Ishwar-kumar Et Al.-precam Res-2013.pdf

Precambrian Research 236 (2013) 227–251

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A Rodinian suture in western India: New insights on India-Madagascar correlations C. Ishwar-Kumar a , B.F. Windley b , K. Horie c , T. Kato d , T. Hokada c , T. Itaya e , K. Yagi f , C. Gouzu f , K. Sajeev a,∗ a

Centre for Earth Sciences, Indian Institute of Science, Bangalore 560 012, India Department of Geology, University of Leicester, Leicester LE1 7RH, UK c National Institute of Polar Research, 10-3 Midori-cho, Tachikawa, Tokyo 190-8518, Japan d Center for Chronological Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan e Research Institute of Natural Sciences, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama 700-0005, Japan f Hiruzen Institute for Geology and Chronology Co., Ltd., 2-5 Nakashima, Naka-ku, Okayama 703-8252, Japan b

a r t i c l e

i n f o

Article history: Received 16 November 2012 Received in revised form 30 July 2013 Accepted 31 July 2013 Available online 11 August 2013 Keywords: Kumta suture Sirsi shelf Karwar block Dharwar block Betsimisaraka suture Rodinia

a b s t r a c t We report detailed evidence for a new paleo-suture zone (the Kumta suture) on the western margin of southern India. The c. 15-km-wide, westward dipping suture zone contains garnet-biotite, fuchsitehaematite, chlorite-quartz, quartz-phengite schists, biotite augen gneiss, marble and amphibolite. The isochemical phase diagram estimations and the high-Si phengite composition of quartz-phengite schist suggest a near-peak condition of c. 18 kbar at c. 550 ◦ C, followed by near-isothermal decompression. The detrital SHRIMP U–Pb zircon ages from quartz-phengite schist give four age populations ranging from 3280 to 2993 Ma. Phengite from quartz-phengite schist and biotite from garnet-biotite schist have K–Ar metamorphic ages of ca. 1326 and ca. 1385 Ma respectively. Electron microprobe-CHIME ages of in situ zircons in quartz-phengite schist (ca. 3750 Ma and ca. 1697 Ma) are consistent with the above results. The Bondla ultramafic-gabbro complex in the west of the Kumta suture compositionally represents an arc with K–Ar biotite ages from gabbro in the range 1644–1536 Ma. On the eastern side of the suture are weakly deformed and unmetamorphosed shallow westward-dipping sedimentary rocks of the Sirsi shelf, which has the following upward stratigraphy: pebbly quartzite/sandstone, turbidite, magnetite iron formation, and limestone; farther east the lower lying quartzite has an unconformable contact with ca. 2571 Ma quartzo-feldspathic gneisses of the Dharwar block with a ca. 1733 Ma biotite cooling age. To the west of the suture is a c. 60-km-wide Karwar block mainly consisting of tonalite-trondhjemite-granodiorite (TTG) and amphibolite. The TTGs have U–Pb zircon magmatic ages of ca. 3200 Ma with a rare inherited core age of ca. 3601 Ma. The K–Ar biotite cooling age from the TTGs (1746 Ma and 1796 Ma) and amphibolite (ca. 1697 Ma) represents late-stage uplift. Integration of geological, structural and geochronological data from western India and eastern Madagascar suggest diachronous ocean closure during the amalgamation of Rodinia; in the north at around ca. 1380 Ma, and a progression toward the south until ca. 750 Ma. Satellite imagery based regional structural lineaments suggests that the Betsimisaraka suture continues into western India as the Kumta suture and possibly farther south toward a suture in the Coorg area, representing in total a c. 1000 km long Rodinian suture. © 2013 Elsevier B.V. All rights reserved.

1. Introduction and background The amalgamation of the Rodinia supercontinent at around 1300–900 Ma involved worldwide orogenic events (Li et al., 2008). Rodinia formed by many collisional events of relatively longlived (1100–700 Ma) Grenvillian age (Pisarevsky et al., 2003). The

∗ Corresponding author. Tel.: +91 80 2293 3404. E-mail addresses: [email protected], [email protected] (K. Sajeev). 0301-9268/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.precamres.2013.07.023

presence and position of the Indian continent in the Rodinia assembly is controversial (Li et al., 2008). In many configurations India is attached to Australia and East Antarctica (e.g., Dalziel and Soper, 2001; Hoffman, 1991; Moores, 1991; Torsvik et al., 1996; Weil et al., 1998). The Grenvillian orogenic belt continues in the Eastern Ghats belt of India, i.e., along the India-East Antarctica margin and extends into the Late Mesoproterozoic Albany Fraser belt of Australia (Torsvik, 2003; Pisarevsky et al., 2003). According to the SWEAT (SW US-East Antarctic) hypothesis (Moores, 1991) the Grenvillian Belt of Laurentia continues around Antarctica, and into India and Australia. However, some of the paleomagnetic studies

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suggest that India was never a part of Rodinia (Fitzsimons, 2000; Torsvik et al., 2001a,b; Powell and Pisarevsky, 2002). Based on paleomagnetic and geochronological data Powell and Pisarevsky (2002) proposed a new model, in which the Eastern Ghats belt of India and the Rayner Complex of Antarctica were not a part of Rodinia, but collided with Gondwanal at around 680–610 Ma. Li et al. (2008) suggested that India became part of Rodinia around 990–900 Ma by the collision of the Eastern Ghats belt of India and the Rayner complex of East Antarctica (e.g., Mezger and Cosca, 1999; Fitzsimons, 2000). Recently, based on major, trace element geochemistry and U–Pb, Ar–Ar geochronology, the Mt. Abu granitoids in NW India are correlated with granitoids of Malani Igneous Suite (Rajasthan, NW India), Seychelles and northern Madagascar and interpreted to represent the western margin of Rodinia (Ashwal et al., 2013). However, the position of Madagascar and its connection with western India during Rodinian time is not well established. Most Rodinia reconstructions have been mainly based on paleomagnetic data, but the reliability of this data has to be treated with caution (Li et al., 2008). The configuration of India and Madagascar in Gondwana is well known compared to that in Rodinia. The Gondwana supercontinent assembled in the Neoproterozoic-Cambrian (Reeves et al., 2002; Meert, 2003; Veevers, 2004), and specifically eastern Gondwana was amalgamated as a result of collisions between East Antarctica, Australia, Sri Lanka, India and Azania (Collins and Pisarevsky, 2005). These collisions led to the formation of major orogenic belts such as the East African Orogen, and to its constituent sutures such as the Betsimisaraka suture (Collins and Windley, 2002; Collins et al., 2003) between the Azania microcontinent, well exposed in central Madagascar, and the Dharwar craton of India (Collins and Pisarevsky, 2005). Sutures that result from the closure of oceans (Burke et al., 1977) provide critical information, not only on paleogeography, but also on material that was transported to the trench, and the pressure-temperature conditions it underwent during subduction and exhumation. Thus, diagnostic high-pressure (HP) rocks such as eclogites, blueschists and whiteschists arguably provide critical evidence that demonstrates that a suture is sited on a former subduction zone (Råheim and Green, 1975; Jiang et al., 1998; Parkinson et al., 2002; Chopin, 2003). Shelf sediments that typically comprise quartzites, carbonates, and shales have been deposited on passive continental margins worldwide since the Neoarchean (Bradley, 2008). In collisional orogenic belts, shelf sediments, or relicts of them, are situated between the craton on which they were deposited and the footwall of a dipping suture zone (Yin and Harrison, 2000). On the eastern side of the west-dipping Betsimisaraka suture, the Sahantaha shelf sediments are only well preserved where they strike E–W. Hottin (1969) described how the sediments are progressively destroyed as they pass around the bend of the Antongil Craton from an EW strike to an NS orientation (Supplementary Fig. S1). In collisional orogenic belts a magmatic arc is commonly situated on the hanging wall side of a dipping suture zone (Yin and Harrison, 2000). On the eastern side of the Azania microcontinent, the Antananarivo active continental margin arc was generated by westwards subduction, and thus is situated on the western side of the Betsimisaraka suture zone (Kröner et al., 1999, 2000). Abundant 825–720 Ma arc-related granitic intrusions were metamorphosed and deformed in the granulite to amphibolite facies during later collision. The Betsimisaraka suture zone in the north-eastern Madagascar extends southwards, reaches the coast near the town of Toamasina, and then extends offshore, presumably below the submarine continental shelf, and it returns on land at c. 180 km farther south, where it is interpreted to separate the Masora Block from Azania (Collins, 2006).

No previous studies have considered where the Betsimisaraka suture zone goes when it passes offshore. The aim of this paper is to demonstrate that it continues in western India, where it contains high-pressure low-temperature schists, which are consistent with subduction-related metamorphism along a suture. In India we introduce the names: Sirsi shelf, Kumta suture, Karwar block and Bondla arc and describe their characteristic features, followed by the petrology and P–T conditions of metamorphic rocks in the suture, the geochronology of various units and finally we discuss the comparable shelf, suture, block and arc in Madagascar.

2. Regional structure of western India Remote-sensing satellite images and aerial photographs are widely employed to determine regional structures. For this study we prepared geological map by compiling information from the Geological Survey of India (1993, 1995, 2005a,b) and modified it with ground truth data (Figs. 1 and 2). Shear zones were delineated based on the regional-scale structural lineaments produced with remote sensing data such as satellite images, digital elevation models and ground truth information (Ishwar-Kumar et al., unpublished data), and integrated with available published literature (e.g., Drury et al., 1984; Ghosh et al., 2004) (Figs. 1 and 2). Landsat Enhanced Thematic Mapper+ orthorectified satellite images (Supplementary Fig. S2), ASTER (Advanced Space-borne Thermal Emission Reflection Radiometer) digital elevation models (Supplementary Fig. S3), Cartosat-2 digital elevation data and their derived products were utilized to map the regional structural lineaments in this study area (Fig. 2, Supplementary Fig. S4). ERDAS Imagine 8.5 software was used for image processing and Arc GIS 10 for information extraction and analysis. Landsat images were obtained from the Global Land Cover Facility/United States Geological Survey (GLCF/USGS) website and blue, green, red, NIR (near infrared), SWIR (shortwave infrared) bands (bands 1, 2, 3, 4, 5 and 7) were stacked to produce color composite images. To obtain maximum energy in particular bandwidths, band combination of 4, 3, 2 standard false color composites (FCC) in which green, red, near-infrared bands are displayed as blue, green, and red respectively. Image enhancement techniques such as contrast stretching, spatial filtering and band ratioing were applied to extract structural information (Lillesand and Kiefer, 1994; Jensen, 1996; Gupta, 2003). Advanced Spaceborne Thermal Emission Reflection Radiometer (ASTER) Digital Elevation Models (DEM) were downloaded from the Land Processes Distributed Active Archive Center/United States Geological Survey (LPDAAC/USGS) website, and Cartosat-2 digital elevation models are from the Bhuvan/Indian Space Research Organization (ISRO) website. DEM-derived products such as slope map, hill-shade map and contour map were used to extract detailed structural information. We followed the imagery laboratory work with many field visits (around 504 field locations, Fig. 2) to determine the corroborative ground truth, and to document, analyze and understand the geological relationships. Near the western coast of India, the structure of the northwestern Karnataka and Goa states (the Karwar area in this paper) has a distinctive arcuate geometry, which is very different from the typical linear NW-trending structures of the Dharwar block to the east (Fig. 2). The narrow Kumta suture zone with dense arcuate structures extending from south to north through Kumta, Joida and Valpoy towns is occupied by heavily sheared and deformed metamorphic rocks. To the east of this zone is the Sirsi shelf with passive margin-type sediments that has an arcuate structure that parallels that of the suture zone. To the west of the suture are randomly oriented structures in homogeneous

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Fig. 1. The regional geology and tectonic framework of southern India. Geology modified after Geological Survey of India (1995) and shear zones after Ishwar-Kumar et al. (unpublished). The rectangle shows location of the study area. Acronyms: TTG-Tonalite-trondhemite-granodiorite, KSZ – Kumta shear zone. CoSZ – Coorg shear zone, ChSZ – Chitradurga shear zone, MKSZ – Mettur-Kolar shear zone, NSZ – Nallamalai shear zone, MSZ – Moyar shear zone, BSZ – Bhavani shear zone, SASZ – Salem-Attur shear zone, CaSZ – Cauvery shear zone, PCSZ – Palghat-Cauvery shear zone, KKPT – Karur-Kambum-Painavu-Trichur shear zone; ASZ – Achankovil shear zone, WDC – Western Dharwar craton, CDC – Central Dharwar craton, EDC – Eastern Dharwar craton, EGMB – Eastern Ghats mobile belt.

