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Geol. Mag.: page 1 of 18.
c
2006 Cambridge University Press 1
doi:10.1017/S0016756805001664
Proterozoic mountain building in Peninsular India: an analysis
based primarily on alkaline rock distribution
C. LEELANANDAM*, K. BURKE†‡§, L. D. ASHWAL†¶ &S.J.WEBB†
*310 Street No. 2, Tarnaka, Secunderabad 500 017, India
†School of Geosciences, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa
‡Department of Geosciences, University of Houston, Houston, TX 77204-5007, USA
§Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road,
Washington, DC 20015, USA
(Received 17 September 2004; accepted 21 July 2005)
Abstract – Peninsular India was assembled into a continental block c. 3 million km
2
in area as a
result of collisions throughout the length of a 4000 km long S-shaped mountain belt that was first
recognized from the continuity of strike of highly deformed Proterozoic granulites and gneisses. More
recently the recognition of a variety of tectonic indicators, including occurrences of ophiolitic slivers,
Andean-margin type rocks, a collisional rift and a foreland basin, as well as many structural and
isotopic age studies have helped to clarify the history of this Great Indian Proterozoic Fold Belt.
We here complement those studies by considering the occurrence of deformed alkaline rocks and
carbonatites (DARCs) in the Great Indian Proterozoic Fold Belt. One aim of this study is to test the
recently published idea that DARCs result from the deformation of alkaline rocks and carbonatites
(ARCs) originally intruded into intra-continental rifts and preserved on rifted continental margins.
The suggestion is that ARCs from those margins are transformed into DARCs during continental,
or arc–continental, collisions. If that idea is valid, DARCs lie on rifted continental margins and on
coincident younger suture zones; they occur in places where ancient oceans have both opened and
closed. Locating sutures within mountain belts has often proved difficult and has sometimes been
controversial. If the new idea is valid, DARC distributions may help to reduce controversy. This paper
concentrates on the Eastern Ghats Mobile Belt of Andhra Pradesh and Orissa, where alkaline rock
occurrences are best known. Less complete information from Kerala, Tamil Nadu, Karnataka, West
Bengal, Bihar and Rajasthan has enabled us to define a line of 47 unevenly distributed DARCs with
individual outcrop lengths of between 30 m and 30 km that extends along the full 4000 km length
of the Great Indian Proterozoic Fold Belt. Ocean opening along the rifted margins of the Archaean
cratons of Peninsular India may have begun by c. 2.0 Ga and convergent plate margin phenomena have
left records within the Great Indian Proterozoic Fold Belt and on the neighbouring cratons starting
at c. 1.8 Ga. Final continental collisions were over by 0.55 Ga, perhaps having been completed at
c. 0.75 Ga or at c. 1 Ga. Opening of an ocean at the Himalayan margin of India by c. 0.55 Ga removed
an unknown length of the Great Indian Proterozoic Fold Belt. In the southernmost part of the Indian
peninsula, a line of DARCs, interpreted here as marking a Great Indian Proterozoic Fold Belt suture,
can be traced within the Southern Granulite Terrain almost to the Achankovil-Tenmala shear zone,
which is interpreted as a strike-slip fault that also formed at c. 0.55 Ga.
Keywords: India, alkaline rocks, carbonatites, continental rifts, continental collisions, suture.
1. Introduction: looking at a great fold belt
inanewway
Radhakrishna & Naqvi (1986, fig. 1) identified a
4000 km long S-shaped mountain belt that weaves
its way across Peninsular India (Fig. 1). That Great
Indian Proterozoic Fold Belt records the Proterozoic
continental and arc-system collisions that assembled
India during a long history of deformation with
discrete episodes that have yielded a wide range
of isotopic and tectonically inferred ages between
c. 2.0 Ga and c. 0.55 Ga (e.g. Chetty & Murthy,
1994; S¸eng
¨
or & Natalin, 1996; Roy & Kataria, 1999;
¶Author for correspondence: ashwall@geosciences.wits.ac.za
Ramakrishnan, Nanda & Augustine, 1998; Srikantia,
1999; Gopalakrishnan, Subramanian & Upendran,
2001; Dobmeier & Raith, 2003), although the idea
of a role for continental and arc collisions in the
formation of the belt has not been fully or universally
accepted (e.g. Mahadevan, 1999; Roy & Kataria, 1999).
For those who accept the idea of collision, questions
remain about such topics as: When and where did rifted
continental margins to the Dharwar, Bhandara (Bastar),
Singhbum and Bundelkhand cratons initially form?
What was the role of arc-systems during continental
margin evolution? Can an Altaid style of evolution
(S¸eng
¨
or & Natalin, 1996) be discerned within the belt?
Did objects of continental dimensions collide with
craton margins more than once during the evolution
2 C. LEELANANDAM AND OTHERS
69º00' E 79º00' E 89º00' E
G-A
1
32-37
30
38-41
42
43-45
47
46
29
28
22-27
7-21
3-6
Bundelkhand
Craton
Eastern
Ghats
Granulite
Terrain
11º00
'N
21º00
'N
Southern
31
Central
Indian
Tectonic
Zone
Dharwar
Craton
Bhandara
(Bastar)
Craton
Singhbhum Craton
2
ARAVALLI - D
ELHI
74º00' E
84º00' E
06º00
'N
16º00
'N
Sausar
Figure 1. Deformed alkaline rocks and carbonatites (DARCs)
in the Great Indian Proterozoic Fold Belt. Forty-seven DARCs
are known from within the 4000 km long mountain belt (inset).
Great Indian Proterozoic Fold Belt boundaries shown by heavy
and dashed lines and numbers of DARCs correspond with those
of Table 1. G-A is the Gorkha-Ampipal Proterozoic DARC of
Nepal which appears unrelated to the Great Indian Proterozoic
Fold Belt. No comparable concentration of DARCs has yet been
recognized in any of the world’s other mountain belts. DARCs
indicate the locations of intra-continental rifts that became rifted
continental margins and on which sutures have later formed
during continental collision (see Fig. 2). The line of DARCs
in this figure defines the approximate location of a suture zone
that formed on the site of rifted continental margins of India’s
Archaean Cratons. The suture zone extends for the length of the
Great Indian Proterozoic Fold Belt.
of the belt? What collisional events are recorded,
particularly in the well-studied Eastern Ghats, for times
before the final assembly of Gondwana? What was the
time of that final collision? Did the final collisional
phase involve only the East Antarctic continent or did
it also involve other continents or arc systems? In an
attempt to contribute to the resolution of some of these
questions we focus on the distribution and significance
of alkaline and carbonatitic rocks (ARCs) in the Great
Indian Proterozoic Fold Belt. All of those ARCs have
been deformed and have become DARCs (Burke,
Ashwal & Webb, 2003). Our analysis concentrates on
the Eastern Ghats Mobile Belt in Andhra Pradesh and
Orissa where alkaline igneous and metamorphosed
rocks are best known (Leelanandam, 1989a, 1998;
Mazumder, Rao & Nathan, 2000). Treatment of the full
length of the Great Indian Proterozoic Fold Belt from
Kerala and Tamil Nadu to the Himalaya is necessarily
less complete (Fig. 1).
In a recent study (Burke, Ashwal & Webb, 2003)
based on the analysis of a comprehensive catalogue
a. Alkaline rocks erupt into
intracontinental rift at
continental rupture
b. Ocean open, alkaline
rocks not active,
preserved on rifted margin
c. Continental collision,
deformation of alkaline
rocks; DARCs develop
DARCs
ophiolites
Figure 2. Sketch illustrating the origin of DARCs by the
deformation of ARCs at continental collision. Alkaline rocks
and carbonatites (ARCs, black dots in a) erupt into an intra-
continental rift while that rift is extending. In (b) the ARCs
are shown (black dots) as inactive but preserved at a rifted
continental margin. When a continental collision occurs (c) the
ARCs (shown as black lenses) are deformed and become
DARCs. Gneissic DARCs, recording both the rifting and the
collisional parts of the Wilson cycle of ocean opening and ocean
closing, define suture zones in the Great Indian Proterozoic Fold
Belt as they do in other mountain belts. All the complexities that
develop during intervals such as that represented by the middle
sketch (b) and of which there is clear evidence in the Great
Indian Proterozoic Fold Belt have been omitted.
of the alkaline rocks of Africa (Woolley, 2001), the
familiar association of alkaline rocks and carbon-
atites (ARCs) with intra-continental rifts and their
consequent common occurrence at rifted continental
margins has been confirmed. A less-expected result
of that analysis was that c. 90 % (28 out of 32) of the
catalogued Deformed Alkaline Rocks and Carbonatites
(DARCs) of Africa lie on Proterozoic suture zones
(Burke, Ashwal & Webb, 2003). The remaining four
occurrences in Africa lie on a line that may also be
a suture zone (Tack et al. 1994). The suggested expla-
nation for the coincidence of DARCs with suture zones
is that alkaline rocks initially emplaced into intra-
continental rifts and later preserved at rifted continental
margins become deformed when involved in collisions
between continents or between arcs and continents
(Fig. 2).
Alkaline rocks when originally emplaced are char-
acteristically without internal structure, although some
carbonatites intruded at shallow depths display sub-
volcanic structures such as cone-sheets. The occur-
rence of metamorphic structure in nepheline syenites
and carbonatites may, therefore, be considered evidence
of their involvement in the intense convergence that
is associated with the establishment of suture zones.
If the Burke, Ashwal & Webb (2003) explanation of
the origin of DARCs is correct, and so far it has only
been tested in Africa where it works very well (Burke,
Ashwal & Webb, 2003; Gerber et al. 2004), then an
Proterozoic mountain building in Peninsular India 3
additional way of recognizing the sites of collisions,
and especially continental collisions, will have become
available. Here we: (1) use the Eastern Ghats Mobile
Belt as a test area to see whether the idea of DARCs
being localized along both ancient rifted continental
margins and suture zones that have later formed on
those margins appears viable on the basis of evidence
from India, and (2) apply the findings in the Eastern
Ghats Mobile Belt to DARC distribution in the full
length of the Great Indian Proterozoic Fold Belt.
