New LA-ICPMS U–Pb ages of detrital zircons from the Highland Complex: insights into late Cryogenian to early Cambrian (ca. 665–535 Ma) linkage between Sri Lanka and India

ABSTRACT Here we report new LA-ICPMS U–Pb zircon geochronology of ultrahigh temperature (UHT) metasedimentary rocks and associated crystallized melt patches, from the central Highland Complex (HC), Sri Lanka. The detrital zircon 206Pb/238U age spectra range between 2834 ± 12 and 722 ± 14 Ma, evidencing new and younger depositional ages of sedimentary protoliths than those known so far in the HC. The overgrowth domains of zircons in these UHT granulites yield weighted mean 206Pb/238U age clusters from 665.5 ± 5.9 to 534 ± 10 Ma, identified as new metamorphic ages of the metasediments in the HC. The zircon ages of crystallized in situ melt patches associated with UHT granulites yield tight clusters of weighted mean 206Pb/238U ages from 558 ± 1.6 to 534 ± 2.4 Ma. Thus, using our results coupled with recently published geochronological data, we suggest a new geochronological framework for the evolutionary history of the metasedimentary package of the HC. The Neoarchean to Neoproterozoic ages of detrital zircons indicate that the metasedimentary package of the HC has derived from ancient multiple age provenances and deposited during the Neoproterozoic Era. Hence, previously reported upper intercept ages of ca. 2000–1800 Ma from metaigneous rocks should be considered as geochronological evidence for existence of a Palaeoproterozoic igneous basement which possibly served as a platform for the deposition of younger supracrustal rocks, rather than timing of magmatic intrusions into the already deposited ancient sediments, as has been conventionally interpreted. The intense reworking of entire Palaeoproterozoic basement rocks in the Gondwana Supercontinent assembly may have caused sediments of multiple ages and provenances to incorporate within supra-crustal sequences of the HC. Further, our data supports a convincing geochronological correlation between the HC of Sri Lanka and the Trivandrum Block of Southern India, disclosing the Gondwanian linkage between the HC of Sri Lanka and Southern Granulite Terrain of India.


Introduction
Sri Lanka represents an integral part in the centre of east Gondwana and hence is an important component of the record of collision and amalgamation of Gondwana. However, the precise geotectonic position of Sri Lanka in East Gondwana with respect to other supercontinent fragments is still controversial (e.g. Kröner et al. 2003;Kehelpannala 2004;references therein). Therefore, it is essential to expand the high-precision geochronology on different types of rocks to explore the capability of correlation of Sri Lanka with its neighbouring Gondwana terrains such as southern India and east Antarctica. The recent studies on metaigneous rocks from Sri Lanka have revealed prominent Neoproterozoic tectonothermal events (e.g. Santosh et al. 2012Santosh et al. , 2014He et al. 2015), which closely correlate the basement of the island with neighbouring Gondwana terrains such as Southern Granulite Terrain of India (e.g. Collins et al. 2007b;Teale et al. 2011;Kröner et al. 2012;Plavsa et al. 2014) and the Lutzöw-Hölm Complex, Antarctica (e.g. Satish-Kumar et al. 2008;Tsunogae et al. 2014Tsunogae et al. , 2015Takamura et al. 2015).
In this study we present LA-ICPMS U-Pb zircon geochronology data of four ultrahigh temperature metasedimentary granulites and two in situ melt

The Kadugannawa Complex
The KC, formerly called 'Arenas' (Vitanage 1972;Almond 1991), forms doubly plunging upright folds in the central part of the country. This unit mainly consists of ca. 900 Ma old hornblende-and biotite-bearing ortho-gneisses, gabbros, diorites, granodioritic to granitic gneisses, charnockites, enderbites, and minor metasediments (Kröner et al. 2003;Willbold et al. 2004). Kleinschrodt et al. (1991) described the KC as a basic layered intrusion at the deepest level. Kriegsman (1994) argued that these intrusions are locally overlain by a thin metasedimentary sequence and contains with calc-silicate, quartzite and itabirites. Based on geochronological, geochemical, and structural evidence some workers (e.g. Kehelpannala 1997;Kröner et al. 2003) suggested that the KC is a part of the WC. More recently, Santosh et al. (2014) argued that the KC is part of a disrupted huge arc magma chamber that was exhumed and transposed along the margin of the WC.

Brief summary of previous geochronological studies
The Highland Complex The HC is the oldest litho-tectonic unit, with Nd-model age of 2000-3400 Ma (Milisenda et al. 1988(Milisenda et al. , 1994. The first geochronological research on Sri Lanka was presented by Boltwood (1907) publishing the first U-Pb ages of ca. 2200 and 900 Ma in thorianites from Galle at the south coast of the southwestern part of the HC. The first detailed geochronological study of the HC was done by Crawford (1969) and Crawford and Oliver (1969) using Rb-Sr and K-Ar methods on whole-rock and mineral samples. The authors suggested that the supracrustal assemblage of the Highland Complex was of Archaean age and the high-grade metamorphism up to granulite facies took place ca. 2000 million years ago. Hölzl et al. (1991Hölzl et al. ( , 1994 and Köhler et al. (1991) presented Rb-Sr whole rock and Sm-Nd garnet ages indicating ca. 2000 Ma. Kagami et al. (1990) reported the Rb-Sr and Sm-Nd isochron ages of 2600-2300 Ma and 500-450 Ma, respectively. Cordani and Cooray (1989) reported Rb-Sr isochron age of 1100 Ma and De Maesschalck et al. (1990) interpreted an Rb-Sr whole rock isochron age of 1930 Ma as the timing of highgrade metamorphism. Hö1zl et al. (1991) and Milisenda (1991) measured garnet-whole rock Sm-Nd ages from para-gneisses and metabasites in the HC and showed the metamorphic ages of ca. 600 Ma. Sajeev et al. (2003) reported a middle Proterozoic internal isochron age of ca. 1500 Ma using garnet core, whole rock, and felsic fraction (quartz + plagioclase) in Sm-Nd system, from a rock having a UHT assemblage. The 'ordinary granulites' of the HC show ca. 550 Ma metamorphic age.
The ages of the detrital zircon spectra are in the range of ca. 3200-2000 Ma (Kröner et al. 1987;Hö1zl et al. 1991Hö1zl et al. , 1994 while metamorphic age has been interpreted as 610-550 Ma. Kröner et al. (1987) reported a Pb loss event around 1100 Ma from metasediment. Using U-Pb isotopes in ilmenite, Burton and O'Nions (1990) measured an isochron age of 1100 Ma, and reported as the metamorphic age. Kröner and Williams (1993) reported zircon U-Pb age of 1900 Ma for granitic gneiss from the Complex as the timing of crystallization of the igneous protolith and the lower intercept of 531 Ma as the age of metamorphism. Sajeev et al. (2007) reported U-Pb zircon metamorphic age of ca. 580 Ma from relatively HP/UHT mafic granulites. In the U-Pb system on zircon and monazite from the Sri Lankan UHT, metasedimentary rocks have ages clustering at 1700 Ma and at ca. 1400-830 Ma, while some of those having overgrowths of ca. 570 Ma (Sajeev et al. 2010). The CHIME dating of monazite by Malaviarachchi and Takasu (2011b) yielded a wide age range of ca. 728-460 Ma. Recently, geochronological studies on metaigneous rocks by Santosh et al. (2014), He et al. (2015) and Takamura et al. (2015) have reported late Neoproterozoic-Cambrian multiple thermal events in the HC. Further, Lu-Hf data of zircon of mafic and intermediate granulites and charnockites show Hf crustal model ages in the range of ca. 1500-2800 Ma He et al. 2015).

