RESEARCH ARTICLE Tectonic Evolution of the Western Margin of the Burma 10.1029/2018TC005049 Microplate Based on New Fossil and Radiometric Key Points: • Naga Metamorphics are of Early Age Constraints Ordovician age, and their protolith Jonathan C. Aitchison1 , Aliba Ao2,3 , Santanu Bhowmik2 , Geoffrey L. Clarke4 , was derived from south Western 5 1 3 1 4 Australian and/or East Antarctic Trevor R. Ireland , Sarah Kachovich , Kapesa Lokho , Denis Stojanovic , Tara Roeder , sources Naomi Truscott4, Yan Zhen6 , and Renjie Zhou1 • Paleocene/Eocene radiolarians in mélange confirm timing of Naga 1School of Earth and Environmental Sciences, University of Queensland, St Lucia, Queensland, Australia, 2Department of Hills Ophiolite (Aptian) Geology and Geophysics, Indian Institute of Technology, Kharagpur, India, 3Wadia Institute of Himalayan Geology, emplacement onto the east Indian 4 5 passive margin Dehra Dun, India, School of Geosciences, University of Sydney, Sydney, New South Wales, Australia, Research School of 6 • The Phokphur Formation contains Earth Sciences, Australian National University, Canberra, ACT, Australia, Institute of Geology, Chinese Academy of enigmatic Permo‐Triassic detritus Geological Sciences, Beijing, China possibly from a continental arc on the northern margin of Gondwana Abstract Results of biostratigraphic and geochronological investigations in eastern Nagaland and Supporting Information: Manipur, NE India, provide new constraints on the tectonic evolution of the western margin of the • Supporting Information S1 • Text S1 Burma microplate. U/Pb zircon ages indicate that the Naga Hills ophiolite developed in a suprasubduction • Figure S1 zone setting as part of an intraoceanic island arc developed during late Early Cretaceous (mid‐Aptian) • Figure S2 ‐ • time and is younger than similar rocks exposed along the Indus Yarlung Tsangpo suture zone. Table S1 fl • Table S2 Radiolarian microfossils provide Jurassic and Cretaceous age constraints for Tethyan ocean oor • Table S3 sediments that were subducted beneath the forming ophiolite. Timing of the emplacement of these rocks onto the passive margin of eastern India is constrained by Paleocene/Eocene radiolarians in sediments over which the ophiolitic assemblage has been thrust. Previously undated schists and gneisses in the Naga Correspondence to: Metamorphics are of Early Ordovician age, and their sedimentary protolith was most likely derived from J. C. Aitchison, ‐ [email protected] sources in the south of Western Australian and East Antarctica. After Barrovian style metamorphism, these rocks were uplifted and eroded becoming an important source of detritus shed into the Eocene Phokphur Formation. This unit also contains abundant clasts sourced from the disrupted basement of the Citation: ‐ ‐ Aitchison, J. C., Ao, A., Bhowmik, S., Naga Hills ophiolite, which it overlies. It also contains Permo Triassic aged detritus eroded off an Clarke, G. L., Ireland, T. R., enigmatic source that was possibly a continental convergent margin arc system somewhere along the Kachovich, S., et al. (2019). Tectonic northern margin of Gondwana. evolution of the western margin of the Burma microplate based on new fossil and radiometric age constraints. Tectonics, 38, 1718–1741. https://doi. org/10.1029/2018TC005049 1. Introduction Received 12 MAR 2018 India's collision with Eurasia provides a well‐studied “type example” of continent‐continent collision. The Accepted 5 APR 2019 Indus‐Yarlung Tsangpo suture zone (IYTSZ) marks the location of remnants of the once extensive Accepted article online 17 APR 2019 Published online 22 MAY 2019 Mesozoic‐age Tethyan Ocean, which formerly lay between India and Asia. This suture extends west to east from Pakistan and NW India across southern Tibet until near 095°E, where it sweeps in a clockwise direction around the “eastern Himalayan syntaxis” of Namche Barwa and turns to the south. It can be traced further through SE Tibet into Myanmar and is dextrally offset by Miocene to Recent displacement along the continental transform Sagaing Fault. The continuation of the Tethyan boundary follows a line of ophiolite occurrences east of the present‐day plate boundary through the Indo‐Burman ranges (IBR) southward toward the Andaman Islands (Acharyya, 2010, 2015; Baxter et al., 2011; Sloan et al., 2017). At the eastern periphery of the spectacular Himalayan orogenic system, the IBR track along the border ©2019. The Authors. between the two nations from which their names are derived. The ranges straddle the present‐day boundary This is an open access article under the between the Indian and Eurasian plates. This mountain chain lies beyond the eastern extremity of the north- terms of the Creative Commons Attribution‐NonCommercial‐NoDerivs ern margin of Indian continental crust (Greater India) and therefore did not develop in direct response to License, which permits use and distri- continent‐continent collision between India and Asia. Consequently, although it appears to be a continua- bution in any medium, provided the tion of the same system, it differs considerably from the Himalaya. Indian sections of the IBR are located original work is properly cited, the use is non‐commercial and no modifica- in SE Arunachal Pradesh and eastern parts of the states of Nagaland, Manipur, and Mizoram. The structural tions or adaptations are made. architecture within this chain of mountains manifests as an imbricate belt of SW‐NE trending thrust slices

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Figure 1. Outline of the geology of Nagaland (modified after Ningthoujam et al., 2012) indicating locations of study areas. ST = Saramati thrust. Inset map shows location of the study area in the Naga Hills (NHO) as well as that of the Chin Hills (CHO) ophiolites. The dashed green line follows the approximate trace of the Neotethyan suture from the Andaman Islands to the Yarlung Tsangpo suture zone (YTSZ).

separated by northwest‐verging thrust faults (Acharyya, 2007; Figures 1 and 2). The present‐day ranges are commonly interpreted to constitute part of an extensive accretionary prism beneath which subduction may still be continuing as the Burma arc along the western edge of the Burma microplate collides with the northeastern Indian passive margin (Sloan et al., 2017; Steckler et al., 2008, 2016). Rock units with Indian, Tethyan and Burman affinity crop out on west to east traverses across the ranges and record an archive of plate interactions. Historical difficulties with access to much of the IBR mean that this area has remained less studied and even now only short‐term visits are possible. This paper presents the results of new field and laboratory investigations into the nature of previously little‐studied rocks along the extension of the suture zone through the IBR in eastern Nagaland and Manipur and examines their ages and provenance. We present the first descriptions of gneisses assigned to the Naga Metamorphics that crop out to the south of Mount

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Figure 2. Distribution of rock units associated with the Naga Hills ophiolite (NHO) in Nagaland and Manipur. (2a) Outline geological map of eastern Nagaland showing broadscale distribution of geological units and locations of radiolarian microfossil and zircon dating samples (modified after Agrawal & Ghose, 1986; Ghose et al., 2014). Mélange zones containing elements of the NHO and other rocks off scraped from the subducting slab cannot be differentiated at the scale of the cross section and are shown accordingly. (2b) Outline geological map of northeastern Manipur showing broadscale distribution of geological units and locations of radiolarian microfossil and zircon dating samples (modified after Ghose et al., 2014; Singh, Devi, et al., 2012; Vidyadharan et al., 1989).

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Figure 3. Field photographs of (a) typical exposures and landscape along the India‐Myanmar border looking northward over and area of Naga Metamorphic outcrop toward the peak of Mount Saramati. (b) Freshly exposed road cutting through Naga Metamorphics exposed between Phokhungri and Avangkhu. Diameter of boulder in bottom right of scene is approximately 135 cm.

