1

Chapter 1: Introduction 1.1. Preamble

The dykes are sheet-like igneous bodies which have larger lateral extent compared

to thickness and commonly cut across the structures of pre-existing rocks in which they

intrude. These generally occur in swarms which are radial or concentric around large

intrusives. The dykes are valued as they commonly inherit the geochemical properties of

the parental magma or the mantle from where they have been derived. Specific stress

regimes such as rifting are required for the emplacement of dykes and therefore dykes are

also studied to infer large geodynamic processes such as continental break-up or plume

eruption. Dykes also preserve characteristic remnant magnetization corresponding to a

well defined time of emplacement and therefore precise geochronological studies along

with the paleomagnetic studies help in reconstructions related to super-continent

assembly and breakup.

Rubin (1995) reviewed the physical mechanisms of magma propagation in form

of dykes and compiled various observations which help in understanding the origin of

several dyke swarms. Thickness to length ratio of most of the dykes studied varies

between 10-2 to10-4 indicating that dykes are generally 100 to 10000 times longer than

their thickness. Rubin (1995) observed that thickness of mafic dykes vary and seem to

have some relationship with the age and type of the terrain. Least thickness observed is

approximately ~10 cm (Nicolas, 1986) in peridotites and widest dykes are found in

Proterozoic terrains (Halls and Fahrig, 1987), with dykes of intermediate thicknesses (1-4

m) are reported from the sheeted dyke complexes of ophiolites (Walker, 1987; Kidd,

1977; Speight, et al., 1982; Gudmundsson, 1990). Dykes of thicknesses as high as 100 m

are also found in the Proterozoic dyke swarms. This highlights the efficiency of dyke

systems to distribute large amount of melt to the surface and within the lithosphere and

thereby influencing the geological evolution of Earth’s crust. (Pollard, 1987; Spence, et

al., 1987; Gudmundsson, 1990; Rubin, 1995).

Dykes of different compositions have been reported but mafic dykes are more

prevalent than the silicic dykes exposing the mantle-derived magmas to the surface

through the networks of fractures in lithosphere in different tectonic settings throughout

Earth history.

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1.2. Sources, magmatic processes and tectonic settings of dykes

Mafic dykes are generally believed to have been generated by partial melting in

mantle. However, subtle variations in different mafic dykes and other mantle-derived

rocks, including mantle-xenoliths has demonstrated that rock composition of mantle-

derived rocks can be a great deal different than the source. Broadly speaking there could

be two main reasons for differences in compositions of mafic rocks, one is due to the

differences in source mantle and the second is due to the differences in processes of

magma generation such as extent of partial melting, or the differences in magmatic

processes such as fractional crystallization and crustal assimilation. For example a source

mantle of peridotitic composition would produce alkali basalt if only <5% by weight is

melted at larger depth corresponding to pressure greater than 3 GPa, but the same source,

if melted to larger degree at shallower level can form basalts of tholeiitic series (Hirose

and Kushiro, 1993). Trace element and isotope studies of various mantle-derived rocks

have also proved that certain variations in composition cannot be explained only by

melting or magmatic processes but reflect the different kinds of mantles. Geochemists

now acknowledge different types of mantles such as N-MORB (Normal Mid Oceanic

Ridge Basalts), E- MORB (Enriched Mid Oceanic Ridge Basalts), OIB-mantle (Ocean

Island Basalt), EM-1 (Enriched Mantle-1), EM-2 (Enriched Mantle-2), HIMU- mantle

with higher 238U/206Pb ratio, etc. (Hoffman, 1997, Palme and O’Neil, 2003).