tonalite-trondhjemite-granodiorites that make-up the Karwar block (Fig. 2). 3. India: the Sirsi shelf, Kumta suture, Karwar block and Bondla arc 3.1. The Sirsi shelf Shelf sediments of the Sirsi passive continental margin crop out in a 40-km-wide, 280-km-long arcuate belt clearly seen on Landsat imagery (Supplementary Fig. S2). This is an almost undeformed and weakly metamorphosed sequence dominated by greywacke, phyllite, sandstones and quartzites that mostly dip shallowly to the west. The shelf sediments have the following upward stratigraphy: a basal quartzitic unit of dominant white pebbly quartzites, which are up to 20–30 km thick, in which the pebbles are closepacked, not matrix-supported, rounded, and about 0.1–0.4 mm across. Bedding units are commonly weakly deformed and typically about 30 cm thick and are sub-horizontal or dip shallowly to

the west (Fig. 3a). Some pebbly sandstones are intercalated with 3-m-thick beds of quartz-pebble conglomerate, and finely-bedded, red haematite-rich quartz sandstone beds. The quartzites contain rare beds of angular flake conglomerate up to about 60-cm-thick, in which the flakes (up to 20 cm long) are of red, haematite-rich sandstone and siltstone, which we interpret as derived cannibalistically from red clastic sediments that are common in the Sirsi shelf. Locally, some 50-m-thick well-bedded turbidites are unmetamorphosed and just tilted; 50 m away these rocks have been deformed in shear zones to steep to vertical slates/phyllites with down-dip lineations (Fig. 3b, Supplementary Fig. S6a). Higher in the stratigraphy the quartzites are succeeded by quartzitic schists, chloritic quartzites, chlorite schists, quartz-muscovite schists with scattered haematite spots, and red haematitic and quartzitic schists. Toward the top of the quartzitic unit there are beds of folded, banded magnetite iron formation at least 10 m-thick (Fig. 3c) containing thin chert beds (Supplementary Fig. S6b). Near the shelf-suture boundary in the west there is a narrow N–S trending zone with almost unmetamorphosed limestones.

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Fig. 2. Sample locations and regional structures of the study area overlain on the geological map (modified after Geological Survey of India, 1993, 1996, 2005a,b and structural lineaments were extracted from Landsat ETM+ satellite imagery and ASTER digital elevation model). The strike and dip of poles of foliations are summarized in a rose diagram (n = 99).

In a sub-horizontal surface the rocks of the cover and basement come to within a meter of each other (Supplementary Figs. S5a and b), where the low-grade meta-sediments like sandstones of the cover dip shallowly to the NW, W or SW, and the high amphibolite facies, biotite-epidote foliated gneisses (granitic to tonalitic compositions) of the basement to the east have discordant strikes and steep to sub-vertical dips; we consider this is consistent with the expected unconformable relations. We document two examples: (1) At IK-121009-05 (75.48◦ E, 15.19◦ N) sandstone (strike 30◦ N–40◦ W, dipping toward west) is in contact with the gneisses of Dharwar block which strikes E–W and dips 38◦ NW (Figs. 2 and 3d; Supplementary Fig. S4 and S5a). Along the contact there is a zone of highly foliated gneiss a few meters wide, which is regarded as a zone of reworked basement created by post-unconformity deformation. (2) At IK-121009-13 (75.58◦ E, 15.06◦ N) weakly metamorphosed phyllites of the cover (strike 35◦ NW, dip 30◦ SW) are in contact with biotite gneisses (strike E–W, dip 35◦ NW) of Dharwar block (Figs. 2 and 3e; Supplementary Fig. S4 and S5b). At this locality there are slices of, probably imbricated, gneiss along the deformed unconformity.

3.2. The Kumta suture The c. 15-km-wide suture zone contains a wide variety of rocks. Near the southwestern margin sub-vertical, 50-m-thick fuchsite schists have a down-dip crenulation lineation. Red quartz-feldspar haematite schists, some tens of meters thick, are intercalated with

30 cm-thick quartz-phengite schist layers associated with 1–2-mthick chlorite-quartz schists (Fig. 3g, Supplementary Fig. S6e and f), 3–4-m-thick layers of quartz pebble sandstone, biotite-augen gneisses up to 6-m thick, and 5-m-thick sheets of granite. The main layer of quartz-phengite schist is 6–10-m thick within chloritequartz schist, and it contains thin layers of talc-sillimanite schist with fine-grained, soft, soapy talc and white needles of sillimanite (Fig. 3g, Supplementary Fig. S6c and d). Some 2–3-m-thick layers of finely banded quartz-phengite schists are bordered by 3 m-thick layers of red clastic haematite-rich schist, and 2 m-thick brown quartz-feldspar schists. Most of these rocks have a general NE strike with a westward dip of 30–65◦ . The main foliation is deformed into broad open folds with axes plunging moderately to the northwest. In the north the suture zone contains a west-dipping layer (about 5 km wide and c. 20 km long) of garnet-biotite schist.

3.3. The Karwar block On the western side of the Kumta suture an arcuate belt of rocks reaches a maximum width of about 60 km (Fig. 2). It is characterized by homogeneous, undeformed to weakly deformed tonalitetrondhjemite-granodiorites that contain layers a few meters wide of amphibolite. These amphibolites have been intruded by veins and sheets of the tonalite-trondhjemite-granodiorite and thus the tonalite-trondhjemite-granodiorite contains many inclusions of amphibolite (Fig. 3f, Supplementary Fig. S6g). Isoclinally folded granitic veins in some amphibolites demonstrate intense local deformation (Fig. 3f).

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Fig. 3. Field photographs illustrating key features across the shelf-suture-arc. (a) Bedded sandstones in the eastern Sirsi shelf. (b) Slate/phyllite in a shear zone in the Sirsi shelf showing down-dip lineations. (c) Metamorphosed and folded banded iron formation in the western Sirsi shelf. (d) Bedded quartzites in the Sirsi shelf near Gojnur near the contact with Dharwar gneisses (Location IK-121009-05, Fig. 1). (e) The unconformity (marked in white) between sub-horizontal phyllites of the Sirsi shelf and dipping gneisses of the Dharwar craton (Location IK-121009-13, Fig. 1). (f) Tonalite-trondhjemite-granodiorites in the Karwar arc intruded into amphibolites that contain isoclinally folded granitic veins. (g) Tectonic stratigraphy within the Kumta suture zone near Kathagal (74.56◦ E, 14.50◦ N). The rocks mostly strike ca. 20◦ N–30◦ E and dip at various angles westwards.

3.4. The Bondla arc The NW/SE-trending, ophiolitic, layered Bondla ultramaficgabbro complex (Fig. 2, Supplementary Figs. S6h, S7) contains gabbro, troctolite, wehrlite, dunite, peridotite, pyroxenite, and chromitite, as well as serpentinite with chromite (Jena, 1980, 1985; Geological Survey of India, 1996; Dessai et al., 2009). The chemistry of chromite from the serpentinites suggests that it evolved in an arc-related tectonic setting (Ishwar-Kumar et al., unpublished data). North of Valpoy town NW-trending, kilometer-size banded iron formations are exploited in open mines, and bordered by poorly exposed psilomelane rocks. 4. Petrography and mineral chemistry Across this shelf-suture-arc belt the main lithologies are as follows (Supplementary Table S1 and S2). In the far east, just west of the high-grade Dharwar Craton sandstones and phyllites of the Sirsi shelf are mostly unmetamorphosed or weakly metamorphosed and have no preferred grain orientation. Close to the Kumta suture greywackes and quartz arenites in the shelf

are slightly deformed. Within the Kumta suture quartz-phengite schists, fuchsite schists, chlorite schists and garnet biotite schists are extremely sheared; the chlorite schist contains augen quartz porphyroblasts in a sheared chlorite-phengite matrix with a pronounced stretching lineation. Tonalite-trondhjemite-granodiorites in the Karwar block contain euhedral to subhedral plagioclases interlocking with quartz with sutured grain contacts (indicating minor interfacial tension) suggesting little deformation or metamorphism. In order to understand the nature and evolution of the Kumta suture and adjacent shelf and arc, constituent minerals were analyzed with a Cameca SX-100 electron microprobe housed at the Geological Survey of India, Bangalore. Analytical conditions were 15 kV accelerating voltage, and a probe current of 15 nA; natural mineral oxides were used as standards. The data were reduced using ZAF correction procedures. SiO2 , TiO2 , Al2 O3 , Cr2 O3 , FeO, MnO, MgO, CaO, Na2 O and K2 O were analyzed for all minerals. X-ray mapping of sample IK-110123-02L (Quartz-phengite schist) was carried out using a JEOL JXA-8530F field-emission hyper-probe in the Department of Materials Engineering, Indian Institute of Science, Bangalore.

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Fig. 4. Photomicrographs. (a) Sandstone containing sub-rounded quartz and feldspar grains in an iron-rich irresolvable matrix. Sirsi shelf. (b) Limestone containing a lens of organic matter circled. Sirsi shelf. (c) Greywacke showing a quartz-rich bed and dark chloritic matrix with cleavage and two sets of displaced quartz-rich veins. Sirsi shelf. (d) Garnet-biotite schist, showing euhedral garnets in a biotite matrix. Kumta suture. (e) Fuchsite schist with well-oriented fabric and elongate augen quartz grains. Kumta suture. (f) Quartz-phengite schist containing alternating bands enriched in quartz and phengite. Kumta suture. (g) Chlorite schist showing a strong fabric with bands rich in quartz and chlorite. Kumta suture. (h) Serpentinite showing grains of olivine in the serpentine. (i) Tonalite-trondhjemite-granodiorite showing undeformed dark sericitized plagioclases and interlocking quartz grains. Qtz – quartz, Cal – calcite, Chr – chromite, Grt – garnet, Fuch – fuchsite, Phe – phengite, Chl – chlorite, Ol – olivine, Srp – serpentine, Pl – plagioclase, Bt – biotite.

4.1. Sandstone Sandstone (IK-110125-01E) contains sub-rounded to wellrounded grains of quartz and feldspar (diameter up to 300 ␮m and ∼70 volume %) cemented by a fine-grained matrix (∼30 volume %) that contains quartz, feldspar and irresolvable matrix ferruginous (haematite) material suggesting the provenance of the sediment was iron-rich. Sandstone (IK-110125-01E) from Dumvad (75.02◦ E, 15.34◦ N), just west of the Dharwar block is unmetamorphosed and lacks any grain orientation (Fig. 4a), but quartz arenite (IK-11012302A1 ) at Kathagal (74.56◦ E, 14.50◦ N) in a graded, fining-upward sequence close to the southern Kumta suture has a preferred grain orientation resulting from mild metamorphism/deformation.

of IK-110126-01 near to the suture is a metamorphosed marble with recrystallized calcite. 4.3. Meta-greywacke and slate/phyllite Meta-greywacke sample IK-110324-01 is predominantly composed of quartz, feldspar, lithic fragments (∼30 volume %) and minor chlorite within a fine-grained matrix (∼60 volume %). The rock is intruded by micro-veins and the chlorite shows a preferred orientation. The greywacke has a deformed texture, and contains several micro-faults, crenulations and mica fish structures, which are small-scale indicators of shearing (Fig. 4c). Slate/phyllite sample IK-110122-02D contains very fine-grained quartz, feldspar, muscovite and chlorite, which are weakly oriented and define a slaty fabric.

4.2. Limestone and marble

4.4. Iron and manganese formations

Limestone sample IK-110126-01 was collected just east of the Kumta suture, near Idagundy, Yellapur, Karnataka. Its microstructure locally shows recrystallized calcite (Fig. 4b) suggesting it is weakly metamorphosed. Sample IK-100911-05 located southwest

Well-banded and folded (IK-100911-04) iron formations consist of quartz (∼60–65 volume %) and magnetite (∼30–35 volume %) in alternate bands that vary in grain size from coarse to fine, but they are mostly fine-grained. These were probably iron-rich sediments

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233

Table 1 Representative mineral chemistry of phengite. Analysis no.

