSZ shows NW-SE trending foliation fabrics with steep dips to southwest. The adjacent

Madurai Block and Trivandrum Block show contrasting lithological and structural

characteristics. The southern boundary of the Madurai Block is marked by the Achankovil

Shear Zone (AKSZ), which also separates the Madurai Block from the Trivandrum Block to

the south. The Trivandrum Block is subdivided on lithological grounds into three tectonic

units; the Kerala Khondalite Belt (KKB), the Nagercoil unit and the Achankovil

metasediments. Trivandrum Block comprises dominantly of metasedimentary gneisses

including garnet-bearing felsic gneisses (known locally as leptynites) and granulite facies

garnet+spinel+cordierite+sillimanite metapelites (termed khondalites). These lithologies

27

constitute a vast sequence of continental margin sediments originally defined as the Kerala

Khondalite Belt (KKB) by Chacko et al. (1987).

The AKSZ is 10-20 km wide and 120 km long and extends along the southern edge of

the charnockite massifs of Cardamom Hills (Drury and Holt, 1980). The AKSZ holds a key

position in juxtaposing the member terranes in the East Gondwana supercontinent

reconstructions. The AKSZ has been correlated with the sinistral Ranotsara shear zone of

Madagascar (Ramakrishnan, 1991; Kriegsman, 1995). However, Sacks et al. (1997) reported

field evidences in support of dextral movements along AKSZ discarding its correlation with

sinistral Ranotsara shear zone. Rajesh et al., (1998) reported structures indicating a sinistral

sense of movement along AKSZ and reiterated its correlation with Ranotsara, which led to

the suggestion of repeated movements along AKSZ (Sacks et al., 1998). The important

lithologies in AKSZ include leptynitic garnet-biotite gneiss, khondalite, cordierite-gneiss, and

minor mafic granulite, calc-silicates, quartzites, and massive granites. Guru Rajesh and

Chetty, (2006) brought out a number of kinematic indicators from the shear zone such as

asymmetric boudins, porphyroblasts, flanking folds, “S” shaped folds, S-C′ fabrics, shear

bands, sub-horizontal stretching lineations, and porphyroclasts etc. They also described two

events of deformation with in the AKSZ; D1- initial dextral deformation and D2- reactivated

and superimposed by sinistral kinematics. The important feature of this shear zone is the

variation in structural trends across the lineament is from N-S on the north east to NW-SE on

the southwest. Santosh et al., (1992) and Bartlett et al., (1995) described that it is a major

Pan-African shear zone in South India as evidenced by 539 Ma (Sm/Nd isochron ages) and

533 Ma (U-Pb ages for zircons) of cordierite-bearing Charnockites.

2.4. Ophiolites

Ophiolites, the remnants of oceanic lithosphere, provide important information on the

evolution of ancient arcs, petrogenetic processes, opening and destruction of ocean basins,

28

and the nature of subduction–accretion–collision tectonics in major orogenic belts (Nicolas,

1989; Şengör, 1990; Searle and Cox, 1999; Dilek and Newcomb, 2003; Beccaluva et al.,

2004). The occurrence of ophiolitic rocks has been reported from various terranes of different

ages on the globe. Although most of the well documented examples come from Phanerozoic

belts (Ishikawa et al., 2002; Dilek and Robinson, 2003; Dilek and Newcomb, 2003; Vaughan

and Scarrow, 2003; Hara et al., 2009; Braid et al., 2010; Isozaki et al., 2010; Pearce and

Robinson, 2010; Zhang et al., 2010), ophiolites of Archean age have also been recorded such

as those from the 2.5 Ga ophiolites in the Dongwanzi, Zunhua and Wutaishan areas in the

North China Craton (Kusky et al., 2001; Polat et al., 2005, 2006; Kusky,2010),and the~3.8

Ga ophiolites from Isua supra crustal belt in southwest Greenland (Furnes et al., 2007, 2009).

Whereas the ophiolites in younger orogenic belts preserve a complete sequence/stratigraphy,

those from Archean and Proterozoic terranes are highly dismembered, preserving only partial

sequences (Kroner, 1985; Barhe, 1990; Dann, 1991; Kusky, 2004). Some of the world's best

examples of Proterozoic ophiolites have been reported from the Arabian–Nubian shield,

Kareliean shield, Capesnot Belt of West Africa and the southwest USA (St Onge et al., 1989;

Abhouchami et al., 1990; Scott et al., 1991, 1992; Boher et al., 1992; Dann, 2004). Some of

the well studied examples for ophiolitic assemblages from Proterozoic terranes include the

Mesoproterozoic Kandra (Vijayakumar et al., 2010), Kanigiri ophiolites (Dharma Rao and

Reddy, 2009; Dharma Rao et al., 2011), and the Neoproterozoic Manamedu ophiolite

complex (Santosh et al., 2009; Yellappa et al., 2010) reported from Peninsular India.

Recently, three important Precambrian ophiolite complexes are reported from the

CSZ that include: Manamedu, Devanur and Agali complexes. Their lithologies, structural

styles, geochemical signatures and geochronological data are described below.