These mantle-types are generally invoked to explain oceanic basalts. Continental

basaltic rocks also thought to have been produced from variable mantle compositions

having additional influences of continental lithospheric mantle (Wyllie and Huang, 1976;

Foley, 1992). Continental mafic rocks have assumed importance due to their close

association with the Archaean greenstone belts, like the komatiite-tholeiite sequences, for

studying mantle evolution with time. Recently mafic dyke swarms in peninsular and

central India have been studied extensively to decipher tectonic evolution of Indian

lithosphere in context of various supercontinent cycles (Meert 2012; Pradhan et al., 2010;

Halls et al., 2007; Kumar et al., 2012; French and Heaman, 2010; etc.). The similarity of

geochronological and paleomagnetic data of geographically widely distributed dyke

swarms in Dharwar craton led Kumar et al. (2012) to suggest an hypothesis of a giant

dyke swarm radiating from a large plume head located somewhere in the west of Indian

3

craton. Beeker and Ernst (2010), has proposed a detailed characteristics of a giant

radiating dyke swarm. It consists of four concentric zones with the plume head at the

center. First zone is ‘proximal zone’ which is within 250 km of the plume head with

layered intrusions tending to occur in this zone. Second zone is ‘near-intermediate zone’

extending out to 750 km from the proximal zone with characteristic concentration of

dykes into separate branches or sub-swarms. Third zone is ‘far-intermediate zone’

ranging from 500 to 1000 km from the center. Dykes in this zone are not many and are

quite large in width (25-50m). The last zone is a ‘distal zone’ far away from the center

(~1000-3000 km) with only a few large dykes being present. If the suggestion of giant

dyke swarm at ~2.3 Ga is validated (Kumar et al., 2012), then the Dharwar dykes would

probably be qualified to be part of near to far-intermediate zone. One importance of such

kind of models is that they help to relate the magmatic activity within a craton with the

magmatic centers which might have been located at far off locations in present time. One

well-studied example of Paleoproterozoic mafic dykes is ‘Scourie’ dykes in Lewisian

gneiss complex of Northwest Scotland. They are shown to demarcate two events of high-

grade metamorphism, the Inverian (~2.5 Ga) and the Laxfordian (~1.7 Ga) and indicating

multiple dyking events spanning more than 400 Ma (Davies et al., 2010). This appears to

be analogous to Dharwar craton, particularly the Northern Block of SGT, where the dykes

occurring in the high grade southern granulite terrain of present study seem to have

intruded between periods of metamorphism.

Recent geochronological and paleomagnetic studies in Archaean-Proterozoic

Dharwar-granulite terrain, have now started putting the dykes of different ages into a

larger context of evolution of craton by multiple events of magmatism and metamorphism

(Halls et al., 2007; French and Heaman 2010; Kumar et al., 2012). However, present data

set of the dykes in peninsular India does not allow to completely validate a particular

model. For example the dykes present to the south of Dharwar craton within the high

grade terrain are previously dated to be of ~1600 Ma (Radhakrishna and Joseph 1996)

could not be related to any tectonic evolutionary model consistent with the proposed giant

Dharwar dyke swarm of ~2.3 Ga age.

The present study attempts to provide new geochemical, isotopic and

geochronological data set of the dyke swarms that are present in the high-grade southern

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granulite terrain just south of Archaean low grade Dharwar craton to understand their

mutual relationship and any petrogenetic relationships.

1.3. Geology of the study area

The peninsular India mainly consists of the Archaean Dharwar craton, the

Cuddapah basin of Proterozoic age, the Eastern Ghats Mobile Belt (EGMB) and the high-

grade Southern Granulite Terrain (Drury et al., 1984) (Fig 1.1).

The SGT is considered to be a mosaic of micro continents or crustal blocks that

are juxtaposed along the shear zones (Chetty et al. 2003). Southern Granulite Terrain

(SGT), exposing a vast tract of high-grade rocks ranging from Ultra High Temperature

(UHT) to Ultra High Pressure (UHP) granulites, has been a key terrain in various

attempts of paleo-supercontinent reconstruction from Paleoproterozoic Columbia to

Neoproterozoic Gondwana (Crawford 1974; Hoffman 1991; Fitzsimons 2000; Cenki and

Kriegsman 2005; Meert 2002, 2012). To decipher the tectono-metamorphic evolution of

SGT has been difficult mainly due to the fact that there has been superimposition of

various deformation and metamorphic events that makes it difficult to correlate this

terrain geologically with other coeval and contiguous terrains of erstwhile

supercontinents. The SGT extends up to the orthopyroxene isograd to the north, first

identified by Fermor (1936) and thereby known as Fermor line. The only prominent

structure within SGT that has been the focus of various evolutionary models is the

Neoproterozoic Palghat Cauvery Shear Zone that has been correlated with Bongolava-

Ranotsara shear zone or Betsimisaraka shear zone in Madagascar in the west and with the

boundary between Rayner and Napier complexes of Antarctica in the east (De Wit et al.,

1995; Harris et al., 1994; Reeves et al., 2002; Meißner et al., 2002; Cenki and Kriegsman

2005).