2. DARCs in the Eastern Ghats Mobile Belt
About 60 alkaline rock complexes, including lampro-
phyric and carbonatitic complexes, have been recog-
nized in southern and eastern India (Mazumder, Rao &
Nathan, 2000, figs 1, 2). We confine our considera-
tion to nepheline syenites and carbonatites because only
those rock-types are recognized to be strongly associ-
ated, when initially erupted, with intra-continental rifts
(Woolley, 2001).
2.a. DARCs of the Eastern Ghats Mobile Belt: 15 to 20
◦
N
Nine nepheline syenite and carbonatite bodies within
this part of the Eastern Ghats Mobile Belt define a
discontinuous NE-trending array nearly 500 km long
(Figs 1, 3; Table 1; Leelanandam, 1989a, 1993, 1998;
Mazumder, Rao & Nathan, 2000). The bodies, all of
which are DARCs, lie on or close to the Sileru shear
zone (Figs 3, 4; Mazumder, Rao & Nathan, 2000, fig 5).
Ramakrishnan, Nanda & Augustine (1998) identified
that shear zone as separating a Western Charnockite
Zone from the rest of the Eastern Ghats Mobile Belt
(Fig 4). We interpret the Western Charnockite Zone
to be composed of up-thrust deep-seated Archaean
rocks of the Bhandara (Bastar) craton that were
caught up and reactivated during one or more of the
several convergent plate boundary events that affected
the Eastern Ghats Mobile Belt between c. 1.8 Ga and
c. 0.55 Ga (cf. Bhadra, Gupta & Banerjee, 2004).
We similarly interpret the Jeypore Province and those
parts of the Krishna Province of Dobmeier & Raith
(2003, table 1) to the west of the Sileru shear
zone as corresponding to the Western Charnockite
Zone and, similarly, to be composed of up-thrust and
reactivated Bhandara craton rocks. In a continental-
collision interpretation, the Western Charnockite Zone
represents a part of the rifted margin of the Bhandara
(Bastar) craton that became involved in convergent
plate-margin tectonics at one or more times between
initial ocean formation and final continental collision
in the Eastern Ghats Mobile Belt. A granulite facies
transition zone marking the Eastern Ghats Front
(Ramakrishnan, Nanda & Augustine, 1998, p. 1)
bounds the Western Charnockite Zone to the west and
the Sileru shear zone marks its boundary in the east. The
Sileru shear zone with its coincident line of nepheline
76º
84ºE
12º
16º
20ºN
Hyderabad
Cuddapah
Basin
Chennai
16º
80º
F
FF
F
DARCs and ARCs of
southern India
and the Eastern Ghats
47
nepheline syenite
carbonatite
SGT
A-T
EGMB
46
45
43
44
42
38-41
31
30
29
28
7-27
37
36
35
34
33
32
Figure 3. DARC (deformed alkaline rock and carbonatite)
occurrences in southern India and the Eastern Ghats. All DARC
occurrences in the Eastern Ghats Mobile Belt crop out within
30 km of a suture zone (hatched line). Fermor’s line (F–F)
coincides with that suture zone in the Eastern Ghats, but in
southern India Fermor’s line (dashes and dots), which forms
the boundary of the Southern Granulite Terrain, is not a suture
zone. The ten DARCs of southern India, which crop out within
the Southern Granulite Terrain lie along a newly postulated
suture zone. The position of the Achankovil-Tenmala shear zone
(A-T) is indicated. Numbers of DARCs are those of Table 1.
Map modified from Figure 1 of Leelananadam (1989a). The
inset shows the DARCs of the Prakasam Alkaline Province
(Leelanandam, 1989b).
syenites and carbonatites (Figs 1, 3, 4) is interpreted,
because of its concentration of DARCs, to mark the
position of the rifted margin of an ancient continent
of which the Bhandara craton forms a part. The Sileru
shear zone marks not only that rifted margin but also a
suture zone that later formed on the rifted margin as a
result of arc or continental collision.
Gneissic structures have been well described from
nepheline syenites of the Prakasam Alkaline Rock
Province (Leelanandam, 1989b; nos 32–37 in Fig. 1
and Table 1, inset to Fig. 3) that lie on or close to
the Sileru shear zone at Elchuru (Leelanandam, 1989b;
Cyzgan & Goldenberg, 1989), Purimetla (Ratnakar &
Leelanandam, 1989), Settupalle and Uppalapadu
(Leelanandam & Krishna Reddy, 1981; Cyzgan &
Goldenberg, 1989). At Elchuru (Fig. 5), Leelanandam
(1989b) has shown: (1) that the gneissic structure and
the boundaries of the nepheline syenite are concordant
with the foliation of the surrounding granodiorite
gneiss and (2) that the foliation is more intense
in the outer subsolvus part of the body. The inner
part of the Elchuru body, consisting of hypersolvus
nepheline syenite gneiss, is less intensely deformed.
Leelanandam’s work at Elchuru shows that individual
outcrops and relatively small areas within DARC
4 C. LEELANANDAM AND OTHERS
Table 1. Occurrences of deformed alkaline rocks and carbonatites (DARCs) in India
Locality Rock type(s) Province Latitude
◦
N Longitude
◦
E Key reference
1 Kishangar Neph. gneiss Rajasthan 26
◦
35
74
◦
53
Srivastava (1989); Roy & Dutt (1995)
2 Newania Carbonatite Rajasthan 24
◦
40
74
◦
00
Srivastava (1989)
3 Beldih Neph. gneiss W. Bengal 23
◦
01
86
◦
15
Mazumder (1978)
4 Sushina Neph. gneiss W. Bengal 22
◦
57
86
◦
37
Battacharyya & Chaudari (1986)
5 Kushunda Neph. gneiss W. Bengal 23
◦
33
86
◦
53
Mazumder (1978)
6 Santuri Neph. gneiss W. Bengal 23
◦
33
86
◦
45
Bhaumik, Mukherjee & Basu (1990)
7 Baradangua Neph. gneiss Orissa 21
◦
04
85
◦
05
Leelanandam (1989a)
8 Kankarakhol Neph. gneiss Orissa 21
◦
20
84
◦
42
Rath, Sahoo & Satpathy (1998)
9 Durhukajharan
10 Chhalak Twelve
11 Hinduja nala named
12 Dalak DARC
13 Kapagola occurrences
14 Sadhubahali between
15 Khandadhuan Kankarakhol
16 Rairatanpur and Rath, Sahoo & Satpathy (1998)
17 Polpani Lodhajari
18 Chingrijharan Mazumder, Rao & Nathan (2000)
19 Burbura
20 Lulang
21 Lodhajari Neph. gneiss Orissa 21
◦
12
84
◦
57
Rath, Sahoo & Satpathy (1998)
22 Rairakhol Neph. gneiss Orissa 21
◦
04
84
◦
20
Panda et al. (1993)
23 Kharsali Neph. gneiss Orissa 21
◦
03
84
◦
24
Mazumder, Rao & Nathan (2000)
24 Kusarimunda Neph. gneiss Orissa 21
◦
04
84
◦
16
Mazumder, Rao & Nathan (2000)
25 Gungijhara Neph. gneiss Orissa 21
◦
05
84
◦
17
Mazumder, Rao & Nathan (2000)
26 Rasibida Neph. gneiss Orissa 21
◦
07
84
◦
12
Mazumder, Rao & Nathan (2000)
27 Machibahal Neph. gneiss Orissa 21
◦
10
84
◦
09
Mazumder, Rao & Nathan (2000)
28 Khariar Neph. gneiss Orissa 20
◦
20
82
◦
38
Mahadevan (1999)
29 Koraput Neph. gneiss Orissa 18
◦
49
82
◦
43
Bose (1970); Sarkar et al. (1989)
30 Kunduluru Neph. gneiss Andhra Pradesh 17
◦
40
81
◦
24
Leelanandam (1989b)
31 Kunavaram Neph. gneiss Andhra Pradesh 17
◦
20
81
◦
10
Ratnakar & Leenanandam (1989); Clark &
Subbarao (1971)
32 Kotappa Konda Neph. gneiss Andhra Pradesh 16
◦
08
80
◦
02
Ratnakar & Leenanandam (1989)
33 Elchuru Nep. gn./Carb. Andhra Pradesh 16
◦
06
79
◦
56
Ratnakar & Leenanandam (1989); Subba
Rao et al. (1989)
34 Settupale Neph. gneiss Andhra Pradesh 16
◦
01
79
◦
52
Ratnakar & Leenanandam (1989)
35 Pasupugallu Neph. gneiss Andhra Pradesh 15
◦
44
79
◦
46
Ratnakar & Leenanandam (1989)
36 Purimetla Neph. gneiss Andhra Pradesh 15
◦
35
79
◦
51
Ratnakar & Leenanandam (1989)
37 Uppalpadu Neph. gneiss Andhra Pradesh 15
◦
35
79
◦
46
Leelanandam & Krishna Reddy (1981)
38 Elagiri Nep. gn.?/Carb. Tamil Nadu 12
◦
31
78
◦
35
Mazumder, Rao & Nathan (2000)
39 Koratti/Sevattur Nep. gn.?/Carb. Tamil Nadu 12
◦
25
78
◦
31
Mazumder, Rao & Nathan (2000); Anil
Kumar & Gopalan (1991)
40 Samalpatti Nep. gn.?/Carb. Tamil Nadu 12
◦
20
78
◦
22
Mazumder, Rao & Nathan (2000)
41 Pikkili Neph. gneiss Tamil Nadu 12
◦
09
78
◦
00
Mazumder, Rao & Nathan (2000)
42 Hogenakal Carbonatite Tamil Nadu 12
◦
07
77
◦
45
Srinivasan (1977)
43 Ariyalur Carbonatite Tamil Nadu 11
◦
05
78
◦
50
Grady (1971)
44 Pakkanadu Carbonatite Tamil Nadu 11
◦
40
77
◦
50
Mazumder, Rao & Nathan (2000)
45 Sivamalai Neph. Gneiss Tamil Nadu 11
◦
03
77
◦
36
Mazumder, Rao & Nathan (2000)
46 Kambammettu Carbonatite Tamil Nadu 09
◦
44
77
◦
44
Mazumder, Rao & Nathan (2000)
47 Munnar Carbonatite Tamil Nadu 10
◦
02
77
◦
03
Santosh et al. (2003)
bodies may show little sign of deformation. Detailed
mapping is in many cases needed before a body can
be identified with confidence as a DARC. Mazumder,
Rao & Nathan (2000) reported observations similar to
those made at Elchuru by Leelanandam (1989b) from
other complexes within the Eastern Ghats Mobile Belt
and commented, in their discussion of the nepheline
syenite bodies of that belt, that ‘.....everyoneofthese
bodies is foliated and concordant ....’ (Mazumder,
Rao & Nathan, 2000, p. 104).
