Browsing by Author "Manavalan Satyanarayanan"

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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.
Petrology and Geochemistry of a Calc-alkaline Lamprophyre-Diorite Dyke from Dancherla Area, in the Vicinity of Wajrakarur Kimberlite Field, Eastern Dharwar Craton
(Geological Society of India, 2025) Ashutosh Pandey; Rohit Pandey; Aparajita Tripathi; Manavalan Satyanarayanan; Aradhi Keshav Krishna; Mahendra Kumar Singh; Deepak Kumar; Kumari Minu Singh
The Eastern Dharwar Craton hosts several Proterozoic volatile-rich mafic-ultramafic alkaline intrusive magmatic rocks. Geological and geochemical studies on such rocks provide significant insights into the nature of the continental lithospheric/sub-lithospheric mantle and Precambrian geodynamic processes. In this study, we present petrology and geochemistry of a newly found calc-alkaline lamprophyre-diorite dyke from Dancherla area in the vicinity of the Wajrakarur Kimberlite Field of the Eastern Dharwar Craton. The lamprophyre consists of a high modal abundance of amphibole (magnesio-hornblende) phenocrysts and microphenocrysts along with clinopyroxene (diopside) microphenocrysts set in a feldspathic groundmass. The inequigranular texture of the lamprophyre gradually transitions into the equigranular (salt and pepper-like) texture of diorite along the trend of the dyke, with amphiboles and feldspar in almost equal proportions in the latter. Changing mineral mode and texture within the dyke points to the derivation of these rocks by crystal fractionation from the same hydrous parental melt. The amphibole and clinopyroxene-dominated crystal fractionation are evident from clustered phenocrysts of these minerals in the lamprophyres, giving rise to the glomeroporphyritic texture. As CaO is preferentially incorporated in clinopyroxene and amphibole, the decrease in CaO contents and CaO/Al2 O3 ratios with decreasing MgO, an index of differentiation supports fractional crystallization of clinopyroxene and amphibole. The predominance of calcic amphiboles, elevated Th/Yb and Zr/Y ratios, and significant depletion of high field strength elements suggest a calc-alkaline nature of the Dancherla lamprophyre. High LILE/HFSE in these rocks, like that found in the global calc-alkaline lamprophyres and arc-related magmatic rocks, reveals a subduction-fluid-aided enriched continental lithospheric mantle source of the parental magma. The timing of enrichment of the mantle source is suggested to be Neoarchean, during which the hot oceanic lithospheres accreted to the Western Dharwar Craton leading to the amalgamation of micro-blocks and cratonization of the Dharwar Craton. © 2025 Geological Society of India, Bengaluru, India.
Petrology of ultramafic and mafic rocks from the South Andaman Ophiolite, Bay of Bengal: Evidence for an arc-related high-pressure origin
(Springer, 2022) Tavheed Khan; Luc Achille Ziem A Bidias; Syed H. Jafri; Rohit Pandey; Nittala V. Chalapathi Rao; Manavalan Satyanarayanan; Drona Srinivasa Sarma
Minor ultramafic (dunite) and mafic (gabbroic) rock occurrences are exposed in South Andaman Island, Bay of Bengal. Dunite is in contact with serpentinite, while gabbroic rocks are in contact with the pyroxenite. Petrographic analysis using a petrographic microscope, major and trace element [including rare earth elements (REE)] analysis using an X-ray Fluorescence (XRF) spectrometer and the High Resolution Inductively Coupled Plasma Mass Spectrometer (HR-ICPMS), and mineral chemistry using an Electron Probe Micro-Analyzer (EPMA) were performed on selected ultramafic and mafic rocks. Petrographically, dunite is composed of olivine, clinopyroxene, and orthopyroxene, while olivine, clinopyroxene, orthopyroxene, and calcic plagioclase are present in olivine–gabbronorite. The bulk rock elemental relationship (Zr versus P2O5 and TiO2 versus Zr/P2O5) indicate that the dunite and olivine–gabbronorite are tholeiitic in composition. The clinopyroxene with high Mg# [Mg2+/(Mg2+ + Fe2+)] and lower TiO2 content is present in dunite, whereas the clinopyroxene with high Mg# and high TiO2 content exists in olivine–gabbronorite. Cr2O3 versus Mg# in the clinopyroxene relationship and negative Nb, Ta, and Ti anomalies in these rocks imply high pressure arc related peridotite mantle source. Our results suggest that the dunite and gabbroic rocks were also intruded in the Andaman Ophiolitic suite of rocks during earlier subduction setting in Late Cretaceous time. Further, it is suggested that these ophiolites have been obducted on to the leading edge of the Eurasian continent during the Mid–Eocene to Late Oligocene event, prior to the current tectonically active Andaman–Java subduction, which was initiated in the Late–Miocene. © 2022, The Author(s), under exclusive licence to Springer-Verlag GmbH Austria, part of Springer Nature.
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.