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Palaeomagnetic and rock magnetic study of charnockites from Tamil Nadu,
India, and the ‘Ur’ protocontinent in Early Palaeoproterozoic times
S. Mondal
a
, J.D.A. Piper
b,*
, L. Hunt
b
, G. Bandyopadhyay
a
, S. Basu Mallik
a
a
Department of Geological Sciences, Jadavpur University, Kolkata 700 032, India
b
Geomagnetism Laboratory, Department of Earth and Ocean Sciences, University of Liverpool, Liverpool L69 7ZE, UK
article info
Article history:
Received 24 January 2008
Received in revised form 30 July 2008
Accepted 7 August 2008
Keywords:
Charnockite
India
Palaeomagnetism
Rock magnetism
Magnetomineralogy
Palaeoproterozoic
Ur protocontinent
abstract
Palaeomagnetic and magnetomineralogical results are reported from charnockites in basement terrane at
the eastern sector of the WSW–ENE granulite belt of South India. Magnetite is the dominant ferromagnet
identified by rock magnetic and optical study; it is present in several phases including large homo-
geneous titanomagnetites and disseminated magnetite in microfractures linked to growth stages ranging
from primary charnockite formation to uplift decompression and exhumation within the interval
2500–2100 Ma. Several components of magnetization are resolved by thermal demagnetization and
summarized by four pole positions; in the northern (Pallavaram) sector these are P1 (33°N, 99°E, dp/
dm= 8/9°) and P2 (79°N, 170°E, dp/dm= 3/6°), and in the southern (Vandallur) sector they are V1
(23°N, 116°E, dp/dm= 8/9°) and V2 (26°S, 136°E, dp/dm= 5/10°). These magnetizations are linked to
uplift cooling of the basement and unblocking temperature spectra suggest acquisition sequences
P1 ?P2 and V1 ?V2 in each case implying movement of the shield from higher to lower palaeolatitudes
sometime between 2500 and 2100 Ma. Palaeomagnetic poles from the cratonic nuclei of Africa, Australia
and India all identify motion from higher to lower palaeolatitudes in Early Palaeoproterozoic times, and
this is dated 2400 and 2200 Ma in the former two shields. The corresponding apparent polar wander
(APW) segments match the magnetization record within the charnockite basement terranes of southern
India to yield a preliminary reconstruction of the ‘Ur’ protocontinent, the oldest surviving continental
protolith with origins prior to 3000 Ma. Although subject to later relative movements these nuclei seem
to have remained in proximity until the Mesozoic break-up of Gondwana.
Ó2008 Elsevier Ltd. All rights reserved.
1. Introduction
Exposures of metamorphic basement to the Precambrian
shields provide the earliest insights into the history of continental
crust and, unless they have been comprehensively overprinted by
later geological events, they may also record the oldest segments
of the palaeomagnetic record and hence the earliest kinematic his-
tory of the shields. The rocks now sampled at the surface have usu-
ally been magnetized at depth during protracted regional uplift
and cooling and record magnetic remanence acquired over long
periods of time.
The metamorphic basement in South India comprises a large
area of deeply exposed continental basement and the rocks
selected for study here come from Pallavaram near the type area
of charnockite (Holland, 1893, 1900; Harley, 1989), a plutonic
mineral assemblage comprising essential orthopyroxene (hyper-
sthene) and potassium feldspar (microcline) with accessory quartz
and magnetite. Charnockite is formed by a range of interrelated
magmatic, metasomatic and metamorphic processes (Chacko
et al., 1987; Thomson et al., 2005) and although closely analogous
to hypersthene granite, it is essentially anhydrous because the
presence of free water and quartz would transform these minerals
into biotite and quartz; as such it preserves a deep crustal signa-
ture of early Precambrian age within the cratonic nuclei. In
the study region it is located within the Madras Block in the north-
ern granulitic segment of the South Indian Shield and is part of
an E–W trending zone running across India (Fig. 1) that separates
the Dharwar Craton to the north from the Madurai Block to the
south.
Archaean and Proterozoic plutonic terranes appear to have been
characterized by rates of uplift much slower than Phanerozoic oro-
genic belts (Watson, 1976) and Fermor (1936) inferred that the en-
tire charnockite region of Peninsular India is a portion of crust
formerly deeply buried to depths in excess of 25 Km subject to
slow exhumation and consolidation during late Archaean and early
Proterozoic times (Crawford, 1969; Crawford and Compston, 1970;
Drury et al., 1984). Protracted cooling results in the lowering
and broadening of blocking temperature spectra in ferromagnetic
minerals (Pullaiah et al., 1975) and the characteristics of magneti-
zations acquired in uplifted plutonic terranes are (i) secular
1367-9120/$ - see front matter Ó2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jseaes.2008.08.004
*Corresponding author. Tel.: +44 151 794 2000.
E-mail addresses: supriya0509@rediffmail.com (S. Mondal), sg04@liverpool.ac.
uk (J.D.A. Piper).
Journal of Asian Earth Sciences 34 (2009) 493–506
Contents lists available at ScienceDirect
Journal of Asian Earth Sciences
journal homepage: www.elsevier.com/locate/jaes
variation is averaged out and polarity inversions can be recorded
within individual samples, (ii) at any given crustal level ferromag-
nets with higher unblocking temperatures are expected to record
earlier parts of the apparent polar wander (APW) record than those
with lower unblocking temperatures, and (iii) if appreciable APW
occurred during cooling, magnetisations from different structural
levels may be distributed along an APW path (Morgan, 1976).
