A palaeomagnetic study of charnockites from Madras Block, Southern Granulite Terrain, India

A palaeomagnetic study of charnockites from Madras Block, Southern Granulite Terrain, India

Gondwana Research 10 (2006) 57 – 65 www.elsevier.com/locate/gr A palaeomagnetic study of charnockites from Madras Block, Southern Granulite Terrain, ...

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Gondwana Research 10 (2006) 57 – 65 www.elsevier.com/locate/gr

A palaeomagnetic study of charnockites from Madras Block, Southern Granulite Terrain, India G.V.S. Poornachandra Rao ⁎, J. Mallikharjuna Rao Palaeomagnetism Laboratory, National Geophysical Research Institute, Hyderabad-500 007, India Received 11 April 2005; accepted 17 November 2005 Available online 3 May 2006

Abstract The South Indian Craton is composed of low-grade and high-grade metamorphic rocks across different tectonic blocks between the Moyar– Bhavani and Palghat–Cauvery shear zones and an elongated belt of eastern margin of the peninsular shield. The Madras Block north of the Moyar– Bhavani shear zone, which evolved throughout the Precambrian period, mainly consists of high-grade metamorphic rocks. In order to constrain the evolution of the charnockitic region of the Pallavaram area in the Madras Block we have undertaken palaeomagnetic investigation at 12 sites. ChRM directions in 61 oriented block samples were investigated by Alternating Field (AF) and Thermal demagnetization. Titanomagnetite in Cation Deficient (CD) and Multi Domain (MD) states is the remanence carrier. The samples exhibit a ChRM with reverse magnetization of Dm = 148.1, Im = + 48.6 (K = 22.2, α95 = 9.0) and a palaeomagnetic pole at 37.5 °N, 295.6 °E (dp/dm = 7.8°/11.8°). This pole plots at a late Archaean location on the Indian Apparent Polar Wander Path (APWP) suggesting an age of magnetization in the Pallavaram charnockites as 2600 Ma. The nearby St. Thomas Mount charnockites indicate a period of emplacement at 1650 Ma (Mesoproterozoic). Thus the results of Madras Block granulites also reveal crustal evolution similar to those in the Eastern Ghats Belt with identical palaeopoles from both the areas. © 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Keywords: India; Pallavaram; Granulites; Palaeomagnetism; Madras Block; Palaeopoles

1. Introduction The igneous and metamorphic rocks exposed over the South Indian Shield span a geological time interval extending from 3500–500 Ma. A large portion of this shield is classified as the Dharwar Craton and represents a landmass that has remained stable over the last 2000 Ma (Naqvi et al., 1974). It is made up of a mosaic of lithologic units, which are coherently juxtaposed crustal segments in which geologic activity can be traced continuously throughout the Precambrian period. On the basis of grade of metamorphism, the craton is divided into a low-grade northern and high-grade southern region with the Palghat– Cauvery lineament as the tectonic boundary. The bulk of this crust was formed prior to 2600 Ma and remobilized during 2600–2000 Ma (Early Proterozoic Mobile Belt, EPMB) and 2000–1500 Ma (Middle Proterozoic Mobile Belt, MPMB) ⁎ Corresponding author. Fax: +91 40 27171564. E-mail address: [email protected] (G.V.S. Poornachandra Rao).

(Radhakrishna and Naqvi, 1986). The EPMB was responsible for producing the low-grade and high-grade metamorphic terrains in the north and south respectively. Systematic palaeomagnetic study of rock units from these terrains can help to constrain the development of events that led to the evolutionary history. With this in mind, we have undertaken a palaeomagnetic study of the entire granulitic region in the South Indian peninsular shield. Earlier results of palaeomagnetic study of charnockites from St. Thomas Mount from the Madras Block suggest a period of magnetization corresponding to the Mesoproterozoic (1650 Ma) era (Poornachandra Rao and Mallikharjuna Rao, 1999). Palaeomagnetic study of several charnockitic and other rocks from the Eastern Ghats Mobile Belt also reveal similar results indicating a contemporaneous evolution (Lakshmipathi Raju and Kedareswarudu, 1992 and references therein). A palaeomagnetic study of charnockites from Dharmapuri area indicates a remanence age of late Archaean (2600 Ma) era (Piper et al., 2003). We have undertaken palaeomagnetic study of charnockites around

