Browsing by Author "Shoraisam Khogenkumar"

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Evidence of intraplate magmatism and subduction magmatism during the formation of Nagaland–Manipur Ophiolites, Indo–Myanmar Orogenic Belt, north-east India
(John Wiley and Sons Ltd, 2022) Sashimeren Imtisunep; Athokpam Krishnakanta Singh; Rajkumar Bikramaditya; Shoraisam Khogenkumar; Monika Chaubey; Naveen Kumar
Mafic extrusive rocks (basalts) and intrusive rocks (gabbros) from the Nagaland–Manipur Ophiolite (NMO) of the Indo–Myanmar Orogenic Belt (IMOB), north-east India, are investigated to understand their magmatic evolution in diverse tectonic environments. Basalts are distinguished into two types: basalt-I and basalt-II. Basalt-I type shows the sub-alkaline character with Nb/Y < 0.50, low Nb/Th (2.36–7.94), and low to moderate La/Sm (1.00–4.12) indicating derivation from a slightly enriched mantle source and also supported by their enriched LREE pattern with flat HREE. They are depleted in HFSEs (Nb and Ti) but enriched in U and Pb, which is indicative of a typical subduction origin derived from an MORB-type mantle source. Investigated samples of basalt-II and gabbros have an equal composition with alkaline characteristics. They have Nb/Y > 0.50, high Nb/Th (8.38–13.37), and highly enriched LREE (La/Sm = 4.41–6.35) pattern. They show typical Ocean Island Basalt (OIB) characters of a plume source. The two sets of basalts and gabbros found in this study have no sign of genetic relationship, and therefore, it strongly suggests that they were derived from two different mantle sources of a plume and a subduction zone mantle wedge. Our study supports the theory that the NMO has records of different magmatic episodes produced ranging from plume-related magmatism, to divergent and convergent plate magmatism that were generated at diverse tectonic settings. © 2022 John Wiley & Sons Ltd.
Formation of the associating high-Al and high-Cr chromitites in the Nagaland-Manipur Ophiolites in northeast India
(Taylor and Francis Ltd., 2025) Monika Chaubey; Athokpam Krishnakanta Singh; Sashimeren Imtisunep; Ibrahim Uysal; Birendra Pratap Singh; Manavalan Satyanarayanan; Bendangtola Longchar; Shoraisam Khogenkumar
The Nagaland-Manipur ophiolites (NMO), part of the Phanerozoic (538.8–0 Ma) Tethyan ophiolites, occur in the NNE-SSW trending Indo-Myanmar Orogenic Belt (IMOB), northeast India. The NMO hosts both high-Al (0.46 < Cr# < 0.53) and high-Cr chromitites (0.71 < Cr# < 0.79). These chromitite bodies are hosted in lherzolite, harzburgite, and dunite and show various textures, including massive, disseminated, nodular, and granular. The high-Al chromitite compositions in conjunction with the calculated Al2O3[melt] (15.66–16.39 wt.%), TiO2[melt] (0.65–0.94 wt.%), and FeO/MgO[melt] (0.65–0.83 wt.%) values indicate that they were derived from the tholeiitic melt that formed at the mid-ocean ridge centre through low-degree partial melting. In contrast, the high-Cr chromitites, coupled with the Al2O3[melt] (11.24–12.99 wt.%), TiO2[melt] (0.21–0.33 wt.%), and FeO/MgO[melt] (0.58–1.54) values show similar geochemical affinities to those derived from boninitic melts produced by partial melting of already depleted mantle due to the subduction of oceanic plate in a supra-subduction zone environment. The total platinum group element (PGE) contents (60–190 ppb) of high-Al chromitites are lower than the total PGE contents (118–2341 ppb) in high-Cr chromitites. Chondrite-normalized PGE patterns in high-Al chromitites are flat from Os to Rh and negatively sloping from Rh to Pd, whereas high-Cr chromitites show strongly fractionated chondrite-normalized PGE patterns. Total PGE contents and low Pd/Ir ratios (0.02–0.64) of chromitites are consistent with typical ophiolitic chromitites. Mineral chemistry and PGE systematics suggest that NMO chromitites were generated in two separate tectonic settings. Thus, we argue that the upper mantle of the NMO of the IMOB has been modified by a substantial amount of supra-subduction zone components after initially being formed in a mid-ocean ridge tectonic environment. © 2024 Informa UK Limited, trading as Taylor & Francis Group.