11

13

3

28

34

36

44

45

50

8

SiO2 TiO2 Al2 O3 Cr2 O3 FeO MnO MgO CaO Na2 O K2 O Total O Si Ti Al Cr Fe Mn Mg Ca Na K Total cation XNa XMg

47.55 0.28 32.35 0.13 1.28 0.00 1.35 0.00 0.36 10.38 93.68

45.85 0.32 30.27 0.24 1.75 0.00 1.13 0.17 0.31 8.58 88.65

46.28 0.09 31.40 0.10 1.47 0.00 1.50 0.00 0.16 10.52 91.53

45.98 0.13 31.01 0.26 1.24 0.00 1.19 0.06 0.19 9.07 89.19

47.43 0.20 31.24 0.39 1.34 0.00 1.21 0.08 0.21 9.79 91.90

47.89 0.31 33.19 0.21 1.23 0.03 1.25 0.08 0.32 10.27 94.78

47.46 0.17 31.79 0.31 1.27 0.00 1.15 0.13 0.18 9.27 91.75

47.20 0.17 31.40 0.16 1.14 0.07 1.08 0.11 0.24 7.71 89.27

46.70 0.07 31.87 0.10 1.08 0.00 1.65 0.12 0.12 9.34 91.03

48.04 0.18 33.44 0.07 1.16 0.01 1.25 0.12 0.21 9.91 94.40

11 3.211 0.014 2.574 0.007 0.072 0.000 0.136 0.000 0.047 0.894 6.955 0.050 0.653

3.252 0.017 2.530 0.013 0.104 0.000 0.119 0.013 0.042 0.776 6.868 0.051 0.534

3.209 0.005 2.566 0.005 0.085 0.000 0.155 0.000 0.022 0.930 6.977 0.023 0.645

3.240 0.007 2.575 0.014 0.073 0.000 0.125 0.004 0.026 0.815 6.879 0.030 0.631

deposited in the inner shelf. Psilomelane (IK-110326-01) sample was collected from the northernmost part of the suture near Valpoy, Goa near ultramafic rocks and banded iron formations; it mainly consists of opaque manganese oxide. 4.5. Garnet-biotite schist Garnet-biotite schist sample IK-130105-04 mainly consists of garnet (∼20 volume %), biotite (∼40 volume %), chlorite (∼20 volume %), amphibole (∼5 volume %), quartz and feldspars (∼5 volume %), epidote, sphene and a few opaque minerals (Fig. 4d). Biotites are partly altered to chlorites. The garnets are mostly euhedral and show a distinct and prominent compositional zoning observed in X-ray maps. 4.6. Fuchsite schist Fuchsite schist sample IK-110123-01 contains fuchsite, chlorite, quartz and plagioclase. The quartz and plagioclase make-up ∼40 volume %, and fuchsite ∼50 volume % (Fig. 4e). A strong mineral lineation is accompanied by an augen structure defined by deformed quartz porphyroblasts sandwiched between fine-grained fuchsite and chlorite layers. 4.7. Quartz-phengite schist Sample IK-110123-02L from Kathagal, Kumta location (74.56◦ E, 14.49◦ N) is a very fine-grained schist composed predominantly of quartz (<250 ␮m) and phengite (<5 ␮m). Quartz porphyroblasts form augen or elongate bands between a fine-grained pale greenish mass of phengite (Fig. 4f). Because the matrix minerals in this sample are so fine-grained, the mineralogy was deciphered by electron microprobe analysis. The results indicate that the greenish phengite matrix (∼35–60 volume %) is enclosing quartz (c. 40–55 volume %) with minor chlorite, chloritoid, rutile, tourmaline and zircon. X-ray mapping also demonstrates that phengite grains contain very fine-grained aluminosilicate inclusions (less than one micron length) (also see Fig. 6). Soft, soapy talc, identified in the field, is so extremely fine-grained that it has proved difficult to confirm petrographically.

3.254 0.010 2.526 0.021 0.077 0.000 0.124 0.006 0.028 0.857 6.904 0.032 0.617

3.192 0.016 2.607 0.011 0.069 0.002 0.124 0.006 0.041 0.873 6.941 0.045 0.644

3.248 0.009 2.564 0.017 0.073 0.000 0.118 0.009 0.024 0.809 6.870 0.029 0.619

3.281 0.009 2.573 0.009 0.066 0.004 0.112 0.008 0.032 0.684 6.778 0.045 0.628

3.220 0.004 2.590 0.005 0.062 0.000 0.170 0.009 0.016 0.822 6.897 0.019 0.731

3.202 0.009 2.627 0.004 0.065 0.001 0.124 0.009 0.027 0.843 6.909 0.031 0.658

Phengite is a solid solution between muscovite KAl2 (AlSi3 O10 )(OH)2 and celadonite K(Mg,Fe)AlSi4 O10 (OH)2 (Rieder et al., 1998). This white mica has a high Si:Al ratio and a fairly large content of Mg and Fe, because Al is substituted mostly by Mg2+ and Fe2+ (Cibin et al., 2008). The Si content of phengite in the Kumta schist ranges from 3.13 to 3.30 a/fu, its XMg = Mg/(Fe + Mg) values vary between 0.37 and 0.73, and XNa = Na/(K + Na) ranges from 0.02 to 0.08 (Table 1, and Supplementary Table S3). The Al, Ti, XMg and XNa values of phengites in the quartz-phengite schist (64 analytical points) are plotted against Si, together with similar values from 21 published, high pressure eclogites (Franz et al., 1986; Hansen, 1992; Carswell et al., 2000; Gouzu et al., 2006; Zhang K et al., 2006; Zhang Z et al., 2006; Song et al., 2007; Cao et al., 2011; Zai et al., 2011) and metapelites (Chopin and Monie, 1984; Theyei and Seidel, 1991; Dempster, 1992; Agard et al., 2001; Brocker et al., 2004; Keller et al., 2005; De Jong et al., 2006; Masago et al., 2009; Rolland et al., 2009; Ganne et al., 2011; Thanh et al., 2011; Rubatto et al., 2011) from well-defined suture zones worldwide (Fig. 5). Also plotted on this figure are the experimental data of Auzanneau et al. (2010) from meta-greywackes and metapelites, which enable comparison with pressure and temperature conditions, determined by thermodynamic modeling. The Si content of phengite is directly proportional to pressure and can be used as a barometer (Velde, 1967; Auzanneau et al., 2010). The Al vs. Si graph of Fig. 5a shows that all data define a clear linear trend, and that the Kumta values are comparable to those of metapelites from other suture zones. The very high Si content (>3.5 a/fu) of phengites reported by Brocker et al. (2004) and De Jong et al. (2006) are associated with calcite-bearing assemblages in marble. From experimental data Auzanneau et al. (2010) demonstrated that the Ti content of phengite is inversely proportional to pressure and directly proportional to temperature, and our rocks clearly fall close to the composition of the lowest P–T values used in these experiments (15 kbar and 800 ◦ C). In Ti vs. Si space (Fig. 5b), Ti values are low to moderate and indicate high pressure and moderate temperature conditions during the phengite formation. The XNa vs. Si relations (Fig. 5c) show a decrease in XNa values with increase in Si, and our samples have the lowest XNa for the corresponding Si values. Because Al is replaced by Mg in phengite, the higher XMg values are closer to the ideal composition of phengite. The XMg vs. Si graph

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Fig. 5. Chemical composition of phengite in quartz-phengite schist compared with the experimental results of Auzanneau et al. (2010), together with the composition of phengites reported from several eclogites and suture zone metapelites worldwide (see text for references). (a) Al vs. Si. (b) Ti vs. Si. (c) XNa = Na/(K + Na) vs. Si. (d) XMg = Mg/(Fe + Mg) vs. Si values (CEVP – metagreywackes, CO – metapelites).

of Fig. 5d demonstrates that for the corresponding Si values our XMg values are high and indicate an ideal phengite composition. 4.8. Chlorite schist Chlorite schist sample IK-100912-08G was collected from the southern end of the Kumta suture. It is mainly composed of quartz (∼50 volume %), chlorite (∼35 volume %), phengite (∼5–10 volume %) and minor rutile. This highly deformed sample shows extensive shear bands with augen structures and sheared quartz porphyroblasts between chlorite bands (Fig. 4g). The Si content of phengite in this chlorite schist is 3.04–3.20 a/fu, which is slightly lower than that of the kyanite-quartz-phengite schist. In the phengites XMg varies from 0.19 to 0.42 and XNa from 0.03 to 0.06, and in chlorites XMg ranges between 0.32 and 0.40. 4.9. Serpentinite/peridotite The ultramafic-gabbro complex at Bondla contains serpentinite/wehrlite (IK-110323-01 and IK-110325-03) that consists of chromian spinel (∼10 volume %), clinopyroxene and ilmenite within the serpentine matrix (altered olivine, ∼80 volume %). Locally olivines are almost entirely pseudomorphed to serpentine chlorite, but preserving their euhedral grain shape. Some serpentine contains no relict olivine (Fig. 4h). Chromian spinels are mostly unmetamorphosed, in some parts weakly metamorphosed, but have preserved euhedral grain shapes. In chromites a strong zoning pattern is observed on X-ray maps and indicated by electron

microproprobe analyses with a growth of a ferrichromite and magnetite rim around the Cr- and Al-rich core. 4.10. Tonalite-trondhjemite-granodiorite and amphibolite Tonalite-trondhjemite-granodiorites (IK-100911-09A, IK120215-05A and IK-120215-12A) mostly contain quartz (∼20 volume %), plagioclase (∼30 volume %), amphibole (∼10 volume %), biotite (∼20 volume %) and K-feldspar (∼5 volume %); zircons occur in some plagioclases. The feldspars are mostly euhedral; some are sericitized. Textures are mainly equigranular. Sutured quartz grains indicate some stress causing tension between grains confirming the rocks are unmetamorphosed or only weakly metamorphosed (Fig. 4i). Amphibolite enclaves within tonalitetrondhjemite-granodiorite (IK-120215-05B and IK-120215-05 C) mainly contain quartz (∼20 volume %), plagioclase (∼10 volume %), amphibole (∼35 volume %) and biotite (∼15 volume %). 5. P–T evolution The composition of phengite can be a sensitive indicator of the pressure of metamorphism, but that also depends on the appropriate accompanying phases (Auzanneau et al., 2010; Ganne et al., 2011). However, due to the absence of such an assemblage, determination of the pressure and temperature conditions of the Kumta quartz-phengite schist necessarily depends on a calculated isochemical phase diagram. Accordingly,

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235

Fig. 6. P–T phase diagram modeled using the calualted bulk composition of quartz-phengite schist in the SiO2 –TiO2 –Al2 O3 –FeO–MnO–MgO–CaO–Na2 O–K2 O–H2 O chemical system.

the phase relations for a calculated bulk composition with a domain rich in phengite were estimated in the chemical system SiO2 –TiO2 –Al2 O3 –FeO–MnO–MgO–CaO–Na2 O–K2 O–H2 O. Bulk compositions were calculated by multiplying the percentage coverage of a mineral in a particular domain with its mean oxide wt.% data and by normalizing it to 100. The phase diagrams (Fig. 6) were constructed using free energy minimization (Connolly, 2005) with end-member thermodynamic data given by Holland and Powell (1998) and revised by the authors in 2002; the solution models are summarized in Supplementary Table S4. The mineral pair phengite ± quartz is stable in all fields (Fig. 6) and kyanite is stable on the high-pressure/temperature side. Chlorite appears when the pressure drops and chloritoid disappears at an even lower pressure. Lawsonite is stable only in the lowtemperature/high-pressure corner of the phase diagram (Okamoto and Maruyama, 1999). There are no significant variations in the composition of phengite in Kumta suture samples. Based on the assemblage (phengite–quartz–rutile–chlorite–chloritoid–H2 O) and the XMg –XNa composition of phengite, the quartz-phengite schist is calculated to have been stable at c. 13 kbar at 525 ◦ C (Fig. 6). The presence of fine-grained alluminosillicates (sillimanite/kyanite) in the phengite matrix (Fig. 6) (revealed by X-ray elemental mapping using a field emission microprobe) extends the P–T segment into the kyanite stability field. Thus, based on the present composition, the rock exhumed from a P–T condition of c. 18 kbar and 550 ◦ C (Fig. 6) in the eclogite facies and underwent re-equilibration in the amphibolites facies (Liou et al., 1998; Oh and Liou, 1998). Such a P–T evolution provides confirmation that the Kumta suture is sited on a subduction zone, which was capable of such subduction and exhumation.