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Pallavaram from the Madras Block with a view of its period of uplift related cooling and magnetization because of its multiple lithologies compared with that of St. Thomas Mount from the same Madras Block and results of our study are presented in this paper. 2. Geology and sampling The Pallavaram–St. Thomas Mount area south of Madras city is the type area for charnockites and their distribution along with other lithologies are shown in Fig. 1 (Sugavanam and Venkata

Rao, 1990). Several workers have studied the rocks in detail and presented mineralogical, petrological, geochemical, structural and tectonic information for these rocks (Subramaniam, 1960). The St. Thomas Mount area contains more basic charnockites and also shows retrograde metamorphic effects whereas the charonckite rocks of the Pallavaram area comprise acidic, basic and intermediate varieties with both prograde and retrograde metamorphic effects. Plagioclase, potash feldspar, orthopyroxene, clinopyroxene and opaque are the essential minerals in these rocks. Magnetite and ilmenite are the opaque minerals occurring as individual elongated grains or released products. Crawford

Fig. 1. Geological and structural map of Pallavaram–St. Thomas Mount area near Madras (after Sugavanam and Venkata Rao, 1990). Inset map shows the study area along with the other tectonic features in South India. The Proterozoic shear zones shown are 1) Mo — Moyar, 2) Bh — Bhavani, 3) N–Ca — Noyil–Cauvery and 4) AK — Achankovil. The other features shown are DT — Deccan Traps, DC — Dharwar Craton, CB — Cuddapah Basin, EGMB — Eastern Ghats Mobile Belt and SGT — Southern Granulite Terrain. Palaeomagnetic sampling sites are also shown.

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(1969) reported an Rb–Sr whole rock isochron age of 2580 ± 125 Ma and Bernard-Griffiths et al. (1987) obtained an Sm–Nd whole rock isochron age of 2555 ± 140 Ma for charnockites from the area. In the Eastern Ghats belt, considered to be continuing southwards into Madras Block, Perraju et al. (1979) reported an Rb–Sr age of 2600 Ma and Crawford (1969) obtained an Rb–Sr age of 1650 Ma. Several other charnockite massifs in South India are dated at around 2600 Ma. These include charnockites of Nilgiri Hills (Crawford, 1969), Biligirirangan Hills (Peucat et al., 1989) and the Palghat–Cauvery shear zone has been identified as the boundary between Archaean craton in the north and Proterozoic terrains in the south whilst some regions also carry imprints of Pan-African event (Santosh et al., 2003; Satyanarayana et al., 2003). Charnockites from the Pallavaram region have been sampled for palaeomagnetic study by collecting oriented block samples from a number of quarries (Fig. 1). The samples were oriented using Solar and Brunton compasses and precision spirit level. A total of 61 block samples at 12 sites were collected and 211 cylindrical specimens of 25.4 mm diameter and 22 mm height prepared in the laboratory to measure their remanent magnetism. 3. Laboratory measurements Natural Remanent Magnetism (NRM) direction and intensity (Jn) of the specimens were measured on a Spinner Magnetometer (Model DSM-2, Schonstedt, USA) and the susceptibility (k) and its variation with temperature was analysed by a Hysteresis and Susceptibility Apparatus (Likhite and Radhakrishnamurty, 1965). The characteristic nature of remanence in the samples has been established by AF and thermal demagnetization methods. For AF demagnetization an AF demagnetizer similar to that described by Creer (1959) has been used and thermal demagnetization has been carried out using an Schonstedt Thermal Specimen demagnetizer (Model TSD-1). The nature of demagnetization results has been analysed by Zijderveld (1967) plots and PCA analysis (Kirschvink, 1980) and averaged using Fisher (1953) statistical methods. NRM directions of samples from these sites exhibit varying amounts of scatter. However, the specimen directions show very good grouping. The most common magnetization has downward and upward shallow to intermediate inclinations with northward pointing declinations. By reference to the present day field this is referred to as ‘normal’. The Characteristic Remanent Magnetism (ChRM) of these charnockite samples was established by both AF and thermal demagnetization tests. Specimens from all the sites representing different NRM groupings were subjected to progressive AF demagnetization on a pilot basis. The specimens were demagnetized at increasing peak alternating fields of 2.5, 5, 10, 15, 20, 25, 30, 40, 50, 60, 80 and 100 mT and their remanent magnetic vectors were measured after each step of demagnetization. The specimens with northward declination changed to southward/southeastward declination and the specimens with upward inclinations changed to downward inclination exhibiting the ChRM in these samples. Few specimens with southeastward declination and downward inclination retain their ChRM direction and

