Scholarly article on topic 'Prograde and retrograde growth of monazite in migmatites: An example from the Nagercoil Block, southern India'

Prograde and retrograde growth of monazite in migmatites: An example from the Nagercoil Block, southern India Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Tim E. Johnson, Chris Clark, Richard J.M. Taylor, M. Santosh, Alan S. Collins

Abstract Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure–temperature–time (P–T–t) evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6–8 kbar and temperatures in excess of 900 °C. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 °C. SHRIMP U–Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palaeoproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P–T–t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.

Academic research paper on topic "Prograde and retrograde growth of monazite in migmatites: An example from the Nagercoil Block, southern India"

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Prograde and retrograde growth of monazite in migmatites: An example from the Nagercoil Block, southern India

Tim E. Johnson3'*, Chris Clarka, Richard J.M. Taylor3, M. Santoshb, Alan S. Collinsc

a Department of Applied Geology, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA 6845, Australia b School of the Earth Sciences and Resources, China University of Geosciences (Beijing), 29 Xueyuan Road, Beijing 100083, China c Tectonics, Resources and Exploration (TRaX), Department of Earth Sciences, University of Adelaide, SA 5005, Australia

ARTICLE INFO ABSTRACT

Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure—temperature—time (P—T—t) evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6—8 kbar and temperatures in excess of 900 °C. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 °C. SHRIMP U—Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palae-oproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P—T—t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.

© 2014, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/

licenses/by-nc-nd/4.0/).

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Article history:

Received 29 October 2014

Received in revised form

3 December 2014

Accepted 6 December 2014

Available online 27 December 2014

Keywords: Monazite Migmatite Zircon

Geochronology

Phase equilibria modelling

Gondwana

1. Introduction

The Neoproterozoic era was a dynamic period in Earth history that witnessed the breakup of the supercontinent Rodinia and the assembly of its successor, Gondwana (Meert, 2003; Boger and Miller, 2004; Collins and Pisarevsky, 2005; Meert and Lieberman, 2008; Nance et al., 2014). The amalgamation of Gondwana occurred through collision of numerous continental blocks to produce some of the most extensive mountain ranges ever built on Earth (Kroner, 1984; Trompette, 1997). The eroded remnants of these orogenic belts expose highly deformed and metamorphosed

* Corresponding author. Present address: Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia. Tel.: +61 8 92667332; fax: +61 8 92663153.

E-mail address: tim.johnson@curtin.edu.au (T.E. Johnson).

Peer-review under responsibility of China University of Geosciences (Beijing)

rocks that permit investigation of the deep crustal processes occurring during supercontinent formation.

Constraining the pressure—temperature—time (P—T—t) evolution of metamorphic rocks is key to unravelling the geodynamic processes involved in their formation (e.g. Brown, 2014). Quantitative information on the P—T conditions of peak and retrograde metamorphism, as well as semi-quantitative constraints on the prograde evolution, is derived via forward modelling of phase equilibria that may be applied to a wide range of common crustal protoliths metamorphosed in a variety of tectonothermal environments under both subsolidus and suprasolidus conditions (e.g. White et al., 2001, 2007; Johnson and Brown, 2004; Evans and Bickle, 2005; Diener et al., 2008; Johnson et al., 2008, 2010, 2014; Rebay et al., 2010; Johnson and White, 2011; Korhonen et al., 2013, 2014). Quantitative constraints on the timing and duration of crustal metamorphism most commonly use U—Pb dating of accessory minerals, in particular zircon and monazite that occur in

http://dx.doi.org/10.1016/j.gsf.2014.12.003

1674-9871/® 2014, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

almost all intermediate to felsic (meta) igneous rocks and silici-clastic (meta) sediments. Increasingly, age data from accessory minerals are used in conjunction with their trace element compositions to place temporal constraints on the timing of formation or consumption of particular phases, including melt, to constrain segments of a metamorphic P—T—t path (Rubatto, 2002; Hermann and Rubatto, 2003; Rubatto et al., 2006; Harley et al., 2007; Clark et al., 2009a; Buick et al., 2010; Taylor et al., 2015).

In migmatites and residual granulites, which represent the most abundant components of the lower crust, new metamorphic growth of zircon and monazite is generally assumed to occur due to saturation of zirconium and light rare earth elements (LREE) in the melt as it cools and crystallises, and the corresponding age data to date high-temperature retrogression (Kelsey et al., 2008; Kelsey and Powell, 2011; Yakymchuk and Brown, 2014). In the case of zircon, the Zr required for new growth is supplied to the melt by dissolution of pre-existing magmatic (in the case of igneous pro-toliths) or detrital (in the case of metasedimentary rocks) zircon, which later precipitates as the SiO2-saturated melt cools and attains Zr saturation. However, monazite requires a source of phosphorus as well as LREE. Thus, monazite may form from the prograde breakdown of pre-existing phosphates, notably apatite, which is a common stable mineral at lower (subsolidus) meta-morphic grades but which may break down incongruently as the rocks begin to melt (Wolf and London, 1994,1995).

Here we combine field and petrographic observations, phase equilibria modelling and in situ U—Pb geochronology of zircon and monazite coupled with trace element analysis of these accessory phases and garnet from a sample of aluminous metapelite. We use these data to provide quantitative constraints on the pressure—temperature—time (P—T—t) evolution of the Nagercoil Block and place the findings within the context of the geological evolution of southern India. Further, we suggest that new growth of monazite can occur during both the prograde (melt producing) evolution of migmatites via incongruent melting reactions consuming apatite, as well as during the retrograde path from crystallisation of melt.

2. Geological setting

Palaeogeographic reconstructions of Gondwana have southern India and its close neighbours, Madagascar, Sri Lanka and Antarctica, located at the junction between several orogenic belts that formed as the continental fragments of India, Australia, Azania, Kalahari and Antarctica collided during the latest Neoproterozoic (Ediacaran) or Cambrian (Torsvik et al., 2001; Powell and Pisarevsky, 2002; Meert, 2003; Boger and Miller, 2004; Collins and Pisarevsky, 2005; Trindade et al., 2006; Bogdanova et al., 2008; Pisarevsky et al., 2008; Scotese, 2009). Southern India has been subdivided into a series of discrete crustal units based on lithological associations, crystallisation and model ages, provenance and tectonothermal histories (e.g. Plavsa et al., 2012; Collins et al., 2014). From north to south these are: the Salem Block (or Northern Granulite Terrane; Clark et al., 2009b), the Palghat—Cauvery shear system(PCSS; Collins et al., 2007a; Clark et al., 2009a), the Madurai Block, which has itself been subdivided into discrete terranes (Plavsa et al., 2012,2014) and, furthest to the south, the Trivandrum and Nagercoil Blocks (Santosh, 1996; Collins et al., 2007b; Santosh et al., 2009; Kröner et al., 2015; Fig. 1). Among these, the Nagercoil Block at the southernmost tip of the Indian peninsula has received comparatively little attention and its role and significance in the evolution of the region are uncertain (Santosh et al., 2003, 2006b; Rajesh et al., 2011).