We conclude on the basis of published studies
and of unpublished field observations (by C.L.) that:
(1) gneissic structures concordant with those in the
surrounding country rocks are universal among the
nepheline syenites of the Eastern Ghats Mobile Belt
and (2) all are therefore DARCs and (3) these DARCs
lie within a suture zone that is close to the Sileru shear
zone and to the shear zone’s along-strike extensions to
the north and the south.
2.b. DARCs of the Eastern Ghats Mobile Belt north
and east of 20
◦
N, 83
◦
E
Craton border structure in the Eastern Ghats Mobile
Belt north and east of 20
◦
N, 83
◦
E differs from that fur-
ther south (Figs 1, 3). In that region Ramakrishnan,
Nanda & Augustine (1998, fig. 2) identified the Western
Charnockite Zone and the adjoining zone transitional
to the craton only in a relatively small area at c. 21
◦
N,
83
◦
E. Along the rest of the 250 km long border of
Proterozoic mountain building in Peninsular India 5
Suture
+
+
+
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+
+
+
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+
+
+
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+
+
+
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+
+
+
+
+
+
+
+
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+
+
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+
+
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+
+
+
+
+
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+
NW
Alkaline Complex
Anorthosite
Migmatitic granitoids
Charnockite
Khondalite
Western Charnockite Zone
Granite + Greenstone (Amphibolite grade)
+
+
+
+
+
SE
+
+
Figure 4. Cross-section of the Eastern Ghats Mobile Belt at about 17
◦
N, modified slightly from Ramakrishnan, Nanda & Augustine
(1998, fig. 6). The alkaline complex shown beneath the word ‘Suture’ marks the position of the Prakasam Province DARCs (nos 32–
37), which lie close to the Sileru shear zone (Mazumder, Rao & Nathan, 2000) and the edge of the Western Charnockite Zone of
the Eastern Ghats Mobile Belt. The rocks of the Western Charnockite Zone are formed from the deep crust of the Bhandara craton
reactivated and thrust to the northwest during convergent events. The alkaline rocks of the Eastern Ghats were erupted into an intra-
continental rift, came to occupy a rifted continental margin, and were finally caught up in a collision at which time they developed
their gneissic structure. Anorthosites that are concentrated close to the suture zone on which the DARCs are localized may represent
parts of the roots of Andean-type volcanoes. The section is generalized. For example, it does not show such features as the occurrence
of amphibolite facies rocks within the region of charnockite, khondalite and anorthosite southeast of the suture zone.
ELCHURU
45º
75º
75º
70º
45º
90º
C
0
1
2
km
+
++
+
+
+
+
+
+
+
+
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79º55'
16º05'
79º55'
Figure 5. Structure of the Elchuru DARC, simplified from
Leelanandam (1989b, fig. 3). A relatively unstructured hy-
persolvus nepheline syenite gneiss (open circles) lies within
a subsolvus nepheline syenite gneiss (crosses) that is itself
surrounded by concordant granodioritic gneiss (unornamen-
ted). Planar structure symbols without numbers indicate the
inclination of gneissic layering with dips of between 50
◦
and
70
◦
. Less abundant gentler and steeper dips are labelled. The
black square (C) marks the location of a carbonatite dyke.
Elchuru is considered to have been erupted as an ARC into
an intracontinental rift and to have been later involved in a
continental collision that generated gneissic structure turning
Elchuru from an ARC into a DARC.
the Eastern Ghats Mobile Belt on either side of that
area, the garnet–sillimanite–gneiss-bearing Western
Khondalite Zone of this belt is directly juxtaposed
against the Bhandara and Singhbhum cratons. This
juxtaposition we attribute to removal of both the
Western Charnockite Zone and the granulite facies
transition zone by intense tectonism at, before, or after
the time of continental collision. Intense tectonism in
the area was emphasized by Bhattacharyya (1997), who
postulated oblique collision from structural evidence
for the existence of a transpressional regime. The area
of the Eastern Ghats Mobile Belt border east of 84
◦
E
was interpreted as showing great tectonic complexity
by Crowe, Cosca & Harris (2001), who considered that
a c. 100 km by c. 200 km area occupying the northern
boundary of this belt east of 84
◦
E was dominated,
at least during its later history, by strike-slip faulting
(see the mapped fault patterns in Crowe, Cosca &
Harris, 2001, fig. 1 and Dobmeier & Raith, 2003,
fig. 3).
40
Ar/
39
Ar isotopic ages from that area record
a range of ages with peaks at c. 750 Ma and 550–
500 Ma subsequent to a dominant 1100–950 Ma meta-
morphism. Both the later events have been associated
with movement on major ESE-trending shear zones
and with fluid flow within these shear zones (Crowe,
Cosca & Harris, 2001). The occurrence within this
region of discrete blocks that have yielded Archaean
ages (Crowe, Cosca & Harris, 2001) indicates that, as is
the case in the Western Charnockite Zone, rocks from
6 C. LEELANANDAM AND OTHERS
the neighbouring craton, in this case the Singhbhum
craton, have been caught up during the evolution of the
Eastern Ghats Mobile Belt.
The nepheline gneisses of Khariar (Fig. 3; Table 1;
Fig. 1, no. 28) crop out within the part of the
Eastern Ghats Mobile Belt that suffered intense NW–
SE collisional tectonism but was not involved in the
later ESE-trending strike-slip dominated movements.
Discontinuous outcrops of the Khariar nepheline
gneisses strike N–S over a distance of c. 30 km (Fig. 3;
Table 1; Mazumder, Rao & Nathan, 2000, figs 3, 4;
Dobmeier & Raith, 2003, fig. 3), and occupy the contact
between Archaean rocks of the Bhandara (Bastar)
craton and the Western Khondalite Zone of the Eastern
Ghats Mobile Belt. We interpret the Khariar DARCs
as occupying a suture zone that we extend along
strike to join the Sileru shear zone suture (Fig. 3).
In terms of the model of Burke, Ashwal & Webb
(2003) and Figure 2, the Khariar rocks record (a) initial
eruption into an intra-continental rift established within
the Bhandara (Bastar) craton, (b) subsequent formation
of a rifted continental margin to that craton and
(c) suturing during a collisional event in the course
of Eastern Ghats Mobile Belt evolution.
Twenty DARCs (nos 8–27 in Fig. 1 and Table 1)
have been mapped in the ESE-trending strike-slip
dominated area of the northeastern Eastern Ghats
Mobile Belt (east of 84
◦
E). These nepheline gneiss
occurrences are distributed c. 100 km to the west of
the Barandagua body (Fig. 3; no. 7 in Fig. 1 and
Table 1). Occurrences forming the Kankarakhol-
Lodhajari complex of Rath, Sahoo & Satpathy (1998;
nos 8–21 of Fig. 1 and Table 1) occupy a c. 30 km
long line close to one of the main strike-slip faults of
the region (Mazumder, Rao & Nathan, 2000, fig. 2;
Dobmeier & Raith, 2003, fig. 3). Six more bodies
(nos 22–27 of Fig. 1 and Table 1) lie about 40 km
to the southwest and near to an ENE-trending left-
lateral strike-slip fault. Before movement on that fault,
DARCs 8 to 27 are suggested to have cropped out
in a single line. The area marginal to the Eastern
Ghats Mobile Belt east of 84
◦
E has a long and
complex geological history (Dobmeier & Raith, 2003,
p. 152), including evidence, in what we suggest to be
a fragment of the Singhbhum Craton, of extensive mag-
matic activity as far back as 2.8 Ga (Crowe, Cosca &
Harris, 2001). Recognition of a rifted margin and
sutured boundary marked by nepheline gneiss DARCs
in the northeastern parts of the Eastern Ghats Mobile
Belt may help in local testing of the model of Figure 2
and in clarification of the regional structure.
3. An Eastern Ghats Mobile Belt suture: ophiolitic
slivers, a gravity anomaly, Fermor’s line and calcic
anorthosites
The linear distribution of DARCs in the Eastern Ghats
(Figs 1, 3) is consistent with the model (Fig. 2) of
F
F
K
S
N
D
Cuddapah
Basin
D = Dalma
N = Nausahi
S = Sukinda
K = Kandra
Ka = Kanigiri
P-G
Rift
12ºN
16º
20º
84ºE80º
Central
Indian
Tectonic
Singhbhum
Craton
Dismembered Ophiolites:
Ka
Zone
Sausar
Belt
Figure 6. Sketch map of eastern India showing rocks considered
to represent fragments of dismembered ophiolites (black lenses).
Suture zones along craton margins have been independently
identified in this paper from their association with DARCs.