The Wanni Complex
The WC yields Nd-model ages of 1000-2000 Ma (Milisenda et al. 1988(Milisenda et al. , 1994 while the intrusion ages of Wanni gneisses range from 790-750 Ma to 1100-1000 Ma. Kröner et al. (1994) reported a Pb-Pb evaporation age of 1330 Ma for detrital zircon indicating that the WC sediments were derived from Mesoproterozoic source rock. The unmetamorphosed granite at Tonigala and Galgamuwa yielded an age of 550 Ma (Hö1zl et al. 1991). Zircons of charnockitic and enderbitic rocks yield ion microprobe U-Pb lower intercept ages between 540 and 590 Ma, correlating with the high-grade metamorphism (e.g. Hö1zl et al. 1991). Recently, Amarasinghe and Collins (2011) reported U-Pb zircon ages of quartzites from 650 to 2745 Ma from the WC. Their major detrital peaks were reported at 650, 1800, and 2700 Ma while metamorphic rims yielded 600-500 Ma. He et al. (2015) proposed that the charnockites from the WC show emplacement age at 1000 Ma, followed by thermal event at 570 Ma. The reported Hf crustal model ages of the WC is in the range of ca. 700-2500 Ma He et al. 2015).

The Vijayan Complex
The VC yields Nd-model ages of 1000-1800 Ma (Milisenda et al. 1988). De Maesschalck et al. (1990 reported Rb-Sr whole-rock isochron age of ca. 800 Ma. Kröner et al. (2003Kröner et al. ( , 2013 interpreted that the emplacement age of the protoliths of the Vijayan gneisses were at 1000-1100 Ma. Zircon U-Pb ages of ca. 590-456 Ma (Kröner et al. 1987;Hö1zl et al. 1991Hö1zl et al. , 1994He et al. 2016) has been interpreted as the age of metamorphism of the VC. In a recent study, Kroner et al. (2013) interpreted that the VC rocks show Pb-loss patterns at 580 and 523 Ma.

The Kadugannawa Complex
The KC yields Nd-model ages ranging between 1000 and 2000 Ma (Milisenda et al. 1988(Milisenda et al. , 1994. Kröner et al. (1987) reported SHRIMP zircon U-Pb age of ca. 1100 Ma. Pb-Pb zircon evaporation ages showed ca. 770-1100 Ma indicating multiple calc-alkaline magmatic activities in the KC (Kröner et al. 2003;Willbold et al. 2004). Most of the dioritic to granodioritic rocks yielded ca. 900 Ma ages. Perera and Kagami (2011) argued that the regional high-grade metamorphism of the KC must be older than 1100 Ma based on a study of Sr-Nd isotope systematics on charnockitic rocks. Santosh et al. (2014) and He et al. (2015) reported U-Pb zircon ages yielding 980-920 Ma as early Neoproterozoic magmatism and metamorphism at 530 Ma. Zircons in their garnet-bearing amphibolite yielded extensive metamorphic recrystallization age of ca. 530-520 Ma. The reported Hf crustal model ages of the KC are in the range of 1200-2800 Ma He et al. 2015).

Sample descriptions and field relations
Six rock samples (sample numbers: UHT 3C, UHT 4D, UHT 6F, UHT 9I, UHT 10J, and UHT 11K) were collected from three localities in the central HC (Figure 1(a,b)). The details of the samples are given below and their field relations are shown in Figure 2.

UHT 3C and UHT 4D
The sampling locality is a quarry in Gampola, which is located slightly close to the ductile shear boundary between the HC and the KC Voll and Kleinschrodt 1991;Cooray 1994, Figure 1 (a,b)). The surrounding area is prominently comprised of garnetsillimanite-biotite-quartz ± graphite gneisses (Khondalites), quartzites, charnockitic gneisses, biotitehornblende gneisses, hornblende-biotite gneisses, marbles, and granitic gneisses (Geological Survey and Mines Bureau 1996a, map sheet No 14). The quarry defines well-foliated rocks with different mineral assemblages as compositional domains. In the rock gradual variation of mineral paragenises without sharp contacts was observed.
Discontinuous layers (approx. 30-40 cm thick) of sapphirine-and kyanite-bearing garnet-sillimaniteorthopyroxene gneiss domains (UHT 3C) occur within the host sillimanite-bearing garnet-orthopyroxene gneiss (UHT 4D) (Figure 2(a,b)). The UHT 3C sample contains mainly medium to coarse subhedral to euhedral garnet (0.25-1.5 cm in diameter; see Supplementary Figure 1 Pyroxene is found associated closely with garnet or as isolated grains in the matrix. Feldspar-rich irregular nabs within ribbon quartz in the matrix might indicate that they were crystallized from a silicate melt. Tiny, disseminated biotite flakes are found in the matrix. The host rock UHT 4D mainly contains porphyroblastic garnet (0.25-2 cm in diameter; see Supplementary Figure 1 (b)), porphyroblastic orthopyroxene (up to 1 cm), ribbon quartz (up to 4 cm in length) and feldspars. Occasionally, ribbon quartz, feldspars, and tiny biotite-rich layers (about 1.5 cm thick) are also present within this host rock.

UHT 6F
Garnet and graphite-bearing quartzo-feldspathic gneiss was collected from a quarry close to Nawalapitiya (Supplementary Figure 1(c)), adjacent to the tectonic contact between the HC and KC. The surrounding lithologies are strongly deformed garnet-sillimanite-biotite ± graphite gneisses (Khondalites), charnockitic gneisses, quartzites, hornblende and biotite gneisses, marbles and granitic gneisses (Geological Survey and Mines Bureau, 1996a, map sheet No. 14). A detailed description of the quarry is reported in Dharmapriya et al. (2015a).
The UHT 6F occurs as fresh and light-coloured massive bands of about up to~10 m thickness within the host charnockitic gneisses (Figure 2(c)). In the charnockite host rock, the UHT 6F occurs parallel to the major foliation of the rock. Quartz-undersaturated domains (corundum-and spinel-bearing garnet-sillimanitegraphite gneisses) frequently occur as patches or boudins with 15 cm to >1 m in diameter (Figure 2(d) and Supplementary Figure 1(d)) within the UHT 6F.
The UHT 6F contains stretched quartz (up to 5 cm in length), recrystallized feldspars, subhedral to euhedral garnet-porphyroblasts (0.25-4 cm in diameter), and fine-to medium-grained graphite flakes. Tiny, acicular biotite flakes occur as thin nabs parallel to the main foliation or around porphyroblastic garnet. Locally occurring biotite, feldspar, and irregular quartz-rich nabs and patches (Supplementary Figure 1(d)) probably have crystallized from a melt.
Discontinuous layers of garnet-bearing quartzofeldspathic rock (UHT 10J) (~30 cm thick) cross-cut nearly parallel to the major foliation of the host charnockite ( Figure 2 Figure 1 (h)). Matrix contains relatively large and medium to coarse feldspars (0.25-1 cm) and irregular-shaped quartz.
Garnet-orthopyroxene bearing coarse-grained samples of UHT 11K were collected from melt patches (~20 cm thick) in the contact between host charnockite and granet-orthopyroxene granulite (UHT 9I, Figure 2 Figure 1(j)). The UHT 11K contains coarse, anhedral garnet (up to 2 cm), coarse anhedral orthopyroxene (up to 2 cm), and coarse grained and irregular shaped quartz. Biotite at the margin of garnet and orthopyroxene could be the late products due to hydration of the host minerals.