The objectives of this study were (a) to evaluate remanence car-
rier(s) in this terrane and relate them to petrology, and (b) to deter-
mine the directional record of magnetic remanence, and thus
whether this palaeomagnetic record can help constrain Early Pro-
terozoic kinematics of the South Indian Shield and relate it to other
shields of similar character and antiquity.
2. Geological framework
The South Indian Shield comprises metamorphic and intrusive
igneous rocks of the provinces of Kerala, Tamil Nadu, Karnataka,
and Andhra Pradesh and is bisected by a west–east zone of gran-
ulitic facies terrane running from Mangalore to Madras near lati-
tude 12°N incorporating extensive development of charnockite
(Fig. 1). This zone separates two contrasting regions of Precam-
brian crust. To the north the Dharwar Schist Belts comprise
supracrustal rocks isoclinally folded into regions of high finite
strain with N–NNW trends and intruded by K-rich granites at
2600–2200 Ma (Crawford, 1969). These are collectively referred
to as the ‘‘Peninsula Gneisses” and form most of the Dharwar Cra-
ton although the protolith is more ancient and Na-rich sialic crust
was present here in early Archaean times (3400–3000 Ma) prior
to formation of the schist belts (Beckinsale et al., 1980; Taylor
et al., 1984). Low to medium metamorphic grades ranging from
upper greenschist to upper amphibolite are typical of this region
and were uplifted from depths of 10–15 km (Rameshwar Rao
et al., 1991) prior to formation of the Cuddapah Basin in the NE
(Fig. 1). Zachariah et al. (1999) determined a 1800 Ma Pb–Pb
age on the Vempalle Limestone near the base of the Cuddapah
succession and Rb–Sr dating of the Cuddapah Traps (Crawford
and Compston, 1970) and microbial evidence (Viswanathiah and
Venkatachalapathy, 1979) both indicate that sedimentation began
here at about 1800 Ma. This is the age of the oldest intrusions
into the succession (Rb–Sr results of Bhaskara Rao et al., 1995)
and defines a minimum age for completion of major crustal uplift
in this sector of the Dharwar Craton; minor subsequent basement
uplift may be recorded by unconformities within the Cuddapah
succession.
Charnockite formation is essentially a static phenomenon that
has overprinted banding and foliation in the host gneisses (Ram-
iengar et al., 1976) and is evidently a late facet of cratonic evolu-
tion with earlier structures guiding the influx of CO
2
-rich fluids
that flushed water from the country rock to produce solid state
recrystallisation into charnockite (Subramaniam, 1959; Drury
and Holt, 1980 Bhattacharya and Sen, 1986). The mineral assem-
blages indicate that charnockite formation occurred at tempera-
tures close to 800 °C and pressures of 7–9 kbars implying depths
of the order of 30–35 km (Weaver, 1980; Condie et al., 1982; Raith
et al., 1982), and isotopic evidence indicates that the source fluids
originated directly from the mantle. Although crossed by a net-
work of Late Proterozoic shear zones (Drury et al., 1984), this re-
gion has been subject to relatively low finite strains (Drury et al.,
1984) and was largely unaffected by the late Archaean-Early Palae-
oproterozoic metamorphism to the north (Fig. 1). Sharp boundaries
between the transition zone and terranes to the north and south
may incorporate metamorphic dehydration fronts (Newton and
Perkins, 1980) and follow a number of major shear zones (Moyar
– Bhavani, Palghat – Cauvery and the Achankovil). These zones
form an anatomizing system developed in Early Palaeoproterozoic
times after charnockite metamorphism and before formation of the
Cuddaph Basin and they divide this high-grade craton into differ-
ent blocks from north to south (Fig. 2): the Palghat – Cauvery Shear
Zone divides the granulite blocks into Northern and Southern seg-
ments. The Northern Granulite segment comprises the Northern,
Nilgiri and Madras Blocks, whilst the Madurai, Periyar and Trivan-
drum Blocks constitute the Southern Granulite Segment (Unni-
krishnan-Warrier et al., 1993 and Fig. 2).
Isotopic age data yield a regional Rb–Sr isotope pattern identi-
fying metamorphism at 2600 Ma (Crawford, 1969) with the age
of charnockite formation also determined as 2600 Ma by Vinogra-
dov et al. (1964) using lead-isochron and Pb
207
/Pb
206
isotope ratios.
Although these determinations are relatively old, this age estimate
of 2600 Ma correlates well with a peak of 2600–2400 Ma in the
histogram of granite-gneiss ages from terranes to the north (Naqvi
et al., 1978, see also Hargraves and Bhalla, 1983) and with an age
for granulite metamorphism of 2555 ± 140 Ma determined by Ber-
nard-Griffiths et al. (1987). Other more recent age studies have
reinforced this assessment: Sm–Nd and Rb–Sr whole rock isoch-
rons yield ages of 2500 Ma along the transition zone (Vidal
et al., 1988); massive charnockites have yielded model ages of
2800 Ma (Peucat et al., 1993), study of incipient charnockite for-
mation has identified a protolith age of 2965 ± 4 Ma and crustal
anatexis dated 2528 ± 5 Ma (Friend and Nutman, 1992; Nutman
et al., 1992, see also Harris et al., 1994). Charnockite formation is
bracketed by field relationships between these two ages, an inter-
val that also correlates with large scale K metasomatism (Divchara
Rao et al., 1982). The subsequent uplift and cooling history is not
well constrained although the shield was probably elevated to high
levels by the time of emplacement of mafic dyke swarms into brit-
tle crust at 2100 Ma (Ikramuddin and Steuber, 1976; Rad-
hakrishna et al., 1995) and levels similar to the present were
reached by the time of deposition of the Cuddapah Supergroup
(1800 Ma); basement consolidation and cooling would have been
complete by this time.