6. Geochronology To understand the chronological relationships between the various tectonic units, we produced SHRIMP U–Pb zircon dates, K–Ar biotite and phengite dates, and Electron Microprobe Analysis (EPMA)-CHIME zircon geochronology. The analytical details of each

method and results of each of the rock types from various tectonic units are given below. 6.1. Analytical procedure 6.1.1. SHRIMP zircon U–Pb geochronology SHRIMP U–Pb zircon dating was carried out on five samples. To establish the age of the Karwar block and the Dharwar block and the time of formation of the Kumta suture between them, U–Pb zircon dates were obtained from three tonalitetrondhjemite-granodiorite samples from the Karwar block, one quartzo-feldspathic gneiss from the Dharwar block and a quartzphengite schist from the Kumta suture. Zircons were separated using crushing and pulverizing, panning and handpicking under a stereo-microscope. Zircons were analyzed using a sensitive highresolution ion microprobe (SHRIMP II) at the SHRIMP Laboratory of the National Institute of Polar Research, Tokyo, Japan. Analytical procedures for the U–Pb analysis follow those of Horie et al. (2012) and Hokada et al. (2013). Zircons from each sample were mounted together with standards on an epoxy resin disk that was polished after cleaning to obtain cross-sections through the grains. Prior to the U–Pb analyses the surfaces of grain mounts were washed with 2% HCl (ultrasonic cleaning-petroleum ether) to remove any lead contamination, and were coated with gold about 135 A˚ thick. To understand the morphology and internal structure of individual grains and to find out the suitable analytical spots backscattered electron (BSE) and cathodoluminescence (CL) images were obtained using a scanning electron microscope (SEM; JEOL JSM-5900 LV) at the National Institute of Polar Research in Tokyo, Japan (Fig. 7). To obtain the SEM images an 0.2 nA electron beam current and a 15 kV acceleration voltage was used on the gold coating. An O2 − primary ion beam of 1.6–2.7 nA intensity was utilized to sputter the analytical spot of 20–25 ␮m diameter (Köhler Ap.: 100–120 ␮m) on the zircons in the polished mount. TEMORA 2 (206 Pb/238 U age = 416.8 Ma; Black et al., 2004) and 91500 (U concentration 81.2 ppm; Wiedenbeck et al., 1995) were used as calibration standard materials for the U–Pb analysis and U concentration. FC1 (reference value: 1099.3 ± 0.3Ma; Paces and Miller, 1993) and OG1 (reference value of Pb–Pb: 3465.4 ± 0.6; Stern et al., 2009) were

236

C. Ishwar-Kumar et al. / Precambrian Research 236 (2013) 227–251

Fig. 7. Concordia plots (Wetherill-total) of zircons analyzed by U–Pb SHRIMP method. Inset shows repersentative CL images for zircons for each sample, probability density graph for quartz-phengite schist and bar charts for tonalite-trondhjemite-granodiorite and quartzo-feldspathic gneiss. (a) IK-120215-12A Tonalite-trondhjemite-granodiorite from the Karwar block, (b) K-120215-05A Tonalite-trondhjemite-granodiorite from the Karwar block, (c) IK-110123-02L Quartz-phengite schist from the Kumta suture, (d) IK-120629-03 Quartzo-feldspathic gneiss from the Dharwar block. All ages are given in Ma.

used as second standards. A correction for common Pb was made on the basis of the measured 204 Pb and the model for common Pb compositions proposed by Stacey and Kramers (1975) for the bulk-crust Pb isotope composition model. The U–Pb data were reduced in a manner similar to that described by Williams (1998), using SQUID2 and Isoplot3 softwares. The pooled ages were made after correction from common Pb using measured Pb, and were calculated using the Isoplot/Ex software (Ludwig, 2008). Only concordant date was (less than 10% discordancy) considered for selecting the ages, and the dates were derived from 207 Pb/206 Pb ratios. Uncertainties reported for individual analyses are at 1␴ level, and error in standard calibration is 0.16–0.33%. 6.1.2. Electron microprobe-CHIME zircon geochronology Electron microprobe-CHIME dates were obtained for zircon grains from a quartz-phengite schist sample. Zircon grains in a conventional thin section were analyzed with a JCXA-733 electron probe microanalyzer (JEOL, Tokyo, Japan) with five wavelengthdispersive spectrometers (radius of the Rowland’s circle = 140 mm) at the Center for Chronological Research, Nagoya University. The instrument operating conditions were 15 kV accelerating voltage, 250 nA probe current and 4 microns probe diameter. The X-ray lines

used were Pb M␤, Th M␣ and U M␤. Standard materials were euxenite by Smellie et al. (1978) for Th and U, and the silicate glass by Suzuki and Adachi (1998) for Pb. Matrix correction was performed by the method of Bence and Albee (1968) with the ␣-factor table by Kato (2005) modified for the Pb M␤ and U M␤ lines. Details of the CHIME dating are described by Suzuki and Adachi (1991) and Suzuki and Kato (2008). To reject geochronologically abnormal results, the analysis points in cracks or inclusions, and points having K2 O > 0.5% m/m or CaO > 0.5% m/m were not included in calculation of the CHIME ages. 6.1.3. K–Ar dating of biotite and phengite minerals K–Ar dating was undertaken on biotite separates from tonalitetrondhjemite-granodiorites and amphibolites from the Karwar block, a gabbro from the Bondla arc and gneiss from the Dharwar block in order to estimate the cooling ages and phengite in quartzphengite schist and biotite in garnet-biotite schist from the Kumta suture to contrain the age of metamorphism and suturing. Fresh and unaltered rock slabs were selected and crushed in a jaw crusher and pulverized in a ball mill, and then sieved to get several fractions, which were washed in pure water to remove any fine particles on grain surfaces, and after that the samples were dried. Biotites

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237

Table 2 U–Pb data of SHRIMP zircon analysis. Spot

% 206 Pbc

U (ppm)

Th (ppm)

4-corr ppm 206 Pb*

232

Th/238 U

206

Pb/238 U Age

207

Pb/206 Pb Age

% discordant

a. IK-120215-12A – tonalite-trondhjemite-granodiorite from Karwar block 70 153 1.1 0.27 329 2.1 2.47 130 43 30 3.1 0.13 209 70 111 5.41 509 159 178 4.1 5.1 2.67 741 12 151 1.90 637 174 145 6.1 2.51 753 18 234 7.1 134 162 8.1 0.61 310 9.1 0.72 308 76 97 0.90 427 6 125 10.1 0.19 306 21 146 11.1 0.45 16 6 9 11.2 0.27 364 137 160 12.1 6.86 309 53 53 12.2 0.41 243 44 108 12.3 0.23 234 56 129 13.1 146 217 14.1 0.33 375 0.38 143 58 91 15.1 0.07 464 1 204 16.1 16.2 0.05 433 87 242 0.15 677 8 165 16.3 8.06 423 7 127 17.1 0.01 417 203 233 20.1 424 197 20.2 0.09 744 21.1 0.07 195 93 100 0.06 90 27 47 22.1 23.1 0.61 517 90 203 24.1 0.65 434 142 181

0.22 0.34 0.34 0.32 0.02 0.28 0.02 0.45 0.25 0.02 0.07 0.43 0.39 0.18 0.19 0.25 0.40 0.42 0.00 0.21 0.01 0.02 0.50 0.59 0.49 0.31 0.18 0.34

2791 1543 3118 2201 1370 1513 1989 3058 2024 1886 2853 3270 2666 1170 2683 3193 3324 3570 2659 3228 1613 1938 3230 1732 3012 3066 2426 2545

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

25 74 35 20 37 20 19 27 99 52 25 76 24 29 37 29 28 35 35 31 82 25 26 15 28 35 21 22

3184 3187 3190 3078 2959 3032 2891 3206 3223 3081 3206 3184 3212 3197 3211 3208 3476 3601 3113 3211 2887 3094 3473 2927 3203 3204 3103 3094

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

5 26 9 18 20 8 7 11 7 6 5 22 6 59 6 5 3 5 4 3 14 24 3 4 5 7 6 9

+15 +58 +3 +34 +59 +56 +36 +6 +43 +45 +14 −3 +21 +69 +20 +1 +6 +1 +18 −1 +50 +43 +9 +46 +7 +5 +26 +21

b. IK-120215-05A – tonalite-trondhjemite-granodiorite from Karwar block 13 18 1.2 0.13 32 1.3 0.09 200 106 110 0.45 227 80 126 2.1 3.1 0.07 69 37 39 0.04 149 46 81 9.2 0.07 316 138 166 11.1

0.41 0.55 0.36 0.55 0.32 0.45

3230 3193 3221 3259 3162 3078

± ± ± ± ± ±

42 27 27 34 29 26

3206 3205 3198 3197 3212 3198

± ± ± ± ± ±

12 5 6 8 9 5

−1 +0 −1 −2 +2 +5

c. IK-120216-07A – tonalite-trondhjemite-granodiorite from Karwar block 1.1 0.09 60 37 33 1.2 0.01 230 73 124

0.629 0.328

3188 ± 36 3147 ± 25

d. IK-110123-02L- Quartz-phengite schist from Kumta suture 0.16 581 321 180 1.1 0.03 135 99 72 2.1 719 432 139 3.1 0.50 4.1 0.13 171 93 54 0.09 378 173 203 5.1 6.1 0.27 733 98 209 0.08 91 59 47 7.1 0.07 330 109 175 8.1 0.43 218 72 62 9.1 0.10 398 129 215 10.1 0.42 898 126 219 11.1 0.10 339 209 163 12.1 0.08 32 30 18 13.1 0.13 600 489 230 14.1 0.10 174 97 75 15.1 0.09 239 117 105 16.1 0.15 240 153 107 17.1 0.17 91 45 45 18.1 0.07 530 404 212 19.1 0.02 92 69 47 20.1 0.02 90 39 42 21.1 0.08 370 197 162 22.1 0.10 100 48 49 23.1 0.17 307 214 140 24.1 0.04 362 147 199 25.1 26.1 0.05 170 127 87 0.09 112 65 56 27.1 0.15 325 82 142 28.1 0.35 427 174 114 29.1 49 64 30.1 0.02 130 31.1 0.10 194 146 100 0.21 522 315 186 32.1

0.57 0.76 0.62 0.56 0.47 0.14 0.67 0.34 0.34 0.33 0.15 0.64 0.96 0.84 0.58 0.50 0.66 0.51 0.79 0.78 0.45 0.55 0.49 0.72 0.42 0.78 0.60 0.26 0.42 0.39 0.78 0.62

1989 3109 1312 2014 3126 1851 3069 3102 1845 3142 1608 2860 3278 2380 2623 2653 2686 2926 2462 3000 2810 2657 2880 2743 3195 3022 2976 2657 1745 2927 3028 2233

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

18 24 17 15 19 11 51 20 24 49 31 18 50 14 34 19 19 29 15 29 28 17 28 18 20 24 27 17 26 25 22 13

3208 ± 8 3201 ± 4 2778 3101 2865 2782 3125 2798 3105 3090 2937 3280 2711 3301 3275 2978 2957 3089 3046 2991 2914 3110 2992 2984 2993 3359 3462 3098 2986 3033 2923 2998 3128 2986

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

14 5 9 6 3 4 8 4 9 4 14 4 17 4 8 5 5 8 4 8 8 4 8 4 3 6 7 5 6 6 5 4

+1 +2 +33 −0 +60 +32 −0 +39 +1 −0 +43 +5 +46 +16 −0 +24 +14 +17 +14 +3 +19 +4 +7 +13 +5 +22 +10 +3 +0 +15 +46 +3 +4 +30

238

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Table 2 (Continued) Spot

% 206 Pbc

U (ppm)

Th (ppm)