Analytical technique
Mineral chemistry Mineral compositions were analysed using a JEOL JXA8530 Field Emission Electron Probe Microanalyzer (FE-EPMA) at the Indian Institute of Science, Bangalore, India. All analyses were carried out using an accelerating voltage of 15 kV and 20 nA beam current with 1-3 μm spot size.
Zircon U-Pb dating After crushing of~1.5 kg of each rock sample using a jaw crusher, zircon grains were separated by gravimetric and magnetic separation methods. Hand picking under the binocular microscope was carried out at the Centre for Earth Sciences, Indian Institute of Science, India.
Subsequently, the selected grains were mounted in epoxy resin discs and polished to expose mid-sections, before the gains were subjected to gold sputter coating followed by taking images under both transmitted and reflected light. To identify internal structures and choose potential target sites for U-Pb analyses, the zircon grains were imaged under cathodoluminescence (CL) at the State Key Laboratory for Continental Dynamics, Department of Geology, Northwest University, Xi'an, China.
U-Pb isotopes in zircons were determined using a Geolas-193 UV laser ablation system coupled with an Agilent 7500a ICP-MS at the State Key Laboratory of Continental Dynamics, Northwest University in Xi'an. Helium was utilized as the carrier gas with a laser beam of 32 μm in diameter and a frequency of 6 Hz. Data acquisition time was 40 s. Zircon 91500 was used as the standard for U-Pb isotopic ratio determination and age calibration. The standard NIST610 was employed as an external standard and Si as the internal standard during trace element analysis. Common Pb was corrected using measured 204 Pb. The degree of discordance of data was <20% during the analysis while in the majority of data with degree of discordance <10%. The detailed analytical procedure is described in Liu et al. (2007). Concordia diagrams and weighted mean calculations were determined using the software Isoplot/Ex Ver3 (Ludwig 2003).

Petrography
A brief description of the textures of the studied rocks is given in the below section. Mineral abbreviations are after Kretz (1983).

The Supplementary
Ribbon quartz (up to 6 cm), recrystallized plagioclase and K-feldspar are present as major constituents of the matrix (Supplementary Figure 2(b)). Biotite and Fe-Ti oxides (exsolved titano-hematite + ilmenite ± rutile) are found as minor minerals while zircon and monazite are present as accessory.
Orthopyroxene, biotite, plagioclase, K-feldspar, and quartz occur as major mineral constituents (Figure 3(b) and, Supplementary Figure 2(c), (d) and (e)) in the matrix. An Fe-Ti oxide (exsolved titano-hematite + ilmenite ± rutile) is the minor constituent. Orthopyroxene often occur close to and/or coexisting with garnet (Supplementary Figure 2(d)). Occasionally garnet is surrounded by biotite-plagioclase symplectite while outer margins of some of the orthopyroxenes are also overprinted by biotite.

UHT 6F
In this rock, quartz, plagioclase, and alkali-feldspar are present as major inclusion phases in garnet. Biotite, rutile, and ilmenite are present as minor mineral phases while zircon, apatite, and monazite are found as accessory minerals (Figure 3(c)). These inclusion phases in garnet suggest the prograde reaction: Bt + Qtz ± Pl = Grt + Kfs + Melt (5).
Less commonly, biotite at the rim of garnet probably indicates the hydration during retrogression via reaction (3). Stretched quartz, strongly recrystallized plagioclase, and alkali-feldspars (Supplementary Figure 3 (f)) occur as major mineral phases in the matrix. Pyrite, ilmenite, and graphite are found as minor mineral phases while zircon and apatite make the set of accessory phases.

UHT 9I
In the UHT 9I, biotite, quartz, and K-feldspar are the major inclusion phases while plagioclase (Figure 3(d)), ilmenite and rutile are the minor mineral phases in garnet. Zircon and apatite are present as accessory phases. The above inclusion phases in garnet suggest that the rock has evolved via the prograde reaction (5). Locally garnet has broken down forming biotite + plagioclase assemblage probably via the reaction (3).
In the matrix, orthopyroxene, quartz, K-feldspar, and plagioclase are present as major mineral phases (Supplementary Figure 3 (g)). Some of the quartz shows slightly elongated nature. Biotite and ilmenite are present as minor phases while zircon and apatite occur as accessory minerals. Coexistence of garnetorthopyroxene is commonly observed (Figure 3(d) and Supplementary Figure 3(g)).

UHT 10J
Garnets of this rock are mostly inclusion free. Occasionally, quartz and K-feldspar inclusions are present (Figure 3(e)). Due to intergrowths of garnet with matrix quartz and K-feldspar, it can be precluded that these garnets could have formed from a melt as a peritectic phase.
As major matrix minerals, K-feldspar and quartz are present while plagioclase occurs as a minor mineral (Supplementary Figure 3(h)). Some of the quartz grains show elongated form. Medium grained subhedral monazite (up to 0.4 cm), ilmenite and zircon are present as accessory phases.

UHT 11K
Garnets of this rock are also mostly inclusion-free and surrounded by quartz and feldspars, probably indicating origin from an existed melt phase, which may have resulted via the prograde melting reaction (reaction 3 and 5) of surrounding rocks ( The matrix of this rock is composed of anhedral coarse to medium orthopyroxene, coarse grained plagioclase, and K-feldspar as major mineral phases (Figure 3 (f) and Supplementary Figure 3(j)) while biotite is found as a minor mineral phase. Ilmenite, monazite, and zircon are present as accessory phases. Frequently, coarse (up to 3 cm) anti-perthite grains are also present (Supplementary Figure 3(j)).

Mineral chemistry
The mineral chemical data are given in Supplementary Tables 1-3 and brief description of mineral compositions is given below.

Garnet
Garnets in all the samples are mainly almandine-pyrope solid solutions. Core and rim compositions of the analysed garnet indicate slight compositional zoning of Fe, Mg, and Ca (Supplementary Table. 1). The garnet cores are slightly depleted in almandine component whereas pyrope content shows opposite behaviour (Supplementary Table 1). The sample UHT 3C contains pyrope-rich garnet and have uniform pyrope content from core to rim (Grt Prp~0 .54). The garnet in samples UHT 9I, UHT 10J, and UHT 11K are characterized by high grossular content (Grt Grs~0 .14, 0.12, and 0.15 at core and 0.11, 0.11, and 0.14 at the rim). All the studied garnets show low spessartine contents (Grs Spe < 0.03).

Pyroxene
The compositions of orthopyroxenes in UHT 3C, UHT 4D, UHT 9I, and UHT 11K are given in the Supplementary Table 2. The X Mg content of orthopyroxene in UHT 3C and UHT 4D are relatively high (X Mg~0 .70-0.72). However, the X Mg in UHT 9I and UHT 10J are~0.51 and~0.49, respectively. The orthopyroxene in sample UHT 3C and UHT 4D shows increased amounts of Al~8.50 wt% and 7.90 wt%, respectively. In both samples, orthopyroxene shows slight zonation from core to rim (Al content of cores are~0.5 wt% richer, compared to rim).

Biotite
The compositional variations of biotite are shown in Supplementary Table 2. Except the biotite inclusions in garnet of UHT 3C, all the other biotites show high TiO 2 content (from~4.53 to~6.80 wt%). The X Mg content is also relatively higher in biotite inclusions in garnet (X Mg 0.56-0.81) compared to biotite in the matrix of all the samples (X Mg 0.56-0.66).

Feldspars
The compositions of plagioclase and K-feldspar are given in Supplementary Table 3. Plagioclase in all the samples is dominated by albite component (Ab 60-80 ). Except for UHT 9I, K-feldspar in all the other samples are rich in orthoclase component (Or 96-80 ). The orthoclase content of the UHT 9I is~0.66.

Spinel
The composition of spinel in the UHT 4D is given in Supplementary Table 3. The X Mg content of spinel is 0.55. Cr content of spinel is around 0.10 wt%. Calculated Fe 3+ of spinel (based on charge balance) is very low (0.005 pfu).