Saramati, the most prominent peak along the border between Nagaland and Myanmar immediately west of the Indo‐Myanmar border (Figure 3a). Contributions by earlier workers, particularly those presented in the pioneering Geology of Nagaland volume (Anonymous, 1986), provided the initial age constraints for rocks in this region. The overall goal of our study is to provide improved biostratigraphic and radio- metric age constraints with which to better understand the tectonic evolution of areas presently located along the eastern margin of the Indian subcontinent. Radiolarian microfossils are used to determine the depositional ages of deep‐water marine sediments. Magmatic zircons constrain the timing of ophiolite generation. Detrital zircons are used to constrain maximum ages for sedimentary and metasedimentary clastic units, as well as to characterize the ages of rocks in source areas and thereby better inform paleogeographic reconstructions.

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2. Regional Geology 2.1. Naga Metamorphics The easternmost and structurally highest unit in Nagaland consists of a zone of schists and gneisses known as the Naga Metamorphics (Brunnschweiler, 1966). These rocks are thrust northwestward as a nappe over an ophiolitic terrane along the high‐angle SE‐dipping Saramati thrust (named herein) and were a particular focus of one of our research expeditions. The age of rocks within this unit was previously unknown, although correlation had been proposed (Socquet et al., 2002) with the Kampetlet schists, a sequence of low‐grade continent‐derived metasediments that include Upper Triassic Halobia‐bearing turbidites, which crop out further to the south along the IBR (Bannert et al., 2012). Our new data indicate this correlation is no longer valid. Recent road construction has exposed a 6‐km‐long transect leading east from the village of Phokhungri to Avangkhu above the Zungki River, a tributary that flows into the Chindwin River in Myanmar. The road cuts across the well‐developed regional foliation within a series of paragneisses (Figure 3b). The rocks have a porphyroclastic texture, defined by a recrystallized feldspar, quartz, and muscovite foliation that envelops detrital plagioclase and microcline grains. An eastward increase in metamorphic grade is indicated by a pro- gressive reduction in the proportion of porphyroclastic grains. A shallow‐dipping early S1 (150/15/W) fabric is overprinted by S2 (190/65/W) associated with schistose shear zones. Further north along regional strike, an old road leading east from ophiolite exposures around the village of Salumi toward Nimi provides an additional 8‐km transect through predominantly psammitic units. These quartzo‐feldspathic schists exhibit a well‐developed S1 foliation (035/65/SW) that envelops strongly recrystallized limestone lenses. They are composed predominantly of quartz, muscovite, and feldspar, accompanied by minor stilpnomelane that together reflect sub‐greenschist to lower greenschist facies meta- morphic conditions. The schists are finely (1‐ to 2‐mm scale) laminated and have a pervasive foliation that is mostly oriented subparallel to bedding (supporting information Figure S1). U‐Pb ages for zircons extracted from these metamorphic rocks allow us to constrain their depositional ages and allow us to speculate regard- ing their origins and history in the context of terrane pathways.

2.2. Naga Hills Ophiolite Ophiolitic rocks crop out extensively within a 200‐km‐long and up to 15‐km‐wide zone in SE Nagaland and NE Manipur (Brunnschweiler, 1966). The Naga Hills ophiolite (NHO) and correlative Manipur Ophiolite Complex referred to collectively herein as the NHO present as zones of heavily disrupted and strongly dis- membered serpentinite‐ and mud‐matrix mélanges (Acharyya et al., 1989, 1990; Anonymous, 1986; Ghose et al., 2014). These rocks are enveloped within a nappe that structurally underlies the Naga Metamorphics and is itself thrust northwestward at a high angle over Indian continent‐derived clastic sedimentary rocks of the Eocene Disang Formation (Imchen et al., 2014) forming a hanging wall to the Waziho thrust (named herein). All elements of a typical “Penrose‐style” ophiolite pseudostratigraphy have been reported, including serpen- tinized peridotite, gabbro, sheeted dike swarms, pillow basalt, and pelagic sedimentary rocks (Acharyya, 2007; Acharyya et al., 1991; Agrawal & Kacker, 1980; Anonymous, 1986; Sengupta et al., 1990). Rare occur- rences of high P/T metamorphic rocks including eclogite and lawsonite‐bearing blueschists (Ao & Bhowmik, 2014; Bhowmik & Ao, 2016; Chatterjee & Ghose, 2010; Ghose et al., 2010; Vidyadharan et al., 1986) are locally associated with the ophiolite especially near Salumi. Garnet‐bearing lherzolite, as well as dunite, pyroxenite, harzburgite, and gabbro are reported in the unit (Ghose et al., 2014; Ghosh et al., 1984). Pillow basalts and red chert are well represented within serpentinite‐matrix mélange that envelops meter‐ to kilometer‐scale mafic lenses and obscures original relationships between the various lithological components. Geochemical studies indicate that mafic volcanic rocks included within the mélange have a range of mid‐ocean ridge basalt, oceanic island basalt, and suprasubduction zone affinities (Devi & Singh, 2011; Kingson et al., 2017, 2019; Sengupta et al., 1989; Singh, Singh, et al., 2012; Singh, Khogenkumar, et al., 2016). Rocks associated with the NHO have been assigned various ages. Late Jurassic (Tithonian) radi- olarians have been extracted from chert associated with pillow basalts near Pungro (Baxter et al., 2011). Laser ablation inductively coupled plasma‐mass spectrometry (ICP‐MS) U/Pb radiometric ages from zircons from plagiogranite in NE Manipur indicate Early Cretaceous magmatism (Singh, Chung, et al., 2017).

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2.3. Cenozoic Sedimentary Units 2.3.1. Phokphur Formation The Phokphur Formation is an ophiolite‐derived polymict conglomerate that rests nonconformably atop the NHO (Acharyya et al., 1986; Jana & Acharyya, 1986). This succession of thickly bedded, commonly conglom- eratic rocks is locally up to several hundreds of meters thick and nonconformably overlies subaerially eroded surfaces developed on top of an already dismembered ophiolite (Figure 4). It is imbricated and cofolded together with the ophiolitic rocks. The conglomerates differ markedly from coeval autochthonous fine‐ grained and rarely conglomeratic sedimentary units of India affinity (Disang Formation). The Phokphur Formation locally includes both terrestrial and paralic facies. A detailed geological map of the type section for this unit is provided by Jana and Acharyya (1986) and reproduced in Acharyya (2015). The conglomerates are commonly red colored due to the presence of oxidized iron in their matrix/cement (Ghose et al., 2014). Clasts range from small pebbles to cobbles. Their compositions depend on outcrop loca- tion and reflect the nature of local sources. Detrital clasts include serpentinite, gabbro, basalt, chert, quartz, sandstone, phyllite, and psammite (supporting information Figure S2). Although basaltic clasts are present, igneous clasts with more felsic compositions such as andesite are noticeably absent. Sandstone petrography indicates a recycled orogenic provenance with a significant contribution of ophiolite‐derived detritus (Roeder, 2015). The gastropod genera Solariella, Pitar, Nerita, Littorina, Turritella, Corbula, Crassatellites, and Arctica have been reported from NE of Phokphur (Acharyya et al., 1986). Where these scarce shallow marine fossils occur, the taxa present are typically long ranging but consistent with a mid‐late Eocene age determined from larger foraminifera (Acharyya et al., 1986). Given the nonconformable relationship in which the Phokphur Formation lies on an eroded surface of ophiolitic mélange, these fossils place minimum age constraints not only on the timing of disruption of ophiolite into mélange but also on its uplift, exposure, and emplacement (Chatterjee & Ghose, 2010). Similar ophiolite‐derived conglomerates of Paleogene age are known from along the length of the IYTSZ (Davis et al., 2002, 2004; Henderson et al., 2011), where they record a history of intraoceanic island arc‐continent collision. 2.3.2. Disang Formation The NHO and structurally underlying sediment‐matrix mélange are together thrust northwestward along the regional‐scale SE‐dipping Waziho thrust over a footwall of Cenozoic Indian margin‐derived clastic sedimentary rocks including the Disang and Barail formations. Together these units comprise the bulk of the IBR. The Eocene Disang Formation has a total thickness of >3,000 m (Dasgupta & Biwas, 2000; Johnson & Alam, 1991) and is characterized by strongly folded flysch‐type sediments dominated by dark gray finely laminated shales inferred to be derived from Proterozoic source rocks in the hinterlands of the NE Indian shield (Ghose et al., 2010). It is dominated by marine sediments variously interpreted to be shallow marine to deltaic or deeper water turbidites (Acharyya, 1996; Acharyya et al., 1986; Brunnschweiler, 1966; Imchen et al., 2014). Planktic foraminifers indicate sedimentation during the late middle to late Eocene (Planktonic foraminiferal zone E14‐16) with accompanying benthic foraminifers in turbidite units indicating outer shelf to upper bathyal paleobathymetry (Lokho et al., 2004; Lokho & Tewari, 2011). Molluscan faunas have been reported from Manipur (Singh, Sijagurumayuma, et al., 2017; 2018). Recent investigations by Imchen et al. (2014) infer from sedimentary geochemistry that the Upper Disang sediments are derived, in part, from the NHO, although our investigations found no evidence to support this hypothesis. 2.3.3. Barail Formation The Upper Disang Formation locally grades into the Oligo‐Miocene Barail Formation, which is dominated by alternations of marine shale and arenaceous sandstone (Vadlamani et al., 2015) indicating a shallowing mar- ine setting. The northwestern front of the Naga Hills is marked by the Naga thrust along which the Disang, Barail, and other units are thrust over younger sedimentary fill in the Assam Basin. The unit is regionally widespread across the Assam Basin and on‐laps onto Nagaland (readers are referred to Alam et al., 2003; Dasgupta & Biwas, 2000; Johnson & Alam, 1991 for further details) but was not a focus of this investigation.