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exhibit only intensity decay. Typical examples of these characteristics in pilot specimens are shown in Fig. 2(a, b, c). Peak fields of 15–20 mT have been selected from these pilot AF studies to recover ChRM directions in these samples using Zijderveld (1967) plots and PCA (Kirschvink, 1980) analysis. Specimens were AF demagnetized at selected peak fields and at least one specimen from each core was demagnetized for ChRM recovery. Behaviors of ChRM direction in the charnockites were also evaluated by thermal demagnetization. Selected specimens representing all groups of NRM features were heated at increasing temperatures of 100, 200, 300, 400, 450, 500, 540, 580 and 600 °C and their remanent vectors measured. Response of specimens to thermal demagnetization is identical to that of magnetic treatment. Remanent magnetic vectors with northward declination change to southward/southeastward declination, and upward inclinations changed to downward inclinations. Specimens without acquisition of secondary/viscous magnetization exhibit ChRM vectors throughout the heating treatment. Typical behaviours of pilot specimens to thermal demagnetization are shown as orthogonal projections in Fig. 2 (d, e, f). Remanent intensity continuously decays at each step of heating identifying titanomagnetite with variable unblocking temperatures. ChRMs from these demagnetization tests are identified using Zijderveld (1967) diagrams and Kirschvink (1980) PCA analysis and peak temperatures of 400–500 °C selected to isolate ChRMs. The specimen and sample mean ChRM vectors were averaged to obtain the site mean ChRM directions using Fisher (1953) statistical analysis. Sample mean remanent directions for all sites are shown in Fig. 3 and listed in Table 1. 4. Susceptibility variation with temperature Radhakrishnamurty and Deutsch (1974) have suggested a simple test in granulometry to quickly identify domain state of the magnetic minerals by a study of susceptibility (K) variation in low temperatures (KLT Study). The magnetite in rocks undergoes a phase transition when the rock is cooled below − 156 °C and exhibits a pattern in susceptibility. Various forms of the susceptibility variation suggest different domain states including Single Domain (SD), Multi Domain (MD), Super Paramagnetic (SP), Cation Deficient (CD) and their mixtures. Nature of remanence carrier in these rocks has been investigated by using this granulometry test of susceptibility variation in low temperatures. Some specimens from each site were cooled in liquid nitrogen, transferred into the susceptibility apparatus and observed the susceptibility variation. In general, these specimens show an enhancement of susceptibility (Relative Susceptibility, RS) value at −196 °C and acquire the room temperature value without peak at − 156 °C on warming. Few specimens show a RS value of 0.34–0.73 at liquid nitrogen temperature and a susceptibility peak at − 156 °C. Susceptibility variation behaviour of these specimens is shown in Fig. 4. Therefore, this K–T variation in the Pallavaram charnockites is inferred to be due to magnetic grains in CD and MD domain states respectively. Magnetite in Multi-Domain state as remanence carrier was also reported by Piper et al. (2003) in

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Fig. 2. Orthogonal plots showing the response of Pallavaram charnockite samples subjected to step-wise AF (a,b,c) and thermal (d,e,f) demagnetization on pilot basis. Solid (open) circles denote projection of end point of the remanent magnetic vector on the E–W horizontal (N–S vertical) plane. Intensities are in mA/m. Numbers refer to Peak AF fields in mT and temperatures in °C intervals of demagnetizing steps used for the pilot study.

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Fig. 3. Stereographic plot of sample mean ChRM vectors from sites of charnockites around Pallavaram area, south of Madras. Solid (open) circles denote downward (upward) pointing inclinations. Circle with horns indicates site mean vectors.

