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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. [ABSTRACT FROM AUTHOR]

We present datasets of the whole‐rock geochemistry and U–Pb zircon geochronology from the Jutogh Thrust (JT) sheet of eastern Himachal Pradesh, NW Himalaya, in order to explain the tectono‐magmatic environment in which they formed. The metapelite and granitoids of the JT sheet have moderate to high SiO2, Al2O3, Na2O, K2O, and rare earth elements (REE). The A/CNK (Al2O3/CaO + Na2O + K2O) ratio with magnitudes ≥1, indicates the peraluminous nature, calc‐alkaline series, and S‐type granitoids. The high values of Rb and Th as compared to Nb and the negative Ba anomaly also suggest the dominance of crustal source material. The distinct enrichment of large‐ion lithophile elements of Rb, Th, K, Ba and Pb and strong depletion in high‐field‐strength elements (HFSE) of Zr, Nb and Ti results in high incompatible elements/HFSE ratios, which suggest that the source is derived from many intra‐crustal tectonic environments. All samples are well‐fractionated REE patterns [(La/Yb)N = 6.06–17.37] and characterized by moderately flat enriched [(La/Sm)N = 2.31–4.71] of light rare earth elements. The negative Eu anomalies (Eu/Eu* = 0.18–0.71), suggest the plagioclase fractionation from the sourced magma and the origin of granitic gneiss through the high temperature (~800°C). The zircon U–Pb ages of granite range between 883.0 ± 15.0 Ma and 1260.0 ± 10.0 Ma, with a consistent peak of ~925.0 Ma. It confirms that the northern edge of the Indian Plate collided with an unnamed microcontinent. Also, in a contemporaneous process, the upper crust underwent partial melting, and the generated magma intruded in an extensional tectonic set‐up of the northern edge (i.e., presently known as the Jutogh and Vaikrita thrust sheets) of the Indian Plate during the Neoproterozoic. The available zircon U–Pb ages of the JT sheet range between 550–2750 Ma. The magmatic nature of zircons, with two peaks of ~825 Ma and ~910 Ma, suggests that episodic magmatic processes have taken place during the Neoproterozoic. This study revealed that the Higher Himalayan Crystalline Sequence was a meta‐sedimentary sequence and was deposited in an active margin set‐up of the northern edge of the Indian Plate. [ABSTRACT FROM AUTHOR]

The ophiolite belt of Nagaland–Manipur states in Northeast India represents a segment of the ocean floor and upper mantle following eastward convergence of the Indian Plate with the Myanmar (Burmese) microplate during the Mesozoic. A variety of high‐pressure metamorphic assemblages have been noted in the metabasics and metacherts from the Phanerozoic ophiolite belt in the central part of the Naga Hills, which constitute the Nagaland Accretionary Prism. The metabasics are represented by very low‐grade assemblages of zeolite, prehnite‐pumpellyite, greenschists, and high‐pressure glaucophane schist and eclogite. We report the occurrence of aegirine‐bearing metabasic rocks, previously not recorded in the region. The metabasics are strongly fractionated and show chemical affinity with low‐K oceanic tholeiite. They are derived from a basic protolith of depleted mantle composition (viz. MORB). Pseudosection modelling reveals that aegirine formed at P–T conditions of c. 1.15 GPa and 490°C in the basic protolith in blueschist facies conditions. Similar P–T conditions have been reported from nearby localities, either as part of retrograde conditions or due to post‐peak cooling changes in the different metamorphic assemblages. However, the P–T observed from our study does not follow the cooling or retrograde path of the reported metabasics. Therefore, we suggest that the aegirine‐bearing metabasics might have formed at an earlier stage in the nascent forearc when temperatures were elevated enough to cause dehydration of the subducting slab to generate Na‐rich fluid fluxes at ambient pressures. [ABSTRACT FROM AUTHOR]