2.4. Subophiolite Sedimentary‐Matrix Mélange An extensive but thin zone of sediment‐matrix mélange lies immediately beneath the NHO nappe sand- wiched between the Salumi and Waziho thrust faults both above and below it, respectively. The mélange

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Figure 4. (a) View (looking SW from 25°33′03.9″N 94°45′52.4″E 1,180 m above sea level) of thickly bedded horizontal layers of conglomerate in Phokphur Formation (top half of photograph) above a nonconformity developed on an eroded surface of serpentinite‐matrix ophiolitic mélange (bottom half of photograph and foreground). (b) Nonconformity between an eroded surface on massive basalts of the Naga Hills ophiolite (NHO) and horizontally bedded basal conglomerates of Phokphur Formation exposed in an outcrop between Zhipu and Mokie (25°38′20.6″N 94°45′38.8″E). Hammer (yellow handle) lies on boundary at center of frame.

incorporates Upper Cretaceous limestone blocks (Singh, Singh, et al., 2016) within a matrix that is variably composed of mudstone and sandstone and commonly exhibits a scaly fabric. It also incorporates a range of ophiolitic blocks together with various deep‐marine cherts and metamorphic blocks of continental affinity (Acharyya et al., 1990). In the vicinity of the Waziho thrust, much of the upper Disang Formation is

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tectonically disrupted into “broken formation” (sensu Hsü, 1968) and zones of mud‐matrix mélange with “exotic” blocks that include pelagic limestones. Although incompletely exposed due to extensive vegetation, the nature of these rocks is reminiscent of mélanges that occur beneath ophiolitic thrust sheets in modern arc‐continent collision zones (e.g., Bobanaro scaly clay in Timor, Indonesia (Barber, 2013), and Lichi and Kenting mélanges in Taiwan (Chang et al., 2009; Huang et al., 2008)). Occurrences of various blocks of fos- siliferous limestone and chert of different ages and from different depositional settings within the mélange complicate regional stratigraphy (Acharyya et al., 1986; Baxter et al., 2011; Lokho et al., 2004; Singh, Singh, et al., 2016). Ocean island basalts potentially scrapped off a downgoing slab as well as pelagic carbonates are also reported (Sengupta et al., 1989). The ages of youngest blocks are inferred to indicate the age of the youngest units in the Disang Formation (Acharyya, 2010, 2015) and place a minimum age constraint on the timing of emplacement of the NHO nappe. Further work is required to resolve details of the former stratigraphy. In this study, radiolarian‐bearing red and green cherts and brown siliceous mudstone were sampled for age determination in order to help resolve this issue.

3. Biostratigraphy Fine‐grained siliceous marine sediments including cherts, siliceous mudstones, and tuffaceous cherts were collected from numerous localities across eastern Nagaland and NE Manipur. (Sample locations are given in supporting information Table S1.) These rocks typically occur as isolated blocks within the mud‐ or serpentinite‐matrix mélanges associated with, or directly underlying, the NHO. Pioneering studies (Acharyya et al., 1986) illustrate numerous fossils, but the images are not of sufficient quality to confirm all taxonomic assignments. Biostratigraphic age ranges in this investigation were determined with reference to other known well‐dated radiolarian occurrences (Baumgartner et al., 1995; Gorican, 1994; Gorican et al., 2006; Hollis, 2006; Jackett et al., 2008; O'Dogherty, 1994; O'Dogherty et al., 2006).

3.1. Red Ribbon‐Bedded Cherts Radiolarian microfossils have previously been reported from red ribbon‐bedded cherts that overlie pillow basalts NE of Pungro in Nagaland approximately 5.5 km N of Salumi (25°50′21.59″N 094°54′20.04″E). The assemblage reported indicates a Late Jurassic (Kimmeridgian–early Tithonian) age range (Baxter et al., 2011). Along regional strike, 28 km further SW large blocks of red ribbon‐bedded chert crop out along the road between Waziho and Ziphre (25°38′30.11″N 94°43′56.33″E). We were able to extract a diverse assemblage of moderately to well‐preserved radiolarians (from sample 13030604) among which the follow- ing taxa, Pantanellium sp. cf. P. riedeli Pessagno, Saitoum levium De Wever, Protunuma europeus O'Dogherty, Goričan, and Dumitrica, Hiscocapsa lugeoni O'Dogherty, Goričan, and Dumitrica, Hsuum amabile (Aita), Hexasaturnalis nakasekoi Dumitrica and Dumitrica‐Jud, and Stichomitra ?takanoensis gr. Aita (illustrated on Figure 5), can be used to assign a Middle Jurassic (latest Bathonian–early Callovian) age range (O'Dogherty et al., 2006). East of the IBR in the western part of the Inner Burman Tertiary Basin, Suzuki et al. (2004) also reported Middle to Late Jurassic radiolarians from chert pebbles in fluvial deposits of the upper member of the Pondaung Formation. This unit contains pebbles of radiolarian chert and ultramafic and metamorphic rocks (Aung, 1999), which suggests that at least in part it is ophiolite‐derived. Extensive mammalian faunas permit assignment of the upper member of these terrestrial sediments to the Bartonian in the upper Middle Eocene (Tsubamoto et al., 2000). Constituent pebbles are considered likely to be derived from the IBR (Suzuki et al., 2004), and a link to the Jurassic radiolarian cherts associated with the Naga Hills ophiolite can be inferred.