The investigated region comprises about 300 km
2
between
Mosque Hill near Pallavaram (80°9
0
E, 12°58
0
N) and Vandallur
(80°5
0
E, Lat. 12°53
0
N) 40 km SW of Madras city and is sited at
the NE perimeter of the Madras Block (Fig. 2). Outcrops are largely
devoid of vegetation and stand out as small ridges trending
roughly NE–SW to NNE–SSW as hillocks rising from flat planes
occupied by Recent cover (Fig. 3). Rock types comprise charnockite
with minor granulite, leptynites, garnet–quartzofeldspathic gneiss,
enderbites and hypersthene diorites intruded by dolerite dykes
and at least three generations of folding predate the static meta-
morphism with the first and third having NNE–NE trends
(Fig. 3); mineral foliations dip fairly steeply at 55°to 75°.
3. Rock magnetic and mineralogical study
3.1. Sampling and laboratory methods
Up to six individually-oriented blocks were collected at site
locations shown in Fig. 3 which are mostly in fresh quarry sections;
up to six cores were drilled from each block in the laboratory. Mag-
netic susceptibilities were measured by Bartington meter and
Minispin magnetometers employed to measure magnetic rema-
nence. Thermal demagnetization utilized MM2D demagnetisers
with measurements and demagnetization performed within large
sets of Rubens coils with the ambient magnetic field continuously
nullified to minimize secondary remanence acquisition. Rock mag-
netic investigation used a vibrating sample magnetometer (VSM)
for hysteresis study and a computer controlled horizontal Curie
Balance for thermomagnetic measurements; a pulse magnetizer
was also used to impart magnetizations and evaluate isothermal
remanent magnetization (IRM) acquisition.
S. Mondal et al. /Journal of Asian Earth Sciences 34 (2009) 493–506 495
3.2. IRMs, magnetic hysteresis and thermomagnetic results
IRMs were applied to samples from selected sites and examples
of remanence acquisition are shown in Fig. 4. There is typically a
sharp rise to 300 mT, a signature of a low coercivity ferromagnet,
presumably magnetite, followed by little or no additional IRM
acquisition in higher fields. Fragmented samples from most sites
were also subjected to cycled magnetic fields up to 1 Tesla in a
VSM to produce hysteresis loops and two examples are shown in
Fig. 4. The characteristic curves again identify a dominant low
coercivity ferromagnet (Fig. 4c) sometimes with significant contri-
butions from paramagnetic silicates presumably including the
orthopyroxene (Fig. 4d). M
rs
/M
s
values lying between 0.02 and
0.5 are typical of pure magnetite in mixed grain sizes with domi-
nant multidomain (MD) and subordinate singledomain (SD) frac-
tions; an additional indication of the latter is provided by the
coercivity values, H
c
, comparable to the typical value of
1.7 10
3
Am
1
for MD magnetite (O’Reilly, 1984).
Thermomagnetic determinations identify predominantly dis-
crete titanomagnetite close to pure magnetite in composition
(Fig. 5a) and as is typical of well annealed and slowly cooled meta-
morphic basement rocks, little or no alteration occurs during the
heating and cooling. Subordinate curve types include significant
contributions from the paramagnetic silicates (Fig. 5b) again with
little or no alteration, and a pyrrhotite signature superimposed
on magnetite, in this case with the latter forming at the expense
of the former on cooling (Fig. 5c).
3.3. Fe–Ti Oxide mineralogy
The charnockites have titanomagnetite and ilmenite as domi-
nant Fe–Ti oxides (Fig. 6) with subordinate rutile and pyrrhotite.
At Pallavaram titanomagnetites occur both as homogeneous and
inhomogeneous grains; ilmenite is present as exsolved lamellae
within titanomagnetite and also as individual grains. The Vandal-
lur region charnockites have homogeneous titanomagnetites with
no ilmenite exsolution lamellae detected and minor pyrrhotite.
Metamorphic magnetites in equilibrium with ilmenite are ex-
pected to contain amounts of titanium increasing with metamor-
phic grade although the Ti appears to be exsolved during re-
equilibration on cooling leaving the pure magnetite evident in
the thermomagnetic analysis (Fig. 5).
Textural relationships in these rocks lead us to identify at least
three generations of ferromagnetic mineral growth. The earliest
comprises the homogeneous titanomagnetites as discrete and
irregular grains occurring either separately or associated with
pyroxene grains (Fig. 6a). Since, complete solid solution of magne-
tite-ulvöspinel can only occur above 600 °C, these grains are likely
to have crystallized near the peak P–T condition of charnockite for-
mation. The second oxide generation (only evident from the rocks
of the Pallavaram region) comprises titanomagnetite grains host-
ing exsolved lamellae of ilmenite (Fig. 6b); these indicate pro-
tracted subsolidus exsolution during slow regional cooling
promoted by a fluid phase. Thirdly fine-grained secondary magne-
tite growth occurs along grain boundaries and microfractures in
050
km
+
+++
+
+
+
+++
+
+
++
++
++
+
+
+
+++
+++
+++
76 78E
12
10
8N
MADURAI
BLOCK
NORTHERN
BLOCK
MADRAS
BLOCK
NILGIRI
BLOCK
enoZraehSrayoM
Bhavani
Shear Zone
Cauver Shear
Zone
Palghat Shear Zone
enoZraehSlivoknahcA
+
Phanerozoic
Cover
Granite-Greenstone
Terranes
High Grade
Archaean Blocks
Early Proterozoic
Shear Zones
Fig. 2. The tectonic framework of South India illustrating the cratonic segmentation by major shear zones. The rectangle is the location of the study area near the NE margin
of the Madras Block.