Sapphirine
The Al contents of sapphirine in rock UHT 4D is around 64%. The composition of sapphirine in the UHT 3C is between end member composition of 2:2:1 and 7:9:3. The X Mg content of sapphirine is~0.74. Calculated Fe 3+ of sapphirine (based on charge balance) is very low (0.052 pfu).
Although the rest of the studied rocks lack a typical mineral or mineral assemblage indicating UHT conditions, Dharmapriya et al. (2015b) reported UHT metamorphism from the same localities. Further, Dharmapriya et al. (2015a) calculated the UHT peak metamorphic conditions in the sample of UHT 6F using conventional thermobarometric methods and psudosection calculations. The authors showed that the rocks in the sampling locality have attained 10-10.5 kbar at 850°C during its prograde evolution. Then the rock has subjected to prograde decompression until the peak metamorphism and the peak metamorphism has been taken place T at 950-975°C and P around 9-9.5 kbar. Subsequently, the rock has undergone near-isobaric cooling stage. During this cooling stage garnet + corundum assemblage was formed via consumption of spinel and sillimanite assemblage, at T~930°C at P~9-9.5 kbar.

UHT 3C
The zircons of sample UHT 3C are colourless or light brown and transparent to translucent and show various morphologies. The grains show elongated, spherical, near-spherical, or irregular morphology with lengths of 40-140 μm and length-to-width ratios of~2:1 to~1:1 in CL images (Figure 4(a)). Most zircons show clear core-rim structures with a dark core, which frequently displays oscillatory zoning, surrounded by bright rim (Figure 4(a)). Structure-less anhedral grains with monotonic grey colour was also found.
A total of 36 spots of 33 zircons were analysed from this rock. The results are shown in Supplementary  Table 4. Thirty-five spots are plotted along the Concordia defining seven age groups (Figure 7(a,b)) distinguished as 'rim ages' and 'core ages'. Out of the rim ages, 14 grains gave four tight groups with weighted mean 206   of the grains varies from 40 to170 μm and length to width ratios varies from 2.5:1 to 1:1. Large numbers of most zircons show clear core-rim texture in CL images (Figure 4(b)). Zircon cores show well preserved oscillatory zoning.

UHT 6E
The zircons from this sample are colourless and transparent to translucent. The anhedral to subhedral grains show near-spherical or irregular morphology. The length of grains is in the range of 25-140 μm and length-to-width ratios varying from 2:1 to 1:1. Most zircons show clear dark core and bright rim in CL images (Figure (5)). Some of the cores contain oscillatory zoning. Occasionally, structure-less discrete grains are present with homogeneous grey colour.
A total of 26 spots of 24 zircons were analysed ( Figure  (5)). The results are shown in Supplementary Table 4. Seventeen spots yielded concordant ages defining two clusters in which seven spots yielded a tight group of weighted mean 206 Pb/ 238 U age of 532.9 ± 3 Ma, (MSWD = 0.74, n = 7; Figure 7(e,f)) with a wide range of Th/U (0.02-0.57). There are 10 spots yielded tight group of weighted mean 206 Pb/ 238 U age of 554.7 ± 2.7 Ma (MSWD = 0.19, n = 10). There was a single grain giving 206 Pb/ 238 U concordant age of 761 ± 10 Ma with Th/U value 1.20. Rest of the spots yielded discordant ages from 807.6 ± 7 to 1872.2 ± 15 Ma (Figure 7(e)).

UHT 9I
The zircons from sample UHT 9I are colourless to light brownish and transparent to translucent. The grains show spherical, near-spherical, or irregular morphology. The length of zircon varies from 40 to 160 μm while the length-to-width ratio varies from 2:1 to 1:1. Most zircons show clear core-rim texture in CL images (Figure 6(a)). Frequently, the grains show dark core and bright rims. Occasionally, discrete grains with uniform grey colour are present.

UHT 10J
Zircons in sample UHT 10J are colourless and transparent to translucent. The subhedral to anhedral grains show near-spherical or irregular morphology. The lengths of the grains vary from 30 to 190 μm and the length-towidth ratios vary in the range of 2:1 to 1:1. Most zircons show clear core-rim texture in CL images (Figure 6(b)). Few grains display uniform grey colour without any specific structure.
Twenty-four spots of 24 zircons were analysed. The results are shown in Supplementary Table 4. All the points define concordant behaviour with ages ranging from 534 ± 4 Ma to 567 ± 4 Ma as two tight groups (Figure 8(c)). Twenty-one spots defined a tight group yielding a weighted mean 206 Pb/ 238 U age 557.9 ± 1.7 Ma (MSWD = 0.89) while the other group yielded a weighted mean 206 Pb/ 238 U age of 536. 3 ± 4 Ma (MSWD = 0.40). The Th/U ratios of zircon yielded relatively higher values in ranges of 0.66 -0.78 and 0.50 -0.91.

UHT 11K
Zircons in sample UHT 11K are colourless and transparent showing mainly an irregular morphology. The length-to-width ratios of zircons range from 30 to 180 μm. Most of the zircon grains were structure-less with homogeneous grey colour in CL images (Figure 6(c)).
A total of 35 spots of 35 zircons were analysed. The results are shown in Supplementary Table 4. The analysed grains define two tightly weighted mean 206 Pb/ 238 U age groups (Figure 8(d)). One group of 11 spots yielded weighted mean 206 Pb/ 238 U age of 533.9 ± 2.4 (MSWD = 0.40 and the other group (24 spots) defined a weighted mean 206 Pb/ 238 U age of 552.6 ± 1.6 Ma (MSWD = 0.87). The Th/U ratios of the two groups were in the ranges of 0.19-0.62 and 0.33-0.90, respectively.