Recently it has also been proposed to be a suture and site of Mozambique ocean

closure culminating the Gondwana amalgamation in Neoproterozoic (Janardhan 1999;

Rambeloson et al., 2003; Cenki and Kriegsman 2005; Collins et al., 2005, 2007;

Raharimahefa and Kusky 2009; Santosh et al., 2009, 2012; Plavsa et al., 2012). However,

the high grade terrain north of Palghat-Cauvery Shear zone and south of the Fermor line

has not been well understood. Even the strato-tectonic position of this block has also been

not clear. According to Drury et al., (1984) this ‘Northern Block’ is part of the granulite

5

terrain that is dissected by crustal scale shear zones. However, another school of thought

maintained that ‘Northern Block’ is part of Dharwar craton which had evolved

independent of the SGT and was juxtaposed with it only during Pan-African time.

Figure 1.1: Regional geological map of peninsular India showing the major rock units and tectonic features; modified from; Geological Survey of India (1981), Meißner et al. (2002), French and Heaman (2010) and the references therein. EGMB – Eastern Ghat Mobile belt; 1 – Mafic Dykes; 2 – peninsular gneises; 3 – Shear zones (AK – Achankovil; B – Bhavali; M – Moyar; Me – Mettur; PC – Palghat-Cauvery); 4 – High grade supracrustal; 5 – Migmatites and Granites; 6 – Younger sediments; 7 – Cuddapha sub-basin; 8 – Granulite terrain; 9 – Closepet batholiths; 10 – Archean supracrustal; 11 – Meso-Neo Proterozoic basin; 12 – Deccan flood basalts.

6

According to this model, the contrasting grade of metamorphism in Dharwar

craton and northern block is result of differential rates of exhumation whereby the high

grade portion is deeper part of the Archaean Dharwar Craton. (Goplakrishna et. al., 1986,

Naha and Srinivasan 1996; Meißner et al., 2002).

Geophysically this region is characterized by a mantle-derived heat flow value of

17 ± 2 mW/m2 which is similar to global average and surface heat flow varies up to 10–

12 mW/m2 within the block (Manglik, 2006). Geochemically the granulite terrain has

similar Sr content as in the transition zone (TZ) charnockite and is depleted in Rb (Allen

et al., 1985; Peucat et al., 1989). This depletion is related to the tonalite emplacement in

the region as tonalite and charnockite both seem to plot on same Rb/Sr isochron. Data

from the Bilgiri Rangan (BR) hills indicate a pre 2500 Ma history in this charnockite

massif. The Pb-Pb system suggests U and Th depletion prior to 2500 Ma which may be

related to a high grade event. The negative values of εNd obtained from the high-grade

rocks have been interpreted as due to previous record of crust prior to ~2500 Ma (Peucat

et al., 1989). The granulites in this region have recorded pressure and temperature of the

order of 7-8 kb and 700-800 °C or above, with depletion of some LIL element (notably

Rb, Th, U, Cs, Pb) in certain localities (Heier 1965; Dury 1978; Wever and Tarney 1980;

Condie and Allen 1982). The role of a CO2 rich fluid phase in the genesis of granulites,

particularly in LIL element depletion has been suggested by many authors. Apart from the

regional high grade rocks, younger granites were also reported and are suggested to have

formed by anatectic and crustal remelting processes during Archaean-Proterozoic

transition. Variation in the grade of metamorphism from the north to south, from low-

grade Dharwar craton to high-grade granulite terrain is suggested to be due to differential

exhumation. Granulites of SGT represent the deeper (>25 km) part of the continental

crust compared to the gneiss-amphibole associations in Dharwar craton (Raith et al.,

1983, 1999). Together these blocks have experienced multiple tectono-metamorphic and

magmatic events and as a result have acquired complex tectonic framework. It is

therefore imperative to generate a large data-set to test and validate the evolutionary

models which are being proposed for Dharwar and granulite terrains.