3.2. Green Ribbon‐Bedded Cherts—Manipur Ophiolite Complex, Manipur Six samples were collected from a decameter‐scale green‐colored ribbon‐bedded chert block within the major river drainage immediately east of Khamasom Village, NE of Ukhrul (25°10′40.85″N 94°31′39.70″ E). The catchment of this river drains exposures of the Manipur Ophiolite Complex. A moderately well‐ preserved fossil assemblage including Pseudodictyomitra carpatica (Lozyniak), Archaeodictyomitra chalilovi (Aliev), A. excellens (Tan), Cecrops septemporatus (Parona), Acanthocircus carinatus (Foreman), Suna hybum (Foreman), Moebicircus furiosus, and Acanthocircus levis (Donofrio and Mostler) was recovered from four samples (M019‐B, M019‐C, and M019‐E). Age‐diagnostic taxa suggest distinct Early Cretaceous affinity (UAZ 17‐21; mid‐Valanginian to early Barremian). The interval UAZ 17‐21 corresponds to the range of

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Figure 5. Composite plate of key biostratigraphically important radiolarian fossils from Jurassic, Cretaceous, and Paleocene/Eocene siliceous sediments in Nagaland and Manipur. Unitary association zones follow Baumgartner et al. (1995) Scale bar = 100 μm: (1) Late Paleocene to early Eocene radiolarians from thinly bedded siliceous mudstone samples M012–M015 collected from the Khamasom villages, Manipur: (1a) Lychnocanoma auxilla Foreman (M015C), (1b) Theocorys ?phyzella Foreman (M014D), (1c) Buryella pentadica Foreman (M013B), (1d) Bekoma demissa robusta Nishimura (M014A), (1e) Phormocyrtis striata exquisita (Kozlova; M014D), (1f) Bekoma ?bidartensis Riedel and Sanfilippo (M014D), (1g) Lychnocanoma costata Nishimura (M013B), (1h) Buryella tetradica Foreman (M013B), and (1i) Orbula comitata Foreman (M013B); (2) Early Cretaceous radiolarians from sample M005, a block of conglomerate (tentatively corre- lated with the Phokphur Formation) from within an extensive zone of mud‐matrix mélange, Ukhrul City, Manipur: (2a) Sethocapsa uterculus (Parona), (2b) Thanarla brouweri (Tan), (2c) Parvicingula cosmoconica (Foreman), (2d) Xitus robustus Wu, (2e) Pantanellium squinaboli (Tan), and (2f) Acanthocircus tri- zonalis dicranacanthos (Squinabol); (3) Early Cretaceous radiolarians from a decameter‐scale green‐colored ribbon‐bedded chert block (M019) in the floor of a ravine through the Manipur ophiolitic mélange complex: (3a) Pseudodictyomitra carpatica (Lozyniak; M019E), (3b) Archaeodictyomitra chalilovi (Aliev; M019B), (3c) Archaeodictyomitra excellens (Tan; M019C), (3d) Cecrops septemporatus (Parona; M019C), (3e) Aurisaturnalis carinatus (Foreman; M019C), (3f) Suna hybum (Foreman; M019E), (3g) Moebicircus furiosus (Jud; M019E), and (3h) Acanthocircus levis (Donofrio and Mostler; M019C); (4) Upper Jurassic radiolarians from red ribbon‐bedded chert block in the Naga ophiolitic mélange complex in the Pungro region, Nagaland (from Baxter et al., 2011): (4a) Xitus gifuensis Mizutani (NG 5.2), (4b) Transhuum maxwelli gr. (Pessagno; NG 5.1), (4c) Zhamoidellum ventricosum Dumitrica (NG 5.1), (4d) Wrangellium okamurai (Mizutani; NG 5.1), (4e) Pseudodictyomitra primitiva Matsuoka and Yao (NG 5.2), and (4f) Williriedellum crystallinum Dumitrica (NG 5.2); (5) Middle Jurassic radiolarians from red ribbon‐bedded chert block (sample 13030604) in the Naga ophiolitic mélange complex, Waziho region, Nagaland: (5a) Pantanellium sp. cf. P. riedeli Pessagno, (5b) Saitoum levium De Wever, (5c) Protunuma europeus O'Dogherty, Goričan, and Dumitrica, (5d) Hiscocapsa lugeoni O'Dogherty, Goričan, and Dumitrica, (5e) Hsuum amabile (Aita), (5f) Hexasaturnalis nakasekoi Dumitrica and Dumitrica‐Jud, and (5g) Stichomitra ?takanoensis gr. Aita.

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Figure 6. Stratigraphic ranges of key radiolarians from Nagaland and Manipur samples calibrated to the Geological Time Scale of Ogg et al. (2016). Age assignments follow Cenozoic, low‐latitude radiolarian zonations of Hollis (2006) and Jackett et al. (2008) and Middle Jurassic to Middle Cretaceous radiolarian zonations of Baumgartner et al. (1995), modified by O'Dogherty et al. (2006, 2017). UAZ = unitary association zone.

Cecrops septemporatus, which occurs in all samples (Figure 6). The first appearance of Archaeodictyomitra chalilovi (Aliev; M019B) and Acanthocircus carinatus (Foreman; M019C) and the last appearance of Pseudodictyomitra carpatica (Lozyniak; M019B) and Moebicircus furiosus (Jud; M019C) appear to further constrain the range to UAZ 20‐21 (mid‐Hauterivian to early Barremian).

3.3. Brown Ribbon‐Bedded Siliceous Mudstones Eleven samples were collected from a large relatively undisrupted zone of thinly bedded pale brown‐colored siliceous mudstones discovered among rocks of upper Disang affinity. Outcrop occurs in road cuttings along a traverse near the villages of Khamasom in Manipur (GPS location 25°11′37.80″N 94°29′55.77″E) 18 km NE

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of Ukhrul City. Radiolarian fossils from these sediments include the following taxa: Bekoma ?bidartensis Riedel and Sanfilippo, Bekoma demissa robusta Nishimura, Buryella pentadica Foreman, Buryella tetradica Foreman, Lychnocanoma auxilla Foreman, Lychnocanoma costata Nishimura, Orbula comitata Foreman, Phormocyrtis striata exquisita (Kozlova), and Theocorys ?phyzella Foreman. Distinct faunal assemblages were recognized among the samples: Sample M015C is defined by the cooccurrence of Lychnocanoma auxilla Foreman and Theocorys phyzella Foreman; sample M014D includes both Phormocyrtis striata exquisita (Kozlova) and Bekoma bidartensis Riedel and Sanfilippo; and sample M013B is characterized by the cooccurrence of Orbula comitata Foreman and Lychnocanoma costata Nishimura. These taxa are indica- tive of age ranges across the late Paleocene to early Eocene. As ophiolitic rocks are thrust over these radiolarian‐bearing sediments, they provide a maximum biostratigraphic age constraint on sediment deposi- tion prior to any ophiolite emplacement event.

3.4. Chert clast—Possible Phokphur Formation Block in Mud‐Matrix Mélange Three red chert pebbles (~5 cm3) were extracted from a decameter‐scale conglomerate block within an extensive zone of mud‐matrix mélange in Ukhrul City, Manipur. The conglomerate exhibits an abundance of serpentinite clasts that are characteristic of the upper Eocene Phokphur Formation observed in Nagaland and distinguish it from nearby exposures of the Disang Formation. One sample (M005) yielded a poorly preserved radiolarian assemblage including Sethocapsa uterculus (Parona), Thanarla brouweri (Tan), Parvicingula cosmoconica (Foreman), Xitus robustus Wu, Pantanellium squinaboli (Tan), and Acanthocircus trizonalis dicranacanthos (Squinabol). The cooccurrence of Xitus robustus Wu and Thanarla brouweri (Tan) indicates an Early Cretaceous (UAZ 13‐14: latest Tithonian–Berriasian) affinity suggesting that the clast may have been derived from cherts associated with blocks present in mélanges of the NHO.