Magma heterogeneity in the generation of ophiolitic mafic rocks on the eastern flank of the Indian plate
(Science Press, 2025) Sashimeren Imtisunep; Athokpam Krishnakanta Singh; Monika Chaubey; Rajkumar Bikrmaditya Singh; Bendangtola Longchar; Shoraisam Khogenkumar; Amrita Dutt
Subduction polarity reversal typically occurs in intra-oceanic arc settings; the existence of an ancient intra-oceanic arc and its associated back-arc system within the Neotethyan plate has been deliberated. In this study, we investigate the possible role of subduction initiation of polarity reversal in the formation of Nagaland-Manipur ophiolite (NMO), evaluate the petrological and geochronological data and compare it with the neighboring natural examples of subduction polarity reversal of the Andaman-Nicobar ophiolite (ANO). The ancient intra-oceanic arc, namely the Incertus-Woyla Arc, and its associated back-arc remnant have been correlated with the back-arc mafic of the ANO. We found that the geochemical signatures of mafic rocks of NMO and ANO are comparable, and the available geochronology data of ~ 145 Ma from the NMO basalt and chert fit well with the evolution and formation of the intra-oceanic arc, i.e., Incertus-Woyla Arc. The evolution and age of the Incertus-Woyla Arc are between 135 and 150 Ma. Although the oldest age of the ANO has been reported from metamorphic sole at about 106.4 and 105.3 Ma, the back-arc affinity of the amphibole has been credited to the back-arc spreading that occurred behind the Woyla Arc. Previous paleomagnetic and geochronological studies have suggested that the development of the back-arc basin behind the Incertus-Woyla Arc was a result of divergent double subduction. Therefore, we have inferred a similar scenario for the development of the back-arc affinity rocks of the NMO behind the Incertus-Woyla Arc and the reinterpretation for the evolution of the supra-subduction zone affinity rocks of NMO and ANO during subduction initiation after subduction polarity reversal. © The Author(s), under exclusive licence to Science Press and Institute of Geochemistry, CAS and Springer-Verlag GmbH Germany, part of Springer Nature 2025.
Magmatic records of Gondwana assembly and break-up in the eastern Himalayan syntaxis, northeast India
(Elsevier Inc., 2022) Govind Oinam; A. Krishnakanta Singh; Amrita Dutt; Shoraisam Khogenkumar; Mallickarjun Joshi; Saurabh Singhal; R.K. Bikramaditya
The Indian sub-continent was an integral part of the Gondwana supercontinent with multiple magmatic episodes during the Gondwana assembly and break-up events. However, most of these vital records to understand the past magmatism were obliterated during the Himalayan orogeny due to the subduction of the Indian plate. In this contribution, we attempt to tackle this issue by investigating the Abor magmatic rocks from the eastern Himalayan syntaxis, which are likely to represent the leftover fragments of the eastern Gondwana continental margin. This study uses zircon U-Pb dating, whole-rock geochemistry, and Sr-Nd isotopic ratios data of the mafic intrusive and felsic volcanic rocks of the Abor magmatism. The mafic intrusive rocks have zircon ages of 500–473 Ma, while the felsic rocks yield ages of 145–132 Ma, indicating two temporally separated episodes of magmatism. The mafic intrusives are sub-alkaline/tholeiitic (Nb/Y < 0.65), with high TiO2 (1.63–3.42 wt%) and ocean island basalt to enriched-mid oceanic ridge basalt affinities. A relatively narrow range of initial 87Sr/86Sr (0.703887–0.705513), 143Nd/144Nd (0.511978–0.512118), and εNd(t) (-0.323–+2.43) of the mafic intrusives suggest fractional crystallization with negligible crustal contamination, generated by low degree (∼3–13 %) partial melting of a primitive mantle (garnet and spinel lherzolite). The felsic rocks display low MgO (0.38–1.17 wt%), CaO (1.06–5.31 wt%), LREE and LILE (Rb, K, Pb) enrichment, depletion in HREE, Sr, Nb, Ti, with strong negative Eu-anomaly (0.48–0.73), high initial 87Sr/86Sr (0.707878–0.717650), and negative εNd(t) (-14.35 to −9.21), suggesting A-type felsic magmatism. The older mafic intrusions were thus attributed to the events of the Gondwana assembly and were inferred to form in an extensional passive margin during the early Paleozoic. However, the younger felsic rocks were likely to have been generated by the interaction of the upwelling Kerguelen mantle plume and the pre-existing crust during the initiation of the eastern Gondwana break-up during the early Cretaceous. Our new findings reveal that two episodic magmatic events related to the eastern Gondwana assembly and the subsequent Gondwana break-up are responsible for the magmatism in the Siang window of eastern Himalayan syntaxis, northeast India. © 2022 International Association for Gondwana Research
New constraints on the tectono-magmatic evolution of the Tidding-Mayodia ophiolites, eastern Himalaya, northeast India