Geochronological data from the northern block of the granulite terrain suggest that

the regional metamorphism is 2500 Ma old (Vidal et al., 1988). The massive charnockite

from the north of Cauvery shear zone has yielded Sm-Nd whole rock isochron age of

7

2490±60 Ma and U-Pb zircon ages of 2499±40 Ma (Bhutani et al., 2007a, b). The Early

Proterozoic granites are recognized around Gingee, Tiruvannamalai and Tirukovilur

along with younger migmatitic granite within the MT terrain and the Rb-Sr age of

migmatitic granites were reported to be 2254 ± 60 Ma (Balasubrahmanyan et al. 1979).

From the perspective of global tectonic events, it was suggested that East

Antarctica and the Eastern Ghat Mobile Belt (EGMB) which borders the Dharwar and

Cuddapah basin to its’ east are complimentary part of an orogeny of Grenvillean time

(Mezger and Cosca 1999). Based on the previously published ages for the dykes

occurring in the Tiruvannamalai, Dharmapuri, Tirupati and adjoining EGMB, Guiting

Hou et al., (2008), had proposed that the EGMB extends all along within SGT and the

dykes emplaced in these regions during Paleo-Mesoproterozoic conjunction. They also

proposed that, India was separated from the supercontinent Columbia in Mesoproterozoic,

while East Antarctica along with other was in the center of the Columbia during 1.85 to

1.20 Ga.

The timing and mechanism of juxtaposition of Dharwar craton with the high grade

granulite terrain has remained an outstanding problem since long. Early suggestions

(Drury et al., 1984) favour thrusting of SGT beneath Dharwar craton during Proterozoic.

Recent studies (Santosh et al., 2010) though envisage a Pacific to Himalayan type of

orogeny during Pan-African time. Srinagesh and Rai (1996) proposed subduction of

Dharwar craton beneath SGT towards south. A Himalayan-type of orogeny was also

proposed by Gupta et al. (2003) based on receiver function analysis. The boundary

between Dharwar craton and SGT has also been debated. Some researchers consider

Palghat Cauvery Shear Zone as the boundary between Dharwar and SGT (Ramakrishnan

2003, and references therein), whereas others consider hypersthene isograd as the

boundary between Dharwar and SGT. Even Moyar Bhavani Shear Zone (MBSZ) also has

been proposed to be the main suture zone (Raith et al., 1999 and Singh et al., 2003), and

yet another group maintains that Karur-Kamban-Painavu-Trichur Shear Zone (KKPTSZ)

is the boundary between Proterozoic ‘Northern block’ and pan-African Madurai block

(Ghosh et al., 2004) Northern block of SGT, appears to be a locale of continental

collision, crustal thickening, exhumation and erosion in all these models.

8

The SGT broadly has two domains of granulite facies, one to the north of Palghat

Cauvery Shear Zone (PCSZ) is of Late Archaean (ca. 2.5 Ga) while one to the south of

PCSZ in Madurai block is of Neo-proterozoic (ca 1.0–0.55 Ga) age.

1.4. Tectonic setting of study area in the Northern Block of Southern Granulite Terrain

The Madras-Tiruvannamalai (MT) terrain, in Northern Block, a triangle shaped

landmass on map, is bounded by Mettur and Palghat-Cauvery shear zones to the

northwest and south respectively and Bay of Bengal to the east. This terrain consists of

hypersthene bearing granulites (also known as charnockite), hornblende biotite gneiss,

granites and mafic dyke swarms (Fig 1.2).

Figure 1.2: Geological map of the north-eastern part of southern granulite terrain (from Geological map of Tamil Nadu and Pondicherry, GSI, 1995) showing the different rock types, dykes and N45°E Mettur shear zone extending through Vaniyambadi; 1- younger sediments; 2-Pyroxenite; 3 – younger granites; 4 – basal boulders/conglomerate; 5 – ultramafics; 6 – syenite complex; 7 – pink migmatite gneisses; 8 – granitoid gneisses; 9 – hornblend-biotite gneisses; 10 – epidote-hornblend gneisses; 11 – charnockite (hypersthene bearing granulite).

