Research article
Journal of the Geological Society
Published Online First
https://doi.org/10.1144/jgs2017-051
U–Pb dating of metamorphic monazite establishes a Pan-African
age for tectonism in the Nallamalai Fold Belt, India
S. Sheppard1*, B. Rasmussen1, Jian-Wei Zi1, V. Soma Sekhar2, D. Srinivasa Sarma2,
M. Ram Mohan2, B. Krapež1,3, S.A. Wilde1 & N.J. McNaughton4
1
Department of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia
CSIR – National Geophysical Research Institute, Uppal Road, Hyderabad 500007, Telengana, India
3
College of Earth Sciences, Guilin University of Technology, 12 Jiangan Road, Guilin 541004, China
4
John de Laeter Centre for Isotope Research, Department of Applied Physics, Curtin University, Kent Street, Bentley, WA
6102, Australia
S.S., 0000-0002-2517-5574; D.S., 0000-0001-5930-4517; M.R., 0000-0002-5455-1823
* Correspondence: stephen.sheppard@curtin.edu.au
2
Abstract: The Nallamalai Fold Belt comprises late Palaeoproterozoic to Mesoproterozoic sedimentary rocks deformed into a
fold-and-thrust belt along the eastern side of Peninsular India. The age of thin-skinned thrusting, folding and low- to mediumgrade metamorphism in the belt is unclear, with estimates ranging from Palaeoproterozoic to early Palaeozoic. A possible PanAfrican age for thrusting has previously been inferred from Rb–Sr dating of muscovite in shear zones from the adjacent Krishna
Province (501 – 474 Ma) but these structures are separated from the Nallamalai Fold Belt by a major thrust. Here, we present in
situ U–Pb dating of metamorphic monazite within a low-grade metasedimentary rock in the Nallamalai Fold Belt at the
Mangampeta barite mine. Our date of 531 ± 7 Ma for the monazite is the first direct evidence that west- to NW-directed nappe
stacking, folding and low-grade metamorphism in the fold belt are related to Pan-African incorporation of India into the
Gondwana supercontinent.
Received 11 April 2017; revised 17 May 2017; accepted 17 May 2017
Peninsular India contains an extensive archive of long-lived
Proterozoic tectonism recorded in several orogenic belts, including
the Eastern Ghats Belt (Fig. 1; Chetty & Santosh 2013). The region
also occupies a key place in the assembly of Gondwana (Dobmeier
& Raith 2003; Collins & Pisarevsky 2005; Chetty & Santosh 2013).
Although considerable advances have been made in establishing the
geochronological record of tectonism in many of the high-grade
metamorphic provinces in Peninsular India (e.g. Mezger & Cosca
1999; Simmat & Raith 2008; Collins et al. 2014), our knowledge
about the timing of tectonothermal events in adjacent lower grade
terranes and sedimentary basins remains sparse (e.g. Dobmeier et al.
2006; Basu & Bickford 2015; Collins et al. 2015). The Eastern
Ghats Belt along the east coast of India comprises a collage of faultbounded tectonic units, some of uncertain provenance, with discrete
geological histories marked by polyphase metamorphism and
deformation (e.g. Chetty & Murthy 1994; Mezger & Cosca 1999;
Dobmeier & Raith 2003). In addition to the structural complexity
within the belt, the contact between the Eastern Ghats Belt and
lower grade terranes and basins to the west is marked by a major
thrust (Vellikonda Thrust Front; Fig. 1), so it is not readily apparent
how, or if, structures in cover rocks relate to those in adjacent
basement terranes.
In the Nallamalai Fold Belt (Fig. 1c), west-verging folds and
thrusts in low-grade metasedimentary rocks of the Nallamalai
Group have been variously interpreted as the product of
Palaeoproterozoic to Mesoproterozoic deformation (Saha 2002),
Mesoproterozoic assembly of Rodinia at c. 990 – 900 Ma (Matin
2014), or latest Neoproterozoic to earliest Palaeozoic deformation
(Dobmeier et al. 2006) in the absence of radiometric dates.