4-corr ppm 206 Pb*

e. IK-120629-03- Quartzo-feldspathic gneiss from Dharwar block – 207 136 83 1.1 222 106 91 3.1 0.05 4.1 0.06 210 99 87 5.1 0.03 231 108 94 0.03 202 91 85 7.1 – 283 163 117 8.1 0.03 195 99 80 9.1 73 69 10.1 0.01 164 10.2 0.02 222 104 93 11.1 0.08 187 94 75 12.1 0.13 228 121 96 0.07 281 173 118 14.1 0.09 249 108 102 16.1 99 95 16.2 0.03 234 19.1 0.02 181 87 74 0.01 233 111 96 20.1 0.03 207 88 87 21.1 49 50 22.1 0.07 123 0.19 308 224 119 23.1 0.02 161 64 67 30.1 31.1 0.06 166 85 69 – 325 181 133 32.1 0.07 130 62 55 38.1 0.11 226 165 88 39.1 82 83 40.1 0.02 196 41.1 0.08 186 83 77 0.05 143 41 62 36.1 42.1 0.02 232 123 96 45.1 0.07 199 94 82 46.1 0.18 311 184 121 48.1 – 179 82 74 49.1 0.03 221 107 92 52.1 0.04 217 103 92 53.1 0.01 234 133 97 55.1 0.11 215 111 88 – 151 69 64 57.1 172 76 72 58.1 0.06 59.1 0.01 225 92 93 60.1 0.09 165 72 71 61.1 0.04 287 128 121 0.08 147 73 62 62.1 64.1 0.03 158 72 66 66.1 0.08 259 145 104 67.1 0.01 194 98 82 70.1 0.02 158 84 67 0.03 283 178 112 71.1 72.1 – 146 71 62 0.01 232 112 96 73.1 0.01 220 104 93 74.1 0.06 92 36 39 76.1 – 198 91 83 77.1 0.07 217 161 91 78.1 0.07 105 52 44 79.1 – 170 74 73 80.1

232

Th/238 U

0.680 0.494 0.488 0.484 0.463 0.594 0.522 0.460 0.486 0.517 0.547 0.635 0.447 0.436 0.494 0.493 0.439 0.412 0.750 0.409 0.527 0.575 0.494 0.752 0.434 0.458 0.295 0.547 0.486 0.611 0.475 0.502 0.491 0.586 0.532 0.469 0.458 0.423 0.449 0.459 0.510 0.472 0.579 0.524 0.551 0.650 0.506 0.501 0.489 0.409 0.477 0.765 0.514 0.450

206

Pb/238 U Age

2480 2510 2537 2506 2560 2538 2528 2570 2564 2473 2569 2565 2520 2502 2510 2523 2559 2492 2388 2539 2548 2505 2555 2414 2586 2529 2641 2545 2515 2411 2535 2547 2586 2534 2506 2569 2544 2542 2606 2563 2566 2540 2468 2576 2587 2442 2580 2540 2584 2576 2567 2561 2565 2609

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

17 17 17 16 17 16 17 18 17 17 17 17 25 16 17 17 17 19 15 18 18 15 20 21 17 17 19 17 17 15 18 21 17 16 17 19 18 18 19 16 19 18 17 18 18 16 19 17 17 22 17 17 21 33

207

Pb/206 Pb Age

2579 2566 2564 2576 2586 2565 2566 2561 2574 2572 2574 2573 2580 2576 2583 2574 2580 2574 2565 2566 2572 2563 2570 2559 2569 2568 2670 2571 2562 2566 2579 2587 2564 2579 2571 2567 2555 2572 2576 2575 2565 2584 2568 2582 2574 2565 2586 2569 2569 2583 2578 2565 2572 2582

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

7 7 7 6 6 5 7 7 6 7 6 6 12 6 7 6 7 9 9 7 7 5 9 6 6 7 8 6 7 6 7 12 6 6 7 7 7 7 8 6 8 8 10 7 7 6 8 6 6 10 6 7 10 7

% discordant +5 +3 +1 +3 +1 +1 +2 -0 +0 +5 +0 +0 +3 +3 +3 +2 +1 +4 +8 +1 +1 +3 +1 +7 −1 +2 +1 +1 +2 +7 +2 +2 −1 +2 +3 −0 +1 +1 −1 +1 −0 +2 +5 +0 −1 +6 +0 +1 −1 +0 +1 +0 +0 −1

Errors are 1s; Pbc and Pb* indicate the common and radiogenic portions, respectively. Common Pb corrected using measured 204 Pb. % discordant denotes the percentage of discordancy (e.g., Song et al., 1996).

were separated using glassy paper and purified again by using a stereo-microscope. Phengite-rich aggregates were separated by handpicking from quartz-phengite schist and then pulverized and sieved to obtain 100–200 mesh fractions. The purity of phengite was confirmed by X-ray diffraction analysis (Supplementary Fig. S8). K–Ar analyses of biotite and phengite separates were carried out at the Research Institute of Natural Sciences, Okayama University of Science, Japan (Nagao et al., 1984; Itaya et al., 1991). Potassium was analyzed by flame photometry using a 2000-ppm Cs buffer with an analytical error within 2% at a 2␴ confidence level. Argon was analyzed on a 15-cm radius sector type mass spectrometer with a single collector system using the isotopic dilution method and 38 Ar spike. Multiple runs of the standard (JG-1 biotite, 91Ma)

indicate that the error of the argon analysis is about 1% at a 2␴ confidence level (Itaya et al., 1991). The decay constants of 40 K to 40 Ar and 40 Ca, and the 40 K content in potassium used in the age calculations are 0.581 × 10−10 /year and 4.962 × 10−10 /year, and 0.0001167, respectively (Steiger and Jäger, 1977). 6.2. Geochronology results 6.2.1. Tonalite-trondhjemite-granodiorites and amphibolites from the Karwar block The zircons from tonalite-trondhjemite-granodiorite (IK120215-12A, IK-120215-05A and IK-120216-07A) are mostly prismatic (a few are slightly rounded) and elongate grains (grain size < 200 ␮m).

C. Ishwar-Kumar et al. / Precambrian Research 236 (2013) 227–251

239

Table 3 K–Ar age results for biotite and phengite minerals. Sample no.

Rock type

Mineral

Potassium content (wt.%)

Rad. 40 Ar (10−8 cc STP/g)

K–Ar age (Ma)

Non-rad. 40 Ar (%)

IK-12021512A IK-12021505A IK-12021607A IK-12021505C IK-130409-01

Tonalite-trondhjemitegranodiorite Tonalite-trondhjemitegranodiorite Tonalite-trondhjemitegranodiorite Amphibolite

7.400 ± 0.148

84583 ± 1314

1746 ± 28

0.5

7.318 ± 0.146

87390 ± 1465

1796 ± 30

0.4

7.635 ± 0.153

91225 ± 1593

1796 ± 30

0.4

8.180 ± 0.164

89502 ± 1733

1698 ± 31

0.4

3.520 to 3.900

36685 ± 383

1644 to 1536

0.5

IK-11012302L IK-130105-04

Quartz-phengite schist

8.079 ± 0.162

61387 ± 767

1326 ± 22

0.2

6.223 ± 0.124

50306 ± 566

1385 ± 22

0.4

IK-110629-03

Quartzo-feldspathic gneiss Quartzo-feldspathic gneiss

Biotite #60-100 Biotite #60-100 Biotite #60-100 Biotite #32-100 Biotite #60-100 Phengite #100-200 Biotite #60-100 Biotite #60-100 Biotite #60-100

7.622 ± 0.152

86060 ± 1385

1733 ± 29

0.4

7.746 ± 0.155

36716 ± 377

933 ± 16

0.7

Gabbro

Garnet-biotite schist

IG-120410-13

IK-120215-12A sample zircons are <200 ␮m and grain shapes are prismatic, but have a slightly round surface. A total of 22 zircon grains was analyzed, 10 spots were used out of 28 analyzed spots in 10 grains (others were discordant). For IK-120215-12A sample, the age is estimated as 3207 ± 4 Ma (MSWD = 1.2; n = 7), and the other three points are 3473 ± 3 Ma, 3476 ± 3 Ma and 3601 ± 5 Ma (Fig. 7a, Table 2a). The zircon textures suggest that the 3601 Ma (i), 3476 Ma, and 3473 Ma (ii) ages correspond to the inheritance age of the zircon, and 3207 ± 4 Ma is the crystallization age. Also the zircons have cracks and fractures on their rims, and their cores are recrystallised; the Concordia graph suggests high lead loss. The 3601 ± 5 Ma inheritance age from this tonalite-trondhjemite-granodiorite is the

oldest age reported from the tonalite-trondhjemite-granodiorites from the Dharwar craton. The K–Ar ages of biotites from IK-12021512A tonalite-trondhjemite-granodiorite sample is estimated to be 1746 ± 28 Ma (Table 3). Zircons from IK-120215-05A are very small <20 ␮m (one grain >400 ␮m), prismatic but slightly rounded, have oscillatory zones and contain many mineral inclusions. 11 grains in total were analyzed, 6 spots were used out of 15 analyzed spots in 5 grains (others were discordant). For IK-120215-05A sample the crystallization age is estimated as 3201 ± 5 Ma (MSWD = 0.69; n = 6) (Fig. 7b, Table 2b). Biotite from IK-120215-05A sample gives a K–Ar cooling age of 1796 ± 30 Ma (Table 3).

Table 4 EPMA-CHIME dating results of Quartz-phengite schist. Spot ID

ThO2

UO2

PbO

Y2 O3

CaO

K2 O

Apparent Age/Ma

Err/Ma (2s)

UO2 *

Group

1-06 1-10 2-01 2-03 2-04 2-06 2-07 2-11 2-12 2-14 2-15 2-18 2-19 2-20 2-21 3-05 4-04 4-08 4-11 4-13 4-16 4-23 5-18 5-25 5-26 5-27 5-28 6-03 7-12 7-15 7-22 7-26

0.0089 0.0062 0.0571 0.0108 0.0096 0.0142 0.0096 0.0122 0.0114 0.0199 0.0359 0.0418 0.0174 0.0173 0.0555 0.0233 0.0282 0.0069 0.0324 0.0376 0.0390 0.0334 0.0317 0.0139 0.0104 0.0079 0.0048 0.0630 0.0076 0.0653 0.0521 0.0151

0.0310 0.0266 0.0303 0.0310 0.0197 0.0330 0.0283 0.0292 0.0236 0.0396 0.0481 0.0429 0.0229 0.0218 0.0476 0.0348 0.0218 0.0134 0.0521 0.0336 0.0260 0.0390 0.0341 0.0214 0.0234 0.0168 0.0140 0.0942 0.0188 0.1351 0.1057 0.0228

0.0312 0.0279 0.0417 0.0248 0.0217 0.0321 0.0306 0.0281 0.0223 0.0411 0.0472 0.0451 0.0212 0.0272 0.0487 0.0418 0.0253 0.0142 0.0166 0.0411 0.0333 0.0125 0.0404 0.0235 0.0244 0.0202 0.0144 0.0305 0.0175 0.0411 0.0321 0.0245

0.0236 0.0201 0.3705 0.0547 0.0551 0.0925 0.0661 0.0931 0.0692 0.0713 0.1121 0.5547 0.2921 0.2922 0.7409 0.1230 0.0615 0.0641 0.1283 0.1116 0.0912 0.1362 0.5359 0.3519 0.2893 0.2452 0.2268 0.2827 0.0414 0.5141 0.3115 0.1310

0.0028 0.0047 0.0103 0.0046 0.0033 0.0043 0.0045 0.0051 0.0059 0.0143 0.0321 0.0036 0.0022 0.0018 0.0200 0.0116 0.0207 0.0037 0.0185 0.0156 0.0061 0.0474 0.0089 0.0037 0.0007 0.0024 0.0024 0.0397 0.0084 0.0429 0.0331 0.0015

0.0241 0.0197 0.0138 0.0318 0.0223 0.0292 0.0326 0.0221 0.0411 0.0324 0.0269 0.0215 0.0242 0.0256 0.0259 0.0434 0.0149 0.0226 0.0288 0.0478 0.0328 0.0361 0.0496 0.0193 0.0170 0.0195 0.0275 0.0349 0.0367 0.0455 0.0388 0.0298

3914 4012 4026 3439 4020 3796 4031 3779 3718 3899 3703 3768 3580 4165 3626 4130 3849 3932 1706 4015 3988 1634 4024 3967 3924 4195 3939 1711 3725 1684 1679 3918

353 392 310 423 503 338 354 385 516 287 241 273 500 443 242 284 457 725 402 271 358 534 291 481 449 563 765 224 633 165 217 451

0.033 0.028 0.045 0.033 0.021 0.036 0.030 0.032 0.026 0.043 0.055 0.051 0.026 0.025 0.062 0.039 0.029 0.015 0.061 0.043 0.036 0.048 0.040 0.024 0.025 0.018 0.015 0.111 0.020 0.153 0.120 0.026

Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 2 Group 1 Group 2 Group 2 Group 1 Group 2 Group 2 Group 2 Group 2 Group 2 Group 1 Group 2 Group 1 Group 1 Group 2

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Fig. 8. PbO vs. UO2 plots of zircons in quartz-phengite schist using the EPMA-CHIME method. a. Group 2 is 3750 Ma and b. Group 1 is 1697 Ma.