4. Zircon Geochronology Zircon geochronology can be used to constrain the magmatic ages of igneous rocks as well as the ages of likely sources for detrital (metasedimentary) sedimentary units. Samples were collected across Nagaland and Manipur with the aim of using such data to better understand potentially different time‐space evolu- tions of the various nappes exposed along this section through the IBR. All zircons dated in this investigation were analyzed using the Sensitive High‐Resolution Ion Microprobe (SHRIMP) at The Research School of Earth Sciences at The Australian National University. (The methodology followed for analysis of the zircon grains is given in supporting information Text S1 and geochronological data are presented in supporting information Tables S2 and S3.) Until now the ages of rocks assigned to the Naga Metamorphics have been unknown with a Paleozoic or older age suggested by Brunnschweiler (1966). Samples were collected along each of the two traverses. Among these, two samples (14NAG04 and 14NAG05) from psammitic schists of the Nimi subunit east of Salumi were analyzed. Samples 14NAG11–14NAG16 were collected from gneisses exposed in new road cut- tings along the Phokhungri‐Avangkhu transect and are also from metasediments, albeit of higher grade. Zircons display a variety of morphologies including doubly terminated prismatic crystals together with rounded and fractured grains. Some evidence of metamorphic overgrowths was observed, but grain rims of this type were too small for analysis. Fresh gabbro and trondhjemite were sampled from sections through the NHO in Manipur to better constrain magmatic ages within this unit. Two gabbro samples yielded sufficient zircons for analysis. Sample M024 is microgabbro collected from west of Poi village (25°17′14.78″N 94°31′51.06″E) approximately 27.5 km NE of Uhkrul. Sample M033 is gabbro collected from a large float boulder in a deeply incised river draining a catch- ment entirely within the NHO. The locality (25°0′50.15″N 94°31′19.91″E) lies 2.5 km east of Singcha and is in the same riverbed as that from which Cretaceous green bedded chert (M019) was collected. The locality is approximately 30 km south of where M024 was collected and 20 km ESE of Ukhrul. Sandstones and conglomerates encountered on our expeditions were collected for analysis of U/Pb ages of detrital zircons in order to better constrain the ages of possible sources. A moderately well‐sorted fine‐ grained sandstone (14NAG17) from the Disang Formation collected along the road traverse NE from Kohima to Pungro (25°44′36.4″N 94°40′ 12.3″E) was selected for analysis. A sandstone (14NAG21) collected from the Barail Formation near Dimapur in the riverbed located along the highway between Dimapur and

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Kohima (25°46′10.3″N 93°48′39.8″E) was also analyzed. Zircons from both these samples are dominated by stubby to elongate crystals with elongation ratios up to 4:1. Morphologies include terminated or flat‐ended prismatic or irregular rounded shapes with fractured edges. Size distribution of the zircon population is con- sistent with the moderately to well‐sorted nature of sediments in which they occur. Thirteen samples were collected from the Phokphur Formation. This unit exhibits considerable local varia- tion reflecting its proximal character especially where it directly overlies dismembered elements of the NHO. Detailed petrographic examination of each sample was undertaken in order to characterize the lithologies and relative abundances of detrital components. Four coarse‐grained sandstone samples (NG01–NG04) were collected from where the formation crops out along the Pungro to Salumi transect (25°49′20.40″N 94°55′5.87″E). They are dominated by basaltic lithic clasts (~35%), serpentinite (~20%), and quartz (27%) together with clinopyroxene and opaque grains. Volcanic lithics are pervasively altered. Serpentinite grains are fibrous and flattened in appearance. Lithic sedimentary grains (up to 25% in sample NG04) are domi- nated by red‐colored radiolarian chert. Most quartz grains exhibit unitary extinction consistent with a volca- nic origin but up to 20% of quartz grains exhibit undulose extinction indicating stress postdating mineral crystallization. The presence of up to 15% clinopyroxene grains consistent with ophiolite derivation. Samples (NG05, NG06A, and NG06B) collected from near Washelo village (25°34′39.72″N 94°45′35.58″E) occur at higher stratigraphic levels within the Phokphur Formation and are dominated by red chert clasts (~65%) in which abundant radiolarians are visible. Quartz (16%) and volcanic lithic clasts (16%) account for most of the other grains in the sample, although opaque minerals are also present (<5%). Samples (NG07–NG09) were collected from the base of an escarpment of Phokphur Formation conglomer- ates further along strike beyond Washelo to the west of the Wazeho to Lachlam (Silloi) lake road (25°33′ 13.91″N 94°45′33.07″E). They are dominated by red radiolarian chert clasts with subordinate serpentinite (7%) and basalt clasts (7%), quartz grains (4%) and minor opaque mineral grains. A nonconformity where Phokphur Formation conglomerates overlie an erosional surface developed atop NHO basalts is well exposed along the road between Ziphu and Mokie (25°38′20.6″N 94°45′38.8″E). Samples NG10 and NG11 from this location are dominated by serpentinite clasts. Other detritus includes basaltic clasts, radiolarian cherts, and clinopyroxene grains and opaque minerals. Further along this road toward Mokie (25°38′50.48″N 94°45′56.36″E), samples NG12 and NG13 were collected from thick, well‐ bedded sandstones, which are dominated by serpentinite (50%) and volcanic lithic (20%) clasts. Chert clasts together with quartz, clinopyroxene and opaque mineral grains constitute the remainder of the samples. Bluffs of thickly bedded Phokphur Formation conglomerates rise immediately to the NE of Mokie. A poly- mict conglomerate sample (14NAG10) was collected from fallen blocks at the foot of this escarpment (25°39′ 30.0″N 94°46′37.9″E). It is dominated by gabbroic clasts but also includes basalt, quartzite, chert, siltstone and vein quartz. The zircon population is morphologically diverse but is dominated by small, irregularly shaped zircons of roughly equant dimensions. Rare elongate crystals (aspect ratios of up to 4:1) are also present.

5. Geochronology Results 5.1. Naga Metamorphics Both samples from the Nimi subunit (14NAG04 and 14NAG05) display strong peaks at 530 Ma, with a range between 800–1,300 Ma peaking at 925 Ma, 1,170 Ma, and a subsidiary peak around 2.45–2.65 Ga and some zircons with ages >3 Ga. Samples of gneiss (14NAG11–14NAG16) collected along the Phokhungri‐ Avangkhu transect are from higher grade metasediments. All samples exhibit a prominent Cambrian‐aged peak at 530 Ma with a Neoproterozoic peak at 925 Ma and another strong late Mesoproterozoic peak around 1,170 Ma. Subordinate populations of Archean‐aged zircons exist around 2.5 and 3.2 Ga (Figure 7). The youngest concordant zircons are around 480 Ma. Together, these results suggest an earliest Paleozoic (Early Ordovician–Tremadocian) maximum depositional age.

5.2. Naga Ophiolite The age of the NHO has previously been estimated based on the ages of fossils in associated sedimentary strata (Baxter et al., 2011). However, the disrupted nature of these rocks obscures or obliterates contacts

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Figure 7. Zircon U/Pb age histograms for samples collected from the Naga metamorphics (all) and for samples on the Phokhungri‐Avangkhu traverse and the Nimi Unit east of Salumi. For comparison, plots of detrital zircons ages from sedimentary units in Myanmar (Sevastjanova et al., 2016), Assam (Vadlamani et al., 2015), Western Australia (Cawood & Nemchin, 2000), and metasedimentary xenoliths in Lesser Himalayan granites (Cawood et al., 2007) are also plotted.

making original relationships difficult to determine. Recent reports of laser ablation ICP‐MS (LA‐ICP‐MS) U/Pb ages for zircons from plagiogranites within the ophiolite (Singh, Chung, et al., 2017) indicate Early Cretaceous (mid‐Aptian) ages that suggest the ophiolitic mélange requires further investigation in order to resolve its complexity. Both of the samples we analyzed using SHRIMP rather than LA‐ICP‐MS yielded similar concordant late Early Cretaceous (mid‐Aptian) ages of 116.63 ± 0.30 Ma (M024) and 117.55 ± 0.35 Ma (M033; Figure 8).