Dharmapuri charnockites through detailed rock magnetic studies. 5. Results and discussion The Pallavaram charnockite samples show variable scatter at NRM level with good grouping of directions in each sample. Groupings greatly improved by AF and thermal demagnetiza-

tion and ChRM directions identify a “reverse” magnetization in the SE quadrant with intermediate downward inclination. Specimen and sample mean remanent directions were averaged to obtain site mean ChRM vectors at all the 12 sites and are shown in Fig. 5. All sites reveal “reverse” magnetic directions with downward inclination with one site (Site 11) showing upward inclination that is excluded in computing the mean ChRM. The site mean ChRM and corresponding palaeopoles are listed in

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Table 1 Palaeomagnetic results from the Pallavaram charnockites, South India Site no. N (n)

Dm Im

K

α95

p°N Lp°E

dp°

dm°

m°S

1 2 3 4 5 6 7 8 9 10 11 12 Mean

160 43 157 42 157 48 182 48 134 37 114 50 144 43 121 49 142 31 176 63 106 −11 153 63 148 48.6

22.39 14.42 14.01 6.94 4.66 20.59 9.88 62.11 4.08 5.27 47 21.51 23.78

10.46 13.04 16.73 37.57 26.46 13.8 18.18 12.56 31.01 30.48 9.13 13.89 8.66

47.3 46.5 42.4 48 33.8 13.1 38.4 21.2 42.2 32.5 16.8 30.6 37.7

13 16 21.8 49.1 31 18.4 22.5 15.6 34.6 47.9 9.3 21.6 7.76

8 9.8 14.3 32.1 18.2 12.3 14 9.6 19.3 37.7 4.7 16.7 11.78

25 24.2 29 29 20.6 30.8 25 25 16.7 44.5 5.6 43.2 28.4

8 (17) 8 (19) 5 (12) 4 (14) 6 (35) 5 (12) 6 (18) 3 (19) 6 (15) 5 (13) 5 (20) 5 (15) 61

287.4 291.4 287.8 257.5 314.1 313.8 302.9 316.6 312.9 263.5 352.5 378 294.6

N = number of samples, n = number of specimens, Dm = mean declination, Im = mean inclination, K = precision parameter, α95 = radius of circle of confidence, p = latitude of VGP, Lp = longitude of VGP, dp = semi-major ellipse of confidence, dm = semi-minor ellipse of confidence.

Table 1. Thus 11 sites in the Pallavaram charnockites reveal a mean ChRM of Dm = 148.1, Im = +48.6 (N = 11, K = 22.2, α95 = 9.0) and a palaeomagnetic pole at λp = 37.5°N, Lp = 295.6°E (dp = 7.8°, dm = 11.8°). This pole plots at a late Archaean (2600 Ma) location on the Indian Apparent Polar Wander Path (APWP) grouping well with other late Archaean poles from peninsular India and the Southern Granulite Terrain (Mishra, 1965; Bhaskara Rao and Lakshmipathi Raju, 1981; Venkatesh et al., 1987; Poornachandra Rao et al., 1989; Poornachandra Rao and Mallikharjuna Rao, 1990; Das et al., 1996). A LANDSAT imagery study by Drury et al. (1984) highlighted the tectonic evolution of the South Indian craton and identified several major tectonic lineaments. Pichamuthu (1967)

Fig. 4. Susceptibility variation pattern with temperature of Pallavaram charnockites. Normalised susceptibilities are plotted against low temperatures obtained by cooling in liquid nitrogen. Some samples show a susceptibility peak at − 156 °C while others show an increase in susceptibility at − 198 °C without a peak at − 156 °C.

observed that regional metamorphism over this craton varies from greenschist through amphibolite to granulite facies from north to south. In order to account for these tectonometamorphic conditions he considered that the Dharwar rocks record a northward plunging anticlinorium. On the other hand Rogers (1986) postulated variations in crustal thickness and thermal gradients to account for the juxtaposition of the granulitic region with the Dharwar Craton. Radhakrishna and Naqvi (1986) propose that the granulitic terrain constitutes a remobilized portion of the peripheral part of the cratonic nucleus and reactivation of the basement rocks on such an extensive scale being brought about by collisional tectonics (Dewey and Burke, 1973). From continuity of Bouguer gravity anomalies over the Dharwar Craton, Hari Narain and Subrahmanyam (1986) suggest that the South Indian granulitic terrain is continuous and extend beneath the low-grade terrain towards the north constituting a continuous crustal block. The gravity map shows a prominent low of the order of few milligals to the south of Palghat–Tiruchirapalli region and an E–W high to the north. The low has been attributed to the crustal thickening and the high to a deep-seated source from the associated anorthosite bodies and the wellknown Bhavani shear zone (Mishra, 1988). A crustal thickness map based on gravity modeling for the southern peninsula also shows a relatively thick crust over an extensive area between 9°N and 11°N latitudes (Subba Rao, 1986–87). The basement as well as the filtered anomaly maps over the Dharwar Craton by Reddy et al. (1988) suggests a zone of profound structural disturbance along the northern edge of the southern granulitic terrain and the Palghat–Tiruchirapalli region has been suggested to be the junction of the northern low-grade greenschist and the southern high-grade granulitic terrains. Palaeomagnetic results from the Eastern Ghats and South Indian shield along with results from the well-constrained