Dismembered ophiolites have been recognized at Dalma (Yellur,
1977; Gupta; Basu & Ghosh, 1980; Sarkar, 1982; Chakraboti
& Bose, 1985), Nausahi and Sukinda (Page et al. 1985;
Leelanandam, 1990) and Kandra and Kanigiri (Leelanandam,
1990; Nagaraj Rao, Katti & Roop Kumar, 1991). P-G Rift –
Pranhita-Godavari rift; F–F – Fermor’s line.
gneissification of alkaline rocks during continental
collision, but there is other evidence from the vicinity of
that line indicating that it marks a suture zone. Previous
investigators have identified the suture zone on the basis
of: (1) outcrops of ophiolitic slivers, (2) a paired gravity
anomaly and (3) the location of ‘Fermor’s line’. We add
to those suture recognition criteria (4) the proximity of
highly calcic anorthosite bodies to the postulated
suture.
3.a. Dismembered ophiolites in the Eastern Ghats
Mobile Belt
Leelanandam (1990) and Nagaraj Rao, Katti & Roop
Kumar (1991) identified a possible suture and collision
site when they showed that the Kandra metamorph-
osed mafic and ultramafic rocks of Andhra Pradesh
(Fig. 6) and nearby rocks at Kanigiri (15
◦
25
N,
79
◦
30
E, Fig. 6) represent dismembered ophiolitic
fragments. The 40 km long outcrop of the Kandra rocks
exposes amphibolitic metabasalts that locally show
relict pillow structures and are partly spilitic. Metach-
erts are associated with the metavolcanic rocks, meta-
gabbroic units show cumulate textures, spinifex tex-
tures have been recognized and ultramafic slivers, now
consisting of talc-chlorite schists, complete the picture
of a dismembered ophiolite. Rocks similar to the
Kandra rocks crop out sporadically for at least 50 km
Proterozoic mountain building in Peninsular India 7
along strike to the north–northeast of the Kandra occur-
rences. The Inukurti anorthosite (Narsimha Reddy et al.
2003), which crops out within c. 20 km of the Kandra
rocks, is an occurrence of high-calcic anorthosite of
the type recognized elsewhere to characterize frag-
ments of older Precambrian ocean floor (Ashwal, 1993,
pp. 5–81). The occurrence of the Inukurti anortho-
sites within ‘voluminous amphibolites’ (meta-basalts)
(Dobmeier & Raith, 2003, p. 153) is consistent with
that idea. Mukhopadyay, Ray & Guha (1994) used geo-
chemical data to suggest that amphibolites in this area
represent rocks formed in a back-arc environment, and
Hari Prasad et al. (2000) interpreted other geochemi-
cal data as indicating the juxtaposition of rocks from an
oceanic island arc with those of a continental margin
arc. These interpretations are both compatible with
occurrence of the rocks in proximity to a suture zone.
Near the northeastern margin of the Eastern Ghats
Mobile Belt, ultramafic rocks that have been suggested
to be fragments of dismembered ophiolite have
been described from Sukinda and Nausahi (Fig. 6)
(Page et al. 1985). These rocks lie on the Sukinda
thrust that Ramakrishnan, Nanda & Augustine (1998,
p. 15) mapped as the northern contact of the Eastern
Ghats Mobile Belt with the Singhbhum Craton, and
Dobmeier & Raith (2003) considered to be the southern
margin of one of their Eastern Ghats Mobile Belt pro-
vinces. The outcrops are near to a major ESE-trending
strike-slip fault (Dobmeier & Raith, 2003, fig. 3). The
general location of the suture zone in this region can be
defined, on the basis of DARC and dismembered ophio-
lite distribution, as occupying a roughly E–W-trending
zone around 21
◦
N that extends eastward from 83
◦
Eto
the unconformity beneath Phanerozic rocks (Fig. 1).
3.b. Calcic anorthosite bodies in the Eastern Ghats Mobile
Belt: possible Andean arc roots
Ashwal (1993, pp. 288–92) suggested that anorthosites
with highly calcic plagioclase (> 90 % An) and asso-
ciated rocks might have formed deep beneath Andean
Arcs. Occurrences of such calcic anorthosites in the
Eastern Ghats Mobile Belt lie to the east of the
Western Charnockite Zone and close to the suture zone
postulated on the basis of DARC distribution (Fig. 4).
Other rocks in the region include charnockites, ender-
bites and a variety of granodiorites and granites that
may also have formed at various depths beneath
Andean arc volcanoes. Dobmeier & Raith (2003,
p. 156) suggested that one body, the Chanduluru com-
posite pluton of Nagasai Sharma & Ratnakar (2000),
has ‘chemical characteristics [that] point to emplace-
ment in an Andean-type arc’.
3.c. Bouguer gravity anomalies of the Eastern Ghats
Mobile Belt
Paired Bouguer gravity anomalies, negative over the
craton and positive over the adjacent mobile belt with
a transition close to the intervening suture zone, were
recognized as characteristic features of Precambrian
collisional mountain belts (Gibb & Thomas, 1976).
A paired Bouguer anomaly signature has long been
recognized in the Eastern Ghats Mobile Belt (e.g.
Subrahmanyam & Verma, 1986; Singh & Mishra,
2002). On the basis of the location of the transition
between positive and negative Bouguer anomalies,
which globally has been recognized to have a typical
horizontal spatial resolution of a few tens of kilometres,
the location of the suture zone from the gravity sig-
nature matches that defined on the basis of the distribu-
tion of DARCs, calcic anorthosites and dismembered
ophiolitic slivers.
3.d. Fermor’s line in the Eastern Ghats Mobile Belt
Fermor (1936) drew a line on a regional map that
bounded charnockite occurrences; this soon came to
be known as ‘Fermor’s line’ (Figs 1, 3; Ramakrishnan,
Nanda & Augustine, 1998, pp. 2–5). The line consists
of two very different parts: a NE-trending part within
the Eastern Ghats Mobile Belt that lies roughly parallel
to the coast, and a more irregular, roughly E–W-
trending part further south that crosses the entire
southern Indian peninsula. In drawing his line within
the Eastern Ghats Mobile Belt, Fermor did not consider
the charnockites of the Western Charnockite Zone. In
Figure 3 we have drawn Fermor’s line in the position
depicted in Leelanandam (1998). Putting the line in that
position places it close to the line of DARCs, the Sileru
shear zone, a concentration of calcic anorthosites, the
line of ophiolitic slivers and the change in sign of the
Bouguer gravity anomaly, all of which we have inter-
preted to indicate the approximate position of a suture
zone. Leelanandam (1990) pointed out that all the
occurrences of DARCs within the Eastern Ghats
Mobile Belt lie within 30 km of Fermor’s line and sug-
gested that the coincidence was likely to be tectonically
significant.
At both its northern end in Orissa and its southern
end in Andhra Pradesh, the NE-trending part of
Fermor’s line turns eastward toward the Bay of Bengal
(Fig. 3), indicating that the continuation of the suture
zone may be sought elsewhere. In Orissa the line lies
close to the margin of the strike-slip-dominated region
of the Eastern Ghats Mobile Belt (Crowe, Cosca &
Harris, 2001; Dobmeier & Raith, 2003, fig. 3).
3.e. Isotopic ages of DARCs of the Eastern Ghats
Mobile Belt
With one exception (Aftalion et al. 2000), published
ages of DARCs of the Eastern Ghats Mobile Belt
are Rb/Sr ages. Unfortunately, in the absence of age
determinations that make use of other isotopic systems
from the same rocks, such as high resolution ages
on zircons, Rb/Sr ages have proved notoriously hard
8 C. LEELANANDAM AND OTHERS
to interpret. We therefore concur with Dobmeier &
Raith (2003, p. 156) who wrote, ‘Overall, however, the
chronology of magmatism and the effects of regional
deformation and metamorphism on the plutons are
poorly constrained’. If the evolution of the Eastern
Ghats Mobile Belt has been as depicted in the model
cross-sections of Figure 2, then the parent ARCs of the
DARCs were emplaced at initial continental rupture,
which could have been as long ago as c. 2 Ga. Final
collision of the Eastern Ghats Mobile Belt cannot
be younger than the final assembly of Gondwana at
c. 0.55 Ga but could also have been at c. 1Ga or
c. 0.75 Ga. The Rb/Sr ages of DARCs of the Eastern
Ghats Mobile Belt, most of which form a group at
1.3 ± 0.1 Ga, seem to us more likely to record a mixture
of eruptive and metamorphic ages than either an initial
rifting or a collisional age. They could, however,
represent a discrete rifting event and a later collisional
event at a time within the long-duration Wilson cycle
represented in the Eastern Ghats Mobile Belt.
3.f. Tectonic evolution of the Eastern Ghats Mobile Belt
related to DARCs
Our reluctance to accept the published Rb/Sr isotopic
ages of the alkaline rocks of the Eastern Ghats Mobile
Belt at their face values has made us unprepared,
as yet, to follow Dobmeier & Raith (2003) fully in
their comprehensive synthesis of the crustal structure
and evolution of this belt. Although a wealth of
isotopic, structural and petrological information has
been brought together in that study, we consider that
it may be premature to distinguish a Krishna Province
including Vinjamuru and Ongole domains from the rest
of the Eastern Ghats Province.
We consider that the initiation of the Wilson cycle
(Burke & Dewey, 1974) recorded in the Eastern Ghats
is, at present, best dated by the observation that
Atlantic-type ocean margin formation at the edge of the
Bhandara (Bastar) craton must have preceded the ini-
tiation of sedimentary deposition within the Cuddapah
basin. This is because sedimentary rock deposition
in the Cuddapah basin appears to have taken place
in a foreland basin environment and to be related to
convergent plate margin activity. The Cuddapah basin
had begun to develop as a foreland basin by 1.8 Ga
(see reviews in Dobmeier & Raith, 2003, p. 147–8;
Chaudhuri et al. 2002; Drury et al. 1984; S¸eng
¨
or &
Natalin, 1996, p. 499). The rifted margin to the
Bhandara craton must, therefore, have formed earlier,
perhaps by c. 2.0 Ga. Initial eruption of ARCs would
have been at that time, or earlier, although later
continental rifting episodes may also have occurred.
