Isotopic systematics of zircon indicate an African affinity for the rocks of southernmost India

Southern India lies in an area of Gondwana where multiple blocks are juxtaposed along Moho-penetrating structures, the significance of which are not well understood. Adequate geochronological data that can be used to differentiate the various blocks are also lacking. We present a newly acquired SIMS U–Pb, Lu–Hf, O isotopic and trace element geochemical dataset from zircon and garnet from the protoliths of the Nagercoil Block at the very tip of southern India. The data indicate that the magmatic protoliths of the rocks in this block formed at c. 2040 Ma with Lu–Hf, O-isotope and trace element data consistent with formation in a magmatic arc environment. The zircon data from Nagercoil Block are isotopically and temporally distinct from those in all the other blocks in southern India, but remarkably correspond to rocks in East Africa that are exposed on the southern margin of the Tanzania–Bangweulu Block. The new data suggest that the tip of southern India has an African affinity and a major suture zone must lie along its northern margin. All of these blocks were finally brought together during the Ediacaran-Cambrian amalgamation of Gondwana where they underwent high to ultrahigh temperature metamorphism.

A single analysis at the rim of a zircon from this sample gave a 206 Pb/ 238 U age of 515 ± 12 Ma that is within uncertainty of the lower intercept age of 550 ± 60 Ma (Fig. 6b). The REE analysis of zircon, garnet and orthopyroxene suggests that the 515 Ma zircon rims were in equilibrium with the garnet and orthopyroxene, whereas the oscillatory-zoned cores show typical igneous patterns and are not in equilibrium with the garnet (Fig. 7b). The discordant garnet leucogranite (IND12-004A) gave a single population of zircon with individual 206 Pb/ 238 U spot ages ranging between 590 Ma and 540 Ma (Fig. 6a). The REE analyses of garnet and zircon from this sample are suggestive of equilibrium between the two minerals (Fig. 7a).  (c) Field photo of the equigranular garnet-bearing charnockite. (d) Field photo of the garnet-bearing leucogranite with large (up to 2 cm) garnets. (e) Photomicrograph of the garnet-bearing charnockite (sample Ind12-04b) with garnet (grt), orthopyroxene (opx), biotite (bi) in a framework of plagioclase (pl) and quartz (q). Note that the orthopyroxene is being partially altered to amphibole. (f) Photomicrograph of garnet-bearing leucogranite (sample Ind12-05A) showing a euhedral garnet within a framework of plagioclase and quartz. (g) Cathodoluminesence (CL) image of at typical oscillatory-zoned (igneous) zircon core mantled by CL-bright (metamorphic) rim from the garnet-bearing charnockite. (h) CL image of a sector zoned zircon from the garnet-bearing leucogranite. Kozhikodupothai. Three samples (two garnet-leucogranites one with patchy charnockitisation and a garnet-bearing charnockite) were analysed at this location. The garnet leucogranites (IND12-006B and IND12-006C) both gave discordant arrays of analyses with poorly defined upper intercept ages of 2012 ± 53 Ma and 2029 ± 65 Ma and lower intercept ages of 571 ± 31 Ma and 536 ± 48 Ma (Fig. 5e,f). There was some minor zircon rim development in the sample that has the patchy charnockitisation (IND12-006C), analyses of these rims returned a weighted mean age of 524 ± 10 (MSWD = 0.107, n = 3; Fig. 6f). The garnet-bearing charnockite (IND12-006A) yielded an age of 2042 ± 45 Ma with some discordance (Fig. 6f). Two rim analyses from this sample gave ages of 540 ± 24 Ma and 525 ± 14 Ma, within error of the lower intercept age of 536 ± 41 Ma and the lower intercept ages in the garnet-leucogranites (Fig. 6f). REE analysis of zircon and garnet suggests younger zircon rims were in equilibrium with the garnet, whereas the oscillatory-zoned cores show typical igneous patterns and are not in equilibrium with the garnet (Fig. 7c,d).