Fig. 5. Stereographic plot of site mean ChRM vectors of Pallavaram charnockites investigated from the Madras Block, Southern Granulite Terrain. Solid (open circle) denote downward (upward) inclinations. Mean direction of 11 sites with downward inclination is indicated with solid circle with horns.

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Table 2 Palaeomagnetic data of some Indian Precambrian rocks Rock/locality

Visakhapatnam charnokite Visakhapatnam charnokite Visakhapatnam charnokite Kondapalle charnokite Devipatnam charnokite Eleswaram charnokite Sadanandapuram calc-granulite Sukinda chromite St. Thomas Mount charnokite Addatigala iron ore Addatigala iron ore Devipatnam iron ore Devipatnam iron ore Hyderabad dyke Kolar dyke Kerala dyke Mysore dyke Kawar volcanics Gwalior traps

Southern granulite block BHQ Pokhra BHJ1 Parkhuri BHJ2 BHJ3 Tamil Nadu dyke swarm Pitepani volcanics Pallavaram charnockites

A B C D

Pole code

Age

VS1 VS2 VS3 KP DP1 EL SP SC STM AT1 AT2 DP2 DP3 HD KL KR MD KV GTA GTB GTC GTD SG BHQ BHJ1 BHJ2 BHJ3 TD PV PC

1650 Ma 1650 Ma 2600 Ma Palaeoprotozoic 1000 Ma Late Archaean Palaeoprotozoic N1650 Ma 1650 Late Archaean Mesoproterozoic Late Archaean 1000 Ma Mesoproterozoic do 1668 × 30 Ma do 2000 Ma 1830 + 200 do do do Palaeaoproterozoic Late Archaean Late Archaean Late Archaean Late Archaean Late Archaean Late Archaean Late Archaean

α95

Vector Dm

Im

280 45 264 45 29 275 278 34 222 305 63 312 35 32 32 225 26 255 78 80 70 78 263 270 289 283 347 294 343 148

+35 +45 +36 − 20 +68 +52 +38 − 21 +36 +20 +16 +49 +64 − 16 − 18 − 49 +50 − 64 +35 +08 +03 +26 +50 − 12 − 09 +60 +03 − 52 − 41 +49

– – 7 – 14 9 13 8.3 8.3 8 7 17 7 7 2 7.2 13 8.3 5 12.5 18 6.3 5.6 12.5 7 12.2 24.1 23 12.5 8.66

VGP

Reference

p

Lp

− 48 1 15 − 49 13 14 − 44 − 37 39 − 28 45 − 49 − 49 − 47 − 45 − 45 − 28 − 19 − 11 − 19 − 16 14 02 15 32 07 13 42 38

332 14 9 290 23 12 37 28 354 352 327 300 25 17 316 318 209 336 349 356 340 45 357 335 356 320 311 284 295

Bhimasankaram (1964) – do – – do – – do – Lakshmipathi Raju (1977) Bhaskara Rao and Lakshmipathi Raju, (1981) Lakshmipathi Raju and Kedareswarudu, (1992) Kumar and Bhalla (1984) Poornachandra Rao and Mallikharjuna Rao (1999) Bhaskara Rao and Lakshmipathi Raju (1979) – do – Lakshmipathi Raju (1977) – do – Verma et al. (1968) Damodara Reddy and Prasad (1979) Radhakrishna et al. (1986) Hasnain and Quereshy (1971) Poornachandra Rao and Mallikharjuna Rao (1996) Klootwijk (1974) – do – – do – – do – Poornachandra Rao et al. (1989) Mishra (1965) – do – Das et al. (1996) – do – Venkatesh et al. (1987) Poornachandra Rao and Mallikharjuna Rao (1990) Present study

Dm = declination, Im = inclination, α95 = radius of circle of confidence, p = latitude of the VGP, Lp = longitude of the VGP.