Published Rb/Sr ages for nepheline syenites listed in the
Eastern Ghats Mobile Belt are considered to represent
something other than, or something in addition to, the
times of initial ARC eruption and rifted continental
margin formation.
As the Eastern Ghats Mobile Belt Wilson cycle
progressed, island arc and Andean margin-type pro-
cesses began to play a role in the evolution of the
rocks now exposed in the belt. Crowe, Cosca &
Harris (2001, p. 239) reviewed a substantial set of
published U/Pb age determinations from the northern
part of the Eastern Ghats Mobile Belt and identified a
c. 1100 Ma to 950 Ma (c. 1 Ga) episode of metamorph-
ism as a highlight. They considered the same c.1Ga
event to be discernable within comparable datasets
from further south. Convergence is recognized in the
gneissification of the alkaline rocks, that is, in DARC
generation, within the Eastern Ghats Mobile Belt, but
at present the timing of that gneissification is not clear.
Convergence in this belt was certainly complete by
the time that eastern Gondwana was finally assembled
(c. 550 Ma: Santosh et al. 2003). By that time the
Napier block of eastern Antarctica had become lodged
against the fully sutured Eastern Ghats Mobile Belt,
perhaps initiating, as S¸ eng
¨
or & Natalin (1996, p. 499)
have suggested, the collision-induced development of
the Pranhita-Godavari rift (P-G in Fig. 9). Published
ages for the Eastern Ghats Mobile Belt are likely to
include some that have been influenced by some of
the later events that contributed to the evolution of the
belt. Our preference at present is to limit ourselves
to considering the rocks of the Eastern Ghats Mobile
Belt as recording a history extending from at least as
long ago as 1.8 Ga to c. 0.5 Ga. That long interval was
dominated by complex convergent and related strike-
slip movements but may also have included rifting
events. The whole tectonic evolution was perhaps
comparable to that recorded in the Altaid mountain
belts further north in Asia between c. 700 and c. 250 Ma
(S¸eng
¨
or & Natalin, 1996). The distinction on the basis
of Nd model ages of areas of the Eastern Ghats Mobile
Belt that contain Archaean material from areas that
contain only material that left the mantle to become
parts of the continental crust during Proterozoic times
and yet other areas that yield both Archaean and
Proterozoic ages is reminiscent of the kind of tectonic
complexity that was involved in the evolution of the
Altaids (Rickers, Mezger & Raith, 2001; Dobmeier &
Raith, 2003, p. 157).
4. The Great Indian Proterozoic Fold Belt within
the Southern Granulite Terrain of southern India
From the time of its first description (Radhakrishna &
Naqvi, 1986; see inset to Fig. 3), the Great Indian
Proterozoic Fold Belt has been recognized to extend
into the Southern Granulite Terrain that crops out over
most of the southernmost part of India. Modern workers
generally concur with that idea, but the exact structure
of the continuation has proved difficult to establish, at
least in part because the Southern Granulite Terrain is
not an easy area in which to work out structure. The
combination of granulite facies rocks, uneven outcrop
Proterozoic mountain building in Peninsular India 9
distribution and prominent late shear zones has proved
particularly challenging for the field geologist.
4.a. Boundary of the Southern Granulite Terrain:
the southern part of Fermor’s line
Rocks of the Archaean Dharwar craton marking the
southwestern boundary of the Eastern Ghats Mobile
Belt reach almost to the shore of the Bay of Bengal
north of Chennai (formerly Madras, Fig. 3). There
the Archaean/Proterozoic Eastern Ghats Mobile Belt
boundary is buried under Late Phanerozoic cover.
Beneath that cover the boundary executes a 180
◦
turn.
As a consequence, a boundary between rocks yielding
Proterozoic isotopic ages to the south and Archaean
ages to the north emerges from under Phanerozoic
cover south of Chennai (Fig. 3). From that point a
somewhat irregular, but generally W-trending, boun-
dary extends across the full width of the Indian
Peninsula. This boundary, which separates granulite-
free Archaean outcrops to the north from the Southern
Granulite Terrain, forms the southern part of Fermor’s
line (Fermor, 1936; Fig. 3). Tectonic interpretation
of the southern part of Fermor’s line has proved
challenging. It is clearly a very different feature from
the structurally concordant northern part of the line
(Srikantia, 1999, p. 154). Many publications (e.g.
Gopalakrishna et al. 1986) and our own field ob-
servations have shown that it is possible to follow
structures across the Fermor’s line boundary from rocks
with charnockitic assemblages into rocks with am-
phibolitic assemblages. For that reason, Fermor’s line
in southern India is unlikely to mark the site of a
structural boundary or a suture zone. The observation
that no line of dismembered ophiolitic slivers and no
paired Bouguer gravity anomaly is associated with the
southern part of Fermor’s line is consistent with that
idea.
It is more appropriate to consider Fermor’s line at
the Southern Granulite Terrain boundary as a product
of static metamorphism (and possibly metasomatism)
because it cuts across geological structure (Fig. 7).
The event that finally established the thermal structure
of the Southern Granulite Terrain has been shown
to date from c. 550 Ma (e.g. Santosh et al. 2003),
and to reflect one of the later episodes in the final
assembly of Gondwana. The distribution of the older
Archaean-aged granulite facies metamorphic rocks in
the Southern Granulite Terrain is more complicated.
Explaining the outcrop pattern represented by those
rocks must await more of the kind of integrated
structural and isotopic studies that are beginning to be
made in the Southern Granulite Terrain, for example,
those of J. G. Ghosh (unpub. Ph.D. thesis, Univ.
Cape Town, 1999). The irregularity of the outcrop
pattern of the E–W-trending sector of Fermor’s line
suggests to us that it represents a metamorphic surface
(‘The SGT (Southern Granulite Terrain) boundary
A
B
CHENNAI
(MADRAS)
F
47
45
42
39
38
44
0
500 km
A
B
Amphibolite
Charnockites
(a)
(b)
~10 km
46
41
40
44
45
47
38 40
42
Figure 7. (a) Sketch of an oblique view of southern India from
the southwest and (b) a sketch cross-section across Fermor’s line
(F) in (a). The DARCs of the Southern Granulite Terrain which
lie on a suture zone may also lie within c. 10 km (vertically
or horizontally) of the projected Fermor’s line surface, which
is suggested to indicate a Late Proterozoic thermal structure.
The distribution of the Archaean charnockites and granulites
of the Southern Granulite Terrain was established earlier and is
unrelated to the Late Proterozoic thermal structure. Numbers of
DARCs correspond to those in Table 1.
surface’) dipping gently to the north, perhaps at < 5
◦
(Fig. 7).
4.b. Shear zones within the Southern Granulite Terrain
E–W- and NW-trending shear zones form major struc-
tural features within the Southern Granulite Terrain
(inset to Fig. 8). Those structures are commonly
grouped as the Moyar-Bhavani, Palghat-Cauvery and
Achankovil-Tenmala shear zones, although other
names and map-patterns for shear zones exist (e.g.
Santosh et al. 2003, fig. 1). Chetty & Bhaskar
Rao (2003, especially fig. 2) interpreted the Palghat-
Cauvery shear zone as a site of Neoproterozoic trans-
pressional tectonics in a crustal-scale flower structure
and Chetty, Bhaskar Rao & Narayana (2003) con-
sidered that the Moyar-Bhavani shear zone could mark
the location of a Palaeoproterozoic thrust boundary
between the Nilgiri block and the Dharwar Craton.
They also suggested that Neoproterozoic transpression
was localized on flower structures in both the Moyar-
Bhavani and Palghat-Cauvery shear zones. Srikantia
(1999) suggested, among other possibilities, that the
three shear zones might be suture zones separating
four gneissic-granulite dominated blocks (Srikantia,
1999, fig. 1). Mukhopadhyay et al. (2001) did not
see evidence of major transcurrent motion along the
Palghat-Cauvery shear zone but did consider that
Neoproterozoic amphibolite facies rocks and Late
Archaean granulites were juxtaposed in that region
10 C. LEELANANDAM AND OTHERS
Bangalore
Madurai
F
e
r
m
o
L
i
r
’
s
n
e
0 50 100 km
?
?
Palghat-
Cauvery
Shear Zone
10º
11º
12º
78º 79º
80º
76º
10º
14º
M-B
P-C
DARCs
Mafic &
Ultramafic
Bodies
Ramnagaram
Granite
Shear
Zones
Suture
Zone
Limits
13º
46
45
41
40
39
38
Moyar
-Bhavani
Shear Zone
Figure 8. Sketch map of an area between Bangalore and
Madurai in which six of the ten DARCs of the Southern
Granulite Terrain crop out, showing their locations along a
newly postulated suture zone. The DARCs are considered to
mark a suture zone by analogy with the line of DARCs in
the Eastern Ghats Mobile Belt because: (1) they form a line;
(2) they are associated with ultramafic/mafic igneous bodies
and (3) they lie close to the abrupt southern termination
(it is here suggested at an ancient rifted margin) of the
c. 500 km long Ramnagaram (formerly Closepet) granite. The
postulated suture zone, which can presently only be defined as
lyingwithinac. 100 km wide belt, may be offset left-laterally
c. 100 km by the Palghat-Cauvery shear zone but is not
apparently greatly offset by the Moyar-Bhavani shear zone.
along a narrow transition. The occurrence of great shear
zones that developed during Late Proterozoic times
(c. 750–500 Ma: Chetty, Bhaskar Rao & Narayana,
2003), and their preservation in granulite facies rocks,
has made it difficult to map older suture zones in
the way that has proved feasible in the Eastern Ghats
Mobile Belt and the Aravalli parts of the Great Indian
Proterozoic Fold Belt, where rocks are preserved in
amphibolite facies and late shearing across the trend of
the mobile belt is less prominent.