496 S. Mondal et al. /Journal of Asian Earth Sciences 34 (2009) 493–506
the silicates; fracturing is also evident in most of the titanomagne-
tite grains at Vandallur. This later stage growth is developed as
numerous fine specks or needle shaped platelets between or along
silicate grain boundaries, or as fine emulsion drops (0.01–
0.03 mm) along silicate boundaries (Fig. 6c). Petrographically this
phase is very complex and presumably includes multiple stages
of oxide precipitation down to microcrystalline iron oxides
(Fig. 6d) emplaced during protracted uplift decompression from
peak pressures of 14–8 Kb and temperatures in the range 850–
750 °C(Newton and Perkins, 1980; Harley, 1989; Thost et al.,
1991). Comparable infilling of microfractures by precipitation of
oxides from metamorphic fluids is reported from other plutonic
metamorphic terranes (Hay et al., 1988; Zhang and Piper, 1994).
The fine needle and blade shaped grains produced at this stage of
uplift decompression will have the pseudo single domain (PSD)
properties able to fix a stable remanence for long periods of geolog-
ical time and it is these grains rather than the large homogeneous
or coarsely exsolved titanomagnetites that are presumably the
PLV-71
PLV-72
PLV-23
PLV-22
PLV-21 PLV-12
PLV-41
PLV-54
PLV-81
VNT-82 VNT-83
VNT-1
VNT-3
F3
F3
F3
F1
F2
F1
F3
F3
VANDALLUR
Tambaram
PALLAVARAM
PLV-53
PLV-24
PLV-25
VNT-2
PLV-51
PLV-61
PLV-62 PLV-42
PLV-32
PLV-13 PLV-11
PLV-55 PLV-31 INDIA
Study
area
Charnockite
Khondalite
Road
Sites
F2
F1
N
TO MADRAS
TO MADRAS
PLV-52
50
100
50
50
12˚55'
80˚10'
Fig. 3. Simplified geological map of the Pallavaram–Vandallur region, Tamil Nadu, showing the distribution of sampling sites. Note that much of the area inferred to be
underlain by chanockite here has recent superficial cover.
S. Mondal et al. /Journal of Asian Earth Sciences 34 (2009) 493–506 497
host of the stable remanence in these rocks. The corollary is that
this magnetism is synchronous with, or later than, decompression
and fracture formation; it is therefore much younger than the mag-
netic fabrics (Section 3.4) and a facet of uplift and exhumation.
3.4. Anisotropy of magnetic susceptibility
Anisotropy of magnetic susceptibility (AMS) was determined
for the cylindrical cores of optimum shape using a Minisep delin-
eator. AMS records the bulk contribution from the ferromagnetic
1000
500
500 °C
mT. a
400
200
500 °C
mT. b
400
500 °C
mT. c
Fig. 5. The three types of thermomagnetic spectra observed in rocks of this study.
Primary homogeneous titanomagnetite
in charnockite showing C1-Stage of
high temperature oxidation (PPL, Oil
immersion X500).
Lamellae of ilmenite in titanomagnetite
showing C3-Stage of high temperature
oxidation (PPL, Oil immersion X 500).
Magnetite occurring as very fine emulsion
drops along silicate grain boundaries
(PPL, Oil immersion, X 500).
Secondary grains (?magnetite) in late
generation cracks within silicate grains
(PPL, Oil immersion, X200).
a b
cd
Fig. 6. Photomicrographs of opaque minerals in the charnockites under oil immersion.
1.0
0.5
200 400 600 800
PLV 254
1.0
0.5
1Mo
200 400 600 800
PLV 214
PLV 623
Am.
mT.mT.
0.5 1.0-0.5-1.0
1000
500
-1000
PLV 254
Am.
0.5 1.0-0.5-1.0
-1000
1000
500
1500
00
0
a
cd
b
Fig. 4. Examples of IRM (a, b) and hysteresis (c, d) behaviors of samples from this
study.
498 S. Mondal et al. /Journal of Asian Earth Sciences 34 (2009) 493–506
grains and the paramagnetic silicates, and in plutonic rocks it is
typically the signature of grain alignment with an ambient stress
field. Since fracturing during decompression is controlled by the
orientations of mineral boundaries and cleavages, secondary pre-
cipitation tends to enhance pre-existing metamorphic fabrics.
AMS is approximated by a triaxial ellipsoid defined by three
orthogonal principal axes of maximum, intermediate and mini-
mum susceptibility (k
1
,k
2
and k
3
).
Anisotropy degrees (k
1
/k
3
) are only rarely greater that 1.1 in
these rocks and the relatively weak fabrics probably reflect the sta-
tic nature of the dominant metamorphism. An implication is that
rock fabric is unlikely to be a significant deflector of magnetic rem-
anence. AMS fabrics in the Pallavaram region are dominated by the
influence of the last (F3) stage of regional deformation: the k
1
axes
conform predominantly to the NE–SW trend of these folds (Sugav-
anam and Venkata Rao, 1990 and Fig. 3) with the remaining axes
(mostly k
2
and k
3
) distributed in a NW–SE girdle (Fig. 7); this indi-
cates tectonic lineation parallel to the maximum extension and
perpendicular to the shortening in the rocks imparted by the last
episode of folding (e.g. Rathore, 1979; Goldstein, 1980).
AMS fabrics in the Vandallur region show only weak directional
properties comprising a faint grouping of k
1
axes dipping NNE and
indications of groupings merging into a girdle of k
2
and k
3
direc-
tions at right angles (Fig. 7). The influence of the NNE–NE folding
is again suggested here although outcrop in this region is too poor
to clarify these structures properly (Fig. 3).