The Nagercoil Block is dominated by massive orthopyroxene-bearing metagranitoids (hereafter charnockites; Rajesh and Santosh, 2012), and has been interpreted by some as a discrete crustal terrane (Santosh, 1996; Santosh et al., 2003). However,

Salem Block Madural Block Trivandrum Block

| Granite-greenstone | | Charnockite | | Supracrustal rocks

| Charnockite | | Migmatite | | Migmatite

| Migmatite \ i_

Shear zones N | | Younger rocks

Figure 1. The distribution of terranes and intervening shear zones in Southern India. The Nagercoil Block, which is dominated by charnockite, is at the southernmost tip of the Indian peninsula. WDC — western Dharwar craton; CDC — central Dharwar craton; EDC — eastern Dharwar craton; MSZ — Moyar—Salem—Atur shear zone; PCSS — Palghat—Cauvery shear system; KKPTSZ — Karur Kambam Painavu Trichur shear zone; TB — Trivandrum block. Isotopic boundary between southern and northern Madurai Blocks from Plavsa et al. (2012). Modified after Plavsa et al. (2014).

U—Pb zircon data from the Nagercoil charnockites show they have Palaeoproterozoic protolith ages similar to those reported from 'old' charnockites in the Trivandrum Block (Ghosh et al., 2004; Kroner et al., 2012), consistent with Nd model ages of 2.2—2.6 Ga (Cenki et al., 2004). In addition, age spectra from zircon and monazite within the Nagercoil charnockites are similar to those from metasedimentary gneisses of the Trivandrum Block to the north (Fig. 1), suggesting they may be part of the same tectonic unit (Santosh et al., 2006b).

Using petrologic and geochemical data, including REE, whole rock oxygen isotopes and stable carbon isotopes extracted from CO2-rich fluid inclusions, Santosh et al. (2009) suggested an igneous origin for the charnockites of the Nagercoil Block. These authors emphasised the adakitic affinities of these rocks and noted their similarities to arc-related charnockites in the Madurai Block, proposing an origin via Pacific-type subduction immediately prior to the amalgamation of Gondwana. In a more recent study Rajesh et al. (2011) interpreted the charnockites to have a metamorphic origin, whose igneous protoliths are suggested to have been granitic plutons derived from partial melting of hydrated basalts (amphibolites) in a subduction zone setting. The high-grade metamorphic event that formed garnet and orthopyroxene is considered to have occurred during the Ediacaran to Cambrian, with garnet growing only in areas where granite had incorporated supracrustal lithologies (Rajesh et al., 2011).

There have been few studies that attempt to constrain the conditions of metamorphism in the Nagercoil Block. Santosh et al. (2003) used conventional thermobarometric methods (garnet— orthopyroxene—plagioclase—quartz barometry and gar-net—orthopyroxene thermometry) on the Nagercoil charnockites to propose peak metamorphic conditions of around 4—6 kbar at

temperatures of 700—900 °C (see their Fig. 5). These authors suggested the retrograde P—T path was characterised by isothermal decompression from 6 to 4 kbar. However, there are no studies constraining the P—T evolution using modern phase equilibria modelling, and there are no detailed datasets constraining provenance or metamorphic history using metasedimentary rocks from the Nagercoil Block.

3. Sample description

The sample selected for detailed study (112—005a) was collected from a metapelitic raft, around 20 m in length, within a charnockite quarry at Kottaram (77°30'57"E, 8°7'48"N) (Fig. 2a). Garnet is enriched in the charnockite adjacent to, and up to several metres from, the contact with the metapelitic raft. Sample 112—005a contains conspicuous porphyroblasts of garnet and prismatic silli-manite, the latter defining a strong foliation that wraps garnet.

In thin section, sample 112—005a comprises coarser-grained layers (generally 5—15 mm thick) rich in feldspar and garnet that are separated by finer-grained and thinner aluminous layers (up to 2 mm thick) dominated by sillimanite, spinel and cordierite, in which sillimanite defines a prominent foliation (Fig. 2b). Garnet forms porphyroblasts up to 10 mm across that are wrapped by this foliation, and some of the porphyroblasts are flattened slightly. Garnet contains inclusions of quartz and feldspar, sillimanite, ilmenite/magnetite, green spinel, zircon and monazite. Sillimanite inclusions are scarce in the cores of garnet where they show no strong preferred orientation, becoming more abundant at the

margins where they are aligned parallel to the matrix foliation (Fig. 2c). Perthite and plagioclase within the coarser-grained garnet-bearing domains are anhedral and generally 0.5—4 mm in diameter. Feldspar grains show weak undulose extinction.

Prismatic sillimanite within the aluminous domains is up to 3 mm long and spatially associated with blocky to cuspate grains of magnetite, composite grains of magnetite and green spinel and rare isolated grains of spinel. In some cases sillimanite is replaced by vermicular intergrowths of spinel and cordierite at its margins. In almost all instances, sillimanite, spinel and magnetite are surrounded by cordierite (Fig. 2c,d). Garnet is variably replaced at its margins by cordierite, particularly where it is near sillimanite and spinel (Fig. 2c,d). Quartz is absent from the matrix and occurs only as inclusions, mainly within garnet. Cordierite only occurs replacing other phases, principally sillimanite, magnetite, spinel and garnet. Biotite forms smaller (<1 mm long) elongate grains that are generally aligned within the foliation, and larger (>1 mm) stubby grains that are at a high angle to the foliation. Both variants appear to have grown across cordierite.

We interpret these petrographic features as evidence for partial melting and melt loss with limited retrograde reaction. The coarser-grained layers are interpreted as deformed K-feldspar-rich leucosomes containing peritectic garnet that grew in equilibrium with sillimanite, spinel and magnetite. The aluminous layers represent the residual portion of the rock. The peak assemblage is interpreted as garnet—K-feldspar—sillimanite—magnetite— plagioclase—ilmenite—spinel—melt. The growth of cordierite replacing sillimanite, spinel—magnetite and garnet, and rare

Figure 2. Field and petrographic features of sample I12—005a. (a) Charnockite quarry at Kottaram with geologists standing on the elongate metapelite raft from which sample I12—005a was collected. (b) Porphyroblast of garnet associated with coarse-grained feldspar (leucosome; bottom and right) and an aluminous layer rich in prismatic sillimanite with magnetite (residuum; left and top). Sillimanite, which defines a foliation that wraps the garnet porphyroblast, is partially replaced by cordierite. (c) Replacement of garnet and spinel by cordierite. The garnet rim contains numerous inclusion of sillimanite with a strong preferred orientation. (d) Sillimanite and garnet (bottom centre) replaced by cordierite. Grains of perthitic feldspar are also present, along with late biotite.

symplectites of spinel—cordierite replacing sillimanite, are suggested to record high-temperature retrograde reaction in the presence of melt. Late biotite probably records crystallisation of the last vestiges of melt.

4. Phase equilibria modelling

Phase equilibria calculations are based on the major element oxide bulk composition of sample 112—005a as determined by XRF analysis, in which the concentrations of ferric and ferrous iron were determined by titration (Table 1). Calculations using THERMOCALC (Powell and Holland, 1988) are in the Na2O— CaO—K2O—FeO—MgO—Al2O3—SiO2—H2O—TiO2—O (NCKFMASHTO) model system and use the end-member thermodynamic data of Holland and Powell (2011) (ds62 dataset as generated on 06/02/14) and a—X models detailed in White et al. (2014a). Although Mn has now been incorporated into these solution models (White et al., 2014b), it has minimal effect at high temperatures and so was not considered.