Lu-Hf results. Hafnium isotopic analyses were carried out on <10% discordant zircon grains and the results are presented in Supplementary Table S4. Data is plotted on epsilon Hf (εHf) vs. age (Ma) plot (Fig. 8a). Two Hf model ages are quoted in Supplementary Table S4, T DM and T DM C , the latter assumes derivation of magma from average continental crust 28 . The evolution of Lu-Hf in a closed system zircon will be different to that in a piece of crust due to the differing proportions of these elements. On Fig. 8a we plot two evolution lines one for continental crust (Lu/Hf = 0.015) and for the average zircon concentration (Lu/Hf = 0.0009) from a starting point of 2.05 Ga, the age of magmatism in the Nagercoil Block. The younger population of c. 0.55 Ga metamorphic zircon have errors in εHf(T) which overlap both of these evolution lines and therefore the Hf data cannot distinguish between a Pb-loss or the introduction of remelted 2.05 Ga continental crust in these younger grains. The εHf(T) values quoted below are calculated for the corresponding U-Pb ages of each individual analysis.
Hafnium isotopic data from charnockites and leucogranites throughout the Nagercoil Block yield two distinct populations in εHf versus U-Pb age space (

Discussion
The new U-Pb data constrain the age of the magmatic protoliths of the Nagercoil Block to ca. 2040 Ma (Fig. 6). The Lu-Hf data demonstrate the juvenile nature of this magmatism and support the interpretation that these formed by melting of basaltic source 22 . Oxygen isotope data are consistent with the incorporation of a supracrustal component (Fig. 8b). We interpret the data to indicate the Nagercoil Block represents the remnants of a previously unidentified Palaeoproterozoic magmatic arc. The REE data from zircon, garnet and orthopyroxene show that garnet and orthopyroxene grew in equilibrium with the zircon rims (Fig. 7). This demonstrates that the charnockite assemblage formed during metamorphism at 530 Ma, coinciding with the amalgamation of Gondwana.
Palaeoproterozoic felsic gneisses, which are interpreted to have magmatic protoliths, occur immediately to the northeast within the Trivandrum Block 7,15 . However, the εHf(T) from these rocks are significantly more evolved than the Nagercoil Block data (Fig. 8a). Further north, the Achankovil Zone and southern Madurai Block have recently been interpreted as a Neoproterozoic suture containing Mesoproterozoic to Neoproterozoic juvenile magmatic and metasedimentary rocks 9,21,27 . The northern Madurai Block is composed predominantly of c. 2500 Ma juvenile magmatic rocks and Proterozoic metasediments 21 . Teale et al. 29 reported middle Neoproterozoic gabbro-anorthosites from this region and also minor Palaeoproterozoic felsic gneisses (Fig. 8a). www.nature.com/scientificreports www.nature.com/scientificreports/ Sri Lanka, southern Madagascar and eastern Africa lie adjacent to the Nagercoil Block in a reconstructed Gondwana e.g. 12 (Fig. 1). The Highland Complex of Sri Lanka has been correlated with southernmost India 6,7 . Limited data have been used to suggest magmatic intrusion between 1.90 to 1.85 Ga 30 and no Hf data are available    31 that have been interpreted to underlie the extensive metasediments in the region 2 . The southeast part of vast Congo Craton, the Bangweulu Block (BB - Fig. 1), lay directly east of southern India/Madagascar in Gondwana (Fig. 1) and is best exposed as the basement to the Irumide Belt of Zambia 32 . These rocks (the Mkushi and Luwalizi gneisses (MLG - Fig. 1)) are deformed 2.04 Ga juvenile orthogneisses that overlap in U-Pb and Hf isotopic composition with the Nagercoil Block samples from this study 32 ; Fig. 3. The isotopic similarities between the Irumide basement and the Nagercoil Block rocks provide a strong argument for correlating these regions and assigning the tectonic affinity of southernmost India to Precambrian Africa (Fig. 8a).
Considerable controversy surrounds the tectonic framework of this key orogen in the Gondwana amalgam. Fitzsimons and Hulscher 4 argued that much of the central EAO originated in Africa and rifted from the Tanzania-Bangweulu continent earlier in the Proterozoic to either collide with India 4 , or back on the African margin at ~650-620 Ma before terminal India-Africa collision at the end of the Ediacaran and into the Cambrian 8 . These models require the existence of multiple oceanic sutures between cratonic India and Africa. In contrast, Tucker et al. 2,31 argue for a Neoproterozoic Greater Dharwar continent and a simple, single suture between 'Indian crust' and ' African crust' to the west of both India and Madagascar. Boger et al. 3 proposed a modified version of this where southern India and central/eastern Madagascar were also part of Neoproterozoic India, but a microcontinent centred around the Androyen Domain of south-central Madagascar collided first with an arc terrane, preserved in the Vohibory Domain (SW Madagascar), then with Neoproterozoic India.