The foliation planes observed in the MT terrain are predominantly trending in the

NE-SW direction and are parallel to the trend of the Mettur Shear Zone (MSZ).

Harinarayana et al., (2006) considered the Mettur shear zone to be the NE extension of

9

the Moyar-Bhavani shear zone that traverses along the amphibolites-granulite transition

zone. Based on the variation in the deep electrical resistivity they conclude that the MSZ

represents a possible collision boundary between the Archaean Dharwar craton in north

and the Proterozoic granulite terrain towards south with high heat flow and low

resistivity. Based on the nature of the seismic reflectors, anomalous low-gravity features

it was proposed that the northern block of SGT has been subjected to collisional processes

and the MSZ represents a collision boundary.

Figure 1.3: a) Trace of the structural features form the geological map of Tamil Nadu and Pondicherry; b) The rose diagram showing the regional dyke trend distribution; c) The contours of the foliation planes in this region.

From the geological map of Tamil Nadu and Pondicherry by GSI, (1995) the

major structural features have been traced out to understand the tectonic deformations

preserved in this region. Based on the faults, shearing and foliation planes in the

a

b c

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northeastern part of SGT it is clear that the Madras-Tiruvannamalai (MT) block had

experienced larger tectonic deformation compared to the Dharmapuri-Krishnagiri (DK)

block (Fig 1.3a). The foliation planes are steeply dipping and are well developed between

Gingee in the east and Tirupattur in the west. The dykes occurring in this region are

predominantly trending in NW-SE direction with a few trending in NE-SW direction

(Fig. 1.3b). The poles of the foliation planes plot near the circumference of the stereonet

forming high density contours in the NW and SE direction (Fig 1.3c).

The contour pattern indicates a tight antiform plunging in the SW direction as

reported in the map. The conjugate sets of dykes appeared to have been emplaced during

single magmatic episode and their relative ages are difficult to determine from the cross-

cutting relationships in the field. The dyke pattern of the NW trending dykes in the

Tiruvannamalai region are perpendicular to the foliation planes and parallel to the

maximum compression direction (Fig. 1.3b). Even though the dykes that are present

within shear zone are not found to be sheared probably indicating that the dyke swarm

has been emplaced in a shear tectonic setting where the dextral shearing was parallel to

the propagating crack (Rubin 1995, and reference therein).

1.5. The dykes in peninsular India

An exclusive map of dykes in the peninsular India was first published by Halls,

1982 (Halls et al., 2007). Episodic emplacement of mafic dykes during Precambrian has

been documented from several parts of the Archean greenstone belts, gneisses and

granites of the Dharwar craton, and southern granulte terrain of peninsular India (Murty et

al., 1987; Mallikarjuna Rao et al., 1995; Murthy, 1995, Radhakrishna and Joseph, 1996).

Most of these dykes, particularly in Dharwar craton, appear to have been emplaced during

Proterozoic (Ikramuddin and Stueber, 1976; Drury and Holt, 1980; Drury, 1984; Murty et

al., 1987, Halls et. al., 2007). Lower parts of Cuddapaph basin are also intruded by

abundant mafic sills, dykes and flows, though the causal links between mafic intrusive

and basin opening is still being developed (Chatterjee and Bhattacharji, 2001; Anand et

al., 2003; French et al., 2008). The available geochronological record of several dyke

swarms on the western margin of the Cuddapah basin indicates towards a complex pattern

of dyking. One study suggests that there could be 18 events of dyking spanning nearly

1500 Ma (Murty et al., 1987). Mallikarjuna Rao et al., (1995), on the other hand group the

11

Dharwar dykes into four time periods 1.9–1.7, 1.4–1.3, 1.2–1.0 Ga and at 650 Ma (Halls

et al., 2007)

The E-W trending dykes including the highly concentrated outcrops of dykes

southwest of the Cuddapah basin (Fig. 1.1), are most prominent mafic dykes in the

