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. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics 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 and Mangampeta barite mine showing the relationship of the large-scale 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 folding and thrusting and indicates that deformation in the fold belt is not a product of the Krishna Orogeny. The monazite date presented here shows that pervasive deformation in the Nallamalai Fold Belt is largely the result of Pan-African tectonism (see 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 I. Fitzsimons for his comments on an earlier version of paper and the editor, K. Schulmann, for helping to improve the paper. Basu, A. & Bickford, M.E. 2015. An alternate perspective on the opening and closing of the intracratonic Purana basins in peninsular India. Journal of the Geological Society of India, 85, 5–25, https://doi.org/10.1007/s12594-0150190-y Biswal, T.K., De Waele, B. & Ahuja, H. 2007. Timing and dynamics of the juxtaposition of the Eastern Ghats Mobile Belt against the Bhandara Craton, India: A structural and zircon U–Pb SHRIMP study of the fold–thrust belt and associated nepheline syenite plutons. Tectonics, 26, TC4006, https://doi.org/ 10.1029/2006TC002005 Chakrabarti, G., Eriksson, P.G. & Shome, D. 2015. Sedimentation in the Papaghni Group of rocks in the Papaghni sub-basin of the Proterozoic Cuddapah Basin, India. In: Mazumder, R. & Eriksson, P.G. (eds) Precambrian Basins of India: Stratigraphic and Tectonic Context. Geological Society, London, Memoirs, 43, 255–267, https://doi.org/10. 1144/M43.17 Chatterjee, C., Vadlamani, R. & Kaptan, O.P. 2016. Paleoproterozoic Cordilleran-style accretion along the south eastern margin of the eastern Dharwar craton: Evidence from the Vinjamuru arc terrane of the Krishna orogen, India. Lithos, 263, 122–142, https://doi.org/10.1016/j.lithos.2016.08. 015 Chaudhuri, A.K., Saha, D., Deb, G.K., Patranabis Deb, S., Kanti Mukherjee, M. & Ghosh, G. 2002. The Purana basins of southern cratonic province of India – a case for Mesoproterozoic fossil rifts. Gondwana Research, 5, 23–33, https:// doi.org/10.1016/S1342-937X(05)70884-4 Chetty, T.R.K. & Murthy, D.S.N. 1994. Collision tectonics in the late Precambrian Eastern Ghats Mobile Belt: mesoscopic to satellite-scale structural observations. Terra Nova, 6, 72–81, https://doi.org/10.1111/j. 1365-3121.1994.tb00635.x Chetty, T.R.K. & Santosh, M. 2013. Proterozoic orogens in southern Peninsular India: Contiguities and complexities. Journal of Asian Earth Sciences, 78, 39–53, https://doi.org/10.1016/j.jseaes.2013.02.021 Collins, A.S. & Pisarevsky, S.A. 2005. Amalgamating eastern Gondwana: The evolution of the Circum-Indian Orogens. Earth-Science Reviews, 71, 229–270, https://doi.org/10.1016/j.earscirev.2005.02.004 Collins, A.S., Clark, C. & Plavsa, D. 2014. Peninsular India in Gondwana: The tectonothermal evolution of the Southern Granulite Terrain and its Gondwanan counterparts. Gondwana Research, 25, 190–203, https://doi. org/10.1016/j.gr.2013.01.002 Collins, A.S., Patranabis-Deb, S. et al. 2015. Detrital mineral age, radiogenic isotopic stratigraphy and tectonic significance of the Cuddapah Basin, India. Gondwana Research, 28, 1294–1309, https://doi.org/10.1016/j.gr.2014.10. 013 Crawford, A.R. & Compston, W. 1973. The age of the Cuddapah and Kurnool systems, southern India. Journal of the Geological Society of Australia, 19, 453–464, https://doi.org/10.1080/00167617308728813 Crowe, W.A., Cosca, M.A. & Harris, L.B. 2001. 