From IK-120216-07A sample only one zircon was obtained on which two ages were measured. The grain is prismatic, has oscillatory zoning and it is about ∼200 ␮m in size. The two spots give ages of 3208 ± 8 Ma and 3201 ± 4 Ma (Table 2c). The K–Ar ages of biotites from IK-120216-07A sample is 1796 ± 30 Ma (Table 3) For amphibolite sample (IK-120215-05C) from the Karwar block the K–Ar age of biotites is estimated as 1698 ± 31 Ma (Table 3).

6.2.2. Quartz-phengite schist and garnet-biotite schist from the Kumta suture Zircons from quartz-phengite schist (IK-110123-02L) sample have rounded to sub-rounded morphology (grain size <100 ␮m) and no metamorphic overgrowths. These are detrital zircons with no metamorphic rims. A total of 32 zircon grains were analyzed, 14 spots out of 32 spots in 14 zircon grains were used. The zircons from quartz-phengite schist (IK-110123-02L) sample gave

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Fig. 9. Geological map of east-central Madagascar (Groups modified after Collins and Windley, 2002; Raharimahefa and Kusky, 2006; De Waele et al., 2011; Key et al., 2011). Major rock types in northeastern Madagascar, the Betsimisaraka suture and surrounding area modified after Bauer and Key, 2005a,b; Raharimahefa and Kusky, 2006; Key et al., 2011. Structural lineaments are extracted from Landsat ETM+ satellite imagery and the ASTER (Advanced Space borne Thermal Emission Reflection Radiometer) digital elevation model.

mainly four age populations, ca. 2993 Ma, ca. 3101 Ma, ca. 3126 Ma, and ca. 3280 Ma (Fig. 7c, Table 2d). The age range of these four populations is quite large, but correlates with the ages of tonalitetrondhjemite-granodiorites from the Karwar block and gneisses in the Dharwar block. This quartz-phengite schist is interpreted as a meta-sediment derived from the Karwar or Dharwar blocks. Because the zircons from the quartz-phengite schist have no metamorphic overgrowths, monazites were dated by the electron microprobe-CHIME method to establish the metamorphic age. But the monazites in this sample could not be dated because they

contain very low Th and Pb. So, EPMA-CHIME dating was carried out on the zircons. In a SHRIMP the spot-size is large, about 20–24 ␮m, but in EPMA-CHIME dating the spot-size is small, thus thin rims can be analyzed. After rejecting points on cracks, inclusions, K2 O > 0.5% m/m and CaO > 0.5% m/m, the CHIME zircon dates mainly give two age groups, Group 1 and Group 2. The Group 2 age is estimated as 3750 ± 219 Ma ((2␴ level), n = 27, MSWD = 0.95, Initial PbO = 20 ± 32 ␮g/g (2␴ level)) (Fig. 8a, Table 4). The Group 1 age is calculated as 1697 ± 49 Ma ((2␴ level), n = 5, MSWD = 0.03, Initial PbO = –2 ± 10 ␮g/g (2␴ level)) (Fig. 8b, Table 4).

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Fig. 10. Map showing the compilation of our new geochronological results and published data from the study area (structural lineaments were extracted from Landsat ETM+ satellite imagery, and ASTER digital elevation model). Geochronological results from present study (U–Pb zircon SHRIMP - brown; K–Ar Bt and Phe - violet; EPMA CHIME zircon - pink) and also compiled from Rekha et al. (2013) (U–Pb–Th monazite - blue) and Balasubramanian (1978), Balasubramanian and Sarkar (1978), Gupta et al. (1988), Russell et al. (1996), French and Heaman (2010), (green). Geochronological details given in Table 5 and discussion given in the text. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Because the quartz-phengite schist has undergone comparatively low temperature metamorphism, the phengite mineral cooling age will be very close to the peak metamorphic age. K–Ar dating of phengite from the quartz-phengite schist gives a metamorphic age of 1326 ± 22 Ma (Table 3). Similarly, the K–Ar analysis of biotite separates from garnetbiotite schist (IK-130105-04) from the Kumta suture gives a metamorphic age of 1385 ± 22 Ma (Table 3). 6.2.3. Quartzo-feldspathic gneiss from the Dharwar block The zircons from quartzo-feldspathic gneiss sample of Dharwar block (IK-120629-03) contains prismatic grains, shows oscillatory zoning and contains many mineral inclusions and cracks (grain size <250 ␮m). Total 75 grains were analyzed, 54 spots out of 77 spots in 52 zircon grains were selected. The age is estimated as 2571 ± 2 Ma (MSWD = 1.1; n = 53). One grain has inheritance age 2670 ± 8 Ma (Fig. 7d, Table 2e). For biotites from gneiss (IK-120629-03) sample K–Ar age is estimated as 1733 ± 29 Ma (Table 3). 6.2.4. Gabbro from the Bondla arc The Bondla ultramafic-gabbro complex contains several gabbro intrusions. K–Ar analysis was carried out on biotite separates from a gabbro (IK-130409-01). The K–Ar cooling age is calculated as 16441536 Ma (Table 3). But, due to sample heterogeneity, biotite in the gabbro has a large error and the K content varies from 3.52, 3.90,

3.59, 3.61 to 3.76 wt.%. The minimum value (3.52 wt.%) gives an age of 1644 Ma and the maximum value (3.90 wt.%) an age of 1536 Ma. 7. Madagascar: the Sahantaha shelf, Betsimisaraka suture, and Antananarivo block It is appropriate here to summarize the key relations of the Malagasy suture, shelf, arc and block (Fig. 9), which, we suggest, are a continuation of the equivalent suture, shelf, arc and block in western India. 7.1. The Sahantaha shelf The Sahantaha shelf sediments were deposited on the northwestern passive margin of the Archean Antongil Craton (Hottin, 1969, 1976), which is a continuation in Madagascar of the Indian Dharwar Craton (Collins and Windley, 2002; Schofield et al., 2010). However, the sediments are only well preserved in the northwestern corner of the block in the Sahantaha region (Supplementary Fig. S1), where the regional strike is EW and where the later major EW compressional (collisional) deformation was least (Windley et al., 1994). The stratigraphy and structure of the Sahantaha Group were most recently studied by De Waele et al. (2011). Low-grade basal quartzites ovelie discordantly high-grade gneisses of the Antongil

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Fig. 11. Schematic geological cross-section of the study area (average dip ∼45◦ ) along XX , with corresponding thin section photomicrographs and hand specimen photos and the possible correlation of Madagascar in the west. The inset map shows the extent of Karwar block, Sirsi shelf, Kumta suture and Bondla arc and Dharwar block.

basement (as Hottin, 1969 described). The Sahantaha Group is dominated by quartzites especially in the lower part where they are up 300-m thick. Overlying quartzites are succeeded upwards by pelitic schists, and minor calcareous sediments. According to De Waele et al. (2011) the sedimentary succession is reminiscent of clastic, quartz-rich fans or lobes that pass out into mud-dominated shelf or slope apron deposits on a passive continental margin, in this case the margin of the Antongil Craton. They interpreted 1771 ± 18 Ma as the maximum age of deposition of the Sahantaha quartzite, and they proposed a minimum age of deposition of 750 or 530 Ma.

7.2. The Betsimisaraka suture This major suture zone extends for more than 600 km along eastern Madagascar bordered by the Antananarivo block to the west and by the Sahantaha shelf and Antogil-Masora blocks to the east (Fig. 9). Because the regional dip is predominantly shallow to the west (mineral lineations down-dip to the west), the outcrop width of the suture may reach >50 km. The suture was defined by Kröner et al. (2000) and Collins and Windley (2002), and briefly mentioned by Tucker et al. (2011a), De Waele et al. (2011) and Key et al. (2011). Structural and remote sensing analysis of the southern and northern segments supported the presence of the suture (Raharimahefa and Kusky, 2006, 2009). The suture zone is largely filled by paragneisses (with common augen, cataclastic and mylonitic fabrics) associated with pelitic mica schists (garnet, staurolite, kyanite, sillimanite and graphite). In the south there are fuchsite mica schists and fuchsite quartzites. These meta-sedimentary rocks enclose abundant maficultramafic lenses of, e.g., garnet meta-gabbro, garnet amphibolite, garnet clinopyroxenite, orthopyroxenite, lherzolite, wehrlite, and harzburgite. These lenses range in width from tens to hundreds of meters and may be up to 2 km long. At Antara a folded ultramafic lens is up to 20–25 km across from limb to limb, and a

15 km × 15 km lens contains peridotite and notably garnet websterite (Supplementary Fig. S9) (Hottin, 1969). The only readily accessible mafic-ultramafic complex within the suture zone is the Ranomena Complex, which is situated 25 km NW of Toamasina city (18◦ 07 46 S: 49◦ 15 47 E) (Fig. 9). This is a ∼700 m × 300 m lens (in garnet-kyanite-sillimanite gneiss) that contains garnet websterite, harzburgite, orthopyroxenite, clinopyroxenite, chromite-layered peridotite, and within the ultramafic rocks is a 10 m-wide layer of two pyroxene-hornblende gabbro. The chromitites were studied by Grieco et al. (2012), who concluded that the mineral chemistry shows strongest affinity with that of chromites in continental layered intrusions. Ishwar-Kumar et al. (under review) report that mafic granulite contains a primary assemblage with clinopyroxene, orthopyroxene, quartz, and Fe/Ti oxide, which have interacted to produce coronas of garnet with or without plagioclase. A P–T isochemical phase diagram computed by free energy minimization (Connolly, 2005) with thermodynamic data of Holland and Powell (1998) and calculated solution models was based on model compositions in the chemical system Na2 O–CaO–K2 O–FeO–MgO–Al2 O3 –SiO2 –H2 O–TiO2 estimated from mineral models and from the compositions of the assemblages. The peak temperature for the garnet-forming reaction between consuming clinopyroxene-orthopyroxene was c. 780 ◦ C at a pressure above 22 kbar. This places the metamorphic recrystallization in the eclogite facies.