4.c. A suture zone within the Southern Granulite Terrain
Gopalakrishnan, Subramanian & Upendran (2001)
identified several parts of what were suggested to have
been a long-lived Andean margin in the area south
of the Palghat-Cauvery shear zone. The idea that the
presence of a long-lived Andean margin is recorded
in the rocks of that area for times between 1.5 Ga and
1.0 Ga and possibly longer is discernable in various
forms in earlier publications, for example, that of
S¸eng
¨
or & Natalin (1996, p. 499). We concur that the
rocks of an Andean margin are preserved within the
Southern Granulite Terrain and favour the idea that
the margin in question occupied the edge of a con-
tinental object which collided with a rifted margin of
the Dharwar Craton.
Gopalakrishnan, Subramanian & Upendran (2001)
distinguished four ‘micro-terranes’ separated by
‘palaeosutures’ within the Southern Granulite Terrain.
They considered, as we do, that alkaline-syenites and
related rocks within the Southern Granulite Terrain
are tectonically significant, but their interpretation con-
trasts with ours in so far as they associated the alkaline
plutons of the Southern Granulite Terrain with abortive
rifting, while we associate those bodies (because they
are DARCs) with intra-continental rifting, ocean open-
ing and later ocean closure. Recognizing that the iden-
tification of suture zones within the Southern Granulite
Terrain is difficult, we here use the distribution of the
DARCs of that terrain to show where we suggest that
a suture zone can be discerned and draw attention to
other evidence which we consider consistent with that
suggestion.
Ten DARCs (Table 1 and Fig. 1, nos 38–47; see
also Figures 3, 8) have been described from within the
Southern Granulite Terrain (Mazumder, Rao & Nathan,
2000; Ratnakar & Leelanandam, 1989). Those DARCs
are mainly carbonatites. They characteristically show
less conspicuous signs of deformation than nepheline
syenites. Rocks at Pakkanadu (no. 44) are strongly
deformed, but some occurrences, such as Hogenakal
(Table 1, no. 42), show only deformed calcite twin
lamellae. An approximately N–S trend in the alignment
of alkaline bodies and carbonatites within the Southern
Granulite Terrain was pointed out by Nair & Santosh
(1984). In interpreting that alignment, Mazumder,
Rao & Nathan (2000, p. 108–9, fig. 14) related four
or more syenite and carbonatite bodies in Tamil Nadu
to a NNE-trending Dharampuri rift or shear zone em-
bodying the Palakoddu and Javadi Hill (Harur) linea-
ments of earlier studies.
We develop the line of thought originated by these
authors by interpreting the ten DARCs within the
Southern Granulite Terrain as occupying a c. 500 km
long by c. 100 km wide NNE-trending belt between
c. 79
◦
E, 13
◦
N and c. 77
◦
E, 10
◦
N. The belt is perhaps
offset c. 100 km at the Palghat-Cauvery shear zone
(Fig. 8). Because the DARCs form a roughly linear
Proterozoic mountain building in Peninsular India 11
array (Figs 3, 8) we infer them, by analogy with our
interpretation of the line of DARCs in the Eastern Ghats
Mobile Belt, to indicate the locations of both an ancient
rifted margin to the Dharwar craton and a Protero-
zoic suture zone within the Great Indian Protero-
zoic Fold Belt. Others (e.g. Nair & Santosh, 1984;
Gopalakrishnan, Subramanian & Upendran, 2001)
have suggested an association between rift structures
and at least some of the alkaline rocks of the
Southern Granulite Terrain. Our suggestion is that the
deformation of the alkaline rocks, which we agree
are likely to have been originally erupted into rifts,
developed during later collisions (Santosh, 1989).
The suture zone that we discern within the Southern
Granulite Terrain is much less well defined than the
suture within the Eastern Ghats Mobile Belt. That
difference we attribute to the tectonic complexity and
to the metamorphic state of the Southern Granulite
Terrain. Nevertheless, as in the Eastern Ghats Mobile
Belt, there are features of our postulated Southern
Granulite Terrain suture zone, besides the occurrence
of a line of DARCs, which can be interpreted to
indicate the location of a rifted margin and a suture. For
example, the 500 km long N–S-trending Ramnagaram
(formerly Closepet) granite outcrop of the Dharwar
craton ends abruptly (Fig. 8). Formation of a rifted
margin at c. 12
◦
N, 77
◦
E could have truncated the
Ramnagaram granite in just that way. The truncation is
in the right place and, within poor resolution, took place
at c. 2 Ga. Dismembered ophiolites have not yet been
discerned within the Southern Granulite Terrain. This is
not surprising because many of the features by which
remnants of ophiolites can be recognized, including
pillow lavas, sheeted dykes and cherts, are unlikely to
survive in the granulite facies conditions represented
in the rocks of the Southern Granulite Terrain. The
dozen or so mafic and ultramafic rock complexes
mapped within the region of the postulated suture zone
(Fig. 8) could represent ophiolitic material, although
mafic and ultramafic rock complexes within granulite
terrains can certainly represent other environments,
such as the roots of arc volcanoes. Geochemical
and isotopic evidence may in the future help in
telling whether ophiolitic material is represented within
the Southern Granulite Terrain. There is no paired
Bouguer gravity anomaly in the vicinity of the
postulated Southern Granulite Terrain suture zone such
as has been considered indicative of the presence
of a suture in the Eastern Ghats Mobile Belt and
elsewhere. That absence may be a consequence of the
Late Proterozoic (c. 550 Ma) tectonic events in the
area.
The zone within the Southern Granulite Terrain in
which ten DARCs and about a dozen mafic and ultra-
mafic complexes occur crosses both the Moyar-Bhavani
and the Palghat-Cauvery shear zones (Fig. 8, inset).
In the region shown in Figure 8, no offset of the
DARC belt across the Moyar-Bhavani shear zone (at
c. 11
◦
30
N) can be recognized, but on the south side of
the Palghat-Cauvery shear zone (at c. 11
◦
N), the belt
containing the DARCs and the mafic and ultramafic
complexes looks to have been offset c. 100 km to
the east by left-lateral movement. Further south,
the Achankovil-Tenmala shear zone (de Wit, 2004;
Braun & Kriegsman, 2003) cannot be seen to affect the
line of DARCs because the most southwestern DARC
within the Great Indian Proterozoic Fold Belt is no. 47
(Munnar or Mannar) at 77
◦
E, 10
◦
N, about 100 km
northeast of that shear zone. Our conclusion is that
the suture zone within the Southern Granulite Terrain
extends from c. 79
◦
E, 13
◦
N as a roughly 500 km long
NNE-trending line to within a few tens of kilometres
of the Achankovil-Tenmala shear zone. Because no
DARCs have yet been identified on the southwestern
side of the Achankovil-Tenmala shear zone, the further
extension of the line of DARCs, the ancient rifted
margin, the Proterozoic suture zone and, indeed, of any
internal structures within the Great Indian Proterozoic
Fold Belt that might serve to indicate offset along the
shear zone are at present unknown.
4.d. Isotopic ages of DARCs and the timing of events
within the Southern Granulite Terrain
Mazumder, Rao & Nathan (2000, table 4) listed a vari-
ety of Rb/Sr and K/Ar isotopic ages obtained by
several different authors from the DARCs of the
Southern Granulite Terrain. Those ages mainly fall in
a range between 750 Ma and 800 Ma, although older
ages in the range from 1920 Ma (Crawford, 1969) to
2000 Ma have also been reported (Natarajan et al. 1994;
see also Anil Kumar et al. 1990). Late Proterozoic
ages were reported by authors at the Symposium
on Carbonatites and Associated Alkaline Rocks of
Tamil Nadu (2001). Of particular interest from that
meeting were electron microprobe ages on monazite
of 750 ± 2 and 759 ± 3Ma (M
¨
oller et al. 2001). Two
data points from those monazites yielded > 1100 Ma
ages and a ‘microcracked’ monazite yielded an age
of 550 Ma (M
¨
oller et al. 2001). A whole rock Pb/Pb
isochron of 801 ± 11 Ma for Sevattur was reported at
the same meeting (Schleicher, 2001). The picture of
the tectonic history of the area now occupied by the
Southern Granulite Terrain that emerges from these
isotopic measurements is of several episodes of Late
Proterozoic activity between c. 0.9 Ga and c. 0.55 Ga,
perhaps all associated with various late collisional
events during the evolution of the Great Indian
Proterozoic Fold Belt. The few ages around 2.0 Ga may
relate to an earlier time when rifting of the Dharwar
craton and initiation of an Atlantic-type ocean margin
to that craton were in progress. Those ages could also
relate to the time of the initial emplacement into the
continental crust, as ARCs, of the DARCs now exposed
within the Southern Granulite Terrain.
12 C. LEELANANDAM AND OTHERS
5. Continuation of the Great Indian Proterozoic Fold
Belt into the Central Indian Tectonic Zone
The northeasternmost part of the Eastern Ghats Mobile
Belt suture zone is defined by the 30 km long set
of nepheline syenite gneiss outcrops that forms the
WNW–ESE-trending Kankarakhol-Lodjahari complex
(nos 8–21 in Table 1) at c.21
◦
15
N, 84
◦
50
E and by
the Barandangua body (no. 7 at 21
◦
04
N, 85
◦
05
E).