Rubidium–strontium muscovite dating of discrete, low-grade shear
zones in the Krishna Province immediately to the east of the
Nallamalai Fold Belt yielded dates of 501 – 474 Ma (Dobmeier
et al. 2006). The dated shear zones have similar kinematics to shear
zones in the Nallamalai Fold Belt and were, therefore, inferred to be
of similar age. Thrust stacking at that time is consistent with
Palaeoproterozoic rocks of the Nallamalai Fold Belt having been
thrust over flat-lying Neoproterozoic sedimentary rocks of the
Kurnool Group (Fig. 1; Saha 2002; Dobmeier et al. 2006), but it is
not clear whether coeval deformation and metamorphism in the
Nallamalai Group cover rocks was pervasive or was confined to
discrete thrusts. To address this problem, we have dated
metamorphic monazite in a low-grade metasiltstone in the
Mangampeta barite mine (Fig. 1c) away from thrust faults in the
Nallamalai Fold Belt.
Regional geology
The eastern side of Peninsular India contains the Archaean Dharwar
and Bastar cratons, which are unconformably overlain by various
Proterozoic intracontinental sedimentary basins, of which two of the
largest are the Cuddapah and Chattisgarh basins (Fig. 1). To the east
of these cratons and basins lies the Eastern Ghats Belt, which
comprises two orogenic belts (Krishna Province and Eastern Ghats
Province) of contrasting ages (Figs 1 and 2). Overall, the Eastern
Ghats Belt records major episodes of tectonism at c. 1760, 1630 –
1600, 1060 – 900, 780 – 660 and 550 – 500 Ma, although not all
these episodes are common to every crustal fragment within the belt
(Dasgupta et al. 2013).
In the southern part of the Eastern Ghats Belt, the Krishna
Province is separated from the weakly deformed fill of the
Cuddapah Basin by the Nallamalai Fold Belt (Figs 1 and 2).
The Nallamalai Fold Belt is bounded on its western side by the
Maidukuru Thrust (or Rudavarum line; Meijerink et al. 1984; Saha
et al. 2010), which thrust the Nallamalai Group to the west over the
Palaeoproterozoic Chitravati and Papaghni Groups of the Cuddapah
Basin and the Kurnool Group to the north (Patranabis-Deb et al.
© 2017 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc.org.uk/permissions.
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S. Sheppard et al.
Fig. 1. Regional setting of the Nallamalai
Fold Belt. (a) Location of the Eastern
Ghats Belt and Cuddapah Basin; (b) main
tectonic units of eastern Peninsular India
showing the Archaean cratons, Proterozoic
basins (dark grey) and the elements of the
Eastern Ghats Belt (Eastern Ghats
Province and Krishna Province);
(c) relationship of the Nallamalai Fold
Belt to surrounding tectonic units and the
location of the Mangampeta barite mine
(grey star). Also shown are the Vinukonda
granite (Dobmeier et al. 2006) and the
Vellaturu granite (Saha 2002). Geology
modified from Henderson et al. (2014),
Collins et al. (2015) and Saha et al.
(2015).
2012). The Cuddapah Basin comprises predominantly fine-grained
siliciclastic rocks and calcarenite with minor dolostone, and mafic
and felsic volcanic rocks (Nagaraja Rao et al. 1987; Chakrabarti
et al. 2015) deposited between c. 1920 and c. 1860 Ma (Sheppard
et al. 2017). To the east, the fold belt is separated from the Eastern
Ghats Belt by the Vellikonda Thrust Front, which marks an abrupt
change in metamorphic grade from low-grade rocks in the
Nallamalai Fold Belt to medium- to high-grade rocks to the east
(Nagaraja Rao et al. 1987; Tripathy & Saha 2008).