Gwalior Traps and other rock formations from peninsular India are listed in Table 2 and plotted in Fig. 6. Different granulite facies rock types along the Eastern Ghats were investigated

by various workers and these are considered here for understanding the evolutionary history of the region in relation to the southern granulite terrain. Palaeomagnetic results on

Fig. 6. Palaeopoles of some Late Archaean–Palaeoproterozoic–Meso Proterozoic formations of the Indian subcontinent plotted on Molleweide's projection. Pole codes are as listed in Table 2. PC — is the palaeopole of Pallavaram charnockites (present study) and STM — is the palaeopole of the St. Thomas Mount Charnockites adjacent to the study area investigated from the Madras Block.

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charnockites, calc-granulites and iron-ores along the Eastern Ghats Mobile Belt are available with palaeopoles confining to Palaeo and Mesoproterozoic time indicating their evolutionary history. Two outcrops of charnockites at Visakhapatnam and Eleswaram and calc-granulites of Sadanandapuram suggest palaeopoles confined to the late Archaean–early Proterozoic eras at 2600 Ma. Another group of palaeopoles from charnockites of Visakhapatnam and Devipatnam and iron-ores from Addatigala and Devipatnam exhibit poles grouping well with results from the Mesoproterozoic Eastern Ghats event. A third set of poles from iron-ores of Addatigala and Devipatnam group well with that of late Proterozoic age at around 1000 Ma. From these data it is inferred that there are three phases of magmatic activity, emplacement and remagnetization recorded in the Eastern Ghats blocks. These are in conformity with the Rb–Sr dating of charnockites of Eastern Ghats Mobile Belt at 2600 Ma (Perraju et al., 1979) and at 1650 Ma (Crawford, 1969). Palaeomagnetic results from the adjacent Pallavaram region of the Madras Block suggest an earlier event at 2600 Ma (Crawford, 1969; Bernard-Griffiths et al., 1987; Santosh et al., 2003). The palaeopole obtained from these charnockites groups well with the BHJ and BHQ poles of late Archaean age (Mishra, 1965; Das et al., 1996) and the palaeopole from the Pitepani Volcanics from Central India (Poornachandra Rao and Mallikharjuna Rao, 1990) that was assigned an age of late Archaean (2600 Ma). Our palaeomagnetic study of charnockites from South India also suggest that they were magnetized during the late Archaean event at 2600 Ma (Poornachandra Rao et al., 1989) since they group well with the palaeopoles of late Archaean age. Geological study in the region suggests that the charnockites of the Madras region form the continuation of the Eastern Ghats Mobile Belt. The charnockitic rocks north of the Noyil–Cauvery Lineament are considered to be of late Archaean age and those south of the lineament bear imprints of the Eastern Ghats and Pan-African events (Unnikrishnan-Warrier et al., 1995; Santosh et al., 2003). Thus the results of palaeomagnetic study are best interpreted in terms of the palaeopole of the St. Thomas Mount charnockites being of about 1650 Ma in age and the palaeopole of Pallavaram charnockites being of about 2600 Ma in age. Palaeomagnetic results of charnockites from SW part of Kerala are not yet available to confirm the Pan-African orogeny event in the Southern Granulitic Terrain. Hence our palaeomagnetic study of the Pallavaram region with a palaeopole located at a 2600 Ma position (late Archaean era) on the Indian APWP suggests that evolution of the Southern Granulitic Terrain started as early as 2600 Ma. Further studies on charnockites from other regions such as the Nilgiri Hills, Biligirirangan Hills, Mangalore, Madurai, Salem and Kerala are underway and aim to further constrain the evolution of the South Indian peninsular shield and the Pan-African event. Acknowledgements We would like express our sincere thanks to Dr. V.P. Dimri, Director, NGRI, Hyderabad, for the continued encouragement and kind permission to publish these results. We are highly

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