4. Palaeomagnetic results
Since magnetism in these rocks was probably acquired by a
thermal process during uplift and cooling, thermal demagnetiza-
tion was considered most appropriate and all cores were treated
by progressive heating, cooling and measurement in steps of 50–
500 °C and then in steps of 20–580 °C, or in some cases to between
630 °C and 680 °C when consistent directional behavior continued
above the Curie point of magnetite. Magnetic vectors at each stage
of treatment were projected onto horizontal and vertical planes to
derive conventional orthogonal vector plots, the components con-
stituting the natural remanent magnetization (NRM) were identi-
fied by eye and equivalent directions calculated by principal
component analysis.
The direction of the mean geomagnetic field direction in this
region (D/I=0°/25°) is recognized in only 20% of NRMs in the
AMS Pallavaram region:
AMS Vandallur region:
N
S
Fig. 7. Contoured distributions of directions of maximum (squares), intermediate
(triangles) and minimum (circles) magnetic susceptibility in the charnockites from
Pallavaram to Vandallur regions.
Fig. 8. Orthogonal projections of magnetizations in the charnockites during
progressive thermal treatment. Projections onto the horizontal plane are shown
as solid squares and projections onto the vertical plane are shown as open squares.
The units of intensity of magnetization are 10
5
Am
2
/kg.
S. Mondal et al. /Journal of Asian Earth Sciences 34 (2009) 493–506 499
collection and indicates that viscous remanent magnetisations
(VRMs) are not prominent in this facies with the exception of some
components in the Pallavaram region (Fig. 10). Unblocking temper-
ature spectra are typically broad and distributed up to the Curie
point of magnetite (Fig. 9), and Total NRM directions prove to be
mostly representative of the Characteristic Remanent Magnetisa-
tions (ChRMs) although second components are sometimes re-
solved above 300 °C(Fig. 8). In dual component NRMs the high
unblocking temperature component is usually of steeper inclina-
tion than the lower unblocking temperature component (PLV421
in Fig. 8 and 242 and 426 in Fig. 9) although this is not invariably
the case (PLV222). The Vandallur charnockites have lower unblock-
ing temperature spectra (Fig. 9) probably reflecting the prevalence
of homogeneous magnetites and concomitant absence of deuteric
oxidation in these rocks (Section 3.3) although smaller fractions
of the NRM survive to the Curie point of magnetite and record dual
components. The lower blocking temperature components are
mostly of SE positive direction whereas the higher blocking tem-
perature components (Fig. 9) have steep negative directions
(Fig. 11).
ChRM directions from the Pallavaram region cluster into two
groups. One (P1) has steep positive inclinations and includes the
majority of the high unblocking temperature components
(Fig. 10). The other (P2) has intermediate positive inclinations
and mostly NNE declinations; only three high unblocking temper-
ature components fall in this group so in the context of uplift re-
lated cooling it is likely to be a later acquisition than P1. The
ChRM directions in the Vandallur region fall into two groups:
one with W intermediate to steep negative directions (V1) includes
all the higher unblocking temperature components where dual
component behaviors are observed and the other is directed SE po-
sitive; this latter distribution is of arcuate form suggestive of pro-
tracted acquisition during an interval of APW. It is characterized by
a mean direction some 30°shallower than V1. Population mean
directions and pole positions are summarized in Table 1.
5. Interpretation
5.1. Proterozoic apparent polar wander of the Indian shield
In the context of the geological history of this terrane involving
slow exhumation and cooling during early Proterozoic times to-
gether with the preservation of fresh titanomagnetites with high
temperature silicates it seems likely that these rocks have pre-
served a magnetization dating from initial cooling. Although no
precise reversals of magnetization have been identified within sin-
Fig. 9. Orthogonal projections of magnetizations in the charnockites and typical demagnetization spectra in Pallavaram and Vandallur samples. Symbols are as for Fig. 7.
500 S. Mondal et al. /Journal of Asian Earth Sciences 34 (2009) 493–506
gle samples they are individually likely to have integrated long
term averages of the palaeofield and therefore record directions
of palaeomagnetic rather than geomagnetic significance. This is
the justification for calculating the overall means in Table 1 at sam-
ple level. Although it is not possible to distinguish the P1, P2, V1
and V2 directions from the entire Phanerozoic field, it is possible
to exclude a major feature of the younger record namely the influ-
ence of the Deccan Igneous Province emplaced near the Mesozoic–
Cenozoic boundary at 65 Ma. This event has produced wide-
spread, and often comprehensive, overprinting in supracrustal
rocks of central and northern India widely discussed in the litera-
ture (see for example Klootwijk, 1979; Hargraves and Bhalla, 1983;
Basu Mallik et al., 1999). Localities studied here are widely sepa-
rated from the remaining Deccan cover and no magnetizations
clustering near either the Deccan normal (D/I= 347°/48°to
339°/57°) or reversed (D/I= 157°/ +57°) directions are clearly rec-
ognized although the limit of the V2 distribution is close to the lat-
ter field. The most problematic magnetization in the context of
Phanerozoic remagnetisation is P2 because it includes direction ly-
ing close to the recent field direction (Fig. 9); an overprinted or
composite origin of this component is therefore likely.
The differences between the directions of P1, P2, V1 and V2 can-
not be explained in terms of later deformation because this is part
of a massive basement terrane spanning hundreds of kilometers
that consolidated well before temperatures fell below ferromag-
netic Curie points. The differences are therefore likely to record dif-
ferent stages of the uplift cooling and/or metamorphic fluid
passage during exhumation; the prevalence of exsolution textures
in titanomagnetites in the Pallavaram region and their absence at
Vandallur is suggestive of contrasting metasomatic histories.