We present integrated in situ zircon U–Pb and Hf isotope data, along with whole‐rock and mineral chemistry data for the Salari granite of western Arunachal Himalaya to constrain its emplacement age, origin, and geodynamic evolution. The investigated Salari granites are high Fe2O3, CaO, and Nb, and low SiO2 and Rb/Sr ratio with fractionated rare earth element patterns ((Ce/Yb)N = 9.90–20.24) and minor negative Eu anomaly (Eu/Eu* = 0.69–0.94). They are metaluminous (molar A/CNK = 0.93–1.07) and have relatively similar FeOt/MgO ratio in biotite (1.58–1.60) to Mg‐biotite, indicating their affinity with I‐type granites. The enrichment of large‐ion lithophile elements with highly depleted negative Nb anomalies is consistent with their origin in a subduction‐related environment. Our zircon U–Pb ages suggest that the magmatic emplacement of the Salari granite took place between 1,791 and 1,768 Ma. The zircon grains have mostly negative εHf(t) values up to −5.5 and yield crustal Hf model ages from 2.4 to 2.8 Ga, suggesting the occurrence of a major crustal growth event in the Neoarchean and re‐melting of the crust during the Palaeoproterozoic. Our new results, that is, zircon U–Pb age and Hf isotope data, in conjunction with the field observations and petro‐mineralogical and geochemical characteristics, suggest that the Salari granite of eastern Himalaya was produced by partial melting of older metabasaltic/metatonalitic rocks in a continental arc setting of the supercontinent Columbia during the Palaeoproterozoic. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We present a comprehensive geochemical data set of whole‐rock geochemistry and mineral phases from the mantle peridotites and mafic intrusives of the Tuting–Tidding Suture Zone (TTSZ) ophiolites, eastern Himalaya, north‐east India. Modal mineralogy, low Al2O3 and CaO, high Cr# of Cr‐spinels, and forsterite number (Fo90–92) in primary olivine indicate the highly refractory nature of the TTSZ peridotites. LREE‐enriched patterns, cumulative olivines in dunites, and Cr‐spinel and olivine compositions further suggest that the peridotites were subjected to high‐temperature melt–rock interaction from a percolating boninitic melt in the nascent forearc of an intra‐oceanic subduction zone. The associated mafic intrusives are tholeiitic in nature and their geochemical characteristics [∑REE = 23.34–59.12; nearly flat rare earth elements (REE) pattern (LaN/YbN = 1.49–2.58); negative anomalies of Nb and Ti] show mid‐oceanic ridge basalt affinity. Trace elemental modelling of the mafic intrusives along with their mineralogy and geochemistry suggests that they formed due to different degrees of partial melting with the involvement of a subduction component in the spreading regime of the forearc region. Based on the presence of hydrous minerals like Cr‐chlorite and tremolite in the peridotites and P–T modelling (525–575°C, 1.05–1.09 GPa) of the mafic intrusives, it is considered that TTSZ ophiolites underwent low‐temperature metamorphism by fluid–rock interaction either during the later phase of subduction (in a cool mature subduction zone) or during Himalayan Orogeny. We also infer that the TTSZ ophiolites resemble other Neo‐Tethyan ophiolites of the Indus‐Tsangpo Suture Zone in terms of their geochemical and petrogenetic aspects. [ABSTRACT FROM AUTHOR]

The Nagaland‐Manipur Ophiolites (NMO) in northeast India is known for its complex geological history. Tough terrain, thick vegetation, and dismembered exposure of ophiolitic suite of rocks in the region made uneasy for geological investigation and put it in a deadlock for a long time. Only in the last decade has seen an appreciable amount of publications but the results boil down to a hot debate between two opposite schools of thoughts of subduction origin versus non‐subduction origin of the NMO. In this article, we revisit the literature data and compare it with our new geochemical data with an attempt to provide fresh insight into the long‐standing debate on the geodynamic evolution of the NMO. Our investigation arrives at the conclusion that the NMO as a whole cannot be considered as 100% subduction or 100% non‐subduction origin. It is indeed a combination of both. The non‐subduction group of mafic rocks shows a high ratio of incompatible elements (Nb/Yb >1), high‐Ti, enriched LILE, and HFSE with primitive mantle normalized values >5. Their bulk‐rock geochemical composition is equivalent to mid‐ocean ridge basalt (MORB) and ocean island basalt (OIB). The subduction group of rocks, on the other hand, shows a low ratio of incompatible elements (Nb/Yb <1), low‐Ti, depleted LILE, and HFSE with primitive mantle normalized values <1, affinity to the fore‐arc depleted N‐MORB type. Similarly, spinels present in subduction‐influenced mantle rocks show high chromium content (Cr# >50) but it is lower (Cr# <50) in non‐subduction abyssal peridotites of the NMO. Such geochemical variations cannot simply be explained by fractional crystallization or variable degree of partial melting of a single source, but rather signifies derivation from different tectonic settings of subduction and non‐subduction magma factories. We further conclude that the primary compressional force of India‐Myanmar Plate collision and secondary strike‐slip faults running along this ophiolite belt jeopardized the accretionary process which led to distortion and dismembering of the rocks like a scrambled bread. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]