Decoding evolutionary history of provenance from beach placer monazites: A case study from Kanyakumari coast, southwest India

Chemical Geology 427 (2016) 83–97

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Decoding evolutionary history of provenance from beach placer monazites: A case study from Kanyakumari coast, southwest India C. Perumalsamy, Subhadip Bhadra, S. Balakrishnan ⁎ Department of Earth Sciences, Pondicherry University, Puducherry 605014, India

a r t i c l e

i n f o

Article history: Received 20 May 2015 Received in revised form 11 February 2016 Accepted 16 February 2016 Available online 18 February 2016 Keywords: Beach placer monazite EPMA chemical age TDM model Nd age Southern Granulite Terrane Provenance proxy

a b s t r a c t Mineral chemical and geochronological studies of beach placer monazites from Kanyakumari coast, SW India were carried out to understand growth history of monazite and its bearing on tectono-thermal evolution of the source rock. In chemically zoned monazites lowest backscattered electron (BSE) response domains (dark gray) comprising the thick core region are mantled by highest BSE response rim domains (light gray region). Dark gray BSE domains are enriched in LREEs (La and Ce), U and Y, and depleted in Th and Pb compared to the light gray BSE domains. Chemical variability between these two domains can be linked with dominantly huttonitic substitution. Two U–Th–total Pb chemical age clusters between 555–578 Ma and 508–536 Ma were obtained respectively for low Th/U monazite cores and high Th/U monazite rims. 147 Sm/144Nd ratios in the analyzed monazites vary from 0.0733 to 0.1039 with an average value of 0.0902. High negative εNd (t = 0) values in the range of −26.5 to −30.8 indicate derivation of monazites from light LREE enriched igneous and metamorphic rocks. TCHUR and TDM model ages vary from 1687 Ma to 2364 Ma and 2035 Ma to 2702 Ma respectively. Average TDM age of 2375 (+ 96/− 44) Ma for placer monazites fits with ~2.4 Ga crustal accretionary episode in Southern Granulite Terrane (SGT). EPMA Sm–Nd ratios of beach placer monazites (~0.153) are similar with the monazites in granites and granodiorites (0.150) of Nagercoil (NG) and Trivandrum Block (TB) of SGT. A comparison with available Th–U–total Pb EPMA monazite ages from various tectonic units within SGT suggests that growth history and crystallization age of monazites also correlate well with the PanAfrican granulite facies metamorphism (570 Ma) and post-peak evolution (535 Ma) of NG and TB. The results obtained in this study augment the growing evidences that beach placer monazites can be used as a proxy for provenance study. A corollary of the study further confirms similar geological history of NG and TB since 2.1 Ga. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Provenances of modern and older clastic sediments are discriminated by comparing mineral assemblage, major and trace elemental abundances, εNd values and Nd model ages (O'Nions et al., 1983; Li and McCulloch, 1996; Jacobsen, 1988; Richards et al., 2005; Chakraborty et al., 2012). However, each of the above methods has limitations in definitively establishing the provenance, for e.g. Nd model ages assume that there was no fractionation of Sm/Nd ratios during deposition of clastic sediments. However, rare earth element and Sm–Nd isotope studies on clastic sediments have found that Sm/Nd ratios in sediments are fractionated relative to their source during transport, deposition and diagenesis (McLennan et al., 1990, 1993; McCulloch and Bennett, 1994; McDaniel et al., 1994; Gruau et al., 1996; Rainbird et al., 1997). The other potential approach is to compare geochemical, isotope and age characteristics of individual heavy minerals with that of the provenance. A number studies have compared U–Pb ages on detrital

⁎ Corresponding author.

http://dx.doi.org/10.1016/j.chemgeo.2016.02.018 0009-2541/© 2016 Elsevier B.V. All rights reserved.

zircons with potential sources to identify their provenance (e.g., McCulloch and Bennett, 1994; Lahtinen et al., 2002; Moecher and Samson, 2006). Although zircons can provide precise crystallization ages of protoliths, timing of metamorphism cannot be determined if they were abraded and rounded to varying degrees during transport (Cliff et al., 1991; McLennan, 2001; Lahtinen et al., 2002; Richards et al., 2005) or the grade of metamorphism is not high enough for zircon crystallization or overgrowth to occur. Hietpas et al. (2011) reported that in situ U–Pb isotope studies on detrital zircons from Appalachian foreland basin sandstones could not identify several younger Palaeozoic tectono-thermal events while detrital monazites unambiguously recorded these events. U–Th–total Pb ages on monazites of clastic sediments can provide information about the timing of monazite crystallization and growth which can be matched with tectono-thermal histories of potential source regions (Maas and McCulloch, 1991; Parrish, 1990; Chen et al., 2006; Iizuka et al., 2010; Hietpas et al., 2011). Whereas, timing of separation of protoliths from mantle can be determined using Sm–Nd isotopic studies on monazite, useful complementary information to identify the provenance. As monazites retain the Sm/Nd ratio of the protolith,

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they truly preserve Nd isotope evolutionary history of provenance better than bulk sediments (Frost and Winston, 1987; Cliff et al., 1991; Ross et al., 1991; McFarlane and McCulloch, 2007; Garcon et al., 2011, 2013). Objective of this coupled study of U–Th–total Pb ages and Sm–Nd isotope system on monazites from beach placers is to test suitability of placer monazites as proxy for crustal and tectono-metamorphic evolution of the provenance. Beach placers commonly host economic concentration of minerals with high density such as monazite, rutile, zircon, ilmenite and garnet (Rao and Misra, 2009; Garcon et al., 2011). Similar to river sediments that are widely used as proxy for isotopic composition and temporal evolution of the continental crust (Gaillardet et al., 1995; Allègre et al., 1996; Goldstein and Jacobsen, 1988; Millot et al., 2004; Kamber et al., 2005), monazite-rich beach sands seem to have immense potential for provenance study. Garcon et al. (2011) documented that monazites, with abundance as low as 0.5 wt.%, dominantly control the Nd isotopic signature in beach placer and are reliable proxy for Nd isotopic composition of the provenance. Beaches along southern and southwestern coast of India are enriched in monazites as well as other heavy minerals. This is due to the presence of suitable lithologies in the provenance, coastal geomorphology, climate, physical and chemical environment, hydro-dynamic regime and sea level fluctuations (Mallik et al., 1987; Chandrasekharan and Murugan, 2001; Kurian et al., 2001; Mohanty et al., 2003a,b; Jayaraju, 2004; Acharya et al., 2009). The hinterland region comprises a collage of Paleoproterozoic (TDM N 2 Ga) granulite terranes (Fig. 1a) with lithologic ensemble such as, orthopyroxene bearing granulites (commonly referred as charnockites), high-grade metasediments, and igneous rocks of syenitic and granitic to intermediate composition. The tectonothermal evolutionary history of the hinterland region is characterized by pervasive Pan-African reworking (650–500 Ma) that coincided with east and west Gondwanaland assembly (Harris et al., 1994; De Wit et al., 1998; Yoshida et al., 1999; Santosh et al., 2005a,b, 2006a,b, 2009b; Kröner et al., 2012; Taylor et al., 2014, 2015). The beach placers of southwest Indian coast provide a natural laboratory to test the suitability of placer monazites as proxy for crustal evolution of the provenance because, a) monazites along with other heavy minerals (ilmenite, sillimanite, garnet, zircon) are derived from granulitic rocks occupying highlands within a short distance of ~ 40 km, b) monazites are least weathered and c) ages of monazites in various rocks and tectono-thermal history of the source region have been well established (Braun et al., 1998; Braun and Bröcker, 2004; Cenki et al., 2004; Collins et al., 2007; Rajesh et al., 2011; Ravindra Kumar and Sreejith, 2010; Santosh et al., 2003, 2004, 2005a,b, 2006a,b, 2009a; Kröner et al., 2012, 2015; Taylor et al., 2014).