Unfortunately the age of regional cooling is only poorly known.
As noted above (Section 2), whole rock isochron studies indicate
that charnockite formation in this region occurred at 2600–
2500 Ma with late cooling suggested by a 2080 Ma K–Ar biotite
age (Balasubramanian, 1975). Thus if the poles derived in this
study date from the time of uplift cooling and sometime after char-
nockite metamorphism they are expected to be 2400–2100 Ma in
age. Whilst this geochronologic control is clearly unsatisfactory, it
is consistent with the age control over the wider area as summa-
rized in Section 2with a climax of charnockite metamorphism at
2550 Ma followed by uplift and consolidation permitting dyke
injection by 2100 Ma and exhumation to near present surface
levels by commencement of Cuddapah sedimentation at
1800 Ma.
The other possible major influence on the magnetic history of
these rocks is the Eastern Ghats Mobile belt, a zone of Late Meso-
proterozoic–Early Neoproterozoic tectonism bordering the eastern
margin of the Indian Shield and broadly correlative with the
1100 Ma Grenville orogeny in the Laurentian Shield. Deformation
associated with this orogeny has deformed the margin of the Cud-
dapah basin (Fig. 1) and proximity to the granulite belt of this
study suggests a possible thermal influence: since activation ener-
gies of magnetic grains are influenced to produce a lowering and
broadening of blocking temperature spectra during prolonged
heating (Pullaiah et al., 1975), it is possible that such a protracted
thermal influence could have remagnetized these rocks. This possi-
bility can be tested by comparing the magnetization record in
these charnockites with the record embraced by the Eastern Ghats
orogeny and results from other contemporaneous rocks. These data
are summarized in Table 2 and show that the pole positions de-
rived here are discrete from all magnetizations attributed to the
1100–1000 Myr time interval correlating with metamorphism
in the Eastern Ghats belt (Radhakrishna and Joseph, 1993). The lat-
ter poles typically plot 60°to the east of the present results. Spe-
cifically results from the St. Thomas charnockites of the Madras
area 50 km to the north along the extension of the granulite belt
and close to the orogenic front have a contrasting magnetization
(Poornachandra Rao and Mallikharjuna Rao, 1999 and Table 2)
and the Visakhapatnam charnockites sited within the orogen also
have remanence consistent with a 1100–1000 Ma age (Rad-
hakrishna and Joseph, 1996; Poornachandra Rao and Mallikharjuna
Rao, 1999). Hence overprinting by the Eastern Ghats orogeny can
probably be excluded as an explanation for the Pallavarm and
Vandallur magnetisations.
Study of the charnockite belt in the Dharmapuri region in
south-central India 150 km south west of the present study has
identified a range of magnetizations linked to exhumation and
All ChRM components, Pallavaram Region:
P1
P2
Fig. 10. Distribution of directions of sample magnetizations in charnockites from
the Pallavaram region. Larger symbols are from higher unblocking temperature
components in samples showing dual component behaviors and the lower figure
shows the contoured data distribution with the populations denoted P1 and P2. The
stars are the directions of the present day mean geomagnetic field axes in this area.
S. Mondal et al. /Journal of Asian Earth Sciences 34 (2009) 493–506 501
cooling of this terrane following charnockite metamorphism (Piper
et al., 2003). The data are summarized in Table 2 with the present
results all broadly linked to the interval 2500–2100 Ma and com-
plemented by two late Archaean-early Proterozoic results from the
Singhbhum Craton of NE India. The collective data permit the def-
inition of a preliminary Early Proterozoic apparent polar wander
(APW) path as shown in Fig. 12. The two results from the Singhb-
hum Craton (KGM, QM1) plot to the east of the results from the
Dharwar Craton and suggest the possibility that movements along
the major shear belts between the two regions such as those
.
V
2
V1
ChRM components,
Vandallur Region
Fig. 11. Directions of sample magnetizations in charnockites from the Vandallur region. Larger symbols are from higher blocking temperature components in samples
showing dual component behaviors and the lower figure shows the contoured data distribution with the populations denoted V1 and V2. The points at D/I = 0/25 and 180/
25 are the directions of the present day average dipole field in this region.
Table 1
Group mean palaeomagnetic directions and pole positions from the granulite charnockite Belt, Madras Region, South India
Group Mean of unblocking
temperature range (°C)
Mean (D°/I°)NR
a
95
(°)kPole position dp/dm(°)
°E°N
P1 206 37.3/75.9 37 35.49 4.9 23.8 99.1 33.3 8.4/9.1
P2 180 11.3/24.4 29 27.80 5.7 23.2 169.6 78.9 3.3/6.1
V1 298 248.6/70.2 31 29.76 5.4 24.2 116.3 22.9 8.0/9.3
V2 148 125.5/40.9 18 17.40 6.6 28.5 135.9 25.5 4.9/8.0
D/Iare the mean declination and inclination derived from Nsamples yielding a resultant vector of length Rand cone of 95% confidence about the mean direction of
a
95;
kis
the precision parameter (=(N1)/NR)) and dpand dmare the semi axes of the oval of confidence about the pole position in the co-latitude direction and at right angles to it
respectively. The second column is the mean unblocking temperature of the component. Component P1 is resolved at sites 13, 21, 22, 23, 24, 32, 42, 52 and 61 (see Fig. 3);
component P2 is resolved at 13, 22, 23, 31, 32, 52 and 61; component V1 is resolved from 1, 2, 3 and 82; component V2 is resolved from sites 1, 2, 3 and 82 (Fig. 3).
502 S. Mondal et al. /Journal of Asian Earth Sciences 34 (2009) 493–506
shown in Fig. 2 may have produced significant relative movement
or that the Singhbhum results have been influenced by Eastern
Ghats orogeny which transects the eastern part of this craton (cf.