40Ar/39Ar geochronolgy and Neoproterozoic tectonics along the northern margin of the Eastern Ghats Belt in north Orissa, India. Precambrian Research, 108, 237–266, https://doi.org/ 10.1016/S0301-9268(01)00132-2 Dasgupta, S., Bose, S. & Das, K. 2013. Tectonic evolution of the Eastern Ghats Belt, India. Precambrian Research, 227, 247–258, https://doi.org/10.1016/j. precamres.2012.04.005 DeCelles, P.G. 2012. Foreland basin systems revisited: Variations in response to tectonic settings. In: Busby, C. & Azor, A. (eds) Tectonics of Sedimentary Basins: Recent Advances. Wiley, Chichester, 405–426. DeCelles, P.G. & Giles, K.A. 1996. Foreland basin systems. Basin Research, 8, 105–123, https://doi.org/10.1046/j.1365-2117.1996.01491.x Dobmeier, C.J. & Raith, M.M. 2003. Crustal architecture and evolution of the Eastern Ghats Belt and adjacent regions of India. In: Yoshida, M., Windley, B. E. & Dasgupta, S. (eds) Proterozoic East Gondwana: Supercontinent Assembly and Breakup. Geological Society, London, Special Publications, 206, 145–168, https://doi.org/10.1144/gsl.sp.2003.206.01.09 Dobmeier, C., Lütke, S., Hammerschmidt, K. & Mezger, K. 2006. Emplacement and deformation of the Vinukonda meta-granite (Eastern Ghats, India) – Implications for the geological evolution of peninsular India and for Rodinia reconstructions. Precambrian Research, 146, 165–178, https://doi.org/10. 1016/j.precamres.2006.01.011 Fletcher, I.R., McNaughton, N.J. & Davis, W.J. and Rasmussen, B. 2010. Matrix effects and calibration limitations in ion probe U–Pb and Th–Pb dating of monazite. Chemical Geology, 270, 31–44, https://doi.org/10.1016/j.chemgeo. 2009.11.003 French, J.E., Heaman, L.M., Chacko, T. & Srivastava, R.K. 2008. 1891–1883 Ma Southern Bastar -Cuddapah mafic igneous events, India: A newly recognized large igneous province. Precambrian Research, 160, 308–322, https://doi.org/10.1016/j.precamres.2007.08.005 Henderson, B., Collins, A.S., Payne, J., Forbes, C. & Saha, D. 2014. Geologically constraining India in Columbia: The age, isotopic provenance and S. Sheppard et al. geochemistry of the protoliths of the Ongole Domain, Southern Eastern Ghats, India. Gondwana Research, 26, 888–906, https://doi.org/10.1016/j.gr.2013. 09.002 Indian Bureau of Mines 2016. Barytes. In: Indian Minerals Yearbook 2015 – Part III: Mineral Reviews, 54th edn. Ministry of Mines, Government of India, Nagpur, 3-1–3-11 (Advance Release), http://ibm.nic.in/writereaddata/files/ 09122016101533IMYB2015_Barytes_12092016_Adv.pdf Joy, S., Jelsma, H., Tappe, S. & Armstrong, R. 2015. SHRIMP U–Pb zircon provenance of the Sullavai Group of Pranhita–Godavari Basin and Bairenkonda Quartzite of Cuddapah Basin, with implications for the Southern Indian Proterozoic tectonic architecture. Journal of Asian Earth Sciences, 111, 827–839, https://doi.org/10.1016/j.jseaes.2015.07.023 Lisker, F. & Fachmann, S. 2001. Phanerozoic history of the Mahanadi region, India. Journal of Geophysical Research: Solid Earth, 106, 22027–22050, https://doi.org/10.1029/2001JB000295 Matin, A. 2014. Tectonics in the Cuddapah fold–thrust belt in the Indian shield, Andhra Pradesh, India and its implication on the crustal amalgamation of India and Rayner craton of Antarctica during Neoproterozoic orogenesis. International Journal of Earth Sciences, 103, 7–22, https://doi.org/10.1007/s00531-013-0957-6 Meijerink, A.M.J., Rao, D.P. & Rupke, J. 1984. Stratigraphic and structural development of the Precambrian Cuddapah basin, S.E. India. Precambrian Research, 26, 57–104, https://doi.org/10.1016/0301-9268(84)90017-2 Mezger, K. & Cosca, M.A. 1999. The thermal history of the Eastern Ghats Belt (India) as revealed by U–Pb and 40Ar/39Ar dating of metamorphic and magmatic minerals: implications for the SWEAT correlation. Precambrian Research, 94, 251–271, https://doi.org/10.1016/S0301-9268(98)00118-1 Nagaraja Rao, B.K., Rajurkar, S.T., Ramlingaswamy, G. & Ravindra Babu, B. 1987. Stratigraphy, structure and evolution of the Cuddapah basin. In: Radhakrishna, B.P. (ed.) Purana Basins of Peninsular India. Geological Society of India, Memoir, 6, 33–86. Narayanaswami, S. 