Browsing by Author "Sashimeren Imtisunep"

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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.
Refertilization of depleted mantle peridotite in the Nagaland–Manipur ophiolite, north-east India: Constraints from PGE, mineral, and whole-rock geochemistry
(John Wiley and Sons Ltd, 2022) Monika Chaubey; Athokpam Krishnakanta Singh; Birendra P. Singh; Sashimeren Imtisunep; Amrita Dutt; Manavalan Satyanarayanan; Kshetrimayum Premi; Sethuraman G. Abhirami
This paper discusses whole-rock geochemistry, mineral chemistry, and platinum group element (PGE) systematics of depleted mantle rocks (harzburgite and dunite) from the northern part of Nagaland–Manipur Ophiolite (NMO), north-east India, to comprehend their source features, fractionation behaviour of PGE during magmatic evolution, and its tectonic origin. The studied ultramafic rocks are characterized by a low concentration of CaO (0.57–0.71 wt%), Al2O3 (0.18–0.92 wt%) with ∑REE of 1.135–2.702 ppm and high concentrations of MgO (38.70–44.21 wt%), Cr (1,843–4,572 ppm), and Ni (894–4,138 ppm). They show U-shaped REE patterns [LREE and HREE enrichment (La/Sm)N = 1.85–4.11, (Dy/Yb)N = 0.51–0.85]. Olivine ranges Fo 88.18 to Fo92.23, whereas Cpx and Opx range En44.84 to En47.89 and En86.37 to En93.37 respectively. The chrome spinel Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe2+)] are 0.47–0.83 and 0.31–0.60, respectively, which indicates recrystallization from a boninitic magma in a Supra-Subduction Zone setting. Conventional thermometry indicates the equilibration temperatures of the dunite sample yielded high temperatures of ~850°C, suggesting their formation due to later interaction with high-temperature percolating melts. The PGE contents in harzburgite are low (125.6–142.8 ppb) as compared to the dunite (248–360 ppb). They have high PPGE/IPGE and negative Pt* (Pt/Pt* = 0.73) anomaly, which is characteristic of re-entry of PPGE into the system via reaction with percolating basaltic melt in the mantle wedge. Significantly higher concentration of PPGEs than IPGEs in the samples, indicating recrystallization of PPGEs with early sulphide fractionation. The presence of significant Rh and Pd enhancements relative to Pt in all samples suggests that Pt was removed during PGE fractionation. This could be one of the reasons for both harzburgite and dunite's sulphide undersaturation. PGE distribution in NMO ultramafic rocks was therefore validated as being governed by sulphide saturation in parental magma and altered not only by partial melting but also by fractionation during their production in the Supra-Subduction Zone environment. © 2022 John Wiley & Sons Ltd.