The data presented here demonstrate that southernmost India has a considerably greater pre-Gondwana affinity with East Africa, than any other block with a magmatic protolith in the central East African Orogen. The major implication of this link is that southernmost India (and Madagascar) is derived from pre-Gondwana Africa and a major strand of the Mozambique Ocean lay to the north-east of the Nagercoil Block. Potential sites of this suture lie in the Palghat-Cauvery shear zone, along the northern margin of the Madurai Block 8 and within the southern Madurai Block/Achancovil Zone 1 with the remnants of the Neoproterozoic ocean-basin sediments and associated magmatism preserved 9,21,33 . In addition, the rocks of the Palghat-Cauvery shear zone contain evidence of Neoproterozoic high-pressure metamorphism 34,35 , interpreted ophiolitic rocks [36][37][38] and has the geohysical characteristics of a mantle penetrating structure 39 . All of these observations are consistent with the PCSS representing a suture zone along which the remnants of the Mozambique ocean were consumed. In addition, these findings reinforce the notion that presented by various workers and summarized by Collins et al. 8 on the detrital provenance of the Palaeoproterozoic sedimentary units that make up the bulk of the Trivandrum and Madurai Blocks are sourced from African protoliths. The Nagercoil Block could be considered the remnant African basement upon which these sediments were deposited.
The Nagercoil Block was part of the Congo-Tanzania-Bangweulu continent (Africa) that was subsequently welded to India during Gondwana amalgamation where it was metamorphosed to granulite-facies resulting in the formation of orthopyroxene (+/− garnet)-bearing gneisses. The African affinity of southernmost India requires a Neoproterozoic oceanic suture to lie within southern India e.g. 1,8 rather than in Madagascar e.g. 3 or to the west of Madagascar 2,31 .

Methods
SHRIMP methods, data and standards. Zircon was separated from crushed rock samples using traditional magnetic and methylene iodide heavy liquid separation techniques. Grains were hand picked and mounted in 25 mm diameter epoxy resin discs. Mounts were carbon coated for imaging on a Tescan MIRA3 scanning electron microscope (SEM) with zircon CL images taken at a working distance of 15 mm and using an accelerating voltage of 10 kV. For SHRIMP analyses the samples were coated with a thin membrane of gold that produced a resistivity of 10-15 Ω across the mount surface.
U-Pb isotopes were analysed on the SHRIMP II at the John de Laeter Centre SHRIMP Facility, Curtin University, Perth, Western Australia. The analytical procedures for the Curtin consortium SHRIMP II have been described by 40 and 41 and are similar to those described by 42 and 43 . For zircon analysis a 25-30 μm diameter spot was used, with a mass-filtered O 2 − primary beam of ~2 nA. Data for each spot were collected in sets of 6 scans through the mass range of 196 Zr 2 O + , 204 Pb + , Background, 206 Pb + , 207 Pb + , 208 Pb + , 238 U + , 248 ThO + , 254 UO + . The 206 Pb/ 238 U age standards used were BR266, a Sri Lankan gem zircon 44 , and Temora-2 a zircon grain separate 45 . The 207 Pb/ 206 Pb standard used to enable correction for instrument induced mass fractionation was OG1 zircon 46 . The common Pb correction was based on the measured 204 Pb 42 . The correction formula for Pb/U fractionation is 206 Pb + / 238 U + = a( 238 U 16 O + / 238 U + ) b 47 using the parameter values of 45 . External spot-to-spot errors on zircon U-Pb calibration sessions were <1% for both sessions, a minimum error of 1% was applied which reflects the long-term performance of the SHRIMP II facility. Uncertainty cited for individual spot analysis in the text and data tables include errors from counting statistics, the common-Pb correction, and the U-Pb calibration error based on reproducibility of U-Pb measurements of the standard, and are at the 2σ level. Uncertainties of weighted mean values for pooled analyses and upper and lower intercepts in the figures are at the 95% confidence level, with MSWD calculated for concordance and equivalence (Fig. 5). Uncertainty ellipses on concordia diagrams are at the 2σ level (Fig. 5).