Protolith, metamorphic, and in situ melt crystallization ages
Zircons in the UHT 3C define four populations of concordant ages (Figure 7(a,b)) from their dark cores (with wellpreserved oscillatory zoning, Figure 4(a)), of 206 Pb/ 238 U age (weighted mean) 728.6 ± 6.4, 837.7 ± 5.6, 883 ± 5, and 957 ± 15 Ma, together with a single grain defining a 206 Pb/ 238 U age of 1007 ± 13 Ma. The ages of these detrital grains reflect the timing of magmatic crystallization in their protoliths. The light colour anhedral grains and light colour rims of darker grains with low Th/U ratios (Th/U = 0.00-0.11, except one spot) are consistent with typical metamorphic origin and could be categorized into four weighted mean 206 Pb/ 238 U age groups as, 632.8 ± 5.5 Ma, 603.5 ± 8.7 Ma, 558.4 ± 8 Ma, and 536.8 ± 7.4 Ma. These age clusters clearly represent the time span of the metamorphism. The single discordant inherited age of 1785 ± 16 Ma may indicate incorporation of sediments that derived from probably Palaeoproterozoic sources.
The sample UHT 4D, collected from the same quarry as the sample UHT 3C was collected also yield four populations of concordant weighted mean 206 Pb/ 238 U ages (Figure 7(c,d)) from their dark cores with well-preserved oscillatory zoning (Figure 7(a)), of 733.5 ± 5.2 Ma, 863.4 ± 6.9 Ma, 931.2 ± 8.2 Ma, and 979 ± 10 Ma. In addition, three single grains define 206 Pb/ 238 U ages of 778 ± 6 Ma, 807 ± 9 Ma, and 899 ± 6 Ma. All these ages reflect the timing of crystallization from the magmatic protoliths of these detrital grains. The zircon rims with low Th/U ratios (except one grain, all others <0.06) indicate that they are of typical metamorphic origin and yield three coherent tight groups with weighted mean 206 Pb/ 238 U ages of 607 ± 7 Ma, 556.6 ± 3.6 Ma, and 534 ± 10 Ma. The oldest discordant inherited age of 1574.7 ± 11 Ma give a clue for contributions of Palaeoproterozoic protoliths sediments. Dharmapriya et al. (2015b) measured LA-ICPMS zircon U-Pb ages of a sapphirine, kyanite, and spinel bearing garnet-sillimanite-orthopyroxene domain of the same quarry from which the UHT 3C and UHT 4D were collected. The authors reported two populations of concordant ages from dark-zircon cores of 834 ± 12 Ma and 722 ± 14 Ma, reflecting the age of magmatic crystallization of the protoliths, while another coherent group yielded a weighted mean 206 Pb/ 238 U age of 535.2 ± 4.8 Ma (MSWD = 0.22, n = 18) representing the metamorphism.
Zircons in the UHT 6F contain low Th/U ratio (mostly <0.10) representing two coherent groups ( Figure 5(a,b)) with weighted mean 206 Pb/ 238 U ages of 554.7 ± 2.7 Ma and 532.9 ± 3 Ma, reflect the metamorphic origin. A single grain with 206 Pb/ 238 U core age of 761.8 ± 10 M with high Th/U ratio (1.20), probably represents a magmatic zircon derived from Neoproterozoic protolith. The detrital zircons in this sample showed discordant ages from 808 to 1873 Ma. Dharmapriya et al. (2015b) measured LA-ICPMS zircon U-Pb ages of the quartz-undersaturated domains (corundum-and spinel-bearing garnet-sillimanite-graphite gneisses) of the same quarry from which the UHT 6F was collected, and reported weighted mean 206 Pb/ 238 U age of 530.2 ± 3.7 Ma as the metamorphic age (MSWD = 0.78, with a wide range of Th/U from 0.02 to 0.57). The detrital zircons of this study yielded inherited ages between 1676 and 1787 Ma with a weighted mean 206 Pb/ 238 U age of 1722 ± 31 Ma (MSWD = 0.6, N = 7, Th/U = 0.07-0.82) along with a concordant age of 759 ± 13 Ma (Th/U = 0.04) defined by a single grain. Hence our data indicate that the studied metasediments in the quarry represent majority of Palaeoproterozic and Neoproterozoic protolith materials.
Zircons of the UHT 9I form three coherent groups with concordant ages of 575.8 ± 6.4 Ma, 611.6 ± 6.7 Ma, and 665.5 ± 5.9 Ma with low Th/U ratios (less than 0.06) reflecting a metamorphic origin. Magmatic zircons with clear oscillatory-zoned core yield discordant ages from 736 to 2303 Ma and concordant ages of 2164.1 ± 18 Ma, 2454.9 ± 15 Ma, 2573.4 ± 16 Ma, and 2862.2 ± 17 Ma inferring the detrital old ages represent Palaeoproterozoic to Neoarchaean protolith materials as their sources. Dharmapriya et al. (2015b) measured the U-Pb ages of corundum bearing granulite collected from the same outcrop from which UHT 9I, UHT 10J, and UHT 11K were collected, and presented a coherent group of 206 Pb/ 238 U ages from metamorphic zircons with a weighted mean age of 578.0 ± 3.7 Ma (MSWD = 0.35, N = 19) with low Th/U ratios (up to 0.16) reflecting typical metamorphic origin. The 206 Pb/ 238 U age of the core in two zircons of this study yielded an age of 2365 ± 31 Ma (Th/U = 1.33) reflecting Palaeoprotorozoic contributions.
The sample UHT 10J yielded two coherent groups of 206 Pb/ 238 U ages of 558 ± 1.6 Ma and 536.3 ± 4 Ma. Th/U ratios of both groups are 0.50-0.81 and 0.66-0.78, respectively, thus they deviate from the typical metamorphic origin. The rock layer occurs as metre scale discontinuous layers (Figure 2(e), Supplementary  Figure 2j) or irregular patches more or less crosscutting the major foliation showing it as crystallized from a melt phase after the peak metamorphism.
The UHT 11K could also have derived from crystallization of in situ melt, which probably derived from partial melting of host charnockite during the late prograde to peak metamorphism. This rock also crosscuts the major foliation of the UHT 9I and host charnockite (Figure 2(f)). The rock also yielded two coherent groups of 206 Pb/ 238 U ages, 552.6 ± 1.6 and 533.9 ± 2.4 Ma. Th/U ratios of the two groups are 0.34-0.90 and 0.36-0.62 (except one grain) clearly indicating magmatic origin.
Our data show crystallization of in situ melt at Nildandahinna (UHT 10J and 11K) has taken place from ca. 558-535 Ma. Tsunogae et al. (2014) have argued that the dissolution of old zircons during partial melting and regrowth of new zircon with coexisting U-and Th-bearing minerals can allow the growth of high Th/U zircon. Dharmapriya et al. (2015b) presented 206 Pb/ 238 U ages of 542 ± 2 Ma (peak metamorphism) from the UHT khondalite at Gampola and Nawalapitiya (close to the present sampling localities at the KC-HC boundary) and a cooling age of 514 ± 3 Ma. Possibly the latter age may represent the timing of melt crystallization in the central HC. Hence it appears that a time span of ca. 25-30 Ma has lapsed for the melt crystallization between the eastern end of the HC (e.g. Nildandahinna) and the central HC (e.g. Kotmale).