Dharwar craton that extend across a large tract of the craton and also include Bangalore

dykes which are studied extensively recently (French et al., 2004; French, 2007; Halls et

al., 2007). Attempts have been made to trace the dominant dykes along strike in a

westerly direction up to Western Ghats as far as 450km crossing the whole Dharwar

craton (Smeeth, 1915; Drury, 1984; Murty et al., 1987). This swarm has also been

identified crossing the southern margin of Dharwar craton and outcropping within the

granulite terrain (Ikramuddin and Steuber, 1976; Halls et al., 2007). Bangalore dyke

swarm would appear to span a period from ∼2.5 to 1.1Ga (Ikramuddin and Steuber, 1976;

Balakrishna et al., 1979; Padmakumari and Dayal, 1987; Murty et al., 1987; Sarkar and

Mallik, 1995; Dayal and Padmakumari, 1995; Mallikarjuna Rao et al., 1995; Zachariah et

al., 1995; French et al., 2004; Halls et al., 2007b). Volcanics within the Cuddapah basin

are dated to be 1817±24 Ma by a Rb–Sr whole rock–mineral isochron method (Bhaskar

Rao et al., 1995) average of Ar-Ar fusion ages of phlogopite from the same volcanics is

1899±20 Ma (Anand et al., 2003).

According to Halls et al., 2007 the easterly trending dykes in Dharwar was

originally emplaced at high latitudes, together with the Widgiemooltha dykes of the

Yilgarn block of Australia. This dyke swarm may have been a segment of a larger

radiating swarm related to a long-lived plume event that was active for about 50My from

2418 to 2367 Ma. This hypothesis is recently supported by Kumar et al., (2012) based on

the similarity of ages between northern Karimnagar dyke swarm and southern Bangalore

dyke swarm. The regional change in the intensity of brown feldspar clouding in the dykes

was explained on the basis of northward tilting of the Dharwar craton, in harmony with

the structure and metamorphism of the Archean rocks discussed above. Rocks which are

in the vicinity of shear zone have feldspars with black clouding instead of brown

clouding. These rocks are suggested to be remagnetized either during the alkali-

carbonatite magmatism at ~800 Ma or during the deformation related to Pan-African

orogeny at ~500 Ma (Halls et al., 2007).

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As far SGT is concerned the dykes are generally medium to coarse grained and

massive tholeiites. Other than Tiruvannamalai area, dykes have also been reported from

Agali-Coimbatore area of granulite terrain (Radhakrishna, 2009). Earlier studies have

reported these dykes to have been emplaced during 1.8 Ga and 1.65 Ga based on the

Ar/Ar and K-Ar isotopic results of dykes in Dharmapuri and Tiruvannamalai regions

respectively. Older ages comparable to those of the Dharwar craton were not known prior

to the present study. Geochemically, these dykes are similar and are grouped as quartz /

hypersthene normative subalkalic tholeiites. The previous studies have reported these

dykes to be enriched in incompatible elements and also exhibiting fractional

crystallization trends (Radhakrishna, 2009)

In light of the recent proposal of Dharwar Giant Dyke swarm the present study

becomes even more important as it provides new data set from the dyke swarms of

Dharmapuri and Tiruvannamalai regions in vicinity of Metur Shear Zone (MSZ) present

in the Northern block of Southern Granulite Terrain. This would help not only to

understand the petrogenesis of these mafic dykes but also to decipher the relationship and

links between the low grade Dharwar craton and high grade Northern block of SGT vis-à-

vis the significance of MSZ. More importantly the present study also focuses on finding

the relationship with proposed Paleoproterozoic Giant Dyke swarm in southern India.

1.6. Objectives

In the light of the above discussion, following objectives could be summarized:

1. To understand the origin and emplacement of the mafic dykes in ‘northern block’ of

southern granulite terrain near Tiruvannamalai region across the NE-SW trending

Shear Zone with the help of detailed geochemical investigation.

2. To correlate the dykes within the granulite terrain with that of within Dharwar

craton.

3. To decipher the precise geochronology and to revisit the existing K-Ar gechronology

which apparently is inconsistent with the recent paleomagneitc studies of Dharwar

dykes.

4. To propose an evolutionary model of the Dharwar and bordering granuilte terrain

together by integrating the available geochemical, geochronological and

paleomagnetic data.