The Nallamalai Group comprises sandstone, shale and dolostone,
as well as felsic tuff, debris-flow conglomerate with clasts of
intrabasinal carbonate, and slumped beds implying synsedimentary
faulting (Chaudhuri et al. 2002). The maximum depositional age of
the Nallamalai Group is not well constrained, relying on youngest
single detrital zircon dates of 1659 ± 22 Ma (Collins et al. 2015) or
1551 ± 12 Ma (Joy et al. 2015). The minimum age for the
Nallamalai Group is also poorly defined; the group is intruded by
the Vellaturu granite (Saha 2002), which was dated at c. 1575 Ma by
Crawford & Compston (1973) using the Rb–Sr method. The U–Pb–
Hf characteristics of detrital zircon grains in the Nallamalai Group
are a close match with potential source rocks in the Krishna
Province to the east and the group may have been deposited at the
same time as tectonism marking the c. 1600 Ma Krishna Orogeny
(Collins et al. 2015). For these reasons, Collins et al. (2015)
interpreted the group as a foreland basin deposit related to the
Krishna Orogeny as southern India and Enderby Land collided.
Metasedimentary rocks of the Nallamalai Group are deformed
about north-trending, inclined to overturned, tight to close folds and
cut by shallowly dipping faults (Saha 2002; Tripathy & Saha 2008;
Matin 2014). These westerly verging folds, associated slaty
cleavage and low-angle faults are interpreted to reflect top-to-thewest thrusting, which in the southern and central parts of the belt is
assigned to D1 (Tripathy & Saha 2008; Matin 2014). However, in
the northern part of the belt Saha (2002) identified an older layerparallel fabric with S–C structures and small recumbent to inclined
folds and, therefore, considered the main fold generation that
dominates the map patterns to belong to D2. Saha (2002) also noted
that the cleavage that is parallel to the axial surfaces of the F2 folds is
commonly a crenulation cleavage. In this paper we follow Saha’s
labelling of the structures and attribute the main folding to D2 rather
than D1. The D2 structures are overprinted by northeasterly trending,
close to open, upright folds (F3) and a steep crenulation cleavage
(S3; Narayanaswami 1966; Saha 2002; Tripathy & Saha 2008).
Matin (2014) interpreted all the folding and thrusting in the
Nallamalai Fold Belt as part of a single progressive deformation
event, although the age of deformation is not well understood. Saha
(2002) suggested that D3 is a much younger event, possibly younger
than deposition of the Kurnool Group. Saha (2002) suggested that
D1 and D2 predated the c. 1575 Ma Vellaturu granite (Fig. 1c)
because the granite contains xenoliths of folded and crenulated
schist with structures similar to those in the rocks intruded by the
pluton. Deformation and metamorphism were further considered to
correlate with higher grade fabrics in the Krishna Province that
predate the c. 1590 Ma Vinukonda granite (Fig. 1c; Dobmeier et al.
2006). The interpretation of the Nallamalai Group as a foreland
basin deposit means that it is entirely possible that some
deformation and metamorphism in the group was related to the
Krishna Orogeny. Discrete low-grade shear zones in the Vinukonda
granite are dated at 501 – 474 Ma and correlated with shear zones
having similar kinematics in the Nallamalai Fold Belt and
interpreted to reflect thrust stacking (Dobmeier et al. 2006).
Local geology and sample details
Local geology
Laminated black siltstone and shale from the Mangampeta barite
mine (Indian Bureau of Mines 2016) were sampled for monazite
geochronology. Barite mineralization at the mine comprises two
stratiform lenses: a northern lens, which hosts most of the resources,
and a smaller southern lens (Neelakantam 1987). The succession in
the mine area and surroundings is folded about NNW–SSEtrending, open folds (F2) with a steeply ENE-dipping cleavage (S2)
parallel to the axial surfaces of the folds. These folds plunge at
Monazite dating of Pan-African tectonism
Petrography
Sample IAS13-32 of shale and siltstone comprises angular to
subangular grains of quartz c. 0.5 mm in diameter in a silty matrix of
carbonate, quartz, chlorite, sericite and albite, with very finegrained carbonaceous material and disseminated pyrite. Thin
sections show a well-developed cleavage nearly orthogonal to
very thin bedding (Fig. 4a and b). The cleavage corresponds to the
S2 cleavage parallel to the axial surfaces of close to open, upright
folds throughout the Nallamalai Fold Belt (Saha 2002; Tripathy &
Saha 2008). There are no indications of a cleavage parallel to
bedding in our samples; that is, the main cleavage is not a
crenulation cleavage (Fig. 4c), which suggests that the part of the
mine sequence we have sampled is a low D1-strain zone. The S2
cleavage is accompanied by strain fringes around pyrite grains
(Fig. 4d and e).