29

2.4.1. Manamedu ophiolitic Complex (MOC)

The Manamedu ophiolite complex (MOC) within the southeastern part of the Cauvery

Suture Zone in Southern India (Yellappa et al., 2010) comprises metamorphosed equivalents

of the following lithological units: (1) an ultramafic group comprising dominantly of

pyroxenite, and dunite, locally preserving cumulate textures; (2) gabbroic rock types

consisting of gabbro, gabbro norite, and anorthosite; (3) sheeted mafic dykes of amphibolite

to andesite categories and (4) plagiogranites and a thin pile of ferruginous cherts. The

succession displays dismembered ophiolite succession comprising actinolite-hornblendite,

hornblendite, pyroxenite, gabbro-norite, gabbro, anorthosite, amphibolite, plagiogranite,

mafic dykes, and associated pelagic sediments such as chert-magnetite bands and carbonate

horizons. The structural studies reveal the anatomy of imbricate thrust sheets and slices of a

dismembered ophiolite suite and pelagic sediments (Chetty et al., 2011). The foliation

trajectory map of MOC reveals inward dipping foliations both in the east and west and shows

isoclinal fold structures in the north and south. A major detachment zone occurs at the

western margin with the development of high amplitude tight folds and intrusive

plagiogranites. Based on the geometry of fold styles, foliation trajectories and large variations

in N-S trending hinges, the MOC can be interpreted as a deformed large scale sheath fold,

associated with south verging back thrust system in a crustal-scale ‘flower structure’

described by Chetty and Bhaskar Rao (2006). Petrological and geochemical characteristics

suggest that these rocks represent the remnants of oceanic crust, developed at shallow levels

from mantle-derived arc magmas probably within a suprasubduction zone tectonic setting

(Yellappa et al., 2010). Geological setting and field observations suggest that these rocks

were obducted on to the continental margin with the closure of ocean basin during

Neoproterozoic period. The dominant population of zircons in the two plagiogranite samples

of MOC yielded weighted mean 206Pb/238U ages of 737 ± 23 Ma and 782 ± 24 Ma. Zircons

30

in the gabbroic anorthosite and gabbro samples show well-defined single clusters on the

concordia with weighted mean 206Pb/238U ages of 744 ± 11 Ma and 786 ± 7.1 Ma (Santosh

et al., 2012). The Manamedu ophiolite units may represent the remnants of the Mozambique

Ocean crust developed during Rodinia breakup which was destroyed during Cambrian period

at the time of Gondwana amalgamation (Santosh et al., 2009).

2.4.2. Devanur ophiolitic Complex (DOC)

The other ophiolitic Complex located at Devanur, about 20km north of MOC,

comprises dismembered outcrops along an east-west trending shear zone and represents

typical oceanic sequences/ocean plate stratigraphy with mafic-ultramafic components,

overlying felsic groups and the hornblende gneisses (Yellappa et al., 2011). The DOC occurs

in the form of a lensoid body and comprises rock types such as pyroxenites, gabbros,

anorthosites, actinolite-hornblendites, amphibolite dykes, dolerites, pyroxene granulites,

trondhjemites/quartz keratophyres and thin layers of ferruginous cherts with varied

dimensions. The DOC extends for over a strike length of ~ 15km and with a maximum width

of less than a km. The field and petrographic studies indicate that these lithologies are highly

altered, sheared and metamorphosed and obducted along shear/thrust planes with in the CSZ.

The geochemistry of mafic dykes shows basaltic-andesitic type magmas with tholeiitic to

calc-alkaline characteristics suggesting that these rocks were generated along a

suprasubduction zone tectonic setting with island arc affinities. They are inferred to have

been incorporated within a Neoarchean accretionary belt associated with continental

collision. Two trondhjemite samples of Devanur complex have yielded SHRIMP zircon

238U-206Pb ages of 2528±61 and 2545±56Ma (Yellappa et al., 2011). Similar ages have

been obtained from magmatic zircons in charnockites, anorthosites and orthogneisses in the

adjacent regions (e.g, Ram Mohan et al., 2012). The suprasubduction zone rock assemblages

31

and arc magmas suggest Neoarchean ocean closure along the southern margin of the Dharwar

craton.

2.4.3. Agali Ohiolite Complex (AOC)

Recently, the discovery of a relatively well-preserved suprasubduction zone ophiolite

suite has been reported from the Agali hill in Attappadi, along the western extension of the

Bhavani Shear Zone (Santosh et al., 2013). The rock sequence from bottom upwards

includes altered ultramafics with vestiges of dunite, thin layer of cumulate pyroxenite, a thick

unit of metagabbro with the upper part grading into anorthositic gabbro and carrying thin

layers of hornblendite, capped by metavolcanics (amphibolites) carrying veins and pools of

trondhjemite. Several fragments of metabasite (dolerite) dykes occur within the gabbroic

body. Metamorphosed banded iron formation also occurs in association with amphibolites.

The lithological distribution in the area has been interpreted to represent a typical ‘Ocean

Plate Stratigraphy’ sequence with arc and exhumed sub-arc mantle material (Santosh et al.,

2013). The common occurrence of magnesite in association with ultramafic units in the area

suggests CO2-induced metasomatism of peridotites in the mantle wedge through fluids

released within the subduction zone. Major, trace and REE data on the Agali Ophiolite

Complex suggest magma derivation in a suprasubduction setting. The U–Pb concordia ages

of zircons in the metagabbro and trondhjemite yielded ~2.5Ga, which are correlatable with

similar age data reported recently from DOC. A tectonic model was proposed envisaging

accretion of oceanic arcs and micro-continents onto the margin of the Dharwar Craton during

Neoarchean, marking an important event of continental growth, and broadly coinciding with

the global crustal growth event at this time.

In the light of the significance of the CSZ in terms of suture zone tectonics and its

correlation with other comparable shear zones in the adjacent fragments of the Gondwana

continent, a small corridor between Namakkal and Mohanur in the south central part of the

32

CSZ has been chosen for the present study. Further, it was also a great opportunity to

investigate the recently dug Rail cutting section near Anniyapuram exposing excellent

geological sections, which otherwise would not have been possible. Detailed field

observations, structural studies, petrographic and geochemical characteristics have been

attempted in the present study. The study area would be described as Namakkal-Mohanur

Corridor (NMC) in the present thesis. The significance of these results in terms of

subduction-accretion and collision in an overall plate tectonic regime has been highlighted.