(John Wiley and Sons Ltd, 2022) Athokpam Krishnakanta Singh; Amrita Dutt; Bibhuranjan Nayak; Raj Kumar Bikramaditya; Govind Oinam; Satyajeet S. Thakur; Rajesh K. Srivastava; Shoraisam Khogenkumar; Manoj Kumar
The Tidding-Mayodia ophiolites (TMO) exposed along the Lohit and Dibang river valleys in eastern Himalaya that have been considered as the extension of the Indus-Tsangpo Suture Zone ophiolites are revisited to review their petrogenetic-tectonic origin. The ophiolites consist of depleted harzburgite and dunite with lesser amounts of mafic rocks (gabbro intrusives, mafic dykes) and carbonates. The serpentinized peridotites consist of antigorite, lizardite, olivine, Cr-spinel, and bastite with minor sulfide minerals. From SEM-EDS studies, sulfide minerals were observed to be associated mainly with magnetites. The main sulfide mineral is pentlandite with minor millerite that exists as inclusions inside the pentlandite grains. Elemental mapping of these sulfides shows that they are mainly Ni-(Co-)-bearing sulfides. The olivines are highly forsteritic (Fo = 95–96) while the Cr-spinels show distinct Cr-magnetite rims with a chromite core (Cr# = ~93). The serpentinized peridotites have whole-rock compositions of SiO2 <47 wt% and high MgO (>36.37 wt%) and low Al2O3 (<1.21 wt%), CaO (<0.82 wt%), indicating the depleted nature of the parent rocks. Highly fractionated LREEs as compared to HREEs [(La/Yb)N = 2.62–13.22], and REE and Cr spinel chemistry modelling suggests that the studied peridotites have formed from ~22% partial melting of a depleted spinel lherzolite source which later underwent interactions with a high-temperature silicate melt that caused enrichment in LREE and Cr of spinels. The parental melt compositions of Cr spinel yield their formation during arc tectonism (Al2O3melt = 6.28–7.65 wt%, FeO/MgO = 1.00–1.33). Furthermore, Mn and Zn concentrations in spinels, the occurrence of Cr magnetite rim in Cr-spinels, presence of secondary olivine with higher Fo (~98), and occurrence of low-temperature re-equilibrated sulfide minerals, indicate that the rocks were subject to low-temperature metamorphism. Based on this evidence, combined with data from previous studies, a tectonic model has been proposed for the genesis of the studied ophiolites. This model shows that the ophiolites have formed from the entrapment of depleted N-MORB mantle in the mantle wedge of an intra-oceanic subduction zone. During the nascent forearc regime, this mantle wedge underwent interactions with high-temperature melts, which caused changes in their chemistry. Moreover, the rocks underwent interactions with low-temperature fluids in the mature forearc, which caused the formation of sulfides and metamorphozed these rocks. © 2021 John Wiley & Sons Ltd.
Plagioclase ultraphyric basalts of the Abor magmatic complex: Implications for a plumbing system at the eastern Himalaya
(Elsevier B.V., 2024) Govind Oinam; A. Krishnakanta Singh; M. Santosh; Mallickarjun Joshi; Amrita Dutt; Shoraisam Khogenkumar; Biraja Prasad Das; R.K. Bikramaditya
Plagioclase ultraphyric basalts (PUBs) are an important unit of the Abor magmatic complex (AMC) of the eastern Himalaya, containing ≥35 vol% plagioclase phenocrysts. Apart from the eastern region, PUBs have not been reported in any other part of the Himalayas. However, very little information is available about their origin and significance in the evolution of the eastern margin of the Indian plate and the Himalayan orogeny. In this contribution, we present the first zircon U[sbnd]Pb age data of the PUBs along with whole-rock geochemistry, Sr[sbnd]Nd isotopic ratios, mineral chemistry, and quantitative textural analysis, to understand the evolutionary history of the AMC and subsurface magma chamber activities. The PUBs formed from highly evolved magma (<6 MgO wt%), having high Fe2O3 (9.06–12.29 wt%) and Ti/Y ratios (>500). Their εNd (t) values (−0.02 to +2.66) suggest plume magma source. A small difference in anorthite contents (<5 mol%) is observed from the thick core (An47–58) with lower anorthite contents towards the rim (An34–47) of the plagioclase phenocrysts, which is an indication of weakly zoned characteristics. Crystal size distribution shows a non-linear and concave upward trend with a relatively gentler slope towards the coarser plagioclase populations, which can be attributed to the hybrid crystallization of plagioclase-bearing magma and its subsequent differentiation with cumulates of plagioclase inside the magma chamber. The zircon U[sbnd]Pb age of these PUBs records two magmatic events - Early Paleozoic (505–473 Ma) at the core and Early Cretaceous (134–126 Ma) at the rim that are consistent with the previously proposed magmatic events of AMC with Gondwana assembly and break-up. Encounter of such dual ages in zircons does not support the usual condition of PUBs formation through crystal floatation in a slow cooling process of a single magma chamber. Therefore, considering the evidences observed in crystal size distribution, core-rim anorthite variation, geochemistry, and age data, we propose that the PUBs of AMC, eastern Himalaya were formed due to injection of a hot and young magma during the Early Cretaceous into an old and cold mush zone containing pre-existing plagioclase phenocrysts formed during the Early Paleozoic. The results further support that the newly injected magma formed the rim of the plagioclase phenocrysts and the groundmass of the PUBs. © 2024