Monazite crystals are up to 200 µm long and typically have their
long axis parallel or subparallel to bedding. They are full of
inclusions of angular to embayed quartz and matrix (Fig. 4f–j),
which is typical of metamorphic monazite in shales (Rasmussen
et al. 2007). The ragged margins of some crystals are aligned within
the cleavage (Fig. 4g and j), and weakly developed strain shadows
are also aligned with the cleavage (Fig. 4f, h and i). Collectively,
these observations imply that monazite grew largely before, but
partly during, development of the main S2 cleavage.
Analytical procedures
Analytical settings
Fig. 2. Simplified time–space plot of the Eastern Ghats Belt modified
from Dobmeier & Raith (2003). Magmatic and tectonic events without
numbers are from Dobmeier & Raith (2003). Numbers for dated events
obtained from the following: 1, Dobmeier et al. (2006); 2, Biswal et al.
(2007); 3, Chatterjee et al. (2016); 4, Collins et al. (2015); 5, Henderson
et al. (2014); 6, Simmat & Raith (2008); 7, Upadhyay (2008); 8,
Vadlamani et al. (2014); 9, French et al. (2008); 10, Sheppard et al.
(2017); 11, Saha (2002); 12, Crawford & Compston (1973).
10 – 15° to the NNW (Neelakantam 1987). The main fold set is
warped by gentle to open, ENE–WSW-trending upright folds (F3)
such that the northern lens outlines a doubly plunging, asymmetric
syncline (Neelakantam 1987).
Plugs, 2 – 3 mm in diameter, containing monazite suitable for
analysis were extracted from the polished thin sections with a
hollow-core rotary drill and mounted in 25 mm diameter epoxy
discs. Monazite reference materials (standards) were set into
separate mounts, which were cleaned and gold coated simultaneously with sample mounts before each analytical session. Standard
and sample mounts were loaded together into the sensitive highresolution ion microprobe (SHRIMP) for concurrent analysis.
Instrument setup followed protocols for small-spot, in situ analysis
of monazite described by Fletcher et al. (2010).
A primary beam of O2− ions was focused through a 70 μm Kohler
aperture (KA, which was changed to 50 μm in the middle of the
session owing to aperture burn-out) to produce an oval spot of
c. 10 μm, with a current intensity of 0.6 nA (0.4 nA after KA
change). The secondary ion system was focused through a 100 μm
collector slit onto an electron multiplier to produce mass peaks with
flat tops and a mass resolution (M/ΔM at 1% peak height) of c. 5400
in all sessions. The post-collector retardation lens was activated to
reduce stray ion arrivals. Monazite was analysed with a 13-peak run
table as outlined by Fletcher et al. (2010), which includes mass
stations for the estimation of La, Ce and Nd (REEPO+2 ), and Y
(YCeO+). Data were collected in sets of eight scans. Count times per
scan for background position 204.045, and Pb isotopes 204, 206,
207 and 208 were, respectively, 10, 10, 10, 30 and 10 s.
Reference materials
Sample details
The sample, IAS13-32, comes from the northeastern part of the
open pit mine (at 14°01′34.3″N, 79°19′16.2″E; datum WGS 1984)
immediately above the northern lens. At this locality the succession
overall dips gently to the SSW, being located on the northeastern
limb of the main syncline. The sampled section comprises gently
dipping black siltstone with a steeply dipping cleavage (S2) almost
perpendicular to bedding (Fig. 3). The black siltstone beds strike at
about 330° and dip 20 – 25° towards either 060° or 240°.