These DARCs decorate the boundary between the
Eastern Ghats Mobile Belt and the Singhbhum craton
close to the place where that boundary disappears under
Phanerozoic sedimentary rocks (Fig. 6). As in Tamil
Nadu, where the Archaean/Proterozoic boundary south
of Chennai executes a 180
◦
turn beneath younger cover,
the boundary between Proterozoic rocks of the Great
Indian Proterozoic Fold Belt and the Singhbhum craton
executes a 180
◦
turn under Phanerozoic sedimentary
rocks to emerge in West Bengal close to 23
◦
N (Fig. 6).
The Great Indian Proterozoic Fold Belt is represented
for the next 1000 km by a partly buried, W-trending
mountain belt that crosses the entire Indian peninsula
(Fig. 1). That belt is commonly called the Central Indian
Tectonic Zone or CITZ (see, e.g. Acharyya, 2003).
In West Bengal and Bihar there are sufficient
occurrences of DARCs, anorthosites and dismembered
ophiolites (Fig. 6) for a suture zone at or close to the
Great Indian Proterozoic Fold Belt margin to be traced
at the surface for c. 300 km. Four nepheline syenite
gneiss bodies cropping out at the Proterozoic/Archaean
contact (no. 6 at 23
◦
33
N, 86
◦
45
E, and no. 3 at
23
◦
01
N, 86
◦
15
W) ornament the northern margin
of the Singhbhum craton in West Bengal (Mazumder,
1978; Mazumder, Rao & Nathan, 2000, pp. 106–7,
figs 1, 8). DARC 3 lies on a 120 km long shear
zone that extends into Bihar. Carbonatites have been
penetrated in mineral exploration drill-holes along that
shear zone (Mazumder, Rao & Nathan, 2000, p. 107).
These four DARCs serve to define the continuation of
the Great Indian Proterozoic Fold Belt suture zone into
the Central Indian Tectonic Zone. Supporting evidence
for the existence of a suture zone comes from: (1) the
association of the 32 km long Bankura anorthosite
with the line of the DARCs on the northern margin
of the Singhbhum craton and (2) the outcrop on the
Singhbhum craton boundary further to the west in Bihar
of the Dalma greenstones (Fig. 6), which have been
considered, on various compositional and structural
grounds, to represent a dismembered ophiolite (Yellur,
1977; Gupta, Basu & Ghosh, 1980; Sarkar, 1982;
Chakraboti & Bose, 1985).
6. Continuation of the Great Indian Proterozoic
Fold Belt from the Central Indian Tectonic Zone
into Gujarat, Rajasthan
S¸eng
¨
or & Natalin (1996, fig. 21.10) elaborated on the
original interpretation of the Great Indian Proterozoic
Fold Belt by Radhakrishna & Naqvi (1986) and we
have followed their model. We show the Great Indian
Proterozoic Fold Belt in Figure 9 as continuing from the
Eastern Ghats Mobile Belt first by occupying the buried
curve around the eastern end of the Singhbhum craton
and then by becoming the Central Indian Tectonic
Zone. The Sausar Mobile belt that crops out between
c. 79
◦
E and 84
◦
E is generally recognized to be part
of the Central Indian Tectonic Zone (e.g. Acharyya,
Bandyopadhyay & Roy, 2001), although not all agree
with Mishra et al. (2000), Yedekar et al. (1990), Jain,
Yedekar & Nair (1991), Rao & Reddy (2002) and Roy &
Prasad (2001) that a suture, the ‘Central Indian Suture’,
occupies the Central Indian Tectonic Zone throughout
its length. Recognition that the last amphibolite facies
metamorphism in the Sausar belt occurred between
800 Ma and 900 Ma and that the records of the Sausar
main granulite belt and the more northern Sausar
granulite belts differ before c. 1100 Ma (Roy et al.
in press) indicates a complex history. Suturing of the
Bundelkhand and Bastar cratons has taken place within
the Central Indian Tectonic Zone. The rocks of the
Sausar belt record convergent plate margin phenomena
over a long interval, perhaps extending from c. 1400 Ma
to c. 800 Ma (Roy et al. in press).
At the western end of the Central Indian Tectonic
Zone, the Great Indian Proterozoic Fold Belt turns to
the northeast to become well exposed in the Aravalli
and Delhi fold belts of northeastern Gujarat and
Rajasthan (Fig. 9). In central Rajasthan, the Kishangar
nepheline syenite gneiss (Table 1, no. 1) at 26
◦
35
N,
74
◦
53
E (Gupta et al. 1997; Roy & Dutt, 1995) is
a DARC exposed in a region where rocks of the
Delhi Group abut directly against rocks of the ancient
banded gneiss complex of the Bundelkhand craton
and the Bhilwara metasedimentary rocks that S¸ eng
¨
or
& Natalin (1996, p. 499) interpreted to be its rifted-
continental-margin cover. The Bhilwara rocks may be
correlatable with the Kisengarh Group of Roy & Dutt
(1995, table 1). Rocks of the Aravalli Group and
the Rakhabdev suture zone that separate the ancient
banded gneiss from the Delhi Group further south
(Fig. 9; Sinha-Roy, 1999, fig. 1) appear to have been
cut out from this area (Gupta et al. 1997) or to be
completely covered by rocks of the South Delhi fold
belt (Sinha-Roy, 1999, p. 95, fig. 6). The Newania
carbonatite (Table 1, no. 2) that crops out about 200 km
south of Kisengarh has been intruded into Bundelkand
gneisses and is possibly the least deformed of all Indian
Proterozoic DARCs. We consider it to be related to the
Kisengarh nepheline syenite gneiss emplacement event
because the two bodies have yielded similar ages of
c. 1.5 Ga (Srivastava, 1989, p. 5). It is clear from
Figure 9 that the occurrence of nepheline syenite gneiss
in the Gorkha-Ampipal area of the Lesser Himalaya
of Nepal (Dhital, 1995) does not lie along strike of
the Kisengarh gneiss. For that reason Gorkha-Ampipal
is not here considered to be part of the Great Indian
Proterozoic Fold Belt array of DARCs.
Proterozoic mountain building in Peninsular India 13
?
Madagascar
Sri
Lanka
Antarctica
Seychelles
Cuddapah
Basin
F
F
F
F
Bundelkhand
Craton
Singhbhum
Craton
Napier
Block
Central
Indian
Tectonic
P-G
Rift
Malani
Delhi
Gp.
Aravalli
and
R.
Suture
P.
Suture
P.
Suture
Dharwar
Craton
Rifted Margin
by 550 Ma
G-A
K
?
Bhandara
Craton
Zone
Bhilwara
A-T
Figure 9. Sketch map (based mainly on S¸eng
¨
or & Natalin, 1996, fig. 21.10 and Torsvik et al. 2001) showing a possible reconstruction of
the Indian Peninsula and then neighbouring continents at c. 750 Ma. DARCs, including those of the Southern Granulite Terrain and the
Eastern Ghats Mobile Belt as well as the Kishangar nepheline gneiss, are shown as black circles. The Malani–Seychelles–Madagascar
Andean arc that was active at c. 750 Ma occupies one continental margin of a c. 3 million km
2
area continental block. Suture zones
involved in the assembly of India include (1) the Rakhabdev suture (R.), (2) the Phulad suture (P.), (3) the Sileru shear zone suture and
the coincident Fermor’s line (F–F) of the Eastern Ghats and (4) the SW-trending suture zone within the Southern Granulite Terrain of
southern India. That suture zone is last discerned entering an area close to the Achankovil-Tenmala shear zone (A–T) near the southern
tip of India. The relationship of India to Eastern Antarctica at this time (shown here as in S¸eng
¨
or & Natalin, 1996, fig. 21.10) remains
uncertain. The Pranhita-Godavari rift (P-G) was suggested by S¸eng
¨
or & Natalin (1996) to be a rift formed as a result of collision in
the Eastern Ghats. The Gorkha-Ampipal nepheline gneiss (G-A) is a DARC in the Lesser Himalaya that appears to be unrelated to the
DARCs of the Great Indian Proterozoic Fold Belt.
DeCelles et al. (2000, fig. 2) have shown that the
ages of detrital zircons in the Nawakot Group of
the Lesser Himalaya of Nepal are concentrated in
the 2.0 to 1.8 Ga range, which is compatible with
the idea that rocks of the Great Indian Proterozoic
Fold Belt may have provided sediment sources for
that unit. The same authors reported a concentration
in Proterozoic rocks of the Greater Himalaya of
detrital zircon ages between 0.8 Ga and 1.0 Ga. That
distinct population could also indicate that rocks of
the Great Indian Proterozoic Fold Belt were involved
as sediment sources. The line separating the two
Himalayan detrital zircon populations is that of the
outcrop of the Main Central Thrust. DeCelles et al.
(2000) suggested that the separation might indicate
the location of a previously unidentified Proterozoic
suture zone in the place at which the Main Central
Thrust presently crops out. The proximity of the
Gorkha-Ampipal nepheline syenite gneiss to the Main
Central Thrust (Dhital, 1995, fig. 2) suggests to us
that if DeCelles et al. (2000) have located a new
suture zone close to the Main Central Thrust then the
Gorkha-Ampipal body may be a DARC lying on that
suture.
14 C. LEELANANDAM AND OTHERS
7. The Great Indian Proterozoic Fold Belt in
a regional context
Our discussions of DARC distribution and of suturing
are here accommodated to studies that place the Great
Indian Proterozoic Fold Belt in a more regional context.
Figure 9, which is based on the work of Radhakrishna &
Naqvi (1986), S¸eng
¨
or & Natalin (1996, especially
figure 21.10), Torsvik et al. (2001) and Ashwal,
Demaiffe & Torsvik (2002), shows part of Eastern
Gondwana as it may have been in Late Proterozoic
times.