7.3. The Antananarivo block The western side of the Betsimisaraka suture is occupied by partly retrogressed granulite facies tonalitic to granitic orthogneisses, the protoliths of which have calc-alkaline chemistry (zircon age of 824–720 Ma; Kröner et al., 1999), and consequently Handke et al. (1999) and Kröner et al. (2000) concluded they belong to a subduction-generated magmatic arc. The presence of

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Table 5 Summary of the available geochronological data from the study area. Rock type

Method

Age (Ma)

Geological unita

Reference

Tonalite-trondhjemite-granodiorite Tonalite-trondhjemite-granodiorite Tonalite-trondhjemite-granodiorite Quartz-phengite schist Quartzo-feldspathic gneiss Tonalite-trondhjemite-granodiorite Tonalite-trondhjemite-granodiorite Tonalite-trondhjemite-granodiorite Amphibolite Gabbro Quartz-phengite schist Garnet-biotite schist Quartzo-feldspathic gneiss Quartzo-feldspathic gneiss Quartz-phengite schist Granite Biotite from Pegmatite Biotite from Pegmatite Granite Limestone Gabbro Peninsular gneiss Peninsular gneiss Phyllite/Schist Phyllite/Schist Phyllite/Schist Phyllite/Schist Phyllite/Schist Tonalite clasts in conglomerate Chlrite-biotite-muscovite-calcite matrix in conglomerate Granitoids Granitoids

SHRIMP U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon SHRIMP U–Pb zircon K–Ar biotite K–Ar biotite K–Ar biotite K–Ar biotite K–Ar biotite K–Ar phengite K–Ar biotite K–Ar biotite K–Ar biotite EPMA – CHIME zircon Rb–Sr (whole rock) K–Ar K–Ar Pb–Pb (whole rock) Pb–Pb (whole rock) U–Pb zircon EPMA – Th–U–Pb monazite EPMA – Th–U–Pb monazite EPMA – Th–U–Pb monazite EPMA – Th–U–Pb monazite EPMA – Th–U–Pb monazite EPMA – Th–U–Pb monazite EPMA – Th–U–Pb monazite EPMA – Th–U–Pb monazite EPMA – Th–U–Pb monazite

3601 ± 5, 3476 ± 3, 3473 ± 3, 3207 ± 4 3201 ± 5 3208 ± 8, 3201 ± 4 3280, 3126, 3101, 2993 2571 ± 2 1746 ± 28 1796± 30 1796 ± 30 1697 ± 31 1644–1536 1326 ± 22 1385 ± 22 1733 ± 29 933 ± 16 3750 ± 219, 1697 ± 49 2669 3552 2564 3014 2639 2180 3138 ± 35 3154 ± 45 3158 ± 110 3067 ± 26 2436 ± 34 2543 ± 66 2625 ± 36 3128 ± 60 2566 ± 53, 2458 ± 34

Karwar block Karwar block Karwar block Kumta suture Dharwar block Karwar block Karwar block Karwar block Karwar block Bondla arc Kumta suture Kumta suture Dharwar block Coorg suture Kumta suture Sirsi shelf Kumta suture Kumta suture Sirsi shelf Sirsi shelf Sirsi shelf Karwar block Karwar block Kumta suture Kumta suture Kumta suture Kumta suture Kumta suture Karwar block Karwar block

Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study Balasubramanian (1978) Balasubramanian and Sarkar (1978) Balasubramanian and Sarkar (1978) Gupta et al. (1988) Russell et al. (1996) French and Heaman (2010) Rekha et al. (2013) Rekha et al. (2013) Rekha et al. (2013) Rekha et al. (2013) Rekha et al. (2013) Rekha et al. (2013) Rekha et al. (2013) Rekha et al. (2013) Rekha et al. (2013)

EPMA – Th–U–Pb monazite EPMA – Th–U–Pb monazite

2924 ± 50 2500 ± 37–2619 ± 37

Karwar block Karwar block

Rekha et al. (2013) Rekha et al. (2013)

a

Geological units classification based on the present study.

2200–2400 Ma upper concordia intercept ages for discordant zircons from Neoproterozoic granites (Paquette and Nédélec, 1998), of zircon dissolution ages of 2590–2503 Ma (Tucker et al., 1999), and zircon xenocryst ages from 2188 Ma to 1007 Ma (Kröner et al., 2000) suggests that this Antananarivo arc developed in an active continental margin rather than an oceanic island arc. In the model of Kröner et al. (2000), Collins and Windley (2002), Collins et al. (2003) and Collins (2006) the Antananarivo arc developed in the hanging wall of the westerly dipping Betsimisaraka subduction zone. The granulite-amphibolite facies metamorphism took place as a result of the final continent-continent collision, when the calc-alkaline protoliths were converted to gneisses and granulites. 8. The Kumta suture and eastern Gondwana paleogeography Current major geotectonic problems within eastern Gondwana include: 1. A close-fit assembly of Madagascar to India was constructed from satellite data (Lawver et al., 1997), and confirmed by geological markers (Reeves and de Wit, 2000); nevertheless its exact fit is still a matter of debate, and requires further geophysical and geological constraints (Ratheesh Kumar et al., under review). 2. The Betsimisaraka suture in eastern Madagascar passes eastwards offshore near Toamasina city and reappears about 180 km farther south (Fig. 9) (Collins and Windley, 2002). However, the question that has not been answered is: where does it go? 3. Tucker et al. (2011a,b) provocatively suggested that the Betsimisaraka suture does not exist, and instead hypothesized that its position is occupied by a closed extensional intra-continental sedimentary basin. If correct, the Dharwar craton did not extend

from India to the Betsimisaraka suture, but continued farther westwards to the western margin of Madagascar, and thus close to the eastern margin of Africa. Answers to these fundamental questions would substantially improve current understanding of the paleogeography of eastern Gondwana. The newly discovered Kumta suture in western India corroborates the existence of the Betsimisaraka suture, and thus helps to resolve the above three problems. A summary of the geochronological results of the present study integrated with published data (Fig. 10) also supports the existence of the Kumta suture. A model for the presence of the Kumta suture is based on the following new information: The basic, overall component-structure of the Kumta suture zone (craton-shelf-suture-arc) (shown in schematic geological cross section – Fig. 11) is the same as that of many suture zones the world over, known since the dawn of the plate tectonic era (Dewey and Bird, 1970; Burke et al., 1977). A common requirement for definition of a suture is that the bordering cratons/terranes should have different isotopic, metamorphic and magmatic histories, and that is satisfied by the fact that the Antongil block is substantially different from the Antananarivo block (for recent discussions see Schofield et al., 2010; De Waele et al., 2011; Key et al., 2011). But additional evidence is required to demonstrate that a suture did involve subduction to depth in a third dimension. In the Kumta suture that evidence is provided by the presence of high-pressure quartz-phengite schist, which equilibrated at peak conditions at 18 kbar and 550 ◦ C. The main components of the Kumta suture have a general dip to the west of 45◦ , suggesting, in the absence of any evidence of later major deformation, that the original suture zone had a westerly dip. A dipping subduction zone can be expected to have a calc-alkaline magmatic arc in its hanging wall; i.e., arcs

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Fig. 12. Reconstruction of India and Madagascar around 120 Ma and correlation of various shear zones based on our new data from the Kumta suture (Modified after Katz and Premoli, 1979; Lawver et al., 1997; Collins and Windley, 2002; O’Neill et al., 2003; Meert, 2003). A1 – Betsimisaraka suture zone, A2 – Kumta suture zone, A3 – Coorg suture zone. B1 – Ranotsara shear zone, B2 – Angavo shear zone, B3 – Chitradurga shear zone, B4 – Kolar shear zone, B6 – Moyar shear zone. C1 – Sahantaha shelf, C2 – Sirsi shelf. D1 – Antananarivo block, D2 – Karwar block; D3 – Coorg block. E1 – Antongil block, E2 – Masora block, E3 – Western Dharwar craton, E4 – Eastern Dharwar craton.

in the Antananarivo and Karwar blocks. The Antananarivo block contains several younger granitic arc intrusions, and the Bondla ultramafic-gabbro complex, on the western margin of the Kumta suture reasonably belongs to a magmatic arc. Thus the Kumta suture meets all the basic criteria necessary for acceptable definition as a suture. And it is important to appreciate that the whole Kumta suture zone is much better preserved than most of the Betsimisaraka suture zone, because it is situated in a re-entrant, and therefore the Sirsi shelf sediments are incredibly well preserved in an almost unmetamorphosed state. In contrast, the Sahantaha shelf sediments are only well preserved where they strike E–W. For most of their extent, they strike NS, and therefore they were exposed to the full-frontal effects of the later EW-oriented collisional thrusting. It has long been known that a re-entrant is a place of least deformation in contrast to an impinging promontory of an advancing craton where collisional deformation is most destructive (S¸engör, 1976). Several geochemical studies have recently been made of greywackes from the Sirsi shelf, which the authors unwittingly considered to belong to the Archaean Dharwar craton. Hegde and Chavadi (2009) concluded that metagreywackes from Ranibennur in the southern Sirsi shelf were derived from a magmatic arc and were deposited near the arc in a continental island arc setting. Devaraju et al. (2010) pointed out that greywackes from the Goa-Dharwar sector of the shelf contain volcanic clasts and suggested that they formed by submarine weathering of felsic volcanics from a magmatic arc, and that they were deposited in a basin,

which progressively changed from a passive to an active island arc-continental margin setting. Budihal and Pujar (2012) concluded that the greywackes and phyllites were sourced from a continental island arc, and suggested that they formed when two tectonic regimes were juxtaposed by subduction processes. Geochronological data are rare in the Kumta area. The only available results are three whole rock ages (Pb–Pb and Rb–Sr), and one U–Pb detrital zircon age from the Sirsi shelf region that ranges from 2180 to 3014 Ma (Fig. 10, Table 5) (Balasubramanian, 1978; Gupta et al., 1988; Russell et al., 1996; French and Heaman, 2010). The detrital zircon ages from the unmetamorphosed shelf region indicate the provenance from which the sediments were transported, probably the Dharwar craton. Balasubramanian and Sarkar (1978) reported K–Ar ages of 3552 and 2564 Ma from a biotite in pegmatite, which lies within the Kumta suture zone (Fig. 10, Table 5). Recently Rekha et al. (2013) reported many Th–U–Pb monazite ages obtained with an electron microprobe (Fig. 10, Table 5). The Th–U–Pb monazite age reported for the Peninsular gneiss is 3138 ± 35 Ma (Rekha et al., 2013). Metamorphic monazites in phyllites, schists and quartzites from the KarwarKumta schist belt have ages of 3158 ± 110 Ma and 3067 ± 26 Ma, and those from Goa schist belt have younger ages of 2436 ± 34 Ma, 2543 ± 66 Ma, and 2625 ± 35 Ma (Rekha et al., 2013). Monazites in tonalite-trondhjemite-granodiorite clasts in conglomerates from the Goa schist belt have mean ages of 3128 ± 60 Ma, which is close to the age of the Peninsular gneiss, and metamorphic monazites in a chlorite-biotite-muscovite-calcite matrix have ages of

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Fig. 13. Time vs. event chart for all major tectonic units related to the Betsimisaraka-Kumta-Coorg suture.

2458 ± 34 Ma and 2566 ± 53 Ma (Rekha et al., 2013). Monazites from Karwar-Chauri granitoids have an age of 2924 ± 50 Ma, and Quepem granitoids range in age from 2500 ± 37 Ma to 2619 ± 37 Ma (Rekha et al., 2013). But detailed geochronological studies based on a structural context, which discriminates various tectonic units in western India, using precise U–Pb zircon spot dating methods are required to establish the age of formation of the suture and emplacement of the arc. This situation can be compared with that of the Betsimisaraka suture zone in Madagascar, which has a wide range of ages from detrital zircons from 783 to 2950 Ma to magmatic zircons ages from 551 to 830 Ma (Kröner et al., 1999, 2000; Key et al., 2011; Tucker et al., 2011a). The tonalite-trondhjemite-granodiorites from the Karwar block are Archaen (ca. 3200 Ma) and indicate a ca. 3200 Ma crystallization event. Being small, the Karwar block in present-day India does not contain any Neoproterozoic or youger intrusions. But gabbros in the Bondla ultramafic-gabbro complex that have a cooling age in the range of 1644–1536 Ma are associated with serpentinites containing chromites with an arc chemical affinity; thus we conclude that the Bondla ultramafic complex is a small relict of an arc, exhumed along the boundary of the suture. Recently Rekha et al. (2013) reported 2436–3154 Ma Th–U–Pb monazite ages from the Karwar block. Madagascar has a similar scenario in so far as the Antananarivo block has an Archean protolith (2500–2700 Ma) intruded by Neoprotrozoic granites (550–850 Ma) (Kröner et al., 2000). The ca. 3200 Ma age of the Karwar tonalite-trondhjemite-granodiorite is correlative with ca. 3200 Ma charnockites (Peucat et al., 2013) from the Coorg block in the south (Fig. 12). Also these ages are close to those of Antongil block (3180–3320 Ma) and Masora block (3100–3300 Ma) (De Waele et al., 2011) from Madagascar, but they are on the other side of the suture. Recently, based on mesoscopic

structures and deformation microstructures and Th–U–Pb monazite ages Rekha et al. (2013) correlated the Karwar block and adjacent units from southern India with the Antongil block and other units in Madagascar. But although we correlate the same tectonic units in our study, we suggest different correlations between India and Madagascar. The ca. 3200 Ma tonalities from Karwar block are correlated with the 3154–3187 Ma Nosy Boraha suite in the Antongil block, the 2436–2625 Ma Goa schist belt is correlated with the 2522–2541 Ma Malagasy Mananara Group, the 3067–3158 Ma Karwar-Kumta schist belt is correlated with the 3178 Ma Fenerivo Group, and the ca. 2500 Ma Quepem granitoids is correlated with the 2147–2542 Ma Masaola suite (Schofield et al., 2010; Rekha et al., 2013) (Fig. 12). Once the relationships between the main tectonic units (blocks, suture and arc) are established, it becomes possible to understand with the appropriate chronological data the tectonic evolution, e.g., the time of suturing, and this helps to constrain inter-continental correlations. The Karwar (ca. 3200 Ma) and Dharwar (ca. 2571 Ma) blocks have different ages and are separated by a thick sedimentary shelf and a high-pressure suture zone. The detrital age of the quartz-phengite schist from the Kumta suture has a wide range (3280–2993 Ma), and is correlatable with the ages of the Karwar and Dharwar block, suggesting that these blocks provided the sediment source. Also the older ages are comparable with those of the Betsimisaraka suture (2950–740 Ma). Neoproterozoic detrital ages are not observed in the quartz-phengite schist from the Kumta suture, because there are no nearby younger granite intrusions in the small Karwar block, as is the case in the larger Antananarivo block, which contains many younger intrusions near the suture. The EPMA-CHIME dating gave two prominent age groups: Group 1 at ca. 1697 Ma, which is close to the K–Ar cooling ages of biotites from