Implications for the depositional history of the HC metasediments
Deposition of sediments of the HC took place from 3200 Ma to 1900 Ma (e.g. Baur et al. 1991: Hö1zl et al. 1991Kröner et al. 1994). Intrusions of most of the granitoid plutons into the HC sediments have taken place at 1800-1900 Ma and very rarely magatic activities are recorded later than 1800 Ma except for a single granitic intrusion at ca. 670 (Baur et al. 1991) and mafic intrusion at ca. 920 Ma (Dharmapriya et al. 2015b). This long silent period without a significant tectono-magmatic activity in the HC was followed by pervasive thermal events of regional granulite facies metamorphism during the Ediacaran period (between 610 and 550 Ma) in response to collision of HC and WC associated with the assembly of Gondwana Supercontinent (e.g. Hö1zl et al. 1991Hö1zl et al. , 1994Kröner et al. 1994). Accordingly, there has been ca. 1500 Ma time span between the last deposition of HC sediments and its high-grade metamorphism without sufficient evidence for magmatic activity, however, remains unexplained (e.g. Perera and Kagami 2011).
Our Palaeoproterozoic to Neoarchaean detrital zircon ages from the sample UHT 9I in this study clearly reflect the ages of magmatic protolith materials of the sample, which is well consistent with the ages derived by previous studies. However, the new age data of the present study (UHT 3C and UHT 4D) and Dharmapriya et al. (2015b), we reveal that the minimum depositional age of the protolith sediments are as young as 720 Ma. Further, incorporation of zircon grains with discordant 206 Pb/ 238 U inherited age of 1575 Ma to 1873 Ma in these two localities precisely indicate contributions from Palaeoproterozoic sources. Previous reports of relict detrital cores with SHRIMP ages of ca. 2500-830 Ma and clusters of ca. 1700 and 1040-830 Ma representing episodes of zircon growth in the Palaeo-Neo Proterozoic protolith sediments (e.g. Sajeev et al. 2010) further support our interpretations.
In Sri Lanka, the oldest inherited age known so far (3200 Ma) was reported by Kröner et al. (1987) from zircons of a metapelite sample close to Polonnaruwa. Subsequently, Amarasinghe and Collins (2011) presented 90% concordant 207 Pb/ 206 Pb ages of the detrital zircons from quartzite of the HC with major detrital peaks at 2200, 2500, and 2700 Ma. Therefore, incorporation of inherited zircons with 206 Pb/ 238 U concordant ages up to 2862 Ma in UHT 9I in this study indicates the extension of the protolith age of the studied rocks into the Neoarchaean. Hence, the new ages of detrital zircon populations of this study imply that shallow marine sediments have derived from Neoarchaean to Palaeoproterozoic multiple age provenances.
In the HC, the U-Pb Concordia upper intercept ages of 1950-1850 Ma of some orthogneisses have been interpreted as the intrusion age of their parent magma (Baur et al. 1991;Hö1zl et al. 1991Hö1zl et al. , 1994Kröner et al. 1994;Santosh et al. 2014). However, we prefer to interpret these ca. 2000-1800 Ma magmatic ages serve best to provide the evidence for existence of a Palaeoproterozoic to Neopreterozoic basement as a platform for deposition of the HC marine sediments. The Ediacaran-Cambrian metamorphic event may have caused the early reworked older basement rocks to be repetitively reworked subsequently.
Our interpretations are further supported by published U-Pb and Lu-Hf isotope data from the HC. Milisenda et al. (1994) He et al. (2015) reported that zircons in a charnockite with crystallization age of 565 Ma in the HC shows negative εHf(t) values in the range of −6.7 to −12.6 with Hf crustal model ages of 2039-2306 Ma suggesting magma derivation through melting of a Palaeoproterozoic source. Also, a metadoirite sample (crystallization age~576 Ma) in the same sampling locality showed εHf(t) range from −11.1 to 1.6 suggesting a mixed source of both older crustal and juvenile material.
Hence, the Palaeoproterozoic to Neoproterozoic igneous basement may have provided the platform for deposition of the HC sediments derived from multiple sources. The Neoproterozoic collisional event around Sri Lanka has resulted in intense reworking of the older crust. The magma derived from melting of the already reworked crust may have given rise to crystallization of granitic, charnockitic and metadioritic rocks with diverse age populations. Subsequently, the remnants of the existed basement rocks (residue after melting of the already reworked crust) could have metamorphosed under granulite facies during the Ediacaran-Cambrian thermal event. The geochemical studies of the HC metasediments indicating that the deposition of sediments has taken place in a stable shelf region of shallow marine environment (e.g. Dissanayake and Munasinghe 1984;Kröner et al. 1994;Prame and Pohl 1994;Santosh et al. 2014) further support above arguments.

Late Cryogenian to early Cambrian multiple thermal events of the Highland Complex and the neighbouring Gondwana terrains
As summarized in previous sections, several metamorphic ages have been proposed for the Sri Lankan basement. From Rb-Sr data the age of metamorphism was considered as ca. 2000-2500 Ma (e.g. Crawford andOliver 1969: De Maesschalck et al. 1990;Kagami et al. 1990). Using U-Pb zircon ages Kröner et al. (1987) earlier, argued that the regional metamorphic event of the HC to be at ca. 1100 Ma. However, there are number of studies showing U-Pb zircon ages confining the age of metamorphism at ca. 610-550 Ma simultaneous with the assembly of Gondwana (Baur et al. 1991;Hö1zl et al. 1991Hö1zl et al. , 1994Kröner and Williams 1993;Kröner et al. 1994Kröner et al. , 2003Kröner et al. , 2013Sajeev et al. 2010). Further, recent studies from the HC further expanded the duration of the time span of this late Neoproterozoic-Cambrian event by providing evidence for multiple thermal activities from 728 to 511 Ma (e.g. Malaviarachchi and Takasu 2011b;Santosh et al. 2014;He et al. 2015;Takamura et al. 2015). This Edicaran-Cambrian highgrade metamorphism has been geochronologically well documented in all the neighbouring Gondwana terrains such as southern and central Madagascar (Paquette et al. 1994;Collin et al. 2012), Southern Granulite Terrain of India (e.g. Collins et al. 2007b;Clark et al. 2009aClark et al. , 2009bBrandt et al. 2011), and East Antarctica (e.g. Black et al. 1992;Shiraishi et al. 1994;Tsunogae et al. 2015) strongly suggesting terrain amalgamation during the Neoproterozoic.
The weighted mean 206 Pb/ 238 U metamorphic ages obtained by our samples are 665.5 ± 5.9 Ma, 611.6 ± 6.7 Ma, and 575.8 ± 6.4 Ma, from UHT 9I, 632.8 ± 5.5 Ma, 603.5 ± 8.7 Ma, 558.4 ± 8 Ma, and 536.8 ± 7.4 Ma from the UHT 3C and 607 ± 7 Ma, 556.6 ± 3.6 Ma, and 534 ± 10 Ma from the UHT 4D. All these ages are representative of multiple thermal events suffered by the UHT granulites of the HC from late Cryogenian to early Cambrian time. The geochronological evidence for the Ediacaran-Cambrian multiple metamorphism from 612 to 534 Ma is well consistent with the reported previous U-Pb metamorphic zircon ages from the Sri Lankan UHT granulites (Sajeev et al. 2007(Sajeev et al. , 2010Dharmapriya et al. 2015b) and HT ordinary granulites (Baur et al. 1991;Hö1zl et al. 1991Hö1zl et al. , 1994Kröner and Williams 1993;Kröner et al. 1994;Santosh et al. 2014;He et al. 2015;Takamura et al. 2015) in the HC. Dharmpriya et al, (2015b) interpreted that the time span of UHT metamorphism in the HC is from 580 to 530 Ma. The sharp peak around 580-530 Ma in the compiled age data probability curve ( Supplementary  Figure 4) of studied samples also further confirm that the late Neoproterozoic UHT metamorphism of the HC. However, our older metamorphic zircons with weighted mean 206 Pb/ 238 U age of 665.5 ± 5.9 Ma and 632.8 ± 5.5 Ma propose a new evidence for early prograde metamorphism of the HC. Kriegsman, (1993Kriegsman, ( , 1996 and Dharmapriya et al. (2014a) argued that the HC underwent low pressure, relatively high temperature amphibolite facies metamorphism prior to crustal thickening. Kriegsman (1993) further argued that this low-pressure high-temperature metamorphism could be due to crustal extension. Baur et al. (1991) reported evidence for 670 Ma intrusions of granitoids in the HC. Malaviarachchi and Takasu (2011b) also presented CHIME dating of monazite yielding a wide age range of~728-460 Ma as a new metamorphic age group for the metapelites of Sri Lanka. Sajeev et al. (2007) showed U-Pb concordant ages from zircon cores up to ca. 661 Ma in a mafic granulite collected from the central HC. Recently, Santosh et al. (2014) presented U-Pb concordant ages of ca. 665 Ma from magmatic zircons in a mafic sill within metasediments of the HC. Hence our early metamorphic ages (665.5 ± 5.9 to 534 ± 10 Ma) could represent the late Cryogenian granitic and mafic magmatism associated with early prograde metamorphism in the HC.
Consideration of East Gondwana terrains, South Kenya, and Tanzania, indicates a major episode of high-grade metamorphism at 640 Ma related with the East African orogeny (Meert 2003). It was suggested earlier that lack of evidence for ca. 640 Ma orogenic episode in Sri Lanka, East Antarctica, and India could be due to late collision of these terrains and hence unaffected by the older deformation (Meert 2003). However, recent studies of Gondwana terrains, which were adjacent to Sri Lanka, reveal rare evidence for ca. 670-620 Ma metamorphism (e.g. Brandt et al. 2011;Sengupta et al. (2015); Johnson et al. 2015). Brandt et al. (2011) showed U-Th-total Pb old monazite ages from 656 ± 65 to 620 ± 59 Ma in Palni Hills, Madurai block, Southern India, Sengupta et al. (2015) reported new zircon growth from 633 to 487 Ma from a metapelite in alkaline granite Complex of Sankagiri, Southern granulite terrain, India and Johnson et al. (2015) presented older metamorphic age ca. 640 Ma from metasediments in the Nagercoil Block of Southern India. Additionally, Paquette and Nédélec (1998), showed that Madagascar collided with East Africa at about 650 Ma and underwent extensional collapse at 630 Ma. Collins (2006) argued that the Antananarivo Block in Madagascar was thermally and structurally reworked between 700 and 532 Ma and during this reworking process, pre-existed rocks were subjected to granulite facies metamorphism. Further, Ashwal et al. (1999) argued that a series of amphibolites facies metamorphic events occurred in the Southern Madagascar between 630 and 530 Ma. De Wit et al. (2001) provided more evidence for this argument based on structural and geochronological studies. Shiraishi et al. (2008) reported evidence for granulite-facies metamorphism at ca. 600-650 Ma in the Northeastern terrene of Sør Rondane Mountains in the Eastern Drowning Maud Land, East Antarctica. They suggested that theses rocks have undergone amphibolite facies metamorphism at ca. 570 Ma. In contrast, Nakano et al. (2013) showed a 655-635 Ma thermal event from the Sør Rondane Mountains related with granulite-facies metamorphism. Thus, late Cryogenian to early Cambrian multiple thermal events of Sri Lanka and its neighbours clearly indicate existence of robust geochronological evidence to correlate the adjacent terrains.