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5.3. Cenozoic Clastic Sedimentary Units Samples from detrital units all reveal multiple zircon populations indicat- ing a variety of possible source terranes; 14NAG17 from the Disang Formation yielded peaks at 100, 200, and between 410–520, 750–900, 1,000–1,150, 1,500–1,750 Ma, 2.4–2.65 Ga and 3.1–3.25 Ga. NG21 from the Barail Formation yielded peaks at 450–510, 750–1,000, 1,200–1,650, 1,750, and 2,450–2,800 Ma. Samples collected from the Phokphur Formation in eastern Nagaland yielded the following age peaks: Pungro to Salumi transect—142 zircons were analyzed from samples NG01–NG04, yielding main peaks of 95, 145, 175, 220, 265–275, 355, 410, 450, 510–530, 1,050, 1,500, and 1,850 Ma and 2.2–2.4 Ga; Washelo village—246 zircons were analyzed from samples NG05, NG06, and NG06A, yielding main peaks of 205– 240, 290, 370, 410, 480–505, 850–1,250, and 1,450–1,750 Ma and 2.1, 2.35, and 2.9 Ga; Wazeho to Lachlam (Silloi) lake road—198 zircons were analyzed from samples NG07–NG09 yielding main peaks of 210–220, 24, 285, 370, 410, 480, 550, 900, 1,200–1,650 Ma and 2.3 Ga and other rare Archean grains; Ziphu and Mokie road nonconformity—108 zircons were analyzed from sample NG10 and NG11, yielding main peaks of 210, 240, 300, 380, 410, 430, 480, 530, 900, and 1,400 Ma, a broad range between 1,550 and 1,950 Ma, and peaks at 2.7 and 3.45 Ga; Mokie road—111 zir- cons were analyzed from samples NG12 and NG13, yielding main peaks of 175,190, 210, 245, 350, 380, 430, 480–500, 530, 750, 1,050, 1,500, 1,550, and 1,750 Ma and 2.3 Ga; Mokie escarpment—85 zircons were analyzed from samples 14NAG10 and 14NAG10A, yielding main peaks of 220, 400, 445, 460, 510–530, 800, 1,050, 1,450, and 1,800 Ma, a broad peak around 2.5, as well as 3.1 and 3.2 Ga. When plotted together (Figure 9), the collective results for 890 zircons from the combined Phokphur Formation samples exhibit strong peaks at 250, 550, 950–1,200, and 1,500–1,750 Ma together with minor diffuse Archean peaks.

6. Discussion The provenance of detrital zircons is considered in light of tectonic recon- structions that constrain the paleogeographic location of Figure 8. Concordia plots for U/Pb ages of zircons from gabbro samples Nagaland/Manipur at the times when sediments now hosting the zircons M024 and M033 collected from ophiolitic mélange in Manipur. The plots were accumulating. India and, by association, Nagaland and Manipur were generated using the IsoplotR program (http://www.ucl.ac.uk/ were constituents of the Gondwana supercontinent until its breakup in ~ucfbpve/isoplotr/). Error ellipses are shown at the one sigma level. the Cretaceous (Acton, 1999). They did not arrive at the Eurasian margin until ~55 Ma at the very earliest (Aitchison et al., 2007; Searle et al., 1987). Thus, any reasonable consideration of zircon provenance needs to be cognizant of Nagaland's position rele- vant to other elements of Gondwana at any given time (Figure 10).

6.1. Naga Metamorphics The combined spectrum of ages from all 523 analyzed zircons from this unit exhibits considerable inheri- tance confirming the protolith of these now metamorphic rocks as detrital sedimentary units. Early Paleozoic minimum peak ages around 480 Ma constrain the maximum age of sedimentation. This probable Early Ordovician depositional age represents both a substantial refinement on the previously suggested Proterozoic age estimate (Ghose et al., 2010) and places constraints on the timing of metamorphism. We note an earlier mention (without illustration) of the mid‐Cretaceous foraminifer Orbitolina from more weakly metamorphosed units in the overlying Nimi Formation (Soonwal and Bhattacharya in Acharyya et al., 1986, p. 72), who favored correlation with Triassic units in the Chin Hills. Acharyya (2007, 2010) considered

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Figure 9. Results of U/Pb analyses for detrital zircons extracted from samples from the Phokphur Formation and Disang/Barail units collected across Nagaland. (a) Data from all Phokphur Formation samples. (b) Data for samples NG01–NG04 from the Pungro‐Salumi road transect. (c) Data for samples NG05, NG06, and NG06A from Washelo. (d) Data for samples NG10 and NG11 from the Zhiphu‐Mokie road nonconformity exposure. (e) Data for samples NG08 and NG09 from to the west of the Wazeho to Lachlam (Silloi) lake road. (f) Data for sample 14NAG10 from cliffs NE of Mokie Cliffs. (g) Data for samples NG12 and NG13 from south of Mokie. (h) Data for sample 14NAG17 from the Disang Formation collected along the road traverse NE from Kohima to Pungro. (i) Data for sample 14NAG21 from the Barail Formation in the riverbed located along the highway between Dimapur and Kohima.

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Figure 10. Time/space plot for lithotectonic units exposed across the Naga Hills area. Potential tectonic (green) and sediment source to sink (red) pathways are indicated by dashed lines. Tectonic reconstructions indicating the likely position of the study area at the time various sedimentary units were accumulating are also shown in order to better illustrate the relative proximity of possible sediment source areas. The present‐day location of Imphal, the administrative capital of Manipur State is shown for reference. In the Gondwana reconstruction a location equivalent to that of modern‐day Imphal lies slightly to the west of the triple junction between India, Antarctica, and Australia. Green dashed lines indicate tectonic pathways (for example, that of elements of the plate subducted beneath the Naga Hills ophiolite as it was forming) and red dashed lines indicate sedimentary pathways (detritus shed into the Phokphur Formation). Tectonic reconstructions were made independent of this study applying a Mollweide projection and magnetic reference frame using the Ocean Drilling Stratigraphic Network establish by GEOMAR Online interactive plate tectonic reconstruction service (http://www.odsn.de/odsn/services/paleomap/paleomap.html). MORB = mid‐ocean ridge basalt; OIB = oceanic island basalt; SSZ = supra‐subduction zone.