Reference materials comprised French (Pb/U age of 514 Ma, Pb/Th
age of 504 Ma, Pb/Pb age of 470 Ma and U concentration of
1000 ppm; Fletcher et al. 2010), 2908 (Pb/Pb age of 1796 Ma) and
2234 (Pb/U age of 1024 Ma) in a separate mount. French was the
primary reference for calibrating Pb/U and Pb/Th ratios and U
concentration. Secondary reference materials 2908 and 2234 were
used to monitor matrix effects, and 2908 was the main monitor of
mass fractionation. French yielded a 1σ external precision of 1.41%
for Pb/U (n = 14 of 15), and 2.37% for Pb/Th (n = 15 of 15). An
insignificant fractionation correction (0.05%) was applied to the
S. Sheppard et al.
Fig. 3. Location of sample site (star) in
the Mangampeta barite mine, looking
west. Bold dashed lines show the trend of
bedding; fine dashed lines show the trend
of the main upright (S2) cleavage. The
steeply dipping cleavage is almost
orthogonal to bedding at this location.
207
Pb/206Pb data, with no augmentation of sample precision
required by the reproducibility of 207Pb/206Pb for the reference
materials. Errors in single analyses are 1σ, whereas errors for pooled
data are 95% confidence limits.
Results
Monazite grains in sample IAS13-32 are mostly >50 μm in
diameter, but all the grains contain abundant inclusions commonly
resulting in high 204Pb. Twenty-six analyses were obtained from 11
monazite grains (Table 1). Most analyses yielded high common-Pb
or large discordance, with only four acceptable analyses ( f206 <1%
and discordance <10%). Two of these analyses (1410F.1-1 and F.12) are from the core of a monazite grain, and have 207Pb/206Pb ages
of 1857 ± 3 Ma and 1825 ± 7 Ma respectively. This monazite core is
characterized by high U and Th concentrations (average >3000 ppm
and >68 000 ppm, respectively), with Th/U values of c. 22, and is
interpreted to be an igneous grain of detrital origin. The two dates
from the core are within error of a c. 1850 Ma population identified
in detrital zircon from the Nallamalai Group by Collins et al. (2015)
and Joy et al. (2015).
Apart from the two spots on the monazite core, all other analyses
yielded much younger dates. The two low common-Pb and
concordant analyses (1410D.2-2, D.2-3) gave a weighted mean
206
Pb/238U age of 523 ± 12 Ma (MSWD = 0.22). Applying a less
restrictive filter, the best six analyses (within 1σ of concordia and
f206 <3%) record a weighted mean 206Pb/238U date of 531 ± 7 Ma
(MSWD = 0.69, probability = 0.63) (Fig. 5). Calculation derived
from 208Pb/232Th ratios of the same six analyses yielded a weighted
mean date of 536 ± 11 Ma (MSWD = 0.59, probability = 0.71),
within uncertainty of the 206Pb/238U date. The more precise and
coherent 206Pb/238U age of 531 ± 7 Ma is taken as the best estimate
of timing of monazite growth. These analyses have an average of
717 ppm U and 2965 ppm Th, and corresponding Th/U of c. 5,
which is more typical of secondary, non-igneous monazite (e.g.
Rasmussen & Muhling 2007).
Discussion
The monazite we have dated at 531 ± 7 Ma grew largely before
development of the main, steeply dipping cleavage (S2) related to
upright folds in the Nallamalai Group at the Mangampeta mine.
Although a bedding-parallel tectonic fabric (S1) is not present in our
dated sample, the monazite crystals are typically elongate parallel to
bedding and may have grown during D1 in a low-strain zone.
Therefore, we interpret our date of 531 ± 7 Ma as the age of very
low-grade metamorphism and thrusting in the Nallamalai Group.
The margins of some monazite crystals are aligned with the S2
cleavage, but these parts of the crystals were not large enough to be
dated. Our date is within uncertainty of the lower intercepts of
discordia for two U–Pb zircon dates (c. 550 and 500 – 400 Ma) from
tuffs in the Cuddapah Basin about 125 km to the NW, interpreted to
reflect Pb loss from Palaeoproterozoic zircon grains during a
thermal event (Sheppard et al. 2017), giving credence to our
interpretation that our monazite date from the Nallamalai Group
indicates the timing of regional metamorphism. This interpretation
is consistent with previous studies demonstrating monazite growth
during low-grade metamorphism in fold-and-thrust belts
(Rasmussen et al. 2005; Rasmussen & Muhling 2007).