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Fig. 14. A tectonic model illustrating the closure of the Betsimisaraka-Kumta-Coorg suture from north to south. (a) The Betsimisaraka-Kumta-Coorg suture with compiled geochronological data, distinguishing juvenile continents, arc intrusions, reworked blocks, and metamorphism along the suture (Age data from Key et al., 2011). (b) 1000 Ma Rodinia reconstruction (after Zhao et al., 2004), shows subduction from the west of Madagascar started during ca. 1300 Ma amalgamation of Rodinia. (c) Tectonic model illustrating the initiation of subduction in the north at 1380 Ma, forming the Sahantaha-Sirsi shelf, the 1644–1536 Ma Bondla arc, and 550–820 Ma granite intrusions in the Antananarivo block. (d) 750 Ma Rodinia reconstruction (after Torsvik, 2003), showing that active subduction started around 1000 Ma from the west of Madagascar and that rifting of Rodinia started around ca. 750 Ma. e. Tectonic model representing subduction at ca. 750 Ma and ocean closure along the Coorg suture. The subduction was enhanced by rifting of Rodinia from the east, and fast collision compared to the north, with no shelf, formation of high-grade metamorphic rocks in the suture, and intrusion of anorthosites, syenites and gabbros in the Coorg arc.

rocks on either side of the suture, and Group 2 at ca. 3750 Ma age, which is close to the zircon crystallization age (SHRIMP U–Pb zircon) (Fig. 8a and b). The 1746 ± 28 Ma and 1796 ± 30 Ma K–Ar ages of biotites in tonalite-trondhjemite-granodiorite from the Karwar block, the 1698 ± 31 Ma K–Ar age of biotites in amphibolite, and the 1733 ± 29 Ma K–Ar age of biotites in gneiss from the Dharwar block are close to the 1697 ± 49 Ma age of zircon rims obtained by EPMA-CHIME dating. In particular, the 1697 Ma K–Ar age of biotites in amphibolite is exactly the same as the EPMA-CHIME age. Iyer et al. (2010) suggested that the chromites in Kankavali and wagda mines of Sindhudurg district (c. 25 km north of Kumta suture), is podyform-type (occur in arc related environment) and emplaced during the NW-SE trending regional deformation prior to the metamorphism related to an orogenic activity. Chemistry of the chromites in serpenitnites from Bondla ultramafic-gabbro complex also suggests the arc realted tectonic setting (Ishwar-Kumar et al., unpublished). Bondla ultramafic-gabbro complex has a biotite (in gabbro) K–Ar cooling age of ca. 1644–1536 Ma and is a small relict of a magmatic arc in the Betsimisaraka-Kumta-Coorg suture (Fig. 12). The complex is located in schistose country rocks on the western limb of a regional synform (Jena, 1985), the axial plane of which is parallel to the Kumta suture suggesting that the complex intruded

prior to the deformation (shearing/suturing), and the metamorphic age of the suture zone rocks is younger than the cooling age of the gabbros of the Bondla ultramafic-gabbro complex. The K–Ar metamorphic ages of phengite from the quartz-phengite schist and of biotite from the garnet-biotite schist are very close (i.e., 1326 Ma and 1385 Ma respectively) that they probably formed during the same metamorphic event (Figs. 13 and 14). In Madagascar the Betsimisaraka suture along the western margin of the Archean Masora block/craton (Fig. 12) contains granitic intrusions with ages of 760–837 Ma (Figs. 13 and 14) (Key et al., 2011); in the comparable block in western India there is also a concentrations of ages around 600–800 Ma. About 200 km south of ca. 3200 Ma Karwar block the Coorg block (Ishwar-Kumar et al., unpublished data) is mainly archean with ages of ca. 3200 Ma (Peucat et al., 2013) (Figs. 13 and 14). Biotite from a mylonitised quartzo-feldspathic gneiss from the Coorg shear/suture zone has a K–Ar cooling age of 933 ± 16 Ma, and many Neoproterozoic intrusions along the Coorg shear zone (Chetty et al., 2012) and in Coorg block have a range of Rb–Sr wholerock ages like, 640 Ma, 680 Ma, 710 Ma, and 750 Ma (Ghosh et al., 2004 and references therein). The age of the Betsimisaraka suture in Madagascar is not well constrained and thus controversial. According to Kröner et al.

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(2000) and Collins and Windley (2002) the Betsimisaraka suture is Neoproterozoic and related to the amalgamation of Gondwana (eastern margin of the East African orogen). But based on U–Pb geochronological results Tucker et al. (2011a,b) proposed that the Greater Dharwar craton formed by the accretion of the Antananarivo block and the Dharwar craton in the Neoarchean (2550–2480 Ma). According to Key et al. (2011) the closure along the Betsimisaraka suture was diachronous; an initial collision in the south between the Masora and Antananarivo blocks was in the period 740–630 Ma, whereas the terminal collision in the north between the Antongil and Antananarivo blocks was in the period 560–490 Ma. Perhaps the Betsimisaraka suture closed in a scissor-like fashion, like the Mongol-Okhotsk suture in Central Asia (Tomurtogoo et al., 2005). Tucker et al. (2011a,b) suggested that the Betsimisaraka suture does not exist, that alternatively a Neoproterozoic extensional sedimentary basin was deposited across the AntongilMasora/Antananarivo boundary, that the present structures that closed the basin formed during 560–490 Ma terminal continentcontinent collision, and that there was never any ocean or plate separation along the Betsimisaraka boundary since the Archean. If that were correct, it is unlikely that any eclogites or other high-pressure rocks would occur in the closed sedimentary basin. However, Ishwar-Kumar et al. (under review) demonstrate that the Ranomena Complex within the suture zone contains high-pressure mafic granulite that equilibrated at c. 22 kbar at 780 ◦ C, thus indicating that subduction to deeper depths has taken place, and logically therefore that the Betsimisaraka suture is sited on a subduction zone. Accordingly, the Kumta suture attains prominence because it likewise contains HP rocks that were exhumed from eclogite facies depths. Therefore, on two fronts we can assert on good evidence that the sedimentary basin model of Tucker et al. (2011a,b) is erroneous and cannot be supported. Based on all the results presented above it is clear that the age of metamorphism of the Kumta suture was from 1385 Ma to 1326 Ma from north to south. Integrating these results with data from the Betsimisaraka suture (Kröner et al., 2000; Key et al., 2011) and the Coorg suture (Ghosh et al., 2004; Peucat et al., 2013), we propose that the Betsimisaraka-Kumta-Coorg suture (Fig. 14) closed from north to south diachronously in the period between ca. 1380 Ma to ca. 750 Ma. The collision between the Antananarivo-Karwar blocks was a soft collision starting at about 1380 Ma in the north. The soft collision is justified by the low-grade metamorphic rocks in the northern part of the suture compared to the high-grade rocks in the south, and the thick sedimentary shelf is present in the north, but absent in the south. Also, Kumta suture contains comparatively high-pressure, low-temperature rocks such as quartz-phengite, garnet-biotite, fuchsite, chlorite-quartz schists, but Coorg suture contains meta-gabbro intercalated with high-grade metapelitic rocks such as kyanite-sillimanite gneiss, garnet-kyanite gneiss and mylonitic hornblend-biotite gneiss. The collision started in the north and resulted in rotation of the continent (e.g., the collision of the Indian plate with the Eurasian plate). According to Li et al. (2008) the amalgamation of Rodinia took place between 1300 Ma and 900 Ma. The amalgamation of the Dharwar block with the Antananarivo-Karwar block began at 1386 Ma, during the start of the major amalgamation of Rodinia. During this period there was active subduction from the western side of Madagascar (Torsvik, 2003). Rifting of Rodinia started at about 750 Ma, and this enhanced the rate of subduction of the southern part of Dharwar/southern India, when subduction from the west of Madagascar was still active (Fig. 14d and e). This rapid movement (because of rifting to the east) and the resistance (because of subduction in the west) made the ocean close faster and probably closed completely at around 750–800 Ma, as evidenced in the southern Betsimisaraka and Coorg sutures. The intrusion age of gabbro in the Bondla arc

ranges between 1644 and 1536 Ma which is situated closest to the sutute zone. However the age of the arc intrusions prograssively gets younger toward west reaching upto ca. 550 Ma for the granite intrusives in Antananarivo block. This westward younging may indicate a progressive subduction front, which happened beneath the Antananarivo-Karwar-Coorg block during various stages of subduction (Fig. 14). This may also indicate shallow subduction of the Dharwar block beneath the Antananrivo-Karwar-Coorg block. It may also intresting to note the absence of older arc related intrusives in the southern part. From the compilation and integration of the present results, published datasets and the tectonic relationships, we conclude that the Kumta suture is clearly an extension of the Betsimisaraka suture, and that closure of the BetsimisarakaKumta-Coorg suture was diachronous from north to south occured between 1380 Ma to 750 Ma, during the formation and break-up of the supercontinent Rodinia (Figs. 12 and 14). 9. Conclusions The regional geology, lithologies, structures, and field relations of the Kumta area are consistent with the presence of a paleosubduction zone, and the gamut of evidence is comparable with that from many well-established suture zones. The occurrence in the suture zone of high-pressure quartzphengite schists and their derived P–T calculations suggests that exhumation was from c. 18 kbar and 550 ◦ C, which is in agreement with the relevant experimental data. The P–T conditions are comparable with those of metapelites from several suture zones with similar composition whiteschists and phengite schists. Based on the integration of field, structural, lithological and geochronological results, the new Kumta suture in western India is interpreted as an eastern extension and continuation of the Betsimisaraka suture zone of Madagascar. Correlation of the Kumta suture with the Betsimisaraka suture, based on structural and geological criteria, supports a best-fit reconstruction of India and Madagascar within the 120 Ma configuration of Gondwana. The Antananarivo-Karwar-Coorg block (2500–3200 Ma) in the west and the Dharwar block (ca. 2571 Ma) in the east were amalgamated along the Betsimisaraka-Kumta-Coorg suture diachronously from north to south in the period 1380–750 Ma. The age of arc intrusions decreases from east to west (1500–550 Ma). The blocks on the either side of the suture were uplifted around 1600–1700 Ma before inception of the plate cycle. Integration of these results suggests the northern part of the suture formed during the amalgamation of Rodinia and the final closure occured around 750 Ma during the rifting of Rodinia. Acknowledgements We are very grateful to Gouchun Zhao for editorial handling of the manuscript and two anonymous reviewers for valuable comments and suggestions. We acknowledge the Geological Survey of India, Bangalore, and the Department of Materials Engineering at IISc in Bangalore for providing Electron Microprobe facility. We also thank K. Shiraishi (NIPR) for SHRIMP facility and S. Umapathy (IPC, IISc) for Raman facility. We thank K.M. Mahesh (GSI), Vandana (ME, IISc) and Vinay (IPC, IISc) for analytical support. IKC thanks Karnataka State Remote Sensing Applications Center (KSRSAC) for support with the ERDAS Imagine software. IKC also thanks B. Amlan, B. Shrema, C. Shipra, R.T. Ratheesh Kumar, P. Ramya, P.M. George, O.S. Vinod, Daniel Dunkley, M. Satish-Kumar and Daniel Harlov for suggestions and constructive discussions. We utilized the laboratory facilities developed through Ministry of Earth Sciences, Government of India project MoES/ATMOS/PP-IX/09. This study is a contribution to ISRO-IISc Space Technology Cell projects ISTC/MES/SK/232 and ISTC/CEAS/SJK/291.

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