Geochronological correlation of Sri Lanka and Southern Granulite Terrain of India
Southern India is composed of a collage of polymetamorphic terrains with prolonged crustal evolution history from early Archaean to late Neoproterozoic (e.g. Santosh and Sajeev 2006). Based on protolith origins and tectonothermal histories, Southern India has been divided into a series of tectonic units (Supplementary Figure 5) from north to south, called: the Salem Block (SB, the Northern Granulite Terrene); the Palghat-Cauvery shear zone (PCSS); the Madurai Block; Achankovil Shear Zone (ACSZ); the Trivandrum Block (the Kerala Khondalite Belt); and the Nagercoil Block (e.g. Gosh et al. 2004;Santosh and Sajeev 2006;Santosh et al. 2009;Collins et al. 2015). The Northern Granulite Terrain is separated by the east-west striking crustal-scale Palghat-Cauvery Shear Zone (e.g. Santosh and Collins 2003a), recognized from the dominantly metasedimentary gneisses and granulites that experienced high-grade metamorphism and pervasive deformation in Neoproterozoic-Palaeozoic times. This southern region is known as the Southern Granulite Terrain (e.g. Santosh and Sajeev 2006).
Approximately 70 km × 400 km east-west extending Palghat-Cauvery Shear zone System (PCSS, Supplementary Figure 5) is characterized by a network of mainly dextral shear zones, located on the southern margin of the Salem Block (Supplementary Figure 5) Tomson et al. 2006;Sato et al. 2011b;Collins et al. 2015). The Madurai Block, which is the largest crustal block in the Southern Granulite Terrain (SGT) and geochronologically, has itself been subdivided into separate terrains as Northern Madurai Block (NMB) and the Southern Madurai Block (SMB, e.g. Plavsa et al. 2012Plavsa et al. , 2014Collins et al. 2015). The Karur-Kamban-Painavu-Trichur lineament (KKPT) is the lithological boundary that separates the Neoarchean northwestern Madurai Block from the rest of the predominantly metasedimentary Madurai Block (Ghosh et al. 2004,). Some workers (e.g. Plavsa et al. 2012Plavsa et al. , 2014Plavsa et al. , 2015Collins et al. 2015) defined an isotopic boundary between Neoarchean to Palaeoproterozoic NMB and Meso-to Neoproterozoic SMB.
The Achankovil Shear Zone (ACSZ) demarcates the southern limit of the Madurai Block, as well as the northern limit of the Trivandrum Block and has traditionally been considered as a typical shear zone (Drury et al. 1984;Sacks et al. 1997;Rajesh et al. 1998;Tadokoro et al. 2008). Based on geophysical surveys across the region, some workers have suggested that the ACSZ is a terraine suture (Santosh et al. 2009;Dhanunjaya Naidu et al. 2011). The southernmost tip of India, which is mainly composed of massive charnockites, has been described by some authors as an isolated tectonic unit called the Nagercoil Block (NB) (Santosh 1996;Santosh et al. 2003b).
Previous workers attempted to correlate the SGT of India and the HC of Sri Lanka based on lithologies (e.g. Fernando 1949;Pichamuthu 1967;Yoshida 1988;Braun and Kriegsman 2003), geological structures and metamorphic grades (e.g. Katz 1974Katz , 1978Braun and Kriegsman 2003), mineralization trends (e.g. Dissanayake and Chandrajith 1999), and geochronological aspects (e.g. Crawford and Oliver 1969;Braun and Kriegsman 2003;Teale et al. 2011). However, lack of sufficient geochronological data was a drawback for appropriate correlation between the two terrains. During the last decade, a number of geochronological studies revealed new magmatic, sediment deposition and crustal evolution ages in the SGT (e.g. Cenki et al. 2004;Ghosh et al. 2004;Collins et al. 2007b;Collins et al. 2015;Teale et al. 2011;Kröner et al. 2012;Plavsa et al. 2012Plavsa et al. , 2014 and the HC (e.g. Sajeev et al. 2007Sajeev et al. , 2010Malaviarachchi and Takasu 2011b;Santosh et al. 2014;He et al. 2015;Takamura et al. 2015;Dharmapriya et al. 2015b) presenting more supportive evidence to trace Gondwanian linkage of Sri Lanka with Southern India. The Figure 9 summarizes the data of crust formation ages (Nd model ages and Hf model ages), magmatic zircon ages, detrital zircon/monazite ages, and metamorphic zircon/monazite ages of the HC and the SGT.
There are a limited number of Hf crustal model ages the have been reported from both Sri Lanka and SGT of India ( Figure 9). The Hf model ages of the SB fall between 2700 and 2930 Ma and εHf(t) between +0.5 and +9.2, indicating the detritus was derived from relatively juvenile Archaean terranes. The NMB yield a wide range of Hf crustal model ages from ca. 2500-3900 Ma (εHf(t) from -36.9 to +8.7) indicating reworking of Archaean crust with possibly mixing with some juvenile material. The Hf crust formation ages of the SMB range between 1.30 and 3.65 Ga and εHf (t) between -31.2 and +7.7 while most of the juvenile signatures (εHf(t) ranging from+3.8 to +5.8) are reported from 1000-1100 Ma detrital grains in metasediments. Kröner et al. (2012) reported Hf crustal model ages ranging from 2680 to 3370 Ma (εHf(t) between −6.1 and −9.2) from a charnockite which define 207 Pb/ 206 Pb upper intercept ages at 1881 ± 14 Ma from samples of TB indicating reworking of Neoarchaean to Palaeoarchaean crust. The reported Hf crustal model ages of the ACSZ are in the range of 2950-3140 Ma (εHf(t) between −6.0 and −17.0) which indicate the reworking of Mesoarchean crust. The Hf crustal model ages of the HC are in the range of 1500-2800 Ma He et al. 2015). The obtained ages from mafic granulite ca. 1500-1650 Ma and interpreted as mixed source from both juvenile Neoproterozoic and reworked Mesoproterozoic components He et al. 2015). The Hf crustal model ages are in the range of 1850-2800 Ma of metagabbro, mafic granulites, and charnockites, and have been interpreted as the age of source material for the magma evolved from reworking of the Neoarchean-Palaeoproterozoic crust. Santosh et al. (2014) reported Hf crustal model ages of 2156-3580 Ma (mean 2614 Ma and εHf(t) between −33.3 to −10.5) for charnockite sample with upper intercept age of 1812 ± 63 Ma, well correlate with the reported upper intercept ages and Hf model ages (Kröner et al. 2012) of the charnockite in the TB.