Our new monazite date indicates that low-grade metamorphism in
the Nallamalai Group is likely to be coeval with intense tectonism in
the Eastern Ghats Belt reflecting collisional events during assembly
of Gondwana. The date of 531 ± 7 Ma provides a maximum age for
the S2 cleavage and the D2 event in the Nallamalai Fold Belt
(Fig. 6). The main S2 cleavage in the dated sample is not a
crenulation cleavage, indicating that an S1 fabric is absent.
West-verging folds (F2) and thrusts in the Nallamalai Fold Belt
have been interpreted as the product of Palaeoproterozoic or
Mesoproterozoic tectonism (Saha 2002; Saha & Chakraborty 2003;
Matin 2014) related to deformation and medium- to high-grade
metamorphism in the Krishna Province to the east at c. 1600 Ma
(Saha & Chakraborty 2003; Dobmeier et al. 2006). However, our
new data indicate that the D2 event in the Nallamalai Fold Belt is no
older than Pan-African and could not have formed during the
Krishna Orogeny. The age of D1 is not constrained as there is no S1
fabric in our sample (Fig. 6). The D1 event could be related to
Mesoproterozoic tectonism as suggested by Saha (2002) from work
in the northern part of the Nallamalai Fold Belt. Alternatively, D1
and D2 may be part of a progressive deformation event (Matin
2014), in which case D1 would also be Pan-African in age. The latter
possibility is consistent with the similar kinematics recorded by
both events.
Collins et al. (2015) showed that the U–Pb–Hf characteristics of
detrital zircon grains in the Nallamalai Group are a close match with
potential source rocks in the Krishna Province and, therefore, they
interpreted the Nallamalai Group as a foreland basin to the Krishna
Orogeny. If this interpretation is correct, then some thickening and
thrust stacking of the group is to be expected at this time because
foreland basins are intimately linked to thrust belts (e.g. DeCelles &
Giles 1996; DeCelles 2012). Nevertheless, there is no evidence for
Mesoproterozoic tectonism in our sample from the southern part of
the Nallamalai Group, and the textures of the dated monazite
crystals presented here suggest a Pan-African age for thrusting and
Monazite dating of Pan-African tectonism
Fig. 4. Textures in sample IAS13-32. (a) Transmitted light image of carbonaceous slate from the Mangampeta barite mine comprising interlaminated beds
of shale and siltstone preserving a cleavage (S2) that is near-perpendicular to bedding (S0). (b) Transmitted light image of slate (see inset (b) in (a))
showing bedding (S0) and a cleavage (S2). (c) Transmitted light close-up image (see inset in (b)) showing cleavage (S2) defined by a strong alignment of
quartz grains and mica crystals. (d) Transmitted light image of slate (inset (d) in (a)). (e) Crossed-polarized light image of strain fringes developed around
pyrite grains in (d). The quartz–chlorite fibres are oriented parallel to the cleavage (S2). (f ) Transmitted light image of metamorphic monazite crystal (mon)
surrounded by strain shadows. (g) Reflected light image of inclusion-rich monazite (mon) in (f ) with monazite fringes aligned in the cleavage (see arrows).
Two oval SHRIMP pits are shown with their corresponding U–Pb date (listed with 1σ analytical errors). (h) Transmitted light image of monazite crystal
(mon). (i) Transmitted light close-up of monazite (mon) crystal in (h) showing metamorphic monazite in the strain fringe (see arrow). ( j) Reflected light
image of inclusion-rich monazite (mon) in (i) showing monazite fringe aligned with S2 fabric (see arrow). Four oval SHRIMP pits are shown with their
corresponding U–Pb date (listed with 1σ analytical errors).
low-grade metamorphism. However, more radiometric dates will be
needed to confirm or refute the presence of structures and
metamorphic assemblages in the group related to the c. 1600 Ma
Krishna Orogeny. It is possible that thrusting took place in the
Nallamalai Group during the Krishna Orogeny, but that the
metamorphic grade was too low for new monazite growth.