LA-ICPMS method, data and standards. Rare earth element (REE) analyses of zircon and garnet were performed at the Curtin University LA-ICP-MS facility using a Resonetics M-50 193 nm excimer laser with an Agilent 7700 mass spectrometer. Zircon was analysed in the grain separate mount used for SHRIMP analysis, while garnet was analysed in thin section. Beam diameter was 23 μm using a repetition rate of 5 Hz which produced a laser power density of ~3 J/cm −2 . Data was collected using time resolved data acquisition and processed using the Iolite software package 48,49 . Where appropriate REE values were normalized to chondritic values 50 . Total acquisition time per analysis was 80 s including 30 s of background time and 40 s of sample ablation, followed by (2020) 10:5421 | https://doi.org/10.1038/s41598-020-62075-y www.nature.com/scientificreports www.nature.com/scientificreports/ a 10 s washout period. Calibration was performed against the NIST 610 standard glass using the coefficients of Pearce, et al. 51 . NIST 610 was run 8 times per sample with 3 analyses at the beginning and end and 2 analyses in the middle of each run. Stoichiometric major elements were used for calibration of trace elements in each phase. Stoichiometric Si was used as the internal standardization element for both zircon (14.76%) and garnet (18%). Precision based on repeated analysis of standards is approximately 5-10%, with detection limits for REE in this study ranging from 0.1 to 0.5 ppm. Due to the depth of the laser ablation pit relative to those associated with SHRIMP analysis, several analyses had to be rejected as they intersected heterogeneous material and/or inclusions of other phases.
Lu-Hf methods, data and standards. Hafnium isotope analyses were conducted on previously dated zircons mounted in epoxy resin using a New Wave/Merchantek LUV213 laser-ablation microprobe, attached to a Nu Plasma multi-collector inductively coupled plasma mass spectrometer (LA-MC-ICPMS). The analyses employed a beam diameter of ∼55 μm and a 5 Hz repetition rate which resulted in ablation pits typically 40-60 μm deep. The ablated sample material was transported from the laser cell to the ICP-MS torch by a helium carrier gas. Interference of 176 Lu on 176 Hf was corrected by measurement of interference-free 175 Lu, and using the invariant 176 Lu/ 175 Lu correction factor 1/40.02669 (DeBievre and Taylor, 1993  O-isotope methods, data and standards. Oxygen isotope ratios ( 18 O/ 16 O) were determined using a Cameca IMS 1280 multi-collector ion microprobe located at the Centre for Microscopy, Characterisation and Analysis (CMCA), University of Western Australia (UWA). Oxygen isotope analyses were performed with a ca. 3 nA Cs + beam with an impact energy of 20 keV focused to a 10-15 µm spot on the sample surface. Instrument parameters included a magnification of 130 × between the sample and field aperture (FA), 400 μm contrast aperture (CA), 4000 μm FA, 110 μm entrance slit, 500 μm exit slits, and a 40 eV band pass for the energy slit with a 5 eV gap toward the high energy side. Secondary O − ions were accelerated to 10 keV and analyzed with a mass resolving power of approximately 2400 using dual Faraday Cup detectors. A normal-incidence electron gun was used to provide charge compensation and NMR regulation was used for magnetic field control.
Ten seconds of pre-sputtering was followed by automatic centering of the secondary beam in the FA and CA. Each analysis consisted of 20 four-second cycles, which gave an average internal precision of 0.2‰ (2 SE). Analytical sessions were monitored in terms of drift and precision using at least four bracketing standards (Temora II; 8.2‰ 52 every 5-10 sample analyses. Instrumental mass fractionation (IMF) was corrected using Temora II following the procedure described in Kita, et al. 53 ). The spot-to-spot reproducibility (external precision) was better than 0.3‰ (2 SD) on Temora II during the analytical session. Propagated uncertainty on each δ 18 O spot has been calculated by propagating the errors on instrumental mass fractionation determination, including the error on the reference value of the standard, and internal error on each sample data point. The resulting uncertainty was typically between 0.2 and 0.3‰ (2 SD).