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this “Nimi Limestone” unit to unconformably overlie the metamorphic rocks, and this interpretation is consistent with our results. Detrital zircon ages range from Archean to Early Paleozoic with major late Mesoproterozoic and late Neoproterozoic peaks and are a characteristic Gondwanan signature, representing Grenville and Pan‐African age detritus, respectively. Early Paleozoic rocks across the Himalayan system on the northern periphery of India record a transition from a stable passive margin to an Andean‐style convergent plate margin (Cawood et al., 2007; Garzanti et al., 1986; Gehrels et al., 2006; Myrow et al., 2006, 2016) first referred to as the Kurgiakh orogeny (Srikantia, 1977). However, during the Cambro‐Ordovician, Nagaland and Manipur lay a considerable distance from this plate margin. Gondwana reconstructions for this time, and indeed until the beginning of the Cretaceous, place it somewhere SW of Western Australia adjacent to East Antarctica (Collins & Pisarevsky, 2005). Derivation of the protolith for the Naga Metamorphics from more proximal sources eroding off the Early Cambrian Pinjara orogen of the south of Western Australia are potentially a good fit (Burrett et al., 2014; Cawood & Nemchin, 2000; Collins, 2003; Collins & Pisarevsky, 2005; Veevers et al., 2006). The next oldest and third highest peak is around 925 Ma and is of Grenville age. This is consistent with a Rayner Province source in the present‐day Prydz Bay area (Fitzsimons, 2000) of Antarctica, which also lays nearby during Cambro‐Ordovician time. Inheritance of additional older zircon populations around 1.1, 2, 2.5, and 3.2 Ga is also consistent with the multicycle Pinjarra pattern or with sources in the Albany‐Fraser orogen of (southern) Western Australia (Clark et al., 2000; Spaggiari et al., 2015) and parts of East Antarctica (Fitzsimons, 2000, 2003). The Albany‐Fraser Orogen is characterized by high‐grade gneisses, granitoid intrusions, and polyphase deforma- tion (Myers, 1990b) with a main episode of magmatic activity from ~1,300–1,150 Ma (e.g., Black et al., 1992; Fitzsimons, 2000; Myers, 1990a). In this area, Fitzsimons (2000) used evidence from detrital zircons to infer distinct orogenic events among six separate provinces. He suggested magmatic activity in the region from 1,330–1,130 Ma and 1,200–1,130 Ma corresponded with collision of the Wilkes and Albany‐Fraser province, 1,090–1,030 Ma in the Maud‐Namaqua‐Natal province, and 990–900 Ma with the Rayner‐Eastern Ghats collision. Metcalfe and Aung (2014) note the presence of a peak around 1.1 Ga in detrital zircon data from NW Australia, Sibumasu, and the High Himalaya that does not appear to be present among data sets for samples from SW Australia. Sircombe and Freeman (1999) have also reported detrital zircon spectra with dominant Neo‐Proterozoic and Meso‐Proterozoic peaks in modern placer deposits from Western Australia as did Cawood and Nemchin (2000) from Permian sandstones in the Perth Basin. The most ancient inheritance in Nagaland/Manipur samples is Archean and possibly represents multicycle zircons originating from the Yilgarn craton; one of two cratons making up Western Australia that has been relatively stable for the past 2.4 Ga (Myers, 1993). Approximately 80% of outcrops within the Yilgarn Craton are granite‐greenstone terranes, most aged 3.0 to 2.6 Ga; the remaining 20% of outcrops are gneisses. The minor zircon peak at 3.2–3.4 Ga is similar to a strong peak in the Jack Hills metasedimentary belt in the Narryer terrane (Pidgeon & Nemchin, 2006; Thern & Nelson, 2012; Wilde, 2010). Notably, no peaks within the range of 1,700–1,800 Ma that might indicate Shilling Plateau or sources among Lesser Himalayan Series protoliths (Webb et al., 2013; Yin et al., 2010) were observed in Naga Metamorphic samples.

6.2. Naga/Manipur Ophiolites Ages from the two NHO samples (M024 [116.63 ± 0.30 Ma] and M033 [117.55 ± 0.35 Ma]) are within the same range as those recently reported (116.4 ± 2.2 Ma and 118.8 ± 1.2 Ma) by Singh, Chung, et al. (2017) from plagiogranites elsewhere in the Nagaland‐Manipur region obtained using the LA‐ICP‐MS method at National Taiwan University. Our investigation applied a different methodology using the ANU SHRIMP but independently obtained similar Aptian/Albian boundary ages around 117 Ma. We note that although ophiolitic elements of the NHO are commonly correlated with other ophiolites that occur within the IYTSZ between India and Asia. The latter are dominated by somewhat older Barremian/Aptian boundary ages clustered around 125 Ma (Chan et al., 2015; Hébert et al., 2012). Although superficially similar, Nagaland/Manipur ages are consistently younger than those from the IYTSZ and do not overlap at the 2σ error level. Lithological and geochemical characteristics of the NHO suggest development of the ophiolitic rocks in a suprasubduction zone setting (Ghose et al., 2014; Kingson et al., 2017, 2019). From this, it can be inferred

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that the NHO was obducted onto the eastern margin of India in response to collision of Greater India with an intraoceanic island arc that developed somewhere in the Neotethyan Ocean. The occurrence of blueschist facies rocks with abundant and well‐preserved lawsonite in association with the typical constituents of an ophiolite pseudostratigraphy suggests rapid and deep burial and subsequent exhumation of some of the ophiolitic material. Ao and Bhowmik's (2014) study of blueschist occurrences suggests that they formed in response to cool, deep subduction of Neotethyan basalts. From metamorphic evidence, it may thus be inferred that the Tethyan intraoceanic subduction invoked to explain the occurrence of ophiolite provinces along the India‐Asia margin (Chatterjee & Ghose, 2010) extended east between Burma and India (Ao & Bhowmik, 2014). Although generation of the Nagaland/Manipur ophiolites occurred slightly later than that of ophiolites, which crop out elsewhere along the IYTSZ, it is not possible to determine if they simply represent younger parts of a single system or are elements of an entirely different intraoceanic convergent plate boundary. As with other zones of ophiolite and ophiolitic mélange along the length of the suture, the ophiolitic rocks (sensu stricto) are mixed together with older elements off scraped from the oceanic plate that was subducted under the intraoceanic arc system in which the ophiolite was generated. In the case of the NHO these include mid‐ocean ridge basalt together with its pelagic sedimentary cover (Jurassic‐aged red chert and Cretaceous‐aged green chert) as well as oceanic island basalt associated with sea mounts and their carbonate carapaces.

6.3. Cenozoic Clastic Sedimentary Units The Disang Formation is interpreted to represent the feather edge of Assam Basin sedimentation (Johnson & Alam, 1991). Biostratigraphic constraints from foraminiferal assemblages (Lokho et al., 2004; Lokho & Kumar, 2008) indicate a latest Eocene (Bartonian to Priabonian) depositional age. The Eocene location of Nagaland/Manipur is well constrained in tectonic reconstructions using marine magnetic anomaly data (Zahirovic et al., 2012). Such reconstructions indicate that any possibility of Australian and/or Antarctic sediment sources can be excluded. A single 100‐Ma grain in the age spectrum hints at a possible contribution of Asian sources related to magmatic activity along the Burmese arc associated with the eastward subduction of the Neotethys (Mitchell et al., 2012). At the time of Disang deposition oceanic crust lay to the east of India and was being subducted beneath the Burma microplate until at least the Miocene (Lee et al., 2016; Steckler et al., 2016). Topographic considerations and the likelihood of an intervening subduction‐related trench appear to mitigate against any contribution from this arc although the possibility of aerial transport of zircons in volcanic ash clouds cannot be totally excluded. An alternative explanation of this young grain is that it might have been shed off a rapidly growing and eroding Himalayan orogenic system and traveled via a long sediment distribution pathway across and through the Assam Basin. Numerous older detrital zircon peaks in the sample are similar to those reported from the western side of the Assam Basin in coeval sediments of the more proximal Kopili Formation (Vadlamani et al., 2015). The sam- ple contains Kurgiakh (senior synonym of Bhimphedian, see Myrow et al., 2016) orogeny‐age components approximately 500 Ma with other broad peaks in the range of 750–900, 100–1,150, 1,500–1,750 Ma and 2.400–2.65 Ga. The 1,500‐ to 1,750‐Ma peak is notable as it is not present in our Naga Metamorphic samples. However, it is similar to detrital zircon peaks for sedimentary units in the Shillong Plateau and Lesser Himalayan Sequence of Arunachal Pradesh (Webb et al., 2013; Yin et al., 2010). Sample 14NAG21 is from the Barail Group for which an Oligocene‐Miocene depositional age has been reported on the basis of micro- fossil assemblages (Johnson & Alam, 1991). Zircon ages are broadly similar to those from the Disang Formation sample, but the size of our sample is small and few young zircons are present. The combined spectra of ages from all 890 zircons analyzed from Phokphur Formation samples provide important insight into the possible provenance of this unit. Major detrital zircon peaks are seen in the Permo‐Triassic, Cambrian, and Mesoproterozoic with all samples also exhibiting various minor Archean peaks. Fossils reported from this unit, which overlies disrupted ophiolitic units of the NHO, suggest Eocene deposition (Acharyya et al., 1986). However, only a few young grains are present. Although the formation rests nonconformably upon the mid‐Aptian age NHO and lowermost beds are dominated by ophiolite‐derived clasts, any detrital zircons sourced from these rocks appear to be rare to absent. We attribute the under representation of any Late Cretaceous ages, as might be expected given the abundance of ophiolite derived clasts to the paucity of zircon within such refractory igneous rocks.