The monazite age of 531 ± 7 Ma for low-grade metamorphism in
the Mangampeta mine is somewhat older than Rb–Sr muscovite
dates of 501 – 474 Ma for discrete, low-grade shear zones in the
Nellore Schist Belt immediately to the east of the Nallamalai Fold
Belt about 220 km NNE of Mangampeta (Dobmeier et al. 2006).
However, the monazite date is within uncertainty of U–Th–Pb dates
of 530 – 470 Ma for monazite (Simmat & Raith 2008) and
40
Ar–39Ar dates of c. 515 Ma and c. 550 – 500 Ma for pseudotachylite and biotite (Crowe et al. 2001; Lisker & Fachmann 2001)
from mylonite zones in the Eastern Ghats Belt.
Table 1. SHRIMP U-Pb data for monazite from sample IAS13-32
Analysis
number
U
(ppm)
Th
(ppm)
Th/
U
f206
(%)
f208
(%)
206
207
Pb*/206Pb*
±
1 207
206
Pb*/238U
±
207
Pb*/235U
±
208
Pb*/232Th
±
Pb*/238U age
(Ma)
±
(Ma)
208
Pb*/232Th age
(Ma)
±
(Ma)
Disc.
(%)
0.3209
0.2960
0.0052
0.0048
5.024
4.555
0.083
0.077
0.0936
0.0875
0.0023
0.0022
18571
18251
26
24
1808
1695
42
42
3
8
0.0849
0.0838
0.0854
0.0862
0.0872
0.0866
0.0014
0.0017
0.0014
0.0015
0.0014
0.0013
0.673
0.670
0.698
0.717
0.729
0.729
0.021
0.021
0.041
0.026
0.029
0.030
0.0268
0.0262
0.0272
0.0265
0.0278
0.0273
0.0006
0.0006
0.0006
0.0007
0.0011
0.0006
525
519
529
533
539
536
8
10
8
9
8
8
522
543
535
529
554
545
12
12
13
13
22
13
−3
2
8
13
14
16
0.0825
0.0874
0.0810
0.0888
0.0771
0.0819
0.0877
0.0850
0.0883
0.0900
0.0889
0.0788
0.0884
0.0873
0.0862
0.0857
0.0814
0.0840
0.0012
0.0015
0.0015
0.0016
0.0012
0.0015
0.0013
0.0014
0.0016
0.0016
0.0014
0.0016
0.0016
0.0019
0.0017
0.0016
0.0034
0.0018
0.758
0.855
0.752
0.831
0.732
0.761
0.726
0.782
1.012
0.889
0.880
0.755
0.851
0.787
1.059
0.686
0.912
1.097
0.031
0.052
0.038
0.054
0.044
0.054
0.110
0.060
0.106
0.116
0.082
0.074
0.148
0.199
0.135
0.072
0.264
0.146
0.0259
0.0287
0.0246
0.0312
0.0234
0.0256
0.0287
0.0270
0.0283
0.0286
0.0280
0.0250
0.0286
0.0276
0.0257
0.0286
0.0241
0.0297
0.0007
0.0008
0.0006
0.0011
0.0007
0.0007
0.0007
0.0007
0.0007
0.0007
0.0007
0.0008
0.0007
0.0010
0.0006
0.0011
0.0006
0.0013
511
540
502
548
479
507
542
526
545
555
549
489
546
540
533
530
504
520
7
9
9
10
7
9
8
8
9
9
9
10
10
11
10
10
20
11
517
572
491
620
467
510
572
538
564
569
557
499
569
550
514
570
481
592
13
16
12
22
14
14
13
13
13
14
13
16
14
19
12
21
11
25
38
43
41
37
46
40
10
37
57
43
44
47
41
31
62
0
59
66
Pb*/ Pb* age.
206
208
204
206
208
206
Pb* indicates radiogenic Pb. f206 and f208 are the proportions of common Pb in Pb and Pb, respectively, determined using the measured Pb/ Pb and Pb/ Pb and common Pb compositions from the Stacey & Kramers (1975) model at the approximate age of
207
206
206
238
207
206
the sample. All Pb isotope data have been corrected for common Pb. Disc. is apparent discordance, as 100(t[ Pb*/ Pb*] − t[ Pb*/ U])/t[ Pb*/ Pb*]. The analysis number comprises the following: 1410 refers to the SHRIMP mount, the letters (A, B, C, etc.)
refer to the plug, the first number refer to the grain, and the second number to the analysis in that grain.