The Kishangar nepheline gneisses (Table 1, no. 1)
and the Newania carbonatite (Table 1, no. 2) appear
to have developed from ARCs erupted into rifts
that later formed an Atlantic-type margin to the
Bundelkhand craton, but all the other DARCs of
Table 1 appear to have developed from ARCs that
had been erupted into rifts that later formed Atlantic-
type margins to the Dharwar, Bhandara (Bastar) and
Singhbhum cratons. The winding S-shaped outcrop of
the Proterozoic fold belts of India (Fig. 9) is reminiscent
of the structure within the Altaid Mountains of Asia
(S¸eng
¨
or & Natalin, 1996). The Malani rhyolite and
the Erinpura granitic rocks of the northwestern part
of the Indian peninsula and comparable rocks in the
Seychelles and Madagascar fit with the Altaid structural
pattern. The Malani–Seychelles–Madagascar rocks
have been interpreted (Ashwal, Demaiffe & Torsvik,
2002; Torsvik et al. 2001) as indicating the location
of a 750 Ma Andean arc. A postulated Malani–
Seychelles–Madagascar Andean arc can be envisaged
as having occupied one margin of a continental object
c. 3 million km
2
or more in area that included the Indian
cratons after they had become wrapped up in the matrix
of Proterozoic mountain belts that makes the Great
Indian Proterozoic Fold Belt. S¸eng
¨
or & Natalin (1996,
p. 500) suggested that the assembled object may also
have included the Lhasa block, but as yet no Proterozoic
rock outcrops are known on that block.
The time of assembly of the 3 million km
2
con-
tinental object that makes up Peninsular India and of
its incorporation into Gondwana and possibly also into
an earlier continental assembly is of great interest to
those considering the times of assembly and rupture
of the postulated ancient super-continents. Our study
is unable to contribute to that subject. We have been
able to map within the Great Indian Proterozoic Fold
Belt, on a reconnaissance scale and over a distance
of c. 4000 km, both a rifted continental margin and
a suture that developed on that margin, but times of
initial continental margin formation and of suturing
cannot be established with available isotopic data.
Isotopic ages from within the Great Indian Proterozoic
Fold Belt show that folding, faulting, igneous activity
and metamorphism extended over an interval from at
least as early as c. 1.8 Ga to c. 0.55 Ga. Episodes
of intense tectonic activity have been discerned in
the Eastern Ghats Mobile Belt, particularly between
c. 1.1 Ga and c. 0.9 Ga, but tectonic activity in the Great
Indian Proterozoic Fold Belt may have been almost
continuous at some level over more than 1 Ga. Dis-
tinguishing episodes of continental assembly and
continental disruption must depend more on inform-
ation about the arrangement of ancient objects of
continental dimensions on the Earth’s surface (that is,
on palaeolatitudinal studies) than on identifying the
timing of particular tectonic events within a mountain
belt. Published isotopic ages and other geological
information indicate to us that one possibility is that
the assembly of India was complete by c. 1 Ga and that
an alternative is that assembly became complete by
c. 750 Ma (Crowe, Cosca & Harris, 2001; Santosh
et al. 2003).
Whether it was assembled at c. 1Gaorc. 750 Ma,
the continental object that now makes up most of the
Indian peninsula became a part of Gondwana at the
time it was assembled. Eastern Gondwana is likely to
have been larger in area at that time than during Early
Palaeozoic times because the Great Indian Proterozoic
Fold Belt strikes northeastward toward the Himalaya
(Fig. 9) and its projection makes a high angle with what
had, by the beginning of Phanerozoic time (550 Ma),
become a rifted margin to Gondwana. The rifted margin
could have been that of a marginal basin.
At the other end of its 4000 km long strike-length, the
suture marking Proterozoic closure in the Great Indian
Proterozoic Fold Belt is last seen entering a region that
contains the Achankovil-Tenmala Proterozoic shear
zone near the southern tip of India (Fig. 9). If final
closure was at c. 750 Ma in the south, as it was
in Rajasthan (Torsvik et al. 2001), then the suture
could have been offset on that c. 550 Ma shear zone
(Fig. 6). There is a single DARC of unknown age at
Makairingobe in Madagascar (Burke, Ashwal & Webb,
2003, fig. 1, inset and table 1; Welter, 1964). It is an
intriguing, although an admittedly remote, possibility
that the nepheline gneiss at Makairingobe marks the
extension of the suture zone that we have discerned
within the Great Indian Proterozoic Fold Belt from its
last discernable location within the Southern Granulite
Terrain.
The assembly of the Indian peninsula had ended
before the time at which Gondwana, with an area of
c. 80 million km
2
, became complete at c. 550 Ma.
How and whether the Indian peninsula was related
to other parts of Gondwana before 550 Ma remains
unresolved. Torsvik et al. (2001, fig. 7) show one
interpretation. The idea that eastern Gondwana was
assembled into its final configuration by c. 750 Ma is
compatible with the conditions sketched in Figure 9.
The possibility has also been suggested that eastern
Gondwana was assembled by as long ago as c. 1Ga
(e.g. Santosh et al. 2003, fig. 18b). Resolving the
timing of assembly of continental objects in the general
region of the Indian peninsula during the Proterozoic
Proterozoic mountain building in Peninsular India 15
awaits new U/Pb age determinations that can be
linked to new information about palaeolatitudinal,
including palaeomagnetic, indicators. Until that time,
there cannot help but be a strong element of speculation
in all histories of reconstruction. With that limitation
in mind we here present tentative tectonic suggestions
about the history of the Great Indian Proterozoic Fold
Belt that embody our observations about DARCs.
We emphasize that these conclusions are presented
so that they can be tested, improved upon or simply
proven wrong when the results of new U/Pb and new
palaeolatitudinal indicator studies become available:
(1) Intracontinental rifts and a rifted continental
margin formed close to the site of the Sileru shear
zone in the Eastern Ghats Mobile Belt and on
the Dharwar craton side of a suture zone within
the Southern Granulite Terrain of southern India.
That rifted margin, which may have extended
through much of the length of the Great Indian
Proterozoic Fold Belt, could have formed as long
ago as c. 2Ga.
(2) Convergent plate boundary processes began to be
involved at that continental margin, as well as in
Gujarat and Rajasthan and possibly throughout
the entire length of the Great Indian Proterozoic
Fold Belt by 1.8 Ga.
(3) A major episode of convergent plate margin
activity at c. 1 Ga has left a record in the Eastern
Ghats Mobile Belt. It is not possible to conclude
from the evidence available whether that event
reflects arc-system or continental collisional
activity.
(4) Ages are scattered widely in the Eastern Ghats
Mobile Belt from 850 Ma into the earliest
Phanerozoic (after 550 Ma). Several peaks in
numbers of analyses have been recognized. At
or before that time the Indian peninsula had
been assembled. These events in the Eastern
Ghats Mobile Belt were roughly contemporary
with both an arc-collisional event and the estab-
lishment of the Malani–Seychelles–Madagascar
Andean arc on the opposite side of the newly
assembled continental object.
(5) Rifting at the Himalayan margin by 550 Ma
and offsetting of the Southern Granulite Terrain
suture zone at c. 550 Ma, possibly into Madagas-
car along the Achankovil-Tenmala shear zone,
complete the picture. By 550 Ma the assembly
of Gondwana was largely complete.
8. Summary and conclusions
(1) Forty-seven DARCs ranging from 30 m to
30 km in length, mainly in the Eastern Ghats
Mobile Belt and the Southern Granulite Terrain,
have been recognized in the Great Indian
Proterozoic Fold Belt (Fig. 1).
(2) Those DARCs are concentrated within a few
tens of kilometres of what may be a single suture
zone (Figs 3, 4, 8).
(3) Proterozoic nepheline syenites and carbonatites
of India are all gneissic and are therefore
DARCs (Fig. 5).
(4) On the hypothesis that DARCs mark suture
zones and that their parent ARCs mark the
sites of intra-continental rifting (Fig. 2; Burke,
Ashwal & Webb, 2003), the Sileru shear zone
and its correlative extensions to the north and
to the south in the Eastern Ghats Mobile Belt
mark a suture zone at which both continental
breakup and continental collision are recorded.
(5) That conclusion is consistent with documented
occurrences within the Eastern Ghats Mobile
Belt of dismembered ophiolitic fragments, a
Bouguer gravity anomaly sign change, the
distribution of calcic anorthosites and Fermor’s
line.
(6) The hypothesis that a line of DARCs marks
a suture zone has been successfully tested in
the Eastern Ghats. This is the first test of the
hypothesis outside Africa.
(7) The location of a suture zone within the
Southern Granulite Terrain has been postulated
on the basis of DARC distribution. That suture
zone is not coincident with Fermor’s line
(Figs 7, 8).
(8) DARCs lie on suture zones throughout the full
length of the Great Indian Proterozoic Fold Belt,
but evidence is less complete than in the Eastern
Ghats Mobile Belt.
(9) The timing of events during the operation of
the Wilson cycle that is recorded in the Eastern
Ghats Mobile Belt and the Southern Granulite
Terrain is at present poorly constrained. Initial
rifting of a continental margin could be as
old as c. 2 Ga. The onset of convergent plate
margin processes appears to have begun by
c. 1.8 Ga. Convergent plate boundary phenom-
ena have left peaks in isotopic records at
c. 1Ga and c. 750 Ma. Final assembly of
Gondwana has left a further record at c. 550 Ma.
(10) Extending consideration to the entire area
of the Indian peninsula and making use of
observations in Gujarat, Rajasthan, Nepal,
the Seychelles and Madagascar helps in the
construction of a tentative tectonic history for
the Great Indian Proterozoic Fold Belt.
Acknowledgements.We thank the South African National
Research Foundation for continued support of our work. The
visits of KB to Johannesburg were generously supported
by De Beers and the University of the Witwatersrand.
Manoj Pandit (University of Jaipur) kindly introduced us
to the nepheline syenite gneiss occurrence at Kishangar
16 C. LEELANANDAM AND OTHERS
(Rajasthan). We deeply appreciate the helpful reviews of
Christoph Dobmeier and John Dewey and the skilled editorial
work of Roger Gibson.
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