Table 2). The charnockite record from the Dharwar Craton com-
mences with poles plotting close to India and then moves to the
south east including the inferred sequences P1 ?P2 and
V1 ?V2. This path embraces the charnockite record with the
exception of poles CG3 and P2; the latter could record a later
excursion of the path although this point is less secure because
these magnetizations lie fairly close to the present field and the
APW path is accordingly not continued to this point. The other
qualification is that poles KGM and CG1 are close to the Late Mes-
oproterozoic–Early Neoproterozoic data (Table 2) and thermal
overprinting by Eastern Ghats orogeny cannot be ruled out as an
explanation of the easterly excursion in the APW path at this point
(Fig. 12).
5.2. The ‘Ur’ protocontinent
There are important geological similarities of Late Archaean–
Early Proterozoic age between Indian (Singhbhum and Dharwar
with the latter incorporating Karnataka), Australian (Kilbaran and
Pilbara) and southern African (Kaapvaal, Zimbabwe) cratons that
have long suggested their close proximity since mid-Archaean
times (e.g. Wingate, 1998; Zegers et al., 1998; Nelson et al.,
1999; Rogers and Santosh, 2004). Rogers (1996) recognized these
as the oldest continental nucleus (‘Ur’) characterized by protolith
that had stabilized by 3000 Ma. This is significantly older than
a contrasting crustal assemblage comprising Siberia, Laurentia,
and Fennoscandia (‘Arctica’) where protolith consolidated later at
2600 Ma with uplift and cooling prior to deposition of supracru-
stal cover after 2400 Ma, and an ‘Atlantica’ nucleus incorporating
the cratons of western Africa and eastern South America which
consolidated at 2200 Ma followed by relatively rapid uplift with
a widespread cover of fluvio-deltaic sediments developing at
2100–2000 Ma (Rogers, 1996; Rogers and Santosh, 2004).
Although the mean 2900–2200 Ma palaeomagnetic poles from
the Indian, Australian and African shields correlate closely with
one another when these shields are brought into close continuity
(Piper et al., 2003) the individual poles remain fairly dispersed pre-
sumably due to the difficulty of defining APW over such a long and
Table 2
Early Palaeoproterozoic and Late Meso–Neoproterozoic Pole positions from the Indian Shield
Code Rock unit Craton Estimated age Pole position dp/dm
°E°N
(i) Early Proterozoic results
QM1 Sukinda Quartz-Magnetite rocks S 2590 ± 40 152 47 5/8
P1 Pallavaram P1 D 25002000 99 33 9/9
CGB Charnockite granulite B D 25002000 85 16 7/7
V1 Vandallur V1 D 25002000 116 23 8/9
KGM Khammano Quartz-Magnetite S 2600 175 6 6/12
CG1 Charnockite granulite A1 D 25002000 163 8 2/5
CG4 Charnockite granulite A4 D 25002000 133 18 4.7
V2 Vandallur V2 D 25002000 136 26 5/8
CG2 Charnockite granulite A2 D 25002000 147 45 2/3
CG3 Charnockite granulite A3 D 25002000 75 73 2/4
P2 Pallavaram P2 D 25002000 350 79 3/6
Rock unit Location Estimated age Pole position A
95
°N°E°E°N
(ii) Late Meso-Neoproterozoic results
St. Thomas Charnockite, Madras 12.5 80.0 ?Eastern Ghats overprint 208 32 8
Visakhapatnam Charnockites 17.5 83.0 ?Eastern Ghats overprint 189 15 15
Qddanchatram Anorthosite 10.5 77.7 11001000 182 10 7
Wajrakarur Kimberlite 15.0 77.4 1090 ± 20 186 18 11
Tiruvannamalai Overprint 12.2 78.8 1000 168 66
Tirupati Dykes 14.0 79.0 980 ± 110 156 54
Cuddapah Sandstone 14.9 78.5 1400 ± 1160 159 22 8
Cuddapah Shale 14.8 78.1 1400 ± 1160 156 23 5
Tiptur dykes 13.4 76.0 11001000 151 14 12
The Early Proterozoic poles are summarized after Piper et al. (2003). The Meso–Neoproterozoic poles are summarized after Satyanarayana et al. (2003) and Radhakrishna and
Joseph (1993); the St. Thomas charnockite result from Madras is from Poornachandra Rao and Mallikharjuna Rao (1999). S = Singhbhum Craton, D = Dharwar Craton.
P1 QM1
V1
CGB KGM
CG1
CG4
V2
CG2
CG3
P2
Indian
Shield
Fig. 12. Preliminary Early Proterozoic apparent polar wander path for the Indian
Shield incorporating 2600–2100 Ma palaeomagnetic poles from basement terr-
anes of southern India; a possible APW swathe is shown by the shaded path and
keys to the poles are given in Table 2.
S. Mondal et al. /Journal of Asian Earth Sciences 34 (2009) 493–506 503
Table 3
2400–2200 Palaeomagnetic poles from central-southern Africa and Western Australia
Code Rock unit Craton Age (Ma) Pole position dp/dm
°E°N
(i) Central-southern Africa
AF1 Garauja-Basuto Gabbro (1360) T (k= 34) 2500 ± 100 341 27 7/9
AF2 Kenya Granites (8) T (k= 29) 2476 ± 50 30 61 12/18
AF3 Post-Kavirondian Granites (8122) T (k= 30) 2420 ± 60 83 41 16/24
AF4 Transvaal Lavas (2429) K (k= 69) 2250 ± 100 14 40 28/28
AF5 Oneguluk Lavas (8250) K (k= 11) 2222 ± 13 91 1 5/5
(ii) Australia
AU1 Ravensthorpe Dykes (1949) Y (k= 76) 2450 ± 100 136 38 26/26
AU2 Widgiemooltha Dykes (1889) Y (k= 50) 2410 ± 2 157 8 8/9
AU3 Wittenoom BIF (7541) P (k= 5) 2200 ± 100 219 36 5/9
AU4 Paraburdoo BIF (7539) P (k= 2) 2200 ± 100 225 41 3/6
The number in brackets by the rock name is the pole reference in the Global Palaeomagnetic Database and kis the palaeolatitude.