S. Sheppard et al.
Detrital grain
1410F.1-1
3683
82810
22
0.06
0.02
0.1136
0.0002
1410F.1-2
2622
54022
21
0.11
0.04
0.1116
0.0004
Main group
1410D.2-2
625
4353
7
0.77
0.89
0.0575
0.0015
1410D.2-3
618
3621
6
2.82
3.55
0.0580
0.0014
1410K.1-2
433
2289
5
0.58
0.55
0.0593
0.0034
1410F.1-3
822
3979
5
2.04
2.84
0.0603
0.0019
1410G.1-1
1088
584
1
2.57 24.15
0.0606
0.0022
1410D.2-4
615
3437
6
2.00
2.36
0.0610
0.0023
Rejected owing to high common Pb (f206 >3%) and/or large discordance (>20%)
1410D.2-1
722
1344
2
1.54
5.33
0.0666
0.0026
1410L.1-1
522
976
2
3.54 11.28
0.0709
0.0042
1410G.1-2
1267
2569
2
3.84 11.75
0.0673
0.0032
1410K.1-1
685
758
1
6.78 28.29
0.0678
0.0043
1410I.1-2
1236
2246
2
6.78 21.09
0.0688
0.0040
1410K.1-3
643
2261
4
7.06 12.55
0.0673
0.0046
1410D.1-3
720
5435
8
7.09
6.11
0.0600
0.0092
1410E.1-1
507
2616
5
8.79 11.03
0.0667
0.0051
1410J.1-2
473
8062
17
9.94
4.08
0.0831
0.0087
1410D.2-5
694
5258
8
10.08
8.87
0.0716
0.0093
1410I.1-1
569
4682
8
11.06
9.20
0.0718
0.0066
1410E.1-2
573
1409
2
11.12 25.24
0.0695
0.0067
1410J.1-1
568
5726
10
11.60
7.81
0.0698
0.0121
1410A.1-4
1017
5454
5
11.96 14.16
0.0654
0.0166
1410L.1-2
449
8531
19
11.98
4.76
0.0891
0.0113
1410B.1-1
750
1217
2
12.11 35.15
0.0580
0.0061
1410D.1-2
399
14,011
35
15.71
3.62
0.0813
0.0235
1410E.1-3
704
1636
2
16.13 32.84
0.0947
0.0126
206
Monazite dating of Pan-African tectonism
Funding This work is the outcome of Australian–Indian collaborative project
(AISRF scheme) ST040010 (Australia) and DST/INT/AUS/P-32/2010 (India).
Scientific editing by Karel Schulmann
References
Fig. 5. Wetherill concordia plot of U–Pb analyses of monazite from
sample IAS13-32. Error ellipses are 1σ.
Fig. 6. Schematic cross-section through the southern Nallamalai Fold Belt
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structures to the dated monazite. Scale bars are approximate. Lower panel
is modified from Matin (2014) and section through the Mangampeta mine
is modified from Neelakantam (1987).
Conclusions
Monazite from the Mangampeta barite mine in the southern part of
the Nallamalai Fold Belt dated at 531 ± 7 Ma grew largely before,
but partly synchronous with, the slaty cleavage that formed during
folding and thrusting (D2). The date provides a maximum age for
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is not a product of the Krishna Orogeny. The monazite date
presented here shows that pervasive deformation in the Nallamalai
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Dobmeier et al. 2006), which is consistent with the observation that
rocks of the Nallamalai Group were thrust over flat-lying
Neoproterozoic sedimentary rocks of the Kurnool Group (Saha
2002; Dobmeier et al. 2006; Collins et al. 2015).
Acknowledgements
We acknowledge the Curtin University SHRIMP
facilities, and the Western Australian Microscopy Centre: each is supported by a
university–government
consortium
and
the
Australian
Research
Council. D. Dunkley is thanked for technical assistance. We thank
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