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Although no depositional contacts upon a surface eroded into a basement of Naga Metamorphics have been reported, angular schistose clasts within the conglomerates must have been deposited in close proximity to their source and no other nearby zone of metamorphic rocks is known. Most of the Cambrian and older inherited zircon populations are consistent with derivation from the Naga Metamorphics. An important and significant difference is the presence of two notable peaks, the first of which is a diffuse and broad peak in the 1,500‐ to 1,800‐Ma range among the 246 zircons analyzed from samples NG05, NG06, and NG06A from Washelo. No similar peak is observed in samples from the Naga Metamorphics. The nearest known possible Indian source is documented from areas such as the Shillong Plateau (Webb et al., 2013; Yin et al., 2010). Interestingly, Lewis and Sircombe (2013) also reported a similar peak in detrital zircons from the Carnarvon Basin in NW Australia. The most significant discovery is a second peak that indicates considerable Permo‐Triassic inheritance in all Phokphur Formation samples. Clearly none of this can be attributed to the Ordovician‐aged Naga Metamorphics. All detrital zircon samples from the Phokphur conglomerate exhibit both early and late peaks or a broad range of ages in the interval between 200 and 300 Ma. In the regional context of the Himalayan orogenic system, until recently few reports of similarly aged zircons existed with very few grains in any given sample (Aikman et al., 2008; Aitchison et al., 2011; Li et al., 2010; Webb et al., 2013). These investigations reported zircons analyzed from Tethyan Himalaya Series rocks collected from the less studied eastern Himalaya region. They have recently been supplemented by additional investigations of the Upper Triassic Langiexue, Songre, and Nieru formations (Cai et al., 2016; Cao et al., 2018) in SE Tibet. Upper Triassic sandstones in the Loi‐an Group among rocks of Sibumasu affinity in the Shan State of Myanmar also exhibit a similar peak (Cai et al., 2017). Zircons in these samples exhibit distinctive ~280‐ to 220‐Ma peaks similar to those found in our Phokphur Formation samples. To provide a better framework within which to consider possible sources for the abundant Permo‐Triassic detrital zircons, it is useful to place Nagaland/Manipur in the context of its location at the time sediments of the Phokphur Formation (Eocene) were deposited. When the abundance of metamorphic clasts and zir- cons attributable to a Naga Metamorphic source are considered together with the structural architecture of this region, it seems likely that much of the Phokphur Formation was derived from sources that lay east of its present location rather than much further to the north in the Himalaya. We note that the inheritance pattern is comparable to that of the Pane Chaung Formation in West Myanmar reported by Sevastjanova et al. (2016), who consider its provenance lay in Sibumasu. Although granites of the widespread SE Asian Tin Belt Ng et al. (2015) are a potential source that is compatible with reconstructions, other possible Permo‐ Triassic sources may also exist among the now dispersed fragments of northern India and NW Australia (Cai et al., 2017; Sevastjanova et al., 2011). In a model presented to explain the presence of Permo‐Triassic detrital zircons within the Lhasa terrane in Tibet, Zhu et al. (2011) postulated the existence of a former con- vergent margin along the NW of western Australia. This gains further support from the existence of Permian arc volcanics (Zhu et al., 2010) and subduction‐related metamorphic rocks (Yang et al., 2009) in the Lhasa terrane. In the context of a Gondwana reconstruction, Veevers and Tewari (1995) reported Late Permian to Early Triassic rhyolites from Western Australia intersected in drill holes in the Carnarvon Basin region. Detrital zircon studies of sedimentary rocks buried on the North West Shelf of Australia also indicate a Permo‐Triassic magmatic component (Lewis & Sircombe, 2013). While a Permo‐Triassic source is clearly an important contributor to younger sediments in the IBR (e.g., Phokphur and Pane Chaung formations), the exact nature and location of that source cannot be resolved using our data alone and further work is necessary. Emplacement of the Naga Metamorphic nappe along the Saramati thrust postdates deposition of the Phokphur Formation. Paleocene/Eocene radiolarians that occur in blocks of thinly bedded siliceous mudstones within mud‐matrix mélange beneath the NHO provide constraints on the timing of emplacement along the Waziho thrust of the NHO and its overlying cover of Eocene fluvial to shallow marine sediments in the Phokphur Formation onto India. Fossils reported from this formation are either long ranging or insuffi- ciently well preserved to allow anything other than a broad Eocene age assignment. However, it was folded together with the NHO mélange prior to emplacement over the Disang Formation. The same radiolarian assemblages have been reported from blocks in mud‐matrix mélanges immediately beneath overthrust ophiolitic nappes in analogous tectonic settings along the length of the IYTSZ (Colchen et al., 1987; Ding, 2003; Li et al., 2018; Liang et al., 2012; Liu & Aitchison, 2002; Wang et al., 2017). This may

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indicate that a Paleocene/Eocene boundary age ophiolite emplacement event was regionally widespread from Ladakh to Manipur. Imchen et al. (2014) cite geochemical data to infer an ophiolite contribution to the Disang sediment budget; however, we were unable to find evidence to corroborate this hypothesis. We note that the two units of Eocene age, Phokphur and Disang, are dominated by markedly different sediment, have different deposi- tional and deformational histories, and are separated by a major structure, the Waziho thrust. Clearly, more investigation is needed to resolve the former relationships between these units. Neither the continental Naga Metamorphics nor the existence of the NHO and its emplacement onto the Indian margin need be implicated in the collision between India and Asia. Plate reconstructions indicate the presence of oceanic crust between India and Myanmar (the Eurasian margin), subduction of which ensued from Eocene through to at least Miocene time. From this, we can infer that oceanic crust and a trench must have lain outboard (to the east) of the NHO and Naga Metamorphic nappes. Although many new data are available through our investigation, they do not provide a unique solution and further investigation is required in order to determine the source(s) of all the clastic sedimentary detritus in the Phokphur Formation and develop a refined tectonic model.

7. Conclusions The Naga Metamorphics are of Early Ordovician age, and their protolith was derived from sources in the south of Western Australian and/or Antarctica. Radiolarians constrain Jurassic to Cretaceous ages for the Tethyan ocean crust subducted beneath the upper plate in which the NHO formed. The NHO formed as part of an intraoceanic island arc system during Early Cretaceous (mid‐Aptian) time. Paleocene/Eocene radiolarians in blocks within mud‐matrix mélange beneath the ophiolite provide a mini- mum age constraint on the timing of NHO emplacement onto the passive margin of eastern India. The Naga Metamorphics were an important source of detritus that was shed into the Phokphur Formation, which overlies an NHO basement. The Phokphur Formation also contains detritus derived from an eroding Permo‐Triassic source that poten- tially includes remnants of a continental convergent margin arc system, which may have developed along the northern margin of Gondwana.

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