Ravensthorpe
dykes 2450 100
CG1
CG4
P1
V1
CGB
Wittnoom BIF
2200 100
Oneguluk lavas
2200 100
Paraburdoo BIF
2200 100
CG2
V2
AF3
Kenya Granites
2476 50
A
F
R
I
C
A
W
.
A
U
S
T
R
A
LI
A
I
N
D
IA
Africa
Australia
India
Dharwar
Pilbara
Yilgarn
Kaapvaal
Zimbabwe
Singhbhum
a
b'Ur' Protocontinent ~ 2400-2200 Ma
Garauja
Gabbro
2500 100
Widgiemooltha
dykes 2410 2
Tanzania
Transvaal Lavas
2250 100
Fig. 13. (a) Reconstruction of the protocontinent of ‘Ur’ in Early Palaeoproterozoic times derived from matching 2400–2200 Ma APW trends from Australia and Africa with
the distribution of poles from the Indian high grade terranes. (b) The configuration of the cratonic nuclei during these times derived from the palaeomagnetic correlation.
504 S. Mondal et al. /Journal of Asian Earth Sciences 34 (2009) 493–506
remote period of geological time. However, the recognition of APW
trends permits some refinement of this conclusion because each of
these shields yields Early Palaeoproterozoic poles (Table 3) identi-
fying higher palaeolatitudes succeeded by poles yielding lower pal-
aeolatitudes. In Africa and Australia these are specifically dated at
2400 and 2200 Ma and thus are broadly correlative with the
uplift magnetizations found in the high grade metamorphic terr-
anes of south India. The corresponding data from Africa and Aus-
tralia are summarized in Table 3 and plotted with the three APW
trends illustrated in Fig. 13a. They can be matched in age and
length to derive a continental configuration and preliminary recon-
struction of the ‘Ur’ protocontinent in early Proterozoic times
(Fig. 13b). The estimated rotational operations retaining Africa in
present day coordinates are 147.0°anticlockwise about an Euler
pole at 84°E, 56°S for Australia and clockwise by 128°about an Eu-
ler pole at 78°E, 32°S for India.
The configuration of these three APW trends indicates that dur-
ing Early Palaeoproterozoic times ‘Ur’ comprised a broad zone of
protolith comprising, in turn, the cratonic nuclei of India, Western
Australia and southern Africa. These nuclei are now isolated by la-
ter tectono-thermal belts and major shear zones. High grade terr-
anes incorporating charnockites occur along the axes of this belt
from central southern India into Australia near the northwest sec-
tor of the Yilgarn nucleus and in south east Africa (Saxena, 1977)
but are in part, preserved within much younger Grenville-age mo-
bile belts and their formation may have occurred much later than
the examples studied here. The orientations of these shields are
somewhat different from their configurations in Neoproterozoic
times (Piper, 2007) with relative motions and clockwise rotation
of India-Africa indicated, in part during Grenville-age orogenesis;
their proximities however, were evidently retained until the Meso-
zoic break-up of Gondwana.
6. Conclusions
Charnockitic rocks within the high grade granulite belt of cen-
tral-southern India possess a stable magnetic record presumed to
be resident largely in fine grained magnetite precipitated in micro-
fractures during uplift decompression following the static meta-
morphism at 2500 Ma. Although no polarity reversals have
been identified the dominant magnetisations yield four poles at
33°N, 99°E, (P1, dp/dm= 8/9°), 79°N, 170°E, (P2, dp/dm= 3/6),
23°N, 116°E, (V1, dp/dm= 8/9°) and 26°S, 136°E, (V2, dp/dm=5
/
10°). Remanence in this terrane does not appear to have been influ-
enced by the Eastern Ghats orogeny (1100–1000 Ma) and is
mostly readily linked to uplift cooling during the interval
2500–2100 Ma. Collective palaeomagnetic results from the high
grade charnockite belt of central-southern India define an arcuate
early Proterozoic APW path for the shield and wider palaeomag-
netic studies of this belt hold the prospect of clarifying the kine-
matics of this shield during the earlier part of Proterozoic times.
The APW swathe defined by present data define motion from high-
er to lower palaeolatitudes during these times in common with the
shields of Western Australia and central-southern Africa, and the
APW trends from the three shields can be matched to yield the first
palaeomagnetic reconstruction of the oldest ‘Ur’ protocontinent
during early Proterozoic times.
Acknowledgements
The University Grants Commission (UGC), India, funded this re-
search project. Jadavpur University provided infrastructure and
laboratory facilities and we especially thank the staffs of the Blue
Earth Section for assistance with coring oriented rock samples.
The study was facilitated by an academic link between Jadavpur
University and Geomagnetism Laboratory of the University of
Liverpool supported by the British Council which funded travel
and subsistence costs. We thank Kay Lancaster for drafting Figs.
1–4,Fig. 11 and 12, Mr. Robi Dey for micro photographic work
and Dr. Ashis Kumar Das for assistance with fieldwork. We are very
grateful to Michael Fuller and an anonymous reviewer for their
comments which helped to improve the paper.
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