2.1. General geology

Fig. 1. a) General geological map of Southern Granulite Terrane (SGT) of India comprising different granulite blocks separated by major shear zones (after Valdiya, 2010). The box indicates the study area. b) Map of the study area showing sample locations (K stands for Kan), lithologies and major streams draining the hinterland of the SW coast line from Colachel to Kanyakumari. Chemistry and geochronology of monazites documented in this study are from the sample locations marked with star. Lithogical index is same for both figures. Acronyms — ACSZ: Achankovil shear zone, KSZ: Karur shear zone, MB: Madurai Block, MSZ: Moyar shear zone, MsB: Madras Block, NG: Nagercoil granulites, PCSZ: Palghat cauvery shear zone, TB: Trivandrum Block.

Tropical climate combined with abundant rainfall over the escarpment along the west coast of India (Western Ghats) facilitate rapid weathering. West flowing, short perennial rivers carry the sediments from the Western Ghats and debouch into the Arabian Sea. The sediments are sorted by waves and currents and heavy minerals concentrated particularly in bays and curvatures formed between the promontories of the southwestern coast (Mallik et al., 1987). Longshore currents, along the southwestern coast, flow southwards during the southwest monsoon (June–September), and northwards in the remaining part of the year with a net southerly longshore drift (Kunte et al., 2001). However, transport of sediments by longshore currents is limited by the presence of bays, headlands and curvatures. This is also supported by the presence of distinct beach placer mineral assemblages in different sectors of the western and southwestern coast. Ratnagiri coast of Maharashtra is dominated by ilmenite with minor hematite and magnetite (Nair, 2001). Northern sector of Kerala is

characterized by pyribole and sillimanite. Southern Kerala contains ilmenite and sillimanite (Mallik et al., 1987; Krishnan et al., 2001). Kanyakumari coast contains ilmenite, garnet and sillimanite (Chandrasekharan and Murugan, 2001). The variation of heavy mineral assemblages along the southwestern coast of India has been attributed to the types of rocks present in the catchment of the rivers. Consequently, Ravindra Kumar and Sreejith (2010) recognized four types of placer deposits showing distinct compositional variation and characteristic of southern Kerala, central Kerala and northern Kerala and Kanyakumari coasts. The hinterland region of the southwestern coast is well known as Southern Granulite Terrane (SGT) that exposes mid to lower parts of continental crust (Mohan and Jayananda, 1999) and consists of clinopyroxene and orthopyroxene bearing granulites, granitoid, gneisses, granites and syenites with intrusive pegmatite (Fig. 1a). The Achankovil shear zone (ACSZ, Fig. 1a) divides the SGT into a northern

2. Geological setting

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Madurai Block (MB), southern Trivandrum Block (TB) and Nagercoil granulites (NG). The Madurai Block is mostly covered with supracrustal sequences, which are metamorphosed up to ultra-high temperature granulite facies conditions in certain areas (Mohan and Jayananda, 1999; Bartlett et al., 1998; Rajesh and Santosh, 2004). Dominant lithologies in the TB include garnet–biotite gneisses (metapelites), garnet–biotite–sillimanite ± cordierite granulite (khondalite) and charnockites with numerous intrusions of pegmatite and granite and few syenite bodies (Santosh et al., 2003; Braun, 2006). Monazite is a common accessory mineral in meta-sedimentary and granitoid rocks of the Trivandrum Block (Santosh et al., 2006a,b; Kröner et al., 2012). The Nagercoil granulites consist of charnockite gneisses with calc-alkaline igneous affinity and metapelitic bands. Achankovil metasediments of ACSZ consist of supracrustal sequences of garnet–biotite gneiss, garnet–sillimanite cordierite gneiss, calc–silicates and quartzite (Bartlett et al., 1998; Taylor et al., 2015). 2.2. Geology of the study area Southwestern beach and sand dune deposits trend NW–SE between Nagercoil granulites and Lakshadweep Sea in Kanyakumari district (N 8°00′ to N.8°10′ and E 77°15′ to 77°30′), Tamil Nadu (Fig. 1b). North to south flowing Pazhayar, Valliyar and Narikkir streams are few tens of km in length and exclusively drain the Nagercoil granulites. Nagercoil granulites are tonalite to granodiorite in composition, and host N5 m wide bands of mafic granulites and b50 cm wide mafic dykes (Rajesh et al., 2011). K-feldspar, plagioclase, quartz, orthopyroxene, biotite and garnet constitute the major mineral phases and ilmenite, apatite, zircon and monazite are the dominant accessory minerals present in the Nagercoil granulites (Rajesh et al., 2011). Heavy minerals are concentrated as discrete patches between the light minerals along the beaches. Beach placers of southwestern coast contains around 3.5 to 4 wt.% monazite, which occurs as transparent,

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honey yellow to pale yellow colored and ellipsoidal to well rounded grains (Pandey and Chandrasekaran, 2010). 3. Analytical techniques 3.1. Sampling and heavy mineral separation The beach sediment samples were collected from 13 locations along a 40 km long traverse from Colachel to Kanyakumari at regular intervals (Fig. 1b) and from a depth of 30 cm using polyvinyl chloride (PVC) pipe of 7.5 cm diameter. In the laboratory, beach sediment samples were washed with water and dried in sunlight. About 2 kg of sample was sieved by hand, with sieve size fractions of 35 (500 μm), 80 (180 μm), and 120 (125 μm) ASTM mesh, for separating mineral fractions of different sizes. The fine fraction of minerals (b125 μm) was subjected to gravimetric separation using bromoform (2.89 g/cm3) to separate light and heavy minerals. Heavy minerals settled at the bottom were washed with acetone and dried in an oven at 70 °C. Further, the heavy minerals (about 10 g) were subjected to magnetic separation using a hand-magnet and Frantz® isodynamic separator (LB1) for separating ferromagnetic and paramagnetic minerals, respectively. Monazites along with leucoxene, pyroxene and garnet were found in 0.8 A magnetic fraction. Monazite grains that were devoid of visible inclusion and alteration were handpicked under a binocular microscope for SEM, EPMA and Sm–Nd isotopic studies. Out of 13 samples only 10 samples contained monazites. 3.2. SEM and EPMA analyses The monazite grains were mounted with epoxy in stainless steel sample holder of 2.4 cm diameter. The mounted grains were polished using diamond paste and carbon coated.

Table 1 Representative EPMA data of beach placer monazite from sample KAN 13, Kanyakumari coast, south west India. Sample

G1/1

G1/2

G1/3

G2/4

G3/2

G4/1

G4/2

G4//3

G4/4

Moacyr (Brazil) monazite standard (this study)

Position Oxides SiO2 CaO P2O5 Y2O3 La2O3 Ce2O3 Pr2O3 Nd2O3 SmO Nd2O3 Tho2 UO2 PbO Total Cations on the basis of 4 oxygen Si Ca P Y La Ce Pr Nd Sm Gd Th U Pb Age (Ma) Uncertainty (2σ) Uncertainty %

Core

Core

Rim

Core

Core

Core

Core

Rim

Rim

2.36 0.91 25.95 0.72 15.71 27.42 2.74 9.16 1.09 0.58 11.38 0.08 0.29 98

2.38 0.9 26.12 0.7 15.73 27.42 2.74 9.22 1.07 0.71 11.23 0.10 0.28 99

2.33 0.9 26.5 0.72 15.73 27.46 2.75 9.19 1.16 0.63 11.21 0.11 0.27 99

2.33 1.69 26.38 0.02 10.23 24.58 3.07 11.96 1.65 0.63 16.8 0.37 0.44 100

1.62 1.03 27.61 bd 14.14 28.94 3.13 10.82 1.16 0.36 10.88 0.12 0.28 100

0.44 1.81 30.07 0.67 12.04 26.64 3.07 11.93 2.04 1.4 9.21 0.58 0.28 100

0.39 1.86 30.3 0.06 12.46 27.42 3.12 11.91 2.01 1.22 9.38 0.54 0.28 101

1.10 2.03 28.07 bd 9.62 25.47 3.34 13.53 2.11 0.83 12.56 0.79 0.35 100

1.05 1.96 27.58 bd 9.82 25.69 3.33 13.26 1.98 0.87 12.16 0.75 0.31 99

1.94 0.4 27.55 1.19 10.37 27.35 3.45 12.85 3.27 1.9 7.516 0.095 0.175 98

1.83 0.41 27.14 1.23 10.37 27.27 3.52 12.9 3.24 1.95 7.659 0.101 0.180 98

1.78 0.41 27.25 1.23 10.36 27.38 3.45 12.85 3.27 1.91 7.606 0.092 0.176 98

0.097 0.040 0.903 0.016 0.238 0.413 0.041 0.134 0.016 0.008 0.106 0.001 0.003 563 29 5.05

0.097 0.039 0.905 0.015 0.237 0.411 0.041 0.135 0.016 0.010 0.105 0.001 0.003 555 29 5.12

0.095 0.039 0.911 0.016 0.236 0.408 0.041 0.133 0.017 0.008 0.104 0.001 0.003 535 29 5.31

0.095 0.074 0.908 0.000 0.153 0.366 0.046 0.174 0.024 0.008 0.155 0.003 0.005 566 22 3.81

0.065 0.044 0.938 0.000 0.209 0.425 0.046 0.155 0.017 0.005 0.099 0.001 0.003 568 30 5.17

0.017 0.076 0.995 0.014 0.174 0.381 0.044 0.167 0.029 0.018 0.082 0.005 0.003 584 32 5.37

0.015 0.077 0.997 0.001 0.179 0.390 0.044 0.165 0.028 0.016 0.083 0.005 0.003 571 32 5.49

0.044 0.087 0.955 0.000 0.143 0.375 0.049 0.194 0.031 0.011 0.115 0.007 0.004 523 25 4.68

0.043 0.086 0.952 0.000 0.148 0.383 0.050 0.193 0.029 0.012 0.113 0.007 0.003 491 25 4.99

0.079 0.017 0.945 0.026 0.155 0.406 0.051 0.186 0.048 0.026 0.069 0.001 0.002 523 36 6.88

0.075 0.018 0.940 0.027 0.156 0.408 0.053 0.188 0.048 0.026 0.071 0.001 0.002 528 35 6.63

0.073 0.018 0.943 0.027 0.156 0.410 0.052 0.188 0.048 0.026 0.071 0.001 0.002 521 36 6.91

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Owing to the availability of a monazite standard (Moacyr standard, see Prabhakar (2013) for detail) that ensures robust U–Th–total Pb chemical dating, monazites were initially analyzed at the EPMA National Facility, IIT Kharagpur using a 4 channel WDS CAMECA — SX-100 equipment. Using one analyzed grain (Kan 13_G4/1, Table 1 and Supplementary Table 1) of the first batch as a secondary standard, remaining samples were subsequently analyzed with a similar instrument at the Central Instrumentation Facility, Pondicherry University. Detail analytical protocols are available in Rekha et al. (2013) and Prabhakar (2013). In brief, NISTREE glass standard A12 was used for REE analysis. Thorianite standard for Th, crocoite for Pb and U glass for U were used. Accelerating voltage of 20 kV and a beam current of 100 nA were applied. U and Th were successively analyzed with a PET crystal on the same wavelength-dispersive spectrometer with a counting time of 225 s and 75 s on peak and background respectively. Pb was analyzed with an LPET crystal using a 300 s counting time on peak. P, Ca, Si and Y were analyzed successively with a PET crystal on the same spectrometer with a 30 s counting time for P and Ca, and 90 s for Si and Y. Light rare earth elements (LREE: La, Ce, Pr, Nd) and Middle rare earth elements (MREEs: Sm, Eu and Gd) were analyzed using an LIF crystal with a counting time of 30 s for La and Ce, 45 s for Pr and Nd, and 60 s for Sm and Gd. The analyzed data were compared with standards in order to quantify the counted data. The L-line was used to analyze U–Th–Pb dating with PET crystal and M-line was used to analyze the LREEs and MREEs (Goncalves et al., 2004). Representative chemical analyses of monazite and the Moacyr standard generated at the EPMA Lab of IIT Kharagpur are provided in Table 1. Remaining analyses that were generated at the CIF, Pondicherry University have been provided in Supplementary Table 1.

Table S1. Since naturally occurring REE-bearing accessory phases fall into four dominant varieties, i.e. monazite: LREE (PO4); brabantite: ThCa(PO4)2; huttonite: ThSiO4 and xenotime: (Y, HREE) (PO4), it was imperative to characterized the exact nature of REE-bearing phases, separated from the beach placers, on the basis of their mineral chemistry. Also, monazite appears either as end member in the monazite– huttonite solid solution or in the monazite–cheralite–brabantite series

3.3. Sm–Nd isotopic analysis Four grains of monazite from each set of nineteen sample fractions (Table 1), hand-picked under binocular microscope, were taken in a 3 ml Savillex® vial and washed with MQ ® and 1 N HCl. Monazites were digested using Aqua Regia solution (HNO3:HCl in the ratio of 1:3) in a Parr® acid pressure dissolution vessel that was kept in an oven at 170 °C. After dissolution, the content of vials dried on a hot plate and split into two fractions for isotope composition (IC) and isotope dilution (ID) analysis. To the ID fraction known amount of Nd and Sm mixed isotope tracer solution added and Nd and Sm separated from each other and also from other REE in HDEHP ion exchange columns following procedure outlined in Anand and Balakrishnan (2010). Nd and Sm were loaded on outgassed Re double filaments along with the 1 N HCI and Nd and Sm isotope ratios measured using a Thermal Ionization Mass Spectrometer (Triton-Thermo Finnigan) at the Department of Earth Sciences, Pondicherry University. The Nd isotope ratios were corrected for mass fractionation using 146Nd/144Nd ratio of 0.721900. Assuming a constant weight for each sample, Sm and Nd concentrations estimated notionally and 147Sm/144Nd ratio calculated as outlined in Faure (1986). Any deviation of notional concentration values from actual values will be in the same magnitude for both Sm and Nd therefore, the parent–daughter isotope ratio thus calculated reflects the actual value. During the course of this study, repeated analysis of Nd isotope standard AMES (n = 16) yielded a mean 143Nd/144Nd ratio of 0.511965 ± 3.4, which is similar to the recommended value (Govindaraju, 1994). The procedural blank values for Nd and Sm were 68 pg and 25 pg, respectively, which are much less than the amount used for their analysis. 4. Results 4.1. Chemical variability of monazites in beach placer 4.1.1. Monazite chemistry Mineral chemical data of monazites from the selected locations (Fig. 1b) have been summarized in Table 1 and Supplementary

Fig. 2. Composition of monazites from beach placers of Kanyakumari, south India. a) Compositional classification showing N80 mol% of monazite component in all analyzed grains. (b) and (c) core (solid symbol) to rim (open symbol) compositional variation in the analyzed grains. Note the variations in (c) and (d) indicate higher huttonite:brabantite ratios in rims relative to cores.

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(Förster and Harlov, 1999). Monazite and xenotime, on the other hand, constitute two immiscible phases in the CePO4–YPO4 system (Gratz and Heinrich, 1997). Consequently, mole fraction, expressed in terms of molecular proportion of cations, of monazite [Xmonazite = LREE / (LREE (La + Ce + Pr + Nd + Sm) + HREE (Gd + Dy) + Y + Si + Ca)], xenotime [Xxenotime = [HREE + Y] / (LREE + HREE + Y + Si + Ca)], brabantite [Xbrabantite = Ca / (LREE + HREE + Y + Si + Ca)] and huttonite [Xhuttonite = Si / (LREE + HREE + Y + Si + Ca)], in any naturally occurring REEbearing phase can be used conveniently to characterize monazite, sensu-stricto, from huttonitic monazite or cheralite (Fig. 2a). It is observed that all the analyzed spots, irrespective of core and rim of the analyzed grains, falls in the monazite field, with 80 to 90 mol% monazite content (Fig. 2a). Distinct compositional differences, in terms of brabantite content, are observed across the samples. Monazites in samples Kan 6 and Kan 9 are chemically similar and characterized by very low brabantite content (b 4 mol%) compared to the other samples that contain 5 to 10 mol% brabantite. Monazites from sample Kan 13 can be grouped into three dominant varieties (Fig. 2a) characterized by low brabantite (b 5 mol%) and high monazite (N85 mol%), high brabantite (N7.5 mol%) and high monazite (N 85 mol%) and intermediate brabantite (5–7.5 mol%) and low monazite (b85 mol%). Xenotime content in the analyzed monazites is less than 2 mol%. In order to ascertain whether the observed mineralogical pattern is related to core to rim chemical variability in the analyzed grains, the mineral chemical data corresponding to core and rim composition of monazite grain from each sample locality were then plotted on a monazite–huttonite– brabantite ternary diagram (Fig. 2b). It is observed that core to rim compositional variation in all the analyzed monazite grains is dominantly linked with huttonitic substitution. This is further manifested from the fact that monazite compositions from all analyzed samples lie between the monazite–brabantite and monazite–huttonite trend lines, with monazite rims characterized by high huttonite:brabantite ratio than the monazite cores (Fig. 2c). These observations are commensurate with established substitution mechanism in monazite,

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viz. Ca2 + + Th4 + → 2LREE3 + and Th4 + + Si4 + → LREE3 + + P5 +, Th4 + + Ca 2 + → 2LREE3 +, and U4 + + Ca 2 + → 2LREE3 + (Mohr, 1984; Chaudhuri and Newesely, 1993; Watt, 1995; Franz et al., 1996; Poitrasson et al., 1996; Ventura and Mottana, 1996; Zhu and O'Nions, 1999a,b ). 4.1.2. REE, U, Th, and Y distribution in monazite For majority of samples, variation in gray shades of back scattered electron (BSE) images reveal either concentric type of patchy type zoning pattern (Zhu and O'Nions, 1999b). In the concentric type (Fig. 3a, b) dark gray shaded (low BSE response) core domain is mantled by light gray shaded (high BSE response) rim domain. The core region is wider compared to the rim region. In patchy zoning (Fig. 3c, d), observed in few samples (e.g. Kan 9) light gray patches of different shapes are hosted in dark gray matrix that comprises the dominant variant. Chondrite normalized REEs, U, Th, Pb and Y spider diagrams (Fig. 4a) reveal enrichment of LREEs over MREEs and distinct peak for Th in all the analyzed grains, irrespective of core and rim domain. However sample wise, dark gray shaded domains in both concentric and patchy types are enriched in LREEs (La, Ce) and Y and depleted in Th and Pb compared to the light gray shaded rim region (Fig. 4b). LREE fractionation decreases systematically with increasing ThO2 between dark gray shaded core domains and light gray shaded rim domains (Fig. 4c). In concentric type, high BSE response (light gray) domains are depleted in U relative to low BSE response (dark gray) domain (Fig. 4a, b). In the patchy type, the light gray domains are either enriched or depleted in U relative to dark gray domain (Fig. 4b). Th/U ratio is consistently high in the light gray domains (24–60) compared to the dark gray domain (8–18). 4.2. Monazite chronometry 4.2.1. U–Th–total Pb monazite geochronology Suzuki and Adachi (1991), Montel et al. (1996) described the principles for determining U–Th–total Pb spot ages using EPMA. 134 spot

Fig. 3. Back-scattered electron (BSE) images of few selected monazite grains. The monazite grains are characterized by concentric (a, b) and patchy type (c, d) zoning of REEs, U, Th, Pb and Y, see text for discussion. Note the low BSE response domains (dark gray shaded region) and high BSE response domains (light gray shaded region) comprise the core and rim regions respectively. U–Th–total Pb spot ages in the analyzed monazite grains are also given for reference. Italics numbers refer to the analyzed spots.

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Consequently, two distinct age domains in the range of 578 Ma to 555 Ma and 536 Ma to 508 Ma (Fig. 5f) can be obtained from the analyzed grains. Monazites from the studied sample localities (Fig. 1b), therefore show similar geochronological record. The older age domain, which is restricted to the dark gray core region, possibly constrains the time of growth of pristine monazite (Fig. 3). The younger age domain, which is confined to the light gray rim domain, seemingly qualifies to post-peak metamorphic compositional modification event (Fig. 3). 4.2.2. Sm–Nd isotopic system Sm–Nd isotopic data for monazites of southwestern coast are given in Table 2. 147Sm/144Nd ratios in the analyzed monazites vary from 0.0733 to 0.1039 with an average value of 0.0902 (n = 19). 143 Nd/144Nd ratios are expectedly low and range from 0.511344 to 0.511061. εNd values [{(143Nd/144Ndsample) / (143Nd/144NdCHUR) − 1} × 10,000] of monazites calculated for present day (t = 0) using the chondrite uniform reservoir Nd isotope ratio (143Nd/144NdCHUR) of 0.512638. They vary from −26.5 to −30.8. High negative εNd values indicate derivation of monazites from light LREE enriched igneous and metamorphic rocks that formed part of Proterozoic continental crust. Model Sm–Nd isotope ages with reference to chondrite uniform reservoir (CHUR), expressed as TCHUR, vary from 1687 Ma to 2364 Ma and depleted mantle Nd model ages (TDM) calculated using parameters defined by DePaolo (1981) vary from 2035 Ma to 2702 Ma. The average TDM age of monazite is 2375 (+96/−44) Ma. For effective comparison, TDM ages for the potential source rocks of TB and NG were recalculated with the above parameters on published Sm–Nd isotope data (Brandon and Meen, 1995; Bartlett et al., 1998; Unnikrishnan-Warrier et al., 1995; Choudhary et al., 1992; Harris et al., 1994; Cenki et al., 2004; Shabeer et al., 2005; Kröner et al., 2015) and the compiled data is given in Supplementary Table S2. 5. Discussion 5.1. Chemical variability of placer monazites and its link to dynamical processes in the source

Fig. 4. The chemical variation diagram for monazite — a) chondrite normalized spider diagram, b) core domain normalized spider diagram. c) Normalized La/Sm vs. ThO2 diagram for core and rim domains of monazite. The suffix ‘2/1’ in parenthesis after each sample location, i.e. Kan3_Gr2 (2/1), in (b) indicates spot 2 (rim) normalized with respect to spot 1 (core).

ages, covering core (dark gray regions) and rim (light gray shaded region) of monazites (Fig. 5) were determined using Geochron protocol of SX 100 EPMA that uses the formulation of Montel et al. (1996). Spot ages with less than 10% uncertainty, where uncertainty % = (2σ uncertainty/age) × 100 (Table 1, Supplementary Table S1) have only been considered. Age data for individual grain, together with their corresponding age uncertainty (2σ) and relative percentage of abundance, have been represented in conventional probability density diagrams (Fig. 5) using the Isoplot software (v. 4.1) of Ludwig (2001). Age uncertainty (2σ) for probability density plots was calculated using ‘unmixing age’ protocol of Isoplot software (Ludwig, 2001). Probability-density analyses for all the analyzed samples display two dominant age peaks that are constrained at 554 ± 14 Ma and 508 ± 22 Ma (Kan 3, Fig. 5a), 555 ± 24 Ma and 517 ± 13 Ma (Kan 6, Fig. 5b), 572 ± 18 Ma and 536 ± 14 Ma (Kan 9, Fig. 5c), 577 ± 17 Ma and 523 ± 9 Ma (Kan 12, Fig. 5d) and 568 ± 12 Ma and 532 ± 20 Ma (Kan 13, Fig. 5e).

This study documents that mineral chemical (Figs. 2, 4), geochronological (Fig. 5) and isotopic (Table 2) signatures of monazites show consistent and predictable, discussed next, relationship between samples collected across a 40 km long southwestern Indian coast, from Colachel to Kanyakumari (Fig. 1b). Depending on the coastal geodynamics, especially the presence of strong southward long shore current (Krishnan et al., 2001; Kunte et al., 2001), monazites in the beach placers may be sourced from a variety of litho-stratigraphic units of TB, NG, and ACSZ (Fig. 1a) that comprise the hinterland region of the southwestern coast. We lack any primary mineral chemical data of monazite for these potential source regions. A review of the existing literature revealed that except very limited work (Harley and Nandakumar, 2014; Johnson et al., 2015), detail mineral chemistry of monazites from different tectonic units of SGT is still unknown that may possibly reflect emphasis of earlier studies towards U–Th–total Pb monazite geochronology. As a result, it was imperative to restrict the target source region so that the objective of linking monazite chemistry with the crustal dynamical processes in the source can be achieved. Consequently, we have selected a small area (Fig. 1a) from the south western coast where majority of sample locations are restricted between two south flowing rivers that dominantly drain through the charnockites of Nagercoil Granulite Block (Fig. 1b). Though lack of adequate information impedes distinguishing the source of monazites solely on the basis of mineral chemistry alone, monazite cores (Fig. 2a) with restricted compositional variability (85 to 90 mol% monazite), dominantly huttonitic substitution between core and rim (Fig. 2b, c) and identical core to rim variation in LREES, MREES, Th, U and Pb (Fig. 4b,c) across the sample localities suggests either a restricted/uniform source or

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Fig. 5. Probability density plot of U–Th–total Pb monazite ages across the sampling sites. Monazites with less than 10% age uncertainty (Table 1 and S1 have only been considered. (c) and (r) indicate core and rim.

identical thermal-metamorphic processes in the source regions of monazite in placer. The latter can be tested from the observed mineral chemical and geochronological results, since tectono-metamorphic evolution of different tectonic units within the SGT is well documented.

Table 2 Sm–Nd isotopic ratios, Nd model ages and εNd values of beach placer monazites from the Kanyakumari coast, south India. Sample

147

Sm

144 Nd

Kan-1a Kan-1b Kan-1c Kan-2a Kan-2b Kan-3a Kan-3b Kan-3c Kan-4 Kan-5 Kan-6 Kan-7 Kan-8 Kan-9a Kan-9b Kan-11a Kan-11b Kan-12 Kan-13

0.089 0.088 0.099 0.087 0.085 0.073 0.094 0.081 0.083 0.096 0.102 0.080 0.092 0.099 0.081 0.097 0.104 0.096 0.089

144 Nd

± (2σ) × 10−6

TCHUR (Ma)

TDM (Ma)

εNd t = 0

εNd t = 550 Ma

0.511249 0.511263 0.511278 0.511200 0.511197 0.511255 0.511102 0.511089 0.511091 0.511200 0.511148 0.511061 0.511244 0.511344 0.511214 0.511180 0.511193 0.511187 0.511087

8 6 6 5 6 4 5 9 4 7 10 6 9 10 5 4 7 6 15

1950 1913 2103 1984 1967 1704 2270 2040 2070 2164 2389 2049 2024 2016 1865 2213 2362 2187 2184

2304 2269 2465 2328 2310 2050 2594 2363 2393 2508 2718 2367 2378 2393 2210 2553 2701 2529 2507

−27.1 −26.8 −26.5 −28.1 −28.1 −27.0 −30.0 −30.2 −30.2 −28.1 −29.1 −30.8 −27.2 −25.2 −27.8 −28.4 −28.2 −28.3 −30.3

−19.5 −19.2 −19.7 −20.3 −20.3 −18.3 −22.8 −22.1 −22.2 −21.0 −22.5 −22.6 −19.9 −18.4 −19.7 −21.4 −21.7 −21.3 −22.7

143

Nd

The studied monazites are characterized by mostly concentric and in few cases by patchy type zoning pattern (Fig. 3). A variety of processes such as overgrowth, regrowth, intergrowth, replacement and recrystallization can give rise compositional zoning in monazite (Zhu and O'Nions, 1999b). As depicted in Fig. 6 and supported by Zhu and O'Nions (1999b), concentric zoning patterns, which is most prevalent in the studied samples (Fig. 3), can be impressed by — a) episodic growth and b) overgrowth around a detrital (xenocrystic) portion. For the first mechanism (Fig. 6a), we have considered a dynamic melting scenario (Bhadra et al., 2007 and references therein) with incongruent growth of monazite along with other phases (Harley and Nandakumar, 2014), as dictated by the melting equilibria. In the second mechanism two possibilities may arise. If the prevailing high temperature induces migmatization in the source rock, overgrowth could be formed due to interaction of detrital monazite with invading melt layer (Fig. 6b). The low temperature scenario (Fig. 6c) is similar to the proposition of Rekha et al. (2013), where fluid focusing can lead to monazite instability at temperature corresponding to greenschist facies condition. During episodic growth (Fig. 6a) and high-T overgrowth (Fig. 6b), increasing silica activity will favor huttonitic–monazite component over pure monazite end member and therefore result in depletion of LREE and enrichment of Th in successive zones towards the monazite rim. Since biotites at granulite grade, which characterizes the peak Pan-African metamorphism in the entire hinterland region (SGT), are fluorine rich, biotite dehydration melting can also lead to an increase in F content in the melt that in turn favors huttonite over monazite (Watt, 1995). Rekha et al. (2013) also documented depletion of LREE

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Fig. 6. Cartoon showing monazite growth resulting from — a) episodic growth concomitant with partial melting, b) high T overgrowth over a xenocrystic monazite and c) low T overgrowth over a xenocrystic monazite. See text for discussion.

and enrichment of Th in the monazite replacement zone. Collectively, the observed trend of core to rim LREE depletion and Th enrichment (Fig. 4b,c) is expected in all three situations. However, some characteristic features such as strong embayment during overgrowth by replacement, in case of second mechanism (Fig. 6b), and enrichment of Y in the alteration zone (Rekha et al., 2013) in case of third mechanism (Fig. 6c), were rare in the observed monazite placers. Variation in the (La/Sm)N ratio in monazite grains from same sample locality (Fig. 4c) can be explained either by successive growths of monazites during a prolonged and episodic melting event, by virtue of strong partitioning of La over Sm in monazite leading to the development of monazites having different (La/Sm)N ratios (Stepanov, 2012), or admixture of monazites from different sources having different (La/Sm)N ratios but similar tectonic evolution or a combination of these two processes. Since extensive migmatization during a regional granulite facies metamorphism and coinciding with Pan-African east and west Gondwanaland assembly is reported from all the lithogical and tectonic units within the Southern Granulite Terrain (Fig. 1a), we tend to believe

that element distribution patterns in the observed monazite placers can be linked with episodic growth (Fig. 6a) concomitant to melting and melt-crystallization in the source rock. Systematic variation with monazite core having high (La/Sm)N ratio and low ThO2, and gradual decrease of (La/Sm)N and complimentary increase ThO2 (Fig. 4c) towards the rim possibly attest to such dynamic melting mechanism during monazite growth (Harley and Nandakumar, 2014). Two distinct U–Th–total Pb ages (Fig. 5) of Pan-African affinity corresponding to the core and rim domain of monazite therefore brackets two significant tectono-thermal episodes, i.e. peak granulite facies reworking and post-peak metamorphic evolution, in the source region. 5.2. Monazite placers as proxy for provenance study In confirmation of the preceding discussion, which establishes that monazites are reliable recorder of crustal dynamical processes in the source rock, it was imperative to constrain the provenance for the

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studied monazites in the placers. The effectiveness of monazite in decoding the provenance, attempted in this study, appears to be a significant contribution because the various tectonic units of the Southern Granulite Terrain, such as MB, TB, NG and ACSZ, which comprise the hinterland region of the southwestern coast have considerable overlapping in protolith and thermal reworking ages (Fig. 7). Collins et al. (2007) reported Paleoproterozoic (2300–1900 Ma) and Cambrian (515 Ma) ion microprobe (SHRIMP) ages respectively from detrital zircon cores and metamorphic zircons in meta-sedimentary granulite gneisses from Madurai and TB (Fig. 1). The authors, based on the study of core and rim of detrital zircons from the metasediments of Achankovil shear zone (Fig. 1a), further documented age of sedimentation and high-grade granulite facies metamorphism respectively at 728 Ma and 513 Ma for Achankovil metasediments. A number of studies, based on isotope and chemical dating of monazites in granites, reported ages ranging from 610 Ma to 420 Ma with few monazites cores recording ages in the range of 1800 Ma–1300 Ma for TB. Neoproterozoic and Cambrian ages were interpreted as the time of igneous intrusion, while Mesoproterozoic monazite cores were considered to be of xenocrystic origin (Braun, 2006). Choudhary et al. (1992) interpreted Sm–Nd garnet (550 Ma) and Rb–Sr biotite–plagioclase (440–460 Ma) ages of gneisses and granulites at Ponmudi in TB to coincide with PanAfrican peak granulite facies metamorphism and subsequent cooling and upliftment of the granulites respectively. Fonarev et al. (2000) documented a short (100 to 160 Ma time span) metamorphic evolution of TB with three distinct phases of metamorphism at 540–600 Ma (M1: peak granulite facies metamorphism), 530 Ma (M2: early retrograde metamorphism with near isobaric cooling) and 440–470 Ma (M3: late retrograde metamorphism with cooling–decompression). EPMA U–Th–total Pb studies on monazites from gneisses of TB reveal Paleoproterozoic (~1.9 Ga) and Neoproterozoic (580 Ma) ages respectively from core and rim (Braun et al., 1998). Emplacement age of associated pegmatites was constrained at 470 Ma (Braun et al., 1998). U–Pb SHRIMP ages of 530 Ma (Shabeer et al., 2005) and U–Pb zircon age of 512 Ma (Miller et al., 1996) are also reported from TB. Kovach et al.

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(1998) reported Pan-African (572 Ma) intrusion age and subsequent cooling (to 415 °C) ages (450 Ma) for the Putteti alkali syenite pluton within host charnockite gneiss in NG. The above reported ages (Fig. 7), from different sources, encompass a broad spectrum and are appeared to be insensitive in resolving the exact provenance. A significant improvement was noticed when the reported U–Th– total Pb of monazites, alone, were considered to discriminate the reworking events (Fig. 8) within different tectonic units of SGT. The data reveal a single reworking event at 570 Ma for Madurai Block (Fig. 8a), multiple source signatures for Achankovil metasediments (Fig. 8b) and TB (Fig. 8c) and two episodes at 564 Ma and 534 Ma (Fig. 8d) for NG. It is observed that the range of U–Th–total Pb ages (Fig. 5) obtained from the beach placer monazites in this study correspond with the reported U–Th–total Pb monazite ages of the TB (Fig. 8c) and NG (Fig. 8d) granulite block. As observed in this study (Fig. 5f), Santosh et al. (2006a,b) reported two distinct age clusters at 564 Ma and ~ 534 Ma (Fig. 8d, reproduced from Santosh et al., 2006a, b) corresponding respectively with core and rim domain of monazites. The authors also interpreted the older ages, restricted to monazite core, to represent timing of peak granulite facies metamorphism in the NG. The younger monazite rims on the other hand was interpreted as growth of monazite during retrograde metamorphism and not due to Pb loss. The latter becomes significant in the context of the two distinct growth episodes (Fig. 6) discussed earlier. A comparison of REE distribution pattern (Fig. 9a, b, c) and La/Nd versus ThO2 (Fig. 9d) with reported rocks of hinterland unequivocally establish granitoids of NG and TB as the source for the placer monazites. By implication, the studied placer monazites are observed to retain the complete mineral chemical and geochemical signature of the source region. Further, overlapping chemical signatures of monazites from these two tectonic units and the placers (Fig. 9d), as well, possibly attest to the fact that NG and TB had similar tectonic-evolutionary history since 2.1 Ga (Johnson et al., 2015; Kröner et al., 2015). Summarily, it may be suggested that careful examination of mineral chemistry vis-à-vis growth history in conjugation with U–Th–total Pb

Fig. 7. A compilation of available age data on Trivandrum Block (TB), Nagercoil granulites (NG) and Achankovil Shear Zone (ACSZ). Source references — 1: Collins et al., 2007; 2: Braun, 2006; 3: Chaudhury et al., 1992; 4: Fonarev et al., 2000; 5: Braun et al., 1998; 6: Shabeer et al., 2005; 7. Millar et al., 1996; 8: Kovach et al., 1998; 9: Santosh et al., 2006a; 10: Santosh et al., 2006b; 11: Santosh et al., 2003; 12: Santosh et al., 2005a; 13: Santosh et al., 2005b; 14: Harley and Nandakumar, 2014; 15: Johnson et al., 2015; 16: Taylor et al., 2014; 17: Taylor et al., 2015; 18: Whitehouse et al., 2014. (c) and (r) indicate core and rim.

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Fig. 8. Probability density plot of U–Th–total Pb monazite ages reported from different lithological units of SGT such as Madurai Block (MB), Achankovil shear zone (ACSZ), Trivandrum Block (TB), and Nagercoil granulites (NG) from published sources.

EPMA ages of beach placer monazites provide a useful proxy for provenance analyses. Since such data are limited in the literature, more exhaustive studies are required to foolproof this technique. 5.3. Sm–Nd systematics of beach placer monazites: determination of provenance and age of crust formation Initial 143Nd/144Nd ratios of monazites, also expressed as ƐNd values at time t, are useful to identify source of crustally derived granite magmas (Tomascak et al., 1998). Nd model ages of light REEs enriched

granitoid rocks and sediments have been extensively used to understand timing of juvenile additions to continental crust and differentiate between terranes with different geological histories (DePaolo and Wasserburg, 1976, 1996; Allegre and Rousseau, 1984; Shirey and Hanson, 1986; Samson et al., 1989; Krogstad et al., 1995; Tomascak et al., 1998; Dey, 2013). Juvenile granitoid magmas have mantle-like Nd isotope compositions. Therefore, ƐNd values of juvenile magmas at the time of their formation would be either zero or positive. Subsequently, during later orogenic events, the juvenile granitoid rocks could have been subjected to partial melting resulting in granites. Also they could have been

Fig. 9. Chondrite normalized REE–Pb–Th–U–Y plot for monazites of (a) TB, (b) NG and (c) beach placers. (d) (La/Nd)N versus ThO2 plot of beach placer monazites compared with monazites of TB and NG. Chondrite values used for normalization are from Anders and Grevesse (1989).

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0.07 and 0.13 with a mean value of 0.1 (McLennan, 2001; Rudnick and Gao, 2003) whereas, a slightly higher value of 0.1193 has been reported by Chauvel et al. (2014). Importantly, monazites also have a similar spread in the 147Sm/144Nd ratio (Iizuka et al., 2011). This feature makes the monazite as a unique tracer of Nd isotope evolution in a variety of LREE enriched continental crustal rocks (Garcon et al., 2011; Iizuka et al., 2011). A comparison of 147Sm/144Nd ratios of monazites from the Kanyakumari coast with gneisses, charnockites and granites of the Trivandrum Block shows that these ratios partially overlap with granites and gneisses (Fig. 10b) whereas, fully with the NG (Fig. 10a). Based on compositional zoning, identified in BSE images (Fig. 3) and U–Th–total Pb age determinations (Fig. 5), two episodes of monazite growth have been identified coinciding with peak metamorphism and subsequent cooling of the host granulites. Iizuka et al. (2011), based on in situ Sm/ Nd isotope study on monazites from metamorphic rocks of Naryar and Jack Hill supracrustal belts of Western Australia, have shown that monazite domains that grew during different phases of metamorphism yielded similar Nd model ages. Furthermore, they found that metamorphic and

Fig. 10. Compiled histograms for reported 147Sm/144Nd isotopic ratios measured on granulites and gneisses from TB, NG and ACSZ. 147Sm/144Nd isotopic ratios of monazites occurring along the Kanyakumari beach are plotted and compared with rocks of — a) NG (b) TB and (c) ACSZ. Sm–Nd isotopic data are from Brandon and Meen (1995), Bartlett et al. (1998), Unnikrishnan-Warrier et al. (1995), Choudhary et al. (1992), Harris et al. (1994), Cenki et al. (2004), Shabeer et al. (2005), and Kröner et al. (2015). Lithological index, as in (b), is same for all the figures.

eroded to yield clastic sediments. If these rocks have 147Sm/144Nd ratio same as their source then all of them will give similar Nd model ages, which will represent the time of formation of the juvenile granitoid rock. Monazites of igneous and metamorphic origin are formed in rocks of low Ca, peralkaline and peraluminous compositions (Parrish, 1990; Spear and Pyle, 2002; Williams et al., 2007; Rasmussen and Muhling, 2007; Vlach, 2010). Although they have high LREE abundances, their 147 Sm/144Nd ratios are similar to the rocks from which they crystallized (Garcon et al., 2011 and references therein). Baldwin et al. (2006) reported that monazites armored in garnet have similar Sm/Nd ratio as that of the host felsic granulites. Bulk of the continental crust is made up of LREE enriched rocks whose 147Sm/144Nd ratio varies between

Fig. 11. Compiled histograms for TDM model ages on granulites and gneisses from (a) NG, TB and (c) ACSZ. TDM ages on monazites occurring along the Kanyakumari beach are plotted for comparison. The legend and sources of Sm–Nd isotope data are given in Fig. 10. TDM ages, recalculated using parameters given in DePaolo (1981), are given in Supplementary Table S2. Uniform lithological index, as in (b), is used.

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detrital monazites yielded similar Nd model ages that helped in identification of source granitoid rocks found around the above supracrustal belts. Thus, dissolution and growth of monazites during different phases of metamorphism (Fig. 6) should not affect the Nd isotope composition and 147Sm/144Nd ratios of monazites. The average Sm/Nd ratio (0.150) for granites and granodiorites from NG and TB (Kröner et al., 2015) is similar to average EPMA Sm/Nd ratio (0.153) of placer monazites of the present study. This indicates that there was no significant fractionation of Sm/Nd ratio between monazite and the whole rock. As a result, Nd model ages of monazites, irrespective of their origin, would be essentially the same as that of the rocks in which they crystallized or grew. Sm–Nd isotope studies on rocks of south India revealed distinct patterns of crustal evolution in the Dharwar craton, Madurai and Trivandrum Blocks and Achankovil shear zone (Brandon and Meen, 1995). Rocks from north of the Palghat-Cauvery shear zone (PCSZ), including Niligiri and Madras granulite Block, yielded Nd TDM ages mostly between 3.2 and 3.5 Ga while some as low as 2.56 Ga (Harris et al., 1994; Bartlett

et al., 1998; Peucat et al., 2013; Tomson et al., 2006, 2013; Collins et al., 2014) whereas, immediately south of the PCSZ in the granulite terrain of North Madurai Block Nd TDM ages ranged between 3.0 and 2.51 Ga (Plavsa et al., 2012). Rocks of the Achankovil shear zone are uniquely characterized by Mesoproterozoic Nd TDM ages (1.2–1.6 Ga) whereas, granulites and gneisses of the Trivandrum Block (also known as Kerala Khondalite Belt) show Nd TDM ages in the range between 2.0 and 3.0 Ga (Bartlett et al., 1998; Cenki et al., 2004; Kröner et al., 2015). Nd model ages range from 2.1 to 3.1 Ga for the NG (Fig. 11a), 2.0 to 3.5 Ga for the TB (Fig. 11b) and 1.3 to 2.0 Ga for the ACSZ (Fig. 11c). Nd TDM ages for beach placer monazites of Kanyakumari coast range from 2.0 to 2.6 Ga, which is similar to that reported for the Nagercoil granulites and rocks of Trivandrum Block (Fig. 11). U–Th–total Pb ages on monazite core and rim, documented in this study (Fig. 5), suggest two growth episodes between 555–578 Ma and 508–536 Ma coinciding with peak prograde metamorphism and subsequent cooling and uplift respectively. However, both the domains must have been in isotopic equilibrium with the whole rock and had similar 147 Sm/144Nd ratios. Therefore, as shown for monazites from Naryar and Jack Hill supracrustal belts, the beach placer monazites that grew at two distinct periods during the Pan African thermal event are expected to have similar Nd TDM ages. Kondalites and felsic charnockites were derived by sedimentary processes and partial melting of preexisted crustal rocks, respectively. Both these rock types along with other igneous rocks (granodiorite–diorite and gabbro) were metamorphosed to granulite facies ~ 550 Ma ago in the Trivandrum Block (Cenki et al., 2004; Kröner et al., 2012, 2015; Taylor et al., 2014). Nd model ages of these rocks, that retain continental crustal 147Sm/144Nd ratios of ~ 0.11, are interpreted to represent the time of juvenile crust formation. As shown in this study placer monazites can provide same information that can be extracted from their source rocks on timing of crust forming episodes. Monazites present in these granulites were crystallized during the early Cambrian, Pan-African thermal event as evidenced from textural criteria and U–Th–Pb ages on monazite (Santosh et al., 2006a,b). Consequently, Nd isotope composition of the granulites and monazites must have been identical at that time of peak Pan-African tectonothermal event, i.e. 570 Ma. ƐNd values calculated at 550 Ma for the beach placer monazites are compared with various rock types in the Nagercoil granulites (Fig. 12a), Trivandrum Block (Fig. 12b) and Achankovil Shear Zone (Fig. 12c). It is observed that the range of calculated ƐNd (t = 550 Ma) values coincide with rocks of Nagercoil granulites and Trivandrum Block (Fig. 12). By implication, beach placer monazites were derived from these two tectonic units of the SGT. Thus, U–Th–total Pb age data combined with Sm–Nd isotope study on the Kanyakumari beach placer monazites demonstrates the utility of monazites to identify the provenance. Further, this study demonstrates the usefulness of monazites also to characterize the average age of extraction from mantle and determine ages of tectono-thermal events that affected the source region and thus provide a snap shot of crustal evolution of geological terranes that may not be easily accessible. Since, Precambrian granulite complexes are made up of a mosaic of terranes with distinct evolutionary histories (Santosh et al., 2012; Collins et al., 2014), timing of crustal accretion and subsequent metamorphic events in the source regions, consisting of granulitic terranes, can be accurately inferred from placer monazites. Geochemical, U–Th– Pb and Nd isotope studies on beach placer monazites found along passive continental margins, such as, east coast of India and north coast of eastern Antarctica can be used to validate models of supercontinent reconstruction. 6. Conclusion

Fig. 12. ƐNd calculated at 550 Ma age is plotted against 147Sm/144Nd for monazites and compared with whole rock data on — a) NG, b) TB and c) ACSZ. The legend and sources of Sm–Nd isotope data are given in Fig. 10. Uniform lithological index, as in (b), is used.

Mineral chemical, geochemical, geochronological and isotopic studies of beach placer monazites from Kanyakumari coast, southwest India revealed that beach placer monazites are useful proxy for provenance

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studies. Element distribution patterns in monazites from the studied beach placers of Kanyakumari coast, SW India suggest episodic growth where core and rim chemical signatures can be linked with respectively melting and melt-crystallization. Element chemistry together with TDM ages restrict TB and NG as the potential source for the beach placer monazites. EPMA chemical ages constrain two age populations between 555–578 Ma and 508–536 Ma that are restricted to respectively core and rim domain and corresponds well with peak Pan-African granulite facies (570 Ma) and retrograde metamorphism (535 Ma) reported from TB and NG. Combined EPMA and Sm–Nd isotope studies on monazites from the southwestern Kanyakumari coast, augment the growing evidences testifying their usefulness in decoding the tectono-thermal evolution of provenance, in particular and crustal accretionary processes, in general. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.chemgeo.2016.02.018. Acknowledgments Central Instrumentation Facility, Pondicherry University extended SEM and EPMA facilities. UGC funded this study through the Special Assistance Program and BSR fellowship. We express our sincere thanks to Prof. Abhijit Bhattacharya for extending support to use the EPMA National facility at the Indian Institute of Technology Kharagpur. Dr. Prabhakar Naraga is thanked for providing analytical assistance and suggestions on monazite chronometry. We thank Dr. T.E. Johnson for providing monazite mineral chemical data on the Nagercoil granulites. We are grateful to Prof. K. Mezger and two anonymous reviewers whose comments helped in improving the manuscript. References Acharya, B.C., Nayak, B.K., Das, S.K., 2009. Heavy mineral placer sand deposits of Kontiagarh area, Ganjam district, Orissa, India. Resour. Geol. 59, 388–399. Allegre, C.J., Rousseau, D., 1984. The growth of the continents through geological time studied by Nd isotope analysis of shales. Earth Planet. Sci. Lett. 67, 19–34. Allègre, C.J., Dupré, B., Négrel, P., Gaillardet, J., 1996. Sr, Nd, Pb isotope systematics in Amazon and Congo River systems: constraints about erosion processes. Chem. Geol. 131 (1), 93–112. Anand, R., Balakrishnan, S., 2010. Pb, Sr and Nd isotope systematics of metavolcanic rocks of the Hutti greenstone belt, eastern Dharwar craton: constraints on age, duration of volcanism and evolution of mantle sources during the late Archean. J. Asian Earth Sci. 39, 1–11. Anders, E., Grevesse, N., 1989. Abundance of elements: meteoritic and solar. Geochim. Cosmochim. Acta 53, 197–214. Baldwin, J.A., Bowring, S.A., Williams, M.L., Mahan, K.H., 2006. Geochronological constraints on the evolution of high-pressure felsic granulites from an integrated electron microprobe and ID-TIMS geochemical study. Lithos 88 (1), 173–200. Bartlett, J.M., Dougherty-Page, J.S., Harris, N.B.W., Hawkesworth, C.J., Santosh, M., 1998. The application of single zircon evaporation and Nd model ages to the interpretation of polymetamorphic terrains: an example from the Proterozoic mobile belt of south India. Contrib. Mineral. Petrol. 131, 181–195. Bhadra, S., Das, S., Bhattacharya, A., 2007. Shear zone-hosted migmatites (Eastern India): the role of dynamic melting in the generation of REE-depleted felsic melts, and implications for disequilibrium melting. J. Petrol. 48 (3), 435–457. Brandon, A.D., Meen, J.K., 1995. Nd isotopic evidence for the position of southernmost Indian terranes within East Gondwana. Precambrian Res. 70 (3), 269–280. Braun, I., 2006. Pan-African granitic magmatism in the Kerala khondalite belt, southern India. J. Asian Earth Sci. 28, 38–45. Braun, I., Bröcker, M., 2004. Monazite dating of granitic gneisses and leucogranites from the Kerala Khondalite Belt, southern India: implications for Late Proterozoic crustal evolution in East Gondwana. Int. J. Earth Sci. 93 (1), 13–22. Braun, I., Montel, J.M., Nicollet, C., 1998. Electron microprobe dating of monazites from high-grade gneisses and pegmatites of the Kerala Khondalite Belt, southern India. Chem. Geol. 146 (1), 65–85. Cenki, B., Braun, I., Bröcker, M., 2004. Evolution of the continental crust in the Kerala Khondalite Belt, southernmost India: evidence from Nd isotope mapping, U–Pb and Rb–Sr geochronology. Precambrian Res. 134 (3), 275–292. Chakraborty, P.P., Das, P., Das, K., Saha, S., Balakrishnan, S., 2012. Regressive depositional architecture on a Mesoproterozoic siliciclastic ramp: sequence stratigraphic and Nd isotopic evidences from Bhalukona Formation, Singhora Group, Chhattisgarh Supergroup, central India. Precambrian Res. 200, 129–148. Chandrasekharan, S., Murugan, C., 2001. Heavy minerals in the beach and coastal red sands (Teris) of Tamil Nadu. Explor. Res. At. Miner. 13, 87–109. Chaudhuri, J.B., Newesely, H., 1993. On the REE-bearing minerals in the beach placers of Puri, Orissa District. J. SE Asian Earth Sci. 8 (1), 287–291.

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