1966. Tectonics of the Cuddapah Basin. Journal of the Geological Society of India, 7, 33–50. Neelakantam, S. 1987. Mangampeta Barytes Deposit, Cuddapah District, Andhra Pradesh. In: Radhakrishna, B.P. (ed.) Purana Basins of Peninsular India. Geological Society of India, Memoir, 6, 429–458. Patranabis-Deb, S., Saha, D. & Tripathy, V. 2012. Basin stratigraphy, sea-level fluctuations and their global tectonic connections – evidence from the Proterozoic Cuddapah Basin. Geological Journal, 47, 263–283, https://doi. org/10.1002/gj.1347 Rasmussen, B. & Muhling, J.R. 2007. Monazite begets monazite: evidence for dissolution of detrital monazite and reprecipitation of syntectonic monazite during low-grade regional metamorphism. Contributions to Mineralogy and Petrology, 154, 675–689, https://doi.org/10.1007/s00410-007-0216-6 Rasmussen, B., Fletcher, I.R. & Sheppard, S. 2005. Isotopic dating of the migration of a low-grade metamorphic front during orogenesis. Geology, 33, 773–776, https://doi.org/10.1130/g21666.1 Rasmussen, B., Fletcher, I. R. & Muhling, J.R. 2007. In situ U–Pb dating and element mapping of three generations of monazite: Unravelling cryptic tectonothermal events in low-grade terranes. Geochimica et Cosmochimica Acta, 71, 670–690, https://doi.org/10.1016/j.gca.2006.10.020 Saha, D. 2002. Multi-stage deformation in the Nallamalai Fold Belt, Cuddapah Basin, South India – implications for Mesoproterozoic tectonism along southeastern margin of India. Gondwana Research, 5, 701–719, https://doi. org/10.1016/S1342-937X(05)70639-0 Saha, D. & Chakraborty, S. 2003. Deformation pattern in the Kurnool and Nallarnalai Groups in the northeastern part (Palnad area) of the Cuddapah Basin, South India and its implication on Rodinia/Gondwana tectonics. Gondwana Research, 6, 573–583, https://doi.org/10.1016/S1342-937X(05) 71008-X Saha, D., Chakraborti, S. & Tripathy, V. 2010. Intracontinental thrusts and inclined transpression along eastern margin of the East Dharwar craton, India. Journal of the Geological Society of India, 75, 323–337, https://doi.org/10. 1007/s12594-010-0019-7 Saha, D., Sain, A., Nandi, P., Mazumder, R. & Kar, R. 2015. Tectonostratigraphic evolution of the Nellore schist belt, southern India, since the Neoarchaean. In: Mazumder, R. & Eriksson, P.G. (eds) Precambrian Basins of India: Stratigraphic and Tectonic Context. Geological Society, London, Memoirs, 43, 269–282, https://doi.org/10.1144/m43.18 Sheppard, S., Rasmussen, B. et al. 2017. Sedimentation and magmatism in the Paleoproterozoic Cuddapah Basin, India: Consequences of lithospheric extension. Gondwana Research, 48, 153–163. Simmat, R. & Raith, M.M. 2008. U–Th–Pb monazite geochronometry of the Eastern Ghats Belt, India: Timing and spatial disposition of polymetamorphism. Precambrian Research, 162, 16–39, https://doi.org/10.1016/ j.precamres.2007.07.016 Stacey, J.S. & Kramers, J.D. 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters, 26, 207–221. Tripathy, V. & Saha, D. 2008. Structure and low grade metamorphism of the east–central part of the Proterozoic Nallamalai Fold Belt, south India – thrust stacking and discontinuous metamorphic gradients along eastern margin of East Dharwar Craton. Indian Journal of Geology, 80, 173–188. Upadhyay, D. 2008. Alkaline magmatism along the southeastern margin of the Indian shield: Implications for regional geodynamics and constraints on craton–Eastern Ghats Belt suturing. Precambrian Research, 162, 59–69, https://doi.org/10.1016/j.precamres.2007.07.012 Vadlamani, R., Hashmi, S., Chatterjee, C., Ji, W.-Q. & Wu, F.-Y. 2014. Initiation of the intra-cratonic Cuddapah basin: Evidence from Paleoproterozoic (1995 Ma) anorogenic porphyritic granite in Eastern Dharwar Craton basement. Journal of Asian Earth Sciences, 79, 235–245, https://doi.org/10. 1016/j.jseaes.2013.09.035