Zircon U–Pb geochronology, Hf isotopic compositions, and petrogenetic study of Abor volcanic rocks of Eastern Himalayan Syntaxis, Northeast India: Implications for eruption during breakup of Eastern Gondwana
(John Wiley and Sons Ltd, 2020) Athokpam Krishnakanta Singh; Sun-Lin Chung; Rajkumar Bikramaditya; Hao-Yang Lee; Shoraisam Khogenkumar
This paper reports new zircon U–Pb ages and Hf isotopic compositions of felsic units of the Abor volcanic rocks (AVR) of Eastern Himalayan Syntaxis (EHS), Northeast India, and discusses their relationship to the Kerguelen plume activity. The AVR are bimodal and predominantly constituted by mafic rocks with minor felsic units. Mafic volcanics are identified as basalt and basaltic andesite with light rare earth elements (LREE) enriched and slightly depleted heavy rare earth elements (HREE) pattern without Eu anomalies. Low concentrations of LILE, high contents of Fe2O3, and other incompatible trace elements ratios reflect enriched nature of these mafic volcanics. Felsic volcanic rocks are dacitic to rhyolitic in composition, which have high REE content, high LREE/HREE, and pronounced negative Eu anomalies. Enriched LREE, high Th/Nb, Ce/Nb ratios, and variations in Rb/Zr, K/Rb, La/Sm ratios with negative anomalies of Ba, Nb, Sr, P, Ti in felsic rocks suggest substantial contribution of crustal contamination at the time of eruption. Zircons from felsic units yield an average U–Pb age of ~132 Ma and unradiogenic (ƐHf(t) < 0) Hf isotope values of −7.0 to −13.3 with model ages between 1.5 and 2.1 Ga, suggesting old crustal assimilation in their genesis. The AVR were emplaced in the continental rift tectonic setting, and depth of the magma source is confirmed as near spinel stability zone. The AVR are positively comparable with other flood basalts that were formed due to the Kerguelen plume activity. Therefore, our combined new geochemical and geochronological data show that the AVR were emplaced at early stage (~132 Ma) of eastern Gondwana breakup due to outbreak of the Kerguelen plume. This study thus supports the idea of the Kerguelen plume affecting a large area of Eastern India, Western Australia, and Antarctica during early stage of Gondwana breakup. © 2019 John Wiley & Sons, Ltd.
Zircon U–Pb geochronology, mineral and whole-rock geochemistry of the Khardung volcanics, Ladakh Himalaya, India: Implications for Late Cretaceous to Palaeogene continental arc magmatism
(John Wiley and Sons Ltd, 2020) Nongmaithem Lakhan; Athokpam Krishnakanta Singh; Birendra Pratap Singh; Koushik Sen; Mutum Rajanikanta Singh; Shoraisam Khogenkumar; Saurabh Singhal; Govind Oinam
In this study, we present new mineral and whole-rock geochemical data with zircon U–Pb ages of the Khardung volcanics (KV) from the western Himalaya and discuss their tectono-magmatic evolution. These volcanics are sandwiched between the Ladakh batholith and Karakoram batholith and classified as intermediate volcanics (basaltic andesite-andesite) and felsic volcanics (dacite-rhyolite). The intermediate volcanics are marked by low SiO2 (52.80–61.31 wt.%), enriched LREEs, and depleted HFSEs (Nb, Ti, Zr), whereas more evolved felsic volcanics exhibit quartz, K-feldspar, and plagioclase as dominant mineral phases and felsic compositions are characterized by high SiO2 (64.52–79.19 wt.%) content with pronounced negative Eu anomalies, enriched LREEs, and depleted HREEs and HFSEs (Nb, Ti). New zircon U–Pb ages of intermediate volcanics (andesite) yield 69.71 Ma, whereas felsic volcanics (rhyolites) range between 62.49 and 66.55 Ma, indicating that the Khardung magmatism overlaps with the last phase of the Ladakh batholith magmatism. Geochemical characteristics indicate that the KV were generated from a same parental magma source through fractional crystallization along with crustal assimilation from an older crust, and they show genetic affinity with the adjacent Ladakh batholith. Therefore, the KV and Ladakh batholith could be considered as a product of the mature stage arc magmatism generated during subduction of the Neo-Tethyan oceanic crust prior to the main collision between the Indian and Eurasian continents. © 2019 John Wiley & Sons, Ltd.