Correlations of magmatic ages
Geochronological studies on SB, PCSS, and NMB indicate that the sedimentation of the protoliths of the metasedimentary rocks have taken place on the Archaean/Palaeoproterozoic gneissic basement over a prolonged period of time (e.g. Ghosh et al. 2004;Clark et al. 2009b;Brandt et al. 2011;Sato et al. 2011a;Plavsa et al. 2012;Yellappa et al. 2012;Sengupta et al. 2015). The 820-750 Ma granites, tonalites, and gabbros intruding in to ca. 2500 Ma basement and supra-crustal rocks have been reported in the NMB (e. However, there is no evidence so far reported for existence of Archaean/Palaeoproterozoic gneissic basement in SMB (e.g. Collins et al. 2015) which contains evidence for Neoproterozoic (1000-790 Ma) magmatism (Plavsa et al. 2012;Sato et al. 2012;George et al. 2015). Evidence for Palaeoproterozoic and Mesoproterozoic (1500-950 Ma) magnetism has been reported in ACSZ (Kröner et al. 2012;Collins et al. 2015;Plavsa et al. 2015). The TB contains evidence for ca. 1800 Ma (Kröner et al. 2012) and ca. 950 Ma (Ghosh et al. 2004)   The 1800-2000 Ma magmatism in the HC is coeval with the Palaeoproterozoic magmatism in the TB and NC (e.g. Kröner et al. 2003). Dharmapriya et al. (2015b) reported rare evidence for ca. 920 Ma magmatism from the HC. In contrast, there is no evidence for ca. 670-650 Ma magmatism in the TB and the NB reported so far, although it has been reported in the HC (Figure 9). However, the WC of Sri Lanka shows Neoproterozoic magmatism from 1000-730 Ma (Hö1zl et al. 1991(Hö1zl et al. , 1994Santosh et al. 2014;He et al. 2015) fitting with the magmatism in the SMB (e.g. Teale et al. M. 2011;Collins et al. 2015).
Correlation of detrital zircon/monazite ages Braun and Kriegsman (2003) discussed that although Sm-Nd fractionation during the partial melting may have occurred, these distinct Nd isotope systematics suggest derivation of the sediment from Palaeo-, Meso-and Neoproterozoic sources in SGT. However, later studies (e.g. Santosh et al. 2003bCollins et al. 2007bCollins et al. , 2015Kooijman et al. 2011;Plavsa et al. 2014Plavsa et al. , 2015Sengupta et al. 2015) revealed the incorporation of Neoarchean to Neoproterozoic sources in the metasediments of the SGT.
The detrital zircon spectra of NMB and SMB display 3400-1700 Ma and 2700-650 Ma, respectively (e.g. Ghosh et al. 2004;Collins et al. 2007bCollins et al. , 2015Kooijman et al. 2011;Teale et al. 2011;Plavsa et al. 2014). The ACSZ contains the detrital zircon ages from ca. 2000 to 650 Ma (Collins et al. 2007b;Sato et al. 2010) and those of TB are variable from 3000 to 1700 Ma. However, detrital zircon ages from 2000 to 1000 Ma  and detrital monazite ages (CHIME method) from 1900-700 Ma (Santosh et al. 2003b) indicated that the deposition of sediment has taken place up to the Neoproterozoic. The NB records detrital zircon ages from 2500-2200 Ma (Collins et al. 2007b).
As discussed in a previous section, most of the detrital zircon ages of the HC metasediments are distributed in a range of 3400-1900 Ma (Supplementary Figure  5). Thus, these Mesoarchaean to Palaeoproterozoic protolith ages are well correlated with protolith ages of NMB and TB. The detrital ages of ca. 2800-720 Ma reported from this study and 2400-830 Ma by Sajeev et al. (2010) also well correlate with those reported in the TB since both terrains provide geochronological evidence for incorporation of Mesoarchaean to Neoproterozoic protolith sediments.
The presence of 2000-1200 Ma Nd model ages (in the south eastern part of the SMB and the eastern margin of the ACSZ), 2800-650 Ma detrital zircon and monazite ages, 1000-750 Ma magmatic zircon ages and 600-500 Ma. Metamorphic ages in the ACSZ and SMB of the SGT and the WC of Sri Lanka may also indicate additional evidence for the Gondwana linkage. Hence, based on above petrological and geochronological relations, the possible Gondwana linkage between Sri Lanka and SGT of India is illustrated in Figure 10 and Supplementary Figure 6.
As shown in the Figure 9, the correlation of crust formation ages, magmatic ages, detrital zircon/monazite ages and metamorphic ages of the TB of India and the HC Sri Lanka clearly set the baseline for correlation of the Gondwana linkage between Sri Lanka and Southern Granulite Terrain of India.
On the other hand, several workers have considered that the Lützow-Holm Complex (LHC) of East Antarctica as an extension of the HC of Sri Lanka (e.g. Yoshida 1988;Yoshida et al. 1992;Shiraishi et al. 1994;Kriegsman 1995;Braun and Kriegsman 2003;Tsunogae et al. 2015). Although detrital zircon ages ca. 1000 Ma have been reported from the LHC (Shiraishi et al. 1994(Shiraishi et al. , 2008, middle to late Neoproterozoic detrital zircon ages have not been reported so far. However, based on Sr, C, and O isotope studies, Satish-Kumar et al. (2008) suggested an apparent age of ca. 730-830 Ma for deposition of protoliths of carbonate rocks (marbles) in the LHC. Hence, the deposition of sediments of the LHC could have taken place during the Neoproterozoic coeval to that of the HC probably providing evidence for a Neoproterozoic suture/orogenic belt extending from TB of India to LHC of Antarctica across the HC of Sri Lanka.

Conclusions
The following conclusions are reached from the present study.
1. The incorporation of ca. 2800-700 Ma detrital zircons in the studied UHT pelitic rocks indicates that the metasediments of the Highland Complex, Sri Lanka have been derived from Neoarchaean to Neoproterozoic multiple provenances. The studied metasedimentary rocks have undergone multiple thermal events from the late Cryogenian to early Cambrian (ca. 665-530 Ma) in which ca. 665 Ma and ca. 630 Ma could represent evidence for prograde metamorphism while the time span of peak UHT metamorphism is from ca. 580-530 Ma.
2. The deposition of sediment in the Highland Complex has taken place at the Neoproterozoic, atop then existing Palaeoproterozoic igneous basement of 2000-1800 Ma age and subjected to intense deformation associated with multiple thermal events during the late Cryogenian to early Cambrian (ca. 665-500 Ma) coeval with the assembly of Gondwana Supercontinent. During these events, the Palaeoproterozoic basement was reworked repetitively forming supra-crustal sequences of multiple ages in the Highland Complex.
3. The presence of similar crust formation ages, magmatic ages, detrital zircon/monazite ages and metamorphic ages in both Sri Lankan Highland Complex and the Trivandrum Block of India clearly set the baseline for correlation of the Gondwana linkage between Sri Lanka and Southern Granulite Terrain of India.
We are thankful to S. Opatha and Thilini Harischandra at the National Institute of Fundamental Studies (NIFS), Kandy for technical helps and support in preparing petrographic thin sections. PLD and SPKM are grateful to the Department of Geology, University of Peradeniya for all supports. C.B. Dissanayake and N.D. Subasinghe are acknowledged for providing additional facilities at the NIFS.