A P—T pseudosection using the loss on ignition (LOI) as determined by XRF as a direct proxy for the H2O content contains fields appropriate to the inferred peak assemblage (with or without magnetite) at conditions of around 6.5—9 kbar and 850—950 °C (Fig. 3a). However, an H2O-saturated ('wet') solidus is predicted to occur between 650 and 700 °C that is inconsistent with the residual nature of the rock and the preservation of high-T anhydrous minerals (White and Powell, 2002). This may be due to post-peak hydration of the sample (as evidenced by minor pinite in cordierite and sericite in feldspar), inclusion of additional volatile species (principally CO2 in cordierite, F in biotite), errors in the analysed LOI and/or issues with the solution model for melt (e.g. Kelsey and Hand, 2015). Consequently, T—X (Fig. 3b) and P—X (Fig. 3c) pseudosections varying the H2O content were calculated in order to constrain the amount of H2O present, such that the calculated solidus intersects appropriate fields containing those minerals preserved within sample 112—005a. In both cases, the phase equilibria are consistent with H2O contents 30% or less of the measured LO1 (Fig. 3b,c).

Fig. 3d shows a P—T pseudosection for a (residual) composition with H2O content [M(H2O)] 0.25 times the measured LOI. This diagram is characterised by an elevated H2O-absent solidus that occurs at temperatures between 775 and 850 °C and by four NCKFMASHTO divariant fields (labelled with emboldened phase abbreviations on Fig. 3) that correspond to, from relatively low-to high-T, the KFMASH univariant reactions:

bi + sill + q = g + cd + ksp + liq (1)

bi + sill + g = cd + sp + ksp + liq (2)

sill + g + ksp = cd + sp + liq (3)

g + cd + ksp = opx + sp + q + liq (4)

A field for the inferred peak assemblage occurs at pressures of around 6—8 kbar and temperatures in excess of900 °C. At pressures

Table 1

Bulk compositions (as mol.%) used in the calculation of the pseudosections shown in Fig. 3.

Figure H2O SiO2 M2O3 CaO MgO FeO K2O Na2O TiO2 O

3a 6.73 53.99 15.91 0.93 5.47 9.90 2.61 2.53 0.75 1.18

3b and 3c (lhs) 6.73 53.99 15.91 0.93 5.47 9.90 2.61 2.53 0.75 1.18

3b and 3c (rhs) 0.01 57.88 17.05 1.00 5.86 10.62 2.80 2.71 0.81 1.27

3d 1.69 56.90 16.77 0.98 5.76 10.44 2.75 2.67 0.79 1.25

below this field cordierite is predicted to be stable and, at lower temperatures, spinel does not occur (although spinel may be stabilized in rocks to lower T by the presence of Zn, which cannot currently be modelled). A field containing the peak assemblage plus cordierite intersects the solidus at around 5 kbar and 800—830 °C; at slightly lower T biotite is predicted. An example P—T path consistent with the petrographic observations is shown in Fig. 3d.

5. Geochronology, trace element and O-isotope geochemistry

Zircon, monazite and garnet from sample 112—005a were analysed in order to derive temporal constraints linking the evolution of the main silicate mineral assemblage with partial melting and accessory phase growth. Measurements of zircon and monazite were made on grain separates mounted in epoxy, whereas garnet was measured in thin section. U—Pb isotopic analysis of zircon and monazite used the Sensitive High Resolution 1on Microprobe (SHRIMP) housed at Curtin University (Tables 2 and 3). Trace elements in zircon, monazite and garnet were measured at the LA—1CP—MS facility at the same institution (Table 4). Oxygen isotopes were measured on the Cameca IMS 1280 ion microprobe at the University of Western Australia. Full details of the analytical methods are given in the Supplementary data.

5.1. Morphology of zircon and monazite

Zircon grains from sample 112—005a are rounded to elongate and typically 50—200 mm in the longest dimension. Cath-odoluminescence (CL) images identify a variety of textures (Fig. 4a—i) with a range of CL responses. Some grains are characterized by oscillatory-zoned cores with variable CL response typical of zircon grown in a magmatic environment (Hoskin and Black, 2000; Corfu et al., 2003) (Fig. 4a—c). Some grains that are generally mid to light grey in CL show evidence for recrystallization and formation of sector zoning (Fig. 4d—f). Other grains show irregular zoning or broad featureless zones with low CL response that, in some cases, mantle oscillatory-zoned igneous cores (Fig. 4b,c) or light grey CL sector zoned grains (Fig. 4g—i).

Monazite grains from sample 112—005a are typically subequant or irregular in shape and 200—400 mm across (Fig. 5). Backscattered electron (BSE) images show that several grains are zoned. Many show irregular lower BSE response central portions surrounded by brighter BSE regions (e.g. Fig. 5a—c). Others exhibit complex textures with multiple zones of variable BSE response (Fig. 5d,e) or little variation in BSE response (Fig. 5f).

5.2. U—Pb geochronology

Twenty-three analyses of zircon were undertaken; 17 were from rims interpreted to have grown during metamorphism and six were from oscillatory-zoned cores interpreted to be of magmatic origin. Zircon ages from oscillatory-zoned cores are highly discordant (Fig. 6a), typical of partial Pb loss during high-grade meta-morphism (Mezger and Krogstad, 1997). The most concordant analysis of an oscillatory-zoned core (analysis 16.1) gives a 207Pb/206Pb spot age of 2064 ± 42 Ma. The remaining core analyses define a poorly constrained discordant array from this age towards the younger concordant rim data (Fig. 6a). Most oscillatory-zoned zircon cores have U concentrations in the range 500—1000 ppm (Table 2) with Th/U ratios of 0.02—0.36.

The data from zircon interpreted to be metamorphic, as identified by mid-grey CL sector zoning and low CL featureless zones, show <4% discordance (206Pb/238U vs. 207Pb/235U, Table 2) and give 206Pb/238U ages that are spread along concordia between around 600 to 510 Ma (Fig. 6b). Metamorphic zircons possess from around

Figure 3. Phase equilibria modelling results based on the composition of the aluminous metapelite sample 112—005. (a) P—T pseudosection assuming an H2O content equivalent to the loss on ignition (LOI) as measured by XRF. The inferred peak field, with or without magnetite, is outlined in bold, although this composition predicts an H2O-saturated solidus suggesting the H2O content is unrealistically high for peak conditions. (b) Isobaric (P = 6 kbar) T—X pseudosection with variable H2O content ranging from that used in the calculation of (a) [M(H2O) = 1] to much lower values of 0.1 mol.%; [M(H2O) = 0]. Cordierite is not predicted to be stable at the solidus for M(H2O) less than around 0.3. (c) Isothermal (T = 850 °C) P—X pseudosection with variable H2O content as described for (b). At 850 °C a solidus is present only for M(H2O) of around 0.25 or less. (d). P—T pseudosection assuming an H2O content one quarter of the value based on measured LOI, a reasonable estimate based on the T—X and P—X diagrams shown in (b) and (c). An example P—T path is shown that is consistent with petrographic constraints, although the prograde segment is largely unconstrained. The bulk compositions used in the calculations of each phase diagram are given in Table 1.

600 to 2900 ppm U and have Th/U ratios of 0.01—0.16 (Fig. 6b). The two main textures identified in the CL images show a correlation with both U—Pb age and Th/U ratio. Mid-grey CL sector zoned zircon (Fig. 4d—f) typically give the oldest ages, ranging from 639 ± 10 to 547 ± 10 Ma (n = 9). The majority of these grains (7 out of 9) have low U concentrations (<800 ppm) and Th/U between 0.01 and 0.02 (Table 2). Metamorphic zircon that is dark in CL and

generally shows broad, featureless zoning (Fig. 4c, g—i) typically give the youngest ages, ranging from 580 ± 6 to 531 ± 5 Ma (n = 8). These grains have the highest U concentrations (1701—2882 ppm) and Th/U between 0.06 and 0.15 (Table 2). Fig. 6b shows that the U—Pb data for metamorphic zircon are correlated with Th/U ratio. Although there are probably insufficient data on the probability density distribution of the metamorphic zircon ages, the most

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concordant data from low CL-response overgrowth show a peak at around 545 Ma (Fig. 6c).

Twenty-nine monazite grains were analysed covering a range of textures identified using BSE imaging. The 2 6Pb/238U data from monazite are spread along concordia from 584 to 5 6 Ma, with all analyses within 2% of concordance (2 6Pb/238U vs. 2 7Pb/235U, Table 3). While the monazite data form an almost continuous spread, two chemically different groups can be identified corresponding to relative probability age peaks at c. 57 and c. 535 Ma (Fig. 6d). There is a morphological (and compositional) correlation within this spread, with older monazite U—Pb ages typically from the dark BSE portions of the grains, corresponding to low Th/U ratios, and younger ages corresponding to higher BSE response and Th/U ratios. Monazite grains with 2 6Pb/238U spot ages from 584 to 563 Ma have Th/U ratios ranging from 5 to 1 , whereas younger monazite with 2 6Pb/238U spot ages from 559 to 5 6 Ma have Th/U ratios ranging from 8 to 19 (Table 3).

5.3. Trace element geochemistry

Due to the complex nature of the zircon grains in this study, including evidence for recrystallisation and multiple zonation, all but 12 laser ablation analyses resulted in mixed analyses (as identified by interrogating time-resolved data). However, these 12 analyses cover the major zircon types identified both using textural criteria and U—Pb isotopic composition.

Three analyses of oscillatory-zoned zircon cores are relatively enriched in Yb (YbN = 215—567) and are characterised by steep positive HREE profiles (YbN/DyN = 2.9—5.0) and Eu/Eu* [=EuN/ (GdN—SmN) x 0.5] of 0.4—0.12 (Fig. 7a). The chondrite-normalised REE composition of metamorphic zircon indicates two different groups, which correspond to subdivisions previously identified using textural and compositional criteria. The first group, with zircon 2 6Pb/238U spot ages from 639 to 586 Ma, contain relatively high concentrations of middle to heavy REE (YbN = 36—143) and show a distinct 'hump' in the MREE (DyN ranging from 150 to 340), resulting in a slightly negative middle to heavy REE slope with YbN/DyN of 0.2—0.5 (Fig. 7a). The second group, with zircon 206Pb/238U spot ages from 559 to 531 Ma, also shows a negative middle to heavy REE slope (YbN/DyN of 0.3—0.6) but contains middle to heavy REE concentrations that are around an order of magnitude lower (Fig. 7a). The two groups also show a clear difference in the magnitude of the negative Eu anomaly; the older metamorphic zircon has Eu/Eu* values of . 9— .11, and the younger metamorphic zircon has values of . 2— . 2.

To further evaluate the importance of the REE data in metamorphic zircon, three garnets from the sample were also analysed in situ; for each grain two cores and two rims were analysed. The three garnet grains exhibit some inter- and intra-grain compositional variation with relatively high, intermediate and low chondrite-normalised HREE concentrations (Fig. 7b). Garnet 1 shows the highest HREE values (YbN = 108—153) and has a slightly positive middle to heavy REE slope with YbN/DyN of 1.2—1.5. Garnet 2 contains the lowest HREE contents (YbN = 21.5—32.3) and has a slightly negative middle to heavy REE slope (YbN/DyN = 0.3—0.5). Neither grain shows any statistically meaningful variation between core and rim compositions. Garnet 3 shows some core to rim variation, with the core of the grain corresponding to the composition of garnet 1 (YbN of 124—129 and YbN/DyN of 1.5—1.6) (Fig. 7b). The rim of garnet 3 is depleted compared to the core (YbN = 46—64; YbN/DyN = 0.5—0.7). All garnet grains exhibit a similar negative Eu anomaly with Eu/Eu* between 0.006 and 0.014.

Monazite grains have variable trace element compositions that are related to the two different age populations. All grains have typical steep negative chondrite-normalised REE slopes and a pronounced but variable negative Eu anomaly (Fig. 7c). Whilst some correlation between the Th/U ratio and U—Pb age data was identified,

Table 3

SHRIMP monazite U—Pb data.

Spot 206Pb (cts/sec) 232Th/238U ±% 206Pb/ age(1) 238U 207Pb/206Pb age(1) 207Pb/ age(1) 235U % Disc. Total 238U/206Pb ±% Total 207Pb/206Pb ±% 207Pb*/235U ±% 206Pb*/238U ±% Corr.

05A-3.2 4569 9 3.0 506 ±9 515 ±15 507 ±10 +0 12.25 1.9 0.0576 0.50 0.65 2.0 0.0816 1.9 0.9

05A-2.2 4728 11 3.0 525 ±10 518 ±22 523 ±11 -0 11.79 1.9 0.0577 0.55 0.67 2.2 0.0848 1.9 0.9

05A-14.1 4650 16 3.0 529 ±10 520 ±16 527 ±11 -0 11.70 1.9 0.0577 0.50 0.68 2.0 0.0855 1.9 0.9

05A-14.2 5269 16 3.0 531 ±10 499 ±15 525 ±11 -1 11.65 1.9 0.0572 0.47 0.68 2.0 0.0858 1.9 0.9

05A-7.2 5132 17 3.0 532 ±10 527 ±27 531 ±12 -0 11.63 1.9 0.0579 1.19 0.69 2.3 0.0860 1.9 0.8

05A-20.2 4869 13 3.0 533 ±10 516 ±14 530 ±11 -1 11.60 1.9 0.0576 0.49 0.69 2.0 0.0862 1.9 0.9

05A-18.2 4024 8 3.0 535 ±10 561 ±14 540 ±11 +1 11.55 1.9 0.0588 0.53 0.70 2.0 0.0866 1.9 0.9

05A-16.2 3892 10 3.0 536 ±10 538 ±15 536 ±11 +0 11.54 1.9 0.0582 0.54 0.70 2.0 0.0866 1.9 0.9

05A-13.2 5168 19 3.1 537 ±10 543 ±22 538 ±12 +0 11.51 2.0 0.0584 0.47 0.70 2.3 0.0869 2.0 0.9

05A-10.2 4523 12 3.0 540 ±10 552 ±12 542 ±11 +0 11.44 1.9 0.0586 0.50 0.71 2.0 0.0874 1.9 1.0

05A-15.2 4302 18 3.0 541 ±10 544 ±16 542 ±11 +0 11.42 2.0 0.0584 0.51 0.70 2.1 0.0876 2.0 0.9

05A-10.1 4539 9 3.0 543 ±10 548 ±14 544 ±11 +0 11.39 1.9 0.0585 0.56 0.71 2.0 0.0878 1.9 0.9

05A-8.2 4068 14 3.0 549 ±10 558 ±14 551 ±11 +0 11.24 1.9 0.0587 0.53 0.72 2.0 0.0890 1.9 0.9

05A-17.2 4553 12 3.0 552 ±10 562 ±13 554 ±11 +0 11.19 1.9 0.0589 0.50 0.73 2.0 0.0894 1.9 1.0

05A-3.1 4582 9 3.0 558 ±10 607 ±12 568 ±11 +2 11.05 1.9 0.0601 0.49 0.75 2.0 0.0905 1.9 1.0

05A-15.1 3359 12 3.0 559 ±10 569 ±17 561 ±12 +0 11.04 1.9 0.0590 0.58 0.74 2.1 0.0906 1.9 0.9

05A-19.2 4458 8 3.0 563 ±10 629 ±13 576 ±12 +2 10.96 1.9 0.0607 0.50 0.76 2.0 0.0913 1.9 1.0

05A-19.1 4864 6 3.0 564 ±10 583 ±11 568 ±11 +1 10.93 1.9 0.0594 0.48 0.75 2.0 0.0915 1.9 1.0

05A-5.1 3928 8 3.0 566 ±10 571 ±12 567 ±11 +0 10.89 1.9 0.0591 0.53 0.75 2.0 0.0918 1.9 1.0

05A-20.1 4145 8 3.0 567 ±10 564 ±11 566 ±11 -0 10.88 1.9 0.0589 0.52 0.75 2.0 0.0919 1.9 1.0

05A-9.1 4573 5 3.0 567 ±10 589 ±11 571 ±11 +1 10.88 1.9 0.0596 0.49 0.76 2.0 0.0919 1.9 1.0

05A-6.2 4506 7 3.0 569 ±10 534 ±12 562 ±11 -1 10.83 1.9 0.0581 0.50 0.74 2.0 0.0923 1.9 1.0

05A-12.1 4127 8 3.0 572 ±11 581 ±21 574 ±12 +0 10.78 1.9 0.0594 0.95 0.76 2.2 0.0928 1.9 0.9

05A-8.1 4642 7 3.0 574 ±11 566 ±12 572 ±11 -0 10.75 1.9 0.0590 0.49 0.76 2.0 0.0931 1.9 1.0

05A-5.2 4691 6 3.0 574 ±11 571 ±11 573 ±11 -0 10.74 1.9 0.0591 0.49 0.76 2.0 0.0931 1.9 1.0

05A-1.2 4434 8 3.0 576 ±11 579 ±13 576 ±12 +0 10.71 1.9 0.0593 0.50 0.76 2.0 0.0934 1.9 1.0

05A-18.1 5340 6 3.0 576 ±11 558 ±11 572 ±11 -1 10.70 1.9 0.0588 0.46 0.76 2.0 0.0934 1.9 1.0

05A-1.1 4134 10 3.0 578 ±11 582 ±13 579 ±12 +0 10.65 1.9 0.0594 0.52 0.77 2.0 0.0939 1.9 1.0

05A-9.2 6162 5 3.0 580 ±11 602 ±9 584 ±11 +1 10.62 1.9 0.0599 0.42 0.78 2.0 0.0942 1.9 1.0

05A-6.1 5420 7 3.0 581 ±11 592 ±19 584 ±12 +0 10.60 1.9 0.0597 0.88 0.78 2.1 0.0944 1.9 0.9

05A-2.1 5200 6 3.0 584 ±11 576 ±10 583 ±12 -0 10.54 1.9 0.0592 0.46 0.78 2.0 0.0949 1.9 1.0

Errors are 1 a; Pbc and Pb* indicate the common and radiogenic portions, respectively. Disc. = discordance, Corr. = correlation. Error in standard calibration was 0.43% (not included in above errors but required when comparing data from different mounts). (1) Common Pb corrected using measured 204Pb.

Table 4

REE concentrations of selected grains of zircon, monazite and garnet normalised to CI chondrite values of McDonough and Sun (1995). Ages for zircon and monazite are 206Pb/238U.

Age (Ma) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Zircon

5A-16.2 531 0.0 2.2 1.0 4.1 30.7 0.1 35 32 26 25 20 19 17 16

5A-12.2 545 0.0 1.6 0.6 2.7 18.4 0.0 27 24 19 11 11 10 11 8

5A-7.1 546 0.1 2.9 1.1 3.6 28.7 0.6 39 36 25 16 21 15 14 14

5A-4.1 559 0.0 1.6 0.9 4.2 30.7 0.4 40 37 31 18 12 10 8 5

5A-5.1 586 bd 0.5 0.2 1.1 20.2 8.3 127 260 300 196 137 90 83 103

5A-11.1 586 0.0 0.4 0.2 0.4 8.6 3.4 62 129 150 85 55 51 36 46

5A-1.1 596 bd 0.7 0.5 1.5 22.2 9.1 147 276 313 230 196 165 143 139

5A-3.1 598 bd 0.7 0.4 1.6 26.3 11.0 170 339 340 194 140 109 86 105

5A-10.1 639 bd 0.5 0.1 0.5 7.7 3.6 70 156 197 140 102 90 70 64

5A-15.1 860 bd 0.9 0.4 2.1 16.9 1.1 45 60 75 63 89 128 215 296

5A-21.1 1000 0.1 2.7 0.2 0.8 9.6 3.8 52 113 159 153 218 322 498 704

5A-19.1 1033 0.0 0.8 0.0 0.7 7.5 2.0 40 71 113 148 216 384 567 794

Monazite

05A-13.2 537 403,068 382,202 374,635 330,681 133,855 423 38,708 8760 1850 394 108 73 18 29

05A-10.2 542 418,833 382,172 362,290 305,924 134,058 547 47,304 10,193 2283 504 130 87 25 36

05A-15.2 545 440,136 382,172 346,801 275,199 126,920 2171 48,576 10,829 2575 629 163 103 25 38

05A-10.1 547 541,116 382,195 310,325 254,863 133,583 594 61,089 18,799 4808 1038 251 114 39 59

05A-8.2 548 470,814 382,172 326,150 258,621 117,403 2289 51,221 12,523 2802 640 165 99 30 42

05A-17.2 549 406,476 382,182 371,605 357,206 129,708 485 22,721 5433 1144 297 102 78 15 20

05A-3.1 552 452,066 382,180 345,791 301,724 113,936 839 25,025 5950 1141 273 86 67 15 23

05A-15.1 553 416,702 382,178 371,942 330,460 111,149 1014 24,654 6691 1533 401 128 73 19 21

05A-19.2 559 458,458 382,172 333,446 249,779 112,848 957 43,947 8603 1821 461 143 91 26 35

05A-19.1 565 491,692 382,172 337,262 248,453 118,015 5613 57,782 14,788 3795 869 210 102 35 46

05A-5.1 565 453,771 382,188 341,302 291,114 136,846 859 49,237 11,680 2625 570 151 86 22 40

05A-20.1 565 525,778 382,190 316,947 251,105 124,541 11,054 59,766 20,496 5665 1336 347 152 60 60

05A-9.1 573 510,439 382,172 327,497 234,306 118,491 6768 61,750 16,309 3885 853 192 100 34 46

05A-6.2 575 470,388 382,187 336,588 279,399 133,923 3304 56,205 16,116 4108 942 230 102 35 49

05A-12.1 576 537,282 382,192 302,694 247,790 118,967 7304 55,341 18,353 5637 1514 459 174 67 59

05A-8.1 577 576,055 382,172 285,859 226,569 120,870 10,175 69,786 24,545 5941 1427 374 154 65 73

05A-5.2 578 546,229 382,200 294,725 222,812 122,502 10,173 69,023 28,375 7042 1642 384 140 59 71

05A-1.2 579 562,846 382,172 287,093 211,096 112,305 10,564 73,347 27,273 6625 1552 378 157 60 71

05A-18.1 579 527,056 382,172 308,081 223,475 111,217 5429 51,628 12,204 2621 538 129 81 26 35

05A-1.1 580 557,733 382,183 305,275 245,137 122,978 8695 62,970 19,890 4998 1094 258 108 41 56

05A-9.2 583 557,307 382,193 293,715 221,927 120,462 10,679 68,566 27,355 6786 1579 388 151 66 70

05A-6.1 585 571,368 382,198 292,480 221,485 127,940 12,679 70,651 32,259 7857 1876 437 157 63 77

Garnet

5a_grt1_rim — 0.0 0.0 1.0 5.3 73.8 0.8 101 88 98 96 128 149 153 154

5a_grt1_rim — 0.0 0.0 0.8 3.6 59.0 0.8 97 91 92 85 108 94 108 104

5a_grt1_core — 0.0 0.0 0.1 2.0 69.3 0.7 101 86 90 96 121 146 130 116

5a_grt1_core — 0.0 0.0 0.1 1.9 59.0 0.9 92 79 85 91 118 112 132 108

5a_grt2_rim — 0.0 0.0 1.1 7.0 78.2 0.9 111 93 85 66 48 40 29 26

5a_grt2_rim — 0.1 0.0 2.0 8.6 91.4 1.4 110 89 79 58 42 38 26 24

5a_grt2_core — 0.0 0.0 0.5 4.0 70.5 0.8 94 82 69 50 44 39 22 33

5a_grt2_core — 0.0 0.1 1.2 4.6 74.7 1.0 93 80 65 46 45 33 32 26

5a_grt3_rim — 0.0 0.3 1.1 6.3 77.1 0.8 114 98 93 79 60 67 46 43

5a_grt3_rim — 0.0 0.0 0.4 2.9 55.0 0.9 97 88 90 73 66 56 65 59

5a_grt3_core — 0.0 0.0 0.1 2.2 67.5 0.7 84 74 80 92 115 120 124 118

5a_grt3_core — 0.0 0.0 0.1 2.3 68.5 0.4 80 73 87 104 119 113 129 123

the clearest chemical criteria for the different monazite age groups are the REE, particularly the Yb concentration and Eu/Eu* ratio (Figs. 6e,f and 7c,d). YbN and Eu anomaly are correlated (Fig. 7d) highlighting two distinct chemical groups, separated by a group of more widely spread analyses, that may represent either partially recrystallized monazite or mixed laser ablation analyses (Fig. 7d). These two clearly defined groups correspond to the two relative probability age groups identified in the SHRIMP U—Pb data. The first group (YbN = 55—75; Eu/Eu* = 0.10—0.13) gave 206Pb/238U spot ages from 584 to 564 Ma with a weighted mean 206Pb/238U age of 572 ± 5 Ma (MSWD = 2.5, n = 12). The second group (YbN = 14—39; Eu/Eu* = 0.005—0.03) yielded 206Pb/238U spot ages from 569 to 506 Ma, with seven out of nine of the analyses forming a distinct age group with a weighted mean 206Pb/238U age of 536 ± 4 Ma (MSWD = 0.83, n = 7). In addition to these differences, there is also a small variation in the concentration of Ca between the two groups of monazite, with the older group containing 6700—4260 ppm (mean = 5345 ppm), and the younger group 5990—3660 ppm (mean = 4224 ppm).

5.4. Oxygen isotope composition

Oxygen isotope analyses from zircon in the metapelite show d18O values of around 9.0—9.6&. These are around 2& higher than those in the enclosing charnockite (d18O = 6.8—7.8&; Clark et al., unpublished data). These data are consistent with the whole rock O-isotope data collected previously from this locality (Rajesh et al., 2011).

6. Discussion

6.1. P—T constraints

Phase equilibrium modelling of sample 112—005a suggests that the interpreted peak metamorphic assemblage records pressures of around 6—8 kbar and temperatures of 900—950 °C. These results are consistent with extreme metamorphism at ultrahigh temperature (Harley, 1998; Kelsey,2008; Clark et al., 2011; Kelseyand Hand, 2015). Although the rock is residual and the modelled composition

Zircon CL images (scale bars = 25 pim)

Figure 4. Cathodoluminesence images illustrating the range of zircon morphologies. The SHRIMP 206Pb/238U ages and 1 s errors are given. Full analytical results are given in Table 2.

is not strictly valid for constraining conditions prior to the meta-morphic peak, the lack of evidence for the prograde presence of kyanite or cordierite (e.g. as inclusions) suggests the high temperature (suprasolidus) segment of the prograde P—T path did not pass through pressures much higher or lower than those at the

metamorphic peak (Fig. 3d). The late growth of cordierite, including cordierite—spinel intergrowths replacing sillimanite, then biotite is consistent with retrograde decompression at high temperature and the crossing of an elevated solidus at conditions of around 5 kbar and 800 °C. The peak conditions and the example clockwise P—T

Monazite BSE images (scale bars = 50 ^im)

Figure 5. Back-scattered electron imaging of selected monazite grains illustrating the range in morphologies. The SHR1MP 206Pb/238U ages with associated errors (1s) are given. Full analytical results are given in Table 3.

Figure 6. U—Pb geochronological data from sample I12—005a. (a) Tera-Wasserburg concordia plot showing all zircon analyses. (b) Tera-Wasserburg concordia plot for metamorphic zircon (i.e. excluding oscillatory-zoned magmatic cores) colour coded for Th/U ratio. (c) Probability-density plot for metamorphic zircon. The onset of growth of new dark CL-response metamorphic zircon began at around 560 Ma. (d) Probability-density plot for monazite showing two distinct age peaks at around 570 and 535 Ma interpreted to represent monazite growth both predating and post-dating the metamorphic peak. (e) Tera-Wasserburg concordia plot for monazite colour coded for Yb. Two distinct groups are recognised that correspond to the age peaks shown in (d), an older higher Yb (HREE) group and a younger lower Yb group. (f) Tera-Wasserburg concordia plot for monazite colour coded for Eu anomaly. Two groups are recognised, an older group with higher Eu/Eu* group and a younger group with lower Eu/Eu*.

path shown on Fig. 3 are similar to those inferred from other regions within southern India (Brown and Raith, 1996; Raith et al., 1997; Nandakumar and Harley, 2000; Sajeev et al., 2001, 2004; Braun and Appel, 2006; Ishii et al., 2006; Prakash et al., 2007; Shimizu et al., 2009; Brandt et al., 2011; Collins et al., 2014) and broadly consistent with the metamorphic evolution of the Nagercoil Block proposed by Santosh et al. (2003).

6.2. Temporal constraints

The morphology of zircon grains in sample I12—005a suggests that most grew during high-grade metamorphism, although a number of grains preserve cores inherited from the igneous rocks from which the sedimentary protoliths were in part sourced (Fig. 4). Development of sector zoning overprinting and replacing

Figure 7. Chondrite-normalised REE concentration of (a) zircon, (b) garnet and (c) monazite. In (c) the data is calibrated against Ce content, which consequently shows no variation. The data for zircon and monazite are colour coded for age. (d) YbN vs. Eu/Eu* in monazite. Two compositionally distinct groups are identified that correspond to the two mean age peaks at around 570 and 535 Ma.

typical oscillatory-zoned igneous cores is consistent with the U—Pb data and indicates extensive isotopic resetting. These data project back along a discordia to an age of around 2000—2100 Ma (Fig. 6a) that, although poorly constrained, is interpreted as the age of the granitoid source rocks. Our data are consistent with the results reported in a recent study by Kröner et al. (2015) where widespread Palaeoproterozoic magmatic protoliths are identified in the Nagercoil and Trivandrum blocks.

The inferred metamorphic U—Pb age data from both zircon and monazite require careful interpretation, with both accessory phases showing a range in age and chemistry that suggest a complex and protracted metamorphic history. In some cases distinct age populations are identified, whereas in other cases the data overlap and/or exhibit more gradational changes that probably represent partial recrystallization and isotopic resetting. Metamorphic zircon ages range from 639 ± 10 to 531 ± 5 Ma and monazite ages range from 584 ± 11 to 506 ± 9 Ma (Fig. 6), data that together describe a long-lived high-grade metamorphic history.

Although the 2s errors overlap with concordia, the data from the oldest sector-zoned metamorphic zircons (639—635 Ma) are imprecise and yield mean ages that are >10% inversely discordant (Fig. 6b). The oldest concordant metamorphic analyses comprise a population of five grains that together give a weighted mean 206Pb/238U age of 585 ± 10 Ma (MSWD = 2.2). This age may represent recrystallisation and partial U—Pb resetting of inherited zircon. The earlier generation of metamorphic zircon is relatively depleted in U but enriched in middle to heavy REE (Fig. 7a). When compared with garnet 1 and the core of garnet 3 (Fig. 7b), this group shows DTm-Lu(zircon/garnet) of close to unity, suggesting that the earliest population of metamorphic zircon was growing with garnet in the rock (e.g. Harley et al., 2001; Hokada and Harley, 2004; Kelly and Harley, 2005), consistent with the high T prograde segment of the P—T path shown in Fig. 3. However, the MREE are strongly partitioned into zircon, which may have been the result of these grains having formed via recrystallization of pre-

existing zircon rather than by new growth. The first phase of monazite growth, characterised by low Th/U and relatively high Eu/ Eu* and Yb values (Figs. 6c,d and 7c,d) gives a mean age of 572 ± 5 Ma (n = 12; MSWD = 2.5). Similar monazite ages have been reported from the crustal blocks of southern India (e.g. Braun et al., 1998; Santosh et al., 2006a; Taylor et al., 2014) and are here interpreted to record prograde growth during high-grade meta-morphism (see following section).

The distinctive dark CL response zones on zircon (Fig. 4c, g—i) form broad, featureless overgrowths that are interpreted to have resulted from new growth during cooling from the metamorphic peak due decreasing solubility of Zr in melt during melt crystallisation (e.g. Kelsey et al., 2008). This interpretation is supported by the more pronounced negative Eu anomaly, which suggests this zircon was growing with plagioclase as melt crystallised (e.g. Bedard, 2006). The dark CL zones have a weighted mean 206Pb/238U age of 542 ± 8 Ma (n = 7), although the high MSWD (6.7) indicates these data are unlikely to represent a single crystallisation age. We suggest that the spread of concordant age data are geologically meaningful and record protracted growth of zircon during slow cooling and melt crystallization (Korhonen et al., 2011). Thus, the oldest concordant age of the dark CL overgrowths, at around 560 Ma, is interpreted to record the onset of melt crystallization and represents the minimum age of the metamorphic peak.

Although the later (lower HREE) garnets (garnet 2 and garnet 3 rims) also have lower middle to heavy REE concentrations, consistent DREE (zircon/garnet) values of ~0.3—0.4 for the middle to heavy REE, they are unlikely to represent equilibrium growth of these two phases based on previous studies (e.g. Harley et al., 2001; Hokada and Harley, 2004; Kelly and Harley, 2005; Rubatto and Hermann, 2007; Taylor et al., 2015). It is likely that those values, which are low compared to high-T equilibrium values [DREE(zircon/garnet) ~ 1], were the result of slow release of HREE during garnet breakdown during high T retrograde decompression, which resulted in relatively depleted zircon HREE concentrations (Fig. 7a).

The youngest ages from the sample come from high BSE response zones within monazite that cut across older, lower BSE response monazite (Fig. 4j,k). These areas, with a weighted mean 206Pb/238U age of 535 ± 4Ma(n = 7; MSWD = 0.83), are interpreted to represent monazite growth during melt crystallisation while garnet was dissolving but still stable. This is supported by the pronounced negative Eu anomaly, which suggests co-crystallisation of this monazite with plagioclase, and relatively depleted HREE contents compared with earlier monazite growth (Fig. 7c).

6.3. Prograde and retrograde monazite growth?

1f the estimate on the age of the metamorphic peak at around 56 Ma is correct, this implies monazite growth during both the prograde and retrograde evolution of the rocks (i.e. during both melt formation and melt recrystallization). Although monazite may grow along the prograde path in metapelites metamorphosed to (subsolidus) amphibolite conditions (e.g. Wing et al., 2 3), it will begin to dissolve as the rocks melt (Rapp and Watson, 1986). Phase equilibria modelling suggests dissolution of all monazite at temperatures of around 800 °C or less, even for extreme contents of LREE (500 ppm; Yakymchuk and Brown, 2014), although such modelling cannot currently incorporate P. Experiments demonstrate that the dissolution of apatite in peraluminous melts is strongly dependent on the aluminium saturation index [AS1 = molar Al2O3/(CaO + Na2O + K2O)] of the melt, in which the solubility of apatite increases by an order of magnitude as the AS1 of the melt increases from around 1.1 to 1.2 (Wolf and London, 1994). However, the melt is unable to carry any significant quantities of REE regardless of AS1 (Wolf and London, 1995). The result is that, during partial melting in which the AS1 of the melt is increasing, REE-rich apatite should break down incongruently to form REErich monazite and REE-depleted melt (Wolf and London, 1995).

Fig. 8 shows the calculated AS1 of the melt produced along a simplified isobaric (7 kbar) heating path from 810 to 950 °C, a firstorder approximation to the high—T portion of the example P—T shown on Fig. 2. Across this interval the AS1 of the melt increases from 1.16 to 1.27. Apatite is likely to have been the main REE carrying phase within the aluminous metapelite sample 112—005a at lower grades. Apatite would have broken down with increasing temperature to form REE-enriched monazite, consistent with the low BSE response monazite cores in sample 112—005a (Figs. 4j—l and 7c). As the melt began to crystallise, any remaining P2O5 dissolved in the melt would have crystallised to form relatively REE-depleted monazite, consistent with the higher BSE response monazite rims (Figs. 4j—l and 7c). The small enrichment in Ca in the older monazite is consistent with the interpretation that the earlier generation grew by the incongruent breakdown of apatite.

Figure 8. Calculated aluminium saturation index [AS1=molar Al2O3/(CaO + Na2O + K2O)] of the melt produced with heating along a prograde path at 7 kbar. Experiments show the solubility of apatite increases by an order of magnitude as the AS1 increases from around 1.1 to 1.2 (Wolf and London, 1994).

6.4. Regional context

Clockwise P—T paths similar to those constrained here for the Nagercoil Block and for other regions within the Southern Granulite Terrane in 1ndia have been interpreted to be the result of protracted Ediacaran—Cambrian orogenesis as Neoproterozoic 1ndia collided with Azania—East Africa in a setting similar to modern day Tibet (Hacker et al., 2000; Collins and Pisarevsky, 2005; Collins et al., 2014). The crustal oxygen isotope values and the absence of any significant volumes of mafic/ultramafic rocks suggest that the heat required for the ultrahigh-temperature metamorphism was derived largely from thickened crust with elevated concentrations of radiogenic heat-producing elements (Clark et al., 2 11, 2 14) rather than from significant mantle-derived melt advection.

The Nagercoil Block is interpreted to be exotic, lying to the south and east of the southern boundary of Neoproterozoic 1ndia, the Palghat-Cauvery shear system (Collins et al., 2 7a,b; Plavsa et al., 2014; Fig. 1). 1t is also inferred to lie south of the southern margin of Azania, the Neoproterozoic belt defined by the southern Madurai Block and Achankovil Belt (Tomson et al., 2013; Collins et al., 2014; Plavsa et al., 2014). 1n these highly deformed and metamorphosed rocks, pre-collisional and orogenic polarity is hard to determine. Neoproterozoic metaigneous rocks in the Madurai Block that have been interpreted to lie above a subduction zone (Teale et al., 2011; Plavsa et al., 2012) were correlated with Cryogenian continental-arc rocks in central Madagascar (Handke et al., 1999; Plavsa et al., 2012) to suggest that Azania formed the upper plate in the Aza-nia/Neoproterozoic 1ndia collision. Whether the collision of the Nagercoil Block (with or without the Trivandrum Block) with Azania had a polarity sympathetic to this is unknown, but the absence of Neoproterozoic magmatic protoliths in the Nagercoil Block suggests that the Nagercoil Block may have formed the lower plate of this collision zone.

The pre-Gondwana affinity of the Nagercoil Block is the subject of considerable conjecture. U—Pb and Hf isotopic data from zircon (Clark et al., unpublished) suggests that the dominant igneous char-nockites are similar to the SE Congo—Tanzania—Bangweulu Block of Africa and therefore the ultimate origin of the Nagercoil Block was as part of this terrane. A second possibility is that the Nagercoil Block correlates with the Androyen domain of southern Madagascar, where similar-aged protoliths have been reported (Tucker et al., 2014). Boger et al. (2014) suggest that this terrane may have formed a Neo-proterozoic microcontinent separate from Azania, Neoproterozoic 1ndia and the Congo—Tanzania—Bangweulu Block.

Data from Clark et al. (unpublished) suggest that the Nagercoil Block has a different tectonic affinity to the other blocks that make up the Southern Granulite Terrane. 1n the previous sections we demonstrated that the timing and conditions of metamorphism were near identical with those experienced by these other crustal blocks, consistent with the observation that it was the terminal suturing of Gondwana that led to the final relationships of these blocks. Post Gondwana breakup, the Nagercoil Block remained attached to 1ndia rather than Africa.

Acknowledgements

Funding for analyses and fieldwork was provided through Australian Research Council (ARC) Discovery and DECRA projects DP 87933 and DE12 1 3 (to CC), Future Fellowship Scheme #FT120100340 (to ASC) and the Australia-1ndia Strategic Research Fund project #ST030046 (to CC and ASC). CC and RT acknowledge support from Curtin University Strategic Research Funding. Zircon and monazite U—Pb analyses were carried out using the SHR1MP-11 1on Microprobe at the John de Laeter Centre for 1sotope Research, Perth, managed by Allen Kennedy on behalf of a consortium

consisting of Curtin University, the Geological Survey of Western Australia, and the University of Western Australia with the support of the ARC. REE analyses were carried out at the LA—ICP—MS Facility at the Department of Applied Geology, Curtin University. We are grateful to Brad Hacker, Qiong-Yan Yang and an anonymous reviewer for comments that led to an improvement in the final version, to Simon Harley for his advice, and to S. Kwon for his proficient editorial handling. Any remaining infelicities are ours. ASC's contribution forms TRaX Record #323. This is contribution 526 from the ARC Centre of Excellence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.gsf.2014.12.003.

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Tim Johnson is a lecturer at Curtin University in Western Australia. His research concentrates on crystalline rocks and what these can tell us about fundamental lithospheric processes. His expertise is in metamorphic petrology, in particular the application of phase equilibria modelling and the generation, segregation and migration of melt in the crust and upper mantle. More recently his research has concentrated on Archaean geodynamics and the generation and modification of Earth's first crust.

Chris Clark is a principal research fellow in metamorphic geology and geochronology at Curtin University in Western Australia. His main research interests are in the linking of geochronology, specifically the U—Pb method using zircon and monazite, with the development of metamorphic assemblages in order to constrain the durations of mountainbuilding events in high-grade metamorphic terranes.

Rich Taylor is a research associate at Curtin University in Western Australia. His expertise lie primarily in the fields of experimental petrology and analytical geochemistry, particularly related to high-grade metamorphism. He uses multi-instrument techniques to evaluate geological processes based on trace element and isotopic compositions of major and accessory phases.

Prof. Alan Collins is a graduate of the Royal School of Mines, Imperial College, London and did his PhD on the multistage obduction and thrust history of the Lycian Nappes in Turkey, based at Edinburgh University, UK. From then he has worked on a variety of tectonic problems, integrating various geological disciplines to best answer tectonic problems. Alan worked in the University of Leicester, UK, Curtin University and the University of Western Australia before moving to the University of Adelaide in 2005. He is presently a professor of geology, an Australian Research Council Future Fellow and the Director of the University of Adelaide's Centre for Tectonics, Resources and Exploration (TRaX).

M. Santosh is Professor at the China University of Geo-sciences Beijing (China), Specially Appointed Foreign Expert of China, and Emeritus Professor at the Faculty of Science, Kochi University, Japan. He obtained his B.Sc. (1978) from Kerala University, his M.Sc. (1981) from the University of Roorkee, his Ph.D. (1986) from Cochin University of Science and Technology, a D.Sc. (1990) from Osaka City University and another (2012) from the University of Pretoria. He is the Founding Editor of Gondwana I Research and the founding Secretary General of the International Association for Gondwana Research. His research ¿Jit^M jggg"-, fields include petrology, fluid inclusions, geochemistry,

geochronology and supercontinent tectonics. He has published over 350 research papers, edited several memoir volumes and journal special issues, and is co-author of the book 'Continents and Supercontinents' (Oxford University Press, 2004). He has been the recipient of a National Mineral Award, an Outstanding Geologist Award, a Thomson Reuters 2012 Research Front Award, a Global Talent Award, an Island Arc Award and a Thomson Highly Cited